Bismuth Related Data Storage Materials

*Bismuth - Fundamentals and Optoelectronic Applications*

[39] Yu BB, Li M, Yang J, Chen W, Zhu Y, Zhang X, et al. Alloy-induced phase transition and enhanced

2019;**7**:8818-8825

photovoltaic performance: The case of Cs3Bi2I9-xBrx perovskite solar cells. Journal of Materials Chemistry A.

Perovskites: A Benign Approach toward Low-Toxicity and Water-Stable Light Absorbers for Photovoltaic Applications. Energy Technology. 2019.

DOI: 10.1002/ente.201901185

Materials. 2015;**27**:6806

[34] Shin SS, Correa Baena JP, Kurchin RC, Polizzotti A, Yoo JJ, Wieghold S, et al. Solvent-engineering method to deposit compact bismuthbased thin films: Mechanism and application to photovoltaics. Chemistry

of Materials. 2018;**30**:336-343

[35] Jain SM, Phuyal D, Davies ML, Li M, Philippe B, De Castro C, et al. An effective approach of vapour assisted morphological tailoring for reducing metal defect sites in lead-free, (CH3NH3)(3)Bi2I9 bismuth-based perovskite solar cells for improved performance and long-term stability. Nano Energy. 2018;**49**:614-624

[36] Lan C, Liang G, Zhao S, Lan H, Peng H, Zhang D, et al. Lead-free formamidinium bismuth perovskites (FA)3Bi2I9 with low bandgap for potential photovoltaic application. Solar

Roknuzzaman M, Yun JH, Hao M, He D, et al. Phenethylammonium bismuth halides: from single crystals to bulkyorganic cation promoted thin-film deposition for potential optoelectronic applications. Journal of Materials Chemistry A. 2019;**7**:20733-20741

[38] Kulkarni A, Singh T, Ikegami M, Miyasaka T. Photovoltaic enhancement of bismuth halide hybrid perovskite by N-methyl pyrrolidone-assisted morphology conversion. RSC Advances.

Energy. 2019;**177**:501-507

[37] Ghasemi M, Lyu M,

[33] Park BW, Philippe B, Zhang X, Rensmo H, Boschloo G, Johansson EMJ. Bismuth based hybrid perovskites A3Bi2I9 (a: Methylammonium or cesium) for solar cell application. Advanced

**98**

2017;**7**:9456-9460

**Chapter 7**

Storage

**Abstract**

*Kuldeep Chand Verma*

Synthesis and Characterization of

Multiferroic BiFeO3 deals with spintronic devices involved spin-charge processes and applicable in new non-volatile memory devices to store information for computing performance and the magnetic random access memories storage. Since multiferroic leads to the new generation memory devices for which the data can be written electrically and read magnetically. The main advantage of present study of multiferroic BiFeO3 is that to observe magnetoelectric effects at room temperature. The

nanostructural growth (for both size and shape) of BiFeO3 may depend on the selection of appropriate synthesis route, reaction conditions and heating processes. In pure BiFeO3, the ferroelectricity is induced by 6s2 lone-pair electrons of Bi3+ ions and the Gtype antiferromagnetic ordering resulting from Fe3+ spins order of cycloidal (62-64 nm wavelength) occurred below Neel temperature, TN = 640 K. The multiferroicity of BiFeO3 is disappeared due to factors such as impurity phases, leakage current and low value of magnetization. Therefore, to overcome such factors to get multiferroic enhancement in BiFeO3, there are different possible ways like changes dopant ions and their concentrations, BiFeO3 composites as well as thin films especially multilayers.

Spintronic devices that electrically store non-volatile information are the poten-

interdependence between magnetization and charge transport phenomenon. Since the capacitor stored information in the form of charges and the electric field moved these charges to transmit information. However, the magnetic recording is caused when the magnetic field used to read or write the information stored in the form of magnetization by measuring local orientation of spins. The behavior started to change in 1988, when the discovery of giant magnetoresistance provides a way for efficient control of charge transport through magnetization [1, 2], for example, the hard-disk recording. Hard disk drives (HDD) with a capacity of 10 MB were sold for \$5300 in the 1980s, and were unaffordable for many during Apple and IBM PC era. However, HDDs with 16 TB capacity are available at the time of writing (2020). The computers in 1980s had memory of hundreds of kB that recently 8 GB random access memory (RAM). Even mobile gadgets have a dynamic random access memory (DRAM) capacity of 4 GB, at the time of writing. This is possible by use of

**Keywords:** electric-driven magnetic switching, chemical synthesis,

tial candidates for high-performance, high-density memories due to their

magnetoelectric, ferroelectric polarization

**1. Introduction**

**101**

Multiferroic BiFeO3 for Data

## **Chapter 7**

## Synthesis and Characterization of Multiferroic BiFeO3 for Data Storage

*Kuldeep Chand Verma*

## **Abstract**

Multiferroic BiFeO3 deals with spintronic devices involved spin-charge processes and applicable in new non-volatile memory devices to store information for computing performance and the magnetic random access memories storage. Since multiferroic leads to the new generation memory devices for which the data can be written electrically and read magnetically. The main advantage of present study of multiferroic BiFeO3 is that to observe magnetoelectric effects at room temperature. The nanostructural growth (for both size and shape) of BiFeO3 may depend on the selection of appropriate synthesis route, reaction conditions and heating processes. In pure BiFeO3, the ferroelectricity is induced by 6s2 lone-pair electrons of Bi3+ ions and the Gtype antiferromagnetic ordering resulting from Fe3+ spins order of cycloidal (62-64 nm wavelength) occurred below Neel temperature, TN = 640 K. The multiferroicity of BiFeO3 is disappeared due to factors such as impurity phases, leakage current and low value of magnetization. Therefore, to overcome such factors to get multiferroic enhancement in BiFeO3, there are different possible ways like changes dopant ions and their concentrations, BiFeO3 composites as well as thin films especially multilayers.

**Keywords:** electric-driven magnetic switching, chemical synthesis, magnetoelectric, ferroelectric polarization

## **1. Introduction**

Spintronic devices that electrically store non-volatile information are the potential candidates for high-performance, high-density memories due to their interdependence between magnetization and charge transport phenomenon. Since the capacitor stored information in the form of charges and the electric field moved these charges to transmit information. However, the magnetic recording is caused when the magnetic field used to read or write the information stored in the form of magnetization by measuring local orientation of spins. The behavior started to change in 1988, when the discovery of giant magnetoresistance provides a way for efficient control of charge transport through magnetization [1, 2], for example, the hard-disk recording. Hard disk drives (HDD) with a capacity of 10 MB were sold for \$5300 in the 1980s, and were unaffordable for many during Apple and IBM PC era. However, HDDs with 16 TB capacity are available at the time of writing (2020). The computers in 1980s had memory of hundreds of kB that recently 8 GB random access memory (RAM). Even mobile gadgets have a dynamic random access memory (DRAM) capacity of 4 GB, at the time of writing. This is possible by use of

charge property of the electron as well the spintronics devices make use of the spin property of an electron. Flash memory is an example, as it is a non-volatile memory as used in mobile applications [3]. However, the electrically induced bistable magnetization switching at room temperature - a necessary requirement for magnetic data storage - is the multiferroics [4]. The room-temperature manipulation of magnetization by an electric field using the multiferroic BiFeO3 represents an essential step toward magnetoelectric (ME) control for spintronics devices [5].

nanoparticles. For example, hard disk drives (use magnetic states to store information). In addition to hard disk drives, the STT-MRAM is a device that written and read electrically without any moving parts. The function of spin-transfer is to write information and such information is read by measuring the device resistance. The magnetoresistance plays role to measure percentage change in resistance between parallel and antiparallel magnetic spins of the electrodes of the magnetic tunnel junction. Such magnetic tunnel junction is made up by a ferromagnetic metal/ insulator/ferromagnetic material [7]. **Figure 1(b)** shows a 1-bit STT-MRAM cell constituted by free layer and reference layer that are magnetized perpendicular to the plane of the junction [7]. The cell is constituted with a word line with a

*Synthesis and Characterization of Multiferroic BiFeO3 for Data Storage*

*DOI: http://dx.doi.org/10.5772/intechopen.94049*

transistor that required for each cell. The biasing voltage could operate the cell with respect to bit lines and such read bias voltage measured the cell resistance to determine the bit state to be low of 100 mV. However, the write bias voltage is higher to allow the magnetic moment of the free layer taken to be reversed by using

The multiferroic BiFeO3 (BFO) has high Curie temperature, Tc 1103 K and Neel temperature, TN 643 K results into simultaneously ferroelectric and antiferromagnetic orders at room temperature. The ferroelectricity in BFO is originated by 6s<sup>2</sup> lone pair electrons of Bi3+ via structural distortion, however, magnetism resulted with Fe-O-Fe superexchange interactions [11]. But the reported study pointed out BFO with low spontaneous polarization and saturation magnetization because superimposition of a spiral spins structure of BFO by antiferromagnetic order. In such spiral spin structure, the antiferromagnetic axis rotates BFO crystal with 62 nm long wavelength, which cancels out the macroscopic magnetization as well as affects ME coupling value. The superexchange between the octahedrally coordinated Fe3+ through the O ligand is responsible for the resulting antiferromagnetism. The presence of oxygen vacancies and the valence fluctuation (Fe2+/Fe3+) believed

to be the main disadvantages causing large electrical leakage in BFO.

(111) plane are rotated clockwise and counterclockwise at 13.8<sup>o</sup>

*1.3.2 Lone-pair mechanism supporting multiferroicity*

The BFO structure is characterized with two distorted perovskite units connected by their body diagonal to build a rhombohedral unit cell as shown in **Figure 2(a)**. For such BFO structure, the two octahedral oxygen connected along

shifted 0.135 Å along same axis from oxygen that present at the octahedral position. The large displacement of Bi ions with respect to the FeO6 responsible to induced ferroelectric polarization [12–14]. The spins in this BFO structure are incommensurate to form antiferromagnetic order. There is also some canting moments to give weak ferromagnetism due to Dzyaloshinskii-Moriya (DM) effect because the moments may oriented perpendicular to the (111) polarization direction to influ-

The spatial asymmetry that caused by anisotropic unbounded valence electrons around Bi3+ might to give lone pair mechanism (**Figure 2(b)**) responsible into room temperature ferroelectric polarization of BFO [12]. In BiFeO3, a pair of Bi3+ valence electrons of the 6 *s* orbital not involved *sp* hybridization to generates a local dipole which resulting into <sup>100</sup> <sup>μ</sup>C cm<sup>2</sup> ferroelectric polarization below TC = 1103 K. A long-range periodic antiferromagnetic structure arises below TN = 643 K. The main

. However, Fe ion is

spin transfer torque.

*1.3.1 BiFeO3 structure*

ence symmetry properties.

**103**

**1.3 Multiferroic BiFeO3**

## **1.1 Multiferroic memories for data storage**

Multiferroic might hold the future for the ultimate memory device. The demonstration of a four-state resistive memory element in a tunnel junction with multiferroic barriers represents a major step in this direction [6–9]. For example, the thin films of lanthanum bismuth manganite remain ferromagnetic and ferroelectric down to thicknesses of 2 nm and, when used as a multiferroic tunneling junction, act as a four-state resistive system. Their spin-filter device as shown in **Figure 1(a)** is the tunnel junctions, which has tunnel barrier height is spin dependent because the bottom level of the conduction band in the ferromagnetic barrier is spin-split by exchange model. This allows the tunneling of electrons that to be efficiently filtered according to their spin. Gajek M *et al.* [10] suggested the large tunnel magnetoresistance in junctions that have a ferromagnetic electrode. The combination of these two effects - magnetoresistance plus electroresistance - yields a four-state resistive memory element. In comparison to the information stored in a capacitor, the resistive memories, on the other hand, can be read more simply, for example, by monitoring the source-drain current in a field-effect transistor. In order to make a multiple-state ME memory, one must be able to access the four states formed by electric polarization P and magnetization M, *i.e.* (+P, +M), (+P, M), (P, +M), and (P, M) [6].

