Chemical pressure controlled colossal magnetoresistance effects in Lao_0.6(Sr_0.4-xCa_x)MnO₃

The effects of structural and paramagnetic to ferromagnetic transition with the isovalent chemical substitution of smaller Ca²⁺ into the bigger Sr²⁺ sites in Lao_0.6(Sr_0.4-x) MnO₃(0≤x≤0.4) are investigated. With increasing x, a change from the rhombohedral cell (space group: R-3c) to an orthorhombic cell (space group: Pbnm) is observed. For x≥0.3, the structural transformation leads to an increase of the Mn-O(1) bond length and the bending of the Mn-O-Mn bond along the c-axis which corresponds to the Jahn-Teller distortion of the distoned MnO₆ octahedra in the orthorhombic cell. Such effects give rise to a decrease in Tc (the temperature of the paramagnetic to ferromagnetic transition) with increasing Ca content.