Role of lanthanum vacancy on the structural, magnetic and magnetocaloric properties in the lacunar perovskite manganites La0.8-: X□xNa0.2MnO3 (0 ≤ x ≤ 0.15)

Abstract

Lacunar La0.8-x□xNa0.2MnO3 (0 ≤ x ≤ 0.15) ceramics where □ is a lanthanum deficiency were synthesized via a sol-gel method. Magnetic phase transitions and magnetocaloric effects of the ceramics have been systematically studied. Structural studies using X-ray diffraction show that all compounds crystallize in the rhombohedral structure with an R3c space group. The lanthanum deficiency does not modify the crystal structure of the pristine compound (x = 0) but results in a slight change of the lattice parameters. The unit cell volume and Mn-O-Mn angle decrease with increasing deficiency content whereas the Mn-O bond length increases. This situation weakens the double-exchange interaction and hence reduces the Curie temperature (TC). By analyzing the temperature and field dependence of magnetization, we find that the La0.8-x□xNa0.2MnO3 family exhibits a second order magnetic phase transition whose critical temperature is tunable near room temperature. A considerable magnetic entropy change is observed in La0.8-x□xNa0.2MnO3 near room temperature. The maximum value of the relative cooling power (RCP) is found to be ∼268 J kg-1 in La0.65□0.15Na0.2MnO3 at 5 T. This value of RCP is about ∼65.3% of that obtained in gadolinium metal, known as one of the most important materials for magnetic refrigeration, at the same magnetic field change of 5 T. With increasing vacancy our manganites exhibit a stable magnetocaloric effect in a wide temperature range. The results suggest that La0.8-x□xNa0.2MnO3 and its composite materials could be expected to have effective applications for magnetic refrigeration near room temperature.