Magnetic Weyl Semimetals for Spintronic Applications

Abstract

Magnetic Weyl semimetals are a category of topological materials that hold promise for spintronic applications due to their unconventional transport properties, which arise from both bulk and surface topological states, as well as the rich interplay between band topology and magnetism. Among the family of semimetallic materials, the antiferromagnetic Weyl semimetals Mn₃X (X=Sn, Ge, etc.) and the ferromagnetic Weyl semimetal Co₂MnGa have attracted significant interest. So far, despite extensive theoretical and experimental investigations, the magnetic dynamics of Mn₃X and the spin-polarized tunneling in Co₂MnGa based spintronic devices remain not fully explored.

In this thesis, I establish a theoretical framework to describe the low energy dynamics of strained Mn₃X. Using perturbation theory, I identify three distinct dynamic modes and derive a Landau-Lifshitz-Gilbert (LLG)-like equation to describe uniform mode dynamics. I also analyze the excitation of dissipative spin waves and the spin superfluidity state in Mn₃X by extending the model to include spatial inhomogeneity. The analytical results are validated against numerical simulations based on fully coupled LLG equations, where good agreement is achieved. In addition, I study fully epitaxial magnetic tunnel junctions (MTJs) composed of Co₂MnGa. By growing Co₂MnGa/MgO/Co₂MnGa stacks under different conditions, I develop a series of MTJs with varying degrees of chemical ordering in the Weyl semimetal electrodes and compare their tunneling magnetoresistance (TMR). I find that the TMR is enhanced with the improvement of the chemical ordering in Co₂MnGa. Our results reveal the relationship between the spin tunneling in MTJs and the chemical order of Co₂MnGa electrodes, offering insights into further enhancing TMR through Weyl semimetal engineering.