A low-temperature Kerr effect microscope for the simultaneous magneto-optic and magneto-transport study of magnetic topological insulators

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 Sb 2 Te 3 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.

Magnetic Topological Insulator Heterostructures: A Review

“…We found that V works as a surfactant which undesirably prevents the substitutional doping of Cr into Sb sites, leading to the formation of the secondary phase Cr 2 Te 3 . [145,179] Apart from co-doping, the other highlighted approach to QAH insulators is modulation doping in heterostructures, [30] that is, periodically inserting a doped Cr:BST layer during the growth of non-doped BST films, a technique that had been applied to GaAs/Al x Ga 1−x . As semiconductor heterostructures to achieve a high carrier mobility.…”

“…In this example, SQUID-on-tip microscopy images show the out-of-plane component of the magnetic stray field of a Cr:BST sample at 250 mK, in an opposite applied magnetic field of ≈300 mT, that is, the reversal of the magnetic domains is visible. c) MOKE is a surface-sensitive technique with a lateral resolution of ≈μm, which can be easily integrated within a cryostat [145]. d) XMCD and XMLD, which are carried out at synchrotrons, rely on the magnetic field or polarization-dependent XAS spectra and allow for the element-selective probing of the different forms of magnetic order.…”

“…f) Electrical transport that probes the AHE is the key technique for studying MTIs, giving a magnetic response integrated over the commonly used, micrometersized Hall bars. AHE and MOKE measurements can be combined in a single system [145]. b) Adapted with permission [24].…”

Controlling the Transverse Magneto-Optical Kerr Effect in Cr/NiFe Bilayer Thin Films by Changing the Thicknesses of the Cr Layer

“…Such behavior is caused mainly by the interferences between the layers, as the optical properties of thin Cr layers (below 20 nm) are found to be typical of dielectric materials. This could be of interest for the development of optical microscopy [23] and sensing [24,25].…”

Exchange Bias in Magnetic Topological Insulator Superlattices

“…To further explore the temperature dependence of the exchange bias in the superlattice, we conducted polar MOKE experiments using the geometry shown in Figure 3 a, 17 where a linearly polarized laser beam is perpendicularly incident on the sample and reflected with a small polarization rotation angle. The experimental procedure follows the step-by-step diagram shown in Figure 3 b.…”