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Noncollinear antiferromagnets can have additional spin Hall effects due to the net chirality of their magnetic spin structure, which provides for more complex spin-transport phenomena compared to ordinary nonmagnetic materials. Here we investigated how ferromagnetic resonance of permalloy (${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$) is modulated by spin Hall effects in adjacent epitaxial ${\mathrm{IrMn}}_{3}$ films. We observe a large dc modulation of the ferromagnetic resonance linewidth for currents applied along the [001] ${\mathrm{IrMn}}_{3}$ direction. This very strong angular dependence of spin-orbit torques from dc currents through the bilayers can be explained by the magnetic spin Hall effect where ${\mathrm{IrMn}}_{3}$ provides novel pathways for modulating magnetization dynamics electrically.

International audience; The study of magnetic bubble configuration in Co/Ni multilayers circular dots under in-plane magnetic field enlightens two magnetic force microscopy (MFM) measurement artifacts. We demonstrate that any tilt of the magnetic field producing out-of-plane field component due to the inhomogeneity of the MFM set up strongly affects the shape and size of the magnetic bubble independently of the in-plane field component intensity. Furthermore, MFM signal variations for in-plane magnetic field larger than 0.1 Tesla can only be understood considering MFM tip magnetization rotation. These two artifacts can have strong impact on MFM images and need to be carefully checked for reliable results in imaging skyrmionic structures with MFM under-field.

Noncollinear antiferromagnets can have additional spin Hall effects due to the net chirality of their magnetic spin structure, which provides for more complex spin-transport phenomena compared to ordinary nonmagnetic materials. Here we investigated how ferromagnetic resonance of permalloy (Ni_{80}Fe_{20}) is modulated by spin Hall effects in adjacent epitaxial IrMn_{3} films. We observe a large dc modulation of the ferromagnetic resonance linewidth for currents applied along the [001] IrMn_{3} direction. This very strong angular dependence of spin-orbit torques from dc currents through the bilayers can be explained by the magnetic spin Hall effect where IrMn_{3} provides novel pathways for modulating magnetization dynamics electrically.

Tailoring Gilbert damping of metallic ferromagnetic thin films is one of the central interests in spintronics applications. Here we report a giant Gilbert damping anisotropy in epitaxial Co$_{50}$Fe$_{50}$ thin film with a maximum-minimum damping ratio of 400 \%, determined by broadband spin-torque as well as inductive ferromagnetic resonance. We conclude that the origin of this damping anisotropy is the variation of the spin orbit coupling for different magnetization orientations in the cubic lattice, which is further corroborate from the magnitude of the anisotropic magnetoresistance in Co$_{50}$Fe$_{50}$., Comment: 13 pages, 14 figures

International audience; The increasing need to store large amounts of information with an ultradense, reliable, low-power, and low-cost memory device is driving aggressive efforts to improve on current perpendicular magnetic recording technology. However, the difficulties in fabricating small-grain recording media while maintaining thermal stability and a high signal-to-noise ratio motivate the development of alternative methods, such as the patterning of magnetic nanoislands and use of energy assistance for future applications. In addition , from both a sensor perspective and a memory perspective, three-dimensional spintronic devices are highly desirable to overcome the restrictions on the functionality in planar structures. Here we demonstrate a three-dimensional magnetic memory (magnetic page memory) based on thermally assisted and stray-field-induced transfer of domains in a vertical stack of magnetic nanowires with perpendicular anisotropy. Use of spin-torque-induced domain multiplication in such a device with periodic pinning sites provides additional degrees of freedom by allowing lateral information flow to realize truly-three-dimensional integration.

