**1.1 Multiferroic BaTiO3**

The magnetoelectric (ME) effect—the induction of magnetization by an electric field and the induction of electric polarization by a magnetic field [1]. Multiferroic nanostructures have given recent advances in new type of memory devices including multistate data storage and spintronics [2]. BaTiO3 (BTO) is a rare single-phase multiferroic. In multiferroics, the magnetic order is due to exchange interactions between magnetic dipoles, which themselves originate from unfilled shells of electron orbitals. Similarly, the electric order is due to the ordering of local electric dipoles, elastic order is due to the ordering of atomic displacements due to strain. The three crystallographic phases of BTO are ferroelectric: rhombohedral <190 K, orthorhombic for 190 K< T < 278 K and tetragonal for 278 K < T < 395 K. At higher temperatures, BTO is a paraelectric. For tetragonal BTO (**Figure 1(b)**) having lattice constants: *a*(Å) = *b*(Å) = 3.99; *c*(Å) = 4.03; space group = P4*mm*, the displacement of Ti4+ ion along *c*-axis might be induced electrical polarization (ferroelectricity). It involved hybridization of charge among Ti cation (3d states) with O anions (2p states). In cubic phase, the Ba2+ is located at the centre of the cube with coordination number 12.

Since to the formation of ME random access memories (MERAMs), the main thing that required ME coupling via interfacial exchange coupling among a multiferroic and a ferromagnet, which can change the magnetization of the ferromagnetic coating with respect to a voltage (**Figure 1(a)**) [3]. For such MERAMs, an electric field is enabled by ME coupling could control the exchange coupling between multiferroic and ferromagnetic at the interface. This exchange coupling at the interface reins the magnetization of the ferromagnetic layer, and therefore the magnetization might be change with multiferroic electrical polarization. Therefore, for perovskite (ABO3) BTO, Ba2+ (A-site cation) induce the required distortion for ferroelectricity, while magnetism can be achieved by the doping such as TM = Cr, Mn, Fe, Co, Ni, Cu along B-site cation [4, 5].

#### **1.2 Spinel ferrites**

The spinel structure typically represented as AB2O4, where 'A' indicates fourfold coordinated tetrahedral sub-lattice sites and 'B' indicates six-fold coordinated octahedral sub-lattice (**Figure 1(c)**). There are 8 A-sites in which the metal cations are tetrahedrally coordinated with oxygen, and 16 B-sites, which possess octahedral coordination. Normal ferrites have divalent cations residing solely as the central ion on the tetrahedral sub-lattice with only trivalent Fe cations occupying octahedral sub-lattice sites. Harrisa and Sepelak [6] suggested superexchange interaction, the JBB is strong and negative indicating antiferromagnetic coupling between Fe3+–O2−–Fe3+ with octahedral sub-lattice cation spins largely canceling out. When

**95**

state is singlet 6

**Figure 1.**

**1.3 Diluted magnetic semiconductors**

*Ferromagnetism in Multiferroic BaTiO3, Spinel MFe2O4 (M = Mn, Co, Ni, Zn) Ferrite…*

the divalent cation resides on the [B] site and A and B ions share the balance of [B] and the totality of (A), the spinel is inverse: (A1-δBδ)[AδB2-δ]O4, where δ is the inversion degree, *i.e.*, NiFe2O4. Ferrite of the type NiFe2O4 (NFO), CoFe2O4 (CFO) and MnFe2O4 (MFO) with the spinel structure are magnetic ceramics which have potential in electronic and magnetic components [7]. The NFO has an inverse spinel structure for which Fe3+ ions occupied tetrahedral A-sites; whereas Fe3+ and Ni2+ ions are sit on octahedral B-sites. This NFO is ferrimagnetic material has magnetization originated by antiparallel spins on A- and B-sites. However, for normal spinel structure of CFO, Co is a divalent atom, occupying tetrahedral A-sites, while Fe is a trivalent atom, sitting on the octahedral B-sites. For MFO, the inverted spinel structure is partial for which the Mn2+ and Fe3+ ions with half-filled 3d shell (ground

*moments and their antiparallel alignment. Adopted from Harrisa and Sepelak [6].*

*(a) Schematic MERAM device in which the binary information is stored by the magnetization direction of the ferromagnetic layer (I & II), read by the resistance, Rp of the magnetic trilayer, and written by applying a voltage across the multiferroic ferroelectric-antiferromagnetic layer (FE-AFM). (b) Tetragonal crystalline structure of BTO, with a lattice ferroelectric distortion. (c) Spinel unit cell. Arrows represent magnetic* 

S with spin = 5/2 with zero orbital momentum) and the crystal field

is not sufficient to split it [8]. The magnetic moment of MFO agrees well with Neel's

Recently, the realization of spin in DMS, semiconductor with substituted magnetic impurities (Fe, Co, Ni, Cu) has attracted great interest in design of spintronics devices like spin field-effect transistors, non-volatile memory devices, and programmable logic gates [10–13]. Among various DMS, ZnO is a promising spin

coupling scheme and has lower resistivity than CFO and NFO ferrites [9].

