*3.7.2 Magnetic ordering in Zn0.996Co0.004O nanoparticles*

and FC measurement with H = 500 Oe. The superimposition of ZFC/FC plots between 150 and 300 K, as well as their clear separation at low temperature with blocking temperature, TB is observed. The observed TB might correspond with Néel

temperature, TN (�42 K) of AF [52]. For more detail, M-H hysteresis is also

enhanced with temperature when going from 300 to 10 K. This is due to the exchange interaction from AF to FM states. It is also shown that for 200–50 K, Hc varies so slowly, but at 10 K, it abruptly increased to 117 Oe, which is smaller than 144 Oe that is observed at room temperature. It means after AF transition, there is some possibility of FM clustered growth in ZFLaO sample [53]. The localization of electrons in magnetic clusters leads to develop high-spin and low-spin intersite electronic transitions. These magnetic clusters may also result from magnetic

). The values Ms and Mr are

) = 0.0276 with Hc(Oe) = 40 Oe. However,

measured at 200, 100, 50, and 10 K (**Figure 8c**<sup>0</sup>

*Magnetic Materials and Magnetic Levitation*

**3.7 DMS ZnO with Co, La, Gd, and Ce ions**

*3.7.1 RTFM in La- and Gd-doped Zn0.95Co0.05O nanostructure*

) = 0.354 and Mr(emu g�<sup>1</sup>

**Figure 9a** shows the M-H hysteresis for Zn0.95Co0.05O (ZCO5),

Zn0.92Co0.05La0.03O (ZCLO53), and Zn0.92Co0.05Gd0.03O (ZCGO53) nanostructure, measured at room temperature [36]. The pure ZCO5 shows weak ferromagnetism

*(a) M-H hysteresis for Co-, La-, and Gd-doped ZnO nanoparticles, measured at room temperature. (b) M(T) and M(H) (inset) for Zn0.996Co0.004O (ZCO04) nanoparticles. (b*<sup>0</sup> *and b*00*) XPS spectra for Co 2p and O 1 s. (c) Temperature-dependent* AC *magnetic susceptibility (χ) of ZFCeO nanoparticles (adopted from [15, 36,*

the La- and Gd-doped ZCO5 result into paramagnetic-type behavior. The weak ferromagnetism in ZCO5 exists due to antiferromagnetic, AF interactions among Co2+ ions [41, 55]. The AF coupling between Co impurities is favored when Co atoms are separated by more than a ZnO unit. While the ferromagnetic coupling is stable if AF interaction in neighboring Co–Co ions falling into contour of BMPs. However, the observed paramagnetism in La- and Gd-doped ZCO5 is related with

polarons [54].

of Ms(emu g�<sup>1</sup>

**Figure 9.**

*40]).*

**124**

The Zn0.996Co0.004O (ZCO04) nanoparticles synthesized with sol–gel process for which free-charge carriers and oxygen vacancies might induce long-range ferromagnetic ordering [15]. The XRD pattern results into wurtzite structure of ZCO04. The ZCO04 crystalline product has nanorod formation with D(nm) = 23 � 3 and L(nm) = 57 � 5. **Figure 9b** (inset) showed the RTFM of Ms (emu g�<sup>1</sup> ) 0.0062 and Mr (emu g�<sup>1</sup> ) = 0.0038 with Hc = 54 Oe. However, the pure ZnO nanorods are diamagnetic [50]. Xu et al. [57] reported RTFM with higher surface-to-volume ratio of nanostructure, which contribute large amount of surface oxygen vacancies defects. It is expected that the RTFM is attributed via exchange interactions among unpaired electron spins arising from either vacancies or surface defects, which is explained on the basis of donor impurity band exchange model form BMPs [58]. It is theoretically investigated that the oxygen vacancies have remarkable change in band structure of host oxides to induce ferromagnetism [59]. For this case of BMPs, the electrons are locally trapped by oxygen vacancies, with the trapped electron occupying an orbital overlapping with the *d* shells of Co neighbors.

To evaluate the origin of RTFM of ZCO04 nanoparticles, the temperaturedependent magnetization is given in **Figure 9b** via ZFC and FC at H = 500 Oe. The separation between ZFC and FC starts increasing with reducing temperature from 300 to 5 K which indicates antiferromagnetic interactions converted to ferromagnetic state. The absence of blocking temperature in ZFC might indicate long-range antiferromagnetism without any cluster growth. The exchange interactions between neighboring magnetic ions mediated by an F-center form a BMP contributing long-range ferromagnetism. At 10 K, the magnetic hysteresis is also shown in the inset of **Figure 9b** with Ms(emu g�<sup>1</sup> ) = 0.0154 and Mr(emu g�<sup>1</sup> ) = 0.002 with Hc(Oe) = 93.

## *3.7.3 Valence states of Co and O ions in Zn0.996Co0.004O nanoparticles*

**Figures 9b**0 **, b**<sup>00</sup> shows the XPS spectra for Co 2p and O 1 s of Zn0.996Co0.004O (ZCO04) nanoparticles. For Co 2p, the doublet is the spin-orbit coupling (2p3/2 and 2p1/2) given in **Figure 9b**<sup>0</sup> . The values of binding energy, Co 2p3/2 � 780.019 eV, 2p1/2 � 795.51 eV, and ΔE � 15.51 eV, and satellite peak (S) � 785.48 eV are observed. The binding energies of Co 2p3/2 and 2p1/2 indicate that the Co ions exist either in +3 or + 2 valence states [60]. The difference ΔE of binding energy among Co 2p3/2 and 2p1/2 levels corresponds well with Co2+ that is homogeneously surrounded by oxygen in tetrahedral coordination [61]. However, the peak S is found in the energy region of 6–8 eV above the principle peak Co 2p3/2 and the value of S � 6 eV. It indicates the formation of multiple coordinations, i.e., tetrahedral or octahedral Co2+ ions. For more clarification, Co 2p peaks shown by octahedral Co2+ (*Co*<sup>2</sup><sup>þ</sup> *<sup>o</sup>* ) and Co3+ and tetrahedral Co2+ (*Co*<sup>2</sup><sup>þ</sup> *<sup>t</sup>* ) are clearly marked.

To find defects/vacancies in ZCO04, the O 1 s spectra is shown in **Figure 9b**00, which deconvoluted into three peaks (Oa, Ob, Oc) [15]. The peak located on lowbinding energy side, Oa � 528.79 eV, is attributed with O2� ions in wurtzite structure. This Oa of O 1 s is associated with Zn-O bonds. The Ob peak at 531.19 eV is associated with O2� ions in the oxygen-deficient regions within the ZnO matrix, which indicate defect formation. The Oc peak at 532.16 eV is attributed to chemisorbed oxygen on the surface of the ZnO.
