Currently, rare-earth permanent magnet Nd-Fe-B has been widely used in all aspects of life, such as magnetic levitation trains, electric cars, wind power generation, stereos, etc. However, the coercivity of sintered Nd-Fe-B products is only 20-30% of the theoretical value (Stoner-Wolfart limit) (often called Brown's paradox). However, the coercivity of sintered Nd-Fe-B products is only 20-30% of the theoretical value (Stoner-Wolfart limit) (commonly known as the Brown's paradox), which seriously limits the application of Nd-Fe-B. Existing theories suggest that the coercivity of sintered NdFeB is mainly determined by the nucleation field required for the creation of reverse magnetic domains near the grain boundaries during the demagnetization process. Therefore, a three-dimensional quantitative analysis of how the grain boundary affects the coercivity is particularly important, which not only deepens the understanding of the mechanism of rare-earth permanent magnet coercivity, but also guides the practical production.

Results

Recently, Associate Professor Rongkun Zheng (corresponding author) and first author Dr. Hansheng Chen of the University of Sydney, together with their team members, used backscattering diffraction, atomic scale 3D atom probe, and micromagnetism simulation based on experimental results as the basis of simulation parameter fitting, to report the further coercivity reduction due to the inhomogeneous composition of crystals in sintered Nd-Fe-B at the nanoscale, and conducted three-dimensional quantitative analyses of the composition of grain boundaries and coercive force. A three-dimensional quantitative analysis of the grain boundary composition and coercivity was performed. It is shown that the ferromagnetic elements (iron and cobalt) in the grain boundary in sintered Nd-Fe-B decrease from 67 at.% to 10 at.% in the 70 nm range. The nucleation field required to generate reverse magnetic domains near the grain boundaries of this inhomogeneous composition is 27% smaller than that required to generate reverse magnetic domains at the boundaries of homogeneous grains containing the same amount of ferromagnetic elements. The results are not only of great significance for industrial production, such as controlling the compositional structure of grain boundaries at the nanometer scale, but the analytical methods used in this paper can also be applied to the study of the relationship between composition and magnetic properties of other magnetic materials. The research result is titled “Coercivity degradation caused by inhomogeneous grain boundaries in sintered Nd-Fe-B permanent magnets” was published in Physical Review Materials.