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

By analyzing the NMR line shape, the transformation of a spatially spin-modulated magnetic structure in BiFeO3 into an ordinary spatially uniform structure of the LaFeO3 orthoferrite in Bi1 - xLaxFeO3 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 Fe-57 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 R3c --> C222. A formula is obtained describing the NMR absorption line shape for the spin-modulated structure with account of local linewidth. Theoretical spectra adequately describe the evolution of the experimental spectrum in the concentration range 0 less than or equal to x less than or equal to 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 LaFeO3. (C) 2003 MAIK Nauka/Interperiodica.