Strain modulation during a two-way shape memory effect (TWSME) in a sputtered nitinol NiTi is used to reliably induce and switch by 90° a uniaxial magnetic anisotropy of a 20 nm thick Ni film during the thermal cycle from 300 K to 400 K. NiTi strain and its distribution are carefully measured by digital image correlation during tensile prestrain and subsequent temperature cycles in order to compare with Ni strain extracted from the magnetometry measurement and from transmission electron microscopy. In a NiTi/Ni bilayer, a variation of 2.7% strain in NiTi during the TWSME generates 1.3% strain in Ni, which results in a transition from −2 × 104 J/m3 in-plane magnetic anisotropy to +1 × 105 J/m3. Such a composite system offers a way to timely ease writability while maintaining high thermal stability at rest in magnetic media.

Recently discovered exotic magnetic configurations, namely magnetic solitons appearing in the presence of bulk or interfacial Dzyaloshinskii-Moriya Interaction (i-DMI), have excited scientists to explore their potential applications in emerging spintronic technologies such as race-track magnetic memory, spin logic, radio frequency nano-oscillators and sensors. Such studies are motivated by their foreseeable advantages over conventional micro-magnetic structures due to their small size, topological stability and easy spin-torque driven manipulation with much lower threshold current densities giving way to improved storage capacity, and faster operation with efficient use of energy. In this work, we show that in the presence of i-DMI in Pt/CoFeB/Ti multilayers by tuning the magnetic anisotropy (both in-plane and perpendicular-to-plane) via interface engineering and postproduction treatments, we can stabilize a variety of magnetic configurations such as N\'eel skyrmions, horseshoes and most importantly for the first time, the recently predicted isolated radial vortices at room temperature and under zero bias field. Especially, the radial vortex state with its absolute convergence to or divergence from a single point can potentially offer exciting new applications such as particle trapping/detrapping in addition to magnetoresistive memories with efficient switching, where the radial vortex state can act as a source of spin-polarized current with radial polarization., Comment: 26 pages, 5 figures

Spin-based memory offers a unique combination of low cost, low power, high speed, and high density solutions to current data storage needs. However, the demand to increase storage density and improve latency is driving an exhaustive study of alternative storage schemes for the future generations of magnetic memory devices. In this chapter, we focus on some of these advanced technologies such as spin torque magnetic random access memory, patterned media, and energy assisted magnetic recording. We will also discuss the possible evolution of magnetic memory devices into three dimensional architectures, such as race-track memory. We conclude with a discussion of the processing challenges for implementation of these devices, which would require a combination of existing lithography technologies and promising new approaches such as nano-imprint lithography and block-copolymer self-assembly.

International audience; The formation conditions of single magnetic bubbles through in-plane field demagnetizationare investigated in an array of Co/Ni circular dots by magnetic force microscopy andcompared to micromagnetic calculations. We demonstrate high success rates in nucleatingstable bubbles. The efficiency of single bubble formation is found to depend not only on thedot size, material thickness and intrinsic material parameters but also on the bubble nucleationpath. Experimental phase diagrams and micromagnetic calculations highlight the influenceof the starting in-plane field amplitude and dipolar interactions in stabilizing the bubble.The identification of a systematic procedure for controlling nucleation of single bubbles,multidomain states or a uniform state is important from a technological point of view, openinga path toward the realization of reprogrammable magnonic crystals for the control of spinwavepropagation.

Solitons are very promising for the design of the next generation of ultralow power devices for storage and computation. The key ingredient to achieving this goal is the fundamental understanding of their stabilization and manipulation. Here, we show how the interfacial Dzyaloshinskii-Moriya Interaction (IDMI) is able to lift the energy degeneracy of a magnetic vortex state by stabilizing a topological soliton with radial chirality, hereafter called radial vortex. It has a noninteger Skyrmion number $S$ ($0.5l|S|l1$) due to both the vortex core polarity and the magnetization tilting induced by the IDMI boundary conditions. Micromagnetic simulations predict that a magnetoresistive memory based on the radial vortex state in both free and polarizer layers can be efficiently switched by a threshold current density smaller than ${10}^{6}\text{ }\text{ }\mathrm{A}/{\mathrm{cm}}^{2}$. The switching processes occur via the nucleation of topologically connected vortices and vortex-antivortex pairs, followed by spin-wave emissions due to vortex-antivortex annihilations.

