Femtosecond Laser Pulse Driven Caustic Spin Wave Beams.

Controlling the directionality of spin waves is a key ingredient in wave-based computing methods such as magnonics. In this Letter, we demonstrate this particular aspect by using an all-optical pointlike source of continuous spin waves based on frequency comb rapid demagnetization. The emitted spin waves contain a range of k vectors and by detuning the applied magnetic field slightly off the ferromagnetic resonance (FMR), we observe X-shaped caustic spin wave patterns at 70° propagation angles as predicted by theory. When the harmonic of the light source approaches the FMR, the caustic pattern gives way to uniaxial spin wave propagation perpendicular to the in-plane component of the applied field. This field-controlled propagation pattern and directionality of optically emitted short-wavelength spin waves provide additional degrees of freedom when designing magnonic devices.

Interfacial Dzyaloshinskii-Moriya Interaction in Pt/CoFeB Films: Effect of the Heavy-Metal Thickness.

We report the observation of a Pt layer thickness dependence on the induced interfacial Dzyaloshinskii-Moriya interaction in ultrathin Pt(d_{Pt})/CoFeB films. Taking advantage of the large spin-orbit coupling of the heavy metal, the interfacial Dzyaloshinskii-Moriya interaction is quantified by Brillouin light scattering measurements of the frequency nonreciprocity of spin waves in the ferromagnet. The magnitude of the induced Dzyaloshinskii-Moriya coupling is found to saturate to a value of 0.45 mJ/m^{2} for Pt thicknesses larger than ∼2 nm. The experimental results are explained by analytical calculations based on the three-site indirect exchange mechanism that predicts a Dzyaloshinskii-Moriya interaction at the interface between a ferromagnetic thin layer and a heavy metal. Our findings open up a way to control and optimize chiral effects in ferromagnetic thin films through the thickness of the heavy-metal layer.

Ferromagnetic resonance measurements of (Co/Ni/Co/Pt) multilayers with perpendicular magnetic anisotropy.

Multilayers of [Co/Ni(t)/Co/Pt]×8 with varying Ni thickness were investigated for possible use as a free layer in magnetic tunnel junctions and spintronics devices. The thickness t of the Ni sub-layer was varied from 0.3 nm to 0.9 nm and the resulting magnetic properties were compared with (Co/Ni) and (Co/Pt) multilayers. As determined from magnetic force microscopy, magnetometry and ferromagnetic resonance measurements, all multilayers exhibited perpendicular magnetic anisotropy. Compared with (Co/Pt) multilayers, the sample with t of 0.9 nm showed almost the same anisotropy field of µ 0 H k = 1.15 T but the damping constant was 40% lower. These characteristics make these multilayers attractive for spin torque based magnetoresistive devices with perpendicular anisotropy.

Phase and frequency-resolved microscopy of operating spin Hall nano-oscillator arrays.

Coherent optical detection is a powerful technique for characterizing a wide range of physical excitations. Here, we use two optical approaches (fundamental and parametric pumping) to microscopically characterize the high-frequency auto-oscillations of single and multiple nano-constriction spin Hall nano-oscillators (SHNOs). To validate the technique and demonstrate its robustness, we study SHNOs made from two different material stacks, NiFe/Pt and W/CoFeB/MgO, and investigate the influence of both the RF injection power and the laser power on the measurements, comparing the optical results to conventional electrical measurements. To demonstrate the key features of direct, non-invasive, submicron, spatial, and phase-resolved characterization of the SHNO magnetodynamics, we map out the auto-oscillation magnitude and phase of two phase-binarized SHNOs used in Ising machines. This proof-of-concept platform establishes a strong foundation for further extensions, contributing to the ongoing development of crucial characterization techniques for emerging computing technologies based on spintronics devices.

Magnetic droplet soliton pairs.

