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Synthetic ferrimagnets based on Co and Gd bear promise for directly bridging the gap between volatile information in the photonic domain and non-volatile information in the magnetic domain, without the need for any intermediary electronic conversion. Specifically, these systems exhibit strong spin-orbit torque effects, fast domain wall motion and single-pulse all-optical switching of the magnetization. An important open challenge to bring these materials to the brink of applications is to achieve long-term stability of their magnetic properties. In this work, we address the time-evolution of the magnetic moment and compensation temperature of magnetron sputter grown Pt/Co/Gd trilayers with various capping layers. Over the course of three months, the net magnetic moment and compensation temperature change significantly, which we attribute to quenching of the Gd magnetization. We identify that intermixing of the capping layer and Gd is primarily responsible for this effect, which can be alleviated by choosing nitrides for capping as long as reduction of nitride to oxide is properly addressed. In short, this work provides an overview of the relevant aging effects that should be taken into account when designing synthetic ferrimagnets based on Co and Gd for spintronic applications., Synthetic ferrimagnets based on Co and Gd bear promise for directly bridging the gap between volatile information in the photonic domain and non-volatile information in the magnetic domain, without the need for any intermediary electronic conversion. Specifically, these systems exhibit strong spin-orbit torque effects, fast domain wall motionand single-pulse all-optical switching of the magnetization. An important open challenge to bring these materials to the brink of applications is to achieve long-term stability of their magnetic properties. In this work, we address the time-evolution of the magnetic moment and compensation temperature of magnetron sputter grown Pt/Co/Gd trilayerswith various capping layers. Over the course of three months, the net magnetic moment and compensation temperature change significantly, which we attribute to quenching of the Gd magnetization. We identify that intermixing of the capping layer and Gd is primarily responsible for this effect, which can be alleviated by choosing nitrides for cappingas long as reduction of nitride to oxide is properly addressed. In short, this work provides an overview of the relevant aging effects that should be taken into account when designing synthetic ferrimagnets based on Co and Gd for spintronic applications.

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.

We introduce a hybrid plasmonic-photonic device for on-chip all-optical switching and reading of ferrimagnet bits with perpendicular magnetic anisotropy in a racetrack spintronic memory, coupled onto an indium phosphide waveguide. The device comprises V-shaped gold plasmonic nanoantennas coupled with a photonic crystal cavity, which enables switching and reading of the magnetic state of nanoscale bits by enhancing the absorbed energy density and polar magneto-optical Kerr effect locally. Using a finite-difference time-domain method, we show that our device can switch and read targeted bits down to 100 nm in the presence of oppositely magnetized background regions in the racetrack with widths up to 120 nm, clearly outperforming a bare photonic waveguide. Our hybrid device provides the missing link between integrated photonics and nanoscale spintronics by tackling the challenges of nonlinear absorption in the waveguide, weak magneto-optics, and size mismatch, leading to the development of ultrafast and energy-efficient advanced on-chip applications.

An ultra-compact, ultra-broadband vertical coupler for dense photonic integrated circuits is reported with a 1.07 × 0.62 μm2 wavelength-scale footprint. This hybrid plasmonic-photonic coupler uses a unique two-plane plasmonic nanoantenna array on a silicon-on-insulator waveguide. The in- and out-of-plane interference of the multipole moments and dual-feed nanoantennas results in efficient, unidirectional coupling. Finite-element simulations show that, for a 0.8 μm diameter Gaussian beam, the maximum coupling efficiency (CE) is −3.4 dB across the telecommunication C-, L- and U-bands with a 3-dB bandwidth of 230 nm. The CE is > 9 dB higher than recently reported ultra-compact plasmonic couplers. The maximum directivity and polarisation extinction ratio across the C- to U-bands are 9.2 and 24.1 dB, respectively. Finally, as an out-coupler, it has a vertical directivity of >8.5 dB, enabling its use for vertical optical interconnects between two vertically separated circuits.

Using spin currents generated by fs laser pulses, we demonstrate excitation of GHz ferromagnetic resonance and THz ferrimagnetic exchange resonances in Co/Gd/Co/Gd multilayers by time-resolved magneto-optic Kerr effect measurements. Varying the Gd layer thickness allows for a tuning of the resonance spectrum by manipulating the total angular momentum and strength of effective exchange fields between the antiferromagnetically coupled layers. Close to the compensation point of angular momentum, a minimum in the frequency of the exchange-dominated mode and a maximum in the frequency of the ferromagnetic resonance mode is observed. Finally, to gain better understanding of the excitation mechanism, we analyze the anomalous variation in the measured exchange mode amplitude as a function of its frequency. A peak in this amplitude in the vicinity of the compensation point of angular momentum is explained using a macrospin model, taking nonlinear effects at finite precession amplitudes into account.

