High-pressure magnetism of the double perovskite Sr2FeOsO6 studied by synchrotron Fe57 Mössbauer spectroscopy

The pressure dependence of the magnetism of the double perovskite ${\mathrm{Sr}}₂{\mathrm{FeOsO}}₆$ was investigated by temperature- and magnetic-field-dependent synchrotron $⁵⁷\mathrm{Fe}$ M\"ossbauer spectroscopy in the energy domain up to $\ensuremath{\sim}50$ GPa. ${\mathrm{Sr}}₂{\mathrm{FeOsO}}₆$ is known to feature antiferromagnetic ordering below ${T}_{\mathrm{N}}\ensuremath{\sim}140$ K and a change in spin structure from AF1 to AF2 near 70 K at ambient pressure. Previous Os ${L}_2,3$ x-ray magnetic circular dichroism (XMCD) spectra [Veiga et al., Phys. Rev. B 91, 235135 (2015)] indicated a pressure-driven change from the antiferromagnetic to the ferrimagnetic state. Raman spectra of ${\mathrm{Sr}}₂{\mathrm{FeOsO}}₆$ collected at room temperature and up to 34 GPa are in agreement with previous x-ray-diffraction studies which verified that the tetragonal ambient pressure crystal structure is retained at high pressure. The M\"ossbauer investigations show that the Fe ions remain in the +3 high-spin $({{t}_2g}³{e}_g²)$ state over the whole pressure range. The magnetic ordering is strongly stabilized by high pressure and for $p>30$ GPa the ordering temperature is increased to above room temperature. Broad magnetic hyperfine patterns as well as coexistence of magnetic and paramagnetic signals at high pressures indicate the formation of inhomogeneous magnetic states. The shapes of applied-field M\"ossbauer spectra at low temperature are similar at 2 and 33 GPa and do not allow resolution of distinct antiferromagnetic and ferrimagnetic components. Pressure-induced spin canting in a basically antiferromagnetic spin structure or magnetic phase separation involving a pressure-induced minority ferrimagnetic phase may be the origin for the ferromagnetic XMCD signal.