Pressure effects on magnetism in Ca2Mn2O5 -type ferrites and manganites

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 ${\mathrm{Sr}}_{2}{\mathrm{Fe}}_{2}{\mathrm{O}}_{5}$ (with a ${d}^{5}$ electronic configuration) and ${\mathrm{Sr}}_{2}{\mathrm{Mn}}_{2}{\mathrm{O}}_{5}$ (${d}^{4}$ configuration), exhibiting the ${\mathrm{Ca}}_{2}{\mathrm{Mn}}_{2}{\mathrm{O}}_{5}$-type oxygen-deficient perovskite structure, with hydrostatic pressure. The ${\mathrm{Ca}}_{2}{\mathrm{Mn}}_{2}{\mathrm{O}}_{5}$-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 ${\mathrm{Sr}}_{2}{\mathrm{Mn}}_{2}{\mathrm{O}}_{5}$ (${d}^{4}$) and G-type antiferromagnetic (AFM-G) for ${\mathrm{Sr}}_{2}{\mathrm{Fe}}_{2}{\mathrm{O}}_{5}$ (${d}^{5}$). We show that hydrostatic pressure enhances the crystal-field splitting and affects the magnetic stability. We find that the AFM-E order exhibited by ${\mathrm{Sr}}_{2}{\mathrm{Mn}}_{2}{\mathrm{O}}_{5}$ is robust over the surveyed ranges of applied pressures, whereas ${\mathrm{Sr}}_{2}{\mathrm{Fe}}_{2}{\mathrm{O}}_{5}$ shows a magnetic transition from AFM-G to ferromagnetic spin order at $\ensuremath{\approx}24.5$ GPa. We also discuss the effect of correlation strength, treated using the Hubbard $U$ correction, which we find suppresses a spin crossover transition in ${\mathrm{Sr}}_{2}{\mathrm{Fe}}_{2}{\mathrm{O}}_{5}$ and shifts it to higher pressures.