## **1.2 Spin-transfer-torque magnetic random access memory**

The spin-transfer-torque magnetic random access memory (STT-MRAM) devices stored information due to use of magnetic orientation in the ferromagnetic

#### **Figure 1.**

*(a) Tunnel junction (electrons tunnel from bottom electrode through barrier into top electrode): Schematic [6]. (b) Spin-transfer-torque magnetic random access memory (STT-MRAM) bit cell. A magnetic tunnel junction (I), a tunnel barrier (II) and a free-layer element (III), with both layers magnetized perpendicular to the plane of the junction (arrows). Bit is selected by a word line and transistor [7].*

*Synthesis and Characterization of Multiferroic BiFeO3 for Data Storage DOI: http://dx.doi.org/10.5772/intechopen.94049*

nanoparticles. For example, hard disk drives (use magnetic states to store information). In addition to hard disk drives, the STT-MRAM is a device that written and read electrically without any moving parts. The function of spin-transfer is to write information and such information is read by measuring the device resistance. The magnetoresistance plays role to measure percentage change in resistance between parallel and antiparallel magnetic spins of the electrodes of the magnetic tunnel junction. Such magnetic tunnel junction is made up by a ferromagnetic metal/ insulator/ferromagnetic material [7]. **Figure 1(b)** shows a 1-bit STT-MRAM cell constituted by free layer and reference layer that are magnetized perpendicular to the plane of the junction [7]. The cell is constituted with a word line with a transistor that required for each cell. The biasing voltage could operate the cell with respect to bit lines and such read bias voltage measured the cell resistance to determine the bit state to be low of 100 mV. However, the write bias voltage is higher to allow the magnetic moment of the free layer taken to be reversed by using spin transfer torque.

## **1.3 Multiferroic BiFeO3**

charge property of the electron as well the spintronics devices make use of the spin property of an electron. Flash memory is an example, as it is a non-volatile memory as used in mobile applications [3]. However, the electrically induced bistable magnetization switching at room temperature - a necessary requirement for magnetic data storage - is the multiferroics [4]. The room-temperature manipulation of magnetization by an electric field using the multiferroic BiFeO3 represents an essential step toward magnetoelectric (ME) control for spintronics devices [5].

Multiferroic might hold the future for the ultimate memory device. The demon-

stration of a four-state resistive memory element in a tunnel junction with multiferroic barriers represents a major step in this direction [6–9]. For example, the thin films of lanthanum bismuth manganite remain ferromagnetic and ferroelectric down to thicknesses of 2 nm and, when used as a multiferroic tunneling junction, act as a four-state resistive system. Their spin-filter device as shown in **Figure 1(a)** is the tunnel junctions, which has tunnel barrier height is spin dependent because the bottom level of the conduction band in the ferromagnetic barrier is spin-split by exchange model. This allows the tunneling of electrons that to be efficiently filtered according to their spin. Gajek M *et al.* [10] suggested the large tunnel magnetoresistance in junctions that have a ferromagnetic electrode. The combination of these two effects - magnetoresistance plus electroresistance - yields a four-state resistive memory element. In comparison to the information stored in a capacitor, the resistive memories, on the other hand, can be read more simply, for example, by monitoring the source-drain current in a field-effect transistor. In order to make a multiple-state ME memory, one must be able to access the four states formed by electric polarization P and magnetization M, *i.e.* (+P, +M),

**1.1 Multiferroic memories for data storage**

*Bismuth - Fundamentals and Optoelectronic Applications*

(+P, M), (P, +M), and (P, M) [6].

**Figure 1.**

**102**

**1.2 Spin-transfer-torque magnetic random access memory**

The spin-transfer-torque magnetic random access memory (STT-MRAM) devices stored information due to use of magnetic orientation in the ferromagnetic

*(a) Tunnel junction (electrons tunnel from bottom electrode through barrier into top electrode): Schematic [6]. (b) Spin-transfer-torque magnetic random access memory (STT-MRAM) bit cell. A magnetic tunnel junction (I), a tunnel barrier (II) and a free-layer element (III), with both layers magnetized perpendicular to the plane*

*of the junction (arrows). Bit is selected by a word line and transistor [7].*

The multiferroic BiFeO3 (BFO) has high Curie temperature, Tc 1103 K and Neel temperature, TN 643 K results into simultaneously ferroelectric and antiferromagnetic orders at room temperature. The ferroelectricity in BFO is originated by 6s<sup>2</sup> lone pair electrons of Bi3+ via structural distortion, however, magnetism resulted with Fe-O-Fe superexchange interactions [11]. But the reported study pointed out BFO with low spontaneous polarization and saturation magnetization because superimposition of a spiral spins structure of BFO by antiferromagnetic order. In such spiral spin structure, the antiferromagnetic axis rotates BFO crystal with 62 nm long wavelength, which cancels out the macroscopic magnetization as well as affects ME coupling value. The superexchange between the octahedrally coordinated Fe3+ through the O ligand is responsible for the resulting antiferromagnetism. The presence of oxygen vacancies and the valence fluctuation (Fe2+/Fe3+) believed to be the main disadvantages causing large electrical leakage in BFO.

## *1.3.1 BiFeO3 structure*

The BFO structure is characterized with two distorted perovskite units connected by their body diagonal to build a rhombohedral unit cell as shown in **Figure 2(a)**. For such BFO structure, the two octahedral oxygen connected along (111) plane are rotated clockwise and counterclockwise at 13.8<sup>o</sup> . However, Fe ion is shifted 0.135 Å along same axis from oxygen that present at the octahedral position. The large displacement of Bi ions with respect to the FeO6 responsible to induced ferroelectric polarization [12–14]. The spins in this BFO structure are incommensurate to form antiferromagnetic order. There is also some canting moments to give weak ferromagnetism due to Dzyaloshinskii-Moriya (DM) effect because the moments may oriented perpendicular to the (111) polarization direction to influence symmetry properties.

#### *1.3.2 Lone-pair mechanism supporting multiferroicity*

The spatial asymmetry that caused by anisotropic unbounded valence electrons around Bi3+ might to give lone pair mechanism (**Figure 2(b)**) responsible into room temperature ferroelectric polarization of BFO [12]. In BiFeO3, a pair of Bi3+ valence electrons of the 6 *s* orbital not involved *sp* hybridization to generates a local dipole which resulting into <sup>100</sup> <sup>μ</sup>C cm<sup>2</sup> ferroelectric polarization below TC = 1103 K. A long-range periodic antiferromagnetic structure arises below TN = 643 K. The main

**Synthesis method**

Coprecipitation method

Hydrothermal process

Solution evaporation method

Microwaveassisted hydrothermal synthesis

Self-catalyzed fast reaction process

Conventional solid state

**105**

Wet chemical methods

Sol-gel method Hours/

day

**Reaction time**

*DOI: http://dx.doi.org/10.5772/intechopen.94049*

Hours Bi

Minutes Fe

Hours Fe

Minutes Bi

Hours/ days

**Precursor salts**

*Synthesis and Characterization of Multiferroic BiFeO3 for Data Storage*

(NO3)3.5H2O, Fe(NO3)3

Bi5H9N4O22, Fe(NO3)3

(NO3)39H2O, Bi2O3

(NO3)35H2O, FeCl36H2O

(NO3)3.9H2O,

(NO3)3.5H2O, HNO3

(NO3)3.5H2O,

(NO3)3.9H2O

Bi2O3, Fe2O3, Co3O4

Bi

Fe

Hours α-Fe2O3, Bi2O3,

Hours/ days

Bi

**Reaction condition Shape**

HNO3 used to adjust pH, Ethylene glycol & carboxylic acid are a polymerizing agent, annealed at 400°C/2 h

Precursor concentration adjusted in 0.05-0.2 M using acetic acid & ethylene alcohol, annealed at 200-500°C

NaOH used as a precipitating agent, maintain pH 12, annealed at 400-600°C/1 h

Precursor salts dissolved in acetone, pH adjusted 10-11 by ammonia solution, Precursor solution transferred into teflon-lined steel autoclave and heated at 180°C for

72 h

Tartaric acid and nitric acid used as precipitating and oxidizing agent, crystallization of the final powder takeplace at 650°C/2 h

NaOH solution and polyethylene glycol were added to the precursors to obtain brown precipitates, solution irradiated by 300 W of MW irradiation for 30 min at 190°C with 2450 Hz

5 mol of tartaric acid (C4H6O6) added to the precursors and heated at 250°C to begin to ignite and violently burn, final powder annealed at 650°C/2 h

Synthesis of BiFeO3 powder from Bi2O3

**control**

Good 13-70 nm particles

Good 10 nm

particles

Poor 200-250 nm particles

Good 22-31 nm particles

Good 20 nm

Poor 100 nm particles

Poor 100-200 nm particles

diameter wires

45-200 nm diameter wires

Very good **Shape/size Ref.**

[15, 16]

[17]

[18]

[19]

[20]

[21, 22]

[23]

[24]

#### **Figure 2.**

*(a) Rhombohedral (*R*3*c*) structure of BiFeO3. (b) Ferroelectricity of BiFeO3 due to lone-pair electrons (iso-surface: red) [12]. (c) Pseudocubic unit cell of BiFeO3, (d) spin cycloid of canted antiferromagnetic alignments [13].*

driving force of the ferroelectric phase transitions seems to be the stereochemical activity of the Bi 6 s lone-pair, resulting into a displacement of Bi and O sublattices. However, the Fe sublattice is also displaced and makes a sizeable contribution to the total electric polarization.

## *1.3.3 Ferroelectric and magnetic order in BiFeO3*

Bulk BiFeO3 crystallized into a slightly distorted rhombohedral structure which commonly described by the pseudocubic unit cell (**Figure 2(c)**). The displacement of Bi ions relative to the FeO6 octahedra gives rise to a strong ferroelectric polarization (100 μC cm<sup>2</sup> ) along one of the [111] directions [13]. However, the magnetization in BiFeO3 involved G-type antiferromagnet order with a cycloidal wavelength, λ 62-64 nm was investigated by high-resolution neutron diffraction. As shown in **Figure 2(d)**, the normalized position of the spin cycloid propagation and the ferroelectric polarization vector might to induce ME coupling effect.

## **2. Synthesis of multiferroic BiFeO3**

The some synthesis methods used for BiFeO3 are summarized in **Table 1**. Some of the listed methods are the derivatives of wet-chemical that deals with chemical reactions in the solution phase using precursors at proper stoichiometric conditions. Each wet-chemical synthesis route differs from the others in the sense that one cannot find a general rule for these kinds of synthesis approaches. Such wet synthesis schemes mostly used for fabrication of 2D nanomaterials. The wet-chemical processes offer a high degree of controllability and reproducibility of the 2D nanostructures. The solvothermal synthesis, template synthesis, self-assembly, oriented attachment, hot-injection, and interface-mediated synthesis are wet-chemical routes. However, the solvothermal and hydrothermal processes are mostly used to synthesize 2D nanostructures due to their simple and scalable steps.


### *Synthesis and Characterization of Multiferroic BiFeO3 for Data Storage DOI: http://dx.doi.org/10.5772/intechopen.94049*

driving force of the ferroelectric phase transitions seems to be the stereochemical activity of the Bi 6 s lone-pair, resulting into a displacement of Bi and O sublattices. However, the Fe sublattice is also displaced and makes a sizeable contribution to the

*(a) Rhombohedral (*R*3*c*) structure of BiFeO3. (b) Ferroelectricity of BiFeO3 due to lone-pair electrons (iso-surface: red) [12]. (c) Pseudocubic unit cell of BiFeO3, (d) spin cycloid of canted antiferromagnetic*

Bulk BiFeO3 crystallized into a slightly distorted rhombohedral structure which commonly described by the pseudocubic unit cell (**Figure 2(c)**). The displacement of Bi ions relative to the FeO6 octahedra gives rise to a strong ferroelectric polariza-

wavelength, λ 62-64 nm was investigated by high-resolution neutron diffraction. As shown in **Figure 2(d)**, the normalized position of the spin cycloid propagation and the ferroelectric polarization vector might to induce ME coupling effect.

