Processing Techniques with Heating Conditions for Multiferroic Systems of BiFeO3, BaTiO3, PbTiO3, CaTiO3 Thin Films

*Kuldeep Chand Verma and Manpreet Singh*

## **Abstract**

In this chapter, we have report a list of synthesis methods (including both synthesis steps & heating conditions) used for thin film fabrication of perovskite ABO3 (BiFeO3, BaTiO3, PbTiO3 and CaTiO3) based multiferroics (in both singlephase and composite materials). The processing of high quality multiferroic thin film have some features like epitaxial strain, physical phenomenon at atomic-level, interfacial coupling parameters to enhance device performance. Since these multiferroic thin films have ME properties such as electrical (dielectric, magnetoelectric coefficient & MC) and magnetic (ferromagnetic, magnetic susceptibility etc.) are heat sensitive, i.e. ME response at low as well as higher temperature might to enhance the device performance respect with long range ordering. The magnetoelectric coupling between ferromagnetism and ferroelectricity in multiferroic becomes suitable in the application of spintronics, memory and logic devices, and microelectronic memory or piezoelectric devices. In comparison with bulk multiferroic, the fabrication of multiferroic thin film with different structural geometries on substrate has reducible clamping effect. A brief procedure for multiferroic thin film fabrication in terms of their thermal conditions (temperature for film processing and annealing for crystallization) are described. Each synthesis methods have its own characteristic phenomenon in terms of film thickness, defects formation, crack free film, density, chip size, easier steps and availability etc. been described. A brief study towards phase structure and ME coupling for each multiferroic system of BiFeO3, BaTiO3, PbTiO3 and CaTiO3 is shown.

**Keywords:** thin films synthesis, magnetoelectric coupling, film-on-substrate geometry

## **1. Introduction**

Multiferroics have simultaneous ferroelectric and magnetic ordering and exhibits unusual physical properties in the sense of heat transport phenomenon at low and higher temperature turns to identify new device applications of spintronics due to their coupling between dual order parameters [1–6]. The first

magnetoelectric (ME) effect studied by Dzyaloshinskii on Cr2O3 in 1960s was discussed in the report [2]. After few decades, the study has been taken on bulk composites of magnetostrictive ferrites and piezoelectric BaTiO3. But the research was halted during number of years due to observed weak value of ME coupling. Now around 2003, the spin-dependent multiferroicity and strong ME effect in TbMnO3 has been reported and the coexistence of antiferromagnetism and ferroelectric polarization in BiFeO3 (BFO) have created much interest in multiferroics [3]. Such perovskite BFO sure multiferroicity due to the fact that magnetism requires unpaired d<sup>n</sup> cations, while ferroelectricity due to d<sup>0</sup> configuration.

### **1.1 Mechanism of multiferroicity (heat sensitive electric and magnetic behavior) occur in perovskites structure**

The ME effect in multiferroic is caused due to switching of magnetization M with an applied electric field E and vice-versa. Moreover, multiferroic switching states should remain (a "persistent" switch, not a transient), and be fast [6]. In single phase multiferroics, the ferroelectricity and ferromagnetism created due to same structural arrangement in ABO3 like BiFeO3, BaTiO3, PbTiO3, CaTiO3 etc., *i.e.* either due to same ion (e.g. Fe3+) and or the ferroelectricity arises due to one ion (e.g. the unpaired electron in Pb or Bi), while the magnetism via a second ion (e.g. Fe in BiFeO3). For memory devices, it is desired that the multiferroic to be highly insulating and function at room temperature with a large switching charge. For a MERAM, it is likely to combine the ultrafast (250 ps) electrical WRITE operation with the non-destructive (no reset) magnetic READ operation, *i.e.* combining the best qualities of FRAM and MRAM. However, the magnetization may small for an effective READ.

