Franz G. Mertens

MAGNON MODES AND MAGNON-VORTEX SCATTERING IN TWO-DIMENSIONAL EASY-PLANE FERROMAGNETS

We calculate the magnon modes in the presence of a vortex in a circular system, combining analytical calculations in the continuum limit with a numerical diagonalization of the discrete system. The magnon modes are expressed by the S-matrix for magnon-vortex scattering, as a function of the parameters and the size of the system and for different boundary conditions. Certain quasi-local translational modes are identified with the frequencies which appear in the trajectory X(t) of the vortex center in recent Molecular Dynamics simulations of the full many-spin model. Using these quasi-local modes we calculate the two parameters of a 3rd-order quation of motion for X(t). This equation was recently derived by a collective variable theory and describes very well the trajectories observed in the simulations. Both parameters, the vortex mass and a factor on the third time derivative of X(t), depend strongly on the boundary conditions.

Vortex polarity switching by a spin-polarized current

The spin-transfer effect is investigated for the vortex state of a magnetic nanodot. A spin current is shown to act similarly to an effective magnetic field perpendicular to the nanodot. Then a vortex with magnetization (polarity) parallel to the current polarization is energetically favorable. Following a simple energy analysis and using direct spin-lattice simulations, we predict the polarity switching of a vortex. For magnetic storage devices, an electric current is more effective to switch the polarity of a vortex in a nanodot than the magnetic field.

Current induced switching of vortex polarity in magnetic nanodisks

It is shown that the vortex polarity can be irreversibly switched by injecting a spin-polarized direct electrical current, which flows perpendicular to the disk plane. Intensive numerical spin-lattice simulations demonstrate that the switching process involves a vortex-antivortex pair creation. This differs from magnets with no dipolar interaction, where the spin dc acts similar to a static magnetic field.The spin torque effect, which is the change of magnetization due to the interaction with an electrical current, was predicted by Slonczewski [1] and Berger [2] in 1996. During the last decade this effect was tested in different magnetic systems [3, 4, 5] and nowadays it plays an important role in spintronics [6, 7]. Recently the spin torque effect was observed in vortex state nanoparticles. In particular, circular vortex motion can be excited by an AC [8] or a DC [9] spin-polarized current. Very recently it was predicted theoretically [10] and observed experimentally [11] that the vortex polarity can be controlled using a spin-polarized current. This opens up the possibility of realizing electrically controlled magnetic devices, changing the direction of modern spintronics [12].

Controllable switching of vortex chirality in magnetic nanodisks by a field pulse

We propose a way of fast switching the chirality in a magnetic nanodisk by applying a field pulse. To break the symmetry with respect to clockwise or counterclockwise chirality, a mask is added by which an inhomogeneous field influences the vortex state of a nanodisk. Using numerical spin-lattice simulations, we demonstrate that chirality can be controllably switched by a field pulse, whose intensity is above some critical value. A mathematical definition for the chirality of an arbitrary shaped particle is proposed.

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

The control of the vortex state magnetic nanoparticle by ultrafast magnetic fields is studied theoretically. Using the micromagnetic simulations for the Permalloy nanodisk we demonstrate that the vortex core magnetization can be irreversible switched by the alternating field, rotating in the disk plane, with the frequency about 10 GHz and intensity about 20 mT. We propose an analytical picture of such phenomena involving the creation and annihilation of vortex-antivortex pairs and calculate the phase diagram of the fields parameters leading to the switching.

An iterative method for the calculation of narrow solitary excitations on atomic chains

Abstract We discuss the quasicontinuum approximation of Collins and give a general condition for its validity. For the solution of the difference-differential equations of the full, discrete model we develop an iteration procedure in Fourier space. The method is tested for solitary excitations with widths of the order of the lattice constant by a comparison with both exact results and computer simulations. An additional result of the new mwthod is a simple derivation of the Collins approximation which allows several generalizations.

Stochastic vortex dynamics in two-dimensional easy-plane ferromagnets: Multiplicative versus additive noise

We study how thermal fluctuations affect the dynamics of vortices in the two-dimensional classical, ferromagnetic, anisotropic Heisenberg model depending on their additive or multiplicative character. Using a collective coordinate theory, we analytically show that multiplicative noise, arising from fluctuations in the local-field term of the Landau-Lifshitz equations, and Langevin-like additive noise both have the same effect on vortex dynamics (within a very plausible assumption consistent with the collective coordinate approach). This is a nontrivial result, as multiplicative and additive noises usually modify the dynamics quite differently. We also carry out numerical simulations of both versions of the model finding that they indeed give rise to very similar vortex dynamics. {copyright} {ital 1999} {ital The American Physical Society}

Internal Mode Mechanism for Collective Energy Transport in Extended Systems

We study directed energy transport in homogeneous nonlinear extended systems in the presence of homogeneous ac forces and dissipation. We show that the mechanism responsible for unidirectional motion of topological excitations is the coupling of their internal and translation degrees of freedom. Our results lead to a selection rule for the existence of such motion based on resonances that explain earlier symmetry analysis of this phenomenon. The direction of motion is found to depend both on the initial and the relative phases of the two harmonic drivings, even in the presence of noise.

Soliton dynamics in damped and forced Boussinesq equations

Abstract:We investigate the dynamics of a lattice soliton on a monatomic chain in the presence of damping and external forces. We consider Stokes and hydrodynamical damping. In the quasi-continuum limit the discrete system leads to a damped and forced Boussinesq equation. By using a multiple-scale perturbation expansion up to second order in the framework of the quasi-continuum approach we derive a general expression for the first-order velocity correction which improves previous results. We compare the soliton position and shape predicted by the theory with simulations carried out on the level of the monatomic chain system as well as on the level of the quasi-continuum limit system. For this purpose we restrict ourselves to specific examples, namely potentials with cubic and quartic anharmonicities as well as the truncated Morse potential, without taking into account external forces. For both types of damping we find a good agreement with the numerical simulations both for the soliton position and for the tail which appears at the rear of the soliton. Moreover we clarify why the quasi-continuum approximation is better in the hydrodynamical damping case than in the Stokes damping case.

Switching between different vortex states in two-dimensional easy-plane magnets due to an ac magnetic field

Using a discrete model of two-dimensional easy-plane classical ferromagnets, we propose that a rotating magnetic field in the easy plane can switch a vortex from one polarization to the opposite one if the amplitude exceeds a threshold value, but the backward process does not occur. Such switches are indeed observed in computer simulations.