Multiscale magnetic domains observed in die-upset melt-spun magnets using magnetic force microscopy

Abstract Magnetic force microscopy (MFM) has been used to examine the magnetic reversal process in samples of Nd 13.9 (Fe 92 Co 8 ) 80.3 B 5.5 Ga 0.5 , one made by hot-pressing and one made by thermomechanical deformation, or die-upsetting. Both samples were imaged in various magnetic states under zero-field conditions. In both the hot-pressed and die-upset samples, the finest scale contrast observable consisted of individual magnetic clusters of approximately 10 grains in volume, on a scale between 150 and 300 nm. In the hot-pressed sample this fine-scale contrast was similar regardless of the magnetic state of the sample and no larger-scale contrast was observed, indicating that exchange interactions do not greatly affect the locality of the magnetic reversal in this sample. The die-upset sample in the thermally demagnetized state showed only fine-scale magnetic contrast. Once the sample was saturated, however, interaction domains formed which extended over many microns, although the fine-scale contrast could still be observed within these larger interaction domains. The increased crystallographic alignment between grains in the die-upset sample as compared with the hot-pressed sample, underlies the hypothesis that intergranular exchange interactions cause the reversal to be localized at the peripheries of previously reversed regions, leading to the creation of large interaction domains.