The response of magnetic vortex cores to subnanosecond in-plane magnetic field pulses was studied by time-resolved x-ray microscopy. Vortex core reversal was observed and the switching events were located in space and time. This revealed a mechanism of coherent excitation by the leading and trailing edges of the pulse, lowering the field amplitude required for switching. The mechanism was confirmed by micromagnetic simulations and can be understood in terms of gyration around the vortex equilibrium positions, displaced by the applied field. DOI: 10.1103/PhysRevLett.102.077201 PACS numbers: 75.40.Gb, 75.60.Jk, 75.75.+a The magnetic vortex is a typical ground state configuration of micron and submicron sized ferromagnetic thin film structures [1]. It minimizes the stray field energy by forming an in-plane curling magnetization. In order to avoid a singularity in the center of the structure, the magnetization turns out of plane, forming the vortex core which can point either up (vortex polarization p ¼ þ1) or down (p ¼ À1). This configuration is very stable; static out-ofplane magnetic fields of about 0.5 T are required to switch the core polarization [2]. The vortex also has a specific excitation mode, the so-called gyrotropic mode, which can be excited by an oscillating in-plane magnetic field [3,4]. It corresponds to a circular motion of the vortex around its equilibrium position.Recently, it was discovered that switching the polarization of the vortex core is not only possible by static fields, but also by excitation of the gyration mode [5]. In this case, only field strengths of a few millitesla are needed. Micromagnetic modeling of these experiments revealed that the vortex switching occurs by the creation and subsequent annihilation of a vortex-antivortex pair [5]. This discovery has triggered a variety of studies on vortex core switching by various excitation methods. In particular, it was found by micromagnetic simulations that the polarization may also be switched by very short in-plane magnetic field pulses. It was shown that switching times as short as 40 ps are possible [6]. Except for a brief report on switching events observed with 2 ns long spin-polarized current pulses [7], the possibilities of such excitations have not been explored experimentally so far.In this work, we have experimentally investigated vortex core switching by in-plane magnetic field pulses, using time-resolved magnetic x-ray microscopy. By taking advantage of the pulsed nature of synchrotron light, stroboscopic imaging was set up at scanning transmission x-ray microscopes [8] at beam line 11.0.2 of the Advanced Light Source and 10ID-1 of the Canadian Light Source. Using the x-ray magnetic circular dichroism [9] as a contrast mechanism, magnetization dynamics could be studied with 25 nm spatial and 70 ps temporal resolution.The samples studied in this work are 500 nm  500 nm and 1 m  1 m square-shaped magnetic Permalloy (Ni 80 Fe 20 ) elements with a thickness of 50 nm. These structures are defined on top of a 2:5 m wide, 1...

A closer look: Investigation of the reduction properties of a single Fischer-Tropsch catalyst particle, using in situ scanning transmission X-ray microscopy with spatial resolution of 35 nm, reveals a heterogeneous distribution of Fe(0), Fe(2+), and Fe(3+) species. Regions of different reduction properties are defined and explained on the basis of local chemical interactions and catalyst morphology.

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