Microstructure and Magnetic Properties of Rapidly Quenched Pr₉Fe_91-xB_x (x=4-12) Nanocomposite Magnets

The microstructure and magnetic properties of rapidly quenched nanocomposites Pr₉Fe_91-xB_x (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 Pr₉Fe_91-xB_x nanocomposites were strongly dependent on the B content. The samples consist of Pr₂Fe₁₄B and α-Fe phases when the B content was low (4-8 at%), and Pr₂Fe₁₄B, α-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, α-Fe and Fe₃B by annealing the sample at temperatures above 750 °C, leading to a more square demagnetization curve. However, the B_r of such nanocomposites decreases slightly due to the excessive grain growth during high-temperature annealing. A comparison of microstructure and magnetic properties between Pr₂Fe₁₄B/α-Fe and Pr₂Fe₁₄B/Fe₃B was also made in this work. It was found that the Pr₂Fe₁₄B/α-Fe exhibited a finer and more uniform microstructure with an average grain size of ∼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/α-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/α-Fe can be attributed to the more favorable intrinsic magnetic properties of α-Fe (higher M_s and lower K) and the much finer microstructure which produces stronger inter-grain exchange coupling between the Pr₂Fe₁₄B and α-Fe phases. The results indicate that RE₂Fe₁₄B/α-Fe nanocomposites are more desirable than RE₂Fe₁₄B/Fe₃B for future development of RE-Fe-B nanocomposite magnets.