X-Ray absorption spectroscopy and X-Ray magnetic circular dichroism studies of transition-metal-codoped ZnO Nano-Particles

Abstract

We report on x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) studies of the paramagnetic (Mn,Co)-codoped ZnO [ZnO:(Mn,Co)] and ferromagnetic (Fe,Co)-codoped ZnO[ZnO:(Fe,Co)] 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. In the case of paramagnetic ZnO:(Mn,Co) nano-particles, the doped Mn and Co atoms are in a mixed-valence (2+, 3+, and 4+) state and the relative concentrations of the high-valence (3+ and 4+) Mn and Co ions are higher in the surface region than in the inner core region. In the case of the ferromagnetic ZnO:(Fe,Co) nano-particles, the doped Fe and Co atoms are found to be in a mixed-valence (2+ and 3+) state and the relative concentrations of the Fe3+ and Co3+ ions are higher in the surface region than in the inner core region. The XMCD spectra show that the Fe3+ ions in the surface region mainly give rise to the ferromagnetism while the doped Co 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 nonmagnetic states due to antiferromagnetic coupling. The present result combined with the previous results on transition-metal-doped ZnO nano-particles suggest that doped holes, probably due to Zn vacancy formation at the surfaces of the nano-particles, rather than doped electrons are involved in the occurrence of ferromagnetism in these systems.