In this paper, we show that magnetic skyrmion nucleation can be controlled using Ga+ ion irradiation, which manipulates the magnetic interface effects (in particular the magnetic anisotropy and Dzyaloshinskii-Moriya interaction) that govern the stability and energy cost of skyrmions in thin film systems. We systematically and quantitatively investigated what effect these changes have on the nucleation of magnetic skyrmions. Our results indicate that the energy cost of skyrmion nucleation can be reduced up to 26% in the studied dose range and that it scales approximately linearly with the square root of the domain-wall energy density. Moreover, the total number of nucleated skyrmions in irradiated devices after nucleation was found to depend linearly on the ion dose and could be doubled compared to nonirradiated devices. These results show that ion irradiation cannot only be used to enable local nucleation of skyrmions, but that it also allows for fine control of the threshold and efficiency of the nucleation process., Comment: Main: 17 pages, 3 figures; Supplemental Material: 7 pages, 5 figures

Funder: Dutch NWO Aspasia program, Funder: University of Colorado Boulder; doi: http://dx.doi.org/10.13039/100007493, Funder: FSE/POPH, Women have made significant contributions to applied physics research and development, and their participation is vital to continued progress. Recognizing these contributions is important for encouraging increased involvement and creating an equitable environment in which women can thrive. This Roadmap on Women in Applied Physics, written by women scientists and engineers, is intended to celebrate women’s accomplishments, highlight established and early career researchers enlarging the boundaries in their respective fields, and promote increased visibility for the impact women have on applied physics research. Perspectives cover the topics of plasma materials processing and propulsion, super-resolution microscopy, bioelectronics, spintronics, superconducting quantum interference device technology, quantum materials, 2D materials, catalysis and surface science, fuel cells, batteries, photovoltaics, neuromorphic computing and devices, nanophotonics and nanophononics, and nanomagnetism. Our intent is to inspire more women to enter these fields and encourage an atmosphere of inclusion within the scientific community.

Spin waves are proposed as information carriers for next-generation computing devices because of their low power consumption. Moreover, their wave-like nature allows for novel computing paradigms. Conventional methods to detect spin waves are based either on electrical induction, limiting the downscaling and efficiency complicating eventual implementation, or on light scattering, where the minimum detectable spin-wave wavelength is set by the wavelength of the laser. In this Article we demonstrate magneto-optical detection of spin waves beyond the diffraction limit using a metallic grating that selectively absorbs laser light. Specifically, we demonstrate the detection of propagating spin waves with a wavelength of 700 nm using a diffraction-limited laser spot with a size of 10 $\mu$m in 20 nm thick Py strips. Additionally, we show that this grating is selective to the wavelength of the spin wave, providing wavevector-selective spin-wave detection. This should open up new avenues towards the integration of the burgeoning fields of photonics and magnonics, and aid in the optical detection of spin waves in the short-wavelength exchange regime for fundamental research., Spin waves are proposed as information carriers for next-generation computing devices because of their low power consumption. Moreover, their wave-like nature allows for novel computing paradigms. Conventional methods to detect spin waves are based either on electrical induction, limiting the downscaling and efficiency complicating eventual implementation, or on light scattering, where the minimum detectable spin-wave wavelength is set by the wavelength of the laser. In this Article we demonstrate magneto-optical detection of spin waves beyond the diffraction limit using a metallic grating that selectively absorbs laser light. Specifically, we demonstrate the detection of propagating spin waves with a wavelength of 700 nm using a diffraction-limited laser spot with a size of 10 μm in 20 nm thick Py strips. Additionally, we show that this grating is selective to the wavelength of the spin wave, providing wavevector-selective spin-wave detection. This should open up new avenues towards the integration of the burgeoning fields of photonics and magnonics, and aid in the optical detection of spin waves in the short-wavelength exchange regime for fundamental research.

Magnetic racetrack memory has significantly evolved and developed since its first experimental verification and is considered as one of the most promising candidates for future high-density on-chip solid state memory. However, the lack of a fast and precise magnetic domain wall (DW) shifting mechanism and the required extremely high DW motion (DWM) driving current both make the racetrack difficult to commercialize. Here, we propose a method for coherent DWM that is free from above issues, which is driven by chirality switching (CS) and an ultralow spin-orbit-torque (SOT) current. The CS, as the driving force of DWM, is achieved by the sign change of Dzyaloshinskii–Moriya interaction which is further induced by a ferroelectric switching voltage. The SOT is used to break the symmetry when the magnetic moment is rotated to the Bloch direction. We numerically investigate the underlying principle and the effect of key parameters on the DWM through micromagnetic simulations. Under the CS mechanism, a fast (~102 m/s), ultralow-energy (~5 attoJoule), and precisely discretized DWM can be achieved. Considering that skyrmions with topological protection and smaller size are also promising for future racetrack, we similarly evaluate the feasibility of applying such a CS mechanism to a skyrmion. However, we find that the CS only causes it to “breathe” instead of moving. Our results demonstrate that the CS strategy is suitable for future DW racetrack memory with ultralow power consumption and discretized DWM.

