Wavevector-dependent magnon accumulation in parametrically populated magnon–phonon spectrum

Abstract Interactions between quasiparticles of different nature, such as magnons and phonons in a magnetic medium, significantly change the spectrum of both types of quasiparticles. In regions where their dispersion curves intersect, forming an avoided crossing, the properties of the two subsystems mix and hybrid states are born, whose characteristics, such as velocity, lifetime and scattering efficiency, are determined by the magnon and phonon portion of their composition. In particular, the influence of the elastic component of quasiparticles allows for the manipulation of the position and even the number of spectral minima of the magnon gas in the wavevector space. Using Brillouin light scattering spectroscopy, we experimentally investigated such a system populated by the parametric pumping of magnons in a film of yttrium iron garnet. We found that the processes of thermalization of exchange and dipole magnons lead to their accumulation in different areas at the bottom of the spin-wave spectrum separated by the magnon–phonon hybridization region. The localization of quasiparticles in the wavevector space eventually leads to their separation in real space.