#### **1.2 Multiferroic heterostructure**

The multiferroic heterostructures is actually the thin film form of the multiferroic. For this, **Figure 1** shown the electric field control and switch of the local magnetism which has two coupling mechanisms exists in ferromagnetmultiferroic heterostructure [4]. In addition to the macroscopic stress, the heteroepitaxy (the growth of a material B on a different material A) caused an intrinsic epitaxial stress so that the crystal symmetry, lattice constant, and/or chemical bonds do not matched perfectly. During the start of the films growth, the substrate lattice constants might to form stresses. Besides to the single-phase multiferroic such as BiFeO3, BaTiO3, PbTiO3, CaTiO3, the composite multiferroic heterostructures constructed in the way to realize an artificial systems in the form of thin film. Wang *et al.* [3] reported an enhancement of polarization in heteroepitaxially constrained BiFeO3 thin film which display a room-temperature spontaneous polarization (50 to 60 μC cm<sup>2</sup> ) that is much higher (in magnitude) than from bulk (6.1 μC cm<sup>2</sup> ). The films thickness is also play an important role to turn multiferroicity in heterostructure.

#### **1.3 Strain mediated magnetoelectric effects**

The ME coupling in two-phase multiferroics is the strain transfer phenomenon occurs among two phases which is schematically constructed for 1–3 nanocomposite (**Figure 2**) [5]. It is the polarization (magnetization) that changed by a magnetic (electric) field. The ME effect in composites is a product tensor property of the magnetostrictive or magnetoelastic effect (magnetization and lattice strain coupling) in one phase and the piezoelectric effect (polarization and lattice strain

#### **Figure 1.**

*Representation (schematic) of electric control of magnetism due to existence of ferroelectricity, antiferromagnetism and ferromagnetism [4].*

**Figure 2.**

*Strain-mediated ME coupling in composites of a ferromagnetic (FM) and a ferroelectric (FE) phases (ΔQ: induces surface charges and ΔM: induces a magnetization change or domain reorientation) [5].*

coupling) in the other phase. This ME coupling is the measurement due to direct or converse ME coefficient which detects an electrical signal with applied magnetic field. The strain generated in the magnetostrictive phase by a magnetic field induces surface charges in the piezoelectric phase. The direct ME voltage coefficient is given by α<sup>E</sup> = ΔE/ΔHac.

## **2. Multiferroic perovskite thin films and their processing techniques**

## **2.1 BiFeO3**

Recently, the multiferroic systems such as BaTi2O4, YMnO3, BiMnO3, LuFe2O4, and BiFeO3 have been widely investigated [7]. Among them, BiFeO3 (BFO) with high Tc 1103 K and TN 643 K attracts much attention due to its simultaneous ferroelectric and antiferromagnetic behaviors exist even at room temperature. This BFO has ferroelectricity occurs due to 6s<sup>2</sup> lone pair of electrons of Bi3+ where structural distortion take-place and the magnetism occurs via superexchange interactions in Fe-O-Fe ions [8, 9]. This suggested to the polarization enhancement in BFO via chemical substitution along A site from rare-earth such as La3+, Sm3+, and Dy3+ ions due to their similar ionic radius and isovalent chemistry with Bi. This substitution of rare-earth into Bi in BFO may also induce a reduction in Tc value and formation of an antiferroelectric phase, but had minimal effect on antiferromagnetism. Moreover, the magnetization in BFO resulted by G-type antiferromagnet order is a cycloidal of wavelength, λ 62–64 nm [10].

## *2.1.1 Multiferroic BiFeO3 thin film*

The bulk BiFeO3 is a room-temperature ferroelectric with a spontaneous electric polarization directed along one of the [111] axes in the perovskite structure as shown in **Figure 3a** [11]. The ferroelectricity due to lattice distortions reduces the symmetry from cubic to rhombohedral to cause ferroelastic strain. With applied an electric field, the ferroelectric polarization in BiFeO3 have eight possible positive

#### **Figure 3.**

*The (001)-oriented BiFeO3 crystal structure (schematic) and the ferroelectric polarization (bold arrows) and antiferromagnetic plane (shaded planes). (a) Polarization with an up out-of-plane component before electrical poling. (b) 180<sup>o</sup> polarization switching with the out-of-plane component switched by an electrical field (no change in antiferromagnetic phase). 109° (c) and 71o (d) polarization switching with the out-of-plane by an electrical field (antiferromagnetic plane changes) [11].*

