Pressure effects on magnetism in Ca_2Mn_2O_5-type ferrites and manganites

摘要

The presence of ordered oxygen vacancies in perovskites governs magnetic phase stability owing to changes in crystal-field splitting with different anion geometries, polyhedral arrangements, and electronic configurations of the transition-metal cations. Here we use density functional theory calculations to assess the magnetic phase stability of Sr_2Fe_2O_5 (with a d~5 electronic configuration) and Sr_2Mn_2O_5 (d~4 configuration), exhibiting the Ca_2Mn_2O_5-type oxygen-deficient perovskite structure, with hydrostatic pressure. The Ca_2Mn_2O_5-type structure is composed of square pyramidal units, the crystal-field splitting and polyhedral connectivities of which support different ground-state magnetic orders depending on d-orbital filling: E-type antiferromagnetic (AFM-E) for Sr_2Mn_2O_5 (d~4) and G-type antiferromagnetic (AFM-G) for Sr_2Fe_2O_5 (d~5). We show that hydrostatic pressure enhances the crystal-field splitting and affects the magnetic stability. We find that the AFM-E order exhibited by Sr_2Mn_2O_5 is robust over the surveyed ranges of applied pressures, whereas Sr_2Fe_2O_5 shows a magnetic transition from AFM-G to ferromagnetic spin order at ≈24.5 GPa. We also discuss the effect of correlation strength, treated using the Hubbard U correction, which we find suppresses a spin crossover transition in Sr_2Fe_2O_5 and shifts it to higher pressures.