# An “s‐Electron” Donor Band Driven Metallic Ferromagnetism in Co‐Doped ZnO Films

## TL;DR

This paper investigates how cobalt-doped zinc oxide films achieve room-temperature ferromagnetism by identifying an s-character electron donor band.

## Contribution

The study experimentally identifies an s-character donor band in Co-doped ZnO, offering new insight into the mechanism of room-temperature ferromagnetism.

## Key findings

- A weak electron donor band with s-character was detected crossing the Fermi level in Co-doped ZnO films.
- The donor band is linked to Zn1+ 4s states mediating ferromagnetism and metallic-like transport.
- The findings suggest a complementary role of dopant-host electronic states in ferromagnetic behavior.

## Abstract

Achieving room‐temperature ferromagnetism (RTFM) in diluted magnetic semiconductors (DMSs) has been a long‐standing challenge, with doping transition metals (TM) into oxide semiconductors being one of the most common approaches. However, the underlying physical mechanisms remain poorly understood, particularly for Co‐doped ZnO (Co:ZnO) films, which exhibit high Curie temperatures (Tc) above 300 K. A promising mechanism proposed for high‐Tc ferromagnetism is the donor impurity band exchange model, in which donor electrons mediate the coupling between TM spins. Despite its theoretical significance, the nature of the donor band electrons has yet to be experimentally identified. In this work, we use polarization‐dependent, bulk‐sensitive hard x‐ray photoemission spectroscopy (HAXPES) to investigate Co‐doped ZnO epitaxial films. Our results reveal the presence of a weak electron donor band, crossing the Fermi level, and from a polarization dependence analysis, it is unambiguously identify it as having “s‐character.” This finding offers new insight into the ferromagnetic mechanism in Co‐doped ZnO, where Zn1+4s
1 states mediate the ferromagnetism, contributing to metallic‐like transport and Co2+ spin ordering. These results not only elucidate the complementary role of dopant‐host electronic states but also open avenues for designing novel room‐temperature magnetic semiconductors, particularly in the context of 2D DMSs.

Hard X‐ray photoemission spectroscopy with polarization dependence enables orbital‐selective probing of buried electronic states. By tuning the incident light's polarization vector, symmetry‐resolved spectral features of core and valence states are revealed, offering deeper insights into orbital hybridization and spin–orbit interactions in functional oxide systems.