Effect of Iron Impurities on the Thermal Conductivity of Magnesium Oxide Single Crystals below Room Temperature

The thermal conductivity between 5 and 300°K of magnesium oxide single crystals containing different amounts of iron impurity has been measured. The iron produces a marked reduction in the thermal conductivity with a minimum at around 80°K in the highly doped (∼ 1%) samples and we attribute this effect to resonant scattering of phonons of energy ∼ 105 ${\mathrm{cm}}^{-1\mathrm{}}$ interacting with two groups of magnetic levels of the ${\mathrm{Fe}}^{++}$ ions. We have estimated the strength of the interaction from earlier spin-lattice relaxation-time measurements. Using an expression due to Callaway for the frequency dependence of the basic phonon heat current, we have then computed the effect of this interaction on the thermal conductivity. We find that we can account substantially for the observed data by using for the resonant interaction a Lorentzian line shape with a full characteristic width equal to that observed in infrared experiments (9 ${\mathrm{cm}}^{-1\mathrm{}}$), but cut off at nine times the width.