Bose–Einstein condensation threshold in a parametrically populated magnon gas (Review article)

When the density of a quasi-equilibrium gas of Bose quasiparticles, such as, polaritons, excitons, or magnons, increases, a macroscopically populated coherent state, the Bose–Einstein condensate (BEC), can form at the bottom of the quasiparticle spectrum. Considering the BEC of magnons, we analytically found an expression for the additional number of magnons that must be created in a magnetic sample to reach the threshold of BEC. This number is determined by the paraprocess, which reduces the number of thermal magnons as the magnetizing bias field increases, and also by the fact that only long-lived dipole-exchange magnons with wavenumbers smaller than 107cm−1 participate in the formation of the BEC. The general results obtained are applied to two particular cases of magnon BEC formation: by parallel parametric pumping in a ferrimagnetic film and by two-magnon scattering processes in a ferrimagnetic sample with a rough surface.