Near-zero-moment ferromagnetism in the semiconductor SmN

Abstract

The magnetic behavior of SmN has been investigated in stoichiometric polycrystalline films. All samples show ferromagnetic order with Curie temperature $({T}_{C})$ of $27\ifmmode\pm\else\textpm\fi{}3\text{ }\text{K}$, evidenced by the occurrence of hysteresis below ${T}_{C}$. The ferromagnetic state is characterized by a very small moment and a large coercive field, exceeding even the maximum applied field of 6 T below about 15 K. The residual magnetization at 2 K, measured after cooling in the maximum field, is $0.035{\ensuremath{\mu}}_{B}$ per Sm. Such a remarkably small moment results from a near cancellation of the spin and orbital contributions for ${\text{Sm}}^{+3}$ in SmN. Coupling to an applied field is therefore weak, explaining the huge coercive field. The susceptibility in the paramagnetic phase shows temperature-independent Van Vleck and Curie-Weiss contributions. The Van Vleck contribution is in quantitative agreement with the field-induced admixture of the $J=\frac{7}{2}$ excited state and the $\frac{5}{2}$ ground state. The Curie-Weiss contribution returns a Curie temperature that agrees with the onset of ferromagnetic hysteresis, and a conventional paramagnetic moment with an effective moment of $0.4{\ensuremath{\mu}}_{B}$ per Sm ion, in agreement with expectations for the crystal-field modified effective moment on the ${\text{Sm}}^{+3}$ ions.