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Perpendicular magnetic tunnel junctions with double-interface free layer (p-DMTJs), which exhibit enhanced tunnel magnetoresistance (TMR) and thermal stability (Δ) at the nanoscale, have received considerable interest as building blocks for spintronic data storage devices. Heat-assisted magnetic recording (HAMR) techniques have been widely employed in mainstream magnetic storage to enable ultrahigh storage density. However, the data access is achieved by sensing the stray field of the selected magnetic element using a mechanical “read head”, resulting in an unfavorable speed limitation and design complexity. To address this issue, integrating laser-assisted switching with a high-performance magnetic tunnel junction has received interest in spintronic R&D; however, it has not yet been achieved. In this study, we experimentally explored femtosecond (fs) laser-assisted switching in a p-DMTJ device using a direct electrical TMR readout. We demonstrate two reconfigurable switching operations, i.e., binary “write” and unidirectional “reset”, by the interplay of the fs laser and synchronized magnetic field sequence. We further explored the joint effect, and a switching phase diagram was obtained. The effect of the stray field of p-DMTJ, as well as laser helicity, on switching is also discussed. Results show the feasibility of fs laser-assisted writing p-DMTJs, which can pave the way in high-density optospintronic storage applications.

As the data modern societies generate and consume increases exponentially, faster and more energy efficient data handling are sought. Innovations integrating photonic components with electronics are vastly improving the performance of data handling, while bottlenecks form where the conversion of electrical to optical signals (or vice-versa) take place. This work presents a hybrid photonic device which mediates direct optical communication with non-volatile magnetic memory and promises on-chip all-optical reading of magnetically stored information.

With information technology consuming a sizeable part of the total energy, “Green IT” information storage and computing technology will have a major impact on addressing societal challenges.

Perpendicular magnetic tunnel junctions are one of the building blocks for spintronic memories, which allow fast nonvolatile data access, offering substantial potentials to revolutionize the mainstream computing architecture. However, conventional switching mechanisms of such devices are fundamentally hindered by spin polarized currents, either spin transfer torque or spin orbit torque with spin precession time limitation and excessive power dissipation. These physical constraints significantly stimulate the advancement of modern spintronics. Here, we report an optospintronic tunnel junction using a photonic-spintronic combination. This composite device incorporates an all-optically switchable Co/Gd bilayer coupled to a CoFeB/MgO-based perpendicular magnetic tunnel junction by the Ruderman-Kittel-Kasuya-Yosida interaction. A picosecond all-optical operation of the optospintronic tunnel junction is explicitly confirmed by time-resolved measurements. Moreover, the device shows a considerable tunnel magnetoresistance and thermal stability. This proof-of-concept device represents an essential step towards ultrafast photonic memories with THz data access, as well as ultralow power consumption.

Design and simulations of an integrated photonic device that can optically detect the magnetization direction of its ultra-thin (&sim, 12 nm) metal cladding, thus &lsquo, reading&rsquo, the stored magnetic memory, are presented. The device is an unbalanced Mach Zehnder Interferometer (MZI) based on InP Membrane on Silicon (IMOS) platform. The MZI consists of a ferromagnetic thin-film cladding and a delay line in one branch, and a polarization converter in the other. It quantitatively measures the non-reciprocal phase shift caused by the Magneto-Optic Kerr Effect in the guided mode which depends on the memory bit&rsquo, s magnetization direction. The current design is an analytical tool for research exploration of all-optical magnetic memory reading. It has been shown that the device is able to read a nanoscale memory bit (400 &times, 50 &times, 12 nm) by using a Kerr rotation as small as 0.2∘, in the presence of a noise &sim, 10 dB in terms of signal-to-noise ratio. The device is shown to tolerate performance reductions that can arise during the fabrication.

Given the development of hybrid spintronic-photonic devices and optical manipulation of chiral magnetic textures, a combined interest in single-pulse all-optical switching (AOS) of magnetization and current-induced domain wall motion in synthetic ferrimagnetic structures with strong Dzyaloshinskii-Moriya Interaction (DMI) is emerging. In this work, we explore the role of the DMI on the AOS process as well as the stability of optically written micromagnetic domains using specially engineered Co/Gd-based multilayer structures. Quantitative insight is obtained by measuring the interesting dynamics in moon-shaped structures written by two successive laser pulses. The stability of domains resulting from an interplay between the dipolar interaction and domain-wall energy are compared to simple analytical models and micromagnetic simulations. A shortening process occurring at ns time scale is confirmed by our computational results, and a stabilizing role of DMI on microscopic AOS-written domains is experimentally demonstrated.

