Composition-induced transition of spin-modulated structure into a uniform antiferromagnetic state in a Bi1−xLaxFeO3 system studied using 57Fe NMR

Abstract

By analyzing the NMR line shape, the transformation of a spatially spin-modulated magnetic structure in BiFeO 3 into an ordinary spatially uniform structure of the LaFeO 3 orthoferrite in Bi 1− x La x FeO 3 solid solutions is studied. The measurements are made using a spin-echo technique at temperatures of 77 and 4.2 K on ceramics with compositions x =0, 0.1, 0.2, 0.61, 0.9, and 1.0 enriched by the 57 Fe isotope. It is shown that the spin-modulated structure disappears near the concentration x =0.2, which corresponds, according to the published data, to the phase transition with a change in the unit-cell symmetry R 3 c → C 222. A formula is obtained describing the NMR absorption line shape for the spin-modulated structure with account of local line-width. Theoretical spectra adequately describe the evolution of the experimental spectrum in the concentration range 0≤ x ≤0.2. Highly nonuniform local magnetic fields in the intermediate compositions make it impossible to detect NMR signals in a sample with x =0.61. A uniform magnetic structure characterized by a single narrow line arises in the range of existence of a phase with the symmetry Pnma typical of the pure orthoferrite LaFeO 3 .