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 ${}^6$H${}_{5/2}$ ground state of the Sm${}^{3+}$ 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${}_{2,3}$ 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$f$ spin moment aligns antiparallel to the field; the very small residual moment in ferromagnetic SmN aligns with the 4$f$ 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.

Figure 1

Measured (symbols) and calculated (lines) XAS and XMCD spectra at (a) the L${}_3$ and (b) the L${}_2$ edges at 15 K in a field of 6 T. XMCD data are quoted relative to the amplitude of the first prominent (white) peak in XAS. The calculated XMCD spectra have been sign reversed and in the case of the L${}_3$ spectrum scaled by a factor 1/3.

Figure 2

Temperature dependence of (a) the L${}_3$ and (b) the L${}_2$ edge XMCD spectra in the ferromagnetic (15, 25 K) and paramagnetic (50 K) phases. The magnetic field was 6 T.

Figure 3

Field dependence of L${}_2$ edge XMCD amplitude at 15 K showing wide hysteresis loop.

Figure 4

SmL${}_3$ edge XMCD in 1.5 nm SmN layers embedded in GdN at 15 K.