Thermal Transport Properties of Manganese Fluoride

Abstract

<p>The thermal transport properties of the antiferromagnet manganese fluoride were investigated using thermal conductivity and heat pulse measurements. With heat flow along the [001] crystallographic direction, the thermal conductivity was measured at temperatures from 0.3 to 4.2 K in magnetic fields ranging from 0 to 60 kOe. Heat pulse signals were measured for pulse propagation along the [001] and [110] directions for an ambient sample temperature of 2.0 K and pulse temperatures between 2.5 and 6.0 K.</p> <p>With a magnetic field applied along the [001] direction the thermal conductivity showed no change that could be attributed to heat transport by the antiferromagnetic magnons. This result was in disagreement with calculations of the boundary limited magnon conductivity. It was demonstrated that the absence of magnon heat conduction could be due to the long magnon-phonon relaxation time of MnF2. This general explanation could also be applied to other magnetic materials.</p> <p>The results for the thermal conductivity as a function of temperature showed a dip at 1.0 K which indicated a resonant interaction of the the phonons with an impurity. The impurity has previously been shown to be OH‾. The attenuation of the heat pulses indicated that only the Eg phonon mode was resonantly scattered. These results were shown to be consistent with the previously unexplained low temperature decrease in the C44 elastic constant of MnF2. This was accounted for by the reduction in the velocity of the Eg phonons caused by their resonant interaction with the OH‾ impurities.</p> <p>Theoretical expressions were fitted to the experimental results. It was found to be important to include phonon focusing effects in these expression. The thermal conductivity, heat pulse, and elastic constant results were all accounted for by a single set of adjustable parameters. From the fits, the level splitting of the OH‾ impurity states was found to be 2.8 cm‾¹, the OH‾ impurity concentration was estimated to be 8x10^16 cm‾³, and the elastic sipole moment of the OH‾ molecule was calculated to be 3.4x10^-24 cm³.</p> <p>When a magnetic field was applied at an angle of 25º to the [001] axis, the conductivity increased below 1.2 K, and decreased for higher temperatures. This was shown to be due to an increase in the resonant frequency of the impurity.</p> <p>A tunneling model was proposed for the OH‾ impurity states in MnF2. The low lying states of the model system were shown to form a tunnel split doublet. The symmetries of the states were such that only Eg phonons would cause transitions between them. In an off-axis magnetic field, the tunnel splitting was shown to increase due to the static strain caused by the magnetoelastic interaction. Thus, it was demonstrated that a tunneling model was consistent with the experimental results.</p>