Resistance drift of the amorphous states of multilevel phase change memory (PCM) cells is currently a great challenge for the commercial implementation of a reliable multiple-bit-per-cell memory technology. This paper reports observation of a stable intermediate state for a multilevel PCM cell that is achieved through nonuniform heating with a square current injection top electrode. Drift coefficient of the intermediate state is an order of magnitude lower than reset and has weaker temperature dependence. Using finite-element simulations and an analytical model for the subthreshold current–voltage characteristics, based on thermally activated hopping of charge carriers across Coulombic donor-like traps, we conclude that the defect density is two orders of magnitude larger in the intermediate state. We attribute the low drift coefficient of the intermediate state to a large number of stable interfacial defects which dominate the electron transport. Current findings give way to a more stable ultrahigh-density PCM device.

The successful operation of spin-based data storage devices depends on thermally stable magnetic bits. At the same time, the data-processing speeds required by today's technology necessitate ultrafast switching in storage devices. Achieving both thermal stability and fast switching requires controlling the effective damping in magnetic nanoparticles. By carrying out a surface chemical analysis, we show that through exposure to ambient oxygen during processing, a nanomagnet can develop an antiferromagnetic sidewall oxide layer that has detrimental effects, which include a reduction in the thermal stability at room temperature and anomalously high magnetic damping at low temperatures. The in situ deposition of a thin Al metal layer, oxidized to completion in air, greatly reduces or eliminates these problems. This implies that the effective damping and the thermal stability of a nanomagnet can be tuned, leading to a variety of potential applications in spintronic devices such as spin-torque oscillators and patterned media.

Transfer of angular momentum from a spin-polarized current to a ferromagnet provides an efficient means to control the dynamics of nanomagnets. A peculiar consequence of this spin-torque, the ability to induce persistent oscillations of a nanomagnet by applying a dc current, has previously been reported only for spatially uniform nanomagnets. Here we demonstrate that a quintessentially nonuniform magnetic structure, a magnetic vortex, isolated within a nanoscale spin valve structure, can be excited into persistent microwave-frequency oscillations by a spin-polarized dc current. Comparison to micromagnetic simulations leads to identification of the oscillations with a precession of the vortex core. The oscillations, which can be obtained in essentially zero magnetic field, exhibit linewidths that can be narrower than 300 kHz, making these highly compact spin-torque vortex oscillator devices potential candidates for microwave signal-processing applications, and a powerful new tool for fundamental studies of vortex dynamics in magnetic nanostructures., 14 pages, 4 figures

Équipe 101 : Nanomagnétisme et électronique de spin; International audience; Dynamical bubble-like solitons have been recently investigated in nanocontact-based spin-torque oscillators with a perpendicular free layer. Those magnetic configurations can be excited also in different geometries as long as they consist of perpendicular materials. Thus, in this paper, a systematic study of the influence of both external field and high current on that kind of dynamics is performed for a spin-valve point-contact geometry where both free and fixed layers present strong perpendicular anisotropy. The usage of the topological density tool highlights the excitation of complex bubble/antibubble configurations. In particular, at high currents, a deformation of the soliton and its simultaneous shift from the contact area are observed and can be ascribable to the Oersted field. Results provide further detailed information on the excitation of solitons in perpendicular materials for application in spintronics, magnonics, and domain wall logic.

Equipe 101 : Nanomagnétisme et électronique de spin; International audience; This paper demonstrates how to excite complex soliton modes in nanomagnets with perpendicular to plane magnetic anisotropy driven by the non-uniform injection of a spin-polarized current. We addressed the study toward two different scenarios, in the first the excitation of two rotating bubble/antibubble pairs is predicted, in the second one, by means of the topological density, we characterized the dissipative droplet recently measured as single constrained bubble/antibubble pair. Our results are important for the theoretical understanding of how to control the spatial structure of soliton modes for application in spintronics, magnonics, and domain wall devices.

