Micromagnetic study of Bloch-point-mediated vortex core reversal

Abstract

We study how micromagnetic calculations can be applied to processes that involve a singularity of the magnetization field, namely, the Bloch point. In order to allow for comparison with recent experiments, we consider Permalloy thin-film disks supporting a vortex magnetic configuration. The structure of the Bloch point at rest in the middle of the core of the vortex is studied first, comparing the evolution of the calculation results under decreasing mesh size to analytical results. The reversal of the core of the vortex under a field applied perpendicularly to the disk plane is then investigated. We apply two different procedures to evaluate switching fields and processes: direct micromagnetic time-dependent calculation, and the evaluation of the energy barrier that separates the two orientations of the vortex core in the configuration space, using a path method. Both methods show the occurrence of Bloch points during reversal. Special attention is paid to the extrapolation towards zero mesh size of the numerical results. The calculations are confronted to experimental values from Okuno et al. ͓J. Magn. Magn. Mater. 240, 1 ͑2002͔͒. We conclude that defects and thermal agitation are likely to assist Bloch-point injection, hence lowering the switching fields.

Limits for the vortex state spin torque oscillator in magnetic nanopillars: Micromagnetic simulations for a thin free layer

“…In Ref. 54, describing the Bloch points responsible for the vortex polarization reversal, considerable changes for different cell sizes are found for the fields needed for reversal. However, big cell sizes ͑but always below exchange length͒ are proposed to be useful for resembling pinning effects similar to those existing in nature, and the changes found for different cell sizes are always smaller than the ones found experimentally.…”

Method for finding mechanism and activation energy of magnetic transitions, applied to skyrmion and antivortex annihilation

“…In many cases, these have been rather simple systems with the magnetic vectors rotating mainly in a plane (with some exceptions, for example [25,28,33]). Several choices of coordinate systems are possible in such calculations.…”

Controlled vortex core switching in a magnetic nanodisk by a rotating field

“…Due to this field the heavy vortex, which is polarized against the field, becomes unstable [14,15] and at some critical value (B s ∼ 1.5πM S [16], M S is the saturation magnetization [34]) it flips to the light vortex state in which the core magnetization is parallel to the field. Note that this switching field should be strong enough, B s ∼ 2.5 kOe [16,17,18,19]. Another possibility to switch the vortex is to use a weak but fast rotating field B(t) [7,8,9,10].…”