Hydrogen-induced anomalous Hall effect in Co-doped ZnO

The electrical transport characteristics and anomalous Hall effect (AHE) were investigated for a hydrogen-injected Co-doped ZnO thin film. Based on the measurements of resistivity and the Hall effect between 5 K and 300 K, the existence of Co-H-Co complexes was observed to introduce the AHE and enable the AHE to persist up to room temperature. The observed H-induced AHE originates from the asymmetric scattering of carrier hopping between the localized states driven by ferromagnetic Co-H-Co complexes, and a theoretical study using first-principle calculations supports the experimental results well. This large ferromagnetic response of charge carriers by the hydrogen-induced AHE on semiconducting oxides will stimulate the further investigation of room-temperature spintronic applications.


Introduction
Ferromagnetic semiconductors have been considered suitable materials for future spintronics due to the beneficial advantage of the strong coupling between their magnetic and electrical transport characteristics [1]. One of the most promising characteristics of ferromagnetic semiconductors is the anomalous Hall effect (AHE), which can provide the electrical manipulation of a ferromagnetically spin-polarized carrier. The AHE signal has also been assumed to be a useful experiment for characterizing the intrinsic ferromagnetism and providing evidence of the presence of a spin-polarized current. For III-V ferromagnetic semiconductors, the electrical-field control of ferromagnetism as well as magnetization reversal and photoinduced ferromagnetism have been achieved using AHE [2][3][4]. However, the low Curie temperature of group III-V materials, which is far below room temperature, has hindered the realization of spintronic devices that are operable at room temperature.
Since the possibility of room-temperature ferromagnetism was first predicted based on wide-band-gap semiconductors, there have been tremendous research efforts focused on transition-metal-doped II-VI semiconductors [5][6][7]. Among these materials, Co-doped ZnO has been intensively studied as a promising candidate for room-temperature ferromagnetism. Several experimental results have provided evidence of the intrinsic ferromagnetism of this material; however, the origin of ferromagnetism still remains unclear, and there are controversial arguments about the extrinsic contributions. Recently, it was reported that nano-clusters of magnetic impurities occur due to spinodal nano-decomposition [8,9]. The nano-clusters are well separated and show super-paramagnetism. However, ferromagnetic behaviour can be observed in the magnetization process at finite temperatures due to the blocking effect [8,9].
In previous papers, we theoretically and experimentally demonstrated the ferromagnetism in ZnCoO:H originating from the ferromagnetic spin-spin interaction of the Co-H-Co complex [10][11][12][13][14]. We also demonstrated that hydrogen (H)-induced ferromagnetism can be reversible and reproducible by tuning the injected H. Currently, the localized ferromagnetic domain has been observed using anodic aluminium oxide templates and the localized hydrogenation method [15]. Theoretical calculations predict that the macroscopic percolation of ferromagnetic Co-H-Co complex units would result in ferromagnetic long-range ordering exhibited as roomtemperature ferromagnetism, which can contribute to the development of spintronic devices that are operable at room temperature [10]. However, the H-injection is simultaneously accompanied by a large change in the electrical characteristics and by the mediation of spin ordering. Therefore, to manipulate the percolation of complexes in ZnCoO:H, it is necessary to investigate the correlations between the changes of electrical transport and magnetic characteristics due to H incorporations.
Here, we report on the clear H-induced AHE and the changes of carrier density (n) and mobility (μ) in Co-doped ZnO thin films with increasing H incorporation from the surface. We also examined the electronic structure of ZnCoO:H using first-principles calculations. Generally, the local density approximation (LDA) is not sufficient to describe the electronic structure of the localized orbitals in the magnetic semiconductors due to the self-interaction effect [16]. Therefore, in this work, we used the LDA+U method, which compensates for the self-interaction. Through the measurements of resistivity and the Hall effect as well as firstprinciples calculations, we demonstrate that the injected H can increase the characteristic AHE due to the localized states formed by the hybridization between the H-s orbital and the Co-t 2 orbitals and that unlike other n-type impurities in ZnCoO, the characteristic nature of H can contribute to the formation of ferromagnetic Co-H-Co complexes and to the enhancement of a spin-dependent asymmetric carrier scattering up to room temperature.   [11][12][13]. Six-contact 1-2-2-1 Hall bars with 300-1000 μm channel sizes were fabricated using conventional UV-lithography with photoresist (PR) patterning ( figure 1(a)). For this study, the thickness of all samples was fixed at 100 nm. In this study, three different types of ZnCoO:H samples were prepared using different H-injection processes. During the dry-etching process using a reactive ion etcher, H was slightly injected into ZnCoO (H-1 sample). For this H-1 sample, additional H injection processes were performed using RF-plasma with a power of 40 W (H-2 sample) and 80 W (H-3 sample) with an Ar-H 2 (10 wt. %) gas mixture for 10 min. After the H-injection process, the Au electrode was evaporated on the fabricated Hall bar using DC-sputtering with a shadow mask ( figure 1(b)).
As presented in our previous report of the secondary ion mass spectroscopy (SIMS) depth profiles [11], H was uniformly distributed on both the surface and the inside of the hydrogenated sample, but the H concentration was reduced by subsequent vacuum annealing. After vacuum annealing and a second hydrogenation process, the H level was the same as that of the sample inside the film prior to subsequent processing. There was no detectable change in the oxygen level. The H was mainly concentrated 20 nm beneath the surface, and the levels of Zn, Co and O changed at the surface during the second hydrogenation process; these results were not seen in our recent study [13], due to optimized H treatment conditions using HIP.
Figures 1(c) and (d) present scanning electron microscope (SEM) and high resolution transmission electron microscopy (HRTEM) images of the fabricated H-3 Hall bar sample, respectively. Generally, an excessive H treatment can induce extrinsic ferromagnetism such as metal clusters. However, as observed in HRTEM image in figure 1(d), no trace of clustering or a secondary phase was observed, even for the highest plasma power for H-injection (H-3 sample). In our previous and current studies, x-ray absorption spectroscopy and synchrotron xray diffraction measurements revealed that the contribution of metallic clusters or crystalline defects is negligible in our optimized H-injection condition [14,17]. Furthermore, the abnormal magnetic behavior in temperature-dependent magnetization attributed to Co nano-clusters [18] was also not observed for the H-2 or H-3 samples.