Ever since its discovery around a decade ago, all-optical magnetization switching (AOS) using femtosecond laser pulses has shown potential for future data storage and logic devices. In particular, single pulse helicity independent AOS in certain ferrimagnetic alloys and multilayers is highly efficient and ultrafast. However, in most cases it is a toggle mechanism, which is not desirable for applications. Here we experimentally demonstrate conversion from toggle switching to a deterministic mechanism by biasing AOS in a Co/Gd bilayer with a spin polarized current which is optically generated in an adjacent ferromagnetic reference layer. We show deterministic writing of an ‘up’ and ‘down’ state using a sequence of one or two pulses, respectively, and demonstrate the non-local origin by varying the magnitude of the generated spin current. Our demonstration of deterministic magnetization writing could provide an essential step towards the implementation of future optically addressable spintronic memory devices., Some magnetic materials can switch magnetization via a single laser pulse. However, while very fast, the mechanism is not deterministic. Here, the authors demonstrate a deterministic method of writing opposite magnetization states optically with a sequence of one or two laser pulses.

All optical switching (AOS) of the magnetization in synthetic ferrimagnetic Pt/Co/Gd stacks has received considerable interest due to its high potential towards integration with spintronic devices, such as magnetic tunnel junctions (MTJs), to enable ultrafast memory applications. Post-annealing is an essential process in the MTJ fabrication to obtain optimized tunnel magnetoresistance (TMR) ratio. However, with integrating AOS with an MTJ in prospect, the annealing effects on single-pulse AOS and domain wall (DW) dynamics in the Pt/Co/Gd stacks haven't been systematically investigated yet. In this study, we experimentally explore the annealing effect on AOS and field-induced DW motion in Pt/Co/Gd stacks. The results show that the threshold fluence (F_0) for AOS is reduced significantly as a function of annealing temperature (T_a) ranging from 100C to 300C. Specifically, a 28% reduction of F_0 can be observed upon annealing at 300C, which is a critical T_a for MTJ fabrication. Lastly, we also demonstrate a significant increase of the DW velocity in the creep regime upon annealing, which is attributed to annealing-induced Co/Gd interface intermixing. Our findings show that annealed Pt/Co/Gd system facilitates ultrafast and energy-efficient AOS, as well as enhanced DW velocity, which is highly suitable towards opto-spintronic memory applications., Comment: 17 pages, 10 figures

The interfacial Dzyaloshinskii-Moriya interaction (iDMI) is of great interest in thin-film magnetism because of its ability to stabilize chiral spin textures. It can be quantified by investigating the frequency nonreciprocity of oppositely propagating spin waves. However, as the iDMI is an interface interaction, the relative effect reduces when the films become thicker, making quantification more difficult. Here, we utilize all-electrical propagating spin-wave spectroscopy to disentangle multiple contributions to spin wave frequency nonreciprocity to determine the iDMI. This is done by investigating nonreciprocities across a wide range of magnetic layer thicknesses (from 4 to 26 nm) in Pt/Co/Ir, Pt/Co/Pt, and Ir/Co/Pt stacks. We find the expected sign change in the iDMI when inverting the stack order and a negligible iDMI for the symmetric Pt/Co/Pt. We additionally extract a difference in surface anisotropies and find a large contribution due to the formation of different crystalline phases of the Co, which is corroborated using nuclear magnetic resonance and high-resolution transmission-electron-microscopy measurements. These insights will open up avenues to investigate, quantify, and disentangle the fundamental mechanisms governing the iDMI, and pave a way toward engineered large spin-wave nonreciprocities for magnonic applications.

We recently described the use of Ti(0) microfibers as an anodization substrate for the preparation of TiO2 nanotubes arrays as porous photoanodes. Here, we report the use of these fibers as a scaffold to build porous photoanodes based on a WO3/BiVO4 heterojunction. The obtained photoelectrodes show promising results under visible light irradiation for water oxidation both in a typical liquid-phase photoelectrochemical setup and in a gas phase reactor (developed in-house) based on a polymeric electrolyte membrane. Keywords: Photoelectrochemical H2 production, Polymeric electrolyte membrane, Gas phase water-splitting, WO3/BiVO4 photoanode, Visible light

Large skyrmion bubbles in confined geometries of various sizes and shapes are investigated, typically in the range of several micrometers. Two fundamentally different cases are studied to address the role of dipole-dipole interactions: (I) when there is no magnetic material present outside the small geometries and (II) when the geometries are embedded in films with a uniform magnetization. It is found that the preferential position of the skyrmion bubbles can be controlled by the geometrical shape, which turns out to be a stronger influence than local variations in material parameters. In addition, independent switching of the direction of the magnetization outside the small geometries can be used to further manipulate these preferential positions, in particular with respect to the edges. We show by numerical calculations that the observed interactions between the skyrmion bubbles and structure edge, including the overall positioning of the bubbles, can be explained by considering only dipole-dipole interactions.

