Two-State Switching in Mesoscale, Ferromagnetic Particles at Thermal Equilibrium*

Two state switching in 250 nm x 250 nm x 10 nm, square, permalloy ferromagnetic particles has been measured.  Dwell times and energy barriers were determined by monitoring the magnetization using the anisotropic magnetoresistance (AMR) to determine the direction of the magnetization. In zero applied magnetic field, the magnetization of a particle prefers being perpendicular to sides of the square from a combination of the exchange and dipole interactions.  An external diagonal magnetic field was used to control the energy creating  two lower energy states.  The thermally activated switching between these two lower energy states was monitored as a function of time to determine the average dwell times for the two states.  Dwell times were measured at temperatures from ~50 – 200K, and fit to the Arrhenius equation. The Arrhenius equation, τ =  τ₀ exp(-U/kT), has long been used to model these types of two- state systems. However, most of these studies have focused on the Boltzmann factor with few experiments studying the prefactor, τ₀. We find prefactors many orders of magnitude smaller than the physical timescales in the system. These results and a possible explanation will be discussed.