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

TL;DR

This study uses X-ray absorption and magnetic circular dichroism techniques to analyze the valence, spin states, and magnetic behaviors of transition-metal dopants in ZnO nanoparticles, revealing surface-specific magnetic properties and the role of doped holes in ferromagnetism.

Contribution

It provides detailed surface and core magnetic state analysis of transition-metal doped ZnO nanoparticles using XAS and XMCD, highlighting the surface-specific ferromagnetism and the role of doped holes.

Findings

Fe$^{3+}$ ions induce ferromagnetism at the surface.

Co and Mn ions are paramagnetic at the surface.

Inner core regions show antiferromagnetic coupling.

Abstract

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.