Search

The photoinduced all-trans to 13-cis isomerization of the retinal Schiff base represents the ultrafast first step in the reaction cycle of bacteriorhodopsin (BR). Extensive experimental and theoretical work has addressed excited-state dynamics and isomerization via a conical intersection with the ground state. In conflicting molecular pictures, the excited state potential energy surface has been modeled as a pure S1 state that intersects with the ground state, or in a 3-state picture involving the S1 and S2 states. Here, the photoexcited system passes two crossing regions to return to the ground state. The electric dipole moment of the Schiff base in the S1 and S2 state differs strongly and, thus, its measurement allows for assessing the character of the excited-state potential. We apply the method of ultrafast terahertz (THz) Stark spectroscopy to measure electric dipole changes of wild-type BR and a BR D85T mutant upon electronic excitation. A fully reversible transient broadening and spectral shift of electronic absorption is induced by a picosecond THz field of several megavolts/cm and mapped by a 120-fs optical probe pulse. For both BR variants, we derive a moderate electric dipole change of 5 ± 1 Debye, which is markedly smaller than predicted for a neat S1-character of the excited state. In contrast, S2-admixture and temporal averaging of excited-state dynamics over the probe pulse duration gives a dipole change in line with experiment. Our results support a picture of electronic and nuclear dynamics governed by the interaction of S1 and S2 states in a 3-state model. [ABSTRACT FROM AUTHOR]

We present time-resolved scanning x-ray microscopy measurements with picosecond photo-excitation via a tailored infrared pump laser at a scanning transmission x-ray microscope. Specifically, we image the laser-induced demagnetization and remagnetization of thin ferrimagnetic GdFe films proceeding on a few nanoseconds timescale. Controlling the heat load on the sample via additional reflector and heatsink layers allows us to conduct destruction-free measurements at a repetition rate of 50 MHz. Near-field enhancement of the photo-excitation and controlled annealing effects lead to laterally heterogeneous magnetization dynamics which we trace with 30 nm spatial resolution. Our work opens new opportunities to study photo-induced dynamics on the nanometer scale, with access to picosecond to nanosecond time scales, which is of technological relevance, especially in the field of magnetism. [ABSTRACT FROM AUTHOR]

All-optical switching of magnetic order presents a promising route toward faster and more energy efficient data storage. However, a realization in future devices is ultimately dependent on the maximum repetition rates of optically induced write/erase cycles. Here, we present two strategies to minimize the temporal separation of two consecutive femtosecond laser pulses to toggle the out-of-plane direction of the magnetization of ferrimagnetic rare-earth transition metal alloys. First, by systematically changing the heat transfer rates using either amorphous glass, crystalline silicon, or polycrystalline diamond substrates, we show that efficient cooling rates of the magnetic system present a prerequisite to accelerate the sequence of double pulse toggle switching. Second, we demonstrate that replacing the transition metal iron by cobalt leads to a significantly faster recovery of the magnetization after optical excitation allowing us to approach terahertz frequency of write/erase cycles with a minimum pulse-to-pulse separation of 7 ps. [ABSTRACT FROM AUTHOR]

Magnetic skyrmions can be created and annihilated in ferromagnetic multilayers using single femtosecond infrared laser pulses above a material-dependent fluence threshold. From the perspective of applications, optical control of skyrmions offers a route to a faster and, potentially, more energy-efficient new class of information-technology devices. Here, we investigate laser-induced skyrmion generation in two different materials, mapping out the dependence of the process on the applied field and the laser fluence. We observe that sample properties like strength of the Dzyaloshinskii–Moriya interaction and pinning do not considerably influence the initial step of optical creation. In contrast, the number of skyrmions created can be directly and robustly controlled via the applied field and the laser fluence. Based on our findings, we propose concepts for applications, such as all-optical writing and deletion, an ultrafast skyrmion reshuffling device for probabilistic computing, and a combined optical and spin–orbit torque-controlled racetrack. [ABSTRACT FROM AUTHOR]