We show that an interlayer Dzyaloshinskii-Moriya interaction in combination with nonlocal damping gives rise to unidirectional magnetic coupling. That is, the coupling between two magnetic layers - say, the left and right layer - is such that the dynamics of the left layer leads to the dynamics of the right layer, but not vice versa. We discuss the implications of this result for the magnetic susceptibility of a magnetic bilayer, electrically actuated spin-current transmission, and unidirectional spin-wave packet generation and propagation. Our results may enable a route towards spin-current and spin-wave diodes and further pave the way to design spintronic devices via reservoir engineering.

Femtosecond laser-pulse excitation provides an energy efficient and fast way to control magnetization at the nanoscale, providing great potential for ultrafast next-generation data manipulation and nonvolatile storage devices. Ferromagnetic van der Waals materials have garnered much attention over the past few years due to their low dimensionality, excellent magnetic properties, and large response to external stimuli. Nonetheless, their behaviour upon fs laser-pulse excitation remains largely unexplored. Here, we investigate the ultrafast magnetization dynamics of a thin flake of Fe$_3$GeTe$_2$ (FGT) and extract its intrinsic magnetic properties using a microscopic framework. We find that our data is well described by our modelling, with FGT undergoing a slow two-step demagnetization, and we experimentally extract the spin-relaxation timescale as a function of temperature, magnetic field and excitation fluence. Our observations indicate a large spin-flip probability in agreement with a theoretically expected large spin-orbit coupling, as well as a weak interlayer exchange coupling. The spin-flip probability is found to increase when the magnetization is pulled away from its quantization axis, opening doors to an external control over the spins in this material. Our results provide a deeper understanding of the dynamics van der Waals materials upon fs laser-pulse excitation, paving the way towards two-dimensional materials-based ultrafast spintronics., Supplementary information included

Synthetic ferrimagnets are an attractive materials class for spintronics as they provide access to all-optical switching of magnetization and, at the same time, allow for ultrafast domain wall motion at angular momentum compensation. In this work, we systematically study the effects of strain on the perpendicular magnetic anisotropy and magnetization compensation of Co/Gd and Co/Gd/Co/Gd synthetic ferrimagnets. Firstly, the spin reorientation transition of a bilayer system is investigated in wedge type samples, where we report an increase in the perpendicular magnetic anisotropy in the presence of in-plane strain. Using a model for magnetostatics and spin reorientation transition in this type of system, we confirm that the observed changes in anisotropy field are mainly due to the Co magnetoelastic anisotropy. Secondly, the magnetization compensation of a quadlayer is studied. We find that magnetization compensation of this synthetic ferrimagnetic system is not altered by external strain. This confirms the resilience of this material system against strain that may be induced during the integration process, making Co/Gd ferrimagnets suitable candidates for spintronics applications., Synthetic ferrimagnets are an attractive materials class for spintronics as they provide access to all-optical switching of magnetization and, at the same time, allow for ultrafast domain wall motion at angular momentum compensation. In this work, we systematically study the effects of strain on the perpendicular magnetic anisotropy and magnetizationcompensation of Co/Gd and Co/Gd/Co/Gd synthetic ferrimagnets. Firstly, the spin reorientation transition of a bilayer system is investigated in wedge type samples, where we report an increase in the perpendicular magnetic anisotropy in the presence of in-plane strain. Using a model for magnetostatics and spin reorientation transition in this type ofsystem, we confirm that the observed changes in anisotropy field are mainly due to the Co magnetoelastic anisotropy. Secondly, the magnetization compensation of a quadlayer is studied. We find that magnetization compensation of this synthetic ferrimagnetic system is not altered by external strain. This confirms the resilience of this material systemagainst strain that may be induced during the integration process, making Co/Gd ferrimagnets suitable candidates for spintronics applications.

Ultrafast laser-induced dynamics in a ferrimagnetic gadolinium iron cobalt (Gd/FeCo) multilayer with a magnetization compensation temperature of TM = 320 K is studied at room temperature as a function of laser-fluence and strength of the applied magnetic field. The dynamics is found to be substantially different from that in archetypical GdFeCo alloys, and depending on the laser fluence one can distinguish two different regimes. At low laser fluence (⩽1.6 mJ cm-2), ultrafast laser excitation of the medium triggers spin precession of an extraordinary large amplitude reaching over 30°. At high laser fluence (⩾2.2 mJ cm-2), the pump heats the medium over the magnetization compensation point, spin precession reduces significantly in amplitude and the process of field-assisted reversal of magnetization of Gd and FeCo is launched. It is argued that such a distinctly different laser-induced magnetization dynamics in the multilayers compared to the alloys is due to the symmetry breaking at the numerous interfaces, giving rise to additional surface anisotropy. The temperature dependence of the latter is found to be the key ingredient in the mechanism of ultrafast laser-induced magnetization dynamics in ferrimagnetic multilayers. Controlling the amount and properties of interfaces in multilayers can thus serve as a mean to achieve efficient ultrafast all-optical control of magnetism.

