Switching dynamics and critical behavior of standard problem No. 4

We report results for μMAG standard problem No. 4, a 500 nm×125 nm×3 nm rectangle of material with properties to mimic Permalloy. Switching dynamics are calculated for fields applied instantaneously to an initial s state: Field 1 at 170° and Field 2 at 190° (−170°) from the positive long axis. Reversal in Field 1 proceeds by propagation of end domains toward the sample center. Reversal in Field 2 involves rotation of the end domains in one direction while the center of the particle rotates in the opposite direction, resulting in collapsing 360° walls with complex dynamics on fine length scales. Approaching the static coercivity, Hc, in small field steps, we find that the ring down frequency, f, and susceptibility, χ, are in approximate agreement with a single spin model that predicts f∝(Hc−H)1/4 and χ∝(Hc−H)−1/2. We show a correlation between the modes of oscillation that become unstable at the critical field and the switching behavior.