*3.3.4 Photoluminescence spectra extract ZnO lattice defects*

The Zn0.95Ni0.05O (ZNiO), Zn0.91Ni0.05Ce0.04O (ZNiO:Ce), Zn0.95Cu0.05O (ZCuO) and Zn0.91Cu0.05Ce0.04O (ZCuO:Ce) nanoparticles were synthesized by a sol-gel process [10]. **Figure 8(d)** show the PL emission for Ni, Cu, Ce substituted ZnO nanoparticles at room temperature. The peak at 369 (3.36 eV) is correlated with surface exciton recombination, which is the near band edge emission of ZnO [59]. The visible emission is formed due to radiative recombination of a photogenerated hole for which an electron occupied oxygen vacancies. The violet emission at 426 nm is the effect of radiative defects related oxygen and Zn vacancies. The peak at 461 and 484 nm is the blue emission, which have two defect level formed due to transition from Zni to valance band or bottom of the conduction band to O interstitial. The peak at 632, 661 and 673 nm is the red emission formed with intrinsic

defect of O [60]. The Ce-doping into ZNiO and ZCuO is the production of oxygen vacancies defects in PL spectra [61].

## *3.3.5 Ferromagnetism at 300 K, 10 K of Zn0.95Ni0.05O and Zn0.91Ni0.05Ce0.04O nanoparticles*

**Figure 9** shows M-H hysteresis at 300 and 10 K, respectively, for ZNiO, ZNiO:Ce nanoparticles [10]. At 300 K, the values of Ms (emu g<sup>−</sup><sup>1</sup> ) = 0.073 and 0.085, and Mr (emu g<sup>−</sup><sup>1</sup> ) = 0.0066 and 0.0187 with Hc (Oe) = 150 and 558, respectively, for ZNiO and ZNiO:Ce. At 10 K, the value of Ms (emu g<sup>−</sup><sup>1</sup> ) = 0.096 and 0.198 and Mr (emu g<sup>−</sup><sup>1</sup> ) = 0.0129 and 0.0642 with Hc (Oe) = 76 and 165, respectively, for ZNiO and ZNiO:Ce. The RTFM in TM substituted ZnO has a great influence of the intrinsic defects such as oxygen vacancies. This is since the BMP model ascribed that the bound electrons (holes) in the defect states can couple with TM ions and cause the ferromagnetic regions to overlap, giving long range ferromagnetic ordering. A theoretical prediction based on first principle calculation in Ni-doped ZnO shows that the ferromagnetic ordering is energetically favorable through double or superexchange mechanisms [10]. The defects such as oxygen vacancies in DMS are responsible in generating carriers for ferromagnetic ordering. The observed values of Ms and Mr at 10 K, are enhanced than at room temperature. However, at low temperature (~10 K), the value of Hc is varies abruptly. This is due to the existence of some ferromagnetic cluster assemblies in the samples, which enlarged Hc, may an indication towards superparamagnetic/spin-glass formation [62].

### *3.3.6 FC/ZFC magnetization of Zn0.95Ni0.05O and Zn0.91Ni0.05Ce0.04O nanoparticles*

The origin of observed RTFM in Ni, Ce substituted ZnO is evaluated by the temperature dependent magnetization [M(T)] with FC at 500 Oe and ZFC measurements (**Figure 9**) [47]. The FC/ZFC curves are separated with decrease in temperature, which usually appears with a coexistent system of antiferromagnetic and ferromagnetic phases. The steep increase of magnetization in FC curve with decreasing temperatures below 50 K is the characteristic of DMS [63]. Decreasing

#### **Figure 9.**

*M-H hysteresis at 300 and 10 K, and M(T) measurement following FC/ZFC at H = 500 Oe for ZNiO, ZNiO:Ce nanoparticles. Inset shows their χ−<sup>1</sup> -T of Curie-Weiss law. Adopted from Verma and Kotnala [10].*

**109**

**Figure 10.**

*Kotnala [47].*

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

temperatures result into an increase in polarons interaction distance, leading to overlap between neighboring polarons and so allowing them to interact through the magnetic impurities, forming correlated polarons clusters. Though the direct interaction among localized carriers induced antiferromagnetism, and the ferromagnetic interaction due to BMP is possible with large magnetic impurities. When the unoccupied 3d states overlap the impurity band might be attributed high Tc value of ZnO. This is explained on the basis of donor impurity band for which oxygen vacancies involved F-centers into host ZnO [64]. The dip in ZFC curve indicates the existence of the blocking temperature, TB. The upward curvature observed in the FC M(T) measurements (inset of **Figure 9**) of Ni and Ce substituted ZnO

*O 1s XPS spectra of pure ZnO, Zn0.95Fe0.05O and Zn0.92Fe0.05La0.03O nanoparticles. Adopted from Verma and* 

*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 10.**

*Electromagnetic Materials and Devices*

vacancies defects in PL spectra [61].

