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

Magnetic force microscopy (MFM) has been used to examine the magnetic reversal process in samples of Nd-13.9(Fe92Co8)(80.3)B5.5Ga0.5, 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. (C) 2001 Elsevier Science B.V. All rights reserved.