and negative orientations along the four cube diagonals, and the direction of the polarization can be switched by 180o , 109o and 71<sup>o</sup> as shown in **Figure 3b**–**d**. Switching of the polarization by either 109o or 71<sup>o</sup> changes the rhombohedral axis in the lattice system due to switching of the ferroelastic domain state. The antiferromagnetic ordering is G-type for which the nearest neighbor Fe moments aligned antiparallel to each other. In bulk BiFeO3, the orientation of the antiferromagnetic vector has a long-wavelength spiral, which may suppress in thin films [12]. The DFT first principle calculations suggested the preferred orientation of the individual spins, in the absence of the spiral, which is perpendicular to the rhombohedral axis. It results into polarization switching by either 71<sup>o</sup> or 109o that should change the orientation of the easy magnetization plane. **Figure 1d** is the ferroelectric switching with 71o which leads to a reorientation of the antiferromagnetic order results into ME switching effects expectable in BFO films.

## *2.1.2 Synthesis techniques for BiFeO3 thin films*

In **Table 1**, we have included a lot of study to summarize chemical synthesis routes for processing of multiferroic BiFeO3 thin films. The observed values of grain size, thickness and quality of film might be depends upon material processing and film coating technique.

## **2.2 BaTiO3**

BaTiO3 (BTO) has shown multiferroicity at room temperature [36]. The three lattice structural phases of BTO are ferroelectric: rhombohedral<190 K, orthorhombic for 190 K < T < 278 K and tetragonal for 278 K < T < 395 K. The paraelectric BTO phase exists at higher temperature. With tetragonal BTO, *a* = *b* = 0.399 nm and *c* = 0.403 nm of P4mm space group. The spontaneous electric polarization of BTO lattice might be related with the displacement of the Ti4+ ion along the *c*-axis which is sensitive to the charge in hybridization of the 3d states of the Ti cation with the 2p states of the surrounding O anions. For single-phase BaTiO3 with doping of transition metal ions, the Fe-doping into BTO is widely reported due to excellent magnetic behavior that originated from unpaired spin of Fe3+ ions [37]. However, the multiferroic nanocomposites may selected from perovskites such as BaTiO3, PbTiO3 and BiFeO3 (due to their large polarization and piezoelectric coefficients) with ferrites which have high magnetostriction value, high resistivity, TN above room temperature and large ferromagnetism [38].

#### *2.2.1 Multi-layered heteroepitaxial multiferroic thin films*

The ME coupling in composite phases is effectively tuned through interfacial strain, exchange-bias, field effects, and so on [39]. It is a way to construct film geometry on the basis of their dimensions. For example, a 0–3 configuration means that there are two phases in the composite, one consisting of zero-dimension particulates, and the other is three-dimensional bulk. However, the ME composites for film-on-substrate, the multi-layered epitaxial thin films (2–2 configuration) were firstly reported that to be results into strong ME coupling with higher quality of crystallography and intimate coherent interface. But the clamping effect of the substrate onto the ferroelectric (FE) phase reduces the magnitude of the ME coupling coefficient in thin films. For 1–3 configuration, *i.e.* the vertical nanostructures from perovskite and spinel systems, like CoFe2O4 nanopillars embedded in a BaTiO3 or BiFeO3 matrix, the substantial enhancement of ME coupling and electric-field








**Table 1.**

*Synthesis methods used to prepare multiferroic BiFeO3 (BFO) thin films.*

induced magnetization switching observed. It is due to the effect of large heteroepitaxial interface and reduced clamping effect. In **Figure 4a**, the new structure that fully epitaxial multilayered nanodot array, *i.e.* 0–0 composite which combines the advantages of 2–2 and 1–3 geometries to obtain a better understanding of extrinsic ME coupling and build prototypes for high density multistate memory devices. In 2–2 type, the horizontal stacking, *i.e.* an epitaxial multilayer or superlattice can provide more flexibility for material design, composition control, and layer arrangement.

