Evolution of cation and spin orders in the double–double-double perovskite series CaxMn2−xFeReO6

Abstract

Variations of perovskite superstructure type and magnetic properties across the high-pressure ${\mathrm{Ca}}_{x}{\mathrm{Mn}}_{2\ensuremath{-}x}{\mathrm{FeReO}}_{6}$ series have been investigated by powder neutron-diffraction, x-ray magnetic circular dichroism, and magnetization measurements. Monoclinic $P{2}_{1}/n\phantom{\rule{0.16em}{0ex}}{A}_{2}B{B}^{\ensuremath{'}}{\mathrm{O}}_{6}$ double perovskite solid solutions with rocksalt-type ordering of $B/{B}^{\ensuremath{'}}\text{\ensuremath{-}}\mathrm{Fe}/\mathrm{Re}$ cations but no $A$-cation order are discovered to exist only close to the end members $0lxl0.17$ and $1.73lxl2$. Compositions in the range $0.74lxl\ensuremath{\sim}1.1$ adopt a tetragonal $P{4}_{2}/n\phantom{\rule{0.16em}{0ex}}A{A}^{\ensuremath{'}}B{B}^{\ensuremath{'}}{\mathrm{O}}_{6}$ double-double perovskite phase based on ${\mathrm{CaMnFeReO}}_{6}$, which has both columnar $A/{A}^{\ensuremath{'}}\text{\ensuremath{-}}\mathrm{Ca}/\mathrm{Mn}$ and rocksalt $B/{B}^{\ensuremath{'}}\text{\ensuremath{-}}\mathrm{Fe}/\mathrm{Re}$ cation orders. Two-phase coexistence of double and double-double perovskites occurs over wide regions between these limits. All samples are ferrimagnetic with Curie temperatures near 500 K. Low-temperature Mn spin order is also observed in the Mn-rich double perovskites and the double-double perovskites. Spin reorientation transitions observed across all of the double perovskites are shown to result from $5{d}^{2}\phantom{\rule{4pt}{0ex}}{\mathrm{Re}}^{5+}$ orbital ordering, but this does not occur in the double-double perovskites. The Ca-rich double perovskites show significant magnetic coercivity at low temperatures.