Dynamics and spontaneous coherence of magnons in ferromagnetic thin films

A theory of the dynamics of the Bose-Einstein 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.