The some synthesis methods used for BiFeO3 are summarized in **Table 1**. Some of the listed methods are the derivatives of wet-chemical that deals with chemical reactions in the solution phase using precursors at proper stoichiometric conditions. Each wet-chemical synthesis route differs from the others in the sense that one cannot find a general rule for these kinds of synthesis approaches. Such wet synthesis schemes mostly used for fabrication of 2D nanomaterials. The wet-chemical processes offer a high degree of controllability and reproducibility of the 2D nanostructures. The solvothermal synthesis, template synthesis, self-assembly, oriented attachment, hot-injection, and interface-mediated synthesis are wet-chemical routes. However, the solvothermal and hydrothermal processes are mostly used to

zation in BiFeO3 involved G-type antiferromagnet order with a cycloidal

synthesize 2D nanostructures due to their simple and scalable steps.

) along one of the [111] directions [13]. However, the magneti-

total electric polarization.

**Figure 2.**

*alignments [13].*

tion (100 μC cm<sup>2</sup>

**104**

*1.3.3 Ferroelectric and magnetic order in BiFeO3*

*Bismuth - Fundamentals and Optoelectronic Applications*

**2. Synthesis of multiferroic BiFeO3**


**3. Results and discussion**

**Table 1.**

**107**

**Synthesis method**

Anodized alumina template technique

Sol-gel based electrospinning **Reaction time**

*DOI: http://dx.doi.org/10.5772/intechopen.94049*

Hours Bi(NO3)3 5H2O, Fe(NO3)3 9H2O

Hours Bi(NO3)3 .5H2O, Fe(NO3)3 9H2O

**Precursor salts**

*Synthesis and Characterization of Multiferroic BiFeO3 for Data Storage*

**Reaction condition Shape**

ultrasonic radiation of 20 kHz, final product heated at 400°C/1 h

Precursor salts are mixed in 2 methoxyethanol, *p*H 4-5 by adding 2 methoxyethanol and nitric acid, anodized aluminum oxide template immersed in precursors for 12 h and heated at 750°C/12 h

Precursors salts are neutralized with 2 methoxyethanol, pH 3-4 adjusted with Ethanolamine, Ethanol, glacial acetic acid, and poly vinyl pyrrolidone (PVP) added, solution was electrospun and the ultrafine fibers spun were collected in glass flake or Pt/Ti/ SiO2/Si substrate, final heating at 550°

**control**

Very good

Very good **Shape/size Ref.**

Wires 50 nm in diameter

Nanofiber diameter in 100-300 nm [28]

[29]

**3.1 Structural analysis of BiFeO3**

*List the synthesis methods used to fabricate BiFeO3.*

effect on multiferroic properties.

*3.1.1 X-ray diffraction of Pb substituted BiFeO3*

**Figure 3(a)** shows the X-ray diffraction (XRD) pattern of Bi1-*x*Pb*x*FeO3 [*x* = 0 (BFO), 0.05 (BPFO5), 0.075 (BPFO75) and 0.1 (BPFO10)] nanostructures measured at room temperature [11]. All reflections are indexed to a rhombohedral structure of *R*3*c* space group. This is based on the character of the single (012) peak

C/2 h

. The

at around 22o and the splitting of the (104) and (110) peaks around 32o

splitting of XRD peaks indicate the structural distortion due to tilting of FeO6 octahedrons. The calculated lattice constants are *a*(Å) = 5.578, 5.577, 5.573 and 5.574 and *c*(Å) = 13.862, 13.893, 13.905 and 13.915, respectively for BFO, BPFO5, BPFO75 and BPFO10. The increase in lattice constant, *c* and decrease in *a* may be due to change in Fe-O bond lengths and Fe-O-Fe bond angles which have a significant


*Synthesis and Characterization of Multiferroic BiFeO3 for Data Storage DOI: http://dx.doi.org/10.5772/intechopen.94049*

**Table 1.**

**Synthesis method**

reaction and thin film deposition of BiFeO3- CoFe2O4

Chemical combustion method

Polymerdirected solvothermal

Sol-gel template process

Sonochemical technique

**106**

**Reaction time**

**Precursor salts**

*Bismuth - Fundamentals and Optoelectronic Applications*

Hours Bi(NO3)3,

Hours Bi(NO3)3 .5H2O, Fe(NO3)3 .9H2O

Hours/ days

Bi

Hours Bi(NO3)3

.5H2O, Fe (NO3)3 .9H2O,

Mn (OOCCH3)2 .4H2O, Cr (NO3)3 .9H2O

(NO3)3.5H2O, Fe(NO3)3. 9H2O

Fe(NO3)3

**Reaction condition Shape**

and Fe2O3 and heated at 800°C, CoFe2O4 powder prepared from Co3O4 and Fe2O3 using ball milling for 24 hours and heated at 1200°C/3 h, RF magnetron sputtering used for thin film deposition

Precursor solution mixed in

polyethylene glycol, Urea added at 70°C and the combustion take-place, annealed at 600°C/5 h

Precursor salt dissolved in HNO3 and dipped by ethanol containing 1 g of PVP, and added 1.2 g of NaOH, transferred precursor solution into Teflon liner steel autoclave and heated at 180°C/6 h

Porous nanochannel alumina (NCA) templates used, precursor salts mixed in nitric acid to get transparent, Citric acid & deionized water added, pH adjusted to be natural by using ammonia, urea added in 1/20th ratio, NCA templates added and heated at 80°C/20 h and annealed at 650°

C/5 h

In sonicated solution of Bi and Fe, add 5 ml of tetraethylene glycol and sonicated for 10 min, pH adjusted to 8 by adding ammonia and irradiated with a high intensity (100 W cm<sup>2</sup>

)

**control**

Poor 50-500 nm

1-D nanoparticlesassembled microrods

Nanotubes 150- 190 nm

Good Nanorods diameter 20-50 nm

Very good

Very good nanostructures

[12]

[25]

[26]

[27]

**Shape/size Ref.**

*List the synthesis methods used to fabricate BiFeO3.*

## **3. Results and discussion**

## **3.1 Structural analysis of BiFeO3**

## *3.1.1 X-ray diffraction of Pb substituted BiFeO3*

**Figure 3(a)** shows the X-ray diffraction (XRD) pattern of Bi1-*x*Pb*x*FeO3 [*x* = 0 (BFO), 0.05 (BPFO5), 0.075 (BPFO75) and 0.1 (BPFO10)] nanostructures measured at room temperature [11]. All reflections are indexed to a rhombohedral structure of *R*3*c* space group. This is based on the character of the single (012) peak at around 22o and the splitting of the (104) and (110) peaks around 32o . The splitting of XRD peaks indicate the structural distortion due to tilting of FeO6 octahedrons. The calculated lattice constants are *a*(Å) = 5.578, 5.577, 5.573 and 5.574 and *c*(Å) = 13.862, 13.893, 13.905 and 13.915, respectively for BFO, BPFO5, BPFO75 and BPFO10. The increase in lattice constant, *c* and decrease in *a* may be due to change in Fe-O bond lengths and Fe-O-Fe bond angles which have a significant effect on multiferroic properties.

However, oxygen atoms dominated with higher frequency modes above 262 cm<sup>1</sup>

**Figure 4(a)** shows the FESEM image of Bi0.9Pb0.1FeO3 nanostructure [11]. The diameter of the nanorods, D = 125 4 nm, and length, L = 900 20 nm. However for pure BFO, the nanoparticles (D = 75 2 nm) are formed [11]. This morphological variation with Pb doping into BFO is explained due to variation in the valence states of Fe ions due to oxygen vacancies. The substitution of Pb2+ into Bi3+ ions induces Fe2+/Fe3+ ions in the BFO matrix that can influence lattice defects (oxygen vacancies) in the rhombohedral BFO phase. It results into an anisotropic growth

**Figure 4(b)** shows a top-view SEM image of a square array with period 83 nm [32]. The bright rectangular islands visible in this image correspond to (111)-faceted tops of the CoFe2O4 pillars, while the darker area corresponds to single crystal

*(a) FESEM image of Bi0.9Pb0.1FeO3 [11]. (b) Top-view SEM image of BFO-CFO nanocomposite [32]. (c and d) AFM surface images of BTO33/BFO67 and BTO67/BFO33 [33]. (e) STEM dark-field image of*

The shifting and broadening of Raman modes with standard values suggests the presence of disorder and oxygen vacancies, and the internal microstrain due to Fe

ions which might to change resulting magnetism.

*Synthesis and Characterization of Multiferroic BiFeO3 for Data Storage*

*3.2.2 BiFeO3-CoFe2O4 self-assembled nanocomposite*

**3.2 Microstructural studies of BiFeO3**

*DOI: http://dx.doi.org/10.5772/intechopen.94049*

*3.2.1 FESEM image of Bi0.9Pb0.1FeO3*

along the *c*-axis.

**Figure 4.**

**109**

*(BaTiO3-BiFeO3) 15 grown on MgO [34].*

.

**Figure 3.**

*(a) XRD pattern of Bi1-*x*Pb*x*FeO3 multiferroic [11]. (b) Glancing angle X-ray diffraction (GAXRD) pattern of BiFeO3/BaTiO3 [thickness of BTO = 100 nm; while BFO = 50 nm (BFBT-5), 100 nm (BFBT-10), 150 nm (BFBT-15), and 200 nm (BFBT-20)] [30]. (c) Raman spectra of BiFeO3 [31].*

## *3.1.2 Crystalline structure of BiFeO3/BaTiO3 bilayer interface*

Glancing angle XRD patterns, recorded at incident angle 1°, on different BFO/BTO bilayer thin films sputtered on Pt/TiO2/SiO2/Si(100) substrates shown in **Figure 3(b)** [30]. Both BFO and BTO layers were found to be polycrystalline in nature without any impurity phase. The (110) plane of BFO appeared in θ-2θ mode only which was not be observed in GAXRD mode. It can be inferred that the bottom BTO layer promotes the formation of pure perovskite phase and high degree of (110) orientation in film texture.

## *3.1.3 Raman spectra of BiFeO3 nanoparticles*

The Raman spectrum of BFO obtained using 488 nm excitation wavelength depicted in **Figure 3(c)** [31]. The spectra have been deconvoluted into 10 individual components for BFO (3 A modes and 7 E modes). It was studied by DFT calculation of first principle that the low frequency Raman modes below 167 cm<sup>1</sup> are due to Bi atoms, and the modes between 152 and 262 cm<sup>1</sup> are due to Fe atoms [31].

However, oxygen atoms dominated with higher frequency modes above 262 cm<sup>1</sup> . The shifting and broadening of Raman modes with standard values suggests the presence of disorder and oxygen vacancies, and the internal microstrain due to Fe ions which might to change resulting magnetism.

## **3.2 Microstructural studies of BiFeO3**

## *3.2.1 FESEM image of Bi0.9Pb0.1FeO3*

**Figure 4(a)** shows the FESEM image of Bi0.9Pb0.1FeO3 nanostructure [11]. The diameter of the nanorods, D = 125 4 nm, and length, L = 900 20 nm. However for pure BFO, the nanoparticles (D = 75 2 nm) are formed [11]. This morphological variation with Pb doping into BFO is explained due to variation in the valence states of Fe ions due to oxygen vacancies. The substitution of Pb2+ into Bi3+ ions induces Fe2+/Fe3+ ions in the BFO matrix that can influence lattice defects (oxygen vacancies) in the rhombohedral BFO phase. It results into an anisotropic growth along the *c*-axis.

## *3.2.2 BiFeO3-CoFe2O4 self-assembled nanocomposite*

**Figure 4(b)** shows a top-view SEM image of a square array with period 83 nm [32]. The bright rectangular islands visible in this image correspond to (111)-faceted tops of the CoFe2O4 pillars, while the darker area corresponds to single crystal

## **Figure 4.**

*(a) FESEM image of Bi0.9Pb0.1FeO3 [11]. (b) Top-view SEM image of BFO-CFO nanocomposite [32]. (c and d) AFM surface images of BTO33/BFO67 and BTO67/BFO33 [33]. (e) STEM dark-field image of (BaTiO3-BiFeO3) 15 grown on MgO [34].*

*3.1.2 Crystalline structure of BiFeO3/BaTiO3 bilayer interface*

*Bismuth - Fundamentals and Optoelectronic Applications*

*(BFBT-15), and 200 nm (BFBT-20)] [30]. (c) Raman spectra of BiFeO3 [31].*

(110) orientation in film texture.