We demonstrate magnetic droplet soliton pairs in all-perpendicular spin-torque nano-oscillators (STNOs), where one droplet resides in the STNO free layer (FL) and the other in the reference layer (RL). Typically, theoretical, numerical, and experimental droplet studies have focused on the FL, with any additional dynamics in the RL entirely ignored. Here we show that there is not only significant magnetodynamics in the RL, but the RL itself can host a droplet driven by, and coexisting with, the FL droplet. Both single droplets and pairs are observed experimentally as stepwise changes and sharp peaks in the dc and differential resistance, respectively. While the single FL droplet is highly stable, the coexistence state exhibits high-power broadband microwave noise. Furthermore, micromagnetic simulations reveal that the pair dynamics display periodic, quasi-periodic, and chaotic signatures controlled by applied field and current. The strongly interacting and closely spaced droplet pair offers a unique platform for fundamental studies of highly non-linear soliton pair dynamics.

Disorder strength and field-driven ground state domain formation in artificial spin ice: experiment, simulation, and theory.

Quenched disorder affects how nonequilibrium systems respond to driving. In the context of artificial spin ice, an athermal system comprised of geometrically frustrated classical Ising spins with a twofold degenerate ground state, we give experimental and numerical evidence of how such disorder washes out edge effects and provide an estimate of disorder strength in the experimental system. We prove analytically that a sequence of applied fields with fixed amplitude is unable to drive the system to its ground state from a saturated state. These results should be relevant for other systems where disorder does not change the nature of the ground state.

Spin-orbit torque-driven propagating spin waves.

Spin-orbit torque (SOT) can drive sustained spin wave (SW) auto-oscillations in a class of emerging microwave devices known as spin Hall nano-oscillators (SHNOs), which have highly nonlinear properties governing robust mutual synchronization at frequencies directly amenable to high-speed neuromorphic computing. However, all demonstrations have relied on localized SW modes interacting through dipolar coupling and/or direct exchange. As nanomagnonics requires propagating SWs for data transfer and additional computational functionality can be achieved using SW interference, SOT-driven propagating SWs would be highly advantageous. Here, we demonstrate how perpendicular magnetic anisotropy can raise the frequency of SOT-driven auto-oscillations in magnetic nanoconstrictions well above the SW gap, resulting in the efficient generation of field and current tunable propagating SWs. Our demonstration greatly extends the functionality and design freedom of SHNOs, enabling long-range SOT-driven SW propagation for nanomagnonics, SW logic, and neuromorphic computing, directly compatible with CMOS technology.

Spin transfer torque driven higher-order propagating spin waves in nano-contact magnetic tunnel junctions.

Short wavelength exchange-dominated propagating spin waves will enable magnonic devices to operate at higher frequencies and higher data transmission rates. While giant magnetoresistance (GMR)-based magnetic nanocontacts are efficient injectors of propagating spin waves, the generated wavelengths are 2.6 times the nano-contact diameter, and the electrical signal strength remains too weak for applications. Here we demonstrate nano-contact-based spin wave generation in magnetic tunnel junctions and observe large-frequency steps consistent with the hitherto ignored possibility of second- and third-order propagating spin waves with wavelengths of 120 and 74 nm, i.e., much smaller than the 150-nm nanocontact. Mutual synchronization is also observed on all three propagating modes. These higher-order propagating spin waves will enable magnonic devices to operate at much higher frequencies and greatly increase their transmission rates and spin wave propagating lengths, both proportional to the much higher group velocity.

Investigation of magnetic droplet solitons using x-ray holography with extended references.

A dissipative magnetic soliton, or magnetic droplet, is a structure that has been predicted to exist within a thin magnetic layer when non-linearity is balanced by dispersion, and a driving force counteracts the inherent damping of the spin precession. Such a soliton can be formed beneath a nano-contact (NC) that delivers a large spin-polarized current density into a magnetic layer with perpendicular magnetic anisotropy. Although the existence of droplets has been confirmed from electrical measurements and by micromagnetic simulations, only a few attempts have been made to directly observe the magnetic landscape that sustains these structures, and then only for a restricted set of experimental parameter values. In this work we use and x-ray holography technique HERALDO, to image the magnetic structure of the [Co/Ni]x4 multilayer within a NC orthogonal pseudo spin-valve, for different range of magnetic fields and injected electric currents. The magnetic configuration imaged at -33 mA and 0.3 T for devices with 90 nm NC diameter reveals a structure that is within the range of current where the droplet soliton exist based on our electrical measurements and have it is consistent with the expected size of the droplet (∼100 nm diameter) and its spatial position within the sample. We also report the magnetisation configurations observed at lower DC currents in the presence of fields (0-50 mT), where it is expected to observe regimes of the unstable droplet formation.

