Critical nucleation size of vortex core for domain wall transformation in soft magnetic thin film nanostrips

Abstract

We have studied the onset process of dynamic transformation of the internal structure of a moving domain wall (DW) in soft magnetic thin film nanostrips, driven by applied magnetic fields larger than the Walker field strength, Hw. It was found that one of the edge-soliton cores of a transverse wall (TW)-type DW should reach a critical nucleation size of the vortex core by moving inward beyond a critical deviation, Δ ycri, in the transverse (width) direction, in order for the transformation from a TW to a vortex wall (VW) (or antivortex wall, AVW) to occur. The value of Δ ycri is estimated to be close to the full width at half maximum of the out-of-plane magnetizations of a stabilized vortex core (or antivortex core). Upon completion of the nucleation of the vortex (antivortex) core, the VW (AVW) is stabilized, accompanying its characteristic gyrotropic motion in a potential well (hill) of a given nanostrip. Field strengths exceeding the Hw, the onset field of DW velocity breakdown, are not sufficient but necessary conditions for the dynamic DW transformations.