**Figure 3.**

**108**

*3.1.3 Raman spectra of BiFeO3 nanoparticles*

Glancing angle XRD patterns, recorded at incident angle 1°, on different BFO/BTO bilayer thin films sputtered on Pt/TiO2/SiO2/Si(100) substrates shown in **Figure 3(b)** [30]. Both BFO and BTO layers were found to be polycrystalline in nature without any impurity phase. The (110) plane of BFO appeared in θ-2θ mode only which was not be observed in GAXRD mode. It can be inferred that the bottom BTO layer promotes the formation of pure perovskite phase and high degree of

*(a) XRD pattern of Bi1-*x*Pb*x*FeO3 multiferroic [11]. (b) Glancing angle X-ray diffraction (GAXRD) pattern of BiFeO3/BaTiO3 [thickness of BTO = 100 nm; while BFO = 50 nm (BFBT-5), 100 nm (BFBT-10), 150 nm*

The Raman spectrum of BFO obtained using 488 nm excitation wavelength depicted in **Figure 3(c)** [31]. The spectra have been deconvoluted into 10 individual components for BFO (3 A modes and 7 E modes). It was studied by DFT calculation of first principle that the low frequency Raman modes below 167 cm<sup>1</sup> are due to Bi

atoms, and the modes between 152 and 262 cm<sup>1</sup> are due to Fe atoms [31].

BiFeO3 matrix. This BFO-CFO nanocomposite was grown by pulsed laser deposition. The film thickness was between 50 and 100 nm.

free electrons. It resulting into a higher conductivity and hence have a harmful influence on the ferroelectricity. The Pb doping into BFO could increase the grain size as well the oriented growth (nanorod-type) in the samples. From **Table 2**, there is a considerable reduction in the leakage current of BFO upon Pb2+ doping, which

**Figure 5(a)** and **(b)** shows the ferroelectric polarization and piezoelectric behavior of epitaxial BiFeO3 thin film [35]. The films display a room-temperature

*(a) A ferroelectric hysteresis of BiFeO3 thin film at 15 kHz. (b) a small signal d33 for a 50-μm capacitor [35]. (c) Room temperature PE loops of BaTiO3/BiFeO3/BaTiO3 trilayer thin films having different BiFeO3 thicknesses [36]. Temperature dependent relative permittivity (εr) of (d) BiFeO3 (e) Bi0.925Pb0.075FeO3 [11]. (f) Frequencies dependent dielectric permittivity at different temperatures for Bi(Co0.4Ti0.4Fe0.2)O3 (insets*

) almost an order of magnitude higher

). These results leads to the observations of

*3.3.2 Ferroelectric and piezoelectric properties of epitaxial BiFeO3 thin film*

*Synthesis and Characterization of Multiferroic BiFeO3 for Data Storage*

indicates reduction in oxygen vacancies.

*DOI: http://dx.doi.org/10.5772/intechopen.94049*

spontaneous polarization (50 to 60 μC cm<sup>2</sup>

than that of the bulk (6.1 μC cm<sup>2</sup>

**Figure 5.**

**111**

*shows lower temperature behavior) [37].*

## *3.2.3 Atomic force microscopy images of BTO/BFO thin films*

The BiFeO3 and BaTiO3 were used to grow homogeneous composite thin films and multilayer heterostructures with 15 double layers by pulsed laser deposition [33]. The thin films are composites grown directly from mixed PLD targets with 67 wt% BTO/33 wt% BFO (BTO67/BFO33) and 33 wt% BTO/67 wt% BFO (BTO33/ BFO67), respectively. **Figure 4(c)** and **(d)** are the AFM surface images of these two different composite films and the nanoparticle sizes are in the ranges 40-100 nm and 100-200 nm, respectively.

## *3.2.4 Scanning transmission electron microscopy (STEM) of BaTiO3-BiFeO3*

Multiferroic (BaTiO3-BiFeO3) 15 multilayer heterostructures show high ME coefficients, αME up to 24 V cm<sup>1</sup> Oe<sup>1</sup> at 300 K [34]. The STEM and SAED mapping results of the (BaTiO3-BiFeO3) 15 multilayer are depicted in **Figure 4(e)**. The STEM cross section is shown 15 double layers BaTiO3-BiFeO3 at smooth interfaces. The octahedral tilt involves both clockwise and counter clockwise rotations around [111], which is parallel to ferroelectric dipole displacements to *R*3*m* phase. The difference between the lattice parameters of the tetragonal BaTiO3 and rhombohedral BiFeO3 layers can be detected by reflection splitting along the growth direction (inset in **Figure 4(e)** top).

## **3.3 Ferroelectric behavior of BiFeO3**

The BiFeO3 is a rhombohedrally distorted perovskite material, which means that the ferroelectric polarization can have orientation along the four pseudo-cubic diagonals (<111>) [35–38]. The largest relative displacements are those of Bi relative to O, consistent with a stereochemically active Bi lone pair that might to induce ferroelectricity of BiFeO3.

## *3.3.1 Ferroelectric polarization of Pb doped BiFeO3 nanoparticles*

The ferroelectric polarization of multiferroic Bi1-*x*Pb*x*FeO3 nanostructures is given in **Table 2** [11]. In BFO, the lone-pair orbital of Bi3+ (6s<sup>2</sup> ) is stereochemically active and responsible for ferroelectric distortion. Here, the distortion is induced by Pb doping and therefore, by tuning the lone-pair activity. Generally, the ferroelectric behavior is weakened due to an increase in oxygen vacancies that form more


**Table 2.**

*Values of spontaneous polarization (Ps), remanent polarization (Pr), electric coercivity (Ec), current density (J) at 20 kV cm<sup>1</sup> , dielectric constant (ε) and MC at 1 kHz and 1 MHz, and ferroelectric phase transition (TFE) at 1 MHz for Bi1* <sup>x</sup>*Pb*x*FeO3 [*x *= 0 (BFO), 0.05 (BPFO5), 0.075 (BPFO75) and 0.1 (BPFO10)] nanostructures [11].*

## *Synthesis and Characterization of Multiferroic BiFeO3 for Data Storage DOI: http://dx.doi.org/10.5772/intechopen.94049*

free electrons. It resulting into a higher conductivity and hence have a harmful influence on the ferroelectricity. The Pb doping into BFO could increase the grain size as well the oriented growth (nanorod-type) in the samples. From **Table 2**, there is a considerable reduction in the leakage current of BFO upon Pb2+ doping, which indicates reduction in oxygen vacancies.

## *3.3.2 Ferroelectric and piezoelectric properties of epitaxial BiFeO3 thin film*

**Figure 5(a)** and **(b)** shows the ferroelectric polarization and piezoelectric behavior of epitaxial BiFeO3 thin film [35]. The films display a room-temperature spontaneous polarization (50 to 60 μC cm<sup>2</sup> ) almost an order of magnitude higher than that of the bulk (6.1 μC cm<sup>2</sup> ). These results leads to the observations of

#### **Figure 5.**

BiFeO3 matrix. This BFO-CFO nanocomposite was grown by pulsed laser deposi-

The BiFeO3 and BaTiO3 were used to grow homogeneous composite thin films and multilayer heterostructures with 15 double layers by pulsed laser deposition [33]. The thin films are composites grown directly from mixed PLD targets with 67 wt% BTO/33 wt% BFO (BTO67/BFO33) and 33 wt% BTO/67 wt% BFO (BTO33/ BFO67), respectively. **Figure 4(c)** and **(d)** are the AFM surface images of these two different composite films and the nanoparticle sizes are in the ranges 40-100 nm

Multiferroic (BaTiO3-BiFeO3) 15 multilayer heterostructures show high ME

mapping results of the (BaTiO3-BiFeO3) 15 multilayer are depicted in **Figure 4(e)**. The STEM cross section is shown 15 double layers BaTiO3-BiFeO3 at smooth interfaces. The octahedral tilt involves both clockwise and counter clockwise rotations around [111], which is parallel to ferroelectric dipole displacements to *R*3*m* phase. The difference between the lattice parameters of the tetragonal BaTiO3 and rhombohedral BiFeO3 layers can be detected by reflection splitting along the growth direction

The BiFeO3 is a rhombohedrally distorted perovskite material, which means that

the ferroelectric polarization can have orientation along the four pseudo-cubic diagonals (<111>) [35–38]. The largest relative displacements are those of Bi relative to O, consistent with a stereochemically active Bi lone pair that might to induce

The ferroelectric polarization of multiferroic Bi1-*x*Pb*x*FeO3 nanostructures is

active and responsible for ferroelectric distortion. Here, the distortion is induced by Pb doping and therefore, by tuning the lone-pair activity. Generally, the ferroelectric behavior is weakened due to an increase in oxygen vacancies that form more

BFO 0.75 0.35 7.66 10.31 6.2 5.1 0.68 0.61 644 BPFO5 1.76 0.63 6.03 6.27 1.9 1.8 2.56 1.59 631 BPFO75 3.55 1.57 8.55 2.72 13 10.6 3.61 0.36 649 BPFO10 6.73 2.63 5.06 1.57 34 12 0.86 0.11 629

*Values of spontaneous polarization (Ps), remanent polarization (Pr), electric coercivity (Ec), current density*

*(TFE) at 1 MHz for Bi1* <sup>x</sup>*Pb*x*FeO3 [*x *= 0 (BFO), 0.05 (BPFO5), 0.075 (BPFO75) and 0.1 (BPFO10)]*

*, dielectric constant (ε) and MC at 1 kHz and 1 MHz, and ferroelectric phase transition*

**J (μA cm<sup>2</sup> )** ) is stereochemically

**ε MC (%) TFE (K) 1 kHz 1 MHz 1 kHz 1 MHz**

*3.3.1 Ferroelectric polarization of Pb doped BiFeO3 nanoparticles*

given in **Table 2** [11]. In BFO, the lone-pair orbital of Bi3+ (6s<sup>2</sup>

**Ec (kV cm<sup>2</sup> )**

**Pr (μC cm<sup>2</sup> )**

*3.2.4 Scanning transmission electron microscopy (STEM) of BaTiO3-BiFeO3*

coefficients, αME up to 24 V cm<sup>1</sup> Oe<sup>1</sup> at 300 K [34]. The STEM and SAED

tion. The film thickness was between 50 and 100 nm.

*Bismuth - Fundamentals and Optoelectronic Applications*

and 100-200 nm, respectively.

(inset in **Figure 4(e)** top).

ferroelectricity of BiFeO3.

