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Scanning transmission x-ray microscopy is employed to investigate experimentally the reversal of the magnetic vortex core polarity in cylindrical Ni81Fe19 nanodisks triggered by two orthogonal monopolar magnetic field pulses with peak amplitude B0, pulse length κ = 60 ps, and delay time Δt in the range from -400 ps to +400 ps between the two pulses. The two pulses are oriented in-plane in the x- and y-directions. We have experimentally studied vortex core reversal as a function of B0 and Δt. The resulting phase diagram shows large regions of unidirectional vortex core switching where the switching threshold is modulated due to resonant amplification of azimuthal spin waves. The switching behavior changes dramatically depending on whether the first pulse is applied in the x- or the y-direction. This asymmetry can be reproduced by three-dimensional micromagnetic simulations but not by two-dimensional simulations. This behavior demonstrates that in contrast to the previous experiments on vortex core reversal, the three-dimensionality in the dynamics is essential here. [ABSTRACT FROM AUTHOR]

We experimentally demonstrate that unidirectional reversal of the magnetic vortex core polarity is possible by excitation with sub-100-ps-long orthogonal monopolar magnetic pulse sequences in a wide range of pulse lengths and amplitudes. The application of such short digital pulses is a favorable excitation scheme for technological applications. Measured phase diagrams of this unidirectional, spin-wave mediated vortex core reversal are in good qualitative agreement with phase diagrams obtained from micromagnetic simulations. The time dependence of the reversal process, observed by time-resolved scanning transmission x-ray microscopy indicates a switching time of 100 ps and fits well with our simulations. The origin of the asymmetric response to clockwise and counterclockwise excitation which is a prerequisite for reliable unidirectional switching is discussed, based on the gyromode-spin-wave coupling. Situations are found in which a three-dimensional dynamics is important, because a vortex-antivortex pair starts to form close to the core of the original vortex in the lower part of the disk without completing the formation across the whole thickness so that it dissolves later on and does not lead to switching of the original vortex core. [ABSTRACT FROM AUTHOR]

We show, by experiments and micromagnetic simulations in vortex structures, that an active "dual frequency" excitation of both the sub-GHz vortex gyromode and multi-GHz spin waves considerably changes the frequency response of spin wave mediated vortex core reversal. Besides additional minima in the switching threshold, a significant broadband reduction of the switching amplitudes is observed, which can be explained by non-linear interaction between the vortex gyromode and the spin waves. We conclude that the well known frequency spectra of azimuthal spin waves in vortex structures are altered substantially, when the vortex gyromode is actively excited simultaneously. [ABSTRACT FROM AUTHOR]