Methods to fabricate and characterize monodisperse magnetic nanoplatelets for fluid/bio-based applications based on spintronic thin-film principles are a challenge. This is due to the required top-down approach where the transfer of optimized blanket films to free particles in a fluid while preserving the magnetic properties is an uncharted field. Here, we explore the use of substrate conformal imprint lithography (SCIL) as a fast and cost-effective fabrication route. We analyze the size distribution of nominal 1.8 μm and 120 nm diameter platelets and show the effect of the fabrication steps on the magnetic properties which we explain through changes in the dominant magnetization reversal mechanism as the size decreases. We show that SCIL allows for efficient large-scale platelet fabrication and discuss how application-specific requirements can be solved via process and material engineering.

Antiferromagnetic (AFM) materials possess a well-recognized potential for ultrafast data processing thanks to their intrinsic ultrafast spin dynamics, absence of stray fields, and large spin transport effects. The very same properties, however, make their manipulation difficult, requiring frequencies in THz range and magnetic fields of tens of Teslas. Switching of AFM order implies going into the nonlinear regime, a largely unexplored territory. Here we use THz light from a free electron laser to drive antiferromagnetic NiO into a highly nonlinear regime and steer it out of nonlinearity with magnetic field from a 33-Tesla Bitter magnet. This demonstration of large-amplitude dynamics represents a crucial step towards ultrafast resonant switching of AFM order.

All-optical switching (AOS) of magnetization by a single femtosecond laser pulse in Co/Gd based synthetic ferrimagnets is the fastest magnetization switching process. On the other hand, He ion irradiation has become a promising tool for interface engineering of spintronic material platforms, giving rise to significant modification of magnetic properties. In this paper, we explore the use of He ion irradiation to enhance single pulse AOS of Co/Gd bilayer-based synthetic ferrimagnets. The intermixing of the constituent magnetic layers by He ion irradiation was both numerically simulated and experimentally verified. We theoretically modeled the effects of intermixing on AOS using the layered microscopic 3-temperature model and found that AOS is enhanced significantly by breaking the pristine Co/Gd interface through intermixing. Following this notion, we studied the threshold fluence of AOS as a function of He ion irradiation fluence. We found that the AOS threshold fluence can be reduced by almost 30%. Our study reveals the control of AOS by He ion irradiation, which opens up an industrially compatible approach for local AOS engineering. This project has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie Grant Agreement Nos. 860060 and 861300. This project is also part of the research programme financed by the Netherlands Organisation for Scientific Research (NWO). Solliance and the Dutch province of Noord-Brabant are acknowledged for funding the TEM facility.

The controlled growth of organic crystalline materials in predefined locations still poses a challenge for functional device application such as phototransistors, photoconductors, or photovoltaic solar cells. This work evidences the use of optical lithography and a fluoroalkylsilane to selectively modify the surface energy and how to create a wettability micro-patterned structure. These are then combined with non-contact printing of the organic solution providing custom-shaped films. To deliver printed films with improved morphological quality, key process parameters for high-performance organic materials are optimized. Particular attention is given to the adjustment of the concentration and solvent mixture to tune the jetting properties, and consequently, slow down the evaporation rate. Continuous films are obtained for an optimized number of droplets and spacing between them. Micro-Raman spectroscopy imaging confirms the crystalline nature of the printed films and the lack of impurities. To validate the method, rubrene and triisopropylsilylethynyl (TIPS)-pentacene are tested using two-terminal optoelectronic devices. TIPS-pentacene rectangular printed micrometric photosensor presents linear behavior and no hysteresis, reaching 0.33 nA under 18.1 mW cm−2. The structural and optoelectronic characterization is in line with other micro-patterned examples, opening doors for new industrial applications.

We introduce a novel integrated hybrid plasmonic-photonic device for all-optical switching and reading of nanoscale ferrimagnet bits. The racetrack memory made of synthetic ferrimagnetic material with a perpendicular magnetic anisotropy is coupled on to a photonic waveguide onto the indium phosphide membrane on silicon platform. The device which is composed of a double V-shaped gold plasmonic nanoantenna coupled with a photonic crystal cavity can enable switching and reading of the magnetization state in nanoscale magnetic bits by enhancing the absorbed energy density and polar magneto-optical Kerr effect (PMOKE) locally beyond the diffraction limit. Using a three-dimensional finite-difference time-domain method, we numerically show that our device can switch and read the magnetization state in targeted bits down to ~100 nm in the presence of oppositely magnetized background regions in the racetrack with widths of 30 to 120 nm, clearly outperforming a bare photonic waveguide. Our hybrid device tackles the challenges of nonlinear absorption in the waveguide, weak PMOKE, and size mismatch between spintronics and integrated photonics. Thus, it provides missing link between the integrated photonics and nanoscale spintronics, expediting the development of ultrafast and energy efficient advanced on-chip applications.

