The use of MFM for investigating domain structures in modern permanent magnet materials

Magnetic force microscopy occupies a special niche in the array of techniques which are currently available for imaging magnetic structures of high energy permanent magnet materials, yielding high resolution data under ambient conditions on bulk samples. The high anisotropies of permanent magnet samples mean that to a good approximation the tip stray held does not modify the magnetic state of the sample. However strong stray fields, of the order of 1 Tesla, may be experienced by a tip in close proximity to the sample. These stray fields are known to perturb the magnetic state of the tip considerably. As a result, contrast may be generated between neighboring domains, or at domain boundary walls or a combination of both, depending on the degree of perturbation of the tip. Examples demonstrating this variation with a series of batch-fabricated tips with various magnetic coatings ranging in coercivity from similar to 1400 Oe to <1 Oe are presented. The origin of the different types of observed contrast is discussed, and it is shown that very low coercivity tips coated with Fe nanoparticles embedded in a SiO2 matrix provide the most readily understood contrast, and have an additional benefit in that they are 'self-focusing'. That is, the active magnetic volume scales with the characteristic dimension of the sample domain structure. Image interpretation is also discussed in the context of specific MFM images from samples of neodymium iron boron (sintered, melt-quenched and nanocomposite) and barium ferrite. (C) 1998 Elsevier Science B.V. All rights reserved.