On the theory of Bose-Einstein condensation of quasiparticles: On the possibility of condensation of ferromagnons at high temperatures

摘要

The Bose condensation of magnons in physical systems of finite size is considered for the case of ferromagnetic thin films. It is shown that in accordance with present-day experimental capabilities, which permit one to achieve densities of long-wavelength spin excitations of ～10{sup}18-10{sup}19 cm{sup}(-3) in such films, the formation of a coherent condensate of such quasiparticles begins at temperatures T～10{sup}2 K (including room temperature). It is found that Bose condensation is accompanied by a scaling phenomenon, according to which the main thermodynamic variable is not the number of particles N but the ratio N/T. This indicates that the Bose condensation of magnons can be observed at relatively low magnon densities (and, accordingly, low pumping). The roles played by the shape of the spectrum of spin excitations and by the film thickness for observation of the phase transition to the state with the Bose condensate are analyzed, and the partial contributions of different groups of quasiparticles to the total spectral distribution of magnons over energies are discussed.