Thin film quantum materials

Abstract:

We report a study on the transition temperature, T_C, of a Cr-doped Bi2Se3 topological insulator thin film, where an increase in the ferromagnetic onset can provide a pathway towards low-power spintronics applications. Arrott plots, obtained by Cr L_2,3 x-ray magnetic circular dichroism as a function of field at various low temperatures, give a T_C ~7 K. This is similar to the bulk value of the sample, which means that there is no indication the spontaneous magnetization is different near the surface. Evaporation of a thin layer of Co onto the pristine surface of the in-situ cleaved sample increases the ordering temperature to ~19 K. X-ray absorption spectroscopy shows that Cr enters the Bi2Se3 host matrix in a divalent state, and is unchanged by the Co deposition.

Abstract:

Transition metal (TM) doped topological insulators have been the focus of many recent studies since they exhibit exotic quantum and magneto-electric effects, and offer the prospect of potential applications in spintronic devices. Here we report a systematic study of the local electronic and structural environment using x-ray absorption fine structure (XAFS) in TM (= Cr, Mn, and Fe) doped Bi2Se3 thin films grown by molecular beam epitaxy. Analysis of the TM K-edge XAFS reveals a divalent character for Cr, Mn, and Fe when substituting Bi in the films, despite the trivalent character of the Bi. All dopants occupy octahedral sites in the Bi2Se3 lattice, which agrees with substitutional incorporation onto the Bi sites. With the incorporation of TM dopants a local structural relaxation of the Bi2Se3 lattice is observed, which strengthens the covalent character of the TM–Se bond. The presence of additional phases and interstitial incorporation for the Mn and Fe dopants is also observed, even at low concentrations.

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.