Unusual Thermal Properties in Magnetic MnFe₂O₄ and FeMn₂O₄ Single Crystals

Materials that form the AB₂O₄-type spinel structure are known to exhibit geometric frustration, as the A sublattice forms diamond-like structure and B sublattice is pyrochlore-like. They are thus promising candidates for studying unconventional thermal and magnetic properties. Here, we report thermal conductivity, thermopower, and specific heat in magnetic spinel MnFe₂O₄ and FeMn₂O₄ single crystals. One remarkable feature is their low thermal conductivity in a wide temperature range (2 K and 400 K) with a maximum value of ~ 2.0 W/K-m, which is usual for single crystalline materials. Furthermore, both the low-temperature thermal conductivity and specific heat exhibit T^3/2 dependence instead of T³ behavior. This strongly suggests that heat in these magnetic spinel materials is predominantly carried by magnons, while phonons exhibit glass-like behavior, conducting little heat. The latter should be attributed to geometric frustration inherent in spinel materials.