Nanofabrication and thin film deposition techniques have been employed to form Ni nanocontacts that have a three-dimensional point contact geometry with a minimum contact diameter in the range of 1–10 nm. The electron transport through these nanocontacts has been demonstrated to be ballistic or at least quasiballistic. Measurements of the magnetoresistance arising from the creation and removal of a domain wall have yielded a small effect

In this work we investigate the highly unexplored PS dynamical regime as well as the effects of additional rf currents on the PS and switching modes. The PS modes are found to interact with the applied rf currents having frequencies in the range of the dc-only driven modes, and under certain conditions are found to frequency lock. In contrast to previous frequency locking results [7, 8, 9], by looking at these effects in the PS regime, we find a measurable dependence of the dc switching threshold on the external rf frequency, finj. In the locking range the rf appears to have a stabilizing effect which hinders switching, however, at frequencies just below this range, we find a measurable reduction in the direct critical current for switching, Ic. While we have already reported this effect [10], an explanation for the origin of this effect has not been given. In this work, by studying the behavior of this system in frequency domain and through macrospin simulations (MS) (using the Landau-Lifshitz-Gilbert equation with Slonczewski STT [2]) we provide a description of the magnetization dynamics originating from the interaction between the dc driven switching modes and the applied rf. The results show good agreement with our experimental data. EXPERIMENT

We study, both experimentally and by numerical modeling, the magnetic dynamics that can be excited in a magnetic thin-film nanopillar device using the spin torque from a spatially localized current injected via a 10s-of-nm-diameter aperture. The current-driven magnetic dynamics can produce large amplitude microwave emission at zero magnetic field, with a frequency well below that of the uniform ferromagnetic resonance mode. Micromagnetic simulations indicate that the physical origin of this efficient microwave nano-oscillator is the nucleation and subsequent steady-state rotational dynamics of a magnetic vortex dipole driven by the localized spin torque. These results show this novel implementation of a spintronic nano-oscillator is a promising candidate for microwave technology applications., Comment: 19 pages, 4 figures

We measure the microwave signals produced by spin-torque-driven magnetization dynamics in patterned magnetic multilayer devices at room temperature, as a function of the angle of a magnetic field applied in the sample plane. We find strong variations in the frequency linewidth of the signals, with a decrease by more than a factor of 20 as the field is rotated from the magnetic easy axis to the in-plane hard axis. Based on micromagnetic simulations, we identify these variations as due to a transition from spatially incoherent to coherent precession., 15 pages, 5 figures

We have measured the critical phase change conditions induced by electrical pulses in Ge2Sb2Te5 nanopillar phase change memory devices by constructing a comprehensive resistance map as a function of pulse parameters (width, amplitude and trailing edge). Our measurements reveal that the heating scheme and the details of the contact geometry play the dominant role in determining the final phase composition of the device such that a non-uniform heating scheme promotes partial amorphization/crystallization for a wide range of pulse parameters enabling multiple resistance levels for data storage applications. Furthermore we find that fluctuations in the snap-back voltage and set/reset resistances in repeated switching experiments are related to the details of the current distribution such that a uniform current injection geometry (i.e. circular contact) favors more reproducible switching parameters. This shows that possible geometrical defects in nanoscale phase change memory devices may play an essential role in the performance of the smallest possible devices through modification of the exact current distribution in the active chalcogenide layer. We present a three-dimensional finite element model of the electro-thermal physics to provide insights into the underlying physical mechanisms of the switching dynamics as well as to quantitatively account for the scaling behaviour of the switching currents in both circular and rectangular contact geometries. The calculated temporal evolution of the heat distribution within the pulse duration shows distinct features in rectangular contacts providing evidence for locally hot spots at the sharp corners of the current injection site due to current crowding effects leading to the observed behaviour.

