Switching Dynamics and Critical Behavior of Standard Problem No. 4
Robert McMichael, Michael J. Donahue, Donald G. Porter, J Eicke
We report switching dynamics for uMAG standard problem no. 4, a 500 nm x 125 nm x 3 nm rectangle of material with properties to mimic Permalloy. Siwtching dynamics are calculated for fields applied instantaneously to an initial s-state: Field 1 at 170o and Field 2 at 190o (-170o) 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 360o walls with complex dynamics on fine length scales. In quasi-static reversal, where the field approaches the static coercivity, Hc, in small steps, we find that the ring down frequency, f, and susceptibility, x are in approximate agreement with an analytical, single-spin model, which predicts f a (Hc - H) 1/4 and x a (Hc - H) -1/2 suggesting exptrapolation methods for determination of critical fields.