Coupled vortex oscillations in spatially separated permalloy squares

We experimentally study the magnetization dynamics of pairs of micron-sized permalloy squares coupled via their stray fields. The trajectories of the vortex cores in the Landau-domain patterns of the squares are mapped in real space using time-resolved scanning transmission x-ray microscopy. After excitation of one of the vortex cores with a short magnetic-field pulse, the system behaves like coupled harmonic oscillators. The coupling strength depends on the separation between the squares and the configuration of the vortex-core polarizations. Considering the excitation via a rotating in-plane magnetic field, it can be understood that only a weak response of the second vortex core is observed for equal core polarizations

Magnetic antivortex-core reversal by circular-rotational spin currents

Topological singularities occur as antivortices in ferromagnetic thin-film microstructures. Antivortices behave as two-dimensional oscillators with a gyrotropic eigenmode which can be excited resonantly by spin currents and magnetic fields. We show that the two excitation types couple in an opposing sense of rotation in the case of resonant antivortex excitation with circular-rotational currents. If the sense of rotation of the current coincides with the intrinsic sense of gyration of the antivortex, the coupling to the Oersted fields is suppressed and only the spin-torque contribution locks into the gyrotropic eigenmode. We report on the experimental observation of purely spin-torque induced antivortex-core reversal. The dynamic response of an isolated antivortex is imaged by time-resolved scanning transmission x-ray microscopy on its genuine time and length scale

Time-Resolved X-ray Microscopy of Spin-Torque-Induced Magnetic Vortex Gyration

Time-resolved X-ray microscopy is used to image the influence of alternating high-density currents on the magnetization dynamics of ferromagnetic vortices. Spin-torque induced vortex gyration is observed in micrometer-sized permalloy squares. The phases of the gyration in structures with different chirality are compared to an analytical model and micromagnetic simulations, considering both alternating spinpolarized currents and the current's Oersted field. In our case the driving force due to spin-transfer torque is about 70% of the total excitation while the remainder originates from the current's Oersted field. This finding has implications to magnetic storage devices using spin-torque driven magnetization switching and domain-wall motion.Comment: 10 pages, 3 figure

Nickel partitioning in biogenic and abiogenic ferrihydrite: the influence of silica and implications for ancient environments

Fe(III) (oxyhydr)oxides are ubiquitous in modern soils and sediments, and their large surface area leads to scavenging of trace elements. Experimental trace element partitioning between Fe(III) (oxyhydr)oxides and aqueous solutions have been used to elucidate the geochemical composition of the Precambrian oceans based on the trace element concentrations in Precambrian banded iron formations (BIFs). However, previous partitioning experiments did not consider the potential influence of microbially-derived organic material, even though it is widely believed that bacterial phytoplankton was involved in Fe(II) oxidation and the deposition of BIF primary minerals. Therefore, the present study focuses on sorption of Ni to, and co-precipitation of Ni with, both biogenic ferrihydrite (Fe(OH)3) precipitated by the freshwater photoferrotroph Rhodobacter ferrooxidans SW2 and the marine photoferrotroph Rhodovulum iodosum, as well as chemically synthesized ferrihydrite. We considered the influence of cellular organic material, medium composition and the availability of dissolved silica. Our results show a preferential association of Ni with ferrihydrite, and not with the microbial cells or extracellular organic substances. We found that the addition of silica (2 mM) did not influence Ni partitioning but led to the encrustation of some cells with ferrihydrite and amorphous silica. The two- to threefold lower Ni/Fe ratio in biogenic as compared to abiogenic ferrihydrite is probably due to a competition between Ni and organic matter for sorption sites on the mineral surface. Additionally, the competition of ions present at high concentrations in marine medium for sorption sites led to decreased Ni sorption or co-precipitation. Based on our data we conclude that, if the Fe(III) minerals deposited in BIFs were – at least to some extent – biological, then the Ni concentrations in the early ocean would have been higher than previously suggested. This study shows the importance of considering the presence of microbial biomass and seawater ions in paleomarine reconstructions

Vortex core switching by coherent excitation with single in-plane magnetic field pulses

The bistability of the core magnetization of nano-scaled magnets with a magnetic vortex configuration has great potential for data storage applications. To exploit this, reliable switching between the two possible states is needed. Time resolved x-ray microscopy was used to study the response of the vortex core to excitation pulses at sub-ns timescales and image the vortex core switching. A reliable switching process by coherent excitation with leading and trailing edges of in-plane magnetic field pulses was found and compared with micromagnetic simulations.Comment: 4 pages, 3 figure

Photofragmentation of \u3ci\u3ecloso\u3c/i\u3e-Carboranes Part 1: Energetics of Decomposition

The ionic fragmentation following B 1s and C 1s excitation of three isomeric carborane cage compounds [closo-dicarbadodecaboranes: orthocarborane (1,2-C2B10H12), metacarborane (1,7-C2B10H12), and paracarborane (1,12-C2B10H12)] is compared with the energetics of decomposition. The fragmentation yields for all three molecules are quite similar. Thermodynamic cycles are constructed for neutral and ionic species in an attempt to systemically characterize single-ion closo-carborane creation and fragmentation processes. Lower energy decomposition processes are favored. Among the ionic species, the photon-induced decomposition is dominated by BH+ and BH2+ fragment loss. Changes in ion yield associated with core to bound excitations are observed