Fast current-induced skyrmion motion in synthetic antiferromagnets.

Magnetic skyrmions are topological magnetic textures that hold great promise as nanoscale bits of information in memory and logic devices. Although room-temperature ferromagnetic skyrmions and their current-induced manipulation have been demonstrated, their velocity has been limited to about 100 meters per second. In addition, their dynamics are perturbed by the skyrmion Hall effect, a motion transverse to the current direction caused by the skyrmion topological charge. Here, we show that skyrmions in compensated synthetic antiferromagnets can be moved by current along the current direction at velocities of up to 900 meters per second. This can be explained by the cancellation of the net topological charge leading to a vanishing skyrmion Hall effect. Our results open an important path toward the realization of logic and memory devices based on the fast manipulation of skyrmions in tracks.

Electrical Detection and Nucleation of a Magnetic Skyrmion in a Magnetic Tunnel Junction Observed via Operando Magnetic Microscopy.

Magnetic skyrmions are topological spin textures which are envisioned as nanometer scale information carriers in magnetic memory and logic devices. The recent demonstrations of room temperature skyrmions and their current induced manipulation in ultrathin films were first steps toward the realization of such devices. However, important challenges remain regarding the electrical detection and the low-power nucleation of skyrmions, which are required for the read and write operations. Here, we demonstrate, using operando magnetic microscopy experiments, the electrical detection of a single magnetic skyrmion in a magnetic tunnel junction (MTJ) and its nucleation and annihilation by gate voltage via voltage control of magnetic anisotropy. The nucleated skyrmion can be manipulated by both gate voltages and external magnetic fields, leading to tunable intermediate resistance states. Our results unambiguously demonstrate the readout and voltage controlled write operations in a single MTJ device, which is a major milestone for low power skyrmion based technologies.

Spiking Dynamics in Dual Free Layer Perpendicular Magnetic Tunnel Junctions.

Spintronic devices have recently attracted a lot of attention in the field of unconventional computing due to their non-volatility for short- and long-term memory, nonlinear fast response, and relatively small footprint. Here we demonstrate experimentally how voltage driven magnetization dynamics of dual free layer perpendicular magnetic tunnel junctions can emulate spiking neurons in hardware. The output spiking rate was controlled by varying the dc bias voltage across the device. The field-free operation of this two-terminal device and its robustness against an externally applied magnetic field make it a suitable candidate to mimic the neuron response in a dense neural network. The small energy consumption of the device (4-16 pJ/spike) and its scalability are important benefits for embedded applications. This compact perpendicular magnetic tunnel junction structure could finally bring spiking neural networks to sub-100 nm size elements.

Gate-controlled skyrmion and domain wall chirality.

Magnetic skyrmions are localized chiral spin textures, which offer great promise to store and process information at the nanoscale. In the presence of asymmetric exchange interactions, their chirality, which governs their dynamics, is generally considered as an intrinsic parameter set during the sample deposition. In this work, we experimentally demonstrate that a gate voltage can control this key parameter. We probe the chirality of skyrmions and chiral domain walls by observing the direction of their current-induced motion and show that a gate voltage can reverse it. This local and dynamical reversal of the chirality is due to a sign inversion of the interfacial Dzyaloshinskii-Moriya interaction that we attribute to ionic migration of oxygen under gate voltage. Micromagnetic simulations show that the chirality reversal is a continuous transformation, in which the skyrmion is conserved. This control of chirality with 2-3 V gate voltage can be used for skyrmion-based logic devices, yielding new functionalities.

Skyrmions in synthetic antiferromagnets and their nucleation via electrical current and ultra-fast laser illumination.

Magnetic skyrmions are topological spin textures that hold great promise as nanoscale information carriers in non-volatile memory and logic devices. While room-temperature magnetic skyrmions and their current-induced motion were recently demonstrated, the stray field resulting from their finite magnetisation and their topological charge limit their minimum size and reliable motion. Antiferromagnetic skyrmions allow to lift these limitations owing to their vanishing magnetisation and net zero topological charge, promising ultra-small and ultra-fast skyrmions. Here, we report on the observation of isolated skyrmions in compensated synthetic antiferromagnets at zero field and room temperature using X-ray magnetic microscopy. Micromagnetic simulations and an analytical model confirm the chiral antiferromagnetic nature of these skyrmions and allow the identification of the physical mechanisms controlling their size and stability. Finally, we demonstrate the nucleation of synthetic antiferromagnetic skyrmions via local current injection and ultra-fast laser excitation.

