Connection of thermopower and giant magnetothermopower with magnetic and structural heterogeneity in Sm0.55Sr0.45MnO3 manganite

Abstract

It was first shown that the thermoelectric power in a magnetic semiconductor Sm0.55Sr0.45MnO3 is caused by ferromagnetic (FM) ferron-type nanoclusters and antiferromagnetic (AFM) CE-type nanoclusters with charge-orbital (CO) ordering. The presence of these clusters is caused by heavily Sr doping of a SmMnO3 compound, which replaced Sm ions. Thermoelectric power S and magnetothermopower ΔS/S has been studied in three samples: ceramics and two single-crystal samples. One of single-crystals was cooled in the air, another one was cooled in an oxygen atmosphere. The annealing in oxygen closes oxygen vacancies and, thus, increases the fraction of the CE-type AFM phase with the CO ordering which displaces the oxygen ions. S(T) curves of single-crystal sample cooled in the air and ceramic sample have a sharp increase starting from Curie temperature (TC) region and a slow decline till 325K. At the same time {ΔS/S}(T) curves have a sharp minimum near TC=126K, reaching the giant absolute value of 87% in the magnetic field H=14.17kOe. This means that thermopower almost vanish with thermal destruction of FM clusters, i.e. thermopower is caused by these clusters in which crystal lattice is compressed. Oxygen cooled single-crystal's S(T) curves have a broad maximum near 270K, including Neel temperature of CE-type clusters TNCE=240K. {ΔS/S}(T) curve has a sharp minimum at the TNCE reaching the absolute value of 50% in H=13.2kOe. The decrease of S is caused by destruction of CO order displacing oxygen ions in CE-type AFM clusters. Thus changed crystal lattice in nanoclusters of ferron type or AFM of CE-type makes the main contribution in thermopower in these three samples.