Examining the oxygen isotope and magnetic field effect on phase separation in Sm_0.5Sr_0.5MnO₃

Magnetic field (MF) dependence of the phase separation (PS) in the manganites Sm0.5 Sr0.5 MnO3 with O 16 and O 18 was studied using high-resolution high-energy x-ray powder diffraction in the temperature range from 4.2 to 240 K and with the MF up to 6 T. Although the two compounds have an identical structure [a paramagnetic (PM) phase with the Pbnm symmetry] at room temperature, they show a significant difference in the PS region below Tps ∼110 K, where the O 16 sample has a structural inhomogeneity in the form of two coexisting ferromagnetic (FM) and antiferromagnetic (AF) phases, while the O 18 -sample phase separates into mixed AF and PM phases. The O 16 → O 18 isotope substitution appears to prevent the formation of the FM phase at low temperature. The application of a magnetic field significantly enhances the FM phase by converting other phases, leading to a homogeneous structure. The effects on the microscopic structure of the oxygen isotope substitution and external magnetic field play the key role in the physics behind the colossal magnetoresistance effect. © 2006 American Institute of Physics.