Professor Thorsten Hesjedal FInstP

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.

Abstract:

We present a structural and density-functional theory study of the interface of the quasi-twin-free grown three-dimensional topological insulator Bi2Te3 on Ge(111). Aberration-corrected scanning transmission electron microscopy and electron energy-loss spectroscopy in combination with first-principles calculations show that the weak van der Waals adhesion between the Bi2Te3 quintuple layer and Ge can be overcome by forming an additional Te layer at their interface. The first-principles calculations of the formation energy of the additional Te layer show it to be energetically favorable as a result of the strong hybridization between the Te and Ge.

Abstract:

Heterostructures composed of ferromagnetic layers that are mutually interacting through a nonmagnetic spacer are at the core of magnetic sensor and memory devices. In the present study, layer-resolved ferromagnetic resonance was used to investigate the coupling between the magnetic layers of a Co/MgO/Permalloy magnetic tunnel junction. Two magnetic resonance peaks were observed for both magnetic layers, as probed at the Co and Ni L3 x-ray absorption edges, showing a strong interlayer interaction through the insulating MgO barrier. A theoretical model based on the Landau-Lifshitz-Gilbert-Slonczewski equation was developed, including exchange coupling and spin pumping between the magnetic layers. Fits to the experimental data were carried out, both with and without a spin pumping term, and the goodness of the fit was compared using a likelihood ratio test. This rigorous statistical approach provides an unambiguous proof of the existence of interlayer coupling mediated by spin pumping.