Second sound and heat conduction of magnons in the ferromagnetic Eu-chalcogenides

The various relaxation times of the thermal magnons in the ferromagnetic Eu-chalcogenides are calculated for temperatures well below the Curie point, and the conditions for the existence of magnon second sound and magnon heat conduction are examined. It is found that for EuS the “frequency window” for magnon second sound exists around 2 MHz at temperatures of about 1 K. In the same material the magnon contribution to the static thermal conductivity should be measurable at temperatures between 1 K and 2 K. Supported in part by the Fonds National Suisse de la Recherche Scientifique.