Enhanced orbital fluctuations in Mg-doped Mn V2 O4 single crystals

We investigated the magnetic and structural transitions of Mn1-xMgxV2O4(x=0, 0.05, 0.1, 0.15) single crystals with a normal spinel structure. These crystals all exhibit a cubic-to-tetragonal structural transition at TS caused by the orbital ordering of V3+ ions, which is followed by a ferrimagnetic ordering transition at TM. We found that TM is equal to TS for x≤0.1, but the ferrimagnetic transition is shifted to lower temperature by Mg doping and TS and TM can be separated clearly for x=0.15. Moreover, a certain degree of orbital fluctuations was brought about by the ferrimagnetic transition at TM, but suppressed below T∗, leading to another transition from weak orbital fluctuations to further orbital ordering at T∗. The orbital fluctuations were caused by the anomalous interchain interaction Jc and can be also enhanced by the substitution of Mg for Mn due to the decrease of the distance between V3+ ions along the c direction cRv-v, which results in the increase of Jc.