Mi-Young Im

Observation of room-temperature magnetic skyrmions and their current-driven dynamics in ultrathin metallic ferromagnets

Magnetic skyrmions are topologically protected spin textures that exhibit fascinating physical behaviours and large potential in highly energy-efficient spintronic device applications. The main obstacles so far are that skyrmions have been observed in only a few exotic materials and at low temperatures, and fast current-driven motion of individual skyrmions has not yet been achieved. Here, we report the observation of stable magnetic skyrmions at room temperature in ultrathin transition metal ferromagnets with magnetic transmission soft X-ray microscopy. We demonstrate the ability to generate stable skyrmion lattices and drive trains of individual skyrmions by short current pulses along a magnetic racetrack at speeds exceeding 100 m s(-1) as required for applications. Our findings provide experimental evidence of recent predictions and open the door to room-temperature skyrmion spintronics in robust thin-film heterostructures.

Tunable room-temperature magnetic skyrmions in Ir/Fe/Co/Pt multilayers

Magnetic skyrmions are nanoscale topological spin structures offering great promise for next-generation information storage technologies. The recent discovery of sub-100-nm room-temperature (RT) skyrmions in several multilayer films has triggered vigorous efforts to modulate their physical properties for their use in devices. Here we present a tunable RT skyrmion platform based on multilayer stacks of Ir/Fe/Co/Pt, which we study using X-ray microscopy, magnetic force microscopy and Hall transport techniques. By varying the ferromagnetic layer composition, we can tailor the magnetic interactions governing skyrmion properties, thereby tuning their thermodynamic stability parameter by an order of magnitude. The skyrmions exhibit a smooth crossover between isolated (metastable) and disordered lattice configurations across samples, while their size and density can be tuned by factors of two and ten, respectively. We thus establish a platform for investigating functional sub-50-nm RT skyrmions, pointing towards the development of skyrmion-based memory devices.

Symmetry breaking in the formation of magnetic vortex states in a permalloy nanodisk

The magnetic vortex in nanopatterned elements is currently attracting enormous interest. A priori, one would assume that the formation of magnetic vortex states should exhibit a perfect symmetry, because the magnetic vortex has four degenerate states. Here we show the first direct observation of an asymmetric phenomenon in the formation process of vortex states in a permalloy nanodisk using high-resolution full-field magnetic transmission soft X-ray microscopy. Micromagnetic simulations confirm that the intrinsic Dzyaloshinskii-Moriya interaction, which arises from the spin-orbit coupling due to the lack of inversion symmetry near the disk surface, as well as surface-related extrinsic factors, is decisive for the asymmetric formation of vortex states.

Logic Operations Based on Magnetic-Vortex-State Networks

Logic operations based on coupled magnetic vortices were experimentally demonstrated. We utilized a simple chain structure consisting of three physically separated but dipolar-coupled vortex-state Permalloy disks as well as two electrodes for application of the logical inputs. We directly monitored the vortex gyrations in the middle disk, as the logical output, by time-resolved full-field soft X-ray microscopy measurements. By manipulating the relative polarization configurations of both end disks, two different logic operations are programmable: the XOR operation for the parallel polarization and the OR operation for the antiparallel polarization. This work paves the way for new-type programmable logic gates based on the coupled vortex-gyration dynamics achievable in vortex-state networks. The advantages are as follows: a low-power input signal by means of resonant vortex excitation, low-energy dissipation during signal transportation by selection of low-damping materials, and a simple patterned-array structure.

Observation of coupled vortex gyrations by 70-ps-time- and 20-nm-space-resolved full-field magnetic transmission soft x-ray microscopy

Observation of coupled vortex gyrations by 70-ps-time- and 20-nm-space- resolved full-field magnetic transmission soft x-ray microscopy Hyunsung Jung 3 , Young-Sang Yu, Ki-Suk Lee, Mi-Young Im, Peter Fischer, 1,a) Lars Bocklage, Andreas Vogel, Markus Bolte, Guido Meier, and Sang-Koog Kim Research Center for Spin Dynamics and Spin-Wave Devices, and Nanospinics Laboratory, Department of Materials Science and Engineering, Seoul National University, Seoul 151-744, Republic of Korea Center for X-ray Optics, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA Institut fur Angewandte Physik und Zentrum fur Mikrostrukturforschung, Universitat Hamburg, Hamburg 20355, Germany We employed time-and space-resolved full-field magnetic transmission soft x-ray microscopy to observe vortex-core gyrations in a pair of dipolar-coupled vortex-state Permalloy (Ni80Fe20) disks. The 70 ps temporal and 20 nm spatial resolution of the microscope enabled us to simultaneously measure vortex gyrations in both disks and to resolve the phases and amplitudes of both vortex-core positions. We observed their correlation for a specific vortex-state configuration. This work provides a robust and direct method of studying vortex gyrations in dipolar-coupled vortex oscillators.

