arXiv subjects

G. M. Wysin

Publications and source records attributed to G. M. Wysin.

25 records · Page 2Linked to original sources

arXiv2003

Vortex-vacancy interactions in two-dimensional easy-plane magnets

For a model with isotropic nearest neighbor exchange combined with easy-plane exchange or single-ion anisotropies, the static effects of a magnetic vacancy site on a nearby magnetic vortex are analyzed on square, hexagonal and triangular lattices. Numerical energy minimization and linear stability analysis using the vortex instability mode are employed. When the vortex is centered on a vacancy, the critical anisotropies where the stable vortex structure switches from out-of-plane to planar form are determined, and the vortex energies and magnetizations are found as functions of anisotropy. Consistent with square lattice calculations by Zaspel et al, the strength of anisotropy needed to stabilize a vortex in the planar form is reduced when the vortex is centered on a vacancy, for all three lattices studied. The vortex-on-vacancy energy is found to be smaller than the typical energy of a vortex centered between lattice sites in a system without vacancies. For a vortex separated from a vacancy, the energy found as a function of separation demonstrates an attractive potential between the two.

cond-mat.mtrl-sci↗

arXiv2000

Relaxation schemes for normal modes of magnetic vortices: Application to S-matrix

Relaxation schemes for finding normal modes of nonlinear excitations are described, and applied to the vortex-spinwave scattering problem in classical two-dimensional easy-plane Heisenberg models. The schemes employ the square of an effective Hamiltonian to ensure positive eigenvalues, together with an evolution in time or a self-consistent Gauss-Seidel iteration producing diffusive relaxation. We find some of the lowest frequency spinwave modes in a circular system with a single vortex present. The method is used to describe the vortex-spinwave scattering (S-matrix and phase shifts) and other dynamical properties of vortices in ferromagnets and antiferromagnets, for systems larger than that solvable by diagonalization methods. The lowest frequency modes associated with translation of the vortex center are used to estimate the vortex mass.

cond-mat.mtrl-sci↗

arXiv1999

Spinwave damping in the two-dimensional ferromagnetic XY model

The effect of damping of spinwaves in a two-dimensional classical ferromagnetic XY model is considered. The damping rate $Γ_{q}$ is calculated using the leading diagrams due to the quartic-order deviations from the harmonic spin Hamiltonian. The resulting four-dimensional integrals are evaluated by extending the techniques developed by Gilat and others for spectral density types of integrals. $Γ_{q}$ is included into the memory function formalism due to Reiter and Solander, and Menezes, to determine the dynamic structure function $S(q,ω)$. For the infinite sized system, the memory function approach is found to give non-divergent spinwave peaks, and a smooth nonzero background intensity (``plateau'' or distributed intensity) for the whole range of frequencies below the spinwave peak. The background amplitude relative to the spinwave peak rises with temperature, and eventually becomes higher than the spinwave peak, where it appears as a central peak. For finite-sized systems, there are multiple sequences of weak peaks on both sides of the spinwave peaks whose number and positions depend on the system size and wavevector in integer units of $2π/L$. These dynamical finite size effects are explained in the memory function analysis as due to either spinwave difference processes below the spinwave peak or sum processes above the spinwave peak. These features are also found in classical Monte Carlo -- Spin-Dynamics simulations.

cond-mat.mtrl-sci↗

arXiv1999

Onsager reaction-field theory for magnetic models on diamond and hcp lattices

The Onsager reaction-field (ORF) theory is extended so that it applies to any three-dimensional Bravais lattice with a basis. The ORF calculation is used to predict the critical temperature for classical Ising, XY, and Heisenberg magnetic models, in particular, on diamond and hexagonal close packed lattices. Results are compared with series extrapolations and other theoretical approaches where available. For the hcp lattice the ORF calculation is seen to be exactly equivalent to a Green's function approach.

cond-mat.mtrl-sci↗

arXiv1999

Low Temperature Static and Dynamic Behavior of the Two-Dimensional Easy-Axis Heisenberg Model

We apply the self-consistent harmonic approximation (SCHA) to study static and dynamic properties of the two-dimensional classical Heisenberg model with easy-axis anisotropy. The static properties obtained are magnetization and spin wave energy as functions of temperature, and the critical temperature as a function of the easy-axis anisotropy. We also calculate the dynamic correlation functions using the SCHA renormalized spin wave energy. Our analytical results, for both static properties and dynamic correlation functions, are compared to numerical simulation data combining cluster-Monte Carlo algorithms and Spin Dynamics. The comparison allows us to conclude that far below the transition temperature, where the SCHA is valid, spin waves are responsible for all relevant features observed in the numerical simulation data; topological excitations do not seem to contribute appreciably. For temperatures closer to the transition temperature, there are differences between the dynamic correlation functions from SCHA theory and Spin Dynamics; these may be due to the presence of domain walls and solitons.

cond-mat↗

arXiv1998

Free vortex and vortex-pair lifetimes in classical two-dimensional easy-plane magnets

We report numerical simulation results for free-vortex lifetimes in the critical region of classical two-dimensional easy-plane ferro- and antiferromagnets having three-component order parameters. The fluctuations in the vortex number density in a spin dynamics simulation were used to estimate the lifetimes. The observed lifetimes are of the same order of magnitude but smaller than the characteristic time-scale above which a phenomenological ideal vortex-gas theory that has been used to account for the central peak in the dynamic structure factor $S^{αα}({\bf q},ω)$ is expected to be valid. % For strong anisotropy, where the vortices are in-plane, the free vortex lifetimes for ferromagnets and antiferromagnets are the same, while for weak anisotropy, where the vortices have nonzero out-of-easy-plane components, the lifetimes in antiferromagnets are smaller than in ferromagnets. The dependence of the free-vortex and total vortex densities on the size dependent correlation length in the critical region is examined. We also determined the lifetimes of vortex-antivortex pairs for $T = T_{\scriptsize KT}$ and well below $T_{\scriptsize KT}$. The observed time-scales are very short, and the observed pair densities are extremely small. These results suggest that pair creation and annihilation are unlikely to play any role in the central peak in $S^{xx}({\bf q},ω)$ observed in computer simulations for the ferromagnetic model for $T \leq T_{\scriptsize KT}$.

cond-mat.stat-mech↗

arXiv1998

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.

cond-mat.mtrl-sci↗