Magnetic properties of lanthanum orthoferrites containing lattice defects

Polycrystalline lanthanum orthoferrites were prepared by firing coprecipitated hydroxides of La³⁺ and Fe³⁺ at elevated temperatures. The materials fired at temperatures below 1100°C were characterized by the coexistence of such lattice vacancies that the concentration ratio of V_La:V_Fe:V_o is always equal to that of the constituent atoms of LaFeO₃. They exhibited a large magnetic susceptibility, a low Néel temperature, and a small spontaneous magnetization, in comparison with lanthanum orthoferrite without these vacancies. These characteristics originated essentially from a local reduction in anisotropic superexchange interaction at vacant sites. The lowering of Néel temperature characteristic of these materials was interpreted in terms of a decrease in the number of Fe³⁺OFe³⁺ linkages due to the existence of oxygen and iron vacancies.