Spin Accumulation and Dynamics in Inversion-Symmetric van der Waals Crystals.

Inversion-symmetric materials are forbidden to show an overall spin texture in their band structure in the presence of time-reversal symmetry. However, in van der Waals materials which lack inversion symmetry within a single layer, it has been proposed that a layer-dependent spin texture can arise leading to a coupled spin-layer degree of freedom. Here we use time-resolved Kerr rotation in inversion-symmetric WSe_{2} and MoSe_{2} bulk crystals to study this spin-layer polarization and unveil its dynamics. Our measurements show that the spin-layer relaxation time in WSe_{2} is limited by phonon scattering at high temperatures and that the interlayer hopping can be tuned by a small in-plane magnetic field at low temperatures, enhancing the relaxation rates. We find a significantly lower lifetime for MoSe_{2} which agrees with theoretical expectations of a spin-layer polarization stabilized by the larger spin-orbit coupling in WSe_{2}.

Hot-Electron-Induced Ultrafast Demagnetization in Co/Pt Multilayers.

Using specially engineered structures to tailor the optical absorption in a metallic multilayer, we analyze the magnetization dynamics of a Co/Pt multilayer buried below a thick Cu layer. We demonstrate that hot electrons alone can very efficiently induce ultrafast demagnetization. Simulations based on hot electron ballistic transport implemented within a microscopic model that accounts for local dissipation of angular momentum nicely reproduce the experimental results, ruling out contribution of pure thermal transport.

Cleaved thin-film probes for scanning tunneling microscopy.

We introduce an alternative type of probe for scanning tunneling microscopy (STM). Instead of using a needle-like tip made from a piece of metallic wire, a sharp-edged cleaved insulating substrate, which is initially covered by a thin conductive film, is used. The sharp tip is formed at the intersection of the two cleaved sides. Using this approach a variety of materials for STM probes can be used, and functionalization of STM probes is possible. The working principle of different probes made of metallic (Pt, Co, and CoB), indium-tin oxide, as well as Cu/Pt and Co/Pt multilayer films are demonstrated by STM imaging of clean Cu(001) and Cu(111) surfaces as well as the epitaxial Co clusters on Cu(111).

Focused-electron-beam-induced-deposited cobalt nanopillars for nanomagnetic logic.

Nanomagnetic logic (NML) intends to alleviate problems of continued miniaturization of CMOS-based electronics, such as energy dissipation through heat, through advantages such as low power operation and non-volatile magnetic elements. In line with recent breakthroughs in NML with perpendicularly magnetized elements formed from thin films, we have fabricated NML inverter chains from Co nanopillars by focused electron beam induced deposition (FEBID) that exhibit shape-induced perpendicular magnetization. The flexibility of FEBID allows optimization of NML structures. Simulations reveal that the choice of nanopillar dimensions is critical to obtain the correct antiferromagnetically coupled configuration. Experiments carrying the array through a clocking cycle using the Oersted field from an integrated Cu wire show that the array responds to the clocking cycle.

Functional nickel-based deposits synthesized by focused beam induced processing.

Functional nanostructures fabricated by focused electron/ion beam induced processing (FEBIP/FIBIP) open a promising route for applications in nanoelectronics. Such developments rely on the exploration of new advanced materials. We report here the successful fabrication of nickel-based deposits by FEBIP/FIBIP using bis(methyl cyclopentadienyl)nickel as a precursor. In particular, binary compounds such as nickel oxide (NiO) are synthesized by using an in situ two-step process at room temperature. By this method, as-grown Ni deposits transform into homogeneous NiO deposits using focused electron beam irradiation under O2 flux. This procedure is effective in producing highly pure NiO deposits with resistivity of 2000 Ωcm and a polycrystalline structure with face-centred cubic lattice and grains of 5 nm. We demonstrate that systems based on NiO deposits displaying resistance switching and an exchange-bias effect could be grown by FEBIP using optimized parameters. Our results provide a breakthrough towards using these techniques for the fabrication of functional nanodevices.

Domain wall depinning governed by the spin Hall effect.

Perpendicularly magnetized materials have attracted significant interest owing to their high anisotropy, which gives rise to extremely narrow, nanosized domain walls. As a result, the recently studied current-induced domain wall motion (CIDWM) in these materials promises to enable a new class of data, memory and logic devices. Here we propose the spin Hall effect as an alternative mechanism for CIDWM. We are able to carefully tune the net spin Hall current in depinning experiments on Pt/Co/Pt nanowires, offering unique control over CIDWM. Furthermore, we determine that the depinning efficiency is intimately related to the internal structure of the domain wall, which we control by the application of small fields along the nanowire. This manifestation of CIDWM offers an attractive degree of freedom for manipulating domain wall motion by charge currents, and sheds light on the existence of contradicting reports on CIDWM in perpendicularly magnetized materials.

