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International audience; In this article here below, we present an electrical model for nanoscale spintronic radio frequency (RF) detectors called spin-torque diodes (STDs). A complete methodology for model parameter extraction is proposed. An equivalent electrical circuit for STD is derived and includes a modeling of the device resistance nonlinearity together with the STD effect. A detailed step-by-step methodology is presented to extract the model parameters using conventional dc measurements, RF scattering parameters (S -parameters), continuous wave (CW), and power characterizations. After parameter extraction, a comparison with measurements of a single STD successfully validates the model. Finally, the proposed STD electrical model is used to anticipate the behavior of a two-STD-based RF detector architecture. Simulation results highlight the interest of the proposed modeling approach to investigate suitable RF detector architectures to enhance RF-to-dc conversion efficiency for single or multiband RF detection.

International audience; This letter presents magnetic tunnel junction based spintronic devices completely implemented in a hybrid semiconductor process that comprises a complementary metal oxide semiconductor and a magnetic tunnel junction technology. To demonstrate the coexistence of both complementary metal oxide semiconductor circuits and magnetic tunnel junction based spintronic devices, a proof-of-concept circuit prototype comprising 40 spintronic devices and a digital complementary metal oxide semiconductor serial peripheral interface is fabricated. According to measurement results, a selected spintronic device from the magnetic tunnel junction array, when surrounded by an external out-ofplane magnetic field of 1 kOe, emitted microwave signals from 2.235 to 2.464 GHz with an output power from 0.88 to 0.72 nW, when the DC current was increased from 0.6 to 1.0 mA. To the authors' best knowledge, this is the first work demonstrating the functionality of spintronic oscillators fully integrated in complementary metal oxide semiconductor circuit implemented in a hybrid complementary metal oxide semiconductor and magnetic tunnel junction process.

et al., Spintronic devices exploit the spin, as well as the charge, of electrons and could bring new capabilities to the microelectronics industry. However, in order for spintronic devices to meet the ever-increasing demands of the industry, innovation in terms of materials, processes and circuits are required. Here, we review recent developments in spintronics that could soon have an impact on the microelectronics and information technology industry. We highlight and explore four key areas: magnetic memories, magnetic sensors, radio-frequency and microwave devices, and logic and non-Boolean devices. We also discuss the challenges—at both the device and the system level—that need be addressed in order to integrate spintronic materials and functionalities into mainstream microelectronic platforms.

International audience; The objective of this study is to co-integrate multiple digital and analog functions together within CMOS by developing a universal magnetic tunneling junction stack (MTJ) capable of realizing logic, memory, and analog functions, within a single baseline technology. This will allow monolithic heterogeneous integration, fast and low-power processing, and high integration density, particularly useful for Internet of Things (IoT) platforms. This unique spintransfer-torque (STT) MTJ is called Multifunctional Standardized Stack (MSS). This paper presents the progress regarding memory, oscillator and sensor functionalities targeted for the technology. We show that a single magnetic stack deposition can be used to obtain these three functionalities on the same wafer.

The correlation of phase fluctuations in any type of oscillator fundamentally defines its spectral shape. However, in nonlinear oscillators, such as spin torque nano-oscillators, the frequency spectrum can become particularly complex. This is specifically true when not only considering thermal but also colored 1/f flicker noise processes, which are crucial in the context of the oscillator’s long term stability. In this study, we address the frequency spectrum of spin torque oscillators in the regime of large-amplitude steady oscillations experimentally and as well theoretically. We particularly take both thermal and flicker noise into account. We perform a series of measurements of the phase noise and the spectrum on spin torque vortex oscillators, notably varying the measurement time duration. Furthermore, we develop the modelling of thermal and flicker noise in Thiele equation based simulations. We also derive the complete phase variance in the framework of the nonlinear auto-oscillator theory and deduce the actual frequency spectrum. We investigate its dependence on the measurement time duration and compare with the experimental results. Long term stability is important in several of the recent applicative developments of spin torque oscillators. This study brings some insights on how to better address this issue.

