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Open Access Publications from the University of California

Scholarly Works (9 results)

  • Thesis
  • Peer Reviewed
UC San Diego Electronic Theses and Dissertations (2016)

The dissertation explores in detail the formation of dipole stabilized skyrmions and skyrmion lattices that we discovered in amorphous Fe/Gd multilayers. The dipole skyrmion phase exists in amorphous Fe/Gd films which is commonly associated as a classical magnetic bubble material. However, as we discuss throughout the thesis, there are clear distinctions between both magnetic textures in topology, magnetic energies required to favor their formation and their response under external perturbations. Bubble domains typically form in films where the uniaxial anisotropy Ku exceeds the demagnetization anisotropy Kd or have a Q = Ku/Kd > 1 whereas the dipole skyrmions and skyrmion lattices we report exist in thin films with Q < 1.

Over the course of the thesis, we present comprehensive studies as individual chapters describing the magnetic, ferromagnetic resonance and transport properties associated with the dipole skyrmions. For each of these studies, we first identified the dipole skyrmion phase using real and reciprocal space imaging techniques and then quantified the respective properties of these films. We then established a one-to-one correlation between magnetic, ferromagnetic resonance and transport measurements and the domain morphologies observed thus allowing us to assign unique attributes to the dipole skyrmion phase. With aid of numerical simulations, we are able to further validate our experimental observations and investigate additional magnetic information that is not easily accessible in experiments.

Overall, dipole skyrmions possess interesting physics that are comparable and complements the physics observed in skyrmions stabilized by the Dzyaloshinskii–Moriya interaction. The sub-100nm features exists over a broad range of temperatures and magnetic fields which can further tuned by modifying the film composition or thickness. The dipole skyrmion phase exists in a material parameter space that had previously not been investigated by neither bubble domain nor skyrmion scientific communities. Last, our results show that a lot of the interesting physics resulting from these textures appears to be a result of topology and not the physical mechanism that forms these chiral cylindrical-like textures.