Multi-channel Andreev reflection in Co-W nanocontacts fabricated using focused electron/ion beam induced deposition.

We report multi-channel electron transport in nano-contacts fabricated using focused electron beam induced deposited (FEBID) cobalt and focused ion beam induced deposited (FIBID) tungsten. Anomalous Andreev reflection (AR) effect is observed to which the conventional Blonder-Tinkham-Klapwijk (BTK) fit cannot be applied. In specific, we have observed multiple number of shoulders near the AR peak, whose origin is unknown in literature. We explain this effect based on a simple model that takes into account the material properties of the FIBID grown W superconductor, as well as the specific interface properties that are an outcome of using FEBID/FIBID as a fabrication technique. We show that numerical calculations using the BTK approximation based on the consideration of multiple channels generate similar shoulders as we observed in the AR experiments. Electrical measurements and x-ray photoemission spectroscopy carried out on FIBID W deposits puts additional evidence towards multi-channel current transport occuring at the interface of the nanocontacts.

Comparing ultrafast demagnetization rates between competing models for finite temperature magnetism.

We investigate a recent controversy in ultrafast magnetization dynamics by comparing the demagnetization rates from two frequently used but competing descriptions for finite temperature magnetism, namely a rigid band structure Stoner-like approach and a system of localized spins. The calculations on the localized spin system show a demagnetization rate and time comparable to experimentally obtained values, whereas the rigid band approach yields negligible demagnetization, even when the microscopic spin-flip process is assumed to be instantaneous. This shows that rigid band structure calculations will never be in quantitative agreement with experiments, irrespective of the investigated microscopic scattering mechanism.

Optical Monitoring of the Magnetization Switching of Single Synthetic-Antiferromagnetic Nanoplatelets with Perpendicular Magnetic Anisotropy.

Synthetic antiferromagnetic nanoplatelets (NPs) with a large perpendicular magnetic anisotropy (SAF-PMA NPs) have a large potential in future local mechanical torque-transfer applications for e.g., biomedicine. However, the mechanisms of magnetization switching of these structures at the nanoscale are not well understood. Here, we have used a simple and relatively fast single-particle optical technique that goes beyond the diffraction limit to measure photothermal magnetic circular dichroism (PT MCD). This allows us to study the magnetization switching as a function of applied magnetic field of single 122 nm diameter SAF-PMA NPs with a thickness of 15 nm. We extract and discuss the differences between the switching field distributions of large ensembles of NPs and of single NPs. In particular, single-particle PT MCD allows us to address the spatial and temporal heterogeneity of the magnetic switching fields of the NPs at the single-particle level. We expect this new insight to help understand better the dynamic torque transfer, e.g., in biomedical and microfluidic applications.

Tunable resistivity of individual magnetic domain walls.

Despite the relevance of current-induced magnetic domain wall (DW) motion for new spintronics applications, the exact details of the current-domain wall interaction are not yet understood. A property intimately related to this interaction is the intrinsic DW resistivity. Here, we investigate experimentally how the resistivity inside a DW depends on the wall width Δ, which is tuned using focused ion beam irradiation of Pt/Co/Pt strips. We observe the nucleation of individual DWs with Kerr microscopy, and measure resistance changes in real time. A 1/Δ(2) dependence of DW resistivity is found, compatible with Levy-Zhang theory. Also quantitative agreement with theory is found by taking full account of the current flowing through each individual layer inside the multilayer stack.

Spin-polarized hot electron transport versus spin pumping mediated by local heating.

A 'toy model'-aimed at capturing the essential physics-is presented that jointly describes spin-polarized hot electron transport and spin pumping driven by local heating. These two processes both contribute to spin-current generation in laser-excited magnetic heterostructures. The model is used to compare the two contributions directly. The spin-polarized hot electron current is modeled as one generation of hot electrons with a spin-dependent excitation and relaxation scheme. Upon decay, the excess energy of the hot electrons is transferred to a thermalized electron bath. The elevated electron temperature leads to an increased rate of electron-magnon scattering processes and yields a local accumulation of spin. This process is dubbed as spin pumping by local heating. The built-up spin accumulation is effectively driven out of the ferromagnetic system by (interfacial) electron transport. Within our model, the injected spin current is dominated by the contribution resulting from spin pumping, while the hot electron spin current remains relatively small. We derive that this observation is related to the ratio between the Fermi temperature and Curie temperature, and we show what other fundamental parameters play a role.

