Microstructure and magnetic properties of rapidly quenched Pr9Fe91-xBx (x=4-12) nanocomposite magnets

The microstructure and magnetic properties of rapidly quenched nanocomposites Pr9Fe91-xBx (x=4, 5, 6, 7, 8, 8.5, 9, 10.5 and 12) have been studied in this work. The results showed that the phases present in the Pr9Fe91-xBx nanocomposites were strongly dependent on the B content. The samples consist of Pr2Fe14B and alpha-Fe phases when the B content was low (4-8 at%), and Pr2Fe14B, alpha-Fe and Fe3B when the B content was sufficiently high (>11 at%). The metastable Pr2Fe23B3 phase appears in the intermediate B content (8-10.5 at%) and is associated with poor magnetic performance. It was further found that this metastable phase can be transformed into Pr2Fe14B, alpha-Fe and Fe3B by annealing the sample at temperatures above 750 'C, leading to a more square demagnetization curve. However, the B, of such nanocomposites decreases slightly due to the excessive grain growth during high-temperature annealing. A comparison of microstructure and magnetic properties between Pr2Fe14B/alpha-Fe and Pr2Fe14B/Fe3B was also made in this work. It was found that the Pr2Fe14B/alpha-Fe exhibited a finer and more uniform microstructure with an average grain size of similar to 10 nm, whereas a relatively coarse microstructure (grain size up to 100 nm) was observed in the Pr2Fe14B/Fe3B. It was also found that the Pr2Fe14B/alpha-Fe yielded higher B-r, (BH)(max) and a more square demagnetization loop, although the H-ci was slightly lower. The superior magnetic properties observed in the Pr2Fe14B/alpha-Fe can be attributed to the more favorable intrinsic magnetic properties of alpha-Fe (higher M-s and lower K) and the much finer microstructure which produces stronger inter-grain exchange coupling between the Pr2Fe14B and alpha-Fe phases. The results indicate that RE2Fe14B/alpha-Fe nanocomposites are more desirable than RE2Fe14B/Fe3B for future development of RE-Fe-B nanocomposite magnets.