    Cover page: Topological Properties of Magnetic Nanostructures in Amorphous Fe/Gd Multilayers
    • Article
    • Peer Reviewed
    UC San Diego Previously Published Works (2020)
    We have studied the effects of electrical current pulses on skyrmion formation in a series of Co/Ni/Pt-based multilayers. Transmission x-ray microscopy reveals that by applying electrical current pulses of duration and current density on the order of 50 μs and 1.7×1010A/m2, respectively, in an applied magnetic field of 50 mT, stripe-to-skyrmion transformations are attained. The skyrmions formed by such pulses remain stable across a wide range of magnetic fields, including zero field. We attribute the transformation primarily to current-induced Joule heating on the order of ∼128 K. Reducing the magnetic moment and perpendicular anisotropy using thin rare-earth spacers reduces the pulse duration, current density, and magnetic field necessary to 25 μs, 2.4×109A/m2, and 27 mT, respectively. These findings show that energetic inputs allow for the formation of skyrmion phases in a broad class of materials, and that material properties can be tuned to yield more energy-efficient access to skyrmion phases.
      Cover page: Energy-efficient generation of skyrmion phases in Co/Ni/Pt-based multilayers using Joule heating
      • Article
      • Peer Reviewed
      UC San Diego Previously Published Works (2018)
      We demonstrate deterministic control of dipole-field-stabilized skyrmions by means of spin-orbit torques arising from heavy transition-metal seed layers. Experiments are performed on amorphous Fe/Gd multilayers that are patterned into wires and exhibit stripe domains and dipole skyrmions at room temperature. We show that while the domain walls and skyrmions are achiral on average due to lack of Dzyaloshinskii-Moriya interactions, the Néel-like closure domain walls at each surface are chiral and can couple to spin-orbit torques. The current-induced domain evolutions are reported for different magnetic phases, including disordered stripe domains, coexisting stripes and dipole skyrmions, and a close-packed dipole skyrmion lattice. The magnetic textures exhibit motion under current excitations with a current density ∼108A/m2. By comparing the motion resulting from magnetic spin textures in Fe/Gd films with different heavy transition-metal interfaces, we confirm spin currents can be used to manipulate achiral dipole skyrmions via spin-orbit torques. We further show the current-induced response of a dipole skyrmion lattice where skyrmions move in channels between pinned regions.
        Cover page: Spin-orbit torque induced dipole skyrmion motion at room temperature
        • Article
        • Peer Reviewed
        UC San Diego Previously Published Works (2022)
          Cover page: Evolution of Novel Chiral Spin Textures in Fe/Gd Based Multilayer Thin Films
          • Article
          • Peer Reviewed
          UC San Diego Previously Published Works (2017)
          Recently, Lorentz transmission electron microscopy (LTEM) has helped researchers advance the emerging field of magnetic skyrmions. These magnetic quasi-particles, composed of topologically non-trivial magnetization textures, have a large potential for application as information carriers in low-power memory and logic devices. LTEM is one of a very few techniques for direct, real-space imaging of magnetic features at the nanoscale. For Fresnel-contrast LTEM, the transport of intensity equation (TIE) is the tool of choice for quantitative reconstruction of the local magnetic induction through the sample thickness. Typically, this analysis requires collection of at least three images. Here, we show that for uniform, thin, magnetic films, which includes many skyrmionic samples, the magnetic induction can be quantitatively determined from a single defocused image using a simplified TIE approach.
            Cover page: Streamlined approach to mapping the magnetic induction of skyrmionic materials
            • Article
            • Peer Reviewed
            LBL Publications (2024)
            Manipulating the topological properties of spin textures in magnetic materials is of great interest due to the rich physics and promising technological applications of these materials in advanced electronic devices. A spin texture with desired topological properties can be created by magnetic monopole injection, resulting in topological transitions involving changes in the topological charge. Therefore, controlling magnetic monopole injection has paramount importance for obtaining the desired spin textures but has not yet been reported. Here, we report the use of reliably manipulated magnetic monopole injection in the topological transition from stripe domains to skyrmions in an Fe/Gd multilayer. An easily tunable in-plane magnetic field applied to an Fe/Gd multilayer plays a key role in the magnetic monopole injection by modulating the local exchange energy. Our findings facilitate the efficient management of topological transitions by providing an important method for controlling magnetic monopole injection.
              Cover page: Manipulation of the magnetic monopole injection for topological transition
              • Article
              • Peer Reviewed
              UC San Diego Previously Published Works (2019)
              Magnetic skyrmions have captivated physicists due to their topological nature and novel physical properties. In addition, skyrmions hold significant promise for future information technologies. A key barrier to realizing skyrmion-based devices has been stabilizing these spin structures under ambient conditions. In this paper, we demonstrate that the tunable magnetic properties of amorphous Fe/Gd mulitlayers enable the formation of skyrmion lattices which are stable over a large temperature and magnetic field parameter space, including room temperature and zero magnetic field. These skyrmions, having a hybrid nature displaying both Bloch-type and Néel-type characteristics, are stabilized by dipolar interactions rather than Dzyaloshinskii-Moriya interactions, typically considered a requirement for the generation of skyrmions. Small angle neutron scattering (SANS) was used in combination with soft x-ray microscopy to provide a unique, multiscale probe of the local and long-range order of these structures. The hexagonal lattice seen in SANS results from the hybrid skyrmion picture obtained with micromagnetic simulations. These results identify a pathway to engineer controllable skyrmion phases in thin film geometries which are stable at ambient conditions.
                Cover page: Realization of ordered magnetic skyrmions in thin films at ambient conditions
                • Article
                • Peer Reviewed
                UC Santa Cruz Previously Published Works (2024)
                The presence of multiple competing periodicities may result in a system to go through states with modulated periodicities, an example of which is the self-similar staircase-like structure called the Devil’s Staircase. Herein we report on a novel staircase structure of domain periodicity in an amorphous and centrosymmetric Fe/Gd magnetic thin film system wherein the reciprocal space wavevector Q due to the ordered stripe domains does not evolve continuously, rather exhibits a staircase structure. Resonant X-ray scattering experiments show jumps in the periodicity of the stripe domains as a function of an external magnetic field. When resolved in components, the length-scale step change along Q x was found to be an integral multiple of a minimum step height of 7 nm, which resembles closely to the exchange length of the system. Modeling the magnetic texture in the Fe/Gd system as an achiral spin arrangement, we have been able to reproduce the steps in the magnetization using a Landau-Lifshitz spin dynamics calculation. Our results indicate that anisotropy and not the dipolar interaction is the dominant cause for the staircase pattern, thereby revealing the effect of achiral magnetism.
                  Cover page: Periodicity staircase in a centrosymmetric Fe/Gd magnetic thin film system
                  • Article
                  • Peer Reviewed
                  UC San Diego Previously Published Works (2020)
                  Topological protection precludes a continuous deformation between topologically inequivalent configurations in a continuum. Motivated by this concept, magnetic skyrmions, topologically nontrivial spin textures, are expected to exhibit topological stability, thereby offering a prospect as a nanometer-scale nonvolatile information carrier. In real materials, however, atomic spins are configured as not continuous but discrete distributions, which raises a fundamental question if the topological stability is indeed preserved for real magnetic skyrmions. Answering this question necessitates a direct comparison between topologically nontrivial and trivial spin textures, but the direct comparison in one sample under the same magnetic fields has been challenging. Here we report how to selectively achieve either a skyrmion state or a topologically trivial bubble state in a single specimen and thereby experimentally show how robust the skyrmion structure is in comparison with the bubbles. We demonstrate that topologically nontrivial magnetic skyrmions show longer lifetimes than trivial bubble structures, evidencing the topological stability in a real discrete system. Our work corroborates the physical importance of the topology in the magnetic materials, which has hitherto been suggested by mathematical arguments, providing an important step toward ever-dense and more-stable magnetic devices.
                    Cover page: Direct Demonstration of Topological Stability of Magnetic Skyrmions via Topology Manipulation
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