Flexibility for interface engineering and access to all-optical switching of the magnetization make synthetic ferrimagnets an interesting candidate for advanced optospintronic devices. Moreover, due to their layered structure and disordered interfaces, they also bear promise for the emerging field of graded magnetic materials. The fastest and most efficient spin–orbit torque driven manipulation of the magnetic order in this material system generally takes place at compensation. Here, we present a systematic experimental and modeling study of the conditions for magnetization compensation and perpendicular magnetic anisotropy in the synthetic ferrimagnetic Co/Gd/Co/Gd system. A model based on partial intermixing at the Co/Gd interfaces of this system has been developed which explains the experiments well and provides a tool to understand its magnetic characteristics. More specifically, this work provides further insight into the decay of the Co proximity-induced magnetization in the Gd, and the role the capping layer plays in the Gd magnetization.

Several rare-earth transition-metal ferrimagnetic systems exhibit all-optical magnetization switching upon excitation with a femtosecond laser pulse. Although this phenomenon is very promising for future opto-magnetic data storage applications, the role of non-local spin transport in these systems is scarcely understood. Using Co/Gd and Co/Tb bilayers we isolate the contribution of the rare-earth materials to the generated spin currents by using the precessional dynamics they excite in an adjacent ferromagnetic layer as a probe. By measuring THz standing spin-waves as well as GHz homogeneous precessional modes, we probe both the high- and low-frequency components of these spin currents. The low-frequency homogeneous mode indicates a significant contribution of Gd to the spin current, but not from Tb, consistent with the difficulty in achieving all-optical switching in Tb-containing systems. Measurements on the THz frequency spin waves reveal the inability of the rare-earth generated spin currents to excite dynamics at the sub-ps timescale. We present modelling efforts using the $s$-$d$ model, which effectively reproduce our results and allow us to explain the behavior in terms of the temporal profile of the spin current., Supplementary information appended

In this paper, we propose a compact integrated hybrid plasmonic-photonic device for optical reading of nanoscale magnetic bits with perpendicular magnetic anisotropy in a magnetic racetrack on top of a photonic waveguide on the indium phosphide membrane on silicon platform. The hybrid device is constructed by coupling a doublet of V-shaped gold plasmonic nanoantennas on top of the indium phosphide waveguide. By taking advantage of the localized surface plasmons, our hybrid device can enable detection of the magnetization state in magnetic bits beyond the diffraction limit of light and enhance the polar magneto-optical Kerr effect (PMOKE). We further illustrate how combining the hybrid device with a plasmonic polarization rotator provides magneto-optical read-out by transforming the PMOKE-induced polarization change into an intensity variation of the waveguide mode. According to the simulation results based on a three-dimensional finite-difference time-domain method, the hybrid device can detect the magnetization states in targeted bits in a magnetic racetrack medium down to ~100×100 nm2, regardless of the magnetization state of the rest of the racetrack with a relative intensity contrast of greater than 0.5% for a ~200×100 nm2magnetic bit. We believe our hybrid device can be an enabling technology that can connect integrated photonics with nanoscale spintronics, paving the way toward ultrafast and energy efficient advanced on-chip applications.

This study presents the design, fabrication and experimental demonstration of a magneto-photonic device that delivers non-volatile photonic memory functionality. The aim is to overcome the energy and speed bottleneck of back-and-forth signal conversion between the electronic and optical domains when retrieving information from non-volatile memory. The device combines integrated photonic components based on the InP membrane on silicon (IMOS) platform and a non-volatile, built-in memory element (ferromagnetic thin-film multilayers) realized as a top-cladding on the photonic waveguides (a post-processing step). We present a design where the phase of the guided light is engineered via two mechanisms: the polar magneto-optical Kerr effect (MOKE) and the propagation in an asymmetrical cross-section (triangular) waveguide. Thanks to its design, the device yields different mode-specific transmissions depending on the memory state it encodes. We demonstrate the recording of the magnetic hysteresis using the transmitted optical signal, providing direct proof for all optical magnetic memory reading using an integrated photonic chip. Using mathematical model and optical simulations, we support the experimental observations and quantitatively reproduce the Kerr signal amplitudes on-chip. A 1% transmitted power contrast from devices is promising indicating that in a shot noise limited scenario the theoretical bandwidth of memory read-out exceeds 50 Gbits/s.

Perpendicular synthetic antiferromagnets (p-SAFs) are of interest for the next generation of ultrafast, high-density spintronic memory and logic devices. However, to efficiently operate their magnetic order by current-induced spin-orbit torques (SOTs), an unfavored high external magnetic field is conventionally required to break the symmetry. Here, we report the field-free SOT switching of a p-SAF through the introduction of an interlayer with Dzyaloshinskii-Moriya interactions (DMIs). We experimentally observe the existence of the DMI interlayer in our SAF sample by an azimuthal angular-dependent anomalous Hall measurement. Deterministic field-free switching is accomplished in such a sample and depicted by macrospin and micromagnetic simulations. The comparison between the uniaxial interlayer DMI and the azimuthal direction-dependent switching behavior strongly suggests its origin from the DMI interlayer. We demonstrate the compatibility of the proposed strategy with magnetic tunnel junction device structure. Our results provide a strategy for p-SAF-based high-performance SOT devices.

