**1. Introduction**

Diluted magnetic semiconductors (DMSs) and oxides raised hopes of applications based on single elements combining the logic functionalities of semiconductors with the information storage capabilities of magnetic elements [1]. The research on DMSs is fascinating during the 1980s on II-VI semiconductors with little magnetic impurities that opened the optical and electronic phenomenon to introduce magnetic field effects [2]. The DMS required high Curie temperature, TC, for a material. Initially, II-VI semiconductors such as CdMnTe and ZnMnSe are widely investigated as DMSs for which the valence of the cation matches with the common magnetic ions, i.e., Mn [3]. Since the hole induced ferromagnetic order in p-type InMnAs and GaMnAs, a lot of research is carried out on III-V-based DMSs that unfortunately failed to give room temperature ferromagnetism (RTFM). After that, a lot of efforts are carried out to result RTFM in III-V-based DMS, i.e., GaN, GaSb, InAs, and oxide-based DMS, ZnO, TiO2, SnO2, In2O3, etc. These DMSs are

attracting potential interest in spin-based information-processing applications. It needs high TC for such DMSs of GaN and ZnO that may relate for their wide-band gap [4]. However, the spintronic applications like spin-valve transistors, spin lightemitting diodes, nonvolatile memory, logic devices, etc. have remarkable interest of RTFM of DMSs [5]. Among DMSs, the Mn-doped GaAs is found to be ferromagnetic with TC 172 K is widely investigated [5].

alternative way as schematized in **Figure 1c**. Such a current would exert torque on the magnetization by exchanging spin-angular momentum with it as it passes through the magnet. Exploitation of this phenomenon, called spin torque, is expected to allow the development of compact magnetic memory devices that can run on low-power consumption. In **Figure 1c**, the metal–insulator–semiconductor device involving a semiconductor—a (GaMn)As film—that has ferro magnetic properties at low temperatures [7]. The device includes a 'gate' electrode isolated electrically from the (GaMn)As film. When a negative voltage is applied to the gate electrode, carriers in the film that have positive charge (electron "holes") are attracted toward the electrode and vice versa. This property allows the density of the electron holes, and thus the magnetic anisotropy in the (GaMn)As film beneath the electrode, to be controlled electrically, resulting in a change in the

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

The researcher has initially found high TC in doped III-V DMS, which for a long time was stuck at 110 K [1]. After that, several groups stressed out the defects mainly Mn atoms that form interstitials rather than substituting for Ga—responsible for this limit, and TC was raised up to 150 K. Dietl [1] proposed a Zener model to perform so many experiments on (Ga, Mn)As, which create problem with higher concentration of Mn due to the interplay between the disorder and localization, and electron–electron correlations have a very influential effect on carrier-mediated ferromagnetism at and above room temperature [11]. For example, for the Codoped ZnO, the Co occupies the Zn sites as Co is paramagnetic and there is no ferromagnetism associated with Co, even when lots of carriers are added by Al codoping and the temperature is very low (5 K) [12]. Moreover, the solubility of Co in ZnO is high, making it is easy to substitute Co for Zn throughout the crystal. However, by considering defect-mediated ferromagnetism, an intrinsic form of high-TC ferromagnetism in dilute magnetic oxides with lots of defects is observed. This is because electrons associated with defects couple antiparallel to dopant spins within the orbital volume of the defect. With high TC, the defect coupling is strong. But, it is difficult to control defects for practical applicability. To realize hightemperature ferromagnetism in DMSs, a wide-band gap ZnO is undoubtedly a major development if the ferromagnetism is unambiguously established to be intrinsic (carrier induced) [13]. Coey et al. [14] proposed that the ferromagnetic exchange is mediated by shallow donor electrons to form bound magnetic polarons that overlap to create a spin-split impurity band. It is reported that the oxygen vacancies might change the band structure of host oxides to induce ferromagnetism [15]. The formation of BMP, which includes electrons locally trapped via oxygen vacancies, with the trapped electron occupying an orbital overlapping with the d shells of transition metal (TM) neighbors, might explain the room temperature ferromagnetism (RTFM) in DMS. Within the BMP model, the greater density of oxygen vacancy yields a greater overall volume occupied by BMP, thus increasing their probability of overlapping more TM ions into the ferromagnetic domains to enhance ferromagnetism. Zhen et al. [16] used first principle calculations on Codoped ZnO and observed the exchange coupling mechanism that accounts

magnetization direction.

**1.3 Ferromagnetic origin in DMS**

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

magnetism with oxygen vacancies.

DMS ZnO has the hexagonal wurtzite structure (direct wide-band gap, Eg 3.3 eV at 300 K) due to its stability at room temperature and normal

**1.4 DMS ZnO**

**111**