Electrical characterizations and Hall effect measurement
The Au electrode was evaporated on the fabricated Hall bar using DC-sputtering with shadow mask. The transport properties of all the samples were measured by a combinations of a Physical Properties Measurement System (PPMS-9, Quantum Design) with function generator (HP 3325B), Lock-in amplifier (Stanford SR830 DSP) and source meter (Keithley 2425C). The temperature was controlled from 5 to 300 K. After fixing temperature, the magnetic field was slowly swept perpendicular to both the sample surface and current direction between −3 T and 3 T during AHE measurements. In order to minimize the contribution of thermal energy, we measured the transverse and longitudinal voltage with lock-in amplifier during the current (10 μA) reversal (19 Hz). For eliminating any magnetic field effects showing the even function of field, anomalous Hall voltage was obtained by The magnetization was measured as function of temperature by vibrating sample magnetometry (VSM) in PPMS.

Results and discussion
3.1. Carrier density(n), mobility(μ) and resistivity(ρ xx ) ∼ cm −3 in our experiment). The H incorporations largely decrease (increase) the ρ xx (σ xx ) over the entire temperature region from 5 K to 300 K, indicating that some of the hydrogen should act as the shallow donor. All of the results clearly indicate a negative temperature coefficient of resistivity without any transition to a metallic phase caused by the hydrogen injections. The thermally excited carrier from the localized state to the conduction band exhibits a temperature dependence of ∼ , where E a is the activation energy and k B is Boltzmanns constant. For the temperature region between 150 K and 300 K, the E a values for the H-1, H-2 and H-3 samples were observed to be 5.7, 3.3 and 2.7 meV, respectively. These small activation energies suggest that the H-related donor states are nearly degenerated on the conduction band minimum (CBM) [19]. In the low temperature region, the variable range hopping (VRH) has well described the transport for ZnO and Co-doped ZnO [20][21][22].
We examined the VRH characteristics of samples using the plot of ln ( ) xx ρ versus T T ( / ) 0 1/4 − , where T 0 is a characteristic temperature that is inversely proportional to the density of state (N 0 ) at the Fermi energy (E F ) in Motts conduction theory [23,24]. Thus, the well-fitted linear slope below 50 K allows us to presume the existence of hopping transport in the lowtemperature regime. However, compared with the reported T 0 for ZnO:H, the fitting results indicated a very small T 0 . This result appears to be due to the hybridization between the H-s orbital and the Co-t 2 orbitals. The electronic structures of the H-1, H-2, and H-3 samples are discussed later in the first-principles calculation results. Figures 2(b) and (c) illustrate the hydrogen effects on the n dependences of σ xx and μ obtained through the Hall effect measurements. The analogous n-dependences in the σ xx -n ( figure 2(b)) and μ-n (figure 2(c)) graphs for the H-1 (black squares) and H-2 (green circles) samples are explained by the dominant scattering of electrons with charged interstitial H. However, σ xx and μ of H-3 have different gradients from those of H-1 and H-2; in particular, the μ of the H-3 sample decreases with increasing n. In the H-1 and H-2 samples treated by relatively lower plasma powers, electrons dominantly scatter with the charged interstitial H near the surface, which results in an increase of μ with increasing n due to the Coulomb interaction. However, because the H in the H-3 sample is incorporated into the deeper layer of the ZnCoO thin film due to the higher plasma power, the contributions of a localized state driven by Co-H-Co complexes can be enhanced, which results in the decrease of μ with increasing n. Figure 3 shows that the increase of H drives H-induced localized states due to the formation of Co-H-Co complexes and that the asymmetric scattering between carriers and the states causes the AHE. Generally, in Hall effect measurements for non-magnetic materials, the transverse voltage arises from a deflection charge caused by the Lorentz force effect (ordinary Hall effect) [25]. In this case, the resultant Hall resistivity (ρ xy ) is proportional to the magnetic field. However, in ferromagnetic materials, the ρ xy additionally acquires the anomalous Hall resistivity ( AHE ρ ), which is proportional to the magnetization (M) of the materials. Empirically, ρ xy can be described as the sum of the ordinary and anomalous Hall resistivity,

