Bose-Einstein Condensation of Magnons in Ferromagnets and Antiferromagnets

Abstract

Bose-Einstein condensation is a collective phenomenon which is the process of transition of many bosons - particles having integer spin - to the same coherent quantum state. One of the examples of bosons are magnons - the quanta of spin waves propagating in magnetic media. Formation of Bose-Einstein condensate (BEC) of magnons has been observed experimentally in ferromagnetic (FM) materials and analyzed in theory. The peculiarity is that BEC of magnons can be formed at room temperature. The BEC of magnons exhibits dynamic properties related to transport of magnon density, which appears to be anisotropic. In addition, the BEC of magnons also possesses nonlinear properties related to interaction of magnons in BEC and responsible for its stability. However, the theory available does not take into account all the important factors. Antiferromagnetic materials (AFM) also allow for existence of magnons, which implies the possibility of BEC formation in AFM materials as well. Although such a possibility has been discussed multiple times, none of the methods proposed has been realized on practice yet. The dissertation first informs about relevant topics, covering principles of magnonics, formation of BEC and its properties. It then describes a model aimed to describe both transport and nonlinear properties of magnons in BEC while accounting for all the peculiarities of magnons. After this, it explores in theory the interaction of magnon gas in the process of BEC formation with an isolated low-frequency mode. Finally, it delves into formation of BEC in AFM samples using a method of rapid cooling. It analyzes conditions required for BEC formation and discusses possible practical realization. The research presented in the dissertation helps to understand complicated phenomena related to formation and existence of BEC. It also opens new possibilities and directions of research both theoretical and experimental ones