We report an experimental study which focuses on the magneto-optic interaction between light and perpendicularlymagnetized ferromagnetic thin-film multilayers. In this study, samples containing multiple Co/Pt interfaces are measured via the polar Magneto-Optic Kerr Effect (MOKE).Thanks to a sample set with a continuous Co thickness variation, and single and double Co layered samples, interface and bulk contributions to the magneto-optic activity areseparately determined. Kerr rotation and ellipticity signals are recorded as a function of the Co thickness. The results are analyzed via a transfer matrix method that accounts for the standard optical and magneto-optic effects in multilayered thin-films. Bulk magneto-optic contribution of Co (QCo;bulk) that is in accordance with literature is consistently determined despite the use of thin-films. Interfacial Co (QCo;int:) and magnetized Pt (QPt) contributions are presented in terms of two models. Interface phenomena of the large spin-orbit coupling between Co and Pt and the proximity induced magnetization in Pt layer are represented in the models. The strength of interfacial magneto-optic activity is interpreted as an indicator of the relative interface quality among the samples.

International audience; 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.

The microscopic mechanism behind the all optical switching (AOS) in ferromagnets has triggered intense scientific debate. Here, the microscopic three-temperature model is utilized to describe AOS in a perpendicularly magnetized ferromagnetic Co/Pt system. We demonstrate that AOS in such a ferromagnet can be explained with the Inverse Faraday Effect (IFE). The influence of the strength and lifetime of the IFE induced field pulse on the switching process are investigated. We found that because of strong spin-orbit coupling, the minimal lifetime of the IFE needed to obtain switching is of the order of 0.1 ps, which is shorter than previously assumed. Moreover, spatial images of the domain pattern after AOS in Co/Pt, as well as their dependence on applying an opposite magnetic field, are qualitatively reproduced.

The future technological impact of smaller, faster and more efficient spintronic devices compared to current technologies inspires the quest of new approaches and strategies. Emerging non-conventional nanofabrication tools are required for this purpose. One attractive technique is focused electron beam induced deposition, a direct-writing process of ferromagnetic nano-objects. Here, we report the fabrication of highly pure iron wires with one-dimensional thickness modulation using diiron nonacarbonyl, Fe2(CO)9 as a starting material. For that purpose, we employ a strategy for the electron beam scanning method, in which the beam spots are separated a certain distance from each other in one direction during the deposition process. Magnetic properties of the wires have been experimentally studied by magneto-optical-Kerr microscopy and supported by micromagnetic simulations. Our results suggest that the thickness modulation induces a local magnetic anisotropy along the short axis on the iron wire, which is not present in wires with a uniform thickness. By the proposed writing strategy, the switching field in such modulated wires could be controlled due to changes in magnetostatic interactions. Based on our outcomes, we conjecture that this procedure can be valuable in research on the impact of one-dimensional thickness modulation in nano-objects and future spintronic devices. Display Omitted New way to fabricate nanowires with 1D thickness modulation by FEBID.Switching field of magnetic wires vs. 1D thickness modulation is controlled.The observed magnetic properties rely on changes in magnetostatic interactions.

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., The high speed switching and energy efficiency nature grant all-optical switching (AOS) great potential for future photonic integrated spintronic devices. Here the authors demonstrate the combination of AOS and domain wall propagation in Pt/Co/Gd synthetic ferrimagnetic racetrack for applications in photonic memory technologies.

The interfacial Dzyaloshinskii-Moriya interaction (iDMI) is attracting great interest for spintronics. An iDMI constant larger than 3 mJ m-2 is expected to minimize the size of skyrmions and to optimize the domain-wall dynamics. In this study, we experimentally demonstrate a giant iDMI in Pt/Co/X/MgO ultra-thin film structures with perpendicular magnetization. The iDMI constants were measured using a field-driven creep regime domain expansion method. The enhancement of iDMI with an atomically thin insertion of Ta and Mg is comprehensively understood with the help of ab-initio calculations. Thermal annealing has been used to crystallize the MgO thin layer to improve the tunneling magneto-resistance (TMR), but interestingly it also provides a further increase of the iDMI constant. An increase of the iDMI constant of up to 3.3 mJ m-2 is shown, which is promising for the scaling down of skyrmion electronics.

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 inter-layer hopping can be tunned 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$_2$.

