Formation of new phase inclusions in the system of quasiequilibrium magnons of high density

Abstract

The paper studies the spatial variation of the magnetization in a nonconducting magnetic sample with an excess number of magnons in comparison to the equilibrium. The phenomenon is considered using the Landau-Lifshits equation with additional terms describing the longitudinal relaxation of the magnetization, the magnon diffusion and the magnon creation by external pumping. The free energy of the system is presented in the mean field approximation. It is shown that, if the pumping exceeds some critical value, regions of the new phase arise where the magnetic moments are oriented opposite to the magnetization of the magnetic sample. The phenomenon is similar to the appearance of droplets of the condensed phase in the supersaturated vapor. The appearance of the new phase either in the form of a single domain or a periodical lattice is demonstrated. The studied process is a competitor to the process of the Bose-Einstein condensation of magnons.

Magnon Bose–Einstein Condensate and Supercurrents Over a Wide Temperature Range

“…The phenomena related to room-temperature Bose-Einstein condensation [1][2][3][4][5][6] and superfluidity [7][8][9] in overpopulated magnon gases constitute a fast evolving field of research, revealing promising features for advanced technological applications [10] and discov-ery of novel physical effects [11][12][13][14][15][16]. Recent examples of such phenomena are magnon supercurrents [7], Bogoliubov [9] and second sound [16] waves, microscaled vorticity [8] in a magnon condensate, thermally induced magnon Bose-Einstein Condensates (BECs) in microsized magnetic structures [17,18], and the interaction of the magnon BEC with accumulated hybrid magnetoelastic bosons [19].…”