Spin wave mediated magnetic vortex core reversal studied by time-resolved x-ray microscopy
Dr. Hermann Stoll (MPI for Intelligent Systems, Stuttgart)

Thin-film soft magnetic platelets of suited micron or sub-micron sizes are characterized by an in-plane closed flux of magnetization, minimizing the dipolar energy. However, at the center, the exchange energy forces the magnetization out-of-plane in a small area of only a few exchange lengths in diameter creating the vortex core with a distinct polarization, either up or down. This can be regarded as a one bit magnetic memory cell.
Essential progress in the understanding of nonlinear vortex dynamics was achieved when low-field vortex core reversal by (sub-GHz) excitation of the vortex gyromode was found using time-resolved scanning transmission X-ray microscopy. This switching scheme, based on the creation and subsequent annihilation of a vortex–antivortex (VA) pair, has been proved to be universal and independent of the type of excitation, e.g., pulsed magnetic fields or spin transfer torque (STT). This discovery not only opened up new routes for using the magnetic vortex core reversal for spintronics applications, e.g. V(ortex)MRAM, but also initiated wide investigations on the physics behind the VA mediated switching mechanism.
Magnetic vortex structures possess azimuthal spin wave modes showing eigenfrequencies in the multi-GHz range. We could demonstrate by experiments and micromagnetic simulations that even much faster unidirectional vortex core reversal can be achieved by exciting azimuthal spin wave modes with (multi-GHz) rotating magnetic fields or electrical currents (STT). In that way we have been able to switch a vortex core within less than 100 ps.
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