Effect of Co and O defects on the magnetism in Co-doped ZnO: Experiment and theory

Abstract

The electronic structure of ${\mathrm{Zn}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}\mathrm{O}$ ($x=0.02$, 0.06, and 0.10) diluted magnetic semiconductors is investigated using soft x-ray emission spectroscopy and first-principles calculations. X-ray absorption and emission measurements reveal that most Co dopants are incorporated at the Zn sites and that free charge carriers are absent over a wide range of Co concentrations. The excess Co interstitials appear in the samples with high Co concentration ($10\phantom{\rule{0.3em}{0ex}}\mathrm{at.}\phantom{\rule{0.2em}{0ex}}%$) but are isolated without any direct exchange interaction with substitutional Co atoms. The lack of free charge carriers and the direct Co-Co interactions is responsible for the absence of ferromagnetism in the samples. First-principles calculations suggest that the exchange interaction between substitutional Co atoms induces only an antiferromagnetic coupling, and strong ferromagnetism in Co-doped ZnO requires not only free charge carriers but also the Co interstitials directly interacting with substitutional Co atoms.