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 lowerature 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.