For data storage purposes, magnetic vortices can be trapped in lithographically defined rectangular or circular magnetic patterns. They are of considerable technological interest because a low stray magnetic field leads to a magnetic stability and minimizes the cross-talk between adjacent vortices—two prerequisites for high storage densities. At low frequencies, magnetic friction (damping) governs the response as the magnetic moments rotate into the direction of an applied field. But at gigahertz frequencies and above, precession is the dominant process. For the microscopic study of such ultrafast magnetization dynamics, the collaborators developed a novel technique based on the 70-picosecond-long x-ray pulses of the ALS, which can be used like light flashes from a strobe to freeze the dynamics and to acquire a snapshot of the motion of the magnetization.

In their experiment at ALS Beamline 7.3.1.1, a light flash from a laser activates a gallium arsenide photo switch, which is integrated into the sample, and launches a 300-picosecond-long electrical current pulse into a waveguide optimized for the transmission of short pulses. The current pulse generates a powerful magnetic field pulse that initiates the dynamics. After a controllable time delay, an x-ray pulse illuminates the sample, resulting in a photoelectron image that is then magnified a thousand-fold by the optics of a photoemission electron microscope (PEEM-2). An electronic (CCD) camera accumulates the signal from millions of x-ray pulses until an image of sufficient quality characterizing the current state of the magnetization of the sample has been obtained.

Laser pulses (red) generate a current pulse, resulting in a magnetic field that initiates the magnetization dynamics. X-ray pulses (blue) probe the sample at 100-picosecond time intervals. The electron image is detected by the photoemission electron microscope.

About 40 magnetic structures of different sizes and shapes were patterned by focused ion beam lithography into a 20-nm thick, magnetically soft cobalt–iron alloy film on a copper waveguide. Four triangular magnetic domains meeting in the center of a structure give rise to a vortex where the domain walls intersect, so that the magnetization curls around the vortex center or core. X-ray magnetic circular dichroism (XMCD) at transition-metal L edges probes the direction and size of the element-specific magnetic moments within the domains. For the cobalt–iron samples, the researchers computed the images at 100-picosecond intervals over several nanoseconds as the ratio of two PEEM images acquired at the cobalt L₃ and L₂ absorption edges.

Top: Domain structure of magnetic vortices showing the directions of the magnetization (white arrows), the vortex handedness (hands), and the out-of-plane core magnetization (green arrows). Bottom: Simulated trajectories of left- and right-handed vortices. Red arrows are the vortex acceleration directions.

In this way, they observed two phases of vortex dynamics: an initial linear acceleration in response to the field pulse, followed by a gyrotropic (spiraling) motion of the vortex core around the pattern center. A "hidden" out-of-plane magnetization in the nanometer-scale vortex core induces a three-dimensional handedness or chirality in the planar magnetic structure. The result is a precessional motion of the core parallel to the subnanosecond in-plane field pulse. The displacement of the core causes an imbalance of the in-plane magnetization within the structure, creating a magnetostatic field perpendicular to the displacement that drives the vortex on a spiraling trajectory. The observed gyrotropic motion corresponds to a subgigahertz mode seen in micromagnetic simulations and recent magneto-optical experiments.

Click on the image above to see a time-resolved PEEM movie of vortex pattern motion over a period of 8 ns after the driving field pulse. The movie shows the original image (left) and gradient image (right), with enhanced contrast of domain walls and vortex core.

Research conducted by S.-B. Choe, A. Scholl, A. Doran, and H.A. Padmore (ALS); Y. Acremann and J. Stöhr (Stanford Synchrotron Radiation Laboratory); and A. Bauer (ALS, SSRL, and Freie Universität Berlin, Germany).

Research funding: U.S. Department of Energy, Office of Basic Energy Sciences (BES), and Laboratory Directed Research and Development Program of Berkeley Lab. Operation of the ALS is supported by BES.

Publication about this research: S.-B. Choe, Y. Acremann, A. Scholl, A. Bauer, A. Doran, J. Stöhr, and H.A. Padmore, "Vortex-driven magnetization dynamics," Science 304, 420 (2004).

ALSNews Vol. 246, October 27, 2004