Abstract
The effects of Pr, Nd and Sm partial substitution for La on the magnetic properties and magnetocaloric effect near the critical phase transition temperatures of La1.4Ca1.6Mn2O7 are analyzed. Polycrystalline powders of La1.2R0.2Ca1.6Mn2O7 double layered manganites (R=La, Pr, Nd, Sm) with grain sizes of 20–30nm were successfully synthesized by a sol–gel method. The substitution of Pr, Nd and Sm for La ions increases the inter-layer lattice distortion, leading to a decrease of the exchange interaction strength. The magnetic measurements evidence a spin-glass-like behavior at low temperatures for all samples, which may be due to frustration of random competing ferromagnetic and antiferromagnetic interactions together with the anisotropy originating from the layered structure. The Mn 3s XPS spectra show an exchange splitting around 4.75eV from which the Mn formal valence is estimated to be around 3.65+, in excellent agreement with the magnetic data obtained at temperatures above the room temperature. A moderate magnetocaloric effect is found for all samples, with the maximum entropy change located at temperatures near the magnetic transition ones. The high RCP(S) values together with the broadened magnetic entropy curves and low magnetic hysteresis suggest the possibility to use these materials for magnetic refrigeration devices.
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