## *2.2.2 SEM images of BaTiO3-CoFe2O4 (BTO-CFO) two-layered nanodots*

A typical scanning electron microscope (SEM) image of the as obtained STO/ AAO substrate is given in **Figure 4b** [39]. There is no gap found between STO and

#### **Figure 4.**

*(a) The film-on-substrate geometry in ME composites. SEM images of (b) as-transferred anodic aluminum oxide (AAO) mask on STO substrate, (c) BTO nanodots with partly removed AAO in first layer deposition, (d) BTO/CFO two-layered nanodot arrays [39].*

the AAO membrane (thickness 120 nm and pore diameters 65 nm). The thin films are synthesized by PLD process. In **Figure 4c** and **d**, the highly ordered nanodot arrays with flat surfaces and diameters around 65 nm were obtained. The AAO was fabricated through a self-assembly process, the periodic area is only μm<sup>2</sup> in size.

### *2.2.3 Synthesis techniques for BaTiO3 thin films*

The thin film processing issues associated with various techniques such as the formation of side phases and the difficulty in controlling stoichiometry, film thickness, and sample crystallinity. For this we have summarized some synthesis method for BaTiO3 based multiferroic thin films described in **Table 2**.

### **2.3 PbTiO3 multiferroics**

Since ferroelectricity in perovskites ABO3 commonly involves B-site transitionmetal ions with a formal electron configuration d<sup>0</sup> (e.g. Ti4+, Nb5+, etc.), while magnetism requires TM cations with partially filled d states. A lot of studies on multiferroics concentrate on the Bi-based perovskites, *i.e.* BiFeO3 and BiMnO3, where the ferroelectricity mainly arises from the lone pair of 6 s electrons [53]. But the basic physics in the ferroelectric thin films is similar to the bulk state in addition to some specific properties in thin-film like interface strain and stress, dead layer effect etc. [54]*.* In perovskites, PbTiO3 also have multiferroic behavior with TM doping or composites substitution and have spontaneous polarization parallel to the *c* axis. The lattice structure of PbTiO3 is tetragonal below the TC and turns into cubic and paraelectric above TC. The value of TC for bulk PbTiO3 lies in 490–493°C which depends upon synthesis process, size or defect effect. The displacement of the Ti





#### **Table 2.**

*Synthesis method used to process BaTiO3 based multiferroic thin films.*

atom in the O octahedron gives the spontaneous polarization of the PbTiO3 due to the vector of polarization from the center of the O octahedron to the Ti atom. Through the substitutions for Pb or Ti or both cations, the ferroelectric properties could be widely modified and the structures could show various characters. Due to scarcity of multiferroics, PbTiO3 is being extensively studied for induction of magnetism. The large 'Pb' cation is coordinated by twelve oxygens in a cub-octahedral way, while the smaller 'Ti' cation is octahedrally coordinated with six oxygens [55]. The large degree of ferroelectricity in PbTiO3 occurs to the contribution of covalent bonds between both Pb 6p - O 2p and Ti 3d - O 2p states. The covalent bond between Pb 6p and O 2p lowers the non-centrosymmetric symmetry towards the ferroelectric state by weakening the ionic core repulsion between Pb and the nearest oxygen ions.

## *2.3.1 ME coupling due to ferroelectric polarization under an external magnetic field*

**Figure 5a** shows spontaneous polarization (P-E loop) under the influence of a magnetic field (H = 0–0.5 T) for Pb0.7Sr0.3(Fe0.012Ti0.988)O3 thin film. This thin film prepared by a MOD method and deposited on Pt/Ti/SiO2/Si substrate using spincoating [56]. The magnetic switching from +Pr to zero at 0.5 T have been observed.

#### **Figure 5.**

*(a) Ferroelectric hysteresis under an external magnetic field (0–0.5 T) for Pb0.7Sr0.3(Fe0.012Ti0.988)O3 thin film. (a*<sup>0</sup> *) Variation of Pr and Ec with applied H. (a ) P* � *0 at 0.5 T (contribution by electrode) [56]. (b and c) F-center exchange mechanism responsible for room-temperature ferromagnetism in Fe-doped PbTiO3 [57].*

In **Figure 5a**<sup>0</sup> , the polarization is continuously increased with increasing H up to 0.3 T which is an indication for the coupling between the polarization and magnetization. When a magnetic field applied to a ME material, the material is strained to induce a stress on the piezoelectrics to generates the electric field. This field related with ferroelectric domains resulting into an increase in polarization. **Figure 5a″** shows a low polarization response with lossy hysteresis at 0.5 T, which may have been caused by the electrode.

