Magnetic Field Dependence of the Thermal Conductivity of Doped MgO

The thermal conductivity of lightly doped single crystals of MgO has been measured over the temperature range from 0.4 to 4.2°K in magnetic fields up to 40 kOe. Resonant absorption of phonons is observed due to both the $\Delta M=1\mathrm{}$ and $\Delta M=2\mathrm{}$ transitions in the ground-state triplet of the ${\mathrm{Fe}}^{2+}$ ion. Theoretical consideration of this ion alone is sufficient to explain the observed data at the lowest temperatures. Deviations from theoretical predictions are observed at higher temperatures, however. These deviations occur at high magnetic fields and are thought to be related to the same scattering source which causes the very unusual dependence of the zero-field conductivity.