Structural and Magnetic Phase Transitions in the Multiferroic HoFe.sub.3.sub.4 Observed by Mössbauer Spectroscopy and X-ray Diffraction

The temperature dependences of the structural parameters in a HoFe.sub.3(BO.sub.3).sub.4 single crystal, studied by X-ray diffraction below and above the structural phase transition at T.sub.S = 365 K, correlate well with behavior of the Mössbauer parameter quadrupole interaction. However, two structural positions Fe1 and Fe2 of iron ions formed in the phase with space group P3.sub.121, which appears at temperatures below T.sub.S, cannot be distinguished by Mössbauer spectroscopy at .sup.57Fe nuclei. This becomes possible only below the Néel temperature T.sub.N. It has been established that below T.sub.N, iron ions form a 3D magnetic order of the Ising type with critical parameters [beta] = 0.283(1) and the dimension of the order parameter n = 1. The refined value of the Néel temperature is T.sub.N = 37.42(1) K. The dynamics of changes in the Mössbauer parameters of the quadrupole shift and the magnetic hyperfine field B.sub.hf, observed near the temperature T = 4.4 K for iron ions in the Fe1 and Fe2 positions, indicates a reorientation of the magnetic moments of iron. This correlates with the spin-reorientation transition of Fe and Ho ions, previously observed by neutron diffraction in HoFe.sub.3(BO.sub.3).sub.4.