Possible surface magnetism in the topological Kondo insulator candidate FeSi

Electrical transport properties of FeSi single crystals, a candidate for a d-electron topological Kondo insulator (TKI), were studied in the phase space of temperature (T), magnetic field (B), and angle (θ) between the electrical current and B. The normalized T-dependent electrical resistance (R) of a successively thinned FeSi single crystal revealed unambiguous evidence for a conducting surface state (CSS) at low T, consistent with previously reported evidence for a CSS based on other measurements [1]. In the CSS, the magnetoresistance (MR) exhibits a hysteresis loop bounded within ± 0.5 T, suggesting two-dimensional magnetic ordering. The hysteretic MR is asymmetric and has an anisotropic character with respect to θ. Further exploration of R(θ) at a fixed field of 9 T reveals a progressive rotation of two-fold rotational symmetry from 2 K to 10 K, then a stabilized two-fold symmetry until at T > 40 K, the anisotropy vanishes, coincident with the disappearance of the CSS. These observations point to a possible magnetically ordered surface state that has been reported in similar systems such as FeSi nanofilms [2] and bulk SmB₆ [3].

[1] Y. Fang et al., Proc. Natl. Acad. Sci. U.S.A. 115, 8558 (2018).

[2] Y. Ohtsuka et al., Sci. Adv. 7, eabj0498 (2021).

[3] Y. Nakajima et al., Nat. Phys. 12, 213 (2016).