Most of today's pump–probe experiments, which rely on two-dimensional detectors, suffer from low read-out rates that prevent the implementation of fast lock-in techniques to increase the signal-to-noise ratio. The first cameras running at true kHz-frame rates are available at large-scale facilities, but require sophisticated data management strategies. Here we present a scheme for high-speed spatial encoding of modulated pump–probe signals using slow area detectors at full repetition rate of a laser system without an increase in data rate or change of the sample environment. Towards that end we block the probe light in front of the detector alternatingly with two inverted masks at the same frequency as the signal is modulated, e.g. the chopping frequency of the pump light. Modulation frequencies up to 500 Hz are demonstrated using a commercial mechanical chopper and have been applied to a time-resolved Faraday microscopy experiment probing all-optical magnetic switching of a GdFe-alloy with femtosecond temporal resolution. We believe that our concept bridges the gap between today's slow area detectors and the upcoming generation of true kHz-frame-rate cameras. [ABSTRACT FROM AUTHOR]

The vision to manipulate and control magnetism with light is driven on the one hand by fundamental questions of direct and indirect photon-spin interactions, and on the other hand by the necessity to cope with ever growing data volumes, requiring radically new approaches on how to write, read and process information. Here, we present two complementary experimental geometries to access the element-specific magnetization dynamics of complex magnetic systems via ultrafast magneto-optical spectroscopy in the extreme ultraviolet spectral range. First, we employ linearly polarized radiation of a free electron laser facility to demonstrate decoupled dynamics of the two sublattices of an FeGd alloy, a prerequisite for all-optical magnetization switching. Second, we use circularly polarized radiation generated in a laboratory-based high harmonic generation setup to show optical inter-site spin transfer in a CoPt alloy, a mechanism which only very recently has been predicted to mediate ultrafast metamagnetic phase transitions. [ABSTRACT FROM AUTHOR]

Further advances in the field of ultrafast magnetization dynamics require experimental tools to measure the spin and electron dynamics with element-specificity and femtosecond temporal resolution. We present a new laboratory setup for two complementary experiments with light in the extreme ultraviolet (XUV) spectral range. One experiment is designed for polarization-dependent transient spectroscopy, particularly for simultaneous measurements of magnetic circular dichroism (MCD) at the 3p resonances of the 3d transition metals Fe, Co, and Ni. The second instrument is designed for resonant small-angle scattering experiments with monochromatic light allowing us to monitor spin dynamics with spatial information on the nanometer scale. We combine a high harmonic generation (HHG) source with a phase shifter to obtain XUV pulses with variable polarization and a flux of about (3 ± 1) × 1010 photons/s/harmonic at 60 eV at the source. A dedicated reference spectrometer effectively reduces the intensity fluctuations of the HHG spectrum to below 0.12% rms. We demonstrate the capabilities of the setup by capturing the energy- and polarization-dependent absorption of a thin Co film as well as the time-resolved small-angle scattering in a magnetic-domain network of a Co/Pt multilayer. The new laboratory setup allows systematic studies of optically induced spin and electron dynamics with element-specificity, particularly with MCD as the contrast mechanism with femtosecond temporal resolution and an unprecedented signal-to-noise ratio. [ABSTRACT FROM AUTHOR]

[Co0.4 nm/Pd0.7 nm]50 multilayers with Pd film thicknesses in the first ferromagnetic maximum of interlayer exchange coupling display almost purely perpendicular-to-plane anisotropy and labyrinth stripe domain patterns in remanence. Their magnetization reversal is characterized by domain nucleation starting at a defined field HN and domain wall movement in a defined magnetic field range. The modification of the magnetization reversal by 10 keV He ion bombardment due to the reduced magnetic anisotropy has been investigated by polar magneto-optical Kerr effect, by vibrating sample magnetometry, and by magnetic force microscopy at room temperature. It is shown that the ion bombardment creates and increases areas with ferromagnetic in-plane anisotropy and proportions of the sample showing superparamagnetism, the latter predominantly in the deeper layers. [ABSTRACT FROM AUTHOR]

Sputter deposited Mn83Ir17(30 nm)/Co70Fe30(10 nm)/Ta thin films have been investigated for their thermal exchange bias field drift at different storage temperatures after 10 keV He+ ion bombardment in an externally applied in-plane magnetic field. It is experimentally shown that the drift coefficient in an intermediate time interval, as given in a recently developed model, is proportional to T and proportional to the initial number of coupling sites in the polycrystalline exchange bias layer system used. [ABSTRACT FROM AUTHOR]

