Observation of ferromagnetism above 900 K in Cr-GaN and Cr-AlN

TL;DR

This study demonstrates ferromagnetism above 900 K in Cr-GaN and Cr-AlN thin films, linking magnetic behavior to hopping conduction and double exchange mechanisms involving Cr impurity bands.

Contribution

It provides the first high-temperature ferromagnetism observations in Cr-GaN and Cr-AlN, and elucidates the conduction and magnetic mechanisms involved.

Findings

Ferromagnetism observed above 900 K in Cr-GaN and Cr-AlN.

Hopping conduction dominates in Cr-GaN, with characteristic mobility and carrier density.

Ferromagnetism attributed to double exchange via Cr impurity bands.

Abstract

We report the observation of ferromagnetism at over 900K in Cr-GaN and Cr-AlN thin films. The saturation magnetization moments in our best films of Cr-GaN and Cr-AlN at low temperatures are 0.42 and 0.6 u_B/Cr atom, respectively, indicating that 14% and 20%, of the Cr atoms, respectively, are magnetically active. While Cr-AlN is highly resistive, Cr-GaN exhibits thermally activated conduction that follows the exponential law expected for variable range hopping between localized states. Hall measurements on a Cr-GaN sample indicate a mobility of 0.06 cm^2/V.s, which falls in the range characteristic of hopping conduction, and a free carrier density (1.4E20/cm^3), which is similar in magnitude to the measured magnetically-active Cr concentration (4.9E19/cm^3). A large negative magnetoresistance is attributed to scattering from loose spins associated with non-ferromagnetic impurities. The results indicate that ferromagnetism in Cr-GaN and Cr-AlN can be attributed to the double exchange mechanism as a result of hopping between near-midgap substitutional Cr impurity bands.