|X-Ray Imaging of the Dynamic Magnetic Vortex Core Deformation|
The existence of the two core polarization states made magnetic data storage (with each vortex storing one bit) an intriguing but unrealistic concept because of the very strong magnetic fields of around half a tesla initially thought to be necessary for reversing the polarization of these highly stable vortices. The vortex core diameter, typically only 10–25 nm, is the source of the so-called vortex gyration mode, corresponding to a circular motion of the vortex. The memory concept received a boost when the group earlier showed that a low-field excitation of this mode can switch the out-of-plane polarization of the core. However, the dynamic process behind the switching could at the time only be inferred from micromagnetic modeling.
For example, theoretical modeling predicted that the reversal is mediated by the creation and annihilation of a vortex–antivortex pair, a mechanism similar to the creation and annihilation of particle–antiparticle pairs. Near a rapidly moving vortex core, a region appears where the magnetization starts to turn towards the opposite direction. In this so-called vortex-core deformation, a vortex–antivortex pair will nucleate. The newly created antivortex then rapidly annihilates with the original vortex, leaving behind only the newly created vortex with a reversed core polarization. Since the proposal of this mechanism, it has been studied by numerous research groups, resulting in more than 100 publications to date. However, experimental support had been lacking until the new magnetic microscopy results.
To obtain high-resolution images of the vortex core dynamics in square-shaped 500-nm-wide permalloy (nickel–iron magnetic alloy) nanostructures, the group used the scanning transmission x-ray microscope at ALS Beamline 11.0.2, which has a resolution of about 30 nm. Using x-ray magnetic circular dichroism (XMCD) for magnetic contrast allowed direct imaging of the vortex core. The vortex was excited with radiofrequency magnetic fields while stroboscopic images of the moving vortex core were recorded by exploiting the pulsed nature of the synchrotron light. The width of the photon flashes limited the time resolution of these images to about 100 ps.
When strongly excited, a vortex exhibited a gyration velocity of 260 m/s, very close to the switching threshold. Recorded images of this rapidly moving core revealed a spot near the vortex core with an opposite magnetization. This spot was identified as the dynamic vortex core deformation, the predicted nucleation site of the vortex-antivortex pair. At slightly higher gyration velocities, the collaboration also experimentally observed the predicted threshold for core reversal.
In sum, the collaboration has provided the first strong experimental support for the microscopic switching model via vortex-antivortex creation and annihilation. This is the first time that "internal" dynamics of the vortex core could be imaged by time-resolved x-ray microscopy. Further improvement of the spatial and temporal resolution may even open the possibility to observe the vortex-antivortex pair creation and annihilation itself.
Research conducted by A. Vansteenkiste (Ghent University, Belgium); K.W. Chou and T. Tyliszczak (ALS); M. Weigand, M. Curcic, V. Sackmann, H. Stoll, G. Schütz, and B. Van Waeyenberge (Max-Planck-Institut für Metallforschung, Germany); and G. Woltersdorf and C.H. Back (Universität Regensburg, Germany).
Research funding: The Institute for the Promotion of Innovation by Science and Technology in Flanders (IWT-Flanders) and the German Research Foundation (DFG), and the Research Foundation Flanders (FWO-Flanders). Operation of the ALS is supported by the U.S. Department of Energy, Office of Basic Energy Sciences.
Publication about this research: A. Vansteenkiste, K.W. Chou, M. Weigand, M. Curcic, V. Sackmann, H. Stoll, T. Tyliszczak, G. Woltersdorf, C.H. Back, G. Schütz, and B. Van Waeyenberge, "X-ray imaging of the dynamic magnetic vortex core deformation," Nature Phys. 5, 332 (2009).