Abstract
Strong interest in nanomagnetism stems from the promise of high storage densities of information through control of ever smaller and smaller ensembles of spins. There is a broad consensus that the Landau-Lifshitz-Gilbert equation reliably describes the magnetization dynamics on classical phenomenological level. On the other hand, it is not so evident that the magnetization dynamics governed by this equation contains built-in asymmetry in the case of broad topology sets of symmetric total energy functional surfaces. The magnetization dynamics in such cases shows preference for one particular state from many energetically equivalent available minima. We demonstrate this behavior on a simple one-spin model which can be treated analytically. Depending on the ferromagnet geometry and material parameters, this asymmetric behavior can be robust enough to survive even at high temperatures opening simplified venues for controlling magnetic states of nanodevices in practical applications. Using micromagnetic simulations we demonstrate the asymmetry in magnetization dynamics in a real system with reduced symmetry such as Pacman-like nanodot. Exploiting the built-in asymmetry in the dynamics could lead to practical methods of preparing desired spin configurations on nanoscale.
Highlights
Strong interest in nanomagnetism stems from the promise of high storage densities of information through control of ever smaller and smaller ensembles of spins
In the section “Simulation of Pacman-like Nanodot” using numerical simulations of the vortex nucleation process we demonstrate that dynamics could induce “directional effects” that can not be observed by taking into account energetics only
We have shown that particular shape of total energy functional in configuration space might lead to dynamics with broken symmetry
Summary
The TES given by equation (2) results in a strong drift in the vicinity of the saddle point and does not exhibit the crossing from asymmetric to symmetric behavior in the probability distribution sense. This crossing from asymmetric to symmetric behavior was observed experimentally in magnetization measurements in nickel nanodots at high magnetic fields and low temperatures[1]. The problem in obtaining symmetric distribution function with TES given by equation (2) is following: if the simulation temperature is chosen to be too high, the trajectories passing in the vicinity of the minimum at M2 appear, but the temperature is high enough for escape from minima M1. At low temperatures (σT < 0.25) the thermal fluctuations are not able to randomize the drift towards the minimum at M1
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