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 $\left(T_C\right)$ of $27\pm 3\text{K}$, 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 $0.035{\mu }_B$ 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 $0.4{\mu }_B$ per Sm ion, in agreement with expectations for the crystal-field modified effective moment on the ${\text{Sm}}^{+3}$ ions.

Figure 1

Temperature dependence of the magnetization of SmN after cooling without (ZFC) and with (FC) an applied field of 0.5 T (sample I). The inset shows the result of the fit of the FC susceptibility $\left(\chi =M/H\right)$ in a Van Vleck approach.

Figure 2

(Color online) Magnetization loops of SmN (sample II) after cooling the film in zero-field (red full circles) and after cooling under 6 T (open black squares). Left: correction for the Si substrate only at (a) 15, (b) 10, and (c) 2 K. Right: additional correction for the paramagnetic signal of the GaN cap layer according to the curve shown in Fig. 3 at (d) 15, (e) 10, and (f) 2 K.

Figure 3

(Color online) Susceptibility $\left(\chi =M/H\right)$ versus temperature (sample II), before (full black circles) and after (open red circles) the correction for the GaN cap layer. The GaN signal used for the correction is shown in small blue dots. The full black line is the Van Vleck fit of the susceptibility to the fully corrected data, setting the multiplet separation $\Delta E=1500\text{K}$.