Effects of La substitution on the crystal structure and magnetization of MM-Fe-B alloy (MM = La, Ce, Pr, Nd)

Permanent magnet materials based on the MM2Fe14B (MM = Misch-metal) are widely noted. However, as a room temperature paramagnetic phase, the existence of CeFe2 phase in MM2Fe14B alloy is unavoidable. In order to eliminate the CeFe2 phase, the La substitution on MM in MM2Fe14B alloys was described. Alloys with nominal composition of (LaxMM1−x)33Fe66B (x = 0, 0.025, 0.05, 0.075, 0.1, 0.15) were prepared, and the crystal structure and magnetization of the (LaxMM1−x)33Fe66B alloys were investigated. XRD results reveal that the lattice parameter a, c, unit-cell volume V, and c/a ratio of (LaxMM1−x)33Fe66B alloys increase linearly with the La increase, indicating that La atoms enter into the main phase. Key to this study is the finding that CeFe2 phase disappears with the La content above 0.075. It is therefore concluded that La substitution could inhibit formation of CeFe2 phase. This is consistent with the observation from SEM-EDS. Another interesting finding is that La substitution would cause the change of Nd, Pr, La and Ce ratio in main phase, which means the ratios of four rare earth elements change with the La substitution. With the increase of x, the (Pr + Nd)/(La + Ce) ratio in main phase of (LaxMM1−x)33Fe66B alloys decreases first, showing the lowest point at x = 0.075, and then increases. As a result, the saturation magnetization (Ms) of (LaxMM1−x)33Fe66B alloys has the same tendency, which decreases from 11.78 kG (x = 0) to 11.35 kG (x = 0.075) first, and then increases to 11.96 kG (x = 0.15). And the Curie temperature (Tc) displays the same trend compared to the Ms. These facts show that the MM has a great potential to prepare the low cost sintered permanent magnets.