Peculiarities of Magnetocaloric Effect in Manganites Connected with Magnetic Heterogeneous State

Abstract

Magnetocaloric effect (MCE) or T-effect was studied in La1-xSrxMnO3, Sm0.55Sr0.45MnO3 and PrBaMn2O6 manganites. It has been found that the maximum value of MCE, measured by direct method, is far less than obtainable by computation from the change of the magnetic entropy in Curie temperature TC. This phenomenon is explained by presence in them of the magnetic two-phase ferromagnetic (F) – antiferromagnetic (AF) state. So in La1-xSrxMnO3 the negative contribution from AF portion of sample lowers MCE and displacesmaximum on T(T)-curve to the higher temperature than TC on 20-40 K. Maximum on T(T) curve of Sm0.55Sr0.45MnO3 is disposed near TC=134 K in the cooling in the air single-crystal and ceramic samples. The cooling inoxygen of single-crystal,restored the Mn-O-Mn broken connections, increases the volume of clusters with CE-type of AF order, results to that maximum on T(T) curve is disposed at Neel temperature TN of this phase (243 K). Magnetic field, applied to sample during T-measurement, transforms AF clusters into F state and theboth types of clusters decompose at TN. The PrBaMn2O6 manganite has two phase transactions: paramagnetic–F at TC=295 K and F–AF at TN=231 K. Phase with spontaneous magnetization obtains the understated magnetic moment. Curve T(T) has the wide maximum near TC and the sharp minimum near TN with the small T-values in both extremes: 0,13 K and - 0,2 K respectively. This is connected with presence of AF interactions in F phase and F interactions in AF phase.