Effect of the structure and composition of grain boundary phases on the pinning strength of domain wall in nanocrystalline PrNd-Fe-B magnets

Abstract

The critical role of grain boundary phases (GBPs) in the magnetic properties, c-axis texture, and pinning strength (PS) of domain walls (DWs) in nanocrystalline PrNd-Fe-B magnets was systematically investigated. The microstructural results show that with rare earth (RE) content increasing, the GBPs are optimized (the volume fraction is enhanced and the composition of GBPs is improved). This leads to an increase of coercivity, squareness factor, and temperature stability. At the thermally demagnetized state, the DWs are straight across the majority of thin ferromagnetism GBPs in low RE content magnets; the DWs are desultory and disappeared at nonferromagnetic thick GBPs in high RE content magnets. Furthermore, in situ observations of the magnetic domain evolutions suggest that the pinning mechanism plays a dominant role in determining the coercivity. With the optimization of GBPs, the coercivity mechanism transforms from inhomogeneous weak pinning to homogeneous strong pinning. Further study found that there exists an exponential function relationship between the average Nd + Pr concentration in the GBPs and PS of DWs. In addition, the weak pinning GBPs are characterized by discontinuous or continuous but thin structure and have low Nd + Pr content. The strong pinning GBPs are characterized by hybrid structure and have high Nd + Pr content, or by the continuous thick GBPs even without high Nd + Pr content.