Microscopic evidence of magnetic and structure phase transition in multiferroic spinel FeV2O4

We report the microscopic evidence for magnetic and structural phase transitions in multiferroic spinel FeV2O4 from the hyperfine magnetic interaction. FeV2O4 sample shows three different crystal structures with the phase transitions from tetragonal to orthorhombic structure around 70 K, from orthorhombic to tetragonal structure around 109 K, and from tetragonal to cubic structure around 140 K. Mössbauer spectra of FeV2O4, obtained at various temperatures, were analyzed with severely distorted 8-line below TC, and doublet at TC. Also, the Mössbauer spectra change from doublet to singlet around TJT ≅ 140 K due to the reduction of Jahn-Teller effect. The value of electric quadrupole splitting (ΔEQ) is 3.05 mm/s at 4.2 K, indicating the noncollinear spin structure with strong polarization from the gap energy of 5T2g band, Δ1 ≅ 0. Whereas, there is collinear spin structure between TS ≅ 70 K < T < TC ≅ 109 K, since Δ1 in this temperature range increases from the value when T < TS due to the non-degenerate energy state with commensuration in the collinear state. Also, we have found that large polar angle θ for T < TS suggests the spin of the Fe2+ cations aligns along c-axis with the distortion in the a-b plane, while the small azimuthal angle θ suggest the direction of the spin is within ab-plane for TS < T < TC.