Control of Magnon Condensates

Abstract

Spintronics is a technology driven field aimed at harnessing the electron spin for novel devices. This is in contrast to traditional electronics, where the electron charge is used as an information carrier. Advances in electronics has to a large extent relied on making components smaller and stacking them closer together. When the components become very small, we enter the quantum mechanical realm, where the properties of the electron are very different. And when we stack the components closer together, we generate a lot of heat. Spintronics may solve some of the challenges traditional electronics face today. Magnons can be used as information carriers in spintronic devices. Magnons are bosons, and are allowed to share the same quantum state. When a large population of magnons thermalize down to the lowest energy state of the system, they may form what we call a Bose-Einstein condensate. The particles in the condensate behave coherently, and they represent a macroscopic manifestation of a single quantum state. These condensates are interesting to fundamental research, and recently we realize that they may also have some exotic practical applications. If we want to utilize the magnon condensates for practical applications, we need to be able to control their properties. This is the main motivation behind the research in the present thesis. The research accompanying this thesis contains theoretical investigations of methods for creating and controlling magnon condensates. We present how the presence of spin-transfer-torques may be used to assist or inhibit magnon creation. We provide a theoretical explanaition of how an out-of.plane easy-axis anisotropy may assist condensate formation, as observed experimentally by Divinskiy et al. We calculate the magnon interactions for condensate magnons in a thin-film ferromagnet, a uniaxial antiferromagnet, and in a biaxial antiferromagnet. We investigate how the two condensate populations in the ferromagnetic film are affected by the external field strength, film thickness and out-of-plane easy-axis anisotropy. We show how these parameters can be used to control properties such as the magnon distribution between the two populations. In Ref. we show how the two condensate populations in the uniaxial film with DzyaloshinskiiMoriya interaction (DMI) are affected by parameters such as the DMI-strength and strength of the in-plane easy-axis anisotropy.