Dynamics and spontaneous coherence of magnons in ferromagnetic thin films

A theory of the dynamics of the BoseEinstein condensate of magnons is presented. It is shown that, despite magnon decay processes, ongoing during the condensate formation, the system manifests spontaneous quantum coherence with a pseudo-spin degree of freedom originated from the presence of two valleys in momentum space. Finally, a real space description of the condensed state is provided, revealing the condensate state as a spin density wave. The effective dynamics of the spinor condensate wavefunction is characterized, and it is shown to display Josephson-like relations.