**Sample Ps**

**Table 2.**

**110**

*(J) at 20 kV cm<sup>1</sup>*

*nanostructures [11].*

**(μC cm<sup>2</sup> )**

**3.3 Ferroelectric behavior of BiFeO3**

*3.2.3 Atomic force microscopy images of BTO/BFO thin films*

*(a) A ferroelectric hysteresis of BiFeO3 thin film at 15 kHz. (b) a small signal d33 for a 50-μm capacitor [35]. (c) Room temperature PE loops of BaTiO3/BiFeO3/BaTiO3 trilayer thin films having different BiFeO3 thicknesses [36]. Temperature dependent relative permittivity (εr) of (d) BiFeO3 (e) Bi0.925Pb0.075FeO3 [11]. (f) Frequencies dependent dielectric permittivity at different temperatures for Bi(Co0.4Ti0.4Fe0.2)O3 (insets shows lower temperature behavior) [37].*

heteroepitaxial, in-plane compressive stress imposed by the epitaxial bottom electrode allows growth of a monoclinic crystal structure in BFO, and the degree of compressive stress progressively decreases with increasing BFO thickness. The piezoelectric hysteresis loop shows a remanent out-of-plane piezoelectric coefficient (d33) value 70 pm V<sup>1</sup> , representing the piezoresponse of the film in the fully clamped state.

cycloidal spin structure could induce uncompensated spins, enhancing the

is represented by the blocking temperature (TB which may attribute via superparamagnetic relaxation, glass transition, TN for antiferromagnetic-

*(a) Magnetization (M) as a function of temperature (T) following ZFC and FC at H = 500 Oe for BiFeO3*

*(Hac = 2.5 Oe without any dc field bias) [11]. (b) Magnetic hysteresis at 300 K for BiFeO3 nanoparticles with different nano-sizes. Inset shows the corresponding magnetization at 50 kOe as a function of 1/*d*. (c) Respective ZFC and FC curves at 200 Oe [39]. (d) Magnetic hysteresis of BiFeO3 nanoparticles grafted on graphene*

�*1*

*(T) following Curie-Weiss*

*ac) at T = 5-200 K*

*(BFO) and Bi0.9Pb0.1FeO3 (BPFO10) multiferroics. The respective insets are χ*

*law. (a: right) The temperature dependent real part of the* ac *magnetic susceptibility (χ*<sup>0</sup>

The ferromagnetic behavior of Pb substituted BiFeO3 is reported in ref. [11] that the maximum value of magnetization, M = 4.73, 8.41, 2.62 and 8.99 emu g�<sup>1</sup>

respectively, for BFO, BPFO5, BPFO75 and BPFO10. The origin of the variation of magnetization is analyzed with temperature dependent zero field (ZFC) and field cooled (FC) magnetization is shown in **Figure 6(a)**. The splitting of the ZFC/FC curves usually appears as the co-existent system of the antiferromagnetic and ferromagnetic phases. With nanostructural dimensions of less than 62 nm, there is a possibility of modification to the cycloidal spin structure of BFO, and that can lead to weak room temperature ferromagnetism. The sharp cusp observed around 65 and 79 K, respectively, for BFO and BPFO10 nanostructures in the ZFC curve

,

*3.5.1 Magnetization in multiferroic BiFeO3 and Bi0.9Pb0.1FeO3*

*Synthesis and Characterization of Multiferroic BiFeO3 for Data Storage*

*DOI: http://dx.doi.org/10.5772/intechopen.94049*

magnetization [45].

**Figure 6.**

**113**

*nanosheets (BiFeO3-*g*-GNS) [40].*

## *3.3.3 Ferroelectric hysteresis of BaTiO3/BiFeO3/BaTiO3*

**Figure 5(c)** represents polarization hysteresis loops of trilayer films of BaTiO3/ BiFeO3/BaTiO3 measured at 50 kV cm<sup>1</sup> applied electric field frequency of 10 kHz [36]. This trilayer thin film was prepared by RF-magnetron sputtering technique at different thicknesses of BiFeO3 layer. The thickness of BTO layer is 20 nm at the top and bottom, and the middle layer BFO is deposited with thicknesses of 20 nm (B-2), 40 nm (B-4), 60 nm (B-6), and 80 nm (B-8), respectively. The film showed maximum remnant electric polarization (2Pr) of 13.5 μC cm<sup>2</sup> and saturation magnetization (Ms) of 61 emu cc<sup>1</sup> at room temperature. The ferroelectric polarization was found to be improved with increasing thickness of BFO layer may be attributed to the reduced oxygen vacancies.

## **3.4 Dielectric properties of BiFeO3**

## *3.4.1 Frequency dependent dielectric properties of Bi(Co0.4Ti0.4Fe0.2)O3*

**Figure 5(f)** correlates the dielectric permittivity versus frequency plot at temperatures from 300 to 773 K of Bi(Co0.4Ti0.4Fe0.2)O3 multiferroic [37]. As the value of dielectric permittivity decreases upon increasing frequency, its nature could be described by Koop's hypothesis and Maxwell-Wagner mechanism. The nano grains with highly resistive grain boundaries might exist in an inhomogeneous medium from which the application of electric field constructs space charge polarization. With low frequency, the grain boundaries influence is more dominant to cause dispersion in dielectric properties. However, the higher frequency reduces the space charge polarization impact because the slow traveling species are not capable to trace an applied electric field.

## *3.4.2 Temperature dependent dielectric permittivity of Pb:BiFeO3*

**Figure 5(d)** and **(e)** shows the temperature dependent relative permittivity (εr) for BiFeO3 and Bi0.925Pb0.075FeO3 (BPFO75) multiferroics [11]. The value of ε<sup>r</sup> starts to increase with temperature because of TC for pure BFO is 1103 K. This change in ε<sup>r</sup> at 600-650 K for both Pb:BFO samples occurs due to occurrence of TN. For pure BFO, the value of TN is 643 K. The value of the ferroelectric phase transition (TFE) is 644 and 649, respectively, for BFO and BPFO75. This observation is an anomaly in the phase transition; TFE around TN confirms the ME coupling, which must be correlated with inverse DM-type interactions and Landau-Devonshire theory of phase transition. The variation in phase transition temperature with frequency for Pb:BFO nanostructures (inset of **Figure 5(d)** and **(e)**) indicates the emergence of the relaxor behavior which explained with an increase cation disorder in the B-site and Bi-site substitution by Pb2+.

## **3.5 Magnetization in BiFeO3**

The magnetization of BiFeO3 is reported in the refs. [39–44]. The cycloidal model of spin ordering in BFO is distorted at low temperatures. Any break to the *Synthesis and Characterization of Multiferroic BiFeO3 for Data Storage DOI: http://dx.doi.org/10.5772/intechopen.94049*

cycloidal spin structure could induce uncompensated spins, enhancing the magnetization [45].

## *3.5.1 Magnetization in multiferroic BiFeO3 and Bi0.9Pb0.1FeO3*

The ferromagnetic behavior of Pb substituted BiFeO3 is reported in ref. [11] that the maximum value of magnetization, M = 4.73, 8.41, 2.62 and 8.99 emu g�<sup>1</sup> , respectively, for BFO, BPFO5, BPFO75 and BPFO10. The origin of the variation of magnetization is analyzed with temperature dependent zero field (ZFC) and field cooled (FC) magnetization is shown in **Figure 6(a)**. The splitting of the ZFC/FC curves usually appears as the co-existent system of the antiferromagnetic and ferromagnetic phases. With nanostructural dimensions of less than 62 nm, there is a possibility of modification to the cycloidal spin structure of BFO, and that can lead to weak room temperature ferromagnetism. The sharp cusp observed around 65 and 79 K, respectively, for BFO and BPFO10 nanostructures in the ZFC curve is represented by the blocking temperature (TB which may attribute via superparamagnetic relaxation, glass transition, TN for antiferromagnetic-

#### **Figure 6.**

heteroepitaxial, in-plane compressive stress imposed by the epitaxial bottom electrode allows growth of a monoclinic crystal structure in BFO, and the degree of compressive stress progressively decreases with increasing BFO thickness. The piezoelectric hysteresis loop shows a remanent out-of-plane piezoelectric coefficient

**Figure 5(c)** represents polarization hysteresis loops of trilayer films of BaTiO3/ BiFeO3/BaTiO3 measured at 50 kV cm<sup>1</sup> applied electric field frequency of 10 kHz [36]. This trilayer thin film was prepared by RF-magnetron sputtering technique at different thicknesses of BiFeO3 layer. The thickness of BTO layer is 20 nm at the top and bottom, and the middle layer BFO is deposited with thicknesses of 20 nm (B-2), 40 nm (B-4), 60 nm (B-6), and 80 nm (B-8), respectively. The film showed maximum remnant electric polarization (2Pr) of 13.5 μC cm<sup>2</sup> and saturation magnetization (Ms) of 61 emu cc<sup>1</sup> at room temperature. The ferroelectric polarization was found to be improved with increasing thickness of BFO layer may be attributed

**Figure 5(f)** correlates the dielectric permittivity versus frequency plot at temperatures from 300 to 773 K of Bi(Co0.4Ti0.4Fe0.2)O3 multiferroic [37]. As the value of dielectric permittivity decreases upon increasing frequency, its nature could be described by Koop's hypothesis and Maxwell-Wagner mechanism. The nano grains with highly resistive grain boundaries might exist in an inhomogeneous medium from which the application of electric field constructs space charge polarization. With low frequency, the grain boundaries influence is more dominant to cause dispersion in dielectric properties. However, the higher frequency reduces the space charge polarization impact because the slow traveling species are not capable to

**Figure 5(d)** and **(e)** shows the temperature dependent relative permittivity (εr) for BiFeO3 and Bi0.925Pb0.075FeO3 (BPFO75) multiferroics [11]. The value of ε<sup>r</sup> starts to increase with temperature because of TC for pure BFO is 1103 K. This change in ε<sup>r</sup> at 600-650 K for both Pb:BFO samples occurs due to occurrence of TN. For pure BFO, the value of TN is 643 K. The value of the ferroelectric phase transition (TFE) is 644 and 649, respectively, for BFO and BPFO75. This observation is an anomaly in the phase transition; TFE around TN confirms the ME coupling, which must be correlated with inverse DM-type interactions and Landau-Devonshire theory of phase transition. The variation in phase transition temperature with frequency for Pb:BFO nanostructures (inset of **Figure 5(d)** and **(e)**) indicates the emergence of the relaxor behavior which explained with an increase cation disorder in the B-site

The magnetization of BiFeO3 is reported in the refs. [39–44]. The cycloidal model of spin ordering in BFO is distorted at low temperatures. Any break to the

, representing the piezoresponse of the film in the fully

(d33) value 70 pm V<sup>1</sup>

to the reduced oxygen vacancies.

trace an applied electric field.

and Bi-site substitution by Pb2+.

**3.5 Magnetization in BiFeO3**

**112**

**3.4 Dielectric properties of BiFeO3**

*3.3.3 Ferroelectric hysteresis of BaTiO3/BiFeO3/BaTiO3*

*Bismuth - Fundamentals and Optoelectronic Applications*

*3.4.1 Frequency dependent dielectric properties of Bi(Co0.4Ti0.4Fe0.2)O3*

*3.4.2 Temperature dependent dielectric permittivity of Pb:BiFeO3*

clamped state.

*(a) Magnetization (M) as a function of temperature (T) following ZFC and FC at H = 500 Oe for BiFeO3 (BFO) and Bi0.9Pb0.1FeO3 (BPFO10) multiferroics. The respective insets are χ* �*1 (T) following Curie-Weiss law. (a: right) The temperature dependent real part of the* ac *magnetic susceptibility (χ*<sup>0</sup> *ac) at T = 5-200 K (Hac = 2.5 Oe without any dc field bias) [11]. (b) Magnetic hysteresis at 300 K for BiFeO3 nanoparticles with different nano-sizes. Inset shows the corresponding magnetization at 50 kOe as a function of 1/*d*. (c) Respective ZFC and FC curves at 200 Oe [39]. (d) Magnetic hysteresis of BiFeO3 nanoparticles grafted on graphene nanosheets (BiFeO3-*g*-GNS) [40].*

ferromagnetic transition). The enhancement in magnetization is explained by Coey JMD *et al.* [46] model of F-center exchange mechanism where spinpolarized electrons were trapped at oxygen vacancies to cause higher magnetic moments. The upward curvature in FC curve of M(T) measurements of Pb:BFO suggested a Curie-Weiss like behavior **Figure 6(a)** (inset). The estimated value of θ is found to be negative which indicate to the formation of antiferromagnetic interactions.

magnetization measured to be 1.98 emu g<sup>1</sup>

*DOI: http://dx.doi.org/10.5772/intechopen.94049*

compromised on GNS grafting.

mica substrate.

