Distinct Evidence of Orbital Order in Spinel Oxide FeV2O4 by 57Fe Mössbauer Spectroscopy

An 57Fe Mössbauer spectroscopy has been conducted on the spinel oxide FeV2O4, where both Fe2+ and V3+ ions bear spin and orbital degrees of freedom. Distinct evidence of the orbital order in FeV2O4 was found. That is, at 65 K, where the transition from the orthorhombic phase to the low-temperature tetragonal phase takes place, the sign of the quadrupole coupling constant e2qQ/2 changes abruptly from negative to positive. The change in the orbital state accompanies a sudden decrease in the hyperfine field Hhf. The small Hhf implies the presence of the orbital angular momentum L, whose value itself does not change considerably through the transition. The spin order in the Fe2+ ion was also obtained. In the ferrimagnetic state below 110 K, the Fe2+ spin aligns along the orthorhombic a-axis (the longest axis). In the canted ferrimagnetic state below 65 K, however, the Fe2+ spin seems to incline slightly from the tetragonal c-axis. The canted V3+ spin may cause the slight canting of the Fe2+ spin.