The system study of 1:13 phase formation, the magnetic transition adjustment, and magnetocaloric property in La(Fe,Co)13−xSix alloys

The effects of Co on the formation of NaZn13-type phase in as-cast and annealed LaFe16−xCoxSi1.4 and LaFe11.6Si1.4−xCox alloys have been investigated systematically by XRD, SEM, and EDS, respectively. In LaFe11.6Si1.4−xCox alloys, the introduction of Co will hamper the formation of 1:13 and LaFeSi phases, and help the formation of αof (Co, Si) solid solution, so there is almost only α-Fe(Co, Si) solid solution when x reaches 0.7 in as-cast and annealed LaFe11.6Si1.4−xCox alloys. Although the amounts of 1:13 phase increases when x reaches 0.7 in as-cast LaFe16−xCoxSi1.4 alloys, there is a small amount of α-Fe in LaFe11.6−xCoxSi1.4 alloys annealed at 1523 K (5 h), which indicates that the annealing time for obtaining a 1:13 single-phase cannot be shortened in our high-temperature and short-time annealing. The studies on the magnetic properties show that the Curie temperature TC goes up from 207 K to 285 K with increase of Co content from x=0.1 to 0.8. The introduction of Co element weakens the first order magnetic phase transition, which results in the change of magnetic transition type from first to second order at about x=0.3-0.5. At the same time, it has effects on the phase transition temperature interval and magnetic filed interval, and the changing rate of magnetic entropy change dependence on the Co content in LaFe16−xCoxSi1.4 alloys. The maximum entropy values of LaFe11.6−xCoxSi1.4 alloys decrease with the increase of Co content, but the relative cooling power does not decrease, the reason of which is that the phase transition temperature interval increases and the first order phase transition character decreases, and the effective refrigeration temperature range becomes big, which is useful to the application of magnetic refrigeration material.