X-ray absorption spectroscopy and X-ray magnetic circular dichroism studies of transition-metal-co-doped ZnO nano-particles

We report on x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) studies of the paramagnetic (Mn,Co)-co-doped ZnO and ferromagnetic (Fe,Co)-co-doped ZnO nano-particles. Both the surface-sensitive total-electron-yield mode and the bulk-sensitive total-fluorescence-yield mode have been employed to extract the valence and spin states of the surface and inner core regions of the nano-particles. XAS spectra reveal that significant part of the doped Mn and Co atoms are found in the trivalent and tetravalent state in particular in the surface region while majority of Fe atoms are found in the trivalent state both in the inner core region and surface region. The XMCD spectra show that the Fe$^{3+}$ ions in the surface region give rise to the ferromagnetism while both the Co and Mn ions in the surface region show only paramagnetic behaviors. The transition-metal atoms in the inner core region do not show magnetic signals, meaning that they are antiferromagnetically coupled. The present result combined with the previous results on transition-metal-doped ZnO nano-particles and nano-wires suggest that doped holes, probably due to Zn vacancy formation at the surfaces of the nano-particles and nano-wires, rather than doped electrons are involved in the occurrence of ferromagnetism in these systems.


Introduction
Various semiconducting oxides such as ZnO [1], TiO 2 [2], and SnO 2 [3] in thin film and nano-particle forms are known to exhibit ferromagnetism at room temperature when they are doped with transition-metal atoms. Current interest in such magnetic nano-particle systems is motivated by unique electronic structures and magnetism at the surfaces of the nano-particles which are different from the inner core region. In the nano-particle form, the structural and electronic properties are modified by surface defects such as Zn and O vacancies with broken chemical bonds and charge imbalance, which may mediate or modify exchange coupling between the doped atoms [4]. For example, in the case of (Mn,Co)-co-doped ZnO [ZnO:(Mn,Co)] nanoparticles [5], high-valence (3+ and 4+) Mn and Co ions are found to be present, probably due to the formation of Zn vacancies (V Zn ) in the surface region. The doped Fe atoms in the ferromagnetic ZnO nano-particles are converted from 2+ to 3+ due to hole doping in the surface regions [4,6,14], resulting in the ferromagnetic interaction between the doped Fe atoms. In the case of Co-doped ZnO systems such as (Co,Ga)-co-doped ZnO [7] and Co-doped ZnO nano-particles [8], on the other hand, oxygen vacancies (V O ), which induce electron doping, are reported to be necessary for ferromagnetism. Recently, room-temperature ferromagnetism was reported for (Fe,Co)-co-doped ZnO [ZnO:(Fe,Co)] in thin film [9] and nano-particle forms [10]. From the first-principle calculations, Karmakar et al. [10] have indicated that V Zn -mediated double exchange interaction plays important role for ferromagnetism in ZnO:(Fe,Co) nano-particles. Indeed, enhance-4 ment of ferromagnetic interaction between transition-metal atoms has been demonstrated in previous first-principles calculations by Gopal and Spaldin [11]. First-principle calculations by Park and Min [12], on the other hand, have suggested the importance of RKKY-type exchange interaction mediated by conduction carriers induced by V O as the origin of ferromagnetism of ZnO:(Fe,Co). Also, calculations by Ghosh et al. [13] have indicated direct exchange interaction mediated by the doped electron carriers at the Fe-V O -Co defect configuration in the surface region of ZnO:(Fe,Co) nano-wires.
Thus, it has been controversial whether the enhancement of exchange interaction comes from electron doping or hole doping. In this paper, we report on x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) studies of paramagnetic ZnO:(Mn,Co) and ferromagnetic ZnO:(Fe,Co) nano-particles. The valence and spin states of the doped ions and their magnetic interaction have been revealed by XAS and XCMD measurements of the transition-metal core levels. Also, both the surfacesensitive total-electron-yield mode and the bulk-sensitive total-fluorescenceyield mode have been employed to extract the valence and spin states of the surface and inner core regions of the nano-particles separately. The experimental results indicate that doped holes rather than doped electrons are involved in the occurrence of ferromagnetism in these systems.
We have made pellets from calcined powders and then sintered them at a temperature of ∼ 570 K for 30 min. The average size of the nano-particles were 7-10 nm [10,14]. ization of x-rays was more than ∼ 90%. All the measurements were performed at room temperature.
Absorption spectra were analyzed using configuration-interaction (CI) cluster-model calculations. The cluster consisted of a transition-metal ion octahedrally and/or tetrahedarally coordinated by O 2− ions. The ground state wave function was expanded in the ψ= α|d n + β|d n+1 L + γ|d n+2 L 2 , where L denotes an ligand O 2p hole. The adjustable parameters of the calculation were the charge-transfer energy ∆, the d-d Coulomb energy U, the p-d transfer integral T , and the crystal field splitting parameters 10Dq. We 6 assumed high-spin states for the calculations, and 10Dq was assumed to be less than 1.0 eV.

Conclusion
In summary, we have investigated the electronic structure and magnetism of the paramagnetic (Mn,Co)-co-doped ZnO and ferromagnetic (Fe,Co)-codoped ZnO nano-particles using 2p→3d XAS and XMCD. In the case of ZnO:(Mn,Co) nano-particles, the doped Mn and Co atoms are in a mixedvalence (2+, 3+, and 4+) state and the relative concentrations of the highvalence (3+ and 4+) Mn and Co ions are higher in the surface region than in the deep core region. Mn and Co 2p→3d XMCD results suggest that the paramagnetism comes from the Co 2+ , Mn 3+ and Mn 4+ states. In the case of the ZnO:(Fe,Co) nano-particles, too, the doped Fe and Co atoms are found to be in a mixed-valence (2+ and 3+) state and the relative concentrations of the Fe 3+ and Co 3+ ions are higher in the surface region than in the inner core region. Fe and Co 2p→3d XMCD signals due to the ferromagnetic Fe ions and paramagnetic Fe and Co ions were observed in the surface region while no appreciable XMCD signals were observed in the inner core region.
From these results, we suggest that the surface region is magnetically active and Fe 3+ contributes to both the ferromagnetism and paramagnetism, and that Fe 2+ contributes only to the ferromagnetism. On the other hand, the ionic Co atoms in the surface region is paramagnetic and that the ferromagnetic component of the Co ions is negligibly small. Considering that the Fe 3+ ions are created due to Zn vacancies, we conclude that the ferromagnetism of ZnO:(Fe,Co) nano-particles comes from the hole-mediated exchange interaction between Fe 3+ (O h ) and Fe 2+ (T d ) in the surface region.

Acknowledgments
The experiment at PF was approved by the Photon Factory Program Advi-