Magnetocaloric effect in manganites

Abstract

The magnetocaloric effect (MCE) in La1 − x Sr x MnO3, Sm0.55Sr0.45MnO3, and PrBaMn2O6 compounds is studied. The maximum values of MCE (ΔT max) determined by a direct method in the second and third compositions and in La0.9Sr0.1MnO3 are found to be much lower than those calculated from the change of the magnetic part of entropy in the Curie temperature (T C) and the Neel temperature (T N) range. The negative contribution of the antiferromagnetic (AFM) part of a sample in the La1 − x Sr x MnO3 system at 0.1 ≤ x ≤ 0.3 decreases ΔT max and changes the ΔT(T) curve shape, shifting its maximum 20–40 K above T C. Lower values of ΔT max are detected in the range T C = 130−142 K in polycrystalline and single-crystal Sm0.55Sr0.45MnO3 samples cooled in air. If such samples were cooled in an oxygen atmosphere (which restores broken Mn-O-Mn bonds and, thus, increases the volume of CE-type AFM clusters), the maximum in the temperature dependence of MCE is located at T N (243 K) for CE-type AFM clusters. A magnetic field applied to a sample during the MCE measurements transforms these clusters into a ferromagnetic (FM) state, and both types of clusters decompose at T = T N. The PrBaMn2O6 composition undergoes an AFM-FM transition at 231 K, and the temperature dependence of its MCE has a sharp minimum at T = 234 K, where MCE is negative, and a broad maximum covering T C. The absolute values of MCE at both extrema are several times lower than those calculated from the change in the magnetic entropy. These phenomena are explained by the presence of a magnetically heterogeneous FM-AFM state in these manganites.