The interfacial Dzyaloshinskii-Moriya interaction (DMI) in ferromagnetic/heavy metal ultra-thin film structures has attracted a lot of attention thanks to its capability to stabilize Néel-type domain walls (DWs) and magnetic skyrmions for the realization of non-volatile memory and logic devices. In this study, we demonstrate that magnetic properties in perpendicularly magnetized Ta/Pt/Co/MgO/Pt heterostructures, such as magnetization and DMI, can be significantly influenced by the MgO thickness. To avoid the excessive oxidation of Co, an ultrathin Mg layer is inserted to improve the quality of the Co-MgO interface. By using field-driven domain wall expansion in the creep regime, we find that non-monotonic tendencies of the DMI field appear when changing the thickness of MgO. With the insertion of a monatomic Mg layer, the strength of the DMI could reach a high level and saturate. We conjecture that the efficient control of the DMI is a result of subtle changes of both Pt/Co and Co/MgO interfaces, which provides a method to optimize the design of ultra-thin structures achieving skyrmion electronics.

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).

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.

In this chapter, we discuss the essence of hyperfine-field induced magnetic field effects in organic semiconductors originating from spindependent reactions between electron and hole polarons. The basic picture we employ is that of two spin-1/2 polarons, each localized on a site in the semiconductor, where one of the polarons incoherently hops to the other site, forming a spin-0 bipolaron or singlet exciton, or a spin-1 triplet exciton. We describe this process within the framework of the stochastic Liouville equation. We introduce the useful simplifications where the hyperfine coupling of the electronic spin with the surrounding nuclear spins is replaced by a classical random hyperfine field (semiclassical approximation) and where the hopping rate is assumed to be much smaller than the hyperfine precession rate (slow-hopping approximation). Within the resulting theoretical framework we discuss the magnetic field effects measured in the conduction and electroluminescence of unipolar and bipolar devices of organic semiconductors at low magnetic field, focusing in particular on the line shapes of the effects. We conclude with a theoretical discussion of a huge organic magnetoresistance that could be realized in donor-acceptor copolymers.

We experimentally demonstrate single-pulse all-optical switching in Pt/Co/Gd stacks using linearly polarized laser pulses. This shows that thermal single-pulse switching is not limited to ferrimagnetic alloys, but is also possible in ferrimagnetic multilayers that are highly suitable for future applications due to easy fabrication and (interface) engineering. Moreover, it is shown that the threshold fluence needed for the optical switch strongly depends on the thickness of the Co layer, with a remarkable low threshold fluence of $\ensuremath{\approx}1.2$ mJ/${\mathrm{cm}}^{2}$ for a Co thickness of 0.8 nm. Lastly, helicity-dependent measurements showed no significant effect of the magnetic circular dichroism in these thin magnetic layers.

We investigate femtosecond spin injection from an optically excited Ni top layer into an Au bottom layer using time-resolved complex magneto-optical Kerr effect (C-MOKE) measurements. Employing the C-MOKE formalism, we are able to follow layer-resolved demagnetization in Ni and the simultaneous spin injection into the adjacent Au film, both occurring within $\ensuremath{\sim}40\phantom{\rule{0.16em}{0ex}}\mathrm{fs}$. We confirm the ballistic to diffusive propagation of the spin transfer process with ab initio theory and superdiffusive transport calculations. In particular, our combined experimental-theoretical effort does allow us to quantify the so far elusive amount of spin injection, and therefore the spin injection efficiency at the interface.

High flux, low energy He plasma exposure is proven to nanostructure iron thin films over their entire thickness to a highly open structure with large surface area. From a large set of plasma exposure parameters, the ion flux, the surface temperature, and the plasma exposure time are found to be the most relevant parameters to process mechanically stable, nanostructured Fe thin films on brittle glass substrates. The nanostructure stays stable during oxidation. Different surface morphologies are found, depending on the location where the plasma plume interacts with the thin film. This method paves the way to a new direction in top down nanostructuring of thin films, which can be adopted for many functional materials in diverse applications that require a high ratio of active to projected surface area.

Magnetic domain wall motion could be the key to the next generation of data storage devices, shift registers without mechanically moving parts. Various concepts of such so-called ‘racetrack memories’ have been developed, but they are usually plagued by the need for high current densities or complex geometrical requirements. We introduce a new device concept, based on the interfacial Dzyaloshinskii-Moriya interaction (DMI), of which the importance in magnetic thin films was recently discovered. In this device the domain walls are moved solely by magnetic fields. Unidirectionality is created utilizing the recent observation that the strength with which a domain wall is pinned at an anisotropy barrier depends on the direction of the in-plane field due to the chiral nature of DMI. We demonstrate proof-of-principle experiments to verify that unidirectional domain-wall motion is achieved and investigate several material stacks for this novel device including a detailed analysis of device performance for consecutive pinning and depinning processes.