Spin-wave-beam driven synchronization of nanocontact spin-torque oscillators.

The synchronization of multiple nanocontact spin-torque oscillators (NC-STOs) is mediated by propagating spin waves (SWs). Although it has been shown that the Oersted field generated in the vicinity of the NC can dramatically alter the emission pattern of SWs, its role in the synchronization behaviour of multiple NCs has not been considered so far. Here we investigate the synchronization behaviour in multiple NC-STOs oriented either vertically or horizontally, with respect to the in-plane component of the external field. Synchronization is promoted (impeded) by the Oersted field landscape when the NCs are oriented vertically (horizontally) due to the highly anisotropic SW propagation. Not only is robust synchronization between two oscillators observed for separations larger than 1,000 nm, but synchronization of up to five oscillators, a new record, has been observed in the vertical array geometry. Furthermore, the synchronization can no longer be considered mutual in nature.

Dynamically stabilized magnetic skyrmions.

Magnetic skyrmions are topologically non-trivial spin textures that manifest themselves as quasiparticles in ferromagnetic thin films or noncentrosymmetric bulk materials. So far attention has focused on skyrmions stabilized either by the Dzyaloshinskii-Moriya interaction (DMI) or by dipolar interaction, where in the latter case the excitations are known as bubble skyrmions. Here we demonstrate the existence of a dynamically stabilized skyrmion, which exists even when dipolar interactions and DMI are absent. We establish how such dynamic skyrmions can be nucleated, sustained and manipulated in an effectively lossless medium under a nanocontact. As quasiparticles, they can be transported between two nanocontacts in a nanowire, even in complete absence of DMI. Conversely, in the presence of DMI, we observe that the dynamical skyrmion experiences strong breathing. All of this points towards a wide range of skyrmion manipulation, which can be studied in a much wider class of materials than considered so far.

Oxidation states and quality of upper interfaces in magnetic tunnel junctions: oxygen effect on crystallization of interfaces.

In this study, we have deposited an 57Fe sensor layer at the upper interface, i.e. the interface between the oxide barrier and the upper electrode in selected magnetic tunnel junctions (MTJs), in order to perform nuclear resonant scattering with the aim of obtaining direct information on the magnetic properties and quality of this interface. This is a unique approach as it makes use of this powerful technique to give information at the atomic level, and specifically from the interface where the sensor layer is deposited. By varying sample tunnel barrier thicknesses and oxidation times in the preparation of this barrier, we have observed that longer oxidation time results in not only an increase of the magnetic hyperfine fields, but also causes an interesting crystallization and smoothing of the interface. We also observed that boron atoms diffuse away from the lower part of the upper FeCoB electrode toward the capping layer. An important observation, which has a crucial effect in tunnel magnetoresistance values, is the absence of any magnetically dead FeO layer at the interface. Another finding is that the deposition of Fe on MgO is much smoother than the deposition of MgO on Fe.

Oxidation states and the quality of lower interfaces in magnetic tunnel junctions: oxygen effect on crystallization of interfaces.

Lower interfaces in magnetic tunnel junctions (MTJs), which are the basic components in many spintronic devices such as magnetoresistive random access memories, have crucial effects on the performance of these devices. To obtain more insight into such interfaces, we have introduced an ultrathin sensor layer of (57)Fe at the interface between the lower electrode and the oxide barrier in selected MTJs. This allowed us to perform nuclear resonant scattering measurements, which provide direct information on the magnetic properties and quality of the interfaces. The application of nuclear resonant scattering to study interfaces in MTJs is a unique approach in the sense that it gives information at the atomic level, and specifically from the interface where the sensor layer is deposited. Samples with different tunnel barrier thicknesses and varied oxidation times in the preparation of this barrier have been studied. These show that oxidation can not only increase the magnetic hyperfine fields but also cause an interesting smoothing and crystallizing of the interface. Another interesting finding is the observation of boron diffusion into the lower part of the FeCoB lower electrode towards the Ta seed layer.

