Observation of thermal switching of a single ferromagnetic particle

A magnetic force microscope (MFM) has been shown to be able to detect fields emanating from submicrometer permalloy particles with magnetic moments as low as 10−13 emu. We have used this MFM equiped with an in situ electromagnet to study the angular dependence of the switching field Hs(θ), and the probability of thermal switching of a single-domain permalloy particle as a function of the time duration of an applied magnetic field H. The angular dependence of Hs(θ) is significantly different from that expected for coherent rotation. For applied magnetic fields close to Hs, the time dependence of the probability for not switching cannot be fit with a simple exponential. These data indicate that multiple-energy barriers of similar height are involved in the thermally activated process responsible for the reversal.