Near-zero-moment ferromagnetism in the semiconductor SmN

The magnetic behavior of SmN has been investigated in stoichiometric polycrystalline films. All samples show ferromagnetic order with Curie temperature (T_C) of 27±3 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 0.035μ_B per Sm. Such a remarkably small moment results from a near cancellation of the spin and orbital contributions for 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=7/2 excited state and the 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 0.4μ_B per Sm ion, in agreement with expectations for the crystal-field modified effective moment on the Sm⁺³ ions.