A study of the magnetic superexchange interactions in the solid-solution series CaxSr1?xRuO3 by ruthenium-99 Mo¨ssbauer spectroscopy

Ruthenium-99 Mo¨ssbauer spectroscopy has been used to examine magnetic superexchange interactions in the distorted perovskite solid-solutions Ca_xSr_1?xRuO₃ (x = 0.1, 0.2, 0.3, 0.4, and 0.5). The end members of this series also have a slightly distorted perovskite structure but CaRuO₃ is Curie-Weiss paramagnetic, with only a single-line Mo¨ssbauer spectrum, whereas SrRuO₃ is ferromagnetic and shows a broad well-resolved hyperfine pattern. For x ? 0.2 a substantial proportion of the ruthenium atoms experience a magnetic flux density (hyperfine magnetic field) close to 35T, but inward collapse of the spectrum suggests that an increasing proportion of ruthenium atoms experience smaller flux densities. For samples with x ? 0.3 there is an intense central “paramagnetic” component which increases rapidly with increasing x. The observed behaviour is incompatible with a conventional localized electron structure but can be interpreted satisfactorily on a collective electron model in which the average spin moment and hence the magnetic flux density at any given ruthenium atom is proportional to the strength of the exchange interactions with the six nearest-neighbour ruthenium atoms. The results imply that the greater electron-pair acceptor strength (Lewis acidity) of Ca²⁺ compared to Sr²⁺ results in a more effective competition with ruthenium for the oxygen anion orbitals involved in the superexchange interaction. It appears that, for a ruthenium to have a coupled spin-moment, it must have at least two exchange interactions through cube faces containing at least three strontium atoms. Possible origins of the reduced magnetic moment of SrRuO₃ are discussed and it is suggested that the latter probably stems from spin-canting rather than from partial overlap of spin-up and spin-down bands.