We measure the magnetic damping parameter a in thin film CoFeB and permalloy (Py) nanomagnets at room temperature using ferromagnetic resonance driven by microwave frequency spin-transfer torque. We obtain $\alpha_{CoFeB} = 0.014 \pm 0.003$ and $\alpha_{Py}=0.010 \pm 0.002$, values comparable to measurements for extended thin films, but significantly less than the effective damping determined previously for similar nanomagnets by fits to time-domain studies of large-angle magnetic excitations and magnetic reversal. The greater damping found for the large amplitude nanomagnet dynamics is attributed to the nonlinear excitation of non-uniform magnetic modes., Comment: 13 pages, 2 figures

We discuss aspects of Andreev reflection (AR) measurements in normal metal-superconductor (N-S) and ferromagnet-superconductor (F-S) devices. We describe the analytical model used to quantify spin polarization from the conductance measurements and discuss the validity of this simple model using parabolic bands as simple surrogates for real band structures. We present (AR) measurements of spin polarization in a $\mathrm{Cu}\text{\ensuremath{-}}\mathrm{Pt}\text{\ensuremath{-}}\mathrm{Pb}$ and $\mathrm{Co}\text{\ensuremath{-}}\mathrm{Pt}\text{\ensuremath{-}}\mathrm{Pb}$ lithographically fabricated nanocontact systems where a scattering layer of has been deliberately added to the interface to enable the study of the effect of pair-breaking scattering on AR conductance and spin polarization. We compare these results to the previously published results from clean $\mathrm{Cu}\text{\ensuremath{-}}\mathrm{Pb}$ and $\mathrm{Co}\text{\ensuremath{-}}\mathrm{Pb}$ devices and argue that the measurements in devices with the Pt layer can be explained by the presence of inelastic-scattering-induced pair-breaking effects. We modify the analytical model to include this effect and show that in some instances, it may be impossible to distinguish between the effects of a finite spin polarization and inelastic scattering. This has implications for AR measurements of spin polarization at disordered or poorly formed F-S interfaces.

This paper deals with micromagnetic model of magnetization reversal in nano scale-point contact devices driven by nonuniform injection of a spin-polarized current. A computational study of the magnetization reversal in the nanosecond regime will be presented considering the influence of the current density distribution below the aperture region on the reversal time. For high current values, a strong dependence of the reversal time on the current distribution has been observed. Finally, results of micromagnetic simulations show that the reversal time versus current behaviour (at T=0K) is monotonic, very different from the switching processes observed in standard spin valves and magnetic tunnel junctions with uniform current injection.

We examine the effect a spatially non-uniform spin current with a component polarized partially out of the plane has on a low saturation magnetization nanomagnet free layer. Micromagnetic simulations indicate that the spin torque efficiency acting upon the reversing nanomagnet can be enhanced through this process, resulting in faster switching with smaller currents. In doing so, we determine that micromagnetic structure within the nanomagnets can be beneficial for reversal processes. We verify this enhancement experimentally in devices with a tapered nanopillar geometry that generates a spin current polarized partly out of plane. Finally, to take even better advantage of these effects, we examine micromagnetically the benefits of a tapered three-magnetic-layer structure that further reduces reversal times while maintaining the thermal stability of the free layer., Comment: 22 pages, 5 figures, submitted to Phys. Rev. B

The high contrast in the electrical resistivity between amorphous and crystalline states of a phase change material can potentially enable multiple memory levels for efficient use of a data storage medium. We report on our investigation of the role of the current injection site geometry (circular and square) in stabilizing such intermediate states within a nanoscale single-phase change material system (Ge2Sb2Te5). We have developed a three dimensional multiphysics model, which includes phase change kinetics, electrical, thermal, thermoelectric, and percolation effects, all as a function of temperature, using an iterative approach with coupled differential equations. Our model suggests that the physical origin of the formation of stable intermediate states in square top contact devices is mainly due to anisotropic heating during the application of a programming current pulse. Furthermore, the threshold current requirement and the width of the programming window are determined by crystallite nucleation and ...

