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Magnetic vortex core reversal by excitation with short bursts of an alternating field

Abstract

The vortex state, characterized by a curling magnetization, is one of the equilibrium configurations of soft magnetic materials1,2,3,4 and occurs in thin ferromagnetic square and disk-shaped elements of micrometre size and below. The interplay between the magnetostatic and the exchange energy favours an in-plane, closed flux domain structure. This curling magnetization turns out of the plane at the centre of the vortex structure, in an area with a radius of about 10 nanometres—the vortex core5,6,7. The vortex state has a specific excitation mode: the in-plane gyration of the vortex structure about its equilibrium position8,9,10. The sense of gyration is determined by the vortex core polarization11. Here we report on the controlled manipulation of the vortex core polarization by excitation with small bursts of an alternating magnetic field. The vortex motion was imaged by time-resolved scanning transmission X-ray microscopy12. We demonstrate that the sense of gyration of the vortex structure can be reversed by applying short bursts of the sinusoidal excitation field with amplitude of about 1.5 mT. This reversal unambiguously indicates a switching of the out-of-plane core polarization. The observed switching mechanism, which can be understood in the framework of micromagnetic theory, gives insights into basic magnetization dynamics and their possible application in data storage.

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Figure 1: Three-dimensional and two-dimensional representation of vortex and antivortex structures.
Figure 2: Schematic overview of the sample and stripline setup.
Figure 3: Experimental results of switching the vortex core polarization.
Figure 4: Schematic representation of different steps in switching the vortex core polarization.

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Acknowledgements

We thank H. D. Carstanjen, D. Goll, S. Komineas, H. Kronmüller and M. Scheinfein for helpful discussions. Financial support was provided by the Deutsche Forschungsgemeinschaft through the priority programme ‘Ultrafast Magnetisation Processes’. The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the US Department of Energy.

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Supplementary information

Supplementary Video 1

This movie shows the dynamic response of the magnetisation in a 1.5 × 1.5 μm2, 50 nm thick Permalloy element imaged before the burst excitation. The element is excited using an in-plane alternating magnetic field with an amplitude of 0.1 mT and a frequency of 250 MHz. The observed contrast is equivalent to Mx. The vortex core moves counterclockwise (MOV 2348 kb)

Supplementary Video 2

This movie shows the dynamic response of the magnetisation in a 1.5 × 1.5 μm2, 50 nm thick Permalloy element imaged after the burst excitation. The element is excited using an in-plane alternating magnetic field with an amplitude of 0.1 mT and a frequency of 250 MHz. The observed contrast is equivalent to Mx. The vortex core movement has changed to clockwise indicating a switching of the vortex core polarisation. (MOV 2280 kb)

Supplementary Video 3

This movie shows the simulated dynamic response of the magnetisation in a 1.5 × 1.5 μm2, 50 nm thick Permalloy element over 40 ns. The left panel shows the in-plane x-component Mx and the right panel shows the out-of-plane z-component Mz. The lower panel indicates the time structure of the excitation and can be followed with a moving red dot. The element is excited using an in-plane alternating magnetic field along the y-axis with an amplitude of 0.1 mT and a frequency of 250 MHz. The field induces the gyrating motion of the vortex core (counterclockwise) and the movement reaches a stable orbit after a few periods. After approximately 12 ns, a short field burst with a length of one monocycle is applied with an amplitude of 3.5 mT. The simulation shows that the vortex core is deformed and becomes unstable. This results in a flip of the core polarisation and the sense of gyration changes to clockwise. The abrupt switch is accompanied by the creation of spin waves i.e. energy is transferred from the vortex gyration mode to the spin wave modes and dissipates over the element. After approximately 28 ns, a same field burst was applied to switch back the vortex core polarisation. (MOV 2275 kb)

Supplementary Video 4

This movie shows the zoomed-in simulated dynamic response of the magnetisation in a 1.5 × 1.5 μm2, 50 nm thick Permalloy element during the switching process. An area of 150 nm × 150 nm, in a timewindow of 720 ps, is shown. The arrows represent the in-plane magnetisation components while the coloured areas represent the out-of-plane component (blue = up, red = down). (MOV 2705 kb)

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Van Waeyenberge, B., Puzic, A., Stoll, H. et al. Magnetic vortex core reversal by excitation with short bursts of an alternating field. Nature 444, 461–464 (2006). https://doi.org/10.1038/nature05240

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