Mössbauer study of the modulated magnetic structure of FeVO4

Abstract

Mossbauer spectroscopy is used to study the FeVO4 multiferroic, which undergoes two magnetic phase transitions at T N1 ≈ 22 K and T N2 ≈ 15 K. The first transition (T N1) is related to transformation from a paramagnetic state into a magnetically ordered state of a spin density wave, and the second transition (T N2) is associated with a change in the type of the spatial magnetic structure of the vanadate. The electric field gradient tensor at 57Fe nuclei is calculated to perform a crystal-chemical identification of the partial Mossbauer spectra corresponding to various crystallographic positions of Fe3+ cations. The spectra measured in the range T N2 < T < T N1 are analyzed on the assumption about amplitude modulation of the magnetic moments of iron atoms μFe. The results of model intersection of the spectra recorded at T < T N2 point to a high degree of anharmonicity of the helicoidal magnetic structure of the vanadate and to elliptic polarization of μFe. These features are characteristic of type-II multiferroics. The temperature dependences of the hyperfine interaction parameters of 57Fe nuclei that were obtained in this work are analyzed in terms of the Weiss molecular field model on the assumption of orbital contribution to the magnetic moments of iron cations.