Helical spin ordering in the multiferroic Ba3NbFe3Si2O14 of the langasite family probed by Mossbauer spectroscopy

The iron-containing crystals of the langasite family A(3)MFe(3)X(2)O(14) (A =Ba, Sr, M = Sb, Nb, Ta, X= Si, Ge) are a new class of multiferroics in which ferroelectricity can be induced by magnetic ordering. We have measured Mossbauer spectra of a powder sample Ba3NbFe3Si2O14 of this family in a wide temperature range and developed a physico-mathematical model for analyzing the complex Mossbauer spectra shape. This model allows establishing a helical magnetic structure at all temperatures in the magnetic ordering region (T < T-N). It is shown that the experimental Mossbauer spectra are well described by the model of a helical magnetic structure with a constant angle of rotation of the moment (rotation step) during its transfer between adjacent planes. The angle value is about 51 degrees, which agrees with the previous results of neutron experiments. The excited states of the magnetic moments that appear at finite (nonzero) temperatures are also taken into account, which makes it possible to study the magnetic dynamics at different temperatures. The value of the angle between the main axis of the electric field gradient (EFG) at the iron nuclei and the crystal axis c (theta = 36 degrees), which does not change with the temperature in the region T < T-N, is established. This indicates that in the region of magnetic ordering of this langasite no structural transitions occur. The developed model promotes a deeper understanding of the magnetic properties of the multiferroic Ba3NbFe3Si2O14 and can be used to study the magnetic structure of other compounds where iron ions form a similar helical structure.