Abstract
Thermoelectric power(TEP) mesurement has been carried out on the series of Pr<sub>0.5</sub>Sr<sub>0.5</sub>Mn<sub>1-x</sub>Ru<sub>x</sub>O<sub>3</sub> (0.0 ≤ x ≤ 0.1) under zero and 6 tesla magnetic fields. In Pr<sub>0.5</sub>Sr<sub>0.5</sub>MnO<sub>3</sub>, a large negative peak of TEP was observed below the ferromagnetic(FM) to antiferromagnetic(AFM) transition temperature, T<sub>N</sub> ~ 165K. Under H = 6 tesla, the magnitude of the negative TEP peak is slightly reduced with decrease of T<sub>N</sub> down to 105K. For Mn-site doped samples with Ru, however, the negative TEP peak is drasically suppressed by only 2% of Ru doping and completely disappears with further Ru doping. This indicates that the FM metallic state is induced more strongly by Ru substitution than by the magnetic field. In the paramagnetic(PM) regime above the Curie temperature, T<sub>C</sub>, it was found that TEP as well as resistivity for Pr<sub>0.5</sub>Sr<sub>0.5</sub>Mn<sub>1-x</sub>Ru<sub>x</sub>O<sub>3</sub> can be described by the polaronic transport mechanism. The systematic changes of TEP in the PM regime with variation of Ru concentration is discussed in relation to the effects of Ru doping at Mn sites which extends the FM phase at low temperatures and increases T<sub>C</sub> similarly to the application of magnetic field.
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