Anomalous Hall resistivity (ρ AHE (H))
where R o is the ordinary Hall coefficient, R s is the anomalous Hall coefficient, and μ 0 is the vacuum permeability. Typically, the AHE is caused by the asymmetric scattering due to the spin-orbit coupling in the presence of spin-polarization. ) of AHE ρ at 5 K agrees with the reported value of the Al-doped ZnCoO sample, which exhibited an almost identical n as the H-1 sample at 5 K ( 5 10 cm 18 3 ∼ × − ) [26,27]. The ρ s AHE of Al-doped ZnCoO also abruptly decreases with increasing n because of the decrease of spin polarization due to the spin-splitted conduction band [26][27][28][29]. Therefore, the large AHE ρ of the H-1 sample is considered to be due to the spin-split conduction band as well as low n, which is similar to the case of Al-doped ZnCoO. In the H-2 sample ( figure 3(b)), where the interstitial H enhances n, AHE ρ deceases unlike that of the H-1 sample. It should be noted that the AHE ρ of Al-doped ZnCoO abruptly decreases with decreasing (increasing) ρ xx (n) [26,27].
However, H-3 has a clear AHE ρ and non-zero value even at room temperature (figure 3(c)), though its ρ xx was much smaller than those of H-1 and H-2 ( figure 1(a)). Figures 3(d) and (e) show the temperature dependencies of M and the fully saturated AHE ρ (ρ s AHE ) at 20 kOe. In particular, ρ s AHE and M of H-3 show a much similar temperature dependency, which clearly indicates a close link between electromagnetic transport and M. The linear behavior between ρ s AHE and M of H-3 is illustrated in figure 3(f), where R s was determined to be 5.5 10 cm Oe 6 1 Ω ∼ × − − over the entire temperature range. Considering the previous reports about hydrogen mediation [10-15, 17, 18], the AHE ρ of H-3 is thought to originate from the asymmetric scattering of carriers by the Co-H-Co complex with parallel alignment of the Cospin. We recently reported the effect of n on ZnCoO and ZnCoO:H via Al doping [30]; higher n did not contribute to greater ferromagnetism in ZnCoO:H. Furthermore, we have also reported that ferromagnetism can be markedly enhanced while maintaining the n [13]. Based on the relationship between the AHE and M, the Co-H-Co complex is one of strong candidates for main contribution to the change in the AHE. Therefore, it is obvious that the H-induced AHE of H-3 persisting up to 300 K has a different physical origin from that of H-1 observed only at low temperature. The small AHE signal for H-2 can be explained by the incorporation of interstitial H at the surface, which led to less formation of ferromagnetic Co-H-Co complexes and limited M to low values. In addition, the superparamagnetic contributions by ferromagnetic clusters (without a percolation process) cannot induce a large AHE, as observed in figure 3(c) [31].

