*3.2.4 Magnetic ordering of ferrites in MFe2O4/BaTiO3 nanocomposite*

The multiferroic MFe2O4/BaTiO3 (MFO/BTO) thin films were fabricated by a MOD method [20]. The addition of ferrite MFe2O4 in BTO results into lattice strain due to tetragonal distortion, unit cell expansion/contraction and lattice mismatch. It enhances ME coupling. The lattice distortion, *c*/*a*, tetragonal phase of BTO, spinel ferrite MFO and the lattice strain in composite phases is studied by XRD pattern [20]. These reported results also shown the AFM images to evaluate the value of average grain size for MnFO/BTO, CFO/BTO, NFO/BTO and ZFO/BTO, which is 25, 102, 24 and 133 nm, respectively. XPS analysis indicate that Fe exist into mixed +2/+3 valence states and O in both O2<sup>−</sup> and deficient states. **Figure 6(a)** shows the room temperature ferromagnetic behavior of MFO/BTO thin films by measuring ferromagnetic hysteresis (M-Hdc). The measured value of saturation magnetization, Ms (kJ T<sup>−</sup><sup>1</sup> m<sup>−</sup><sup>3</sup> ) = 1.29, 20.25, 27.64 and 6.77, remanent magnetization Mr (kJ T<sup>−</sup><sup>1</sup> m<sup>−</sup><sup>3</sup> ) = 0.03, 3.75, 6.76 and 1.49 and with magnetic coercivity, Hc (× 105 Am<sup>−</sup><sup>1</sup> ) = 0.013, 0.079, 0.167 and 0.135, respectively, for MnFO/BTO, CFO/BTO, NFO/BTO and ZFO/BTO nanocomposite. An enlarged view of M-Hdc as the inset of **Figure 6(a)** is also shown for more clarification. This magnetic measurement of MFO/BTO is compared with magnetization of a single phase MnFO (5.40 kJ T<sup>−</sup><sup>1</sup> m<sup>−</sup><sup>3</sup> ), CFO (33.50 kJ T<sup>−</sup><sup>1</sup> m<sup>−</sup><sup>3</sup> ), NFO (50.60 kJ T<sup>−</sup><sup>1</sup> m<sup>−</sup><sup>3</sup> ) and ZFO (230 kJ T<sup>−</sup><sup>1</sup> m<sup>−</sup><sup>3</sup> ), which indicate abrupt reduction in nanocomposite sample. This is because non-magnetic BTO phase exist in nanocomposite, which reduced the magnetization of ferrite. Since the non-magnetic elements weaken the A-B superexchange interaction which result into an increase the distance between the magnetic moments in A and B sites in spinel structure. Since the magnetization decreases/increases in ferrites is a relaxation process that might be concerned with the redistribution of oxygen vacancies reported by Wang et al. [36]. The presence of a magnetic dead or antiferromagnetic coating on the nanostructural surface have reducible value of saturation magnetization of ferrite nanoparticles [37]. Vamvakidis et al. [38] suggested that the reduction in inversion degree (~0.22) of MnFe2O4 due to the partial oxidation of Mn2+ to Mn3+ ions results into weaker superexchange interactions between the tetrahedral and octahedral sites within the spinel structure. Bullita et al. [39] reported the cation distribution of ZnFe2O4 at the nanoscale level, which is contributed by partial inverted spinel structure results into

**103**

(emu g<sup>−</sup><sup>1</sup>

**Figure 6.**

*Adopted from Refs. [19, 20, 35].*

(Ms (emu g<sup>−</sup><sup>1</sup>

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

an increase magnetization. Peddis et al*.* [40] indicates inversion degree of CoFe2O4 nanoparticles, which has a better correlation between spin canting and cationic

*(a) M-Hdc hysteresis of MFe2O4/BaTiO3 nanocomposite thin films at 300 K. (b) Room temperature M-Hdc measurement for pure CFO and BTO-CFO nanoparticles. (c) the M-H measurement of LSF thin films.* 

The multiferroic 0.25BTO–0.75CFO nanocomposite is prepared by hydrothermal process at 180°C/48 h [19]. The spinel CFO and tetragonal BTO structure is studied by XRD pattern. The 0.25BTO–0.75CFO nanocomposite has nanoparticles formation with average particles size from FESEM is 80 ± 10 nm and from TEM is 76 ± 13 nm. **Figure 6(b)** shows the M-Hdc hysteresis for pure CFO and 0.25BTO–0.75CFO nanoparticles, measured at room temperature. The value of Ms

and 149, respectively, for CFO and 0.25BTO–0.75CFO nanoparticles. These measured values of Ms and Hc for pure CFO are smaller than reported for bulk CoFe2O4

