Spin/orbit moment imbalance in the near-zero moment ferromagnetic semiconductor SmN

TL;DR

This study investigates the near-zero net magnetic moment in SmN, revealing the spin and orbital moment balance and how Gd substitution influences the magnetic alignment and magnitude.

Contribution

It provides experimental and theoretical insights into the spin and orbital moment alignment in SmN and how Gd doping affects its magnetic properties.

Findings

SmN has a nearly canceled spin and orbital magnetic moments.

The spin moment aligns antiparallel to the magnetic field.

Gd substitution reverses the moment alignment due to exchange interactions.

Abstract

SmN is ferromagnetic below 27 K, and its net magnetic moment of 0.03 Bohr magnetons per formula unit is one of the smallest magnetisations found in any ferromagnetic material. The near-zero moment is a result of the nearly equal and opposing spin and orbital moments in the 6H5/2 ground state of the Sm3+ ion, which leads finally to a nearly complete cancellation for an ion in the SmN ferromagnetic state. Here we explore the spin alignment in this compound with X-ray magnetic circular dichroism at the Sm L2,3 edges. The spectral shapes are in qualitative agreement with computed spectra based on an LSDA+U (local spin density approximation with Hubbard-U corrections) band structure, though there remain differences in detail which we associate with the anomalous branching ratio in rare-earth L edges. The sign of the spectra determine that in a magnetic field the Sm 4f spin moment aligns antiparallel to the field; the very small residual moment in ferromagnetic SmN aligns with the 4f orbital moment and antiparallel to the spin moment. Further measurements on very thin (1.5 nm) SmN layers embedded in GdN show the opposite alignment due to a strong Gd-Sm exchange, suggesting that the SmN moment might be further reduced by about 0.5 % Gd substitution.