Although all-optical magnetization switching with fs laser pulses has garnered much technological interest, the ultimate data rates achievable have scarcely been investigated. Recently, it has been shown that after a switching event in a GdCo alloy, a second laser pulse arriving 7 ps later can consistently switch the magnetization. However, it is as of yet unknown whether the same holds in layered ferrimagnetic systems, which hold much promise for applications. In this work, we investigate the minimum time delay required between two subsequent switching events in synthetic ferrimagnetic Co/Gd bilayers using two fs laser pulses. We experimentally demonstrate that the minimum time delay needed for consistent switching can be as low as 10 ps. Moreover, we demonstrate the importance of engineering heat diffusion away from the magnetic material as well as control over the laser pulse energy. This behavior is reproduced using modeling, where we find that the second switch can occur even when the magnetization is not fully recovered. We further confirm that heat diffusion is a critical factor in reducing the time delay for the second switch, while also confirming a critical dependence on laser energy.

Spin-orbitronics and single pulse all-optical switching (AOS) of magnetization are two major successes of the rapidly advancing field of nanomagnetism in recent years, with high potential for enabling novel, fast, and energy-efficient memory and logic platforms. Fast current-induced domain wall motion (CIDWM) and single shot AOS have been individually demonstrated in different ferrimagnetic alloys. However, the stringent requirement for their composition control makes these alloys challenging materials for wafer scale production. Here, fast CIDWM and energy efficient AOS in a synthetic ferrimagnetic system based on multilayered [Co/Gd]2are simultaneously demonstrated. First, it is shown that AOS is present in its full composition range. It is found that current-driven domain wall velocities over 2000 m s-1at room temperature, achieved by compensating the total angular momentum through layer thickness tuning. Furthermore, analytical modeling of the CIDWM reveals that Joule heating needs to be treated transiently to properly describe the CIDWM for oursub-ns current pulses. The studies establish [Co/Gd]-based synthetic ferrimagnets to be a unique materials platform for domain wall devices with access to ultrafast single pulseAOS.

We theoretically investigate laser-induced spin transport in metallic magnetic heterostructures using an effective spin-Transport description that treats itinerant electrons and thermal magnons on an equal footing. Electron-magnon scattering is included and taken as the driving force for ultrafast demagnetization. We assume that in the low-fluence limit, the magnon system remains in a quasiequilibrium, allowing a transient nonzero magnon chemical potential. In combination with the diffusive transport equations for the itinerant electrons, the description is used to chart the full spin dynamics within the heterostructure. In agreement with recent experiments, we find that in the case the spin-current-receiving material includes an efficient spin dissipation channel, the interfacial spin current becomes directly proportional to the temporal derivative of the magnetization. Based on an analytical calculation, we discuss that other relations between the spin current and magnetization may arise in the case the spin-current-receiving material displays inefficient spin-flip scattering. Finally, we discuss the role of (interfacial) magnon transport and show that, a priori, it cannot be neglected. However, its significance strongly depends on the system parameters.

Optically-induced spin currents have proven to be useful in spintronics applications, allowing for sub-ps all-optical control of magnetization. However, the mechanism responsible for their generation is still heavily debated. Here we use the excitation of spin-current induced THz spin-waves in noncollinear bilayer structures to study optically-induced spin currents in the time domain. We measure a significant laser-fluence dependence of the spin-wave phase, which can quantitatively be explained assuming the spin current is proportional to the time derivative of the magnetization. Measurements of the absolute spin-wave phase, supported by theoretical calculations and micromagnetic simulations, suggest that angular momentum transfer via the s-d interaction in combination with ballistic interlayer transport is sufficient to fully explain spin-current generation and transport in our experiments. Finally, we show that the damping-like optical STT dominates THz spin-wave generation. Our findings suggest laser-induced demagnetization and spin-current generation share the same microscopic origin.

Recently it has been shown that not only GdFeCo alloys exhibit single-pulse helicity-independent all-optical switching (HI-AOS), but that this effect is also seen in Co/Gd bilayers. However, there have been no reports on the explicit time dynamics of the switching process in these bilayers as of yet. Furthermore, time-resolved measurements of switching of other materials are typically done with a constant applied field to reset the magnetization between consecutive pulses and thus ensure repeatable behavior. In this paper we experimentally resolve the explicit dynamics of the switching process in Co/Gd, and the influence of applied magnetic fields on the switching process. We observe that after a switch within several picoseconds, the magnetization switches back at a timescale of hundreds of picoseconds. This backswitch includes a strong dependence on the magnetic field strength even at sub-tesla fields, significantly smaller than the exchange fields that govern the switching dynamics. This surprising behaviour is explained by a combination of longitudinal switching (on a picosecond timescale), precessional switching (on a nanosecond timescale) and domain-wall motion (on a timescale of 10 ns and beyond). We discuss these different switching regimes and their relative importance using simple model calculations.