0.25BTO–0.75CFO has abrupt decrement in Ms value, which is explained on the basis of BTO coating over CFO. For nanostructural CFO, the inversion degree must be swung with surface spin-canting and finite size-effect, to execute magnetic disorder. The change in bond angle and bond length may also influence the magnetization of CFO in composite. Since the spin coupling due to 3d unpaired electrons in Co2+ and Fe3+ along A and B-sites of spinel structure would lead to the ferrimagnetism of CoFe2O4. Due to such spin arrangement by unpaired electrons, there exist FeB–O–FeB, FeB–O–CoB, FeB–O–FeA, CoB–O–FeA and CoA–O–FeB superexchange interactions to induce magnetic ordering. In this, the antiferromagnetic interactions are strongest via A–O–B superexchange (inter-sublattice) and the weak ferromagnetism is attributed with A–O–A and B–O–B superexchange (intra-sublattice). It

) =73 and Hc(kOe) = 5, at 300 K). However, the nanocomposite of

) = 19.724 and 1.545 with Hc (Oe) = 1226

*3.2.5 Magnetization in CFO and 0.25BTO–0.75CFO nanoparticles*

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

distribution to get competitively higher Ms.

) = 39.027 and 6.199, Mr (emu g<sup>−</sup><sup>1</sup>

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

**Figure 6.**

*Electromagnetic Materials and Devices*

*3.2.3 AFM images of NFO, CFO and MFO thin films*

ferrite thin films, were synthesized by MOD method [9].

*3.2.4 Magnetic ordering of ferrites in MFe2O4/BaTiO3 nanocomposite*

esis (M-Hdc). The measured value of saturation magnetization, Ms (kJ T<sup>−</sup><sup>1</sup>

) and ZFO (230 kJ T<sup>−</sup><sup>1</sup>

0.135, respectively, for MnFO/BTO, CFO/BTO, NFO/BTO and ZFO/BTO nanocomposite. An enlarged view of M-Hdc as the inset of **Figure 6(a)** is also shown for more clarification. This magnetic measurement of MFO/BTO is compared with

tion in nanocomposite sample. This is because non-magnetic BTO phase exist in nanocomposite, which reduced the magnetization of ferrite. Since the non-magnetic elements weaken the A-B superexchange interaction which result into an increase the distance between the magnetic moments in A and B sites in spinel structure. Since the magnetization decreases/increases in ferrites is a relaxation process that might be concerned with the redistribution of oxygen vacancies reported by Wang et al. [36]. The presence of a magnetic dead or antiferromagnetic coating on the nanostructural surface have reducible value of saturation magnetization of ferrite nanoparticles [37]. Vamvakidis et al. [38] suggested that the reduction in inversion degree (~0.22) of MnFe2O4 due to the partial oxidation of Mn2+ to Mn3+ ions results into weaker superexchange interactions between the tetrahedral and octahedral sites within the spinel structure. Bullita et al. [39] reported the cation distribution of ZnFe2O4 at the nanoscale level, which is contributed by partial inverted spinel structure results into

20.25, 27.64 and 6.77, remanent magnetization Mr (kJ T<sup>−</sup><sup>1</sup>

and 1.49 and with magnetic coercivity, Hc (× 105

magnetization of a single phase MnFO (5.40 kJ T<sup>−</sup><sup>1</sup>

m<sup>−</sup><sup>3</sup>

m<sup>−</sup><sup>3</sup>

 m<sup>−</sup><sup>3</sup> ),

) = 0.03, 3.75, 6.76

) = 0.013, 0.079, 0.167 and

), CFO (33.50 kJ T<sup>−</sup><sup>1</sup>

), which indicate abrupt reduc-

m<sup>−</sup><sup>3</sup>

Am<sup>−</sup><sup>1</sup>

m<sup>−</sup><sup>3</sup>

m<sup>−</sup><sup>3</sup>

) = 1.29,

CoFe2O4 (CFO) and MnFe2O4 (MFO) thin films [9]. These ferrites were prepared by a MOD method using spin coating. The thickness of all the film is ~700 nm. It is also reported that the miller indices of cubic spinel ferrites structure are (2 2 0), (3 1 1), (2 2 2), (4 0 0), (3 3 1), (4 2 2) and (5 1 1), respectively, detected with diffraction angle 2θ = 30.48, 34.99, 37.48, 42.58, 48.02, 51.23 and 55.85° for NFO, for CFO, 2θ = 30.25, 34.87, 37.36, 42.58, 47.20, 51.23 and 55.84° and 2θ = 29.77, 34.64, 37.36, 41.75, 47.08, 51.23 and 57.04° for MFO. The higher coercivity value of CFO than NFO and MFO is the effect of higher magneto-crystalline anisotropy of Co cations than Ni and Mn. The observed values of Ms, Mr and Hc of the NFO, CFO and MFO films are quite smaller than bulk form [9]. For this, the decrease in Ms in ferrite nanoparticles is the canted spins in the surface layers [33]. Also, it observed very smaller values of Mr, Ms and Hc of MFO because lowering number of magnetic domains and the rate of alignment of the spins with the applied field [34, 35].

