Many striking non-equilibrium phenomena have been discovered or predicted in opticallydriven quantum solids 1 , ranging from light-induced superconductivity 2,3 to Floquetengineered topological phases 4-8 . These effects are expected to lead to dramatic changes in electrical transport, but can only be comprehensively characterized or functionalized with a direct interface to electrical devices that operate at ultrafast speeds 1-8 . Here, we make use of laser-triggered photoconductive switches 9 to measure the ultrafast transport properties of monolayer graphene, driven by a mid-infrared femtosecond pulse of circularly polarized light. The goal of this experiment is to probe the transport signatures of a predicted light-induced topological band structure in graphene 4,5 , similar to the one originally proposed by Haldane 10 . We report the observation of an anomalous Hall effect in the absence of an applied magnetic field. We also extract quantitative properties of the non-equilibrium state. The dependence of the effect on a gate potential used to tune the Fermi level reveals multiple features that reflect the effective band structure expected from Floquet theory. This includes a ∼60 meV wide conductance plateau centered at the Dirac point, where a gap of approximately equal magnitude is expected to open. We also find that when the Fermi level lies within this plateau, the estimated anomalous Hall conductance saturates around ∼1.8±0.4 e 2 /h.Optical driving has been proposed as a means to engineer topological properties in topologically trivial systems 4-8 . One proposal for such a 'Floquet topological insulator' is based on breaking time-reversal symmetry in graphene through a coherent interaction with circularly polarized light 4 . In this theory, the light field drives electrons in circular trajectories through the band structure (Fig. 1a). Close to the Dirac point, these states are predicted to acquire a non-adiabatic Berry phase every optical cycle, which is equal and opposite for the upper and lower band. This time-averaged extra phase accumulation amounts to an energy * These authors contributed equally to this work

Magnetic transmission x-ray microscopy is used to directly visualize the influence of a spin-polarized current on domain walls in curved permalloy wires. Pulses of nanosecond duration and of high current density up to 1.0x10(12) A/m(2) are used to move and to deform the domain wall. The current pulse drives the wall either undisturbed, i.e., as composite particle through the wire, or causes structural changes of the magnetization. Repetitive pulse measurements reveal the stochastic nature of current-induced domain-wall motion.

The theoretical foundation for a nonvolatile memory device based on magnetic vortices is presented. We propose a realization of a vortex random-access memory (VRAM) containing vortex cells that are controlled by alternating currents only. The proposed scheme allows to transfer the vortex into an unambiguous binary state regardless of its initial state within a subnanosecond time scale. The vortex handedness defined as the product of chirality and polarization as a bit representation allows direct mechanisms for reading and writing the bit information. The VRAM is stable at room temperature.

Abstract:The stochastic field-driven depinning of a domain wall pinned at a notch in a magnetic nanowire is directly observed using magnetic X-ray microscopy with high lateral resolution down to 15 nm. The depinning-field distribution in Ni 80 Fe 20 nanowires considerably depends on the wire width and the notch depth. The difference in the multiplicity of domain-wall types generated in the vicinity of a notch is responsible for the observed dependence of the stochastic nature of the domain wall depinning field on the wire width and the notch depth. Thus the random nature of the domain wall depinning process is controllable by an appropriate design of the nanowire.

In experiments the distinction between spin-torque and Oersted-field driven magnetization dynamics is still an open problem. Here, the gyroscopic motion of current-and field-driven magnetic vortices in small thinfilm elements is investigated by analytical calculations and by numerical simulations. It is found that for small harmonic excitations the vortex core performs an elliptical rotation around its equilibrium position. The global phase of the rotation and the ratio between the semi-axes are determined by the frequency and the amplitude of the Oersted field and the spin torque. PACS numbers: 75.60.Ch, 72.25.Ba Recently it has been found that a spin-polarized current flowing through a magnetic sample interacts with the magnetization and exerts a torque on the local magnetization. 1,2 A promising system for the investigation of the spin-torque effect is a vortex in a micro-or nanostructured magnetic thinfilm element. Vortices are formed when the in-plane magnetization curls around a center region. In this few nanometer large center region 3 , called the vortex core, the magnetization turns out-of-plane to minimize the exchange energy. 4 It is known that these vortices precess around their equilibrium position when excited by magnetic field pulses 5,6 and it was predicted that spin-polarized electric currents can do the same. 7 The spacial restriction of the vortex core as well as its periodic motion around its ground state yield an especially accessible system for space-and time-resolved measurements with scanning probe and time-integrative techniques such as soft X-ray microscopy or X-ray photoemission electron microscopy. 5,6,8,9,10 Magnetic vortices also occur in vortex domain walls. The motion of such walls has recently been investigated intensively. 11,12 Understanding the dynamics of confined vortices can give deeper insight in the mechanism of vortex-wall motion. 13 An in-plane Oersted field accompanying the current flow also influences the motion of the vortex core. For the interpretation of experimental data it is crucial to distinguish between the influence of the spin torque and of the Oersted field. 14 (a) l X (b) FIG. 1: (a) Scheme of the magnetization in a square magnetic thinfilm element with a vortex that is deflected to the right. (b) Magnetization of a vortex in its static ground state. The height denotes the z-component while the gray scale corresponds to the direction of the in-plane magnetization.In this paper we investigate the current-and field-driven gyroscopic motion of magnetic vortices in square thin-film elements of size l and thickness t as shown in Fig. 1 and present a method to distinguish between spin torque and Oersted field driven magnetization dynamics. In the presence of a spinpolarized current the time evolution of the magnetization is given by the extended Landau-Lifshitz-Gilbert equationwith the coupling constant b j = P µ B /[eM s (1 + ξ 2 )] between the current and the magnetization where P is the spin polarization, M S the saturation magnetization, and ξ the degree o...