Localized surface nucleation of magnetization reversal

Standard treatments of the magnetization reversal of fine particles yield a reversal mechanism that begins with nucleation of a nonuniform or uniform energy barrier state that involves virtually the entire sample volume. Accordingly, the barrier height is essentially proportional to the sample volume. Such strong volume dependence is not observed, especially in measurements of thermally activated reversal. Numerical micromagnetic analysis also shows a variety of surface reversal modes depending on the ratio of the particle size to the exchange length. An analytic model for surface nucleation is presented here for an idealized system to illustrate this phenomenon. The corresponding barrier height does not depend on sample volume, though it may depend on surface area. Any additional size dependence will arise from the finite front velocity with which the reversal propagates. An example of thermally activated reversal is given that shows “effective” reversal volumes that correspond to these surface reversal modes, and thus, can be much smaller than the sample volume.