Field dependence of the thermal conductivity of CoBr2.6H2O

The thermal conductivity of CoBr26H2O has been measured in the temperature range 1.5-30 K and in magnetic fields up to 90 kOe. The specific field dependence of the data, especially with the field along the easy axis, indicates that the magnons do not contribute to the heat transport, but mainly act as a scattering source for the phonons. The data can be understood qualitatively by confronting them with calculated field-dependent magnon spectra. The actual scattering mechanism seems to be governed by the two-magnonone-phonon process.