Mutually synchronized bottom-up multi-nanocontact spin-torque oscillators.

Spin-torque oscillators offer a unique combination of nanosize, ultrafast modulation rates and ultrawide band signal generation from 100 MHz to close to 100 GHz. However, their low output power and large phase noise still limit their applicability to fundamental studies of spin-transfer torque and magnetodynamic phenomena. A possible solution to both problems is the spin-wave-mediated mutual synchronization of multiple spin-torque oscillators through a shared excited ferromagnetic layer. To date, synchronization of high-frequency spin-torque oscillators has only been achieved for two nanocontacts. As fabrication using expensive top-down lithography processes is not readily available to many groups, attempts to synchronize a large number of nanocontacts have been all but abandoned. Here we present an alternative, simple and cost-effective bottom-up method to realize large ensembles of synchronized nanocontact spin-torque oscillators. We demonstrate mutual synchronization of three high-frequency nanocontact spin-torque oscillators and pairwise synchronization in devices with four and five nanocontacts.

Spin torque-generated magnetic droplet solitons.

Dissipative solitons have been reported in a wide range of nonlinear systems, but the observation of their magnetic analog has been experimentally challenging. Using spin transfer torque underneath a nanocontact on a magnetic thin film with perpendicular magnetic anisotropy (PMA), we have observed the generation of dissipative magnetic droplet solitons and report on their rich dynamical properties. Micromagnetic simulations identify a wide range of automodulation frequencies, including droplet oscillatory motion, droplet "spinning," and droplet "breather" states. The droplet can be controlled by using both current and magnetic fields and is expected to have applications in spintronics, magnonics, and PMA-based domain-wall devices.

Direct observation of a propagating spin wave induced by spin-transfer torque.

Spin torque oscillators with nanoscale electrical contacts are able to produce coherent spin waves in extended magnetic films, and offer an attractive combination of electrical and magnetic field control, broadband operation, fast spin-wave frequency modulation, and the possibility of synchronizing multiple spin-wave injection sites. However, many potential applications rely on propagating (as opposed to localized) spin waves, and direct evidence for propagation has been lacking. Here, we directly observe a propagating spin wave launched from a spin torque oscillator with a nanoscale electrical contact into an extended Permalloy (nickel iron) film through the spin transfer torque effect. The data, obtained by wave-vector-resolved micro-focused Brillouin light scattering, show that spin waves with tunable frequencies can propagate for several micrometres. Micromagnetic simulations provide the theoretical support to quantitatively reproduce the results.

Partition controlled delivery of hydrophobic substances in toxicity tests using poly(dimethylsiloxane) (PDMS) films.

Interpretation of toxicity test results may be hampered when doubt exists about the actual exposure concentration. Processes that are responsible for differences between the nominal and the actual concentration in aqueous test systems may include sorption, precipitation, volatilization, chemical and biological degradation, and uptake into biological or test tissue. In this study, the use of a poly(dimethylsiloxane) (PDMS) film containing the test compound is introduced as a versatile technique for partition controlled delivery of hydrophobic compounds to aqueous toxicity tests. Two methods developed produced preloaded films, having toxicant added to the PDMS prepolymer solution before film deposition and curing, and postloaded films, which are created by the addition of toxicant in a solvent to an already-polymerized PDMS film. Preloaded films were generally more easily prepared, may better accommodate larger molecules, and have a higher capacity than postloaded films. Postloaded films provided film-solution partition coefficients with higher precision and allowed for the use of films from stock and thus for a more portable technique. Chemical analysis showed that equilibrium between films and the aqueous solution was established within 7-10 min and was maintained for a suite of aromatic compounds (log Kow ranging from 2.8 to 6.1). The reliability of the film technique was demonstrated by application to the Microtox bacterial toxicity tests of solutions of polycyclic aromatic hydrocarbons (PAHs).