In a set of papers, the authors present a comprehensive investigation of the magnetic, structural, and dynamic properties of thin film Ni-Co, Ni-Fe, and Co-Fe alloys and uncover new phenomena in systems previously thought to be well understood and commonplace. High-precision measurements of saturation magnetization, effective magnetization, spectroscopic $g$ factor, inhomogeneous linewidth broadening, and damping are performed, allowing precise testing of \textit{ab initio} calculations for the orbital magnetic moment and the intrinsic magnetic damping, which is the most integral motivation of this work. Quantitative comparison between experiment and theory is shown and discussed for magnetic damping spanning a broad range of material parameters. Furthermore, a large set of data has been created that will serve as a glossary for alloy magnetic properties in thin films.

A systematic experimental study of Gilbert damping is performed via ferromagnetic resonance for the disordered crystalline binary 3d transition-metal alloys Ni-Co, Ni-Fe, and Co-Fe over the full range of alloy compositions. After accounting for inhomogeneous linewidth broadening, the damping shows clear evidence of both interfacial damping enhancement (by spin pumping) and radiative damping. We quantify these two extrinsic contributions and thereby determine the intrinsic damping. The comparison of the intrinsic damping to multiple theoretical calculations yields good qualitative and quantitative agreement in most cases. Furthermore, the values of the damping obtained in this study are in good agreement with a wide range of published experimental and theoretical values. Additionally, we find a compositional dependence of the spin mixing conductance.

The Rashba effect leads to a chiral precession of the spins of moving electrons, while the Dzyaloshinskii-Moriya interaction (DMI) generates preference towards a chiral profile of local spins. We predict that the exchange interaction between these two spin systems results in a “chiral” magnetoresistance depending on the chirality of the local spin texture. We observe this magnetoresistance by measuring the domain wall (DW) resistance in a uniquely designed Pt/Co/Pt zigzag wire and by changing the chirality of the DW with applying an in-plane magnetic field. A chirality-dependent DW resistance is found, and a quantitative analysis shows a good agreement with a theory based on the Rashba model. Moreover, the DW resistance measurement allows us to independently determine the strength of the Rashba effect and the DMI simultaneously, and the result implies a possible correlation between the Rashba effect, the DMI, and the symmetric Heisenberg exchange.

Focused-electron-beam-induced deposition (FEBID) is employed to create freestanding magnetic nanostructures. By growing Fe nanopillars on top of a perpendicular magnetic domain wall (DW) conduit, pinning of the DWs is observed due to the stray fields emanating from the nanopillar. Furthermore, a different DW pinning behavior is observed between the up and down magnetic states of the pillar, allowing to deduce the switching fields of the pillar in a novel way. The implications of these results are two-fold: not only can 3-dimensional nano-objects be used to control DW motion in applications, it is also proposed that DW motion is a unique tool to probe the magnetic properties of nano-objects.

Organic materials provide a unique platform for exploiting the spin of the electron—a field dubbed organic spintronics. Originally, this was mostly motivated by the notion that because of weak spin-orbit coupling, due to the small mass elements in organics and small hyperfine field coupling, organic matter typically displays a very long electron spin coherence time. More recently, however, it was found that organics provide a special class of spintronic materials for many other reasons—several of which are discussed throughout this issue. Over the past decade, there has been a growing interest in utilizing the molecular spin state as a quantum of information, aiming to develop multifunctional molecular spintronics for memory, sensing, and logic applications. The aim of this issue is to stimulate the interest of researchers by bringing to their attention the vast possibilities not only for unexpected science but also for the enormous potential for developing new functionalities and applications. The six articles in this issue deal with some of the breakthrough work that has been ongoing in this field in recent years.

Recently, it was discovered that the current through an organic semiconductor, sandwiched between two non-magnetic electrodes, can be changed significantly (up to 25%) by applying a small (a few millitesla) magnetic field. At present, the microscopic mechanisms underlying this so-called organic magnetoresistance (OMAR) are intensively being debated. One of the mechanisms which can successfully describe the magnetic field effects on the current in pristine organic semiconductor devices uses the reactions of triplet excitons and polarons. Here, we present a proof of concept study in which we tune these interactions in the device by deliberately doping our devices with fullerene, creating additional charge transfer states (CTS). By engineering devices with different energetic alignments of the CTS and triplet exciton, we can influence the triplet exciton density in the device. We correlate pronounced changes in the magnetic field effect magnitude and lineshape to the energy of the CTS with respect to the triplet exciton. 2014 Elsevier B.V. All rights reserved.

In spintronic devices relying on magnetic domain-wall (DW) motion, robust control over the DW position is required. We use electric-field control of perpendicular magnetic anisotropy to create a voltage-gated pinning site in a microstructured Pt/Co/AlOx DW conduit. A DW pins at the edge of a gate electrode, and the strength of pinning can be tuned linearly and reversibly with an efficiency of 0.22(1) mT/V. This result is supported by a micromagnetic model, taking full account of the anisotropy step at the gate edge, which is directly caused by a change in the electron density due to the choice of material.

