Professor Thorsten Hesjedal FInstP

Abstract:

Magneto-optical Kerr effect (MOKE) microscopy is a surface-sensitive probe of magnetisation with micron-sized lateral resolution. Here, we present a low-temperature, focused polar MOKE microscope for the simultaneous magnetooptical and magneto-transport measurements, which has a temperature range of 1.6-300 K and is equipped with a magnet capable of delivering a field of up to 9 T. In this microscope, all optical components are integrated in a free-standing probe, allowing for the straightforward incorporation into many non-optical cryostat systems. Two-dimensional magnetisation scans on patterned ferromagnetic [CoFeB/Pt]n films demonstrate a magnetisation sensitivity of 10 µrad (Kerr angle) and a spatial resolution of 2.2 µm. The combination of optical and electrical measurements provides complementary temperature-dependent information, as demonstrated by the study of magnetic topological insulator thin films with out-of-plane magnetic anisotropy. Using this complementary approach, we study the effects of a secondary phase in Cr and V co-doped Sb2Te3 thin films, which show a combination of weak antilocalization and anisotropic magnetoresistance effects above 70 K. Our results highlight the virtue of MOKE and electrical transport to optimise exotic topological magnetic materials, paving the way for energy-efficient spintronic devices.

Abstract:

Magnetic topological insulators (TIs) are an ideal playground for the study of novel quantum phenomena building on time-reversal symmetry broken topological surface states. By combining different magnetic TIs in a heterostructure, their magnetic and electronic properties can be precisely tuned. Recently, we have combined high-moment Dy:Bi₂Te₃with high transition temperature Cr:Sb₂Te₃ in a superlattice, and found, using x-ray magnetic circular dichroism (XMCD), that long-range magnetic order can be introduced in the Dy:Bi₂Te₃ layers. Accompanying first-principles calculations indicated that the origin of the long-range magnetic order is a strong antiferromagnetic coupling between Dy and Cr magnetic moments at the interface extending over several layers. However, based on XMCD alone, which is either averaging over the entire thin film stack or is surface sensitive, this coupling scenario could not be fully confirmed. Here we use polarized neutron reflectometry (PNR), which is ideally suited for the detailed study of superlattices, to retrieve the magnetization in a layer- and interface-resolved way. We find that the magnetization is, in contrast to similar recent studies, homogeneous throughout the individual layers, with no apparent interfacial effects. This finding demonstrates that heterostructure engineering is a powerful way of controlling the magnetic properties of entire layers, with the effects of coupling reaching beyond the interface region.