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Influence of Domain Wall Pinning on the Dynamic Behavior of Magnetic Vortices Print
Wednesday, 27 August 2008 00:00


Soft magnetic, micron-sized thin-film structures with magnetic vortices are intriguing systems that may one day be used in ultrafast computer memories. In such systems, the otherwise in-plane magnetization turns perpendicular to the plane at the center of the vortex, forming the vortex core. Because such a core has two possible polarizations (up or down) and can be switched between these two states by a small alternating magnetic field, it could serve as a memory bit in future magnetic memory devices. However, these magnetic structures often contain numerous imperfections such as domain wall pinning sites, which have to be taken into account for the practical application of such systems. To study how these defects affect the dynamics of magnetic vortices, researchers from Belgium, Germany, and the United States investigated square-shaped and disk-shaped thin-film structures with artificially introduced imperfections in the form of nanometer-sized holes. They used time-resolved scanning transmission x-ray microscopy (STXM) at ALS Beamline 11.0.2 to determine the frequency at which these vortices vibrate (their eigenfrequency). The imperfections were found to cause a higher vibrational frequency in square-shaped structures, but did not influence the disk-shaped structures. Knowledge of the frequency is crucial for vortex-based memories, since the electric signal for writing data needs to be precisely tuned to it.

Finding the Right Frequency

A big leap in humankind's ability to remember was our discovery that we could house information externally: in songs and stories, on paper and, eventually, on computers. Then digital computing applied the revolutionary on-off switch. Since information is represented using a combination of binary digits (bits, 0s and 1s), ultrasmall systems with binary configurations could be the next thing in ultrafast computer memory storage. Micron-sized magnetic vortex structures are suitable candidates because the polarization at their vortex core can point up or down, a state preserved without any power. In contrast, today's computers lose the contents of their fast random-access memory (RAM) when turned off and only preserve the contents of their much slower hard disk. Magnetic RAM, however, can preserve content without using power and still work much faster than a hard disk. "Magnetic memory" computers could boot-up instantaneously, access files rapidly, require little power, and withstand power outages.

Switching the vortex core (turning "1" to "0" ) can be done very fast with a low-power magnetic field pulse tuned to the intrinsic frequency of the vortex. However, nanometer-sized imperfections introduced in production affect that frequency, making tuning difficult. Researchers using the ALS recreated a common type of magnetic thin-film defect and investigated its influence on frequency. Their results will make tuning this system easier, bringing us closer to the use of these ultrasmall structures in ultrafast computers.

Top: Scanning Electron Microscope (SEM) images of a square- and a disk-shaped structure, with four antidots created by focused ion beam etching. Center: Corresponding STXM images showing the x-component of the magnetization (the arrows also indicate the direction of the magnetization). Bottom: Micromagnetic simulation of the magnetization in these structures.

The in-plane magnetization in a square-shaped structure with a vortex configuration forms four domains separated by domain walls. In a disk-shaped structure, on the other hand, the magnetization curls around the center in a continuous way and no domain walls are formed. In both cases, the magnetic vortex is characterized by a gyrotropic mode, which can be excited using in-plane magnetic fields. When the magnetic field is tuned to the eigenfrequency of the vortex system, the core polarization can easily be switched between the up and down states. This would correspond to writing a "1" or a "0" to a vortex-based memory bit. For such applications, it is thus important to know the eigenfrequency of the system in order to efficiently tune the magnetic field to it.

However, when these thin-film structures are manufactured, numerous imperfections are inevitable due to high production volumes, and these can affect the eigenfrequency. It is already well known that magnetic defects can pin a vortex core as well as the domain walls. When the core is trapped, however, gyration movement is totally suppressed. Therefore, to study the effect of defects on dynamic movement and frequency, the researchers needed to leave the core free to gyrate. They created pinning sites in the form of antidots ("holes") in Permalloy (Ni80Fe20) thin-film structures by means of focused ion bean (FIB) etching and, to avoid the core, they placed the antidots further away from the structure center. In this way, naturally occurring domain wall pinning sites were mimicked in a controlled manner.

Experimentally determined vortex gyration frequencies of a structure with and without domain wall pinning sites. The Fourier transform of the position of the vortex core after a fast magnetic field pulse reveals the eigenfrequency of the mode. All structures with pinning sites were found to have significantly higher eigenfrequencies than unmodified ones.

The magnetic systems were then imaged at ALS Beamline 11.0.2 with STXM. X-ray magnetic circular dichroism (XMCD) was used as a contrast mechanism. The ALS storage ring delivers short but intense photon flashes every 2 ns, which allowed the researchers to record stroboscopic time-resolved images of the magnetic dynamics. The time-resolved images of the vortex gyration obtained in this way allowed them to precisely determine the resonance frequency of the structures. The team discovered that the domain wall pinning sites altered the vortex eigenfrequency of the square-shaped systems by increasing it but did not significantly affect the disk-shaped structures, where no domain walls are present. These results demonstrate that domain wall pinning does cause an increase in frequency. This can be explained by the confinement of the domain wall motion to the portion of the structure that is circumscribed by the holes.



Research conducted by: A. Vansteenkiste, J. De Baerdemaeker, B. Van Waeyenberge (Ghent University, Belgium), K.W. Chou, H. Stoll, M. Curcic, G. Schutz (Max Planck Institute for Metals Research, Germany), T. Tyliszczak (ALS), G. Woltersdorf,C. H. Back (University Regensburg, Germany).

Research funding: The Institute for the Promotion of Innovation by Science and Technology in Flanders (IWT-Flanders), 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 (BES).

Publication about this research: A. Vansteenkiste, J.D. Baerdemaeker, K.W. Chou, H. Stoll, M. Curcic, T. Tyliszczak, G. Woltersdorf, C.H. Back, G. Schutz, and B.V. Waeyenberge, "Influence of domain wall pinning on the dynamic behavior of magnetic vortex structures: Time-resolved scanning x-ray transmission microscopy in NiFe thin film structures," Phys. Rev. B 77, 144420 (2008).