We investigate the possibility of realizing a spintronic memristive device based on the dependence of the tunnel conductance on the relative angle between the magnetization of the two magnetic electrodes in in-plane magnetized tunnel junctions. For this, it is necessary to design a free layer whose magnetization can be stabilized along several or even any in-plane direction between the parallel and the antiparallel magnetic configurations. We experimentally show that this can be achieved by exploiting antiferromagnet-ferromagnet exchange interactions in a regime where the antiferromagnet is thin enough to induce enhanced coercivity and no exchange bias. The frustration of exchange interactions at the interfaces due to competing ferro- and antiferromagnetic interactions is at the origin of an isotropic dissipation mechanism yielding isotropic coercivity. From a modeling point of view, it is shown that this isotropic dissipation can be described by a dry friction term in the Landau-Lifshitz-Gilbert equation. The influence of this dry friction term on the magnetization dynamics of an in-plane magnetized layer submitted to a rotating in-plane field is investigated both analytically and numerically. The possibility to control the free layer magnetization orientation in an in-plane magnetized magnetic tunnel junction by using the spin transfer torque from an additional perpendicular polarizer is also investigated through macrospin simulation. It is shown that the memristor function can be achieved by the injection of current pulses through the stack in the presence of an in-plane static field transverse to the reference layer magnetization, the aim of which is to limit the magnetization rotation between 0{\deg} and 180{\deg}., Comment: 24 pages, 9 figures

International audience; In the field of spintronics, ferromagnetic/non-magnetic metallic multilayers are core building blocks for emerging technologies. Resonance experiments using stripline transducers are commonly used to characterize and engineer these stacks for applications. Up to now in these experiments, the influence of eddy currents on the excitation of the dynamics of ferromagnetic magnetization below the skin-depth limit was most often neglected. Here, using a coplanar stripline transducer, we experimentally investigated the broadband ferromagnetic resonance response of NiFe/Cu bilayers a few nanometers thick in the sub-skin-depth regime. Asymmetry in the absorption spectrum gradually built up as the excitation frequency and Cu-layer thickness increased. Most significantly, the sign of the asymmetry depended on the stacking order. Experimental data were consistent with a quantitative analysis considering eddy currents generated in the Cu layers and the subsequent phaseshift of the feedback magnetic field generated by the eddy currents. These results extend our understanding of the impact of eddy currents below the microwave magnetic skin-depth and explain the lineshape asymmetry and phase lags reported in stripline experiments.

We report a ferromagnetic resonance study of an electrically insulating magnetic bi-layer consisting of two yttrium iron garnet (YIG) films epitaxially grown on both sides of a non-magnetic gadolinium gallium garnet (GGG) slab. We show that standing transverse sound waves couple the coherent magnetization dynamics of the two YIG films half millimeter apart through the magnetoelastic interaction, periodically modulating the microwave absorption as a function of frequency. Constructive and destructive interferences between the dynamics of the two YIG layers is observed. This long range coherent coupling by phononic angular momentum currents through non-magnetic dielectric waveguide brings new functionalities to insulator hybrid spin circuits and devices.

Low frequency noise close to the carrier remains little explored in spin torque nano oscillators. However, it is crucial to investigate as it limits the oscillator's frequency stability. This work addresses the low offset frequency flicker noise of a TMR-based spin-torque vortex oscillator in the regime of large amplitude steady oscillations. We first phenomenologically expand the nonlinear auto-oscillator theory aiming to reveal the properties of this noise. We then present a thorough experimental study of the oscillator's $1/f$ flicker noise and discuss the results based on the theoretical predictions. Hereby, we connect the oscillator's nonlinear dynamics with the concept of flicker noise and furthermore refer to the influence of a standard $1/f$ noise description based on the Hooge formula, taking into account the non-constant magnetic oscillation volume, which contributes to the magnetoresistance., 10 pages

Vortex based spin torque nano oscillators (STVOs) can present more complex dynamics than the spin torque induced gyrotropic (G) motion of the vortex core. The respective dynamic modes and the transition between them can be controlled by experimental parameters such as the applied dc current. An interesting behavior is the stochastic transition from the G- to a dynamic C-state occurring for large current densities. Moreover, the C-state oscillations exhibit a constant active magnetic volume. We present noise measurements in the different dynamic states that allow accessing specific properties of the stochastic transition, such as the characteristic state transition frequency. Furthermore,we confirm, as theoretically predicted, an increase of flicker noise with $I_{dc}^2$ when the oscillation volume remains constant with the current. These results bring insight into the potential optimization of noise properties sought for many potential rf applications with spin torque oscillators. Furthermore, the investigated stochastic characteristics open up new potentialities, for instance in the emerging field of neuromorphic computing schemes.