Explaining the paradoxical diversity of ultrafast laser-induced demagnetization.

Pulsed-laser-induced quenching of ferromagnetic order has intrigued researchers since pioneering works in the 1990s. It was reported that demagnetization in gadolinium proceeds within 100 ps, but three orders of magnitude faster in ferromagnetic transition metals such as nickel. Here we show that a model based on electron-phonon-mediated spin-flip scattering explains both timescales on equal footing. Our interpretation is supported by ab initio estimates of the spin-flip scattering probability, and experimental fluence dependencies are shown to agree perfectly with predictions. A phase diagram is constructed in which two classes of laser-induced magnetization dynamics can be distinguished, where the ratio of the Curie temperature to the atomic magnetic moment turns out to have a crucial role. We conclude that the ultrafast magnetization dynamics can be well described disregarding highly excited electronic states, merely considering the thermalized electron system.

Internal electron diffraction from atomically ordered subsurface nanostructures in metals.

We demonstrate that a part of interface at a subsurface nanocavity in Cu(110) can efficiently induce electron scattering back to the surface even if it is inclined with respect to the surface, if the condition for electron diffraction is fulfilled. This backscattering induces oscillations of electron local density of states at the surface versus electron energy. In agreement with our model calculations, the diffraction is assigned to a specific atomic structure at the interface, and is found to be significantly enhanced by focussing of electron waves for propagation along the [110] direction.

Magnetic-field dependence of the electroluminescence of organic light-emitting diodes: a competition between exciton formation and spin mixing.

We explore the magnetoelectroluminescence (MEL) of organic light-emitting diodes by evaluating the magnetic-field dependent fraction of singlet excitons formed. We use two- and multisite polaron-hopping models with spin mixing by hyperfine fields and different singlet and triplet exciton formation rates k(S) and k(T). A huge MEL is predicted when exciton formation is in competition with spin mixing and when k(T) is significantly larger than k(S). This competition also leads to a low-field structure in the MEL that is in agreement with recent experiments.

Spin-spin interactions in organic magnetoresistance probed by angle-dependent measurements.

The dependence of organic magnetoresistance (OMAR) on the orientation of the magnetic field has been investigated. In contrast with previous claims, a finite and systematic change in magnitude is observed when the orientation of the field is changed with respect to the sample. It is demonstrated that, to explain these effects, spin-spin interactions have to be included in the models previously suggested for OMAR. Dipole coupling and exchange coupling are introduced in combination with either an anisotropy of the orientation of the spin pairs or an anisotropy in the hyperfine fields.

Fe:O:C grown by focused-electron-beam-induced deposition: magnetic and electric properties.

We systematically study the effect of oxygen content on the magneto-transport and microstructure of Fe:O:C nanowires deposited by focused-electron-beam-induced (FEBID) deposition. The Fe/O ratio can be varied with an Fe content varying between ∼ 50 and 80 at.% with overall low C content (≈16 ± 3 at.%) by adding H(2)O during the deposition while keeping the beam parameters constant as measured by energy dispersive x-ray (EDX) spectroscopy. The room-temperature magnetic properties for deposits with an Fe content of 66-71 at.% are investigated using the magneto-optical Kerr effect (MOKE) and electric magneto-transport measurements. The nanostructure of the deposits is investigated through cross-sectional high-resolution transmission electron microscopy (HRTEM) imaging, allowing us to link the observed magneto-resistance and resistivity to the transport mechanism in the deposits. These results demonstrate that functional magnetic nanostructures can be created, paving the way for new magnetic or even spintronics devices.

Associations between vascular co-morbidities and depression in insulin-naive diabetes patients: the DIAZOB Primary Care Diabetes study.

