Magnetic force microscopy of NdFeB hard magnetic alloys

The principal aim of this investigation was to use magnetic force microscopy (MFM) to determine the stray magnetic field structure of NdFeB alloys in both single crystal and nanocrystalline form. A detailed study of the effect of microscope operating parameters on image morphology was undertaken which allowed reproducible images to be obtained. However, major differences in MFM images could result from scanning the same area with two tips from nominally the same batch. Therefore, an image was only accepted if it could be reproduced with a second tip. A range of sensing tip coatings were used, namely ∼50nm thick CoCr, ∼10nm thick CoCr, NiFe, FeSiB and Fe/SiO2 providing a range of properties from high coercivity to superparamagnetic. The high and low coercivity tips were found to give complementary information and therefore provided an important aid to image interpretation. Nanocrystalline NdFeB alloys were examined, with grain size in the range 30-40nm. Low-Nd (Nd9.5Fe84.5B6) containing Nd2Fe14B and α-Fe phases, stoichiometric (Nd11.8Fe82.3B5.9) and high-Nd (Nd18Fe76B6) containing Nd2Fe14B and Nd-rich phases, all with approximately similar grain sizes were investigated. The magnetic stray field consisted of interaction domains of a size substantially greater than grain size (e.g. 350 ± 30nm for stoichiometric alloy). Within the dominant magnetic structure, a finer scale structure was apparent, of a size comparable to the grain size. The fine scale structure was shown to be true magnetic contrast and is believed to represent the single domain grains which have similar, but not identical spins, which make up an interaction domain. Both the domain correlation length and the tip-sample interaction 'strength' were found to decrease with increasing Nd-content and increase by magnetising the sample to remanence. In addition, increasing the grain size was also found to result in the increase of the domain correlation length. The single crystal NdFeB sample was examined on planes parallel and perpendicular to the easy axis of magnetisation. On the plane perpendicular to the easy axis, a rick-rack domain structure, spike domains and star shaped domains were observed. The rick-rack pattern contained fine contrast on a scale down to ∼10nm, which was shown to be at least in part true magnetic contrast. Scanning on a plane parallel to the easy axis of magnetisation revealed a stripe domain structure with clear evidence of domain subdivision as the surface or grain boundary is approached. (author)