ИСТИНА

Magnetocaloric effect (MCE) or deltaT-effect was studied in La(1-x)SrxMnO3, 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 is because the magnetic two-phase ferromagnetic (F) – antiferromagnetic (AF) state presents in them. So in La(1-x)SrxMnO3 the negative contribution from AF portion of sample lowers MCE and displaces maximum on deltaT(T)-curve to the higher temperature than TC on 20-40 K. Maximum on deltaT(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 in oxygen of single-crystal, restored the Mn-O-Mn broken connections, increase the volume of clusters with CE-type of AF order, results to that maximum on deltaT(T) curve is disposed at Neel temperature TN this phase (243 K). Magnetic field, applied to sample at deltaT-measurement, transfers AF clusters in F state and in TN both are destroyed. 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 dltaT(T) has the wide maximum near TC and the sharp minimum near TN with the small deltaT-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.