The thermal exchange bias field drift of sputter deposited Mn83Ir17(15 nm)/Co70Fe30(10 nm)/Ta thin films at room temperature after 10 keV He+ ion bombardment in an externally applied in-plane magnetic field for different ion fluences was studied. Although field cooling of the layer system resulted in a temporally stable exchange bias field at room temperature the exchange bias field starts to drift after ion bombardment like in non-annealed samples. Between 1 and 648 h after ion bombardment a logarithmic increase in the absolute magnitude of the exchange bias field is observed. A tentative model is presented for its description based on noninteracting domains in the antiferromagnet. A comparison between experimental data and the model reveals the delicate interplay between the ion bombardment modified average antiferromagnetic anisotropy constants, exchange coupling constants, and relaxation time distributions in the polycrystalline layer system influencing the thermal drift velocities. [ABSTRACT FROM AUTHOR]

A quantitative analysis of magnetic force microscopy (MFM) images taken in external in-plane magnetic fields is difficult because of the influence of the magnetic field on the magnetization state of the magnetic probe tip. We prepared calibration samples by ion bombardment induced magnetic patterning with a topographically flat magnetic pattern magnetically stable in a certain external magnetic field range for a quantitative characterization of the MFM probe tip magnetization in point-dipole approximation. [ABSTRACT FROM AUTHOR]

Exchange-biased IrMn/CoFe full films are magnetically structured with He-ion bombardment into stripes with antiparallel-aligned loop shift. The results indicate that the longitudinal magnetization reversal through head-on domain-wall motion and partial penetration of magnetization from neighboring strips is highly asymmetric.

A new plasma ion source for in situ keV He ion bombardment of solid state samples or thin films was designed and built for ion fluences between 1 × 1012 and 1 × 1017 ions/cm2. The system was designed to be mounted to different diffraction chambers for soft x-ray resonant magnetic scattering. Without breaking the vacuum due to He-ion bombardment, structural and magnetic modifications of the samples can be studied in situ and element specifically. [ABSTRACT FROM AUTHOR]

1. Sodium 2-[5-(4-chlorophenyl)-pentyl]-oxirane-2-carboxylate (B807–27 or POCA) inhibits ketogenesis from endogenous and exogenous long-chain fatty acids and 14CO2 production from [U-14C]palmitate, but not from [U-14C]palmitoylcarnitine or octanoate, and inhibits gluconeogenesis from lactate and pyruvate in perfused livers of starved rats. 2. Inhibition of ketogenesis, 14CO2 production and gluconeogenesis was complete at concentrations of 10 μmol/l POCA, but onset was more rapid for inhibition of ketogenesis and CO2 production than for gluconeogenesis. Infusion of octanoate abolished inhibition of all three processes. 3. Experiments with isolated rat liver mitochondria showed that carnitine palmitoyltransferase I (EC 2.3.1.21) is inhibited by POCA-CoA. The inhibitory process is dependent on time and concentration, and more pronounced in mitochondria from fed than from fasted rats. Concentrations required for 50% inhibition after 20 min preincubation with POCA- CoA are 0.02, 0.06 and 0.1 μmol/l in liver mitochondria from fed, 24-h-fasted and 48-h-fasted rats, respectively. The inhibitor appears to be tightly bound to the enzyme. 4. The extent of inhibition of carnitine palmitoyltransferase I correlates well with the hypoglycaemic and hypoketonaemic effects of the compounds in fasted rats. We conclude that specific inhibition of the enzyme leads, due to inhibition of long-chain fatty acid utilization, to depressed ketogenesis and gluconeogenesis and, in consequence, to hypoglycaemic and hypoketonaemia in vivo under gluconeogenic and ketogenic conditions. [ABSTRACT FROM AUTHOR]

