Measuring the coercivity of individual sub-micron ferromagnetic particles by Lorentz microscopy

The Foucault mode of Lorentz microscopy has been applied to detect the polarity of the magnetic field produced by ferromagnetic particles in the single-domain size range. One can follow the polarity of any given particle through multiple cycles of removal and return of the sample from the transmission electron microscope. Hence, one can determine the particle coercivity by applying a sequence of known fields to the sample. The proposed technique makes it possible to obtain unambiguous data for the dependence of an individual particle´s coercivity on the orientation of the applied field, and the particle´s dimensions. Both these data are needed to determine the magnetization reversal mechanism in single-domain particles. The authors demonstrate the ability to detect the polarity and measure the coercivity of nearly ellipsoidal γ-Fe₂O₃ particles which have a moment of ~10^-13 emu