Spin currents can be generated on an ultrafast time scale by excitation of a ferromagnetic (FM) thin film with a femtosecond laser pulse. Recently, it has been demonstrated that these ultrafast spin currents can transport angular momentum to neighboring FM layers, being able to change both the magnitude and orientation of the magnetization in the adjacent layer. In this paper, both the generation and absorption of these optically excited spin currents are investigated. This is done using noncollinear magnetic bilayers, i.e., two FM layers separated by a conductive spacer. Spin currents are generated in a Co/Ni multilayer with out-of-plane (OOP) anisotropy, and absorbed by a Co layer with an in-plane (IP) anisotropy. This behavior is confirmed by careful analysis of the laser-pulse induced magnetization dynamics, whereafter it is demonstrated that the transverse spin current is absorbed very locally near the injection interface of the IP layer (90% within the first ≈2 nm). Moreover, it will also be shown that this local absorption results in the excitation of THz standing spin waves within the IP layer. The dispersion measured for these high-frequency spin waves shows a discrepancy with respect to the theoretical predictions, for which an explanation involving intermixed interface regions is proposed. Lastly, the spin current generation is investigated by using magnetic bilayers with a different number of repeats for the Co/Ni multilayer, which proves to be of great relevance for identifying the optical spin current generation mechanism.

This article reviews static and dynamic interfacial effects in magnetism, focusing on interfacially-driven magnetic effects and phenomena associated with spin-orbit coupling and intrinsic symmetry breaking at interfaces. It provides a historical background and literature survey, but focuses on recent progress, identifying the most exciting new scientific results and pointing to promising future research directions. It starts with an introduction and overview of how basic magnetic properties are affected by interfaces, then turns to a discussion of charge and spin transport through and near interfaces and how these can be used to control the properties of the magnetic layer. Important concepts include spin accumulation, spin currents, spin transfer torque, and spin pumping. An overview is provided to the current state of knowledge and existing review literature on interfacial effects such as exchange bias, exchange spring magnets, spin Hall effect, oxide heterostructures, and topological insulators. The article highlights recent discoveries of interface-induced magnetism and non-collinear spin textures, non-linear dynamics including spin torque transfer and magnetization reversal induced by interfaces, and interfacial effects in ultrafast magnetization processes., 193 pages, including 28 figures inserted after text and references. Paper accepted in Reviews of Modern Physics

We experimentally investigate to what extent superdiffusive spin transport contributes to the ultrafast demagnetization of ferromagnets by pumping Ni thin films from the front side and back side of the sample. Within the experimental accuracy, the temporal evolution of the magnetization for front-side and back-side pumping is identical, hence no influence of transport was detected. Furthermore, adding a conducting buffer layer to enhance spin transport does not alter the magnetization dynamics significantly either. In order to explain the experimental results, angular momentum has to be locally dissipated on femtosecond timescales, supporting heating of the spin system as the main driving force for ultrafast demagnetization in the investigated ferromagnetic films.

We present our experimental and theoretical study on Co diffusion in the first few atomic layers of Cu(001). While the diffusion of Co atoms in Cu(001) is usually expected to be intense at a temperature $T\ensuremath{\ge}800$ K, in the vicinity of the surface, it is already activated at a considerably lower temperature $T\ensuremath{\sim}650$ K whereas the diffusion in the deep bulk region is still inhibited. This intense near-surface diffusion provides an accumulation of Co atoms in the first six atomic layers upon a single stage Co deposition. The details of the near-surface diffusion are studied by analyzing the distribution of the location depth of the buried single Co atoms. The depth of location of single Co atoms is deduced from scanning tunneling microscopy (STM) data. The subsurface Co atoms induce a perturbation of the electron density of states of the surface which is observable as apparent ringlike ripples in the STM images of the atomically flat Cu surface. The depth of location of the buried Co atoms is determined from the diameter of those rings. The largest amount of Co atoms is found to be in the third layer, emphasizing that the near-surface diffusion should be described by diffusion parameters different from those for bulk diffusion. A model that describes the embedding process of Co atoms into Cu(001) layers is developed. The model assumes Co atoms to diffuse via the vacancy and ring-exchange mechanisms. The energy barriers for the interlayer Co diffusion via those mechanisms are calculated using the nudged elastic band method. The model satisfactorily explains the experimental results. Our study reveals that the energy barriers for Co diffusion in the first five atomic layers of Cu(001) are lower than those in the bulk. This defines the region in which Co diffusion should be considered as a near-surface one.

