A systematic investigation of Pr-rich Pr-(Fe,Co)-B material system: Phase formation, microstructure and magnetic property

Abstract

Praseodymium (Pr) doping is known to increase the coercivity of Nd-Fe-B permanent magnets without significant reduction in magnetization. However, there is still a lack of studies on the specific impact of Pr in comparison of Nd on phase formation and microstructure, which is crucial for the development of high-performance Pr-based magnets. In this study, we investigate the phase formation, microstructure, magnetic properties and coercivity mechanisms in Pr-rich Pr15(Fe1-xCox)78B7 (x = 0 – 1) alloys using advanced synthesis, high-resolution electron microscopy, and modeling techniques. The alloys with x = 0 and 0.2 are comprised of a Pr2(Fe,Co)14B matrix phase with a non-magnetic Pr-rich intergranular phase, and this microstructure is stable up to 1150 °C. Due to the Pr-rich phase, the magnetization reversal in Pr2(Fe,Co)14B grains is hindered, resulting in high coercivity of 1.8 ∼ 2.0 T. For the Co concentration of x = 0.4 - 0.8, the Pr-rich intergranular phase gradually disappears, and ferromagnetic Pr(Fe,Co)4B, Pr(Fe,Co)2 or Pr(Fe,Co)3 phases are exist instead. The soft-magnetic Pr(Fe,Co)2 phase acts as a weak link and tends to induce the nucleation of the reversal interaction domain inside the Pr2(Fe,Co)14B grain near the Pr(Fe,Co)2 / Pr2(Fe,Co)14B interface, resulting in degraded coercivity of 0.7 ∼ 1.0 T. In the Pr15Co78B7, the Pr2Co14B, PrCo4B and PrCo5 phases co-exist as independent exchange-coupled grains and the weak link was found at the PrCo4B / Pr2Co14B interface, where the reversal interaction domains nucleate and then propagate into Pr2Co14B grains, resulting in a coercivity of approximately 1.2 T. Our experimental findings were compared with micromagnetic modeling, which reveals that the change in the direction of magnetization at the interfaces of exchange-coupled grains often has a very complex topology, and the transition regions between two interaction domains is “pinned” at the structural inhomogeneities. Thus, by using the Pr-Fe-Co-B alloys as a model system, we highlight the significance of gaining a deeper understanding of these processes at the micro- and nano-levels. This knowledge may assist in constructing more accurate models of magnetization reversal in nanocrystalline magnets.