Magnetic properties of the topological Kondo insulator SmB$_6$: localized magnetic moments and Pauli paramagnetism

The magnetic properties of the topological Kondo insulator SmB$_6$ are studied in the temperature range $T < 10$ K in an applied magnetic field up to $5$ T. The analysis of the nonlinear part of the field dependence of the magnetization indicates that the nonlinear contribution to the magnetization at low temperatures is in general related not only to localized magnetic moments but also to the Pauli paramagnetism of surface electrons. It has been shown for the first time that a decrease in temperature in the range of $T^*\sim 5.5-6$ K gives rise to the crossover between two modes determining the magnetic properties of SmB$_6$. In the range $T > T^*$, the magnetization qualitatively corresponds to the model of decay of Kondo singlets and is determined by the Pauli paramagnetism, which includes both the linear and nonlinear contributions. In the range $T < T^*$, the Pauli paramagnetism becomes supplemented in the threshold manner by the contribution of localized magnetic moments. The performed estimate of effective magnetic moments arising at $T < T^*$ gives anomalously high values $\mu^*\sim7{-}14\,\mu_{\text{B}}$ ion, far exceeding the values $\mu^*\sim3{-}5\,\mu_{\text{B}}$ expected for a separate magnetic Sm$^{3+}$ ion. This is a signature of the possible spin-polaron nature of localized magnetic moments in the topological Kondo insulator SmB$_6$.