A giant magnetocaloric effect with a tunable temperature transition close to room temperature in Na-deficient La0.8Na0.2−x□xMnO3 manganites

Abstract

A giant magnetocaloric effect induced by sodium-deficiency in a La_0.8Na_0.2−x□_xMnO₃ (0.00 ≤ x ≤ 0.15) polycrystalline sample is reported in the present research work. The ability to tune the temperature transition close to room temperature is revealed to be possible by changing the sodium-deficiency content as well. All samples were synthesized using solid–solid reaction. X-ray diffraction and magnetization measurements were performed to investigate their crystallographic structure and magnetocaloric properties. A rhombohedral structure with the Rc space group is deduced from pattern diffraction refinement of all samples. A second-order magnetic phase transition from the ferromagnetic to the paramagnetic state at the Curie temperature (T_c) is observed. Such a T_c decreased from 335 to 260 K when the sodium deficiency rate increased. Besides, the magnetic entropy change and the related Relative Cooling Power (RCP) values, sensitive to sodium-deficiency, were estimated. The magnetic entropy change ranges increased from 2.38 J kg⁻¹ K⁻¹ to 3.48 J kg⁻¹ K⁻¹ under a magnetic field of 2 T, whereas the RCP varied from 210 to 235 J kg⁻¹ under an applied magnetic field of 5 T, when x increased from 0.00 to 0.15. The comparison of the values reported in the reference Gd material underlines that the proposed oxide material has substantial advantages for magnetic refrigeration.