Phase relation of LaFe _11·6 Si _1·4 compounds annealed at different high-temperature and the magnetic property of LaFe _{11·6– x} Co _x Si _1·4 compounds

Abstract

LaFe _11·6 Si _1·4 compounds are annealed at different high temperatures from 1323 to 1623 K. The powder X-ray diffraction patterns show that large amount of NaZn₁₃-type phase begins to be observed in LaFe _11·6 Si _1·4 compound after being annealed at 1423 K for 5 h. In the temperature range from 1423 to 1523 K, the $\boldsymbol{\alpha}$-Fe and LaFeSi phases rapidly decrease to form 1:13 phase. LaFeSi phase is rarely observed in the XRD pattern in the LaFe _11·6 Si _1·4 compound annealed at 1523 K (5 h). With annealing temperature increasing to 1573 K and 1673 K, La ₅ Si ₃ phase is detected, and there is a certain amount of LaFeSi phase when the annealing temperature is 1673 K. The amount of impurity phases in the LaFe _11·6 Si _1·4 compound annealed by the two-stage annealing consisting of high temperature ($\boldsymbol{>}$1523 K) and 1523 K is larger than that of the single stage annealing at 1523 K under the same time. According to the results of different high-temperature annealing, LaFe$_{{\bf 11{\cdot}6}-\boldsymbol{x}}$Co$_{\boldsymbol{x}}$Si _1·4 ($\boldsymbol{0{\cdot}1} \boldsymbol{\le} \boldsymbol{x} \boldsymbol{\le} \boldsymbol{0{\cdot}8}$) compounds are annealed at 1523 K (5 h). The main phase is NaZn13-type phase, and the impurity phase is a small amount of $\boldsymbol{\alpha}$-Fe in LaFe$_{{\bf 11{\cdot}6}-\boldsymbol{x}}$Co$_{\boldsymbol{x}}$Si _1·4 compounds. With increase in Co content from $\boldsymbol{x} \boldsymbol{=} \boldsymbol{0{\cdot}1}$ to $\boldsymbol{0{\cdot}8}$, the Curie temperature $\boldsymbol{T}_{\!\boldsymbol{\rm C}}$, goes up from 207 to 285 K. The introduction of Co element weakens the itinerant electron metamagnetic transition, and also results in the change of magnetic transition type from first to second order at about $\boldsymbol{x = 0{\cdot}5}$. The magnetic entropy change decreases from 19·94 to 4·57 J /kg K with increasing Co concentration at a low magnetic field of 0–2 T. But the magnetic hysteresis loss around $\boldsymbol{T}_{\!\boldsymbol{\rm C}}$ reduces remarkably from 26·2 J /kg for $\boldsymbol{x = 0{\cdot}1}$ to 0 J /kg for $\boldsymbol{x} \boldsymbol{=} \bf 0{\cdot}8$.