Magnetic order of tetragonal CuO ultra-thin films

TL;DR

This study reveals that ultra-thin tetragonal CuO films exhibit robust antiferromagnetic order above 360K, with complex magnetic behavior involving pinned and paramagnetic moments, differing from bulk predictions.

Contribution

It provides the first detailed experimental investigation of magnetic order in ultra-thin tetragonal CuO films, highlighting complexities not captured by existing theories.

Findings

Antiferromagnetic order persists above 360K.

Pinned Cu moments have a large orbital magnetic contribution.

Magnetic properties differ from bulk CuO, indicating complex behavior.

Abstract

We present a detailed low-energy muon spin rotation and x-ray magnetic circular dichroism (XMCD) investigation of the magnetic structure in ultra-thin tetragonal (T)-CuO films. The measured muon-spin polarization decay indicates an antiferromagnetic (AFM) order with a transition temperature higher than 200K. The XMCD signal obtained around the Cu $L_{2,3}$ edges indicates the presence of pinned Cu$^{2+}$ moments that are parallel to the sample surface, and additionally, isotropic paramagnetic moments. The pinning of some of the Cu moments is caused by an AFM ordering consisting of moments that lie most likely in the plane of the film. Moreover, pinned moments show a larger orbital magnetic moment contribution with an approximate ratio of $m_{orb}/m_{spin} = 2$, indicating that these spins are located at sites with reduced symmetry. Some fractions of the pinned moments remain pinned from an AFM background even at 360K, indicating that $T_N >$ 360K. A simple model could explain qualitatively these experimental findings; however, it is in contrast to theoretical predictions, showing that the magnetic properties of ultra-thin T-CuO films differ from bulk expectations and is more complex.