Integration of Spin Torque Nano-Oscillators STNO's in conventional microwave circuits means that the devices have to meet certain specifications. One of the most important criteria is the phase noise, being the key parameter to evaluate the performance and define possible applications. Phase locking several oscillators together has been suggested as a possible means to decrease phase noise and consequently, the linewidth. In this work we present experiments, numerical simulations and an analytic model to describe the effects of thermal noise in the injection locking of a tunnel junction based STNO. The analytics show the relation of the intrinsic parameters of the STNO with the phase noise level, opening the path to tailor the spectral characteristics by the magnetic configuration. Experiments and simulations demonstrate that in the in-plane magnetized structure, while the frequency is locked, much higher reference currents are needed to reduce the noise by phase locking. Moreover, our analysis shows that it is possible to control the phase noise by the reference microwave current (IRF) and that it can be further reduced by increasing the bias current (IDC) of the oscillator, keeping the reference current in feasible limits for applications. Fil: Tortarolo, Marina del Carmen. Comisión Nacional de Energía Atómica. Gerencia del Área de Investigaciones y Aplicaciones no Nucleares. Gerencia de Física (Centro Atómico Constituyentes); Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Lacoste, B.. International Iberian Nanotechnology Laboratory; Portugal. Universite Grenoble Alpes; Francia Fil: Hem, J.. Universite Grenoble Alpes; Francia Fil: Dieudonné, C.. Universite Grenoble Alpes; Francia Fil: Cyrille, M. C.. Universite Grenoble Alpes; Francia Fil: Katine, J.A.. Hitachi Global Storage Technologies; Estados Unidos Fil: Mauri, D.. Hitachi Global Storage Technologies; Estados Unidos Fil: Zeltser, A.. Hitachi Global Storage Technologies; Estados Unidos Fil: Buda Prejbeanu, L.D.. Universite Grenoble Alpes; Francia Fil: Ebels, U.. Universite Grenoble Alpes; Francia

The conditions of field and voltage for inducing steady state excitations in fully perpendicular magnetic tunnel junctions (pMTJs), adapted for memory applications, were numerically investigated by the resolution of the Landau-Lifshitz-Gilbert equation in the macrospin approach. Both damping-like and the field-like spin transfer torque terms were taken into account in the simulations, as well as the contribution of the second order uniaxial anisotropy term (K2), which has been recently revealed in MgO-based pMTJs. An in-plane applied magnetic field balances the out of plane symmetry of the pMTJ and allows the signal detection. Using this model, we assessed the states of the free layer magnetization as a function of strength of K2 and polar theta_H angle of the applied field (varied from 90 to 60 deg.). There are two stable states, with the magnetization in-plane or out of plane of the layer, and two dynamic states with self-sustained oscillations, called in-plane precession state (IPP) or out of plane precession state (OPP). The IPP mode, with oscillation frequencies up to 7 GHz, appears only for positive voltages if theta_H = 90 deg. However, it shows a more complex distribution when the field is slightly tilted out of plane. The OPP mode is excited only if K2 is considered and reaches a maximum oscillation frequency of 15 GHz. Large areas of dynamic states with high frequencies are obtained for strong values of the field-like torque and K2, when applying a slightly tilted external field toward the out of plane direction. The non-zero temperature does not modify the phase diagrams, but reduces drastically the power spectral density peak amplitudes., 31 pages with figures and figure captions, 5 figures, article

In the last two decades Spintronics has been an important driver for Information Technologies with the introduction in 1996 of magneto-resistive field sensors in data storage and in 2006 of magnetic random access memories. Spintronics can also bring alternative solutions for rf components used in Information-Communication Technologies. By virtue of their specific spin polarized transport properties spintronic devices become active, multifunctional, compact, low power, low cost, CMOS compatible rf components that combine functions of microwave signal generation (DC-to-RF converter), modulation and frequency selective microwave signal detection (RF-to-DC converter). In this presentation an overview will be provided for the realization of rf spintronic components and their corresponding rf properties with respect to different functions such as signal generation, modulation and detection. The results reported here were obtained in a collaborative effort within the FP7 project MOSAIC [1] including different laboratories SPINTEC-UGA/CEA/CNRS (F); CEA-LETI (F); INL, Braga (Pt); TUD (G); UMPhy CNRS/THALES (F); THALES TRT (F)

Spin transfer magnetization dynamics have led to considerable advances in Spintronics, including opportunities for new nanoscale radiofrequency devices. Among the new functionalities is the radiofrequency (rf) detection using the spin diode rectification effect in spin torque nano-oscillators (STNOs). In this study, we focus on a new phenomenon, the resonant expulsion of a magnetic vortex in STNOs. This effect is observed when the excitation vortex radius, due to spin torques associated to rf currents, becomes larger than the actual radius of the STNO. This vortex expulsion is leading to a sharp variation of the voltage at the resonant frequency. Here we show that the detected frequency can be tuned by different parameters; furthermore, a simultaneous detection of different rf signals can be achieved by real time measurements with several STNOs having different diameters. This result constitutes a first proof-of-principle towards the development of a new kind of nanoscale rf threshold detector.

