Orbital-spin-coupled fluctuations in spinel vanadate MnV₂O ₄

The elastic properties of a spinel vanadate MnV₂O₄ that has an orbital degree of freedom in the triply degenerate t_2g orbital at V3⁺(d2) sites are investigated by ultrasonic pulse-echo measurement. Considerable softening of the shear elastic constant (C ₁₁-C₁₂)/2 is observed as the temperature approaches the first-order transition from the high-temperature orbital-disordered cubic phase to the low-temperature orbital-ordered tetragonal phase. The softening is attributed to fluctuations between d_yz and d_zx. Moreover, the elastic anomaly is found to be sensitive to the external magnetic field, revealing a coupling between spin and orbital fluctuations. The elastic anomaly is well reproduced theoretically based on Landau theory and elucidates a characteristic precursor phenomenon in MnV₂O₄.