Increase of ferromagnetic ordering temperature by the minority-band double-exchange interaction in SrRu_1−xCr_xO₃

Structural, magnetic, and resistive properties of the chromium substituted SrRu_1−xCr_xO₃ perovskites with increased ferromagnetic transition temperature from 163 to 188 K were studied for polycrystalline samples synthesized in air and at high pressure. The Cr solubility limit found at x=0.15 in air is controlled by geometrical constraints of the tolerance factor and can be extended to higher values through high-pressure synthesis at 3 GPa. A linear decrease of the lattice parameters and bond lengths and an increase of the bond angles with x are consistent with the substitution of smaller Cr^4+∕3+ for Ru^4+∕5+. Similar decreases of the magnetic susceptibility χ_m in the paramagnetic region above T_c and the high-field magnetization at 5 K with increasing x also indicate the presence of the Cr^4+∕3+ and Ru^4+∕5+ ions. This chemical substitution creates a possible Ru^4+∕5+(d^4∕3)‐O²⁻‐Cr^4+∕3+(d^2∕3) minority band double-exchange interaction which involves the Cr³⁺ and Cr⁴⁺ in magnetic ordering that enhances the ordering temperature. A reduced coercive field determined from the magnetization curves and a minimum of the resistivity point to decreased disorder for slightly substituted compositions x∼0.025.