*DOI: http://dx.doi.org/10.5772/intechopen.82437*

*Ferromagnetism in Multiferroic BaTiO3, Spinel MFe2O4 (M = Mn, Co, Ni, Zn) Ferrite… DOI: http://dx.doi.org/10.5772/intechopen.82437*

#### **Figure 1.**

*Electromagnetic Materials and Devices*

**1.1 Multiferroic BaTiO3**

at localized magnetic impurities and electron–electron interactions. To use DMSs for practical spintronic devices, a relatively high concentration of magnetic elements needed in the semiconductor host, and a large ferromagnetism is required with a Curie temperature (Tc) above room temperature. The transition metal (TM) ferrites with a spinel structure (MFe2O4; M = Co2+, Ni2+, Cu2+, Zn2+, etc.) are used in a wide variety of technological applications such as magnetic memory devices and biomedicine. However, these spinel ferrites are the candidate materials for multiferroic heterostructure due to their excellent magnetic response. For such multiferroic heterostructures, the perovskite (BaTiO3, PbTiO3, BiFeO3, etc.) has an opportunity of higher piezoelectric coefficient that may pool with magnetostrictive materials

The magnetoelectric (ME) effect—the induction of magnetization by an electric field and the induction of electric polarization by a magnetic field [1]. Multiferroic nanostructures have given recent advances in new type of memory devices including multistate data storage and spintronics [2]. BaTiO3 (BTO) is a rare single-phase multiferroic. In multiferroics, the magnetic order is due to exchange interactions between magnetic dipoles, which themselves originate from unfilled shells of electron orbitals. Similarly, the electric order is due to the ordering of local electric dipoles, elastic order is due to the ordering of atomic displacements due to strain. The three crystallographic phases of BTO are ferroelectric: rhombohedral <190 K, orthorhombic for 190 K< T < 278 K and tetragonal for 278 K < T < 395 K. At higher temperatures, BTO is a paraelectric. For tetragonal BTO (**Figure 1(b)**) having lattice constants: *a*(Å) = *b*(Å) = 3.99; *c*(Å) = 4.03; space group = P4*mm*, the displacement of Ti4+ ion along *c*-axis might be induced electrical polarization (ferroelectricity). It involved hybridization of charge among Ti cation (3d states) with O anions (2p states). In cubic phase, the Ba2+ is located at the centre of the cube with coordination number 12. Since to the formation of ME random access memories (MERAMs), the main

thing that required ME coupling via interfacial exchange coupling among a multiferroic and a ferromagnet, which can change the magnetization of the ferromagnetic coating with respect to a voltage (**Figure 1(a)**) [3]. For such MERAMs, an electric field is enabled by ME coupling could control the exchange coupling between multiferroic and ferromagnetic at the interface. This exchange coupling at the interface reins the magnetization of the ferromagnetic layer, and therefore the magnetization might be change with multiferroic electrical polarization. Therefore, for perovskite (ABO3) BTO, Ba2+ (A-site cation) induce the required distortion for ferroelectricity, while magnetism can be achieved by the doping such as TM = Cr,

The spinel structure typically represented as AB2O4, where 'A' indicates fourfold coordinated tetrahedral sub-lattice sites and 'B' indicates six-fold coordinated octahedral sub-lattice (**Figure 1(c)**). There are 8 A-sites in which the metal cations are tetrahedrally coordinated with oxygen, and 16 B-sites, which possess octahedral coordination. Normal ferrites have divalent cations residing solely as the central ion on the tetrahedral sub-lattice with only trivalent Fe cations occupying octahedral sub-lattice sites. Harrisa and Sepelak [6] suggested superexchange interaction, the JBB is strong and negative indicating antiferromagnetic coupling between Fe3+–O2−–Fe3+ with octahedral sub-lattice cation spins largely canceling out. When

Mn, Fe, Co, Ni, Cu along B-site cation [4, 5].

**1.2 Spinel ferrites**

(CoFe2O4, NiFe2O4, ZnFe2O4, etc.) via lattice strain effect.

