Skyrmions and Magnetic Textures
Topological spin textures in chiral magnets are of interest both to fundamental science, through Berry phase-induced Hall effects, and for potential device applications, including magnetic racetrack memories and neuromorphic computing. Competing magnetic interactions lead to spin textures such as helices, where spins periodically tumble with a characteristic pitch length, and magnetic skyrmions, which are whirling localized textures with a total pi-rotation of the spins. In many cases of interest, the competition is between ferromagnetic exchange, favoring aligned spins, and the Dzyaloshinskii-Moriya interaction (DMI), which favors perpendicular spins, and which arises from spin-orbit coupling in the presence of broken inversion symmetry. DMI can arise in bulk crystals with e.g. chiral structure, and in interface systems. Spin-polarized scanning tunneling microscopy (SP-STM) is uniquely suited to probe the rich physics of such nanoscale spin textures in real space, and provides microscopic insights that complement those obtained from ensemble techniques that may average over different magnetic domains.
The groups’ efforts in this area have demonstrated SP-STM imaging of skyrmions and magnetic textures in both bulk (i.e., MnGe, FeGe thin films) and interface DMI systems (i.e., SRO/SIO oxide and metallic /Pt/Cu heterostructures). The principal focus is to understand how aspects of the surface, interface and film structure influence the size and distribution of skyrmions, chiral domain walls and new topological textures.
SP-STM image showing a variety of magnetic textures in the chiral magnet, MnGe
Topographic and SP-STM image of a Cu/Co/Pt heterostructure correlating the magnetic contrast associated with a nanoscale skyrmion and the local grain structure.
Magnetic Topological Insulators
Introducing magnetism to topological insulators (TIs) breaks time-reversal symmetry and leads to a variety of interesting phenomena such as magnetoelectric effects, Weyl semimetal phases, efficient coupling of quantum spin defects, and the quantum anomalous Hall effect (QAHE). Some approaches to create magnetic TIs include: Doping with magnetic impurities, proximity to magnetic layers, and intrinsic magnetism. The QAHE has been observed in each of these cases, but only at temperatures much lower than the Curie temperature. The physics underlying this discrepancy is not well understood but may be due to disorder associated with random doping or interfaces. SP-STM is a natural choice to better understand the role of disorder at the atomic scale in magnetic TIs.
Our group’s focus so far have been on the van der Waals (vdW) ferromagnet Fe3GeTe2 (FGT) coupled to vdW topological insulator Bi2Te3. Integration of TIs with vdW ferromagnets offers a route to minimize interfacial disorder, as the vdW gap eliminates dangling bonds and facilitates epitaxial growth of materials with disparate lattices. Among the growing family of vdW ferromagnets, Fe3GeTe2 (FGT) is particularly attractive to couple to TIs such as Bi2Te3 due to its relatively similar layered structure and large bulk Curie temperature (TC ~ 220 K) which persists down to the monolayer limit (TC~ 130 K) and may even further increase up to room temperature in FGT/Bi2Te3 heterostructures.
Bi2Te3 and Single quintuple layer FGT interface. (a) Image of FGT and Bi2Te3 atomic lattices (-0.8 V, 0.3 nA). The moiré pattern is visible in the FGT region. (b) The fast-Fourier transformation of (a) with peaks corresponding to the FGT and Bi2Te3 Brillouin zone. The inset shows the peaks corresponding to the moiré periodicity in the center. (c) A linescan along the peaks of (b), used to calculate the FGT, Bi2Te3, and moiré lattice constants of 390 ± 10 pm, 430 ± 10 pm, and 4.3 ± 0.4 nm respectively.