Lorentz microscopy of small ferromagnetic particles

Abstract

Several authors have shown theoretically that a ferromagnetic particle with a diameter less than a critical value will be in a uniformly magnetized, single-domain state. Interesting transition states for particles just above the critical size are also predicted. Using special techniques, specimens have been prepared which contain discrete islands of NiFe film on a nonmagnetic substrate. Each specimen contained islands, usually circular, several hundred angstroms thick with a wide range of diameters (∼1-50 μ). The islands were thus two-dimensional analogs of three-dimensional particles. Lorentz microscopy gave direct evidence of the configurations predicted for small uniaxially anisotropic islands: (a) a circular magnetization configuration, (b) a 180°domain wall separating two oppositely directed domains, and (c) a single-domain, uniformly magnetized state (observed for elongated islands). The magnetization in islands in state b was not uniform, but "curled" around the ends of the domain wall near the edges of the islands. The transition from state b to a was noted below a critical island diameter Dc if all other parameters were fixed; theoretical estimates of Dc were made. In addition, the distortion of all three states in the presence of applied fields was observed. Elongated single-domain islands reversed by a process involving "locking" of the magnetization. © 1965 The American Institute of Physics.