AQP Seminar: Skyrmions and Antiferromagnetic Skyrmions in Metals and Insulators
Magnetic Skyrmions are topological textures in magnetization of certain non-centrosymmetric magnets that were predicted in the 1980s, and have been discovered during the last decade. In order to control these topologically protected textures for a possible use in memory devices, it is important to understand their microscopic origin. Conventionally, such textures are described with the help of phenomenological Landau free energy functionals including various anisotropic interactions. In recent years, there have been efforts towards achieving a microscopic electronic description of these topological textures. I will discuss our recent efforts to understand the formation of Skyrmions and Antiferromagnetic Skyrmions in non-centrosymmetric metals and insulators within a unified approach. Our approach relies on two key concepts: strong Hund's rule coupling and spin-orbit coupling. We consider a model of electrons hopping on a two-dimensional lattice, experiencing spin-orbit coupling of Rashba or Dresselhaus type and a strong coupling to a background of classical spins. The model is directly simulated using a hybrid Monte Carlo technique and magnetic states possessing isolated Skyrmions as well as Skyrmion lattices are obtained. In order to achieve an understanding of these, we derive an effective spin-only model by integrating out the electrons. The model turns out to be a spin-orbit modified version of the famous double-exchange model that is very well studied in the context of magnetoresistive manganites. A different effective model, similar to a spin-orbit modified t-J model in the Mott limit, is obtained in the insulating limit. This limit, in fact, supports formation of antiferromagnetic Skyrmions -- configurations that can be viewed as a superpositions of staggered antiferromagnet and skyrmions. Finally, we discuss how interface engineering can help inducing the antiferromagnetic-skyrmion states in the metallic regime.