Effects of a Magnetic Field on Heat Conduction in Some Ferrimagnetic Crystals

The thermal conductivities of single-crystal bars of several ferrites have been measured over the temperature range 1.4° to 25°K in both zero external magnetic field and in a field of about 9.4 kilo-oersteds. For each speciment the zero-field conductivity is a smooth function of the temperature over the entire range considered. The magnitudes of the conductivities of the manganese and cobalt-zinc ferrite specimens are quite low, approximaely 1.5 mw/cm-°K at 4°K. Upon application of a magnetic field parallel to the direction of heat current flow, the conductivities of the manganese ferrite and cobalt-zinc ferrite specimens are increased. The magnetoconductivities are rather large. For example, $\frac{\Delta K}{K_0}=0.2\mathrm{}$ for manganese ferrite at 2°K. Such a field produces no detectable change in the conductivity of the manganese-zinc ferrite. The experimental results are interpreted on the basis of the assumption that the heat current is primarily a phonon current limited by phonon-magnon scattering.