Thomas GUILLET: Topological insulator-based heterostructures

Abstract:
During this seminar, I will present the main results I obtained during my post-doc at the Catalan Institute of Nanoscience and Nanotechnology as well as the main frame of the research project I intend to present to the CNRS, backed up by the IJL.
The project I am leading at ICN2 deals with the interface problems that exist in topological insulator-based (TI) heterostructure. Those materials own very peculiar properties: their bulk are insulating whereas their surface is conducting, the nature of these surface states is topological and grant these materials a large spin polarization. One of the central mechanisms in modern spintronics is the spin-orbit torque (SOT). The idea is to harness the strong spin-orbit coupling of heavy metals such as Pt to generate a large transversal spin current (spin Hall effect) and use it to manipulate the magnetization of an adjacent ferromagnet (FM). During the last few years, the community naturally tried to employ TIs to enhance the devices efficiency with inconsistent result. It now appears that the interface sharpness and the high chemical affinity between Bi-based TIs and classical 3d FMs is a major obstacle to reach the predicted breakthrough in magnetization switching power-efficiency.

My current work can be summarized in two approaches:

• The first approach consists in inserting a non-magnetic spacer between the TI and the FM. The growth of high crystalline quality and low-doped ternary TIs (BST) is achieved by molecular beam epitaxy on a BaF2 substrate. I studied the effect of the intercalation of an ultrathin layer of Al on the intensity of the SOT effective fields using harmonic magnetotransport. These results are still preliminary but clearly show an increase of the SOT with the thickness of the spacer, as well as a modification of the transport mechanisms in the structure.

• The second approach is more ambitious and aims at using novel 2D ferromagnetic materials. The interest lies in the van der Waals force that exists between the TI and the magnetic 2D. This weak interaction helps to reduce the problems of chemical reactions and hybridization at the interface between the two layers, at the benefit of the SOT. In practice, I developed the MBE growth of the 2DFM Fe 3 GeTe 2 (FGT) on a TI thin film (BST). The second harmonic measurements then showed a very strong SOT generation, which resulted in an ultra-low power magnetization switching of the FGT.