Lorentz Microscopy of Small Ferromagnetic Particles

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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.

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