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

SmN is ferromagnetic below 27 K, and its net magnetic moment of 0.03 Bohr magnetons per formula unit is one of the smallest magnetizations found in any ferromagnetic material. The near-zero moment is a result of the nearly equal and opposing spin and orbital moments in the super(6) H sub(5/2) ground state of the Sm super(3+) 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 L sub(2,3) 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 determines that in a magnetic field the Sm 4[functionof] spin moment aligns antiparallel to the field; the very small residual moment in ferromagnetic SmN aligns with the 4[functionof] 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.