*nanoparticles*

Mr (emu g<sup>−</sup><sup>1</sup>

Mr (emu g<sup>−</sup><sup>1</sup>

defect of O [60]. The Ce-doping into ZNiO and ZCuO is the production of oxygen

**Figure 9** shows M-H hysteresis at 300 and 10 K, respectively, for ZNiO, ZNiO:Ce

) = 0.0066 and 0.0187 with Hc (Oe) = 150 and 558, respectively, for

) = 0.0129 and 0.0642 with Hc (Oe) = 76 and 165, respectively, for

ZNiO and ZNiO:Ce. The RTFM in TM substituted ZnO has a great influence of the intrinsic defects such as oxygen vacancies. This is since the BMP model ascribed that the bound electrons (holes) in the defect states can couple with TM ions and cause the ferromagnetic regions to overlap, giving long range ferromagnetic ordering. A theoretical prediction based on first principle calculation in Ni-doped ZnO shows that the ferromagnetic ordering is energetically favorable through double or superexchange mechanisms [10]. The defects such as oxygen vacancies in DMS are responsible in generating carriers for ferromagnetic ordering. The observed values of Ms and Mr at 10 K, are enhanced than at room temperature. However, at low temperature (~10 K), the value of Hc is varies abruptly. This is due to the existence of some ferromagnetic cluster assemblies in the samples, which enlarged Hc, may an

) = 0.073 and 0.085, and

) = 0.096 and 0.198 and

*3.3.5 Ferromagnetism at 300 K, 10 K of Zn0.95Ni0.05O and Zn0.91Ni0.05Ce0.04O* 

nanoparticles [10]. At 300 K, the values of Ms (emu g<sup>−</sup><sup>1</sup>

ZNiO and ZNiO:Ce. At 10 K, the value of Ms (emu g<sup>−</sup><sup>1</sup>

indication towards superparamagnetic/spin-glass formation [62].

*3.3.6 FC/ZFC magnetization of Zn0.95Ni0.05O and Zn0.91Ni0.05Ce0.04O nanoparticles*

The origin of observed RTFM in Ni, Ce substituted ZnO is evaluated by the temperature dependent magnetization [M(T)] with FC at 500 Oe and ZFC measurements (**Figure 9**) [47]. The FC/ZFC curves are separated with decrease in temperature, which usually appears with a coexistent system of antiferromagnetic and ferromagnetic phases. The steep increase of magnetization in FC curve with decreasing temperatures below 50 K is the characteristic of DMS [63]. Decreasing

*M-H hysteresis at 300 and 10 K, and M(T) measurement following FC/ZFC at H = 500 Oe for ZNiO, ZNiO:Ce* 

*-T of Curie-Weiss law. Adopted from Verma and Kotnala [10].*

**108**

**Figure 9.**

*nanoparticles. Inset shows their χ−<sup>1</sup>*

*O 1s XPS spectra of pure ZnO, Zn0.95Fe0.05O and Zn0.92Fe0.05La0.03O nanoparticles. Adopted from Verma and Kotnala [47].*

temperatures result into an increase in polarons interaction distance, leading to overlap between neighboring polarons and so allowing them to interact through the magnetic impurities, forming correlated polarons clusters. Though the direct interaction among localized carriers induced antiferromagnetism, and the ferromagnetic interaction due to BMP is possible with large magnetic impurities. When the unoccupied 3d states overlap the impurity band might be attributed high Tc value of ZnO. This is explained on the basis of donor impurity band for which oxygen vacancies involved F-centers into host ZnO [64]. The dip in ZFC curve indicates the existence of the blocking temperature, TB. The upward curvature observed in the FC M(T) measurements (inset of **Figure 9**) of Ni and Ce substituted ZnO

nanoparticles, involve a Curie–Weiss behavior related with susceptibility (χ) as: <sup>χ</sup> <sup>=</sup> \_\_\_\_ *<sup>C</sup> T* − θ ; where C is the material specific Curie constant, T is the absolute temperature, and θ is the Weiss constant (in K) [10]. The estimated value of θ is found to be negative, which indicate antiferromagnetic interactions. But χ−<sup>1</sup> (T) displays a notable deviation from the Curie-Weiss law that might be attributed by short-range ferromagnetism, antiferromagnetism, or a spin-glass system [65].
