Orbital fluctuations in spinel Mn(V _1-xAl _x) ₂O ₄ studied by thermal conductivity measurements

We measured the thermal conductivity of spinel MnV ₂O ₄ and Al-doped samples with an applied magnetic field, and they exhibited orbital and spin ordering at low temperatures. We found that the thermal resistivity (reciprocal thermal conductivity) in the orbital-disordered phase is enhanced when the system approaches the critical point of the orbital ordering under the variation of both temperature and magnetic field, but it is suppressed when it crosses the critical point and goes into the orbital-ordered phase. This result indicates that the thermal conductivity of these compounds is dominated by the fluctuation of V t _2g orbitals.