Semiquantum thermodynamics of complex ferrimagnets

High-quality magnets such as yttrium iron garnet (YIG) are electrically insulating and very complex. By implementing a quantum thermostat into atomistic spin dynamics we compute YIG's key thermodynamic properties, viz., the magnon power spectrum and specific heat, for a large temperature range. The results differ (sometimes spectacularly) from simple models and classical statistics, but agree with available experimental data.