Currently, much attention is being paid to patterned multilayer systems in which there exists a perpendicular magnetic anisotropy, because of their potential applications in spintronics devices and in a new generation of magnetic storage media. To further improve their performance, different patterning techniques can be used, which render them suitable also for other applications. Here we show that He+ 10 keV and Ar+ 100 keV ion bombardment of (Ni80Fe20-2 nm/Au-2 nm/Co-0.6 nm/Au-2 nm)10 multilayers through colloidal mask enables magnetic patterning of regularly arranged cylindrical magnetic domains, with perpendicular anisotropy, embedded in a non-ferromagnetic matrix or in a ferromagnetic matrix with magnetization oriented along the normal. These domains form an almost perfect two-dimensional hexagonal lattice with a submicron period and a large correlation length in a continuous and flat multilayer system. The magnetic anisotropy of these artificial domains remains unaffected by the magnetic patterning process, however the magnetization configuration of such a system depends on the magnetic properties of the matrix. The micromagnetic simulations were used to explain some of the features of the investigated patterned structures. [ABSTRACT FROM AUTHOR]

The controlled movement of magnetic beads trapped on a surface in the moving inhomogeneous stray fields of moving domain walls between artificial domains of exchange bias layer systems has been applied for mixing of two aqueous fluids in a microfluidic device of small volume. The mixing of the two fluids can be considerably accelerated by transporting full rows of beads and use them as micro stirrers. The mixing speed in the current experiment is tripled in the first 6 min of mixing as compared to normal diffusion even for ratios of 250 between container height and bead diameter. [ABSTRACT FROM AUTHOR]

In this work, we take a step toward nanometer resolution magnetic tomography, demonstrating the use of resonant magnetic x-ray scattering for three-dimensional (3D) characterization of Co/Pd multilayers. Trading coherence for x-ray flux, we measured magnetic scattering out to full period resolutions of 20 nm for incidence angles up to 75° from normal incidence, which was sufficient for a semiquantitative analysis of the 3D magnetic domain structure of such films. For the analysis of such patterns, we developed a scattering model based upon multiple plane propagation of the beam through the sample, taking into account the effects of multiple scattering not accounted for with Born or Rytov approximation based models. Our results demonstrate that 3D statistical characterization of nanoscale magnetic features is currently possible, and it is expected that the advent of high brilliance sources such as MAX IV in Sweden and SIRIUS in Brazil will provide the coherent flux necessary to permit the generalization of this work to nanometer resolution magnetic tomography. [ABSTRACT FROM AUTHOR]

The photoinduced all-trans to 13-cis isomerization of the retinal Schiff base represents the ultrafast first step in the reaction cycle of bacteriorhodopsin (BR). Extensive experimental and theoretical work has addressed excited-state dynamics and isomerization via a conical intersection with the ground state. In conflicting molecular pictures, the excited state potential energy surface has been modeled as a pure S[Formula: see text] state that intersects with the ground state, or in a 3-state picture involving the S[Formula: see text] and S[Formula: see text] states. Here, the photoexcited system passes two crossing regions to return to the ground state. The electric dipole moment of the Schiff base in the S[Formula: see text] and S[Formula: see text] state differs strongly and, thus, its measurement allows for assessing the character of the excited-state potential. We apply the method of ultrafast terahertz (THz) Stark spectroscopy to measure electric dipole changes of wild-type BR and a BR D85T mutant upon electronic excitation. A fully reversible transient broadening and spectral shift of electronic absorption is induced by a picosecond THz field of several megavolts/cm and mapped by a 120-fs optical probe pulse. For both BR variants, we derive a moderate electric dipole change of 5 [Formula: see text] 1 Debye, which is markedly smaller than predicted for a neat S[Formula: see text]-character of the excited state. In contrast, S[Formula: see text]-admixture and temporal averaging of excited-state dynamics over the probe pulse duration gives a dipole change in line with experiment. Our results support a picture of electronic and nuclear dynamics governed by the interaction of S[Formula: see text] and S[Formula: see text] states in a 3-state model., Competing Interests: Competing interests statement:The authors declare no competing interest.

Magnetic skyrmions are quasiparticles with nontrivial topology, envisioned to play a key role in next-generation data technology while simultaneously attracting fundamental research interest due to their emerging topological charge. In chiral magnetic multilayers, current-generated spin-orbit torques or ultrafast laser excitation can be used to nucleate isolated skyrmions on a picosecond time scale. Both methods, however, produce randomly arranged skyrmions, which inherently limits the precision on the location at which the skyrmions are nucleated. Here, we show that nanopatterning of the anisotropy landscape with a He + -ion beam creates well-defined skyrmion nucleation sites, thereby transforming the skyrmion localization into a deterministic process. This approach allows control of individual skyrmion nucleation as well as guided skyrmion motion with nanometer-scale precision, which is pivotal for both future fundamental studies of skyrmion dynamics and applications.

