Systematic changes of the electronic structure of the diluted ferromagnetic oxide Li-doped Ni$_{1-x}$Fe$_x$O with hole doping

TL;DR

This study investigates how Li doping affects the electronic structure of Ni$_{1-x}$Fe$_x$O, revealing changes in Fe oxidation states and increased conductivity, and proposes bound magnetic polarons as the source of ferromagnetism.

Contribution

It provides detailed spectroscopic analysis of Li-doped Ni-Fe oxide and suggests a new mechanism for ferromagnetism involving bound magnetic polarons.

Findings

Fe$^{3+}$ increases with Li doping but only slightly

Doped holes mainly enter O 2p and Ni 3d$^8$L states

Electrical conductivity increases with Li content

Abstract

The electronic structure of Li-doped Ni$_{1-x}$Fe$_x$O has been investigated using photoemission spectroscopy (PES) and x-ray absorption spectroscopy (XAS). The Ni $2p$ core-level PES and XAS spectra were not changed by Li doping. In contrast, the Fe$^{3+}$ intensity increased with Li doping relative to the Fe$^{2+}$ intensity. However, the increase of Fe$^{3+}$ is only $\sim 5%$ of the doped Li content, suggesting that most of the doped holes enter the O $2p$ and/or the charge-transferred configuration Ni $3d^8\underline{L}$. The Fe 3d partial density of states and the host valence-band emission near valence-band maximum increased with Li content, consistent with the increase of electrical conductivity. Based on these findings, percolation of bound magnetic polarons is proposed as an origin of the ferromagnetic behavior.