Crystallographic and magnetic properties of the mixed-valence oxides Ca Ru 1 − x Mn x O 3

The crystallographic and magnetic properties of $\mathrm{Ca}{\mathrm{Ru}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}{\mathrm{O}}_{3}$ $(0\ensuremath{\le}x\ensuremath{\le}1.0)$ were investigated in detail by x-ray powder diffraction, magnetization, and magnetic Compton scattering measurements. The lattice parameters show considerable deviation from Vegard's law. Ferromagnetism appears at a relatively large Mn concentration $(x\ensuremath{\ge}0.2)$, and the magnetization and the Curie temperature have a maximum at a Mn content near $x=0.7$. The magnetic Compton scattering measurement revealed that Mn makes a dominant contribution to the magnetization and the Mn moment is antiparallel to the Ru moment, which is induced by Mn doping. The anomalous change in the unit cell volume and the occurrence of ferromagnetism were discussed on the basis of the mixed-valence model of ${\mathrm{Mn}}^{3+}$, ${\mathrm{Mn}}^{4+}$, ${\mathrm{Ru}}^{4+}$, and ${\mathrm{Ru}}^{5+}$ ions. The Mn-composition dependence of the spontaneous magnetization was explained semiquantitatively assuming (1) ferromagnetic coupling between ${\mathrm{Mn}}^{3+}$ and ${\mathrm{Mn}}^{4+}$ ions, (2) antiferromagnetic coupling between ${\mathrm{Ru}}^{5+}$ and Mn ions, and (3) the theoretical spin moments of ${\mathrm{Mn}}^{3+}$, ${\mathrm{Mn}}^{4+}$, and ${\mathrm{Ru}}^{5+}$. The ferromagnetic interaction between ${\mathrm{Mn}}^{3+}$ and ${\mathrm{Mn}}^{4+}$ ions seems to make a dominant contribution to the Curie temperature. The $\mathrm{Ca}{\mathrm{Ru}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}{\mathrm{O}}_{3}$ system is considered to be a ferrimagnet induced through competition between the ferromagnetic interaction between Mn ions and the antiferromagnetic interaction between ${\mathrm{Ru}}^{5+}$ and Mn ions.