**Figure 7.**

**115**

*BiFeO3/BaTiO3 thin films [30].*

reported data [49]. It means the magnetic properties of BiFeO3 are not

*Synthesis and Characterization of Multiferroic BiFeO3 for Data Storage*

*3.5.4 Magnetization of BiFeO3-CoFe2O4 (BFO-CFO) bulk heterojunction*

*3.5.5 Ferromagnetism in BiFeO3/BaTiO3 bilayer interface*

for those observed in BFBT-15 (15 emu cc<sup>1</sup>

**Figure 7(a)** exhibits the magnetic hysteresis in isotropic magnetic behavior along in plane and out of plane directions [41]. The value of Ms of BFO-CFO/mica is 237 emu cm<sup>3</sup> with Hc 2 kOe, which is smaller than from epitaxial CFO/STO (3 kOe). This may due to the effect of an effective relaxation of clamping from the

**Figure 7(b)** shows the ferromagnetic behavior of BiFeO3/BaTiO3 (BTO thickness = 100 nm; BFO = 50 nm (BFBT-5), 100 nm (BFBT-10), 150 nm (BFBT-15), and 200 nm (BFBT-20) films [30]. The observed ferromagnetism in the bilayer thin films can be interpreted due to creation of unbalanced spins at the interface. Maximum magnetization value Ms 33 emu cc<sup>1</sup> was observed in BFBT-5. The value of saturation magnetization is 20 emu cc<sup>1</sup> for BFBT-10 sample which is higher than

because with smaller antiferromagnetic nanoparticles, the size reduction has incomplete surface compensation of long-range antiferromagnetic ordering which result into increase magnetic moment at comparatively smaller size nanograins.

*(a) M-H hysteresis for BiFeO3-CoFe2O4 composite at room temperature [41]. (b) M-H loops of bilayer*

. These values well matched with

) and BFBT-20 (8 emu cc<sup>1</sup>

). This is

The *ac* magnetic susceptibility of Pb substituted BFO is measured at 1 Hz, 100 Hz, 1 kHz 10 kHz, and the temperature dependent real (*χ*<sup>0</sup> *ac*) *ac* magnetic susceptibility (χac) is shown in **Figure 6(a)**(right). The applied oscillating field, Hac = 2.5 Oe without any *dc* bias in T = 5-200 K. A quite sharp cusp is observed in both the samples. This *ac* magnetic measurement at different frequencies revealed the peak positions of *χ*<sup>0</sup> *ac*(T) curve shift toward higher temperature and the peak magnitudes drop down with rising frequency. Such behavior is expected for a spin glass system. The dynamic susceptibility measurements can thus be used to confirm such spin glass or superparamagnetic by using frequency dependence of Tf(ω) in the expression, *<sup>Δ</sup><sup>p</sup>* <sup>¼</sup> *<sup>Δ</sup><sup>T</sup> <sup>f</sup> <sup>T</sup> <sup>f</sup>Δ*ð Þ log *<sup>ω</sup>* . The calculated peak shift (Δp) per decade of frequency shift has a value 0.014 and 0.019, respectively, for BFO and BPFO10. These values of Δp are lower than those observed for superparamagnetic system (Δp is �0.1.54).

## *3.5.2 Nano size dependent magnetism of BiFeO3*

The SQUID results as shown in **Figure 6(b)** suggest that a magnetic response in BiFeO3 can be initiated when the size of the system is less than about 95 nm [39]. A plot of the magnetization, measured at the maximum applied field of Happl � 50 kOe as a function of 1/d, is given (inset of **Figure 6(b)**). Neel L [47] attributed the magnetic moment of small antiferromagnetic particles for the incomplete magnetic compensation between these two spin sublattices. For single-domain antiferromagnetic particles, the magnetization is expected to scale as �1/d(diameter), that is, as the surface to volume ratio [39]. For particles ranging in diameter from 95 to 41 nm, a linear dependence is observed, indicating that the simple Neel model is applicable [48]. The smallest nanoparticle is 14 nm that deviates from the expected behavior which indicates that such 14 nm nanoparticle may diminish the model for superposition of an antiferromagnetic core and a ferromagnetic surface. The maximum magnetization, obtained as Ms � 1.55 emu g�<sup>1</sup> for the 14 nm particles. **Figure 6(c)** shows the magnetization measurements as a function of temperature at an applied field strength of 200 Oe following ZFC and FC process. It is noted that the apparent sharp cusps observed in the magnetization curves at 50 K are reproducible for BFO samples with particle dimensions over 95 nm (e.g. 245 nm and bulk). For BiFeO3 nanoparticles possessing diameters of ≤95 nm, associated data curves exhibit a broad magnetization maximum around Tmax = 85 K. Tmax represents a spin-glasslike freezing temperature due to high packing volume fraction as well as a complex interplay between finite size effects, interparticle interactions, and a random distribution of anisotropy axes.

## *3.5.3 Temperature dependent magnetization of BiFeO3-g-GNS nanoparticles*

Magnetic properties of BiFeO3 grafted on graphene nanosheets (BiFeO3-*g*-GNS) is studied using SQUID VSM with an applied field of 5 T at different 10, 297 and 380 K **Figure 6(d)** [40]. Magnetic moments of nanoparticles get distorted at higher temperature. The maximum observed value of magnetization is 2.52 emu g�<sup>1</sup> at 10 K, while the minimum is 1.78 emu g�<sup>1</sup> at 380 K. At room temperature,

magnetization measured to be 1.98 emu g<sup>1</sup> . These values well matched with reported data [49]. It means the magnetic properties of BiFeO3 are not compromised on GNS grafting.

## *3.5.4 Magnetization of BiFeO3-CoFe2O4 (BFO-CFO) bulk heterojunction*

**Figure 7(a)** exhibits the magnetic hysteresis in isotropic magnetic behavior along in plane and out of plane directions [41]. The value of Ms of BFO-CFO/mica is 237 emu cm<sup>3</sup> with Hc 2 kOe, which is smaller than from epitaxial CFO/STO (3 kOe). This may due to the effect of an effective relaxation of clamping from the mica substrate.

## *3.5.5 Ferromagnetism in BiFeO3/BaTiO3 bilayer interface*

**Figure 7(b)** shows the ferromagnetic behavior of BiFeO3/BaTiO3 (BTO thickness = 100 nm; BFO = 50 nm (BFBT-5), 100 nm (BFBT-10), 150 nm (BFBT-15), and 200 nm (BFBT-20) films [30]. The observed ferromagnetism in the bilayer thin films can be interpreted due to creation of unbalanced spins at the interface. Maximum magnetization value Ms 33 emu cc<sup>1</sup> was observed in BFBT-5. The value of saturation magnetization is 20 emu cc<sup>1</sup> for BFBT-10 sample which is higher than for those observed in BFBT-15 (15 emu cc<sup>1</sup> ) and BFBT-20 (8 emu cc<sup>1</sup> ). This is because with smaller antiferromagnetic nanoparticles, the size reduction has incomplete surface compensation of long-range antiferromagnetic ordering which result into increase magnetic moment at comparatively smaller size nanograins.

**Figure 7.**

*(a) M-H hysteresis for BiFeO3-CoFe2O4 composite at room temperature [41]. (b) M-H loops of bilayer BiFeO3/BaTiO3 thin films [30].*

ferromagnetic transition). The enhancement in magnetization is explained by Coey JMD *et al.* [46] model of F-center exchange mechanism where spinpolarized electrons were trapped at oxygen vacancies to cause higher magnetic moments. The upward curvature in FC curve of M(T) measurements of Pb:BFO suggested a Curie-Weiss like behavior **Figure 6(a)** (inset). The estimated value of θ is found to be negative which indicate to the formation of antiferromagnetic

The *ac* magnetic susceptibility of Pb substituted BFO is measured at 1 Hz,

susceptibility (χac) is shown in **Figure 6(a)**(right). The applied oscillating field, Hac = 2.5 Oe without any *dc* bias in T = 5-200 K. A quite sharp cusp is observed in both the samples. This *ac* magnetic measurement at different frequencies revealed

magnitudes drop down with rising frequency. Such behavior is expected for a spin glass system. The dynamic susceptibility measurements can thus be used to confirm such spin glass or superparamagnetic by using frequency dependence of Tf(ω) in

quency shift has a value 0.014 and 0.019, respectively, for BFO and BPFO10. These values of Δp are lower than those observed for superparamagnetic system (Δp is

The SQUID results as shown in **Figure 6(b)** suggest that a magnetic response in BiFeO3 can be initiated when the size of the system is less than about 95 nm [39]. A plot of the magnetization, measured at the maximum applied field of Happl � 50 kOe as a function of 1/d, is given (inset of **Figure 6(b)**). Neel L [47] attributed the magnetic moment of small antiferromagnetic particles for the incomplete magnetic compensation between these two spin sublattices. For single-domain antiferromagnetic particles, the magnetization is expected to scale as �1/d(diameter), that is, as the surface to volume ratio [39]. For particles ranging in diameter from 95 to 41 nm, a linear dependence is observed, indicating that the simple Neel model is applicable [48]. The smallest nanoparticle is 14 nm that deviates from the expected behavior which indicates that such 14 nm nanoparticle may diminish the model for superposition of an antiferromagnetic core and a ferromagnetic surface. The maximum magnetization, obtained as Ms � 1.55 emu g�<sup>1</sup> for the 14 nm particles. **Figure 6(c)** shows the magnetization measurements as a function of temperature at an applied field strength of 200 Oe following ZFC and FC process. It is noted that the apparent sharp cusps observed in the magnetization curves at 50 K are reproducible for BFO samples with particle dimensions over 95 nm (e.g. 245 nm and bulk). For BiFeO3 nanoparticles possessing diameters of ≤95 nm, associated data curves exhibit a broad magnetization maximum around Tmax = 85 K. Tmax represents a spin-glasslike freezing temperature due to high packing volume fraction as well as a complex interplay between finite size effects, interparticle interactions, and a random distri-

*3.5.3 Temperature dependent magnetization of BiFeO3-g-GNS nanoparticles*

Magnetic properties of BiFeO3 grafted on graphene nanosheets (BiFeO3-*g*-GNS) is studied using SQUID VSM with an applied field of 5 T at different 10, 297 and 380 K **Figure 6(d)** [40]. Magnetic moments of nanoparticles get distorted at higher temperature. The maximum observed value of magnetization is 2.52 emu g�<sup>1</sup> at 10 K, while the minimum is 1.78 emu g�<sup>1</sup> at 380 K. At room temperature,

*ac*(T) curve shift toward higher temperature and the peak

*<sup>T</sup> <sup>f</sup>Δ*ð Þ log *<sup>ω</sup>* . The calculated peak shift (Δp) per decade of fre-

*ac*) *ac* magnetic

100 Hz, 1 kHz 10 kHz, and the temperature dependent real (*χ*<sup>0</sup>

*Bismuth - Fundamentals and Optoelectronic Applications*

interactions.

�0.1.54).

the peak positions of *χ*<sup>0</sup>

the expression, *<sup>Δ</sup><sup>p</sup>* <sup>¼</sup> *<sup>Δ</sup><sup>T</sup> <sup>f</sup>*

bution of anisotropy axes.

**114**

*3.5.2 Nano size dependent magnetism of BiFeO3*

## **3.6 Magnetocapacitance**

The ME behavior due to Magnetocapacitance (MC) effect in BiFeO3 is given in the refs. [50–54]. The MC effect is the change in the capacitance with an external applied magnetic field. This MC/magnetodielectric effect has a resistive origin that arises from the Maxwell-Wagner effect and magnetoresistance [55].

## *3.6.1 Dielectric constant and MC of Pb substituted BiFeO3*

**Table 2** shows the magnetic field affected dielectric constant of Pb:BFO nanostructures measured at room temperature. The frequency dependent relative permittivity (εr) of Pb:BFO in the frequency region 20 Hz-10 MHz under *dc* magnetic field (H = 0, 1 kOe) is given [11]. From **Table 2**, the improvement in dielectric constant with the substitution of Pb2+ for Bi3+ provides a larger vibration space to a larger dipole moment. Besides the oxygen vacancies due to ionic formation of Fe2+/ Fe3+ valence states, the shape/size of BFO grains might to influence the dielectric behaviors [11]. It is also observed from **Table 2** that the capacitance varies with applying magnetic field of 1 kOe, which indicate a positive/negative MC effect. The applied magnetic field leads to local stresses (or strains) and consequently changes in the polarization of the ferroelectric phase due to the piezoelectric effect. The values of MC {[ε(H) - ε(0) = Δε]/ε(0)} at a frequency of 1 MHz is 0.61, 1.59, 0.36 and 0.11%, respectively, calculated for BFO, BPFO5, BPFO75 and BPFO10 multiferroic.

