EVOLUTION OF MAGNETIC MICROSTRUCTURE IN HIGH-COERCIVITY PERMANENT-MAGNETS IMAGED WITH MAGNETIC FORCE MICROSCOPY

Magnetic force microscopy (MFM) has been shown to give high-resolution imaging of magnetic domain structures in a variety of high-coercivity permanent magnets [Folks er ah., J. Magn. Magn. Mater. (in press)]. W e show that this technique can be extended by the application of exter nal fields during imaging, thus allowing direct observation of submicr on microstructural evolution as a function of field. Electromagnets mo unted on the MFM supplied fields up to 7 kOe laterally and 3 kOe verti cally. In sintered materials, submicron processes such as depinning of domain walls at grain boundaries, domain fragmentation, and hysteresi s were observed. MFM tips having very low coercivity highlighted domai n walls, whereas higher-coercivity tips suffered unpredictable rotatio n of their magnetic moment due to both the sample and applied fields, leading to images which are difficult to interpret. For imaging of the finer-grained melt-quenched magnets, however, relatively high-coerciv ity tips were superior. These results show promise for the direct obse rvation of the submicron-scale processes that dictate bulk magnetic pr operties, and the quantification of their field dependence. (C) 1997 A merican Institute of Physics.