Magnetism of Lanthanum Orthoferrites Containing Lattice Defects

Abstract

Lanthanum orthoferrites were prepared by firing co-precipitated hydroxides of La3+ and Fe3+ at various temperatures. The materials fired below 700°C apparently had a cubic perovskite-type structure imperfectly crystallized, and the firing temperature elevation changed it to the orthorhombic phase of the GdFeO3-type structure. The chemical analysis and the density measurements showed that the thermally metastable crystals fired below 1100°C more or less contain such lattice vacancies that the vacancy concentration ratio of VLa:VFe:Vo is always equal to that of the constituent atoms of LaFeO3. To clarify the origin of the remarkable line broadening of the X-ray diffraction peaks, the effect of crystallite size on half-height width, β, was separated by plotting β cos θ vs. sin θ. A fairly large fluctuation of interplaner spacing, Δd/d, decreased with increasing firing temperature. Lanthanum orthoferrites containing lattice vacancies were also characterized by a large magnetic susceptibility, a low Neel temperature, a small spontaneous magnetization, and moreover, “diffuse magnetic anomaly” in the vicinity of antiferromagnetic-paramagnetic transition compared with lanthanum orthoferrites without vacancies. Essentially, these characteristics were interpreted in terms of a local decrease in anisotropic superexchange interaction at vacant sites.