## *3.6.2 Magnetocapacitance effect in BiFe0.95Sc0.05O3*

The magnetic field dependent capacitance for BiFe0.95Sc0.05O3 system to induce MC effect at 30 kHz is shown in **Figure 8(a)** [50]. The value of MC is 0.04% at applied magnetic field of 5 T which is higher than from pure BFO (0.007%). This type of MC behavior might be correlated with P<sup>2</sup> M2 in a Ginzburg-Landau free energy leads to a quadratic dependent dielectric constant in respect to magnetization. The observed results of MC in **Figure 8(a)** have magnetization-linear dielectric behavior which may proportional to P2 M of a linear MC effect.

## *3.6.3 Piezoelectric properties of BiFeO3/Na0.5Bi4.5Ti4O15 composite films*

Lead-free BiFeO3/Na0.5Bi4.5Ti4O15 (BFO/NBTO) composite films were deposited on Pt(100)/Ti/SiO2/Si substrates using chemical solution deposition [51]. A giant ME voltage coefficient has maximum α<sup>E</sup> = 136 mV cm<sup>1</sup> Oe<sup>1</sup> at Hbias = 8.0 kOe. **Figure 8(b)** shows the piezoelectric coefficient (d33) and surface displacement (d) vs. applied voltage (V) for BFO/NBTO films. A typical butterfly curve from D-V characteristics shows maximum value 4.06 nm at 15 V of 2.63% highest ratio of strain. The occurrence of d33-V piezoelectric hysteresis from D-V curve is the result converse piezoelectric effect. The d33-V loop clearly shows that BFO/NBTO composite films are switchable and the ferroelectricity is retained. The piezoelectric coefficient d33 of BFO/NBTO films is as high as 285 pm V<sup>1</sup> at 20 V, which suggests the strong piezoelectric effect for BFO/NBTO films.

observed, and at 6 kOe, the contraction is approximately 20 ppm. BaFe12O19 is a ferromagnetic material and can have a magnetostrictive response. However, due to the weak ferromagnetic nature of the BFO-BTO ceramics, it was difficult to detect the strain below 6 kOe. Because only a small amount of BaFe12O19 is generated in the Bi-deficient ceramics, the magnetostriction is also very small in the materials.

*(a) Magnetocapacitance of BiFe0.95Sc0.05O3 [50]. (b) Piezoelectric response of BiFeO3/Na0.5Bi4.5Ti4O15 (BFO/NBTO) composite [51]. (c) Magnetostriction of BiFeO3-BaTiO3, Bi0.8FeO2.7-BaTiO3, and BaFe12O19 measured at room temperature [52]. (d) P-E hysteresis under 0-0.6 T field for BFO-BTO nanoparticles. (Inset of d) P 0 at 0.6 T [53]. PFM study on BiFeO3 thin film: (e) AFM (f) amplitude image (g) amplitude*

*Synthesis and Characterization of Multiferroic BiFeO3 for Data Storage*

*DOI: http://dx.doi.org/10.5772/intechopen.94049*

The mechanism for the spin driven ferroelectricity must be involved the spincurrent model or inverse DM interaction due to the local electric polarization, p ∝ eij (Si Sj), where eij is the unit vector of adjacent spins, Si and Sj [56]. These cycloidal spin structures produced the macroscopic electric polarization P due to helicity of spins. The ferroelectric hysteresis under an external magnetic field and the flop near 0.6 T reveal a strong ME coupling in BFO-BTO multiferroics as shown

*3.6.5 ME coupling: ferroelectric polarization in an applied magnetic field*

**Figure 8.**

**117**

*behavior at an applied bias [54].*

## *3.6.4 Magnetostriction of BiFeO3-BaTiO3, Bi0.8FeO2.7-BaTiO3, and BaFe12O19*

The magnetostriction of BFO-BTO, B0.8FO2.7-BTO, and BaFe12O19 ceramics was measured using a resistive strain gauge when a dc magnetic field was applied to the materials (**Figure 8(c)**) [52]. A contraction induced by the magnetic field is

*Synthesis and Characterization of Multiferroic BiFeO3 for Data Storage DOI: http://dx.doi.org/10.5772/intechopen.94049*

#### **Figure 8.**

**3.6 Magnetocapacitance**

multiferroic.

**116**

The ME behavior due to Magnetocapacitance (MC) effect in BiFeO3 is given in the refs. [50–54]. The MC effect is the change in the capacitance with an external applied magnetic field. This MC/magnetodielectric effect has a resistive origin that

The magnetic field dependent capacitance for BiFe0.95Sc0.05O3 system to induce MC effect at 30 kHz is shown in **Figure 8(a)** [50]. The value of MC is 0.04% at applied magnetic field of 5 T which is higher than from pure BFO (0.007%). This

Lead-free BiFeO3/Na0.5Bi4.5Ti4O15 (BFO/NBTO) composite films were deposited on Pt(100)/Ti/SiO2/Si substrates using chemical solution deposition [51]. A giant ME voltage coefficient has maximum α<sup>E</sup> = 136 mV cm<sup>1</sup> Oe<sup>1</sup> at Hbias = 8.0 kOe. **Figure 8(b)** shows the piezoelectric coefficient (d33) and surface displacement (d) vs. applied voltage (V) for BFO/NBTO films. A typical butterfly curve from D-V characteristics shows maximum value 4.06 nm at 15 V of 2.63% highest ratio of strain. The occurrence of d33-V piezoelectric hysteresis from D-V curve is the result converse piezoelectric effect. The d33-V loop clearly shows that BFO/NBTO composite films are switchable and the ferroelectricity is retained. The piezoelectric coefficient d33 of BFO/NBTO films is as high as 285 pm V<sup>1</sup> at 20 V, which suggests

energy leads to a quadratic dependent dielectric constant in respect to magnetization. The observed results of MC in **Figure 8(a)** have magnetization-linear dielec-

*3.6.3 Piezoelectric properties of BiFeO3/Na0.5Bi4.5Ti4O15 composite films*

*3.6.4 Magnetostriction of BiFeO3-BaTiO3, Bi0.8FeO2.7-BaTiO3, and BaFe12O19*

materials (**Figure 8(c)**) [52]. A contraction induced by the magnetic field is

The magnetostriction of BFO-BTO, B0.8FO2.7-BTO, and BaFe12O19 ceramics was measured using a resistive strain gauge when a dc magnetic field was applied to the

M2 in a Ginzburg-Landau free

M of a linear MC effect.

arises from the Maxwell-Wagner effect and magnetoresistance [55].

**Table 2** shows the magnetic field affected dielectric constant of Pb:BFO nanostructures measured at room temperature. The frequency dependent relative permittivity (εr) of Pb:BFO in the frequency region 20 Hz-10 MHz under *dc* magnetic field (H = 0, 1 kOe) is given [11]. From **Table 2**, the improvement in dielectric constant with the substitution of Pb2+ for Bi3+ provides a larger vibration space to a larger dipole moment. Besides the oxygen vacancies due to ionic formation of Fe2+/ Fe3+ valence states, the shape/size of BFO grains might to influence the dielectric behaviors [11]. It is also observed from **Table 2** that the capacitance varies with applying magnetic field of 1 kOe, which indicate a positive/negative MC effect. The applied magnetic field leads to local stresses (or strains) and consequently changes in the polarization of the ferroelectric phase due to the piezoelectric effect. The values of MC {[ε(H) - ε(0) = Δε]/ε(0)} at a frequency of 1 MHz is 0.61, 1.59, 0.36 and 0.11%, respectively, calculated for BFO, BPFO5, BPFO75 and BPFO10

*3.6.1 Dielectric constant and MC of Pb substituted BiFeO3*

*Bismuth - Fundamentals and Optoelectronic Applications*

*3.6.2 Magnetocapacitance effect in BiFe0.95Sc0.05O3*

type of MC behavior might be correlated with P<sup>2</sup>

the strong piezoelectric effect for BFO/NBTO films.

tric behavior which may proportional to P2

*(a) Magnetocapacitance of BiFe0.95Sc0.05O3 [50]. (b) Piezoelectric response of BiFeO3/Na0.5Bi4.5Ti4O15 (BFO/NBTO) composite [51]. (c) Magnetostriction of BiFeO3-BaTiO3, Bi0.8FeO2.7-BaTiO3, and BaFe12O19 measured at room temperature [52]. (d) P-E hysteresis under 0-0.6 T field for BFO-BTO nanoparticles. (Inset of d) P 0 at 0.6 T [53]. PFM study on BiFeO3 thin film: (e) AFM (f) amplitude image (g) amplitude behavior at an applied bias [54].*

observed, and at 6 kOe, the contraction is approximately 20 ppm. BaFe12O19 is a ferromagnetic material and can have a magnetostrictive response. However, due to the weak ferromagnetic nature of the BFO-BTO ceramics, it was difficult to detect the strain below 6 kOe. Because only a small amount of BaFe12O19 is generated in the Bi-deficient ceramics, the magnetostriction is also very small in the materials.

## *3.6.5 ME coupling: ferroelectric polarization in an applied magnetic field*

The mechanism for the spin driven ferroelectricity must be involved the spincurrent model or inverse DM interaction due to the local electric polarization, p ∝ eij (Si Sj), where eij is the unit vector of adjacent spins, Si and Sj [56]. These cycloidal spin structures produced the macroscopic electric polarization P due to helicity of spins. The ferroelectric hysteresis under an external magnetic field and the flop near 0.6 T reveal a strong ME coupling in BFO-BTO multiferroics as shown in **Figure 8(d)** [53]. Without applying H, values of Pmax = 24.80 μC cm<sup>2</sup> , Pr = 15.13 μC cm<sup>2</sup> and Ec = 53.6 kV cm<sup>1</sup> are observed. Switching from +Pr to -Pr by E, and magnetic switching from +Pr to zero at 0.6 T have been observed. The inset of **Figure 8(d)** shows a low polarization response with lossy hysteresis of BFO-BTO composite at 0.6 T, which may cause by the electrode. When a magnetic field is applied to a ME material, the material is under strain to induce a stress on the piezoelectrics (ferroelectric) to orient ferroelectric domains, leading to enhance polarization.

## *3.6.6 ME effect in BiFeO3 by PFM*

**Figure 8(e)** shows AFM, out of plane piezoresponse amplitude of the BFO thin film which leads to agglomeration of the nanoparticles of average size 10 nm [55]. It can be seen from **Figure 8(f)** that BFO nanoparticles exhibits positive and negative polarization components and the ferroelectric domains are constrained at grain boundaries. The PFM amplitude under bias voltage is shown in **Figure 8(g)**. It observed ferroelectric hysteresis for dc voltage sweeps in +0.5 V to 0.5 V to +0.5 V which indicates that sub - 5 nm BFO nanoparticles retains their ferroelectric behavior that might be usable for read-write operation.

## **3.7 Magnetoelectric (ME) coupling**

## *3.7.1 ME coupling in Bi0.88Dy0.12Fe0.97Ti0.03O3+<sup>δ</sup> (BDFO) and BiFeO3*

The ME effect of BFO and BDFO was measured, and the results are shown in **Figure 9(a)** [57]. It can be seen that the pure BiFeO3 exhibits no ME signal under bias magnetic field because spatially incommensurately modulated spin structure that cancels out the linear ME effect. In contrast, BDFO exhibits a strong ME signal under bias magnetic field at 300 K due to the ME coefficient αE-magnetic field (αE-H) hysteresis to include the features of saturation at field of 250 Oe. This is because the switching of electric polarization by either 109o or 71<sup>o</sup> leads to switching of the ferroelastic domain states [58–60]. The incorporation of Dy3+ into BFO suppresses the spiral spin structure, leads to weak ferromagnetism. Because the magnetic moment lags behind the variation of magnetic field, the electric polarization induced by magnetic field through ME effect is also lagged behind the variation of magnetic field, giving αE(H) hysteresis.

