Non-Equilibrium Two-State Switching in Mesoscale, Ferromagnetic Particles

Abstract

There has been much theoretical study attempting to expand upon the Arrhenius law, $f=f_o exp(U/kT)$, which describes the switching rate in thermally activated, two-state systems, but few experiments to verify it. This is especially true for ferromagnetic particles. Most of the previous experiments performed attempting to study the Arrhenius law focus on the effect the Boltzmann factor, exp(U/kT), has on the switching rate since it dominates any measurement due to its exponential dependence on temperature. This has made it difficult to probe the underlying physics of the prefactor in front of the exponential. Using square, ferromagnetic particles of sizes 250 nm x 250 nm x 10 nm and 210 nm x 210 nm x 10 nm, controlling the barrier height using an applied field, and measuring the average dwell times in each individual state has allowed us to focus on these prefactors. Our measured prefactors vary by twenty five orders of magnitude, and they are smaller than those predicted by previous theories for particles of this size. They become so small as to reach unphysically short timescales. We attribute these unexpectedly small prefactors to our magnetic particles being multidomain and undergoing transitions before the particles have time to reach thermal equilibrium. We show that our particles have a higher probability of transitioning the less time they have been in a state which we attribute to the magnetization spending most of its time near the barrier allowing faster transitions.