Brooke Mesler

University of California, Berkeley, Berkeley, California, United States

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Publications (11)12.56 Total impact

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    ABSTRACT: We report experimental evidence of nonlinear gyrotropic vortex core motion. Using soft x-ray transmission microscopy we observed the time-averaged dynamic response of a magnetic vortex core in a 2 lm diameter, 100 nm thick permalloy (Ni 80 Fe 20) disk as a function of the amplitude and frequency of an applied RF magnetic field. At lower amplitude fields a single resonance was observed, but two distinct resonances, above and below the low amplitude resonance frequency, were observed when higher amplitude fields were applied. The results are discussed in the context of a nonlinear vortex energy potential. V C 2012 American Institute of Physics. [doi:10.1063/1.3678448] Magnetic vortex structures, which form in micronsized circular permalloy (Ni 80 Fe 20) disks, 1 consist of an in-plane circulating magnetic domain with two possible chiralities and an out-of-plane singularity in the center, the vortex core (VC), with two possible polarities. 2–4 The dynamic response of VCs to applied fields and currents has received significant attention 5,6 because of its relevance to future technologies, 6 and as a model system for fundamental studies of nanoscale magnetism. 7–9 Recently it has been shown experimentally and theoreti-cally that the core polarity can be flipped by an applied field or current pulse, 10 particularly when the VC is undergoing resonant gyrotropic motion. The gyrotropic resonance fre-quencies are usually several hundreds of MHz. 11,12 Open questions remain, especially concerning its driven motion for high amplitude driving fields. Here we use soft x-ray transmission microscopy to examine the dynamic behavior of the VC in 2 lm diameter, 100 nm thick Ni 80 Fe 20 disks. We record time-averaged images of the VC while applying a RF magnetic field to explore the gyrotropic VC motion as a function of driving magnetic field frequency and amplitude. For low amplitude applied fields we find the expected response of the core, that is, amplified motion at the gyrotropic resonance frequency. However, at higher driving magnetic amplitudes, there appear to be two distinct frequency ranges where the core has a dynamic response. The experiments were conducted at XM-1, the full field transmission soft x-ray microscope at beamline 6.1.2 at the Advanced Light Source (ALS) in Berkeley, CA. XMCD pro-vides an element-specific magnetic contrast mechanism. 14 Using Fresnel zone plate optics, XM-1 has achieved a better than 12 nm (Ref. 15) spatial resolution; for this experiment the resolution was about 25 nm. Using e-beam lithography, a Ni 80 Fe 20 disk with a 2 lm diameter and a thickness of 100 nm was fabricated on a gold waveguide structure on an x-ray transparent substrate, a 100 nm thick silicon nitride membrane. A RF current flowed through the waveguide, generating an in-plane oscillating magnetic field. The RF field was applied to the sample with various frequencies and amplitudes over the course of the experiment. The applied field peak amplitudes, H 0 , ranged from 4 Oe to 10 Oe and typical frequencies were on the order of several hundred MHz. Images were taken with normal inci-dence, providing images with the core clearly shown as a dark or bright spot in the sample. An image of the sample with no field applied is shown in Fig. 1. The VC diameter is 52 nm, similar to previous VC studies. 4 Sample preparation and ex-perimental technique are discussed in detail elsewhere. 16
    Journal of Physics Conference Series 01/2012; 111:07D311.
  • Peter Fischer, Mi‐Young Im, Brooke L. Mesler
    11/2010: pages 7 - 37; , ISBN: 9783527632282
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    ABSTRACT: We observed a motion of magnetic vortex core in a hexagonal Permalloy pattern by means of soft x-ray microscopy. Pump-probe stroboscopic observation on a picosecond timescale has been carried out after exciting a ground state vortex structure by an external field pulse of 1 ns duration. Vortex core is excited off from the center position of the hexagonal pattern but the analysis of the core trajectory reveals that the motion is nongyrotropic.
    Journal of Applied Physics 06/2010; · 2.21 Impact Factor
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    ABSTRACT: Soft x-ray microscopy offers high spatial and temporal resolution imaging with element specific magnetic contrast. As such, it is an ideal method for studying nanoscale spin dynamics, such as vortex core dynamics. At XM-1, the full field soft x-ray transmission microscope at the Advanced Light Source in Berkeley, a technique has been developed for pinpointing vortex dynamics without time resolution. In addition, a phase-locked setup has been used to conduct time resolved experiments of vortex core dynamics. The samples in this study were 100 nm thick, 2 μ m diameter Ni <sub>80</sub> Fe <sub>20</sub> disks. Analysis of nontime resolved images suggested that resonant vortex core dynamics were excited by ac magnetic fields close to 340 MHz . This behavior was confirmed with time resolved imaging and gyrotropic motion of the vortex core was observed.
    Journal of vacuum science & technology. B, Microelectronics and nanometer structures: processing, measurement, and phenomena: an official journal of the American Vacuum Society 02/2010; · 1.36 Impact Factor
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    ABSTRACT: We have investigated a coupled motion of two parallel vortex cores in ferromagnetic/nonmagnetic/ferromagnetic trilayer cylinders by means of micromagnetic simulation. Dynamic motion of two vortices with parallel and antiparallel relative chiralities of curling spins around the vortex cores have been examined after excitation by 1 ns pulsed external field, revealing a nontrivial coupled vortices motion.
    Applied Physics Letters 10/2009; 95(14):142509-142509-3. · 3.79 Impact Factor
  • Brooke Mesler, Dong-Hyun Kim, Peter Fischer
