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

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 nearzero 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 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.

On the ferromagnetic ground state of SmN

“…Another study by Morari et al [26] used a variety of LSDA+U implementations and parameters to find even larger values for the net moment, though these were also spin-dominant. The ground state was furthermore antiferromagnetic, contradicting recent experiments unambiguously displaying ferromagnetism [5,17]. Other band structure calculations on EuN [7] and TmN [27] suggest that dynamical meanfield theory in the Hubbard-I approximation provided better agreement with experiment than LSDA+U , suggesting alternative techniques should be explored.…”

“…To a good approximation, the integral over the Sm L 3 EQ spectrum scales linearly with S z and L z , which can be seen from the XMCD sum rules derived for EQ transitions [46]. As discussed in Refs [15,17], the sign change between spectra indicates the magnetization of the thin SmN layers are forced into alignment with the GdN by strong interface-exchange in the superlattice. In addition to the sign change, the ED signal is enhanced by a factor of 3.1 in the superlattice, while the EQ feature is only enhanced by a factor of 1.6.…”

“…To date there has been no convincing description of SmN which accounts for both the magnitude and orbitaldominant sign of the ordered moment. Furthermore, XMCD experiments on bulk SmN and a SmN/GdN superlattice show a 60% enhancement in the magnitude of the Sm 4f polarization in the superlattice compared to bulk SmN, caused by interface exchange with highly spinpolarized GdN [15,17]. This implies that the bulk Sm 4f spin (m 4f S ) and orbital (m 4f L ) moments are not fully polarized (= 5 µ B ), but are significantly reduced from full alignment, consistent with the 4f electrons being dominated by spin-orbit coupling and crystal field energies.…”

“…There is a report of an unconventional twisted magnetization phase occurring in SmN films exchange coupled to GdN, observed via x-ray magnetic circular dichroism (XMCD) [15], and the discovery of superconductivity coexisting with ferromagnetic order at temperatures of 3-5 K in SmN films and SmN/GdN superlattices, thought to be due to a spin-triplet pairing mechanism [16]. These phenomena are intimately related to the ferromagnetic state of SmN, which displays ferromagnetic order below T C ≈ 27 K with a near-zero moment of 0.035 µ B per Sm 3+ , with the unusual situation that the orbital moment is marginally dominant, and defines the net magnetization direction [4,5,17].…”

Surface- and pressure-induced bulk Kondo breakdown in SmB6

“…4(b), recorded at 40 K, P = 0.6 GPa, and μ 0 H = 1.3 T. Two well defined peaks are clearly present in both XAS and XMCD spectra. The quadrupolar contribution as a result of 4f/5d hybridization is strong in Sm L 3 edge but highly reduced in L 2 edge XMCD spectra in SmN [48]. Thus these spectral features with separated peaks from Sm 2+ and Sm…”