Magnetization Reversals of Nd-Fe-B-Based Magnets with Different Microstructural Features

In order to develop permanent magnets with coercivities close to their physical limit, it is essential to obtain a better understanding of their magnetization reversal processes by directly observing their magnetic domains dynamically under an external magnetic field. Here, we review recent magneto-optical Kerr effect microscopy studies for correlating the magnetization reversal processes of Nd-Fe-B-based permanent magnets with different microstructural features. Angular--dependent coercivity measurements of conventional Nd-Fe-B sintered magnets with a ferromagnetic intergranular phase (IGP) suggest that the coercivity is governed by the pinning of magnetic domains at grain boundaries. Micromagnetic simulations based on the model created from experimentally observed microstructures predicted that the alternation of IGP to weak/non-ferromagnetic hinders the cascade-type domain wall propagation through an intergranular phase. Nevertheless, the angular-dependent coercivity of Ga-doped Nd-rich sintered magnets, that shows thick Nd-rich IGP partially isolating Nd2Fe14B grains, indicates that the reversal mechanism is still the pinning type. Hence, further exchange decoupling would lead to larger coercivity in Nd-Fe-B sintered magnets. We discuss how the magnetism of IGPs and tuning the anisotropy field of the IGP/Nd2Fe14B interfaces in ultra-fine-grained hot-deformed magnets alters the magnetization reversal process to reach the large coercivity of 2.5 T.

Automated summary

Understanding the magnetization reversal processes of permanent magnets is essential for developing magnets with high coercivity. Recent studies have used magneto-optical Kerr effect microscopy to observe the dynamic magnetic domains under an external magnetic field. Different microstructural features, such as grain boundaries and interfaces, influence the magnetization reversal processes of Nd-Fe-B-based permanent magnets. By manipulating these factors, it is possible to achieve magnets with higher coercivity.