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

Abstract

We have applied the Foucault mode of Lorentz microscopy to detect the polarity of the magnetic field produced by ferromagnetic particles in the single-domain size range. We can follow the polarity of any given particle through multiple cycles of removal and return of the sample from the Transmission Electron Microscope(TEM). Hence, we can determine the particle's coercivity by applying a sequence of known fields to the sample. Our 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. In the present paper we demonstrate the ability to detect the polarity, and measure the coercivity, of nearly-ellipsoidal ϒ-Fe<inf>2</inf>O<inf>3</inf> particles which have a moment of ~10^-13emu. © 1991 IEEE