Abstract:

Nitrogenation of La_0.5Pr_0.5Fe_11.4Si_1.6 with a particle size of 100-150 μm was performed in a high-purity N₂ atmosphere of 40 MPa. La_0.5Pr_0.5Fe_11.4Si_1.6N_0.3 nitrogenated at 480 °C exhibits an increase in the Curie temperature T _C from 187 to 195 K. Moreover, for a field change from 0 to 1.5 T, the maximum hysteresis loss at T _C is remarkably reduced from 53 to 3 J kg^-1 and a large magnetic-entropy change ΔS _m of over 10 J kg^-1 K^-1 is maintained. For a La_0.5Pr_0.5Fe_11.4Si_1.6N_1.2 sample, nitrogenated at 550 °C, the XRD pattern clearly exhibits two 1:13 phases and, accordingly, the thermal magnetic curves exhibit two transitions, at 210 and at 295 K. The two transitions lead, for a field change of 5 T, to a ΔS _m larger than 2 J kg^-1 K^-1 over a large temperature range from 200 to 310 K with a maximum value of 5.3 J kg^-1 K^-1 at 225 K. Upon further increase in the nitrogenation temperature to 650 °C, the amount of the nitrogen-poor phase strongly decreases and a large amount of α-Fe precipitates, resulting in a large reduction in the MCE.

Key words: Metals and alloys, Magnetocaloric effect, Magnetic hysteresis