Correlation between magnetism and crystal symmetry in the mixed valence system of Sr1−xCaxRu0.5Mn0.5O3

The crystallographic, magnetic and transport properties of Sr1−x Ca x Ru0.5Mn0.5O3 were studied to reveal the effects of crystal symmetry on the magnetism and the transport properties of antiferromagnetic SrRu1−y Mn y O3 (y > 0.2). The Ca-content dependences of the lattice constants and volume suggest that the mixed valence state of Mn3+, Mn4+, Ru4+, and Ru5+ ions in SrRu0.5Mn0.5O3 is held for Ca substitution. However, a structural transformation from tetragonal to orthorhombic symmetry occurs at a Ca content of x ∼ 0.2, and a ferromagnetic state appears simultaneously. The Weiss temperature of all samples has a positive value regardless of the magnetic state. The resistivity decreases as the Ca content increases, and the high temperature part of the resistivity could be fitted by using the variable range hopping (VRH) model. The localization length increases as soon as ferromagnetism appears. The Curie temperature seems to depend on the distance between transition metal ions. The antiferromagnetic state was explained by using the competition between a ferromagnetic interaction along the c-axis and an antiferromagnetic interaction in the ab-plane, and the orthorhombic ferromagnetic state was explained by using a three-dimensional ferromagnetic interaction between Mn ions.