Dynamics of vortex nucleation in nanomagnets with broken symmetry

Abstract

We investigate fundamental processes that govern dynamics of vortex nucleation in sub-100 nm mesoscopic magnets. We focus on a structure with broken symmetry -Pacman-like nanomagnet shape -in which we study micromagnetic behavior both by means of a simple model and numerically. We show that it is possible to establish desired vortex chirality and the polarity of vortex core by applying only quasi-static in-plane magnetic field along specific directions. We identify the modes of vortex nucleation that are very robust against external magnetic field noise. These vortex nucleation modes are common among wide range of sub-100 nm magnets with broken rotational symmetry. 75.60.Jk, 75.78.Cd Confinement leads to fundamental changes in physical behavior of materials due to increased role of the surface. In mesoscopic magnetic materials such changes in the energy landscape could lead to novel magnetic spin configurations such as vortices. Equilibrium properties of these new topological states are governed by both the properties of the magnetic material and the geometry of the object. On the other hand, confinement also leads to distinct dynamic behavior of the new topological states, since the available energy levels are very much limited. The transition probabilities between different states can thus be controlled by careful engineering of the geometry of the mesoscopic object. Here we show that by tailoring the geometry of the mesoscopic magnet one can produce deterministic dynamic switching between well defined degenerate topological states using only in-plane magnetic pulses. We present a simple model that explains the mechanism of the vortex nucleation and origin of robustness of the vortex polarization. Confirmation of the model is accomplished by using numerical simulations.Controlled manipulation of magnetic domains in ferromagnet nanostructures have recently opened opportunities for novel fast, high-density, and low-power memories with novel architectures 1-3 . Any perspective magnetic memory architecture, such as sub-micron non-volatile magnetic memory, has to have (1) a well-defined switching field used to set the memory bits, and (2) a reproducible switching behavior using simple sequence of external magnetic field pulses. Therefore, the dynamics of the switching between different ground states has to be understood in detail.Recent advances in fabrication technology at nanoscale have enabled studies of magnetic systems that are well defined in all three dimensions on a nanometer length scale (< 100 nm) 4,5 . The size reduction of nanomagnets leads to novel spin topological states such as vortex state, C-state, S-state, flower state, etc.6 , and to simplified transitions between these states in external magnetic field. The transition between ground states in such nanomagnets is of fundamental importance 7 . It is governed by competition between the magneto-static energy and exchange energy, and it is influenced by the magnetic material used and by the choice of the nanomagnet shape. For example, the magneti...

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