Professor Thorsten Hesjedal FInstP

Abstract:

In the field of spintronics, the generation of a pure spin current (without macroscopic charge flow) through spin pumping of a ferromagnetic (FM) layer opens up the perspective of a new generation of dissipation-less devices. Microwave driven ferromagnetic resonance (FMR) can generate a pure spin current that enters adjacent layers, allowing for both magnetization reversal (through spin-transfer torque) and to probe spin coherence in non-magnetic materials. However, standard FMR is unable to probe multilayer dynamics directly, since the measurement averages over the contributions from the whole system. The synchrotron radiation-based technique of x-ray detected FMR (XFMR) offers an elegant solution to this drawback, giving access to element-, site-, and layer-specific dynamical measurements in heterostructures. In this work, we show how XFMR has provided unique information to understand spin pumping and spin transfer torque effects through a topological insulator (TI) layer in a pseudo-spin valve heterostructure. We demonstrate that TIs function as efficient spin sinks, while also allowing a limited dynamic coupling between ferromagnetic layers. These results shed new light on the spin dynamics of this novel class of materials, and suggest future directions for the development of room temperature TI-based spintronics.

Abstract:

We present the results of transverse field (TF) muon-spin rotation (μ+SR) measurements on Cu2OSeO3, which has a skyrmion-lattice (SL) phase. We measure the response of the TF μ+SR signal in that phase along with the surrounding ones, and suggest how the phases might be distinguished using the results of these measurements. Dipole field simulations support the conclusion that the muon is sensitive to the SL via the TF line shape and, based on this interpretation, our measurements suggest that the SL is quasistatic on a time scale τ>100 ns.

Abstract:

Breaking the time-reversal symmetry (TRS) in topological insulators (TIs) through ferromagnetic doping is an essential prerequisite for unlocking novel physical phenomena and exploring potential device applications. Here, we report the successful growth of high-quality (Dy(x)Bi(1-x))2Te3 thin films with Dy concentrations up to x = 0.355 by molecular beam epitaxy. Bulk-sensitive magnetisation studies using superconducting quantum interference device magnetometry find paramagnetic behaviour down to 2 K for the entire doping series. The effective magnetic moment, μeff, is strongly doping concentration-dependent and reduces from ∼12.6 μ(B) Dy(-1) for x = 0.023 to ∼4.3 μ(B) Dy(-1) for x = 0.355. X-ray absorption spectra and x-ray magnetic circular dichroism (XMCD) at the Dy M4,5 edge are employed to provide a deeper insight into the magnetic nature of the Dy(3+)-doped films. XMCD, measured in surface-sensitive total-electron-yield detection, gives μ(eff )= 4.2 μ(B) Dy(-1). The large measured moments make Dy-doped films interesting TI systems in which the TRS may be broken via the proximity effect due to an adjacent ferromagnetic insulator.