Perpendicular synthetic antiferromagnets (SAFs) are of interest for the next generation ultrafast, high density spintronic memory and logic devices. However, to energy efficiently operate their magnetic order by current-induced spin orbit torques (SOTs), an unfavored high external field is conventionally required to break the symmetry. Here, we theoretically and experimentally demonstrate the field-free SOT switching of a perpendicular SAF through the introduction of interlayer Dzyaloshinskii-Moriya interaction (DMI). By macro-spin simulation, we show that the speed of field-free switching increases with the in-plane mirror asymmetry of injected spins. We experimentally observe the existence of interlayer DMI in our SAF sample by an azimuthal angular dependent anomalous Hall measurement. Field-free switching is accomplished in such a sample and the strength of the effective switching field demonstrates its origin from interlayer DMI. Our results provide a new strategy for SAF based high performance SOT devices.

The impact of Gd addition on the structural and magnetic properties of [Formula: see text]-FePt alloy thin films, which were sputter-deposited on MgO(001) substrates at 800[Formula: see text]C, was investigated. A rapid deterioration of [Formula: see text] chemical ordering along with a strong amorphization effect is observed with addition of Gd. At more than 20 at. % Gd, additional crystalline phases occur. Due to the diminishing [Formula: see text] chemical order, the perpendicular magnetic anisotropy (PMA) gets strongly reduced from about 3 down to 1 MJ/m[Formula: see text] with addition of 14 at. % Gd. For higher Gd concentrations, the easy axis of magnetization turns fully in-plane due to dominating magnetic shape anisotropy. Furthermore, the saturation magnetization gets reduced with Gd addition due to the antiferromagnetic coupling between the Fe and Gd magnetic moments. Also, the Curie temperature can be lowered. Interestingly, with addition of Gd, a change in film morphology takes place, changing from an isolated island structure to a more continuous film morphology, which is of particular interest for samples with low Gd concentration as these films exhibit still strong PMA but rather low coercive fields.

We present a new approach to tuning fork-based atomic force microscopy for utilizing advanced "tip-on-chip" probes with high sensitivity and broad compatibility. Usually, such chip-like probes with a size reaching 2 mm x 2 mm drastically perturb the oscillation of the tuning fork, resulting in poor performance in its intrinsic force sensing. Therefore, restoring initial oscillatory characteristics is necessary for regaining high sensitivity. To this end, we developed a new approach consisting of three basic steps: tuning fork rebalancing, revamping holder-sensor fixation, and electrode reconfiguration. Mass rebalancing allows the tuning fork to recover the frequency and regain high Q-factor values up to 10E4 in air and up to 4 x 10E4 in ultra-high vacuum conditions. The floating-like holder-fixation using soft wires significantly reduces energy dissipation from the mounting elements. Combined with the soft wires, reconfigured electrodes provide electrical access to the chip-like probe without intervening in the force-sensing signal. Finally, our easy-to-implement approach allows converting the atomic force microscopy-tip from a passive tool to a dedicated microdevice with extended functionality., 11 pages including references, 8 figures

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.

We use transient absorption spectroscopy with circularly polarized x-rays to detect laser-excited hole states below the Fermi level and compare their dynamics with that of unoccupied states above the Fermi level in ferromagnetic [Co/Pd] multilayers. While below the Fermi level an instantaneous and significantly stronger demagnetization is observed, above the Fermi level the demagnetization is delayed by 35+/-10 fs. This provides a direct visualization of how ultrafast demagnetization proceeds via initial spin-flip scattering of laser-excited holes to the subsequent formation of spin waves., 10 pages, 4 figures. The following article has been submitted to Applied Physics Letters. After it is published, it will be found at https://publishing.aip.org/resources/librarians/products/journals/

Skyrmions are topologically protected chiral spin textures that have shown promise as data carriers in future spintronic applications. They can be stabilized by the interfacial Dzyaloshinskii-Moriya interaction (iDMI) in material systems with inversion asymmetry and spin-orbit coupling, such as Ir$|$Co$|$Pt multilayers. The ability to locally tune such interface interactions, and hence the skyrmion energy, could greatly enhance the nucleation and control of skyrmions in racetrack type devices. In this work, we investigate local tuning of the iDMI and perpendicular magnetic anisotropy (PMA) using focussed Ga$^{+}$ ion beam irradiation, in an Ir$|$Co$|$Pt multilayer system. We show that the magnitude of the interface contribution to both effects can be significantly reduced by the irradiation with Ga$^{+}$ ions. This leads to a reduction by a factor two of the domain wall energy density, while still preserving the N\'{e}el character of the domain walls. Hence, we postulate that Ga$^{+}$ ion irradiation is an effective way to locally reduce the energy barrier for skyrmion nucleation, providing a novel pathway for targeted skyrmion nucleation in racetrack type devices.