The value of average grain's size from AFM (**Figure 5(c–e)**) is 46, 61 and 75 nm, and the surface roughness 2.5, 4 and 2 nm, respectively, for NFO, CFO and MFO

The multiferroic MFe2O4/BaTiO3 (MFO/BTO) thin films were fabricated by a MOD method [20]. The addition of ferrite MFe2O4 in BTO results into lattice strain due to tetragonal distortion, unit cell expansion/contraction and lattice mismatch. It enhances ME coupling. The lattice distortion, *c*/*a*, tetragonal phase of BTO, spinel ferrite MFO and the lattice strain in composite phases is studied by XRD pattern [20]. These reported results also shown the AFM images to evaluate the value of average grain size for MnFO/BTO, CFO/BTO, NFO/BTO and ZFO/BTO, which is 25, 102, 24 and 133 nm, respectively. XPS analysis indicate that Fe exist into mixed +2/+3 valence states and O in both O2<sup>−</sup> and deficient states. **Figure 6(a)** shows the room temperature ferromagnetic behavior of MFO/BTO thin films by measuring ferromagnetic hyster-

**102**

NFO (50.60 kJ T<sup>−</sup><sup>1</sup>

*(a) M-Hdc hysteresis of MFe2O4/BaTiO3 nanocomposite thin films at 300 K. (b) Room temperature M-Hdc measurement for pure CFO and BTO-CFO nanoparticles. (c) the M-H measurement of LSF thin films. Adopted from Refs. [19, 20, 35].*

an increase magnetization. Peddis et al*.* [40] indicates inversion degree of CoFe2O4 nanoparticles, which has a better correlation between spin canting and cationic distribution to get competitively higher Ms.

### *3.2.5 Magnetization in CFO and 0.25BTO–0.75CFO nanoparticles*

The multiferroic 0.25BTO–0.75CFO nanocomposite is prepared by hydrothermal process at 180°C/48 h [19]. The spinel CFO and tetragonal BTO structure is studied by XRD pattern. The 0.25BTO–0.75CFO nanocomposite has nanoparticles formation with average particles size from FESEM is 80 ± 10 nm and from TEM is 76 ± 13 nm. **Figure 6(b)** shows the M-Hdc hysteresis for pure CFO and 0.25BTO–0.75CFO nanoparticles, measured at room temperature. The value of Ms (emu g<sup>−</sup><sup>1</sup> ) = 39.027 and 6.199, Mr (emu g<sup>−</sup><sup>1</sup> ) = 19.724 and 1.545 with Hc (Oe) = 1226 and 149, respectively, for CFO and 0.25BTO–0.75CFO nanoparticles. These measured values of Ms and Hc for pure CFO are smaller than reported for bulk CoFe2O4 (Ms (emu g<sup>−</sup><sup>1</sup> ) =73 and Hc(kOe) = 5, at 300 K). However, the nanocomposite of 0.25BTO–0.75CFO has abrupt decrement in Ms value, which is explained on the basis of BTO coating over CFO. For nanostructural CFO, the inversion degree must be swung with surface spin-canting and finite size-effect, to execute magnetic disorder. The change in bond angle and bond length may also influence the magnetization of CFO in composite. Since the spin coupling due to 3d unpaired electrons in Co2+ and Fe3+ along A and B-sites of spinel structure would lead to the ferrimagnetism of CoFe2O4. Due to such spin arrangement by unpaired electrons, there exist FeB–O–FeB, FeB–O–CoB, FeB–O–FeA, CoB–O–FeA and CoA–O–FeB superexchange interactions to induce magnetic ordering. In this, the antiferromagnetic interactions are strongest via A–O–B superexchange (inter-sublattice) and the weak ferromagnetism is attributed with A–O–A and B–O–B superexchange (intra-sublattice). It

is reported for an ideal spinel CFO, the bond angle is 90° of B–O–B, 120° of A–O–B and 80° of A–O–A magnetic exchange interactions. In the present case, the Rietveld analysis [19], calculated the values of bond-angle/length between A and B sites of CFO in 0.25BTO–0.75CFO nanocomposite, *i.e.*, bond angle = 91.07° for FeB–O–FeB, 133.57° for FeA–O–CoB and 76.13° of FeA–O–FeA. The values of bond length (*l*), *l*A-A = 0.3626(1) nm, *l*B-B = 0.2961(2) nm and *l*A-B = 0.34713 (2) nm. It is reported that the superexchange interactions might be related with bond-angle/length among bonds in Fe, Co and O atoms [41]. For the bond angle of 90° (FeB—O–FeB bonds), the ferromagnetic super-exchange interaction are resulted. When this bond angle is increased, the antiferromagnetic super-exchange interactions are there and generally, bond 120° of super-exchange FeA–O–FeB is the dominant interaction of magnetite. Above 120° of bond angle, the antiferromagnetic strength is increased which is maximum at 180° [42]. Hence the calculated values of bond angles for Co and Fe ions along A and B-sites are deviated with theoretical one that might be indicated the distorted spinel lattice to influence resulting magnetic behavior.
