Crystal structures and magnetic order ofLa0.5+δA0.5−δMn0.5+ϵRu0.5−ϵO3(A=Ca,Sr,Ba): Possible orbital glass ferromagnetic state

Abstract

The crystallographic and magnetic properties of ${\mathrm{La}}_{0.5+\ensuremath{\delta}}{A}_{0.5\ensuremath{-}\ensuremath{\delta}}{\mathrm{Mn}}_{0.5+ϵ}{\mathrm{Ru}}_{0.5\ensuremath{-}ϵ}{\mathrm{O}}_{3}\phantom{\rule{0.3em}{0ex}}(A=\mathrm{Ca},\mathrm{Sr},\mathrm{Ba})$ were investigated by means of neutron powder diffraction. All studied samples show the orthorhombic perovskite crystal structure, space group $Pnma$, with regular $(\mathrm{Mn},\mathrm{Ru}){\mathrm{O}}_{6}$ octahedra and no chemical ordering of the ${\mathrm{Mn}}^{3+}$ and ${\mathrm{Ru}}^{4+}$ ions. Ferromagnetic spin structures were observed below ${T}_{c}\ensuremath{\sim}200--250\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, with an average ordered moment of $\ensuremath{\sim}1.8--2.0{\ensuremath{\mu}}_{B}∕(\mathrm{Mn},\mathrm{Ru})$. The observation of long-range ferromagnetism and the absence of orbital ordering are rationalized in terms of a strong $\mathrm{Mn}\mathrm{Ru}$ hybridization, which may freeze the orbital degree of freedom and broaden the ${e}_{g}$ valence band, leading to an orbital glass state with carrier-mediated ferromagnetism.