Susan Kempinger

Giant anisotropic magnetoresistance in a quantum anomalous Hall insulator

When a three-dimensional ferromagnetic topological insulator thin film is magnetized out-of-plane, conduction ideally occurs through dissipationless, one-dimensional (1D) chiral states that are characterized by a quantized, zero-field Hall conductance. The recent realization of this phenomenon, the quantum anomalous Hall effect, provides a conceptually new platform for studies of 1D transport, distinct from the traditionally studied quantum Hall effects that arise from Landau level formation. An important question arises in this context: how do these 1D edge states evolve as the magnetization is changed from out-of-plane to in-plane? We examine this question by studying the field-tilt-driven crossover from predominantly edge-state transport to diffusive transport in Crx(Bi,Sb)2−xTe3 thin films. This crossover manifests itself in a giant, electrically tunable anisotropic magnetoresistance that we explain by employing a Landauer–Büttiker formalism. Our methodology provides a powerful means of quantifying dissipative effects in temperature and chemical potential regimes far from perfect quantization.

Visualization of superparamagnetic dynamics in magnetic topological insulators.

The ferromagnetic state of topological insulators showing quantum anomalous Hall effect is surprisingly superparamagnetic. Quantized Hall conductance is a generic feature of two-dimensional electronic systems with broken time reversal symmetry. In the quantum anomalous Hall state recently discovered in magnetic topological insulators, time reversal symmetry is believed to be broken by long-range ferromagnetic order, with quantized resistance observed even at zero external magnetic field. We use scanning nanoSQUID (nano–superconducting quantum interference device) magnetic imaging to provide a direct visualization of the dynamics of the quantum phase transition between the two anomalous Hall plateaus in a Cr-doped (Bi,Sb)2Te3 thin film. Contrary to naive expectations based on macroscopic magnetometry, our measurements reveal a superparamagnetic state formed by weakly interacting magnetic domains with a characteristic size of a few tens of nanometers. The magnetic phase transition occurs through random reversals of these local moments, which drive the electronic Hall plateau transition. Surprisingly, we find that the electronic system can, in turn, drive the dynamics of the magnetic system, revealing a subtle interplay between the two coupled quantum phase transitions.

Characterization of switching field distributions in Ising-like magnetic arrays

The switching field distribution within arrays of single-domain ferromagnetic islands incorpo- rates both island-island interactions and quenched disorder in island geometry. Separating these two contributions is important for disentangling the effects of disorder and interactions in the magnetization dynamics of island arrays. Using sub-micron, spatially resolved Kerr imaging in an external magnetic field for islands with perpendicular magnetic anisotropy, we map out the evolution of island arrays during hysteresis loops. Resolving and tracking individual islands across four different lattice types and a range of inter-island spacings, we extract the individual switching fields of every island and thereby determine the relative contributions of interactions and quenched disorder in the arrays. The width of the switching field distribution is well explained by a simple model comprising the sum of an array-independent contribution (interpreted as disorder-induced), and a term proportional to the maximum field the fully polarized array could exert on a single island. We conclude that disorder in these arrays is primarily a single-island property.