## *2.3.2 Mechanism for ferromagnetism in Fe-doped PbTiO3*

F-center exchange (FCE) mechanism is used to evaluate the magnetic origin in Pb(Ti1-*x*Fe*x*)O3 thin films as shown in **Figure 5b** and **c** [54, 57]. As the tetravalent Ti4+ is replaced by trivalent Fe3+ cations, there is generation of oxygen vacancies to cause charge neutrality in the multiferroic. An electron trapped in the oxygen vacancy might to produce an F center, where the electron occupies an orbital to overlaps the *d* shells of both iron neighbors. As the unoccupied spin orbitals are minority in the 3d<sup>5</sup> of Fe3+, the trapped electron and the iron neighbors have opposite spin direction. Therefore, the system becomes ferromagnetic due to super-exchange effect generates antiferromagnetism.

## *2.3.3 Synthesis techniques for PbTiO3 thin films*

To the investigation of bulk ferroelectric materials, the compositional adjustment has plays a major role to study new ferroelectric. However for thin films, the suitability of synthesis method for processing of epitaxial ferroelectric plays key feature to change the polarization behavior due to some technical features related with film geometry on substrate. In **Table 3**, we have introduced some typical synthesis routes for PbTiO3-based multiferroic thin films fabrication.

#### **2.4 Multiferroic CaTiO3 systems**

Since CaTiO3 (CTO) is prototype for the perovskite structure [67]. At room temperature, CTO has the orthorhombic GdFeO3 type crystal structure (*a* = 5.38 Å,




*Bold emphasis has more significance.*

#### **Table 3.**

*Synthesis method used to prepare multiferroic PbTiO3 based thin films.*

#### **Figure 6.**

*(a) Spontaneous polarization in 0.85BiTi0.1Fe0.8Mg0.1O3–0.15CaTiO3 (BTFM-CTO) thin film. (b) PFM image after electric writing by 10 V, and corresponding. (c) MFM image without an external magnetic field [69].*

*b* = 5.44 Å, *c* = 7.65 Å) and exhibits cubic (*Pm*3*m*) one above 1575 K. Below this temperature, CTO has tetragonal (I4/mcm) phase for which transitions of orthorhombic (Pnmb) phase exist below 1525 K [67]. This orthorhombic structure could create an epitaxial strain in thin films due to changing symmetry at the phase transitions by tilting TiO6 oxygen octahedral away from cubic one. CTO has possessed its nonpolarity to antiferrodistortive (AFD) distortions of the TiO6 octahedral rotations and cation displacements that resulting into an orthorhombic (Pbnm) symmetry and a a c<sup>+</sup> rotations in glazer notation [68]. The ferroelectric polarizations and transition temperatures are significantly enhanced in classical ferroelectric perovskite like BaTiO3 and PbTiO3, and the enhancement in lattice strain might to turn ferroelectricity. Otherwise nonpolar ferroelectrics such as SrTiO3 and

CaTiO3 reported strain-induced ferroelectric transition temperatures only at room temperature or below. It is also reported that the simultaneous doping from Ca and Ti ions into BiFeO3 results to decrease electric conductivity and stabilize the polar rhombohedral phase, and the magnetic properties are enhanced by protect oxygen stoichiometry with Fe in +3 oxidation state [68].

## *2.4.1 Ferroelectricity in 0.85BiTi0.1Fe0.8Mg0.1O3-0.15CaTiO3 thin film*

**Figure 6a** shows the ferroelectric polarization (P) for 0.85BiTi0.1Fe0.8Mg0.1O3– 0.15CaTiO3 thin film at room temperature [69]. This P-E hysteresis loop shows a


## **Table 4.**

*Synthesis methods used for fabrication of CaTiO3 based multiferroic thin film.*

large remanent polarization of 2*P*r = 191.6 μC cm<sup>2</sup> and coercive field (Ec) 2Ec = 681.8 kV cm<sup>1</sup> without excluding the contribution from the leakage current.