**94**

*(a) Schematic MERAM device in which the binary information is stored by the magnetization direction of the ferromagnetic layer (I & II), read by the resistance, Rp of the magnetic trilayer, and written by applying a voltage across the multiferroic ferroelectric-antiferromagnetic layer (FE-AFM). (b) Tetragonal crystalline structure of BTO, with a lattice ferroelectric distortion. (c) Spinel unit cell. Arrows represent magnetic moments and their antiparallel alignment. Adopted from Harrisa and Sepelak [6].*

the divalent cation resides on the [B] site and A and B ions share the balance of [B] and the totality of (A), the spinel is inverse: (A1-δBδ)[AδB2-δ]O4, where δ is the inversion degree, *i.e.*, NiFe2O4. Ferrite of the type NiFe2O4 (NFO), CoFe2O4 (CFO) and MnFe2O4 (MFO) with the spinel structure are magnetic ceramics which have potential in electronic and magnetic components [7]. The NFO has an inverse spinel structure for which Fe3+ ions occupied tetrahedral A-sites; whereas Fe3+ and Ni2+ ions are sit on octahedral B-sites. This NFO is ferrimagnetic material has magnetization originated by antiparallel spins on A- and B-sites. However, for normal spinel structure of CFO, Co is a divalent atom, occupying tetrahedral A-sites, while Fe is a trivalent atom, sitting on the octahedral B-sites. For MFO, the inverted spinel structure is partial for which the Mn2+ and Fe3+ ions with half-filled 3d shell (ground state is singlet 6 S with spin = 5/2 with zero orbital momentum) and the crystal field is not sufficient to split it [8]. The magnetic moment of MFO agrees well with Neel's coupling scheme and has lower resistivity than CFO and NFO ferrites [9].

#### **1.3 Diluted magnetic semiconductors**

Recently, the realization of spin in DMS, semiconductor with substituted magnetic impurities (Fe, Co, Ni, Cu) has attracted great interest in design of spintronics devices like spin field-effect transistors, non-volatile memory devices, and programmable logic gates [10–13]. Among various DMS, ZnO is a promising spin

source, since it epitomizes DMS with Tc well above room temperature. The DMS ZnO is a wide band gap that gained recent research in spintronics, due to an ability to change its optical and magnetic behavior with doping of TM = Fe, Co, Mn, Ni, Cu, Cr, or V ions and/or by intrinsic defects, such as oxygen vacancy (VO) and zinc vacancy (VZn). The DMS ZnO has wide applications included:

#### *1.3.1 Magnetic recording*

For read heads in magnetic disk recorders (computer components), read head senses the magnetic bits that are stored on the media, which is stored as magnetized regions of the media, called magnetic domains, along tracks (**Figure 2(a)**) [13]. Magnetization is stored as a "0" in one direction and as a "1" in the other. Although, there is no magnetic field emanating from the interior of a magnetized domain itself, uncompensated magnetic poles in the vicinity of the domain walls generate magnetic fields (sensed by the GMR element) that extend out of the media.

### *1.3.2 Nonvolatile memories*

The "Nonvolatile" refers to information storage that does not "evaporate" when power is removed from a system, *i.e.*, magnetic disks and tapes. Prinz [13] has recently demonstrated that GMR elements can be fabricate in arrays with standard lithographic processes to obtain memory that has speed and density approaching that of semiconductor memory, but is nonvolatile. A schematic representation of RAM that is constructed of GMR elements is shown in **Figure 2(b)**. The spindependent scattering of the carriers (electrons & holes) is minimized for parallel magnetic moment of the ferromagnetic layer, to induce lowest value of resistance. However, the highest resistance is the result of maximized spin-dependent scattering carriers via anti-aligned ferromagnetic layers. An external magnetic field could give the direction of magnetic moments, applied to the materials. The spin-valve structures of GMR set into series using lithographic (wires) termed as a sense line. This sense line has information storage due to resistance (resistance of elements). The sense line runs the current which is detected at the end by amplifiers because resistance changes in the elements.

#### **Figure 2.**

*A schematic representation of (a) GMR read head (I) that passes over recording media containing magnetized regions, (b) RAM that is constructed of GMR elements. Adopted from Prinz [13], (c) bound magnetic polaron (BMP), VZn, Oi trapped carriers couple with the 3d shell spins of TM ions within its hydrogenic orbit.*

**97**

tization, Ms (emu g<sup>−</sup><sup>1</sup>

coupling constant, αME (mV cm<sup>−</sup><sup>1</sup>

14.6, χ<sup>2</sup>

*Ferromagnetism in Multiferroic BaTiO3, Spinel MFe2O4 (M = Mn, Co, Ni, Zn) Ferrite…*

because of bound magnetic polaron (BMP) model [14]. The localized spins of the dopant ion interact with the charge carriers that are bound to a small number of defects such as oxygen vacancies, resulting into a magnetic polarization of the surrounding local moments. Since the magnetism in TM ions doped ZnO nanoparticles relates with exchange interactions between unpaired electron spins, that arising from the lattice imperfections such as oxygen vacancies, VO at the surface of the nanoparticles. Pal et al*.* [15] described BMP formation in Co doped ZnO and shown in **Figure 2(c)**. The electrons trapped in the defect vacancies undergo orbital coupling with the *d* shells of the adjacent divalent dopant ion and form BMP. In BMP model, the bound electrons (holes) hold in defect states that coupled through TM ions to overlap ferromagnetic regions, which responsible into high TC (due to formation of long-range ferromagnetic ordering) [16]. When the dopant ions are donors or acceptors, the exchange interactions are sp-d that would lead BMPs formation.