Focused beams of helium ions are a powerful tool for high-fidelity machining with spatial precision below 5 nm. Achieving such a high patterning precision over large areas and for different materials in a reproducible manner, however, is not trivial. Here, we introduce the Python toolbox FIB-o-mat for automated pattern creation and optimization, providing full flexibility to accomplish demanding patterning tasks. FIB-o-mat offers high-level pattern creation, enabling high-fidelity large-area patterning and systematic variations in geometry and raster settings. It also offers low-level beam path creation, providing full control over the beam movement and including sophisticated optimization tools. Three applications showcasing the potential of He ion beam nanofabrication for two-dimensional material systems and devices using FIB-o-mat are presented., (Copyright © 2021, Deinhart et al.)

The electric-field enhancement in terahertz (THz) antennas designed for nonlinear THz spectroscopy of soft matter is characterized by spatially resolved electrooptic sampling. To mimic the relevant interaction geometry, metallic, resonant bow-tie antennas are deposited on a thin zinc telluride crystal of 10 µm thickness. The THz electric field transmitted through the antenna gap is recorded by electrooptic sampling. By focusing the 800 nm, sub-20 fs sampling pulses, we achieve a spatial resolution of some 3 µm, which is 1/3 to 1/8 of the antenna-gap width. The THz field in the gap displays an enhancement by a factor of up to 4.5 with a pronounced spectral variation, depending sensitively on the antenna-arm length and the gap width. By scanning the 800 nm probe spot laterally through the antenna gap, the spatial variation of the enhancement is determined, reaching the highest values at the edges of the gap. The results are in agreement with simulations of the electric-field distributions by finite-element calculations.

Magnetic skyrmions are stabilized by a combination of external magnetic fields, stray field energies, higher-order exchange interactions and the Dzyaloshinskii-Moriya interaction (DMI). The last favours homochiral skyrmions, whose motion is driven by spin-orbit torques and is deterministic, which makes systems with a large DMI relevant for applications. Asymmetric multilayers of non-magnetic heavy metals with strong spin-orbit interactions and transition-metal ferromagnetic layers provide a large and tunable DMI. Also, the non-magnetic heavy metal layer can inject a vertical spin current with transverse spin polarization into the ferromagnetic layer via the spin Hall effect. This leads to torques that can be used to switch the magnetization completely in out-of-plane magnetized ferromagnetic elements, but the switching is deterministic only in the presence of a symmetry-breaking in-plane field. Although spin-orbit torques led to domain nucleation in continuous films and to stochastic nucleation of skyrmions in magnetic tracks, no practical means to create individual skyrmions controllably in an integrated device design at a selected position has been reported yet. Here we demonstrate that sub-nanosecond spin-orbit torque pulses can generate single skyrmions at custom-defined positions in a magnetic racetrack deterministically using the same current path as used for the shifting operation. The effect of the DMI implies that no external in-plane magnetic fields are needed for this aim. This implementation exploits a defect, such as a constriction in the magnetic track, that can serve as a skyrmion generator. The concept is applicable to any track geometry, including three-dimensional designs.

Regularly arranged magnetic out-of-plane patterns in continuous and flat films are promising for applications in data storage technology (bit patterned media) or transport of individual magnetic particles. Whereas topographic magnetic structures are fabricated by standard lithographical techniques, the fabrication of regularly arranged artificial domains in topographically flat films is difficult, since the free energy minimization determines the existence, shape, and regularity of domains. Here we show that keV He(+) ion bombardment of Au/Co/Au layer systems through a colloidal mask of hexagonally arranged spherical polystyrene beads enables magnetic patterning of regularly arranged cylindrical magnetic monodomains with out-of-plane magnetization embedded in a ferromagnetic matrix with easy-plane anisotropy. This colloidal domain lithography creates artificial domains via periodic lateral anisotropy variations induced by periodic defect density modulations. Magnetization reversal of the layer system observed by magnetic force microscopy shows individual disc switching indicating monodomain states.