*1.4.2 Theoretical survey on magnetism of DMS ZnO with TM = Cr, Mn, Fe, Co, and Ni ions*

Wang et al. [19] reported that the Cr, Fe, Co, and Ni dopants in ZnO occupy the Zn sites and couple antiferromagnetically, while Mn exhibits no site preference and distributes uniformly in ZnO lattice. For hexagonal ZnO, the lattice constants are *a* = *b* = 3.249 Å and *c* = 5.205 Å [space-group P63mc (No. 186)]. The ZnO thin film containing 28 formula units (Zn28O28) is shown in **Figure 1c** [19]. To find the magnetic coupling among TM ions, the two Zn atoms are replaced with two TM (= Cr, Fe, Co, and Ni) atoms with a dopant concentration of 14.28%. The preferred magnetic coupling between the TM atoms is determined with ferromagnetic (FM) and antiferromagnetic (AF) coupling by comparing their total energies, ΔE=EAF EFM. Positive ΔE means that the FM state is lower in energy than the AF state. In **Figure 1c**, when the two TM atoms are at the nearest neighbor sites on the surface, the corresponding magnetic couplings are AF. It is also reported that the total-energy difference between FM and AF states is reduced to 0.006–0.032 eV, when the distance between two TM atoms are increased to about 5.60 Å. It means that the AF interactions are short ranged in TM/ZnO. Srinivasulu et al. [23] suggested various 3d TM such as Ti, V, Cr, Mn, Fe, Co, Ni, and Cu that are also tried as dopants in ZnO to improve its optical and electrical behavior. Among these dopants, V, Cr, Mn, Co, Ni, and Cu are recognized as suitable dopants of ZnO for spintronic and magneto-optical communication devices due to their RTFM. In TM ions, the magnetization arises from partially filled 3d shells, and most of the cases since total orbital magnetic moment is zero, the magnetic moment is only due to the spin component, and hence total magnetic moment per atom is less [24]. Among TM/ZnO, Co deserved a special attention due to its highest magnetic moments (4.8 μB) and a positive magnetic exchange coupling constant [25]. Coey et al. [14] explained ferromagnetism in intrinsically n-type semiconductors and insulators by a model, where shallow donor electrons, created due to intrinsic defects in the semiconductors, form bound magnetic polarons with magnetic cations, which finally give rise to the ferromagnetic interaction. For BMPs, the localized spins of the dopant ion interact with the charge carriers such as oxygen vacancies, resulting in a magnetic polarization of the surrounding local moments [26]. The mediated oxygen vacancies are dependent upon dopant level and nanostructural formations.

#### *1.4.3 Rare earth ions attributed ferromagnetism in DMS ZnO*

In rare earth (RE) elements, magnetization appears due to unfilled 4f orbitals leading to higher magnetic moment per atom, though 4f electrons interacted with 5d or 6 s electrons [24], but exhibits weak exchange interaction with other RE ions, which is contrast to TM ions 3d electrons are directly interacted. The RE ion-doped ZnO has ferromagnetism that is induced by p-f hybridization via defect carriers [27]. Compared with 3d TMs, 4f RE ions have larger magnetic moments. The intrinsic defects such as oxygen vacancies play an important role on the magnetic properties of RE/ZnO. However, the exchange interaction by simultaneous doping from TM and RE ions in ZnO is 4f-5d-3d, which is antiferromagnetic when the 5d band is less than half full and the 3d band is more than half full. The first principle calculations revealed that the superexchange interaction between two magnetic Nd ions is mediated by the nonmagnetic O ions responsible for higher magnetic moment of ZnO [28]. This approach of doping RE elements with intrinsic strong

atmospheric pressure. The atomic arrangement of the wurtzite structure is comprised of four zinc ions (Zn2+) occupying the corner of a tetrahedral coordinate with one oxygen ion (O2�) located at the center and vice versa (**Figure 2a**) [20]. The particle size, doping, and co-doping are used to induce the band gap of ZnO [17]. Dietl et al. [1] reported RTFM for DMS for which 3d ions substituted ZnO. However, the nanocrystals of pure ZnO also produce RTFM [21]. Gao et al. [22] suggested oxygen vacancies locating at the surface of ZnO nanoparticles are responsible for RTFM. It is also found that the ZnO nanoparticles had absorbed certain organic molecules to modify the electronic structure to give RTFM without

