Composition-Induced Transition of Spin-Modulated Structure into a Uniform Antiferromagnetic State in a Bi_(1-x)La_xFeO_3 System Studied Using ~(57)Fe NMR

摘要

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_xFeO_3 solid solutions is studied. The measurements are made using a spin-echo technique at temperature of 77 and 4.2 K on ceramics with composition 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 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 experiential 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 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.