Effects of Nd-doping on the magnetic properties and magnetocaloric effects (MCEs) of Nd x La 1 x Fe 11.5 Al1.5 have been investigated. Substitution of Nd leads to a weakening of the antiferromagnetic (AFM) coupling and an enhancement of the ferromagnetic (FM) coupling. This in turn results in a complex magnetic behaviour for Nd 0.2 La 0.8 Fe 11.5 Al1.5 characterized by the occurrence of two phase transitions at ～188 K (PM–AFM) and ～159 K (AFM–FM). As a result, a table-like MCE (9 J/kg·K) is found in a wide temperature range (160–185 K) for a field change of 0–5 T around the transition temperature, as evidenced by both the magnetic and calorimetric measurements. Based on the analysis of low-temperature heat capacity, it is found that the AFM–FM phase transition modifies the electron density significantly, and the major contribution to the entropy change comes from the electronic entropy change.
Microstructure dependent on silicon and formation of 1:13 phase in LaFe13-ySiyC0.2 compounds was investigated. C and Si ele-ments played different roles in assisting the formation of 1:13 phase. Si could inhibit the growth of α-Fe. The volume fraction of La-rich phase increased with the increase of Si content in the LaFe13-ySiyC0.2 ingots. When Si content was lower in LaFe13-ySiyC0.2 (y≤1.0), α-Fe was excess and grew very large in the initial annealing process. As a result, a large amount of α-Fe remained even after a long time annealing process. Carbon doping could accelerate the formation of 1:13 phase in the LaFe13-ySiyC0.2 compounds. The amount of the 1:13 phase reached ~90 vol.% in LaFe13-ySiyC0.2 (y≥1.2) after annealing at 1353 K for only 3 d. After optimized annealing, large magnetic entropy changes were ob-tained in LaFe13-ySiyC0.2 compounds (18.6 and 15 J/(kg·K) in 0-2 T field change for y=1.2, 1.4, respectively).
The powder X-ray diffraction patterns of LaFe11.5Si1.5 compounds annealed at different high temperatures from 1323 K (5 h) to 1623 K (2 h) show that a large amount of 1:13 phase begins to form in LaFe11.5Si1.5 compound annealed at 1423 K (5 h). In the temperature range from 1423 to 1523 K, α-Fe and LaFeSi phases rapidly decrease to form 1:13 phase. LaFeSi phase is rarely observed, and the most amount of 1:13 phase is obtained in the compound annealed at 1523 K (5 h). With the annealing temperature increasing to 1573 and 1623 K, LaFeSi is detected again in the LaFe11.5Si1.5 compound. According to the results of annealing at different high-temperatures, the La1-xCexFe11.5Si1.5 compounds are annealed at high temperatures of 1373 K (2 h) + 1523 K (5 h). The main phase is NaZn13-type phase, and the impurity is a small amount of α-Fe in La1-xCexFe11.5Si1.5 compounds with 0 ≤ x ≤ 0.35, and there is a large amount of Ce2Fe17 phase in La0.5Ce0.5Fe11.5Si1.5. It indicates that the substitution of cerium atoms for La in LaFe11.5Si1.5 compounds has limit. At the same time, the substitution of Ce for La has large effect on magnetocaloric properties. With increasing Ce content from x = 0 to x = 0.35, the Curie temperature decreases linearly from 196 to 168 K, the magnetic entropy change increases from 16.5 to 57.3 J·kg-1·K-1 in a low magnetic field change of 0-2 T, and the thermal hysteresis also increases from 3 K to 8 K.