Anomalous Hall conductivity (σ AHE (H))
Based on most theoretical predictions, the Hall conductivity ( /( ) xx xy 2 2 xy xy σ ρ ρ ρ = + ) is considered to be an essential measurement in investigating the origin of AHE, where the Hall conductivity σ xy consists of OHE σ and AHE σ , and the strength of AHE σ depends on both the contents of the carrier and the spin [7]. Figure 4(a) shows the temperature dependences of σ s AHE for all samples. H-1 only exhibits a small σ s AHE because of its low σ xx . Although the interstitial H slightly enhanced the σ xx value, H-2 still exhibited no remarkable enhancement of σ s AHE . However, H-3 exhibited a remarkable enhancement of σ s AHE , especially at low temperatures, and a non-zero value of σ s AHE persisted up to room temperature. In particular, it should be noted that σ s AHE increased with decreasing temperature, though the σ xx of H-3 clearly decreased with decreasing temperature (the inset of figure 1(a)). These results indicate that the σ s AHE of H-3 is strongly related to the formation of the Co-H-Co complex and its M (the inset figure of figure 4(a)). As observed in figure 4(b), the σ s  If AHE σ has a scaling relation proportional to σ xx , the AHE is ascribed to skew (asymmetric) scattering. If AHE σ is proportional to σ 2 xx , the AHE is ascribed to the side-jump mechanism [32]. Recently, the characteristic scaling relation of AHE σ ∝ xx 1.6 σ has been intensively investigated in many transition-metal-doped oxides as well as in GaAs-based magnetic semiconductors [1,[32][33][34][35]. σ − (red dashed line). Hence, we now see that the origin of the AHE of H-3 is different from that of the other magnetic semiconductors, showing the 1.6 scaling relation because the observed H-induced AHE originates from the asymmetric scattering of carrier hopping between the localized states driven by ferromagnetic Co-H-Co complexes.

Localized state driven by hybridization between the H−1s and Co-t 2 orbitals
We examined the electronic structure of ZnCoO:H using first-principles calculations, which were performed using the projector augmented wave (PAW) method [36] of the Vienna ab initio simulation package (VASP) [37].
The Perdew-Burke-Ernzerhof exchange-correlation functional (PBE) [38] approach utilizing the generalized gradient approximation (GGA) scheme was employed, and the local spin density approximation (LSDA)+U method was used to compensate for the Coulomb interaction in the localized semi-core Zn-3d and Co-3d orbitals [39]. We used U = 5 eV. The state of the capture of H between two nearest Co Zn , which is compared with the state without H capture and the density of electronic states, is shown in figures 5(b) and (a), respectively. A previous study indicated that H can be strongly captured between two Co ions and that the Co- spin can be parallelly aligned by the H capture, which was suggested to lead to the ferromagnetism of ZnCoO [10].
In transition metal-doped ZnO, the competition between the ferromagnetic doubleexchange interaction and the antiferromagnetic superexchange interaction has been considered [40,41]. It has also been reported that the ferromagnetic state is stabilized by electron doping in Co-doped ZnO [40,41]. Electrically, H increases n by acting as a shallow donor. Based on the Zeners double-exchange mechanism, the enhanced n contributes to ferromagnetism. However, in ZnCoO:H, H serves as an additional strong magnetic channel for short-range ferromagnetic spin-spin interactions between Co dimers; this interaction dominates the expected contribution from the double-exchange interaction at room temperature [10]. In ZnCoO:H, the Zeners double-exchange mechanism is expected, although it does not lead to room temperature ferromagnetism [10]. In the ferromagnetic state, which can be mediated by H, the free carriers at the conduction band edge can be partially spin-polarized by the spin-carrier double-exchange interaction.
In figures 5(a) and (b), a remarkable finding is that the localized state driven by the Co-H-Co center is generated at the CBM. The localized state originates from the hybridization between the H-s orbital and the Co-t 2 orbitals, which indicates that the electron carrier mobility can be significantly reduced by the hydrogen contamination and that the electron carrier can be transported by the hopping mechanism between the localized states. The electron carrier is not provided by the Co-H-Co center; however, it can be generated by the interstitial H in the presence of a high concentration of H. These results indicate that the observed H-induced AHE originates from the asymmetric scattering of carrier hopping between the localized states formed by a hybridization between the H-s orbital and Co-t 2 orbitals, which is driven by the Co-H-Co complex persisting ferromagnetic spin ordering up to room temperature.

Conclusion
In summary, we report the H-induced electrical transport characteristics and the unique AHE attributed to a ferromagnetic spin ordering of Co-H-Co complexes in Co-doped ZnO thin films. In H-1 with the low H-injection condition, the interstitial H causes a significant decrease in ρ xx with the decreasing AHE originating from the spin-split conduction band, which is similar to the contributions of other n-type dopants in ZnCoO. However, as the injected H forms ferromagnetic Co-H-Co complexes with a higher H-injection process, the characteristic AHE increases. Furthermore, the H-induced ferromagnetic response of the carrier persists up to room temperature while being proportional to the amplitude of M due to the ferromagnetic complexes. The origin of the AHE of H-induced ferromagnetism is different from that of other magnetic semiconductors because it originates from the asymmetric scattering of carrier hopping between the localized states driven by ferromagnetic Co-H-Co complexes. The theoretical calculation results indicate that the localized state driven by the Co-H-Co center is generated at the CBM, which supports very well the idea that the AHE in a hydrogen-mediated system is attributed to the asymmetric scattering of carrier hopping between the localized states driven by ferromagnetic Co-H-Co complexes.