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    ABSTRACT: Soft X-ray microscopy provides element specific magnetic imaging with a spatial resolution down to 15nm. At XM-1, the full-field soft X-ray microscope at the Advanced Light Source in Berkeley, a stroboscopic pump and probe setup has been developed to study fast magnetization dynamics in ferromagnetic elements with a time resolution of 70ps which is set by the width of the X-ray pulses from the synchrotron. Previous studies of patterned permalloy elements have revealed complex magnetization dynamics. Results obtained with a 2mum x 4mum x 45nm rectangular permalloy sample exhibiting a seven domain Landau pattern reveal dynamics up to several nsec after the exciting magnetic field pulse. Domain wall motion, a gyrotropic vortex motion, and a coupling between vortices in the rectangular geometry are observed. On going studies of patterned trilayer elements, composed of magnetic permalloy and cobalt layers separated by a copper spacer layer, will probe the dynamics of the trilayer system. Of particular interest is observing how the coupling between the magnetic layers affects the vortex dynamics.
    03/2009;
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    ABSTRACT: We have investigated a coupled motion of two vortex cores in ferromagnetic/nonmagnetic/ferromagnetic trilayer cynliders by means of micromagnetic simulation. Dynamic motion of two vortex with parallel and antiparallel relative chiralities of curling spins around the vortex cores have been examined after excitation by 1-ns pulsed external field. With systematic variation in non-magnetic spacer layer thickness from 0 to 20 nm, the coupling between two cores becomes significant as the spacer becomes thinner. Significant coupling leads to a nonlinear chaotic coupled motion of two vortex cores for the parallel chiralities and a faster coupled gyrotropic oscillation for the antiparallel chiralities.
    Applied Physics Letters - APPL PHYS LETT. 01/2009;
  • Conference Paper: Soft x-ray microscopy
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    ABSTRACT: form only given. Soft X-ray microscopy is at the forefront of research with spatial resolution approaching 10 nm, and wide ranging applications to the physical and life sciences, including the dynamics of magnetic nanostructures, three-dimensional biotomography at the sub-cellular level, elemental and chemically specific environmental studies. Examples of recent work are shown below in figures 1-3. Figure 1 shows a general layout of the soft X-ray microscope XM-1 at the Advanced Light Source (ALS) synchrotron facility at Lawrence Berkeley National Laboratory. Figure 2 shows an image, at 15 nm spatial resolution, of nanomagnetic structures in a CoCrPt alloy as revealed by X-ray magnetic circular dichroism (XMCD) using synchrotron radiation tuned to the cobalt L3-edge at 778 eV (1.59 nm wavelength). Figure 3 shows a natural contrast tomographic reconstruction of a whole yeast cell imaged in the water window at 500 eV (2.4 nm wavelength).
    IEEE Lasers and Electro-Optics Society, 2008. LEOS 2008. 21st Annual Meeting of the; 01/2008
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    ABSTRACT: Magnetic soft X-ray microscopy images magnetism in nanoscale systems with a spatial resolution down to 15 nm provided by state-of-the-art Fresnel zone plate optics. X-ray magnetic circular dichroism (X-MCD) is used as the element-specific magnetic contrast mechanism similar to photoemission electron microscopy (PEEM), however, with volume sensitivity and the ability to record the images in varying applied magnetic fields which allows study of magnetization reversal processes at fundamental length scales. Utilizing a stroboscopic pump–probe scheme one can investigate fast spin dynamics with a time resolution down to 70 ps which gives access to precessional and relaxation phenomena as well as spin torque driven domain wall dynamics in nanoscale systems. Current developments in zone plate optics aim for a spatial resolution towards 10 nm and at next generation X-ray sources a time resolution in the fs regime can be envisioned.
    Surface Science 10/2007; · 1.84 Impact Factor
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    ABSTRACT: Time-resolved magnetic X-ray microscopy combines in a unique way spatial resolution down to 15 nm with a temporal resolution of less than 100 ps. The former is enabled through state-of-the-art Fresnel zone plates used as X-ray optical elements and the latter is limited by the inherent time structure of current synchrotron radiation X-ray sources. Using a stroboscopic pump and probe scheme spin dynamics in magnetic systems of confined geometries can be imaged with great detail. The magnetization of the elements is excited with fast electronic pulses with a rise time of 100 ps that are launched into waveguide structures. Varying the delay time between the pump and the X-ray probing pulses one can image in real space the temporal evolution of the magnetic configuration in nanoscale elements. This paper is based on an invited talk given at the ICM2006 conference and provides an overview of the current status of time-resolved soft X-ray microscopy. Studies of vortex dynamics in rectangular structures are reported as a specific example.
    Journal of Magnetism and Magnetic Materials 01/2007; · 2.00 Impact Factor
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    ABSTRACT: Soft X-ray microscopy provides element specific magnetic imaging with a spatial resolution down to 15nm. At XM-1, the full-field soft X-ray microscope at the Advanced Light Source in Berkeley, a stroboscopic pump and probe setup has been developed to study fast magnetization dynamics in ferromagnetic elements with a time resolution of 70ps which is set by the width of the X-ray pulses from the synchrotron. Results obtained with a 2 {micro}m x 4 {micro}m x 45nm rectangular permalloy sample exhibiting a seven domain Landau pattern reveal dynamics up to several nsec after the exciting magnetic field pulse. Domain wall motion, a gyrotropic vortex motion, and a coupling between vortices in the rectangular geometry are observed.
    Journal of Vacuum Science & Technology B Microelectronics and Nanometer Structures 01/2007; 25(6):2598-. · 1.36 Impact Factor