**1.1 DMS opens new window for spintronics**

Even though low transition temperatures, ferromagnetism in diluted magnetic semiconductors, DMSs is essential to explore new ideas to develop spintronic technology, which is actually the electrical manipulation of magnetism [6]. A small Mn percentage in III-V semiconductors of (In, Mn)As and (Ga, Mn)As replaces Ga or In atoms to stabilize the exchange interaction between charge carriers and localized spins. Due to low carrier concentration in these DMSs, it is possible to control a considerable portion of carriers by external electric fields using metal–insulator– semiconductor or p-n junction configuration (**Figures 1a, b**). It has a thin ferromagnetic semiconductor layer, to the extent that the field significantly alters the stability of the ferromagnetic phase and other magnetic properties. The value of TC is controlled in a ferromagnet with the application of an electric field which was observed for metal–insulator–semiconductor configuration of (In, Mn)As thin films [8]. Later, electrical manipulation of the coercive field (HC) is also possible for (In, Mn)As which means an applied electric field changes the magnetic anisotropy [9]. This is the exchange interaction which splits the carrier states according to the spin-orbit interaction [10].

#### **1.2 DMS made up as a computer memory**

For low-power-consumption computer memory devices, the DMSs influencing the magnetization direction to achieve magnetic data-storage and memory devices of hard disks directs with the direction of magnetization [7]. In a hard disk, the data is stored on a disk-shaped magnet in local magnetization form; to write information, a pulse of current is applied to a small electromagnet that scans the disk. In this process, an energy is wasted due to a magnetic field exists between the current and the manipulated magnetization. However, a spin-polarized current is applied directly to the magnet instead of using current to generate the magnetic field is an

#### **Figure 1.**

*(a, b) Electric control of ferromagnetism (schematic representation). (c) a spin-polarized current changes the magnetization direction, i.e., (GaMn)As (adapted from [6, 7]).*

*Diluted Magnetic Semiconductor ZnO: Magnetic Ordering with Transition Metal… DOI: http://dx.doi.org/10.5772/intechopen.90369*

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 magnetization direction.

#### **1.3 Ferromagnetic origin in DMS**

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

Even though low transition temperatures, ferromagnetism in diluted magnetic semiconductors, DMSs is essential to explore new ideas to develop spintronic technology, which is actually the electrical manipulation of magnetism [6]. A small Mn percentage in III-V semiconductors of (In, Mn)As and (Ga, Mn)As replaces Ga or In atoms to stabilize the exchange interaction between charge carriers and localized spins. Due to low carrier concentration in these DMSs, it is possible to control a considerable portion of carriers by external electric fields using metal–insulator– semiconductor or p-n junction configuration (**Figures 1a, b**). It has a thin ferromagnetic semiconductor layer, to the extent that the field significantly alters the stability of the ferromagnetic phase and other magnetic properties. The value of TC is controlled in a ferromagnet with the application of an electric field which was observed for metal–insulator–semiconductor configuration of (In, Mn)As thin films [8]. Later, electrical manipulation of the coercive field (HC) is also possible for (In, Mn)As which means an applied electric field changes the magnetic anisotropy [9]. This is the exchange interaction which splits the carrier states according to the

For low-power-consumption computer memory devices, the DMSs influencing the magnetization direction to achieve magnetic data-storage and memory devices of hard disks directs with the direction of magnetization [7]. In a hard disk, the data is stored on a disk-shaped magnet in local magnetization form; to write information, a pulse of current is applied to a small electromagnet that scans the disk. In this process, an energy is wasted due to a magnetic field exists between the current and the manipulated magnetization. However, a spin-polarized current is applied directly to the magnet instead of using current to generate the magnetic field is an

*(a, b) Electric control of ferromagnetism (schematic representation). (c) a spin-polarized current changes the*

of RTFM of DMSs [5]. Among DMSs, the Mn-doped GaAs is found to be

ferromagnetic with TC 172 K is widely investigated [5].

**1.1 DMS opens new window for spintronics**

*Magnetic Materials and Magnetic Levitation*

spin-orbit interaction [10].

**Figure 1.**

**110**

**1.2 DMS made up as a computer memory**

*magnetization direction, i.e., (GaMn)As (adapted from [6, 7]).*

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 magnetism with oxygen vacancies.

#### **1.4 DMS ZnO**

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

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 any magnetic impurity ions [21].

[18]. The electrons in the impurity band will be localized by the influence of electronic correlations and potential fluctuations associated with the dopant cations.

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

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

and antiferromagnetic (AF) coupling by comparing their total energies,

*1.4.3 Rare earth ions attributed ferromagnetism in DMS ZnO*

**113**

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)

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.

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

*and Ni ions*

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

#### *1.4.1 Ferromagnetism of ZnO with transition metal ions*

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)

#### **Figure 2.**

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

*Diluted Magnetic Semiconductor ZnO: Magnetic Ordering with Transition Metal… DOI: http://dx.doi.org/10.5772/intechopen.90369*

[18]. The electrons in the impurity band will be localized by the influence of electronic correlations and potential fluctuations associated with the dopant cations.
