Influence of the magnetic and structural heterogeneities on the thermopower, magnetothermpower, and spontaneous generation of electric voltage in the manganite Sm_0.55Sr_0.45MnO₃

Abstract

The thermopower α and magnetothermopower Δα/α have been studied on single-crystal Sm_0.55Sr_0.45MnO₃ samples consisting of three types of magnetic clusters (ferromagnetic clusters with the Curie temperature T _C = 134 K, the A-type antiferromagnetic clusters with the Néel temperature T _NA ≥ T _C, and the CE-type antiferromagnetic clusters with T _NCE = 240 K). The temperature dependences of α and Δα/α have extremes in the vicinity of T _NCE, namely, a wide maximum in curve α(T) and a sharp minimum in curve{Δα/α}(T). The negative magnetothermopower in the minimum has a giant value of 50% in the magnetic field of 13.2 kOe. The thermopower is shown to be mainly due to the existence of the CE-type antiferromagnetic clusters, in which there is a charge-orbital ordering that displaces the oxygen atoms. The changed crystal lattice inside these clusters changes the value of the thermopower inside them. This thermopower influences the voltage drop across the sample during measuring the thermopower and, thus, the effective value of α of the whole sample. The application of a magnetic field near T _NCE accelerating the destruction of the CE-type antiferromagnetic order causes the sharp decrease in the thermopower of the whole sample. This implies that the CE-type antiferromagnetic clusters with the charge-orbital ordering make main contribution to the thermopower of the whole sample.