Investigation Of The Magnetic Entropy Change In Rear-Earthtransition Metal La-Fe-Si Compounds

Room magnetic refrigeration is based on the magnetocaloric effect (MCE). Compared with the conventional gas-compression refrigeration, it has some advantages, such as energy saving and environmental protection. In order to obtain a high performance of magnetic refrigeration, a great deal of prototype materials and intermetallic compounds with large MCE were studied. Among the numerous potentially useful magnetic refrigeration materials, the La-Fe-Si compounds with cubic NaZn13-type structure are more promising due to their excellent soft ferromagnetism, high magnetization, and low cost. In this paper, the magnetism, phase transition and magnetic entropy change of LaFe11.7Si1.3,LaFe11.6Si1.4 and LaFe11.5Si1.5 compounds are well investigated by the means of X-ray power differaction (XRD) and Physical Property Measurement System (PPMS). The magnetic mechanisms of these compounds were also analysed. The major results are summarized as follows:XRD measurement results confirmed LaFe11.7Si1.3,LaFe11.6Si1.4 and LaFe11.5Si1.5 compounds to be of single phase of cubic NaZn13-type structure. The magnetic measurement results suggested that the present compounds exhibit a thermal induced first-order phase transition from ferromagnetic state to paramagnetic state around TC and a field induced itinerant-metamagnetic (IEM) transition from the paramagnetic state to ferromagnetic state above TC. The thermal induced first-order magnetic phase transition and the field induced IEM transition are ultimaterly related with the double minima of the magnetic-free energy structure.The shapes of magnetic entropy change vs temperature of LaFe11.7Si1.3,LaFe11.6Si1.4 and LaFe11.5Si1.5 compounds consist a pike around TC and a plateau above TC.. It should be noted that the shape and magnitude of the spike is independent of the applied field from 1 to 9T while the plateau broadens asymmetrically with the increasing of the applied magnetic field. The thermal induced magnetic transition contributes mainly to the presentation of the pike while the field induced IEM transition leads to the plateau part above TC. The Curie temperature is increased, while the magnetization and the maximum magnetic entropy changes are decreased with reducing Fe concentration. The critical field needed to induce the IEM transition is linearly with increasing temperature, which reveals that it is more difficult to induce an IEM transition in higher temperatures. The magnetic refrigeration capacity is proportional to the applied magnetic field change, which means using a higher magnetic field can get a larger refrigeration capacity. According to the Landau-Ginzburg metamagnetism theory, the condition for the appearance of the IEM transition above TC is discussed: the Landau coefficients a, b, c should be satisfied with a>0,c>0,b<0,and 3/16