Anisotropic magnetism at the surface of a non-magnetic bulk insulator

Abstract

The potential for topological Kondo insulating behavior in d-electron systems has attracted interest in studying the surface states of the correlated insulators FeSb2 and FeSi. While detailed studies and theoretical description of a spin-orbit coupled ferromagnetic surface state have been applied to FeSi, the magnetic properties of the surface states of FeSb2 have not been addressed. Here, we report on the surface magnetic properties of FeSb2, utilizing the surface area dependence of magnetic susceptibility to separate the surface Curie-Weiss temperature dependence from the bulk spin-gap susceptibility. We use these results to further extract the surface magnetic anisotropy of a thin, rough-surfaced single-crystal FeSb2 to compare with the observed magnetotransport anisotropy, and find good agreement between the anisotropy in the surface magnetization and surface magnetotransport. We conclude with evidence of an anomalous Hall contribution to the low-temperature surface transport.

Figures (4)

• Figure 1: Magnetization scaling of the powder magnetic susceptibility with surface area (i.e. powder grain size). (a) Magnetization field dependence scaling with each powder step labeled by average diameter. (b) histograms of powder grain size for each powder grind step as calculated from powder images. (c) $B$ = 5 T, $T$ = 2 K magnetization for each average grain size. (d) $M$ vs $T$ at $B$ = 10 mT for the most coarse ($\langle d\rangle=325\mu m$) and most fine powders with inset for the finest ($\langle d\rangle=2.5\mu m$) showing a bifurcation between zero field cooled (ZFC) and field cooled (FC) data.
• Figure 2: $M/B$ vs $T$ and magnetization with linear field dependence subtracted for clarity of hysteresis, $\Delta M$ vs $B$, is shown for fine FeSb$_2$ powder. These results show (a) field dependence of ZFC vs FC bifurcation and (b) temperature dependence of magnetic hysteresis.
• Figure 3: Anisotropy of surface magnetization compared with surface magnetotransport. (a) magnetic susceptibility of a thin, primarily (001) surface, rough polished single crystal for field along $a$ and $b$-axes. Dashed line represents a fit, which is a linear combination of bulk spin-gap susceptibility and surface Curie-Weiss susceptibility. (b) Magnetoresistance ($MR=100\%*(R(B)-R(0))/R(0)$) of a thin, primarily (001) surface sample with current along $c$-axis and magnetic field along $a$ (red) and $b$ (blue) axes. Dashed line represents fit to linear combination of hopping conduction (positive MR) and magnetic scattering (negative MR).
• Figure 4: Hall effect showing some small magnetic correlations in the surface dominated transport regime. (a) resistance vs temperature of primarily (101) surface with current along $b$-axis. (a, inset) inverse temperature dependence of resistance showing the crossover from primarily bulk electrical conduction around 10 K to primarily surface conduction below 5 K. (b) Hall effect in the primarily surface conduction regime. Dashed line represents linear combination of single channel Hall and anomalous Hall effect component.