Probing and controlling magnetism in 2D materials

Abstract

A central goal of condensed matter physics is the discovery and design of materials systems with properties and behavior that have the potential to advance technology. The rapidly expanding field of two-dimensional materials provides unprecedented opportunities for rational materials design due to both the interesting physics that emerge in pristine nanoscale crystals, and the unprecedented tunability of these systems which enables on-demand control over the emergent phenomena. In this thesis, we will explore tunable coupling between spin, charge, and lattice in the layered magnetic semiconductor CrSBr. The first half of this thesis will establish that the excitonic optical response and electronic transport in CrSBr are strongly dependent on the magnetic state, a desired behavior of magnetic semiconductor systems which have been pursued for over two decades due to their promise for energy-efficient spintronic devices. The second half will explore techniques for applying large strains to 2D materials using a piezoelectric strain cell at cryogenic temperatures. In addition to establishing new techniques for engineering interlayer strain transfer in 2D materials generally, we found that CrSBr has excellent mechanical properties which allow for the application of extreme strains. As strain is increased along the a axis, the interlayer magnetic exchange coupling is continuously tuned from a negative antiferromagnetic (AFM) value all the way to a positive ferromagnetic (FM) one, resulting in a reversible strain-induced magnetic phase transition. We then harnessed this AFM to FM phase transition to realize strain-controlled magnetic tunnel junction devices. The strain tuned magnetism enables many discoveries including novel magnon modes at intermediate strains, dynamic layer-wise magnetization flipping and control of multiple stable magnetic states, and the creation of stochastic domains with a strain-tunable sigmoidal response curve. These results combined with the developed strain techniques provide a plethora of opportunities for probing and controlling 2D magnetism and a wide range of other quantum phenomena in 2D materials and heterostructures.