The magnetic interfacial Dzyaloshinskii-Moriya interaction (DMI) in multi-layered thin films can lead to exotic chiral spin states, of paramount importance for future spintronic technologies. Interfacial DMI is normally manifested as an intralayer interaction, mediated via a paramagnetic heavy metal in systems lacking inversion symmetry. Here we show how, by designing synthetic antiferromagnets with canted magnetization states, it is also possible to observe interfacial interlayer-DMI at room temperature. The interlayer-DMI breaks the symmetry of the magnetic reversal process via the emergence of noncollinear spin states, which results in chiral exchange-biased hysteresis loops. This work opens up yet unexplored avenues for the development of new chiral spin textures in multi-layered thin film systems., 15 pages, 4 figures

The exchange interaction governs static and dynamic magnetism. This fundamental interaction comes in two flavours-symmetric and antisymmetric. The symmetric interaction leads to ferro- and antiferromagnetism, and the antisymmetric interaction has attracted significant interest owing to its major role in promoting topologically non-trivial spin textures that promise fast, energy-efficient devices. So far, the antisymmetric exchange interaction has been found to be rather short ranged and limited to a single magnetic layer. Here we report a long-range antisymmetric interlayer exchange interaction in perpendicularly magnetized synthetic antiferromagnets with parallel and antiparallel magnetization alignments. Asymmetric hysteresis loops under an in-plane field reveal a unidirectional and chiral nature of this interaction, which results in canted magnetic structures. We explain our results by considering spin-orbit coupling combined with reduced symmetry in multilayers. Our discovery of a long-range chiral interaction provides an additional handle to engineer magnetic structures and could enable three-dimensional topological structures.

The water splitting activity of hematite is sensitive to the film processing parameters due to limiting factors such as a short hole diffusion length, slow oxygen evolution kinetics, and poor light absorptivity. In this work, we use direct current (DC) magnetron sputtering as a fast and cost-effective route to deposit metallic iron thin films, which are annealed in air to obtain well-adhering hematite thin films on F:SnO2-coated glass substrates. These films are compared to annealed hematite films, which are deposited by reactive radio frequency (RF) magnetron sputtering, which is usually used for depositing metal oxide thin films, but displays an order of magnitude lower deposition rate. We find that DC sputtered films have much higher photoelectrochemical activity than reactive RF sputtered films. We show that this is related to differences in the morphology and surface composition of the films as a result of the different processing parameters. This in turn results in faster oxygen evolution kinetics and lower surface and bulk recombination effects. Thus, fabricating hematite thin films by fast and cost-efficient metallic iron deposition using DC magnetron sputtering is shown to be a valid and industrially relevant route for hematite photoanode fabrication.

An ultra-compact and ultra-broadband polarization-selective bidirectional vertical coupler is proposed. The coupler is designed by coupling unique two-plane chiral π -shaped plasmonic nanoantennas on top of a silicon-on-insulator waveguide. By taking advantage of the spin-directional locking principle and coupled nanoantennas, a circularly polarized incident light can be coupled into a specific direction through the waveguide, depending on its spin. Based on the full-vectorial finite element method, for a beam radius of 0.53 µm, the coupler with a size of 1.07 × 0.5 µ m 2 provides maximum coupling efficiency and an extinction coefficient of − 6.9 d B and 16.9 dB across the telecommunication C to U bands, respectively, with a 2 dB bandwidth of > 160 n m . The proposed coupler has potential applications in telecommunications, vertical cross-chip communications, and quantum information processing.

One of the most important challenges in antiferromagnetic spintronics is the readout of the Néel vector state. High current densities up to 108 Acm−2 used in the electrical switching experiments cause notorious difficulty in distinguishing between magnetic and thermal origins of the electrical signals. To overcome this problem, we present a temperature dependence study of the transverse resistance changes in the switching experiment with CoO | Pt devices. We demonstrate the possibility to extract a pattern of spin Hall magnetoresistance for current pulses with a density of [Formula: see text] that is present only below the Néel temperature and does not follow a trend expected for thermal effects. This is the compelling evidence for the magnetic origin of the signal, which is observed using purely electrical techniques. We confirm these findings by complementary experiments in an external magnetic field. Such an approach can allow determining the optimal conditions for switching antiferromagnets and be very valuable when no imaging techniques can be applied to verify the origin of the electrical signal.

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 currents4, 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 optomagnetic tunnel junction using a spintronic-photonic 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 optomagnetic 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 spintronic memories with THz data access, as well as ultralow power consumption., Comment: 18 pages, 3 figures

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.