Double magnetic tunnel junctions with a switchable assistance layer for improved spin transfer torque magnetic memory performance.

This paper reports the first experimental demonstration of a new concept of double magnetic tunnel junctions comprising a magnetically switchable assistance layer. These double junctions are used as memory cells in spin transfer torque magnetic random access memory (STT-MRAM) devices. Their working principle, fabrication and electrical characterization are described and their performances are compared to those of reference devices without an assistance layer. We show that thanks to the assistance layer, the figure of merit of STT-MRAM cells can be increased by a factor of 4 as compared to that of STT-MRAM based on conventional stacks without the assistance layer. A detailed discussion of the results is given supported by numerical simulations. The simulations also provide guidelines on how to optimize the properties of the assistance layer to get the full benefit from this concept.

Route towards efficient magnetization reversal driven by voltage control of magnetic anisotropy.

The voltage controlled magnetic anisotropy (VCMA) becomes a subject of major interest for spintronics due to its promising potential outcome: fast magnetization manipulation in magnetoresistive random access memories with enhanced storage density and very low power consumption. Using a macrospin approach, we carried out a thorough analysis of the role of the VCMA on the magnetization dynamics of nanostructures with out-of-plane magnetic anisotropy. Diagrams of the magnetization switching have been computed depending on the material and experiment parameters (surface anisotropy, Gilbert damping, duration/amplitude of electric and magnetic field pulses) thus allowing predictive sets of parameters for optimum switching experiments. Two characteristic times of the trajectory of the magnetization were analyzed analytically and numerically setting a lower limit for the duration of the pulses. An interesting switching regime has been identified where the precessional reversal of magnetization does not depend on the voltage pulse duration. This represents a promising path for the magnetization control by VCMA with enhanced versatility.

Helium Ions Put Magnetic Skyrmions on the Track.

Magnetic skyrmions are deemed to be the forerunners of novel spintronic memory and logic devices. While their observation and their current-driven motion at room temperature have been demonstrated, certain issues regarding their nucleation, stability, pinning, and skyrmion Hall effect still need to be overcome to realize functional devices. Here, we demonstrate that focused He+-ion-irradiation can be used to create and guide skyrmions in racetracks. We show that the reduction of the perpendicular magnetic anisotropy and Dzyaloshinskii-Moriya interaction in the track defined by ion-irradiation leads to the formation of stable isolated skyrmions. Current-driven skyrmion motion experiments and simulations reveal that the skyrmions move along the irradiated track, resulting in the suppression of the skyrmion Hall effect, and that the maximum skyrmion velocity can be enhanced by tuning the magnetic properties. These results open up a new path to nucleate and guide magnetic skyrmions in racetrack devices.

Corrigendum: Room-temperature chiral magnetic skyrmions in ultrathin magnetic nanostructures.

This corrects the article DOI: 10.1038/nnano.2015.315.

Fabrication of nanotweezers and their remote actuation by magnetic fields.

A new kind of nanodevice that acts like tweezers through remote actuation by an external magnetic field is designed. Such device is meant to mechanically grab micrometric objects. The nanotweezers are built by using a top-down approach and are made of two parallelepipedic microelements, at least one of them being magnetic, bound by a flexible nanohinge. The presence of an external magnetic field induces a torque on the magnetic elements that competes with the elastic torque provided by the nanohinge. A model is established in order to evaluate the values of the balanced torques as a function of the tweezers opening angles. The results of the calculations are confronted to the expected values and validate the overall working principle of the magnetic nanotweezers.

Room-temperature chiral magnetic skyrmions in ultrathin magnetic nanostructures.

