Emergent phenomena in two-dimensional magnetic crystals

Abstract

Two-dimensional (2D) van der Waals (vdW) crystals and their heterostructures offer a simple, yet powerful platform for discovering emergent phenomena and implementing device structures in the atomically thin limit. Fervent work over the past several years has pushed this frontier to include magnetism. These advances have brought forth a new wave of layered materials that intrinsically possess a wide variety of magnetic properties and are significant in integrating exchange and spin-orbit interactions into vdW heterostructures. In this thesis, we will focus on exploring the tantalizing physics that has been enabled by the discovery of one of these 2D magnetic vdW crystals, namely the magnetic insulator chromium triiodide (CrI3). While its bulk form is ferromagnetic, emergent layer-dependent magnetism is observed when CrI3 is cleaved down to just a countable number of layers – it is a ferromagnet in the monolayer, but a layered antiferromagnet when it possesses two to four layers. We leverage this unprecedented magnetic behavior to demonstrate the electrical control of 2D magnetism in bilayer CrI3, an important step towards realizing efficient, low-power spintronics devices for computing and memory storage applications. We also uncover a host of magneto-optical phenomena that arise from symmetry-breaking in a 2D magnetic insulator: electrical generation of Kerr signal in a layered antiferromagnet; giant polarization rotation and magnetoelectrical manipulation of Raman phonon selection rules; and discrete angular momentum conservation and its imprint on 2D magnon optical selection rules.