Abstract
Three-dimensional micromagnetic simulations of the switching of a magnetic vortex core in a cylindrical nanodisk are performed, for excitations with out-of-plane fields (symmetric switching) or with various types of time-dependent in-plane fields (asymmetric switching). Although the switching mechanisms are different in detail, all switching events must involve the movement of a Bloch point through the disk, because the switching leads to a change of the Skyrmion number which is a topological invariant as long as there is no action of a Bloch point. The momentary magnetization configurations are different in different layers of the disk. Because of the three-dimensionality it is often difficult to decide whether the asymmetric switching is caused by the splitting of the dip close to the vortex core into a vortex-antivortex pair, and the annihilation of the original vortex with the antivortex (whereby a Bloch point moves). It is suggested that there are situations for which such a switching occurs by the formation of a Bloch point in a configuration which is already similar to a vortex-antivortex configuration, but by a movement of this Bloch point before the formation of a complete pair and without the annihilation of the original vortex with an antivortex.
Highlights
A magnetic vortex is the magnetic ground state of a circular nanodisk of diameter 2R and thickness D
The switching mechanisms are different in detail, all switching events must involve the movement of a Bloch point through the disk, because the switching leads to a change of the Skyrmion number which is a topological invariant as long as there is no action of a Bloch point
V A, counterclockwise excitation) give a hint to the fact that the asymmetric switching is not always performed by the mechanism in which a Bloch point moves through the disk during the annihilation of the original vortex with the antivortex of opposite polarization [17] resulting from a vortex-antivortex (VA) pair which is formed by the excitation
Summary
A magnetic vortex is the magnetic ground state of a circular nanodisk of diameter 2R and thickness D. Because the vortex polarity is very stable, such structures could be used in advanced information storage and data processing devices For this it necessary to change the polarity in a short time. For asymmetric switchings after excitation with two orthogonal monopolar in-plane pulses [13] three-dimensional simulations have been performed, which will be discussed in more detail in the present paper. The simulations for an excitation by a special type of dynamical in-plane field (see Sec. V A, counterclockwise excitation) give a hint to the fact that the asymmetric switching is not always performed by the mechanism in which a Bloch point moves through the disk during the annihilation of the original vortex with the antivortex of opposite polarization [17] resulting from a vortex-antivortex (VA) pair which is formed by the excitation. There may be asymmetric switching events in which a magnetic structure similar to a VA pair is formed, but a Bloch point appears in this structure (which is energetically already favorable for this formation) and moves through the disk before the creation of a fully developed VA pair and without the annihilation of the original vortex with an antivortex
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