Itinerant-to-localized electron transition in CaRu1-xSnxO3 and SrRu1-xPbxO3.

Abstract

SrRu${\mathrm{O}}_{3}$ and CaRu${\mathrm{O}}_{3}$ are perovskites with an orthorhombic distortion and are believed to have a narrow itinerant band. SrRu${\mathrm{O}}_{3}$ is the only known ferromagnetic conductor in $4d$ transition-metal oxides, while CaRu${\mathrm{O}}_{3}$ is a paramagnetic conductor. Substituting Pb and Sn for Ru in SrRu${\mathrm{O}}_{3}$ and CaRu${\mathrm{O}}_{3}$, respectively, preserves the crystal structure, while removing electrons from the $4d$ band, resulting in an itinerant-to-localized electron transition and also substantially altering the magnetic coupling, giving rise to a new magnetically ordered phase. In particular, it appears that the paramagnetic CaRu${\mathrm{O}}_{3}$ is on the verge of magnetic ordering and readily evolves into a magnetically ordered phase when slightly doped with Sn (4-10%). Such an ordered phase is not stable and disappears when Sn concentration is higher than 10%. On the other hand, the Curie temperature (${T}_{C}=165$ K) of SrRu${\mathrm{O}}_{3}$ seems to be sensitive to Pb doping, and increases to 210 K when the Pb concentration in the compound is higher than 30%. For both SrRu${\mathrm{O}}_{3}$ and CaRu${\mathrm{O}}_{3}$ the transition to the magnetic state is accompanied by an itinerant-to-localized electron transition due to the $4d$-electron depletion. These results are based on measurements of magnetic susceptibility and electrical resistivity. This study reveals striking observations which underscore the importance of the interplay between the conduction band and (anti)ferromagnetism.