Vortex motion in a finite-size easy-plane ferromagnet and application to a nanodot

We study the motion of a non-planar vortex in a circular easy-plane ferromagnet, which imitates a magnetic nanodot. Analysis was done using numerical simulations and a new collective variable theory which includes the coupling of Goldstone-like mode with the vortex center. Without magnetic field the vortex follows a spiral orbit which we calculate. When a rotating in-plane magnetic field is included, the vortex tends to a stable limit cycle which exists in a significant range of field amplitude B and frequency $\omega$ for a given system size L. For a fixed $\omega$, the radius R of the orbital motion is proportional to L while the orbital frequency $\Omega$ varies as 1/L and is significantly smaller than $\omega$. Since the limit cycle is caused by the interplay between the magnetization and the vortex motion, the internal mode is essential in the collective variable theory which then gives the correct estimate and dependency for the orbit radius $R\sim B L/\omega$. Using this simple theory we indicate how an ac magnetic field can be used to control vortices observed in real magnetic nanodots.Comment: 15 pages (RevTeX), 14 figures (eps

Switching between different vortex states in 2-dimensional easy-plane magnets due to an ac magnetic field

Using a discrete model of 2-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.Comment: 4 pages, 4 figures, submitted to Phys. Rev. Let

Noise-induced switching between vortex states with different polarization in classical two-dimensional easy-plane magnets

In the 2-dimensional anisotropic Heisenberg model with XY-symmetry there are non-planar vortices which exhibit a localized structure of the z-components of the spins around the vortex center. We study how thermal noise induces a transition of this structure from one polarization to the opposite one. We describe the vortex core by a discrete Hamiltonian and consider a stationary solution of the Fokker-Planck equation. We find a bimodal distribution function and calculate the transition rate using Langer's instanton theory (1969). The result is compared with Langevin dynamics simulations for the full many-spin model.Comment: 15 pages, 4 figures, Phys. Rev. B., in pres

Internal Modes and Magnon Scattering on Topological Solitons in 2d Easy-Axis Ferromagnets

We study the magnon modes in the presence of a topological soliton in a 2d Heisenberg easy-axis ferromagnet. The problem of magnon scattering on the soliton with arbitrary relation between the soliton radius R and the "magnetic length" Delta_0 is investigated for partial modes with different values of the azimuthal quantum numbers m. Truly local modes are shown to be present for all values of m, when the soliton radius is enough large. The eigenfrequencies of such internal modes are calculated analytically on limiting case of a large soliton radius and numerically for arbitrary soliton radius. It is demonstrated that the model of an isotropic magnet, which admits an exact analytical investigation, is not adequate even for the limit of small radius solitons, R<<Delta_0: there exists a local mode with nonzero frequency. We use the data about local modes to derive the effective equation of soliton motion; this equation has the usual Newtonian form in contrast to the case of the easy-plane ferromagnet. The effective mass of the soliton is found.Comment: 33 pages (REVTeX), 12 figures (EPS

Effect of Homogenous and Localized Supply of Various N-forms on Growth and Mineral Element Composition of Rape Cultivars in a Split-root System

In most agricultural soils of high fertility, nitrate is the predominant nitrogen (N) form in the soil solution and is, therefore, supposed to be the most important N source for plant uptake. On the other hand, plants usually perform best on mixtures of both N-forms compared to sole nitrate or ammonium. The aim of this study was to determine the effects of homogenous (HS) and localized (LS) supply of various N-forms on growth, morphology and mineral element composition of two oilseed rape cultivars. Two winter oilseed rape cultivars (Apex, Bristol) were grown in a double split-root vessel (2 x 5.5 L) filled continuously aerated nutrient solution. One from each cultivar, 2 plants were grown under 1000 &micro;M N concentration with six different N form combinations [(AA ,NN, MM, NA, A0, N0); (A: ammonium, N: nitrate, M: ammoniumnitrate 0: no N)] and with four replications. As the N source, Ca(NO3)2, and (NH4)2SO4 was used. The pH was maintained neutral by adding CaCO3 and to hamper nitrification, a nitrification inhibitor was applied. HS supply of MM increased the shoot and root growth compared to HS supply of NN and LS supply of NA. Strong reduction in shoot and root dry weight occurred when ammonium was supplied as HS (AA) or LS (A0). Apex showed more, but Bristol less sensitivity to ammonium. No significant differences existed among two cultivars. Similar to shoot and root dry weight, total root length significantly decreased under HS supply of AA or LS supply of A0. In contrast to dry weight reduction, total root length was increased at LS supply of N0. HS supply of AA and LS supply of A0 increased considerably the shoot N concentration whereas strongly reduced Ca2+ and K+ concentrations. In contrast to ammonium, the shoot Ca2+ and K+ concentrations were increased at HS supply of MM, NN and LS supply of NA and N0. Experimental results showed that shoot and root growth were enhanced when ammonium and nitrate were homogenously supplied whereas a slight decline occurred due to LS supply of both N-forms. Therefore, for a better yield both N-forms should be evenly distributed in the soil or in other growth mediums. Under HS supply of AA the root growth was impeded and shoot N accumulation was increased while the uptake of Ca2+ and K+ was decreased and may therefore plants showed severe ammonium toxicity. However, this problem could not be solved by supplying of ammonium only at one side of the compartment (A0).&nbsp;Keywords: Nitrogen, cultivar, N-form, split-root morphology</p