AIMS/HYPOTHESIS: The aim of the study was to determine the prevalence of depression in insulin-naive diabetes patients and to investigate the associations between different forms of vascular co-morbidity and depression. METHODS: Cross-sectional data were used from a primary-care sample of 1,269 insulin-naive (i.e. not using insulin therapy) diabetes patients participating in the DIAZOB Primary Care Diabetes study. Demographics, vascular co-morbidities, clinical and lifestyle characteristics, and psychosocial factors were assessed. Depression symptoms were measured with the Edinburgh Depression Scale, with a score >11 defined as depression. The chi (2) and Student's t tests were used to compare groups with and without vascular co-morbidities. Rates and odds ratios of depression were calculated for each vascular co-morbidity, with diabetes only as the reference group, correcting for age and sex. Single and multiple logistic regression analyses were performed to test a more comprehensive model regarding the likelihood of depression in diabetes. RESULTS: The prevalence of depression was 11% in the total sample with little difference between the groups with and without any vascular co-morbidity (11.2% vs 10.0%). Single vascular co-morbidities were not associated with increased rates of depression. The final model predicting depression included: having multiple vascular co-morbidities compared with none; having less social support; having experienced a recent stressful life event; female sex; and being a smoker. CONCLUSIONS/INTERPRETATION: Rates of depression in those with one additional vascular co-morbidity did not differ from patients with diabetes only. Vascular co-morbidities were only associated with higher depression scores in case of multiple co-morbidities.

Magnetoresistance in hybrid organic spin valves at the onset of multiple-step tunneling.

By combining experiments with simple model calculations, we obtain new insight in spin transport through hybrid, CoFeB/Al2O3(1.5 nm)/tris(8-hydroxyquinoline)aluminium (Alq3)/Co spin valves. We have measured the characteristic changes in the I-V behavior as well as the intrinsic loss of magnetoresistance at the onset of multiple-step tunneling. In the regime of multiple-step tunneling, under the condition of low hopping rates, spin precession in the presence of hyperfine coupling is conjectured to be the relevant source of spin relaxation. A quantitative analysis leads to the prediction of a symmetric magnetoresistance around zero magnetic field in addition to the hysteretic magnetoresistance curves, which are indeed observed in our experiments.

Integrating all-optical switching with spintronics.

All-optical switching (AOS) of magnetic materials describes the reversal of the magnetization using short (femtosecond) laser pulses, and received extensive attention in the past decade due to its high potential for fast and energy-efficient data writing in future spintronic memory applications. Unfortunately, the AOS mechanism in the ferromagnetic multilayers commonly used in spintronics needs multiple pulses for the magnetization reversal, losing its speed and energy efficiency. Here, we experimentally demonstrate on-the-fly single-pulse AOS in combination with spin Hall effect (SHE) driven motion of magnetic domains in Pt/Co/Gd synthetic-ferrimagnetic racetracks. Moreover, using field-driven-SHE-assisted domain wall (DW) motion measurements, both the SHE efficiency in the racetrack is determined and the chirality of the optically written DW's is verified. Our experiments demonstrate that Pt/Co/Gd racetracks facilitate both single-pulse AOS as well as efficient SHE-induced domain wall motion, which might ultimately pave the way towards integrated photonic memory devices.

Constitutive loss and acute pharmacological manipulation of ErbB4 signaling do not affect attention and inhibitory control in mice.

The receptor tyrosine kinase ErbB4 and its ligand trophic factors of the neuregulin (NRG) family have been associated with schizophrenia and other mental disorders in human genetic studies. In vivo studies in mice have shown how abnormal Nrg-ErbB4 signaling leads to deviant behaviors relevant to distinct aspects of schizophrenia, including hyperactivity, sensory gating deficits, working and spatial memory deficits and impaired social behavior. However, so far little is known on the role of ErbB4 in attention and inhibitory control, two aspects of executive functions that are impaired in schizophrenia. Here we investigated the effects of constitutive loss of ErbB4 in the central nervous system of mice on performance in a 5-choice serial reaction time task (5CSRTT) assessing attention and inhibitory control. In this task, ErbB4-/- mice did not show deficits in various parameters of attention, and premature responses as measure of inhibitory control. Nonetheless, ErbB4-/- mice recapitulated a specific set of behavioral phenotypes associated with schizophrenia, including a deficit in spatial learning and memory in the Barnes Maze and in contextual fear learning, and a trend for a deficit in sensorimotor gating. Furthermore, we investigated the effect of acute pharmacological inhibition of ErbB tyrosine kinase receptor using the pan-ErbB kinase inhibitor JNJ-28871063 (JNJ), in an automated version of the 5CSRTT. JNJ did not affect attention and inhibitory control. In conclusion, our data suggest no direct involvement of a classical Nrg-ErbB4 pathway in attention and inhibitory control in mice, while it confirms the involvement of this pathway in other domains relevant to schizophrenia.