Magnetic topological insulator heterostructures: a review
Advanced Materials Wiley (2021)
Abstract:Topological insulators (TIs) provide intriguing prospects for the future of spintronics due to their large spin–orbit coupling and dissipationless, counter-propagating conduction channels in the surface state. The combination of topological properties and magnetic order can lead to new quantum states including the quantum anomalous Hall effect that was first experimentally realized in Cr-doped (Bi,Sb)2Te3 films. Since magnetic doping can introduce detrimental effects, requiring very low operational temperatures, alternative approaches are explored. Proximity coupling to magnetically ordered systems is an obvious option, with the prospect to raise the temperature for observing the various quantum effects. Here, an overview of proximity coupling and interfacial effects in TI heterostructures is presented, which provides a versatile materials platform for tuning the magnetic and topological properties of these exciting materials. An introduction is first given to the heterostructure growth by molecular beam epitaxy and suitable structural, electronic, and magnetic characterization techniques. Going beyond transition-metal-doped and undoped TI heterostructures, examples of heterostructures are discussed, including rare-earth-doped TIs, magnetic insulators, and antiferromagnets, which lead to exotic phenomena such as skyrmions and exchange bias. Finally, an outlook on novel heterostructures such as intrinsic magnetic TIs and systems including 2D materials is given.
Periodically modulated skyrmion strings in Cu2OSeO3
npj Quantum Materials Springer Nature 6:2021 (2021) 73
Abstract:Magnetic skyrmions are vortex-like spin textures, which are usually treated as two-dimensional objects. In their lattice state, they form well-ordered, hexagonal structures, which have been studied in great detail. To obtain a three-dimensional (3D) skyrmion crystal, these planes can be envisaged to be stacked up forming skyrmion strings in the third dimension. Here, we report the observation of a 3D skyrmion phase in Cu2OSeO3 by carrying out reciprocal space mapping in resonant elastic x-ray scattering. We observe regions in the magnetic field-cooling phase diagram in which the skyrmion phase apparently coexists with the conical phase. However, such a coexistence is forbidden due to symmetry arguments. Instead, the skyrmion strings themselves are periodically modulated along their axes, as confirmed by micromagnetic simulations. The periodic modulation is in fact a necessary consequence of the evolution of the skyrmion phase out of the conical state, and should therefore be a universal property of skyrmion strings in chiral helimagnets.
Deriving the skyrmion Hall angle from skyrmion lattice dynamics
Nature Communications Springer Nature 12 (2021) 2723
Abstract:Magnetic skyrmions are topologically non-trivial, swirling magnetization textures that form lattices in helimagnetic materials. These magnetic nanoparticles show promise as high efficiency next-generation information carriers, with dynamics that are governed by their topology. Among the many unusual properties of skyrmions is the tendency of their direction of motion to deviate from that of a driving force; the angle by which they diverge is a materials constant, known as the skyrmion Hall angle. In magnetic multilayer systems, where skyrmions often appear individually, not arranging themselves in a lattice, this deflection angle can be easily measured by tracing the real space motion of individual skyrmions. Here we describe a reciprocal space technique which can be used to determine the skyrmion Hall angle in the skyrmion lattice state, leveraging the properties of the skyrmion lattice under a shear drive. We demonstrate this procedure to yield a quantitative measurement of the skyrmion Hall angle in the room-temperature skyrmion system FeGe, shearing the skyrmion lattice with the magnetic field gradient generated by a single turn Oersted wire.
Study of spin pumping through α-Sn thin films
Physica Status Solidi: Rapid Research Letters Wiley 15:6 (2021) 2100137
Abstract:Elemental tin in the α-phase is an intriguing member of the family of topological quantum materials. In thin films, with decreasing thickness, α-Sn transforms from a three-dimensional (3D) topological Dirac semimetal (TDS) to a two-dimensional (2D) topological insulator (TI). Getting access to and making use of their topological surface states is challenging and requires interfacing to a magnetically ordered material. Here, we report the successful epitaxial growth of α-Sn thin films on Co, forming the core of a spin-valve structure. We carried out time- and element-selective ferromagnetic resonance experiments to investigate the presence of spin pumping through the spin-valve structure. We applied a rigorous statistical analysis of the experimental data using a Landau-Lifshitz-Gilbert-Slonczewski equation based model. A strong exchange coupling contribution was found, however no unambiguous proof for spin pumping. Nevertheless, the incorporation of α-Sn into a spin-valve remains a promising approach given its simplicity as an elemental TI and its room temperature application potential.
Detailed crystallographic analysis of the ice V to ice XIII hydrogen-ordering phase transition.
The Journal of chemical physics 154:13 (2021) 134504