We report enhanced spin-torque oscillator results obtained in spin-valve nanopillars. When biased within the optimal range of a moderate, ≤600 Oe, hard axis field, the spin-torque-driven oscillations exhibit a sharp increase in power and a sharply narrowed linewidth, ≤10 MHz, which, based on micromagnetic simulations, we ascribe to a transition from incoherent to coherent dynamics. The simulations indicate that the coherent dynamics are enabled by the combination of strong coupling between the two oscillator end modes of the magnetic free layer and strong non-linear damping arising from a non-uniform magnetization that leads to a spatially varying anti-damping spin torque.

International audience; We demonstrate a measurement technique with zero-applied magnetic field to deduce and spatially map the activation energy barrier distribution of strongly exchange-coupled magnetic-multilayer thin films, which is otherwise inaccessible with conventional methods in the presence of an applied magnetic field. Our technique involves the analysis of magnetic force microscopy images of magnetic microwires, whose magnetizations have been subject to thermal decay due to Joule heating from applied nanosecond scale current pulses. Fitting the results of such measurements on CoNi/Pd magnetic-multilayer microwires to a modified Arrhenius–Neel formalism yields an energy barrier distribution with 8% sigma, in good agreement with complementary fits of the switching-field-distribution measurements on patterned CoNi/Pd magnetic-multilayer islands. In order to achieve reliable operation of nanoscale magnetic devices, it is paramount to understand the role of spatial inhomogeneity of intrinsic magnetic properties, such as mag-netization, exchange, anisotropy, and activation energy for reversal in the overall performance of these devices. This evaluation is also necessary to separately diagnose the complementary contributions to the element-to-element performance variations from extrinsic ͑i.e., patterning induced͒ factors, such as ion-mill edge damage 1 and sidewall oxidation effects. 2

In low resistance-area product MgO magnetic tunnel junction nanopillars, we observe high integrated power (up to 43nW), narrow linewidth (down to 10MHz), spin transfer induced microwave emission at frequencies up to 14GHz due to precession of the free layer magnetization at room temperature. Although all devices were fabricated on the same wafer, they present bimodal transport and precessional characteristics. The devices in which the narrowest linewidths were observed exhibited low resistance and tunneling magnetoresistance (30%), while maintaining large integrated power.

We have used scanning transmission electron microscopy and electron energy-loss spectroscopy techniques to study the nature of interfacial interactions in a Ta∕CuOx stack that give rise to a smooth surface morphology, which can be utilized for seeding thin magnetic multilayer devices. Our measurements reveal that the interfacial smoothing is mainly due to the preferential reaction of Ta with O at the Ta∕CuOx interface assisted by grain boundary diffusion of oxygen which thereby acts to smooth out the surface roughness created by the large crystalline grains of Cu.

We have used nanofabrication techniques to incorporate an ∼20–30nm diameter nanoaperture within a 150×250nm2 elliptical magnetic multilayer to enable the localized injection of spin-polarized currents into a thin film nanomagnet. This results in very low spin transfer currents being required for at least partial nanomagnet reversal as well as for onset of dynamic precession. Micromagnetic simulations using Landau-Liftshitz-Gilbert equation with a spin-torque term indicate that reversal occurs via domain nucleation at the injection site followed by domain wall propagation away from the aperture, with the nanomagnet ending in one of several different states depending upon the current amplitude.

We have fabricated permalloy∕copper∕permalloy nanopillar spin valves designed to reduce the critical current for spin-transfer switching while maintaining thermal stability of the free layer. Pulsed current amplitudes necessary for switching a 4.5‐nm-thick permalloy free layer range from 0.4mA for a 100ns pulse to 2mA for a 1ns pulse, showing that the magnetization must be overdriven to achieve switching on short time scales. Comparisons to Landau–Lifshitz–Gilbert simulations indicate an effective damping parameter ≈0.03 and spin-torque efficiencies for parallel-to-antiparallel and antiparallel-to-parallel switching that are more symmetric than predicted by recent theoretical models.