Preparation of New Composite Magnetocaloric Compounds by Modifying the Annealing Temperature of La_0.8Ca_0.2−x □ _x MnO₃ Perovskite

Abstract

In this paper, a new method to broaden the range of the magnetic refrigeration temperature by changing the annealing temperature was proposed. Series of La_0.8Ca_0.2−x □ _x MnO₃(0.00≤x≤0.20) compounds were prepared by solid-state reaction and annealed firstly at a temperature of 1473 K (S1) and then at 1073 K (S2). Morphologic and structural studies have revealed that the decrease of the annealing temperature modifies the grain size and the structure of these compounds. The magnetic measurements have shown that the annealing at low temperature (1073 K) increases the magnetization and enhances the one-electron bandwidth, which induces an increase of the Curie temperature for 183 K (S1) to 241 K (S2) for the x=0.00 sample. The magnetocaloric investigation has exposed that the decrease of the annealing temperature induces a change from a second-order magnetic phase transition to first-order one for S1 and S2 compounds, respectively. Also, we have found that the Relative Cooling Power (RCP) factor remains almost constant as a function of calcium-deficiency concentration (x) and the annealing temperature. Finally, we have deduced that we can use composite magnetocaloric compounds, exploiting a mixing of the same compounds annealed at two different temperatures (1473 K; S1) and (1073 K; S2), for refrigeration over the temperature range 175–264 K.