Unmasking the interior magnetic domain structure and evolution in Nd-Fe-B sintered magnets through high-field magnetic imaging of the fractured surface

Conventional magnetic imaging techniques have observed magnetic domains in the polished surface of Nd-Fe-B sintered magnets, but the mechanical processing involved introduces numerous defects that facilitate the nucleation of reversed domains, thereby masking the interior domain structure. Here, we utilize high-field synchrotron x-ray magnetic circular dichroism microscopy to map the elemental and magnetic distributions in the polished and fractured surfaces of a Nd-Fe-B sintered magnet throughout its entire demagnetization process. As the applied field is varied, the domains in the fractured surface behave completely differently from those in the polished surface, thereby unmasking the interior domain structure and behavior. The area-averaged fractured surface coercivity is mu(0) H-c(frac) = 0.85 T which is much higher than the area-averaged polished surface coercivity, mu(0) H-c(pol) = 0.5 T. Most of the local magnetic hysteresis loops are positively or negatively biased from the zero of the field axis. The highest coercivity grains, some of which exceed 2 T, are almost always located in the vicinity of strongly oppositely biased adjacent grains. This indicates that these oppositely biased grains are strongly influencing the magnetostatic field at the sample surface.