Magnetic skyrmions are chiral spin structures with a whirling configuration. Their topological properties, nanometre size and the fact that they can be moved by small current densities have opened a new paradigm for the manipulation of magnetization at the nanoscale. Chiral skyrmion structures have so far been experimentally demonstrated only in bulk materials and in epitaxial ultrathin films, and under an external magnetic field or at low temperature. Here, we report on the observation of stable skyrmions in sputtered ultrathin Pt/Co/MgO nanostructures at room temperature and zero external magnetic field. We use high lateral resolution X-ray magnetic circular dichroism microscopy to image their chiral Néel internal structure, which we explain as due to the large strength of the Dzyaloshinskii-Moriya interaction as revealed by spin wave spectroscopy measurements. Our results are substantiated by micromagnetic simulations and numerical models, which allow the identification of the physical mechanisms governing the size and stability of the skyrmions.

Chiral damping of magnetic domain walls.

Structural symmetry breaking in magnetic materials is responsible for the existence of multiferroics, current-induced spin-orbit torques and some topological magnetic structures. In this Letter we report that the structural inversion asymmetry (SIA) gives rise to a chiral damping mechanism, which is evidenced by measuring the field-driven domain-wall (DW) motion in perpendicularly magnetized asymmetric Pt/Co/Pt trilayers. The DW dynamics associated with the chiral damping and those with Dzyaloshinskii-Moriya interaction (DMI) exhibit identical spatial symmetry. However, both scenarios are differentiated by their time reversal properties: whereas DMI is a conservative effect that can be modelled by an effective field, the chiral damping is purely dissipative and has no influence on the equilibrium magnetic texture. When the DW motion is modulated by an in-plane magnetic field, it reveals the structure of the internal fields experienced by the DWs, allowing one to distinguish the physical mechanism. The chiral damping enriches the spectrum of physical phenomena engendered by the SIA, and is essential for conceiving DW and skyrmion devices owing to its coexistence with DMI (ref. ).

Spin-orbit torque magnetization switching controlled by geometry.

Magnetization reversal by an electric current is essential for future magnetic data storage technology, such as magnetic random access memories. Typically, an electric current is injected into a pillar-shaped magnetic element, and switching relies on the transfer of spin momentum from a ferromagnetic reference layer (an approach known as spin-transfer torque). Recently, an alternative technique has emerged that uses spin-orbit torque (SOT) and allows the magnetization to be reversed without a polarizing layer by transferring angular momentum directly from the crystal lattice. With spin-orbit torque, the current is no longer applied perpendicularly, but is in the plane of the magnetic thin film. Therefore, the current flow is no longer restricted to a single direction and can have any orientation within the film plane. Here, we use Kerr microscopy to examine spin-orbit torque-driven domain wall motion in Co/AlOx wires with different shapes and orientations on top of a current-carrying Pt layer. The displacement of the domain walls is found to be highly dependent on the angle between the direction of the current and domain wall motion, and asymmetric and nonlinear with respect to the current polarity. Using these insights, devices are fabricated in which magnetization switching is determined entirely by the geometry of the device.

Fast current-induced domain-wall motion controlled by the Rashba effect.

The propagation of magnetic domain walls induced by spin-polarized currents has launched new concepts for memory and logic devices. A wave of studies focusing on permalloy (NiFe) nanowires has found evidence for high domain-wall velocities (100 m s(-1); refs,), but has also exposed the drawbacks of this phenomenon for applications. Often the domain-wall displacements are not reproducible, their depinning from a thermally stable position is difficult and the domain-wall structural instability (Walker breakdown) limits the maximum velocity. Here, we show that the combined action of spin-transfer and spin-orbit torques offers a comprehensive solution to these problems. In an ultrathin Co nanowire, integrated in a trilayer with structural inversion asymmetry (SIA), the high spin-torque efficiency facilitates the depinning and leads to high mobility, while the SIA-mediated Rashba field controlling the domain-wall chirality stabilizes the Bloch domain-wall structure. Thus, the high-mobility regime is extended to higher current densities, allowing domain-wall velocities up to 400 m s(-1).