Probing the spin polarization of current by soft x-ray imaging of current-induced magnetic vortex dynamics

Time-resolved soft x-ray transmission microscopy is applied to image the current-induced resonant dynamics of the magnetic vortex core realized in a micron sized Permalloy disk. The high spatial resolution better than 25 nm enables us to observe the resonant motion of the vortex core. The result also provides the spin polarization of the current to be 0.67+/-0.16 for Permalloy by fitting the experimental results with an analytical model in the framework of the spin-transfer torque.

Magnetic Microstructure of Rolled‐Up Single‐Layer Ferromagnetic Nanomembranes

The magnetic microstructure of rolled-up magnetic nanomembranes is revealed both theoretically and experimentally. Two types of nanomembranes are considered, one with a non-magnetic spacer layer and the other without. Experimentally, by using different materials and tuning the dimensions of the rolled-up nanomembranes, domain patterns consisting of spiral-like and azimuthally magnetized domains are observed, which are in qualitative agreement with the theoretical predictions.

Pinning induced by inter-domain wall interactions in planar magnetic nanowires

Pinning Induced by Inter-Domain Wall Interactions in Planar Magnetic Nanowires T.J. Hayward 1 , M.T. Bryan 1 , P.W. Fry 2 , P.M. Fundi 1 , M.R.J. Gibbs 1 , D.A. Allwood 1 , M.-Y. Im 3 and P. Fischer 3 Department of Engineering Materials, University of Sheffield, Sheffield, UK Nanoscience and Technology Centre, University of Sheffield, Sheffield UK Center for X-ray Optics, Lawrence Berkeley Natl Lab, Berkeley, CA, USA PACS: 07.85.Tt, 75.60.Ch, 75.75.+a, 85.70.Kh We have investigated pinning potentials created by inter-domain wall magnetostatic interactions in planar magnetic nanowires. We show that these potentials can take the form of an energy barrier or an energy well depending on the walls’ relative monopole moments, and that the applied magnetic fields required to overcome these potentials are significant. Both transverse and vortex wall pairs are investigated and it is found that transverse walls interact more strongly due to dipolar coupling between their magnetization structures. Simple analytical models which allow the effects of inter- domain wall interactions to be estimated are also presented. There is great interest in developing memory [1] and logic [2] devices based upon the controlled motion and interaction of domain walls (DWs) in ferromagnetic planar nanowires. Such domain walls have particle-like properties which allow them to be propagated around complex circuits using rotating magnetic fields [3,4] or short electric current pulses [5], and hence they may be used to represent binary data in a similar way to electric charge in conventional microelectronics. DWs in planar magnetic nanowires have head-to-head (H2H) or tail-to-tail (T2T) character (Fig 1(a)), and consequently they carry a net monopole moment (i.e. a localised excess of north (H2H) or south (T2T) magnetic poles). Therefore, to a first approximation DWs in adjacent nanowires will interact via a Coulomb-like potential: if the DWs have like monopole moments there will be a repulsive interaction, whereas if they have opposite monopole moments their interaction will be attractive. Understanding these effects and how they affect DW propagation is likely to be important to the development of DW based devices, where large nanowire densities will be desirable. So far there have been relatively few investigations into these effects, with studies characterizing attractive coupling between walls with opposite monopole moments for a limited range of nanowire geometries and DW structures [6,7]. We have also previously demonstrated that DW interaction energies are dependent to some degree on the DWs magnetization structure and chirality [8].

Tailoring magnetic energies to form dipole skyrmions and skyrmion lattices

Author(s): Montoya, SA; Couture, S; Chess, JJ; Lee, JCT; Kent, N; Henze, D; Sinha, SK; Im, MY; Kevan, SD; Fischer, P; McMorran, BJ; Lomakin, V; Roy, S; Fullerton, EE | Abstract:

Switchable Cell Trapping Using Superparamagnetic Beads

Ni81Fe19 microwires are investigated as the basis of a switchable template for positioning magnetically labeled neural Schwann cells. Magnetic transmission X-ray microscopy and micromagnetic modeling show that magnetic domain walls can be created or removed in zigzagged structures by an applied magnetic field. Schwann cells containing superparamagnetic beads are trapped by the field emanating from the domain walls. The design allows Schwann cells to be organized on a surface to form a connected network and then released from the surface if required. As aligned Schwann cells can guide nerve regeneration, this technique is of value for developing glial-neuronal coculture models in the future treatment of peripheral nerve injuries.