Generally, the room temperature ferromagnetism (RTFM) in DMS ZnO is given

The multiferroic, DMS and ferrites materials might be synthesized by methods

such as a sol-gel [17], chemical combustion [18], hydrothermal [19], metalloorganic decomposition (MOD) [20], conventional solid-state reaction [21], sol-gel

The sol-gel method is used to prepare pure BaTiO3 and BaTM0.01Ti0.99O [TM = Cr (BTO:Cr), Mn (BTO:Mn), Fe (BTO:Fe), Co (BTO:Co), Ni (BTO:Ni), Cu (BTO:Cu)] nanoparticles [4]. The TM ions in perovskite BTO structure highly influenced lattice constants to induce lattice strain and unit cell expansion, which responsible into defects vacancies formation. **Figure 3** shows the Rietveld refinement (Full-Prof program) of X-ray diffraction (XRD) patterns for pure and BaTM0.01Ti0.99O nanoparticles measured at room temperature. A polycrystalline with tetragonal BTO phase (space group:P4*mm*) is detected. The fitting parameters, Rp(%) = 6.7, 7.1, 6.7, 4.9, 9.1, 8.2 and 9.8, Rwp(%) = 3.2, 9.9, 9.2, 10.0, 12.5, 1.2 and

 = 1.1, 1.4, 0.81, 1.4, 3.3, 3.3 and 0.9 and distortion ratio, (*c*/*a*) = 1.00959, 1.00932, 1.00688, 1.00909, 1.00776, 1.00625 and 1.00508, respectively, refined for pure BTO and BTO with Cr, Mn, Fe, Co, Ni, Cu doping. Also, **Figure 3(a)** shows the tetragonal splitting of (200) diffraction peak at 2θ = 44.3–45.7°. The diffraction peak of pure BTO has shifted towards a lower diffraction angle with TM doping which supports the lattice strain in BTO. Such splitting of (200) peak might be confirmed the tetragonal phase formation. It is due to electrostatic repulsions between 3d electrons of Ti4+ ions and 2p electrons of O2<sup>−</sup> ions, the structure becomes distorted. It is also reported in Ref. [4] that the average particles size, from TEM, DTEM (nm) = 20 ± 3, 13 ± 1, 33 ± 5, 35 ± 3, 17 ± 1 and 47 ± 7, the value of saturation magne-

Oe<sup>−</sup><sup>1</sup>

respectively, measured with Cr, Mn, Fe, Co, Ni, Cu doping into BTO.

) = 0.056, 0.042, 0.066, 0.035, 0.013 and 0.021, and the ME

) = 25.91, 11.27, 31.15, 16.58, 11.61 and 16.48,

*3.1.1 X-ray diffraction for BaTiO3 and BaTM0.01Ti0.99O3 nanoparticles*

*1.3.3 Ferromagnetism in DMS ZnO due to bound magnetic polarons*

*DOI: http://dx.doi.org/10.5772/intechopen.82437*

**2. Experimental methods**

**3. Result and discussion**

**3.1 Multiferroic systems of BaTiO3**

precipitation [22], thermal evaporation [23], etc.

*Ferromagnetism in Multiferroic BaTiO3, Spinel MFe2O4 (M = Mn, Co, Ni, Zn) Ferrite… DOI: http://dx.doi.org/10.5772/intechopen.82437*

*1.3.3 Ferromagnetism in DMS ZnO due to bound magnetic polarons*

Generally, the room temperature ferromagnetism (RTFM) in DMS ZnO is given because of bound magnetic polaron (BMP) model [14]. The localized spins of the dopant ion interact with the charge carriers that are bound to a small number of defects such as oxygen vacancies, resulting into a magnetic polarization of the surrounding local moments. Since the magnetism in TM ions doped ZnO nanoparticles relates with exchange interactions between unpaired electron spins, that arising from the lattice imperfections such as oxygen vacancies, VO at the surface of the nanoparticles. Pal et al*.* [15] described BMP formation in Co doped ZnO and shown in **Figure 2(c)**. The electrons trapped in the defect vacancies undergo orbital coupling with the *d* shells of the adjacent divalent dopant ion and form BMP. In BMP model, the bound electrons (holes) hold in defect states that coupled through TM ions to overlap ferromagnetic regions, which responsible into high TC (due to formation of long-range ferromagnetic ordering) [16]. When the dopant ions are donors or acceptors, the exchange interactions are sp-d that would lead BMPs formation.