From the survey of many theoretical studies, it has been found that a slight doping of TM metal ions is likely �5%, induce ferromagnetic ordering that

observed at room temperature [21]. Venkatesan et al. [18] postulated on the basis of spin-split donor impurity-band model to observe RTFM in DMS ZnO with 5% of Sc, Ti, V, Fe, Co, or Ni, but not Cr, Mn, or Cu ions. For Cr, Mn, Cu, or Zn, no moment appreciably greater than the experimental uncertainty (<0.1 μB) is observed at room temperature. The basic action in a spintronic device is that the electrons are traveling from a ferromagnetic metal, through a normal metal, to a second ferromagnetic metal. When the magnetizations of the two ferromagnetic metals are in an aligned state, the resistance is low, whereas the resistance is high in the antialigned state. For the light 3d elements, the 3d<sup>↑</sup> states lie high in the 2*p*(O)-4 *s*(Zn) gap, overlapping the donor impurity band which is spin split (**Figure 2b**). In the middle of the TM series, there is no overlap with the 3d levels and exchange is weak, but toward the end of the series, the 3d<sup>↓</sup> states overlap the impurity band, which then has the opposite spin splitting for the same occupancy. The high TC is found whenever unoccupied 3*d* states overlap the impurity band, but not otherwise. The likely origin of the donor impurity band in ZnO films is lattice defects, such as oxygen vacancies, which have trapped between one and two electrons (F<sup>0</sup> centers)

*(a) Hexagonal wurtzite ZnO unit cell. Density of states (schematic) of Zn1-*x*TM*x*O,TM = Ti (b), Mn (b*<sup>0</sup>

*Co (b*00*), for which the Fermi level lies in a spin-split donor impurity band. (c) Energy differences, ΔE=EAFM* � *EFM, for Zn0.857TM0.143O (TM = Cr, Mn, Fe, Co, and Ni) supercell; Zn(light gray spheres),*

*),*

any magnetic impurity ions [21].

*Magnetic Materials and Magnetic Levitation*

**Figure 2.**

**112**

*O(red), and TM (purple) (adopted from [17–19]).*

*1.4.1 Ferromagnetism of ZnO with transition metal ions*

magnetic anisotropy and tailoring the coupling between dopants and defects should be a general approach toward stable ferromagnetic order in ZnO nanomaterials. Among RE ions, Sm3+ with five 4f electrons offers a unique possibility to induce the bifunctional properties for RTFM as well as visible luminescence in ZnO, making suitable material in spin transport properties and spin-LEDs [29].

*lZn*�*<sup>O</sup>* ¼

*DOI: http://dx.doi.org/10.5772/intechopen.90369*

where

**Figure 3.**

*[27, 40, 41]).*

**115**

system is calculated using.

*a*2 3 þ

*<sup>u</sup>* <sup>¼</sup> *<sup>a</sup>*<sup>2</sup>

where u is a positional parameter. The volume per unit cell for the hexagonal

The calculated values of the lattice parameters are *a*(Å) = 3.257, 3.256, 3.260, and 3.261; *c*(Å) = 5.207, 5.206, 5.214, and 5.217; c/a = 1.5987, 1.5988, 1.5994, and 1.5998;

*(a) XRD pattern of Zn0.94Fe0.03Ce0.03O (ZFCeO) and Zn0.94Co0.03Ce0.03O (ZCCeO) nanoparticles. (b, d) Raman and UV-visible absorption (for energy band calculation) spectra of Zn0.95Ni0.05O (ZNiO), Zn0.91Ni0.05Ce0.04O (ZNiO/Ce), Zn0.95Cu0.05O (ZCuO), and Zn0.91Cu0.05Ce0.04O (ZCuO/Ce) nanoparticles. (c) Photoluminescence spectra for ZnO with Co, Mn, and Fe nanoparticles (adapted from*

s

*Diluted Magnetic Semiconductor ZnO: Magnetic Ordering with Transition Metal…*

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

∗ *c*<sup>2</sup>

<sup>3</sup>*c*<sup>2</sup> <sup>þ</sup> <sup>0</sup>*:*<sup>25</sup> (3)

<sup>V</sup> <sup>¼</sup> <sup>0</sup>*:*<sup>866</sup> � a2 � <sup>c</sup> (4)

(2)

1 <sup>2</sup> � *<sup>u</sup>* � �<sup>2</sup>
