MAGNETIC FORCE MICROSCOPY IMAGES OF HIGH-COERCIVITY PERMANENT-MAGNETS

The stray magnetic field distributions at the Surfaces of isotropic an d anisotropic NdFeB-type magnets have been imaged by magnetic force mi croscopy. Polished surfaces of bulk samples produced by melt-quenching , hot-rolling, and sintering were examined using two techniques which provided complementary information on the structure of the domains at the surface. For the first technique a high-coercivity magnetic tip wa s used, and for the second, a soft magnetic tip was used. The structur es which have been observed are compared with those reported by other workers using techniques of Lorentz electron microscopy and optical Ke rr microscopy and generally appear to be in good agreement. Preliminar y analysis of the images collected from the hot-rolled material sugges ts the presence of circular 'bubble' domains, alongside the more regul arly observed stripe domains. These appear to be reverse domains, invo lving only a small amount of material, which have arisen at the expose d surface to minimize the magnetostatic energy at the surface. Imaging of the die-upset material proved to be somewhat more difficult than f or the other materials due to strong stray fields at the surface that affected the tip's magnetic moment during imaging. This problem was la rgely overcome by the use of special high-coercivity tips. The results nonetheless are in accordance with recently published Kerr micrograph s for the same material.