## *2.4.2 ME coupling in 0.85BiTi0.1Fe0.8Mg0.1O3-0.15CaTiO3 thin film*

The ME coupling might be studied by considering the magnetic field (H) effect on the ferroelectric polarization and magnetization (magnetic field on the domain switching) that shown in **Figure 6b** and **c** [69]. **Figure 6b** is the PFM image after electric lithography which is observed magnetic domain switching after the electric lithography (**Figure 5c**). Although, the magnetic domains are not completely switched, indicating that the ME coupling is stable at room temperature.

## *2.4.3 Synthesis techniques for CaTiO3 thin films*

**Table 4** shown the chemical synthesis methods used in the preparation of multiferroic systems of CaTiO3 thin films.

## **3. Conclusion and comparative features for synthesis methods used for BiFeO3, BaTiO3, PbTiO3 and CaTiO3 thin films**

The chemical synthesis methods for thin film fabrication of perovskite BiFeO3, BaTiO3, PbTiO3 and CaTiO3 multiferroic systems and their composites have been described on the basis of their brief synthesis procedure, time of reaction, composition, heating conditions and morphology. Since thin film composite structures are mostly suitable for chip-device implementation. In various thin film processing methods of multiferroics, the vacuum based deposition like PLD may provide a large ME coupling, enabling the realization of complex microstructures including epitaxy, texture, or columnar distribution of magnetic nanopillars into a ferroelectric matrix. PLD enables the growth of high quality epitaxial thin films with substrate area for coated are small but to the making of uniform films on the wafer level is challenging.

The BFO films made via CSD needs richer oxygen environments for annealing and obtain crystalline films at relatively low temperatures. This CSD method has low cost, easily control of stoichiometry and easy to operate for large area films for a complex-shaped substrates. Among various CSD techniques, polymer assisted deposition (PAD) is most suitable to produce high quality epitaxial complex and multilayer-structured films (film thickness below 100 nm).

The multiferroic thin films deposited with RF sputtering performs high reproducibility with an accurate stoichiometry in a controlled process (but the film growth rate is low and the cost of fabrication is high), and considered to be the most suitable method of thin film preparation due to a smooth surface and dense structure that usable to fabricate the integrated circuit device.

Other chemical synthesis is sol–gel which has good reproducibility, low cost, thickness uniformity and moreover large area deposition and commonly used spin coating deposition. It simply prepared BFO thin films. Photosensitive sol–gel method can integrate the preparation with the fine patterning of the film which can simplify the lithography process remarkably.

The atomic layer deposition (ALD) provides additional degrees of freedom in design and fabrication of devices depending on domain wall optimization, adding the option of conformity in deposition of various geometries (it easily embedded nanoparticles, embedded nanopillars, and nanolaminates according to

configurations, *i.e.* 0–3, 1–3, and 2–2) with unique thickness control and also to fabricate high-quality ultrathin films.

Another chemical technique is the chemical vapor deposition (CVD) which highly usable for BFO thin films deposition which has excellent substrate coverage, low-cost, ease of scale-up, control over thickness and morphology. Aerosol assisted CVD is not rely upon the use of highly volatile precursors, essential for typically high molecular weight heterometallic cluster compounds.

For shape control in multiferroics, hydrothermal method could provide an alternative approach to the synthesis of high-quality epitaxial BFO and BTO thinfilm with low-cost. The hydrothermal reaction typically occurs at low temperature (<250°C) and high pressure (<15 MPa) originating from water vapor pressure. The spray pyrolysis method has been implemented for the production of porous and layer by layer films.

## **Acknowledgements**

The author K.C. Verma thankfully acknowledges the financial support by UGC of Dr. DS Kothari Post Doctorate Fellowship [No. F4-2/2006(BSR)/PH/16-17/0066] and CSIR-HRDG for SRA (Pool Scientist) fellowship Grant No. B-12287 [SRA (Pool No): 9048-A].

## **Author details**

Kuldeep Chand Verma1,2\* and Manpreet Singh3

1 Materials Science and Sensor Applications (MSSA), CSIR-Central Scientific Instruments Organisation, Chandigarh, India

2 Department of Physics, Panjab University, Chandigarh, India

3 Department of Chemistry, Eternal University, Baru Sahib, Himachal Pradesh, India

\*Address all correspondence to: dkuldeep.physics@gmail.com; kcv0309@gmail.com

© 2021 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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## **Chapter 4**
