Spin wave mediated unidirectional vortex core reversal by two orthogonal monopolar field pulses: The essential role of three-dimensional magnetization dynamics

Matthias Noske,Manfred Fähnle,Ajay Gangwar,Georg Woltersdorf,Hermann Stoll,Georg Dieterle,Andrei Slavin,Markus Weigand,Johannes Förster,Gisela Schütz,Christian H Back

doi:10.1063/1.4948354

Abstract

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 τ=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.

Highlights

Numbers n and m indicate the number of nodes in radial and azimuthal directions of the mode profile and the sign of m defines the rotational sense of the mode (m < 0: clockwise, and m > 0: counter clockwise)
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 s 1⁄4 60 ps, and delay time Dt in the range from À400 ps to þ400 ps between the two pulses
We have experimentally studied vortex core reversal as a function of B0 and Dt

Summary

Introduction

Numbers n and m indicate the number of nodes in radial and azimuthal directions of the mode profile and the sign of m defines the rotational sense of the mode (m < 0: clockwise, and m > 0: counter clockwise) Due to their symmetry, dipolar spin-wave modes with azimuthal mode numbers jmj 1⁄4 1 can be efficiently excited by magnetic in-plane fields. The fact that digital pulses can be used for fast switching instead of rotating field bursts is an attractive aspect for potential technological applications For all these types of excitations, the dynamic switching mechanism is, in general, the same (see Ref. 12 and references therein). We report on experimental results for the switching of the vortex core by two orthogonal monopolar in-plane magnetic field pulses in x- and y-directions with delay times Dt between the two pulses which are considerably longer than the half width of the pulses. We compare two-dimensional and threedimensional simulations with the experimental findings to demonstrate the importance of a three-dimensional treatment of the problem

Methods

Results

Discussion

Conclusion