X-Ray Imaging of the Dynamic Magnetic Vortex Core Deformation

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Magnetic thin-film nanostructures can exhibit a magnetic vortex state in which the magnetization vectors lie in the film plane and curl around in a closed loop. At the very center of the vortex, a small, stable core exists where the magnetization points either up or down out of the plane. Three years ago, the discovery of an easy core reversal mechanism at the ALS not only made the possibility of using such systems as magnetic memories much more realistic, it also initiated investigation of the core switching mechanism itself. Now, a Belgian–German–ALS collaboration has used high-resolution, time-resolved, magnetic x-ray microscopy to experimentally reveal the first step of the reversal process: the dynamic deformation of the vortex core. The group also measured a critical vortex velocity above which reversal occurs. Both these observations provide the first experimental support for the postulated reversal mechanism.

The Twitch Before the Switch

In digital electronics, data storage comes down to switching between two physical states such as the presence or absence of an electrical charge (a memory microchip) or the direction of the magnetization in a magnetic material (a hard drive). One state represents the binary 0 and the other the binary 1. Driven by the consumer's insatiable demand for inexpensive devices that store more data in a smaller area and access it faster while consuming less power, the data storage industry is ever on the lookout for new materials with new switching mechanisms.

Uncovered by basic research into the fundamentals of magnetism, one such candidate consists of miniscule magnetic vortices like miniature magnetic whirlpools in nanometer-scale magnetic films. At the core of each vortex, the magnetization can point vertically up or down out of the film, thereby providing a possible new data storage scheme, provided that the vortex polarization can be easily and reliably switched from one state to the other. Recently, magnetic microscopy studies of permalloy (a nickel–iron magnetic alloy) revealed a way to easily switch the vortex polarization with a small alternating electrical current, but how the switching actually occurred has remained speculative in the absence of further experimental evidence. Vansteenkiste et al. have now provided the evidence by detailed magnetic imaging that captures the initial stage of the process, revealing features that confirm the leading theory.

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.

Left: Scanning electron micrographs of the sample structure. A radiofrequency current through a copper stripline (light line) on which the square-shaped permalloy nanostructures (center) are placed provides the magnetic excitation. A very thin silicon nitride membrane (dark area), transparent to x rays, supports the structure. Right: Magnetic STXM image of the out-of-plane magnetization component of a rapidly gyrating vortex. The white spot indicates the core points up. The black contrast is the region where the vortex core is dynamically deformed.

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.

Top: Simulated and experimentally obtained images of the moving vortex core (red spot inside circle) and deformation with opposite polarization (blue spot) just before a vortex-antivortex pair nucleates next to the core. Because of the differential imaging required for dichroism measurements, the core and the deformation appear twice, but the agreement between experiment and simulation is clear. Bottom: Out-of-plane magnetization profiles of the dynamically deformed vortex core, from experiment and simulation. The region with opposite magnetization is clearly visible next to the core. The color scale measures the out-of-plane magnetization.

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).

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