Auger spectroscopy of rare earth-transition metal-boron magnets in relation to the mechanism of coercivity

Magnets of composition Nd₁₅Fe₇₈B₇ were examined by SEM (scanning electron microscopy) and Auger spectroscopy. The sample was fractured in the ultra-high-vacuum chamber of the spectrometer, and examination of the fracture surfaces provided information which is taken to be representative of the internal structure and composition of Nd-Fe-B magnets. The grains were found to be coated with an Nd-rich film that was presumably generated by the high reactivity between oxygen and Nd. The technique provided information as to the changes occurring during postsintering. The two main effects are reduction in grain size by about a factor of two and the development of a more homogeneous (Nd-rich) film. Prior to postsintering, a small amount of Nd was observed on the grain surface. Afterward, the Nd content had risen sharply (a factor of three or more). The results are used to provide fresh insight into the mechanism of coercivity. The Nd-rich film is thought to be a magnetic dead layer which insulates the particles and makes the system behave as an assemblage of weakly interacting fine particles. Nucleation, which is a rare event, is isolated by the dead layer in the grain of origin, giving rise to the large observed coercivity