Correlation between thermoelectric properties and magnetic phases in the charge-ordered Pr_0.5Sr_0.5-xCa_xMnO₃

We have performed measurements of thermoelectric power (TEP), electrical resistivity, DC magnetization and X-ray powder diffraction in polycrystalline samples of the charge-ordered manganite Pr_0.5Sr_0.5-xCa_xMnO₃ for x = 0, 0.1 and 0.2. All the studied samples revealed three magnetic phases: paramagnetic at high temperatures, ferromagnetic below T_C down to T_N, below which the antiferromagnetic phase is established. At the paramagnetic-ferromagnetic transition, the TEP presents a clear break, and the three samples show metallic resistivity below T_C. On the other hand, below T_N all the samples turn to an insulating-like resistivity, but the TEP shows striking differences between the x = 0 sample and the other two. These differences are compatible with the different lattice and magnetic structures of the samples. These results support the idea that below T_N a 2D-metal is present and no charge-order accompanies the A-type antiferromagnetic phase.