Structural and magnetic properties of the iron-containing langasite family A_3MFe_3X_2O_(14) (A = Ba,Sr; M = Sb, Nb, Ta; X = Si, Ge) observed by Moessbauer spectroscopy

摘要

The occurrence of the long-range magnetic order of high-spin Fe~(3+) ions is found in iron-containing langasite family crystals A_3MFe_3X_2O_(14) (A = Ba, Sr; M = Sb, Nb, Ta; X = Si, Ge) at T≤ 40 K. The Neel temperature (T_N) values (27.1-37.1 K) depend on the composition and significantly increase with the strengthening covalence of Fe-O bonds in the 3f tetrahedra. Splitting of iron positions into two magnetic sublattices is established in Ba_3TaFe_3Si_2O_(14) and Ba_3NbFe_3Si_2O_(14) crystals at T < T_N. Iron magnetic moments in two sublattices are directed at different angles to the local crystal axes with a mean value ≈45°. At the temperatures between 4.2 and 20 K. the iron spins in two sublattices rotate in opposite directions. It can be related to magnetic chirality in the helicoidal magnetic structure. The appearance of nonequivalent Fe~(3+) sites is associated with the structural phase transition P321 → P3 (or P321 → C2) induced by the magnetic transition. The presence of the polar threefold axis at this transition provides the conditions for the occurrence of the ferroelectric state. Such crystals can be considered as magnetically induced multiferroics.