## *3.7.2 ME coupling of 15 (10 nm BaTiO3-5 nm BiFeO3)*

The ME coefficient was measured as a function of dc bias field in **Figure 9(b)** for 15 (10 nm BaTiO3-5 nm BiFeO3) [58]. The ME coefficient reaches its maximum, off - resonance, value of 60.2 V cm<sup>1</sup> Oe<sup>1</sup> at a bias field of 2 T. This higher value is described due to the rotation of the oxygen octahedra with the antiferrodistortion vector Ω and shows that the linear ME effect depends on the derivative of Ω with respect to the external field. This leads to giant values of αME.

multilayer additionally contains piezoelectric/piezomagnetic interfaces, an extra ME coupling in multilayers may occur via the horizontal interfaces through strainmediated interface coupling. The magnetostrictive stress is produced in the weak ferromagnetic BiFeO3 layer and is transferred to ferroelectric BaTiO3 layer through the interface (**Figure 9(d)**). This mechanical stress generates an electric potential

*(a) Magnetoelectric P(H) hysteresis at 300 K, showing ME coefficient of Bi0.88Dy0.12Fe0.97Ti0.03O3+<sup>δ</sup> (BDFO) and BFO multiferroic [57]. (b) ME coefficient of the multilayers 15* � *(10 nm BaTiO3 - 5 nm BiFeO3) at room temperature [58]. (c) ME coefficient αME of 15* � *[BaTiO3/BiFeO3] multilayer film and pure BiFeO3. (d) Ferroelectric-multiferroic multilayer (left) showing the ME effect through magnetostrictive-piezoelectric interface coupling (arrows represent magnetostrictive stress); and a BiFeO3 film (right) [59]. (e) MEimpedance response of BaTiO3/BiFeO3/BaTiO3 trilayer film measured at room temperature (magnetomodulus*

*3.7.4 Magnetoimpedance and ME effects of BaTiO3/BiFeO3/BaTiO3 heterostructure*

an equivalent circuit of two series RC-elements. Two well resolved semicircles

The ME effect for BaTiO3/BiFeO3/BaTiO3 heterostructure is also investigated by

) under applied magnetic fields (**Figure 9(e)**) [36]. The data were fitted with

) as well as complex modulus plots (M<sup>00</sup>

difference in the ferroelectric layer via a piezoelectric effect.

*plots: inset). (f) ME coefficient for BTO/BFO/BTO trilayer films [36].*

*Synthesis and Characterization of Multiferroic BiFeO3 for Data Storage*

*DOI: http://dx.doi.org/10.5772/intechopen.94049*

analyzing complex impedance (�Z<sup>00</sup> vs. Z<sup>0</sup>

vs. M<sup>0</sup>

**119**

**Figure 9.**

## *3.7.3 ME coupling of 15 [BaTiO3/BiFeO3] multilayer*

The effect of strain and interfaces on the ME coupling in BiFeO3 was studied for a number of thin films and multilayers of 15 [BaTiO3/BiFeO3] as shown in **Figure 9(c)** [33, 59]. To this end, a direct longitudinal ac method was used to measure the ME coefficient αME as a function of static magnetic field. It is clear that αME of the multilayer is notably larger than that of the BFO film. Since the

*Synthesis and Characterization of Multiferroic BiFeO3 for Data Storage DOI: http://dx.doi.org/10.5772/intechopen.94049*

#### **Figure 9.**

in **Figure 8(d)** [53]. Without applying H, values of Pmax = 24.80 μC cm<sup>2</sup>

polarization.

*3.6.6 ME effect in BiFeO3 by PFM*

ior that might be usable for read-write operation.

*Bismuth - Fundamentals and Optoelectronic Applications*

variation of magnetic field, giving αE(H) hysteresis.

*3.7.2 ME coupling of 15 (10 nm BaTiO3-5 nm BiFeO3)*

*3.7.3 ME coupling of 15 [BaTiO3/BiFeO3] multilayer*

**118**

*3.7.1 ME coupling in Bi0.88Dy0.12Fe0.97Ti0.03O3+<sup>δ</sup> (BDFO) and BiFeO3*

**3.7 Magnetoelectric (ME) coupling**

μC cm<sup>2</sup> and Ec = 53.6 kV cm<sup>1</sup> are observed. Switching from +Pr to -Pr by E, and magnetic switching from +Pr to zero at 0.6 T have been observed. The inset of **Figure 8(d)** shows a low polarization response with lossy hysteresis of BFO-BTO composite at 0.6 T, which may cause by the electrode. When a magnetic field is applied to a ME material, the material is under strain to induce a stress on the piezoelectrics (ferroelectric) to orient ferroelectric domains, leading to enhance

**Figure 8(e)** shows AFM, out of plane piezoresponse amplitude of the BFO thin film which leads to agglomeration of the nanoparticles of average size 10 nm [55]. It can be seen from **Figure 8(f)** that BFO nanoparticles exhibits positive and negative polarization components and the ferroelectric domains are constrained at grain boundaries. The PFM amplitude under bias voltage is shown in **Figure 8(g)**. It observed ferroelectric hysteresis for dc voltage sweeps in +0.5 V to 0.5 V to +0.5 V which indicates that sub - 5 nm BFO nanoparticles retains their ferroelectric behav-

The ME effect of BFO and BDFO was measured, and the results are shown in **Figure 9(a)** [57]. It can be seen that the pure BiFeO3 exhibits no ME signal under bias magnetic field because spatially incommensurately modulated spin structure that cancels out the linear ME effect. In contrast, BDFO exhibits a strong ME signal under bias magnetic field at 300 K due to the ME coefficient αE-magnetic field (αE-H) hysteresis to include the features of saturation at field of 250 Oe. This is because the switching of electric polarization by either 109o or 71<sup>o</sup> leads to

switching of the ferroelastic domain states [58–60]. The incorporation of Dy3+ into BFO suppresses the spiral spin structure, leads to weak ferromagnetism. Because the magnetic moment lags behind the variation of magnetic field, the electric polarization induced by magnetic field through ME effect is also lagged behind the

The ME coefficient was measured as a function of dc bias field in **Figure 9(b)** for 15 (10 nm BaTiO3-5 nm BiFeO3) [58]. The ME coefficient reaches its maximum, off - resonance, value of 60.2 V cm<sup>1</sup> Oe<sup>1</sup> at a bias field of 2 T. This higher value is described due to the rotation of the oxygen octahedra with the antiferrodistortion vector Ω and shows that the linear ME effect depends on the derivative of

The effect of strain and interfaces on the ME coupling in BiFeO3 was studied for

a number of thin films and multilayers of 15 [BaTiO3/BiFeO3] as shown in **Figure 9(c)** [33, 59]. To this end, a direct longitudinal ac method was used to measure the ME coefficient αME as a function of static magnetic field. It is clear that

αME of the multilayer is notably larger than that of the BFO film. Since the

Ω with respect to the external field. This leads to giant values of αME.

, Pr = 15.13

*(a) Magnetoelectric P(H) hysteresis at 300 K, showing ME coefficient of Bi0.88Dy0.12Fe0.97Ti0.03O3+<sup>δ</sup> (BDFO) and BFO multiferroic [57]. (b) ME coefficient of the multilayers 15* � *(10 nm BaTiO3 - 5 nm BiFeO3) at room temperature [58]. (c) ME coefficient αME of 15* � *[BaTiO3/BiFeO3] multilayer film and pure BiFeO3. (d) Ferroelectric-multiferroic multilayer (left) showing the ME effect through magnetostrictive-piezoelectric interface coupling (arrows represent magnetostrictive stress); and a BiFeO3 film (right) [59]. (e) MEimpedance response of BaTiO3/BiFeO3/BaTiO3 trilayer film measured at room temperature (magnetomodulus plots: inset). (f) ME coefficient for BTO/BFO/BTO trilayer films [36].*

multilayer additionally contains piezoelectric/piezomagnetic interfaces, an extra ME coupling in multilayers may occur via the horizontal interfaces through strainmediated interface coupling. The magnetostrictive stress is produced in the weak ferromagnetic BiFeO3 layer and is transferred to ferroelectric BaTiO3 layer through the interface (**Figure 9(d)**). This mechanical stress generates an electric potential difference in the ferroelectric layer via a piezoelectric effect.

## *3.7.4 Magnetoimpedance and ME effects of BaTiO3/BiFeO3/BaTiO3 heterostructure*

The ME effect for BaTiO3/BiFeO3/BaTiO3 heterostructure is also investigated by analyzing complex impedance (�Z<sup>00</sup> vs. Z<sup>0</sup> ) as well as complex modulus plots (M<sup>00</sup> vs. M<sup>0</sup> ) under applied magnetic fields (**Figure 9(e)**) [36]. The data were fitted with an equivalent circuit of two series RC-elements. Two well resolved semicircles


**BiFeO3**

**121**

**composition**

0.5BiFeO3/0.5MnFe

BiFeO3/BiMnO3

BiFeO

3 0.7BiFeO3/

0.3MgLa0.025Fe1.975

0.9BiFeO3-0.1PbTiO3

BaFe12

BiFeO3/CoFe

O2 4 0.7BiFeO3-0.3PbTiO3

0.67BaTiO3–0.33BiFeO3

BaTiO3/BiFeO3/BaTiO3

Bi0.8FeO2.7-BaTiO3

BiFeO3/Na0.5Bi4.5Ti4O15

**Table 3.**

*Summary of multiferroic* *(diameter (D) and length (*l*)), ferroelectric*

*magnetoelectric*

 *coefficient (αME)].*

 *properties of BiFeO3 [phase structure (monoclinic*

 *spontaneous*

 *polarization*

 *(Ps), remanent polarization*

 *(M),* 

*rhombohedral*

 *(R), tetragonal (T),* 

 *(Pr), saturation* 

*orthorhombic*

*magnetization*

 *(M),* 

*antiferromagnetic*

 *(AF),* 

*magnetocapacitance*

 *(MC), and*

 *(O), cubic (C), hexagonal (H)),* 

*nanostructural*

 *size*

Chemical solution deposition

RF-magnetron

 sputtering

Solid-state

R/T

 R/O

D =

BFO = 879 nm

NBT = 545 nm

1 μm

48/30.2

62.1/38.7

 4.68

—

136

 [51]

5.5

—

120

 [52]

R/T

 BTO = 20 nm BFO = 40 thick

thin film

O19/BiFeO3

Sol–gel solid state

Mechano-chemical

Solvothermal

Solid-state Pulsed laser

R/T

H/R R/spinel

R/T R/T

 1850 nm thick film, D = 40 nm 400/230 102

D = 37 nm

D = 588 nm D = 285 nm

—

3.8 105

2.8/1.4

2.1/1 1.1/0.2

Cm2

8/3.2

37

 5.95

 457

 [36]

 AF

2 103

μoT

—

20.75

 [33]

—

 —

[84]

30.1

—

8.5

 [83]

28

—

11.9

 [82]

*Synthesis and Characterization of Multiferroic BiFeO3 for Data Storage*

0.06

—

0.2

 [81]

O4

Sol–gel auto combustion

BaTiO3

O2 4

Sol–gel Pulsed laser

RF-magnetron

 sputtering

 (110) plane: high

degree

 distorted R

D = 56 nm

0.28/0.08

 5.28

—

 —

[80]

*DOI: http://dx.doi.org/10.5772/intechopen.94049*

auto-combustion

 R/spinel Pseudo-cubic

Superlattices:

50 nm thick BiFeO3

32 nm thick

D = 110 nm

0.64/0.97

26 15.5/9

33

 4.96

 61

 [30]

10

—

 —

[79]

 15.65

—

 —

[78]

**Synthesis method**

 **Phase structure**

**Nano-structure**

 **Ps/Pr (μC cm2)**

 **M**

**MC**

**αME**

**Ref.**

**(emu g1**

**)**

**(%)**

**mVcm1 Oe1**

#### *Bismuth - Fundamentals and Optoelectronic Applications*


## *Synthesis and Characterization of Multiferroic BiFeO3 for Data Storage DOI: http://dx.doi.org/10.5772/intechopen.94049*
