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 rever sal 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, Bo th samples were imaged in various magnetic states under zero-field conditio ns. In both the hot-pressed and die-upset samples, the finest scale contras t observable consisted of individual magnetic clusters of approximately 10 grains in volume, on a scale between 150 and 300 nm. In the hot-pressed sam ple this fine-scale contrast was similar regardless of the magnetic state o f the sample and no larger-scale contrast was observed, indicating that exc hange interactions do not greatly affect the locality of the magnetic rever sal in this sample. The die-upset sample in the thermally demagnetized stat e showed only fine-scale magnetic contrast. Once the sample was saturated, however, interaction domains formed which extended over many microns, altho ugh the fine-scale contrast could still be observed within these larger int eraction domains. The increased crystallographic alignment between grains i n the die-upset sample as compared with the hot-pressed sample, underlies t he hypothesis that intergranular exchange interactions cause the reversal t o be localized at the peripheries of previously reversed regions, leading t o the creation of large interaction domains. (C) 2001 Elsevier Science B.V. All rights reserved.