Anomalous surface transport, magnetism and doping-induced bulk magnetotransport in the correlated insulator FeSb2

Abstract

This dissertation investigates the magnetotransport and magnetic properties of FeSb$_2$ and its Cr- and Co-substituted derivatives, motivated by recent reports of metallic surface states in FeSb$_2$ and FeSi, surface magnetism in FeSi, and theoretical predictions of unconventional magnetism in doped FeSb$_2$. Building on the background laid out in Chapters 1-4, the experimental work discussed in Chapters 5-7 of this dissertation are separated into three parts: surface magnetotransport properties of FeSb$_2$, surface magnetic properties of FeSb$_2$, and the magnetic and magnetotransport properties of doped FeSb$_2$. In the first part of this work, discussed in Chapter 4, the low-temperature surface magnetotransport of FeSb$_2$ was studied using a Corbino disk geometry to isolate contributions from individual crystal facets. The anisotropic magnetoresistance observed on [110] and [101] surfaces reveals a magnetocrystalline anisotropy arising from local moment scattering, with a well-defined [100] easy axis, and no evidence for a 2D Drude-like response. These results, together with observations of 3D variable-range hopping below the bulk-to-surface crossover, indicate that the surface conduction channel in FeSb$_2$ is a thin but 3D metallic layer, inconsistent with the expectation for a strong topological insulator. In the second part of this work, discussed in Chapter 5, we explored subsequent magnetic susceptibility measurements that reveal that the FeSb$_2$ surface hosts a distinct magnetic contribution following a Curie–Weiss temperature dependence, separable from the bulk spin-gap behavior. The extracted surface magnetic anisotropy aligns closely with that of the surface magnetotransport, and an anomalous Hall contribution to the surface transport further supports the presence of magnetic correlations in the surface transport. The final part of this work, discussed in Chapter 6, examines the magnetic and magnetotransport properties of Cr- and Co-doped FeSb$_2$, motivated by theoretical predictions of incipient unconventional magnetism with anomalous transport properties. In the Cr-doping regime that this unconventional magnetism was predicted in, we find enhanced effective moment associated with the breakdown of the spin-gap present in pure FeSb$_2$, although an absence of an insulator-to-metal transition is required to realize anomalous transport. However, with 1% Te-doping, Cr-doped FeSb$_2$ samples exhibit metallic low-temperature transport which is accompanied by a clear anisotropic anomalous Hall effect. In Co-doped FeSb$_2$, weak ferromagnetism and metallic transport is observed, accompanied by a large anomalous Hall effect and field-induced metamagnetic transitions for field along the b-axis, which hint at an underlying complex magnetic ordering. The magnetotransport and magnetism studied in this thesis have advanced the current understanding of the surface states and bulk incipient magnetism within the correlated insulator FeSb$_2$. In addition to the first report of magnetism at the surface of FeSb$_2$, this work represents a unique demonstration of magnetocrystalline anisotropy at the surface of a non-magnetic bulk insulator. Our work on the magnetotransport of doped FeSb$_2$ shows an interesting interplay between the incipient magnetic and transport properties which has potentially uncovered (1) an anisotropic anomalous Hall effect induced by the bulk insulator-to-metal transition and (2) a field-induced magnetic transition in an ordered phase which was only previously understood to be a simple ferromagnet.