Controlling the relaxation of magnetization in magnetic nanostructures is key to optimizing magnetic storage device performance. This relaxation is governed by both intrinsic and extrinsic relaxation mechanisms and with the latter strongly dependent on the interactions between the nanostructures. In the present work we investigate laser induced magnetization dynamics in a broadband optical resonance type experiment revealing the role of interactions between nanostructures on the relaxation processes of granular magnetic structures. The results are corroborated by constructing a temperature dependent numerical micromagnetic model of magnetization dynamics based on the Landau-Lifshitz-Bloch equation. The model predicts a strong dependence of damping on the key material properties of coupled granular nanostructures in good agreement with the experimental data. We show that the intergranular, magnetostatic and exchange interactions provide a large extrinsic contribution to the damping. Finally we show that the mechanism can be attributed to an increase in spin-wave degeneracy with the ferromagnetic resonance mode as revealed by semianalytical spin-wave calculations.

Domain-wall (DW) motion in magnetic nanostrips is intensively studied, in particular because of the possible applications in data storage. In this work, we will investigate a novel method of DW motion using magnetic field pulses, with the precession torque as the driving mechanism. We use a one dimensional (1D) model to show that it is possible to drive DWs in out-of-plane materials using the precession torque, and we identify the key parameters that influence this motion. Because the DW moves back to its initial position at the end of the field pulse, thereby severely complicating direct detection of the DW motion, depinning experiments are used to indirectly observe the effect of the precession torque. The 1D model is extended to include an energy landscape in order to predict the influence of the precession torque in the depinning experiments. Although preliminary experiments did not yet show an effect of the precession torque, our calculations indicate that depinning experiments can be used to demonstrate this novel method of DW motion in out-of-plane materials, which even allows for coherent motion of multiple domains when the Dzyaloshinskii-Moriya interaction is taken into account.

To increase the efficiency of current electronics, a specific challenge for the next generation of memory, sensing and logic devices is to find suitable strategies to move from two- to three-dimensional (3D) architectures. However, the creation of real 3D nano-objects is not trivial. Emerging non-conventional nanofabrication tools are required for this purpose. One attractive method is focused-electron-beam induced deposition (FEBID), a direct-write process of 3D nano-objects. Here, we grow 3D iron and cobalt nanopillars by FEBID using diiron nonacarbonyl Fe2(CO)9, and dicobalt octacarbonyl Co2(CO)8, respectively, as starting materials. In addition, we systematically study the composition of these nanopillars at the sub-nanometer scale by atom probe tomography, explicitly mapping the homogeneity of the radial and longitudinal composition distributions. We show a way of fabricating high-purity 3D vertical nanostructures of ~50 nm in diameter and a few micrometers in length. Our results suggest that the purity of such 3D nanoelements (above 90 at% Fe and above 95 at% Co) is directly linked to their growth regime, in which the selected deposition conditions are crucial for the final quality of the nanostructure. Moreover, we demonstrate that FEBID and the proposed characterization technique not only allow for growth and chemical analysis of single-element structures, but also offers a new way to directly study 3D core–shell architectures. This straightforward concept could establish a promising route to the design of 3D elements for future nano-electronic devices.

Iron nanostructures grown by focused electron beam induced deposition (FEBID) are promising for applications in magnetic sensing, storage and logic. Such applications require a precise design and determination of the coercive field (H C), which depends on the shape of the nanostructure. In the present work, we have used the Fe2(CO)9 precursor to grow iron nanowires by FEBID in the thickness range from 10 to 45 nm and width range from 50 to 500 nm. These nanowires exhibit an Fe content between 80 and 85%, thus giving a high ferromagnetic signal. Magneto-optical Kerr characterization indicates that H C decreases for increasing thickness and width, providing a route to control the magnetization reversal field through the modification of the nanowire dimensions. Transmission electron microscopy experiments indicate that these wires have a bell-type shape with a surface oxide layer of about 5 nm. Such features are decisive in the actual value of H C as micromagnetic simulations demonstrate. These results will help to make appropriate designs of magnetic nanowires grown by FEBID., This work was supported by Spanish Ministry of Economy and Competitivity through project No. MAT2011-27553-C02, MAT2011-28532-C03-02 and MAT2014-51982-C2-1-R, including FEDER funds and by the Aragon Regional Government.

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. © 2016 IOP Publishing Ltd.

