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 La_0.8−x□_xNa_0.2MnO₃ (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 Rc 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 (T_C). By analyzing the temperature and field dependence of magnetization, we find that the La_0.8−x□_xNa_0.2MnO₃ family exhibits a second order magnetic phase transition whose critical temperature is tunable near room temperature. A considerable magnetic entropy change is observed in La_0.8−x□_xNa_0.2MnO₃ near room temperature. The maximum value of the relative cooling power (RCP) is found to be ∼268 J kg⁻¹ in La_0.65□_0.15Na_0.2MnO₃ 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 La_0.8−x□_xNa_0.2MnO₃ and its composite materials could be expected to have effective applications for magnetic refrigeration near room temperature.