Ferromagnetism induced by chromium substitution in theCaRuO3perovskite

Abstract

Chromium has been substituted for ruthenium in $\mathrm{Ca}\mathrm{Ru}{\mathrm{O}}_{3}$ up to 35%. It is found that chromium induces a ferromagnetic contribution showing a maximum fraction for $x=0.15$ in $\mathrm{Ca}{\mathrm{Ru}}_{0.85}{\mathrm{Cr}}_{0.15}{\mathrm{O}}_{3}$ with a Curie temperature $({T}_{\mathrm{C}})$ of $\ensuremath{\sim}115\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. This change of magnetic ground state goes with an electronic localization, the resistivity at $5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ increasing by more than 6 orders of magnitude as $x$ increases from $x=0.06$ to $x=0.35$. Concomitantly, a negative magnetoresistance is induced by the chromium substitution. In order to explain the chromium effect, it is suggested that each trivalent chromium couples antiferromagnetically to its ${\mathrm{Ru}}^{4+∕5+}$ nearest neighbors. When surrounded by six ruthenium cations, this creates ``ferrimagnetic'' polarons in which ${T}_{\mathrm{C}}$ is governed by the ${\mathrm{Ru}}^{4+∕5+}--\mathrm{O}--{\mathrm{Cr}}^{3+}$ antiferromagnetic exchange $({T}_{\mathrm{C}}\ensuremath{\sim}115\phantom{\rule{0.3em}{0ex}}\mathrm{K})$. However, as the ${\mathrm{Cr}}^{3+}$ concentration increases, the ${\mathrm{Cr}}^{3+}\text{\ensuremath{-}}\mathrm{O}\text{\ensuremath{-}}{\mathrm{Cr}}^{3+}$ antiferromagnetic exchange (energy) $({T}_{\mathrm{N}}\ensuremath{\sim}290\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ creates an antagonistic effect. This explains the existence of an optimum chromium content at $x=0.15$.