As the first magnetic random access memories are finding their way onto the market, an important issue remains to be solved: the current density required to write magnetic bits becomes prohibitively high as bit dimensions are reduced. Recently, spin–orbit torques and the spin-Hall effect in particular have attracted significant interest, as they enable magnetization reversal without high current densities running through the tunnel barrier. For perpendicularly magnetized layers, however, the technological implementation of the spin-Hall effect is hampered by the necessity of an in-plane magnetic field for deterministic switching. Here we interface a thin ferromagnetic layer with an anti-ferromagnetic material. An in-plane exchange bias is created and shown to enable field-free S HE-driven magnetization reversal of a perpendicularly magnetized Pt/Co/IrMn structure. Aside from the potential technological implications, our experiment provides additional insight into the local spin structure at the ferromagnetic/anti-ferromagnetic interface., Future information storage technology may exploit electrical currents to write the states of ferromagnetic nanoelements via spin torque effects. Here, the authors demonstrate such behaviour promoted by exchange bias from an interfaced antiferromagnet, which may help overcome practical device limitations.

The movement of magnetic domain walls can be used to build a device known as a shift register, which has applications in memory1 and logic circuits2, 3. However, the application of magnetic domain wall shift registers has been hindered by geometrical restrictions, by randomness in domain wall displacement and by the need for high current densities or rotating magnetic fields. Here, we propose a new approach in which the energy landscape experienced by the domain walls is engineered to favour a unidirectional ratchet-like propagation. The domain walls are defined between domains with an out-of-plane (perpendicular) magnetization, which allows us to route domain walls along arbitrary in-plane paths using a time-varying applied magnetic field with fixed orientation. In addition, this ratchet-like motion causes the domain walls to lock to discrete positions along these paths, which is useful for digital devices. As a proof-of-principle experiment we demonstrate the continuous propagation of two domain walls along a closed-loop path in a platinum/cobalt/platinum strip.

In this work we investigate the frequency dependence of organic magnetoresistance (OMAR) both in small molecule-based (Alq3) and polymer (PPV derivative) materials, and investigate its thickness dependence. For all devices, we observed a strong decrease in magnetoconductance (MC) with increasing frequency of the ac component of the applied magnetic field. Moreover, we observed a strong reduction of the cut-off frequencies for increasing film thickness. By means of admittance spectroscopy and device simulations, we show that the cut-off frequency is related to the inverse transit time of the minority charge carriers. These observations confirm the important role of minority carriers in OMAR, and show that changes in OMAR are not only due to microscopic mechanisms, but also device physics is of significant relevance. Keywords: Organic semiconductor; Magnetoresistance; Carrier mobility; Minority carriers; Transit time

It is still an open question whether the statistical ratio of 1:3 for the formation of singlet vs. triplet excitons in organic semiconductors can be violated. In this work, we use a two-site and a multi-site model to investigate this problem. We show that spin mixing by hyperfine fields leads to a violation of the statistical ratio if singlet and triplet excitons are formed with a different rate. This only occurs if the exciton formation rate is slow enough compared to the hyperfine precession frequency. The violation of the statistical ratio can be suppressed by applying an external magnetic field. In the multi-site model we demonstrate how disorder and Coulomb interactions affect the fraction of singlet excitons formed. © 2010 Elsevier B.V. All rights reserved.

We investigate electrically pumped, distributed feedback (DFB) lasers, based on gap-plasmon mode metallic waveguides. The waveguides have nano-scale widths below the diffraction limit and incorporate vertical groove Bragg gratings. These metallic Bragg gratings provide a broad bandwidth stop band (~500 nm) with grating coupling coefficients of over 5000/cm. A strong suppression of spontaneous emission occurs in these Bragg grating cavities, over the stop band frequencies. This strong suppression manifests itself in our experimental results as a near absence of spontaneous emission and significantly reduced lasing thresholds when compared to similar length Fabry-Pérot waveguide cavities. Furthermore, the reduced threshold pumping requirements permits us to show strong line narrowing and super linear light current curves for these plasmon mode devices even at room temperature.

Organic semiconductors are interesting materials for spintronics applications because of their long spin lifetimes. In addition, organic spintronics offers the possibility to add magnetic functionality to existing organic electronics. Two main topics of organic spintronics are discussed. First, in organic spin valves, spin transport occurs through an organic spacer layer. As a main source of loss of spin polarization the interactions of the spins with random hyperfine fields originating from surrounding hydrogen nuclei has been identified. Recent progress in the development of organic spin valves and related approaches are discussed, addressing conductivity mismatch and the question whether the observed magnetoresistance is from spin injection or direct tunneling. Second, an intrinsic magnetoresistance is observed in many organic semiconductors. This, so-called, organic magnetoresistance (OMAR) is interesting because it shows large effects at room temperature, making it interesting for applications, and poses fundamental questions about spin transport and spin interactions in these organic materials. An overview of the main characteristics of OMAR is given and the three main models are discussed. These models are the bipolaron model, the electron–hole pair model and the exciton–charge interaction model. Finally, a comparison is made between the proposed models and experimental results, where it is concluded that the exact origin of OMAR is still open for debate.