Magnetic structure and microstructure of die-upset hard magnets RE13.75Fe80.25B6 (RE = Nd, Pr): A possible origin of high coercivity

In situ transmission electron microscopy magnetizing experiments combined w ith Lorentz magnetic microscopy in Fresnel-Foucault modes were used to char acterize the magnetic structure of die-upset, high energy-product hard magn ets Nd13.75Fe80.25B6 and Pr13.75Fe80.25B6. Experimental observations indica te a well-aligned grain structure and quasiperiodic nonaligned "extended de fect'' layers transverse to press direction. The local remanence of the "de fect'' layers is far from saturation when the external field is removed. Th e layers are enriched with inclusions of approximate composition Nd7Fe3, ge nerally with a polygonal shape, and are associated with the original ribbon interfaces. They may be responsible for a high coercivity mechanism, since the motion of reverse domains can be impeded by these layers, even when th ey are nucleated. Thus, a delayed nucleation of reversed domains seems to b e a limiting factor for magnetization reversal and coercivity force. Both L orentz magnetic imaging and high-resolution microscopy highlight the role o f magnetocrystalline anisotropy for domain wall-grain boundary interactions and pinning. Local remanence was estimated directly from magnetic moment s ensitive Foucault images. (C) 1999 American Institute of Physics.