Effect of Magnons on Thermal Transport in Insulators

Abstract

Magnons can influence the thermal conductivity of insulators either by interacting with the phonons or by transporting heat. In a ferromagnet or ferrimagnet the magnon energies are raised by a magnetic field. The magnon-phonon interaction can suppress the contribution of phonons whose energy is close to that at which the magnon and phonon dispersion curves cross. A magnetic field moves the crossover to higher phonon energies. The phonon contribution is peaked at about an energy of 4 kT in a good crystal. Thus the phonon conductivity initially decreases with increasing field, then recovers to its zero field value at sufficiently high fields, but the magnon part does not. Similar effects occur in antiferromagnets. Using a 3He cryostat capable of reaching 0.2°K and a magnet capable of fields up to 50 kOe, we have observed these effects in MnCl2·4H2O, GdCl3, YIG, and Li Ferrite. In all the crystals except YIG and Li Ferrite, recovery to the zero field value eventually occurred, indicating that the magnon contribution had to be less than 2% of the heat current at zero field. In YIG the magnons contributed about 66% of the heat current. In Lithium Ferrite they contribute about 40%.