Kunz, Andrew

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Marquette Universitye-Publications@MarquettePhysics Faculty Research and Publications Physics, Department of9-24-2010Improved Magnetic Domain-wall Control withTransverse FieldsAndrew KunzMarquette University, andrew.kunz@marquette.eduPublished version. SPIE Newsletter (September 2010), DOI. © 2010 Society of Photo-OpticalInstrumentation Engineers. Used with permission.One print or electronic copy may be made for personal use only. Systematic reproduction anddistribution, duplication of any material in this paper for a fee or for commercial purposes, ormodification of the content of the paper are prohibited.10.1117/2.1201008.003204Improvedmagnetic domain-wallcontrol with transverse fieldsAndrew KunzFull control of fast, high-density, nonvolatile data-storage devicesenables reliable injection, motion, and positioning of domain walls.Understanding the motion of domain walls along planar nano-wires is of interest because of their potential for creating newnonvolatile, fast, high-density memory and logic devices.1, 2 Theviability of such devices depends on our ability to quickly andreliably control an individual domain wall from creation to stor-age in the presence of other domain walls. Control is possiblewhen the system’s structure is known. This knowledge is cru-cial for avoiding Walker breakdown,3 which occurs at a criticaldriving field and limits the domain-wall speed by altering itsmagnetic structure.To avoid Walker breakdown, the process of injecting, moving,and storing the structure must be completed by applying smallmagnetic fields to the system. Unfortunately, large magneticfields are typically necessary for injection into the wire. In addi-tion, when two domain walls are placed into a wire, they movein opposite directions because of the magnetic field. To avoidthese difficulties, we show how an additional in-plane field com-ponent, applied perpendicularly to the driving field, can be usedin combination with pads and notches placed along the wire, toquickly and reliably control individual domain walls.The additional, transverse magnetic-field component givesrise to a significant improvement in our ability to control thesestructures. We use the transverse field to create a domain wallwith a known magnetic configuration.4 Knowing the magneticstate is necessary because it is possible to speed up the systemby maintaining the transverse field parallel to the magneticmoments within the wall, and to slow it down by configuringthe transverse field antiparallel.5 Controlling the speed of thesystem enables precise placement of the wall along the wire.Figure 1 shows our nanopad and wire structure, as well asthe simulated magnetic-field configuration (represented by ar-rows). A domain wall is found in the region between oppositelyoriented domains (represented by the red and blue shading).Starting with the magnetic state shown in Figure 1(a), we applyFigure 1. Physical nanopad and nanowire, with arrows representingthe magnetic-field structure. The red and blue shading is primarily in-tended to orient the eye to domain-wall transitions. A combination ofmagnetic fields is used to control the fast injection, motion, and posi-tioning of domain walls in the wire. t: Time.a transverse field oriented upward and a small driving fieldpointing to the right. The field combination and structural shapefacilitate fast injection of a domain wall with a magnetic config-uration pointing upward: see Figure 1(b).We use the notches along the top edge of the nanowire tocapture the domain wall for storage. A fast-moving system hasenough energy to overcome the potential-energy barriers set bythe notches. By maintaining a transverse field oriented upward,the structure’s speed remains high until it has passed the firstand second notch. Removal or reversal of the transverse fieldleads to a decrease in domain-wall speed. Figure 1(c) shows thatthe wall is captured at the center notch.Continued on next page10.1117/2.1201008.003204 Page 2/2Figure 2. The three-notch configuration leads to eight distinct mag-netic states. We arbitrarily assign a ‘1’ if a domain wall is captured ata notch and a ‘0’ otherwise. Transverse fields can be used to assist a fasttransition between states with the same number of domain walls (e.g.,101! 011).Once a domain wall is captured at a notch, a reversal of thetransverse and driving fields allows injection of a second domainwall—see Figure 1(d)—and to move it to its desired location: seeFigure 1(e). Importantly, the combination of field reversals leavesthe original domainwall in its location as the new systems are in-jected andmoved around. The three-notch configuration enableseight different magnetic states (see Figure 2), each of which canbe quickly created from the original state.In summary, a driving field is necessary to quickly move a do-main wall along the axis of a planar nanowire, but for accuratecontrol a transverse field is essential. Application of a transversefield leads to a significant reduction in the injection field neededto create the system (thus avoiding Walker breakdown), guaran-tees the magnetic structure of the domain wall (which improvesour control of the structure’s speed), and enables control of itsfinal resting site in the wire. This improved level of control forfield-driven domain walls impacts the potential for high-speeddata storage. We continue to work on optimizing the scaling andimproving timing.This work is supported by the National Science Foundation (DMR-0706194).Author InformationAndrew KunzMarquette UniversityMilwaukee, WIAndrew Kunz received his PhD from the University of Min-nesota in 2000 and is currently an assistant professor of physics.His expertise is in magnetization dynamics using micromag-netic simulation. He has six refereed articles in the last two yearswith undergraduate-student co-authors related to domain-wallmotion.References1. D. A. Allwood, G. Xiong, C. C. Faulkner, D. Atkinson, D. Petit, andR. P. Cowburn,Magnetic domain-wall logic, Science 309, pp. 1688–92, 2005.2. S. S. P. Parkin, M. Hayashi, and L. Thomas, Magnetic domain wall racetrack mem-ory, Science 320, pp. 190–194, 2008.3. N. L. Schryer and L. R. Walker, The motion of 180 degree domain walls in uniformDC magnetic fields, J. Appl. Phys. 45 (12), pp. 5406–21, 1974.4. A. Kunz and S. C. Reiff, Dependence of domain wall structure for low field injectioninto magnetic nanowires, Appl. Phys. Lett. 94, p. 192504, 2009.5. A. Kunz and S. C. Reiff, Enhancing domain wall speed in nanowires with transversemagnetic fields, J. Appl. Phys. 103, p. 07D903, 2008.c 2010 SPIE

Improved Magnetic Domain-wall Control with Transverse Fields

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Improved Magnetic Domain-wall Control with Transverse Fields

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