International audience; A comprehensive numerical study of the spin toque driven dynamic states is presented for a synthetic ferrimagnet. For this, the Landau-Lifshitz-Gilbert equation has been solved simultaneously for the two coupled layers of the synthetic ferrimagnet in a macrospin approach including the spin transfer torque term from an external polarizer for one of them. It is shown that a large variety of dynamic modes (in-plane precession (IPP) and out-of-plane precession) can be established, upon varying the strength of the exchange bias field that pins one of the layers of the SyF as well as the Ruderman-Kittel-Kasuya-Yosida interlayer coupling strength. The current—field state diagrams are presented as well as the frequency current dependencies of the most important mode which is the IPP mode. A characteristic feature of the IPP mode for the coupled system (as compared to single layer excitations) is the change, increase or decrease of the frequency, with current upon increasing field. It is shown that this strongly depends on the asymmetry of the internal fields that the two layers experience, upon varying either their thickness or the exchange bias field.

In this manuscript the general concepts of spin wave theory are adapted to the dynamics of a self-polarized system based on two layers coupled via interlayer exchange (conservative coupling) and mutual spin torque (dissipative coupling). An analytical description of the non-linear dynamics is proposed and validated through numerical simulations. In contrast to the single layer model, the phase equation of the coupled system has a contribution coming from the dissipative part of the LLGS equation. It is shown that this is a major contribution to the frequency mandatory to describe well the most basic features of the dynamics of coupled systems. Using the proposed model a specific feature of coupled dynamics is addressed: the redshift to blueshift transition observed in the frequency current dependence of this kind of exchange coupled systems upon increasing the applied field. It is found that the blueshift regime can only occur in a region of field where the two linear eigenmodes contribute equally to the steady state mode (i.e. high mode hybridization). Finally, a general perturbed Hamiltonian equation for the coupled system is proposed., 16 pages, 7 figues

International audience; STT-MRAM are foreseen as the best contender forDRAM replacement. STT-MRAM could also be used for SRAMapplications if switching time below 1ns could be realized in areliable way. In this study, we demonstrate that sub-ns switchingwith final state determined by the current polarity through thestack can be achieved in STT-MRAM cells comprising two spinpolarizinglayers having orthogonal magnetic anisotropies [1],[2].We carried out a thorough experimental and modeling study ofthese ultrafast STT-MRAM. We demonstrated that a quitereliable switching can be achieved by increasing the cell aspectratio (AR) or advantageously by applying an in-plane statictransverse field on the cell. Switching in 200ps could bedemonstrated with write energy less than 100fJ.

International audience; Spin Transfer Nano-Oscillators (STNOs) are promising candidates for telecommunications applicationsdue to their frequency tuning capabilities via either a dc current or an applied field. Thisfrequency tuning is of interest for Frequency Shift Keying concepts to be used in wireless communicationschemes or in read head applications. For these technological applications, one importantparameter is the characterization of the maximum achievable rate at which an STNO can respond toa modulating signal, such as current or field. Previous studies of in-plane magnetized STNOs onfrequency modulation via an rf current revealed that the maximum achievable rate is limited by theamplitude relaxation rate Cp, which gives the time scale over which amplitude fluctuations aredamped out. This might be a limitation for applications. Here, we demonstrate via numerical simulationthat application of an additional rf field is an alternative way for modulation of the in-plane magnetizedSTNO configuration, which has the advantage that frequency modulation is not limited bythe amplitude relaxation rate, so that higher modulation rates above GHz are achievable. This occurswhen the modulating rf field is oriented along the easy axis (longitudinal rf field). Tilting the directionof the modulating rf field in-plane and perpendicularly with respect to the easy axis (transverse rffield), the modulation is again limited by the amplitude relaxation rate similar to the responseobserved in current modulation.

Inertial magnetization dynamics have been predicted at ultrahigh speeds, or frequencies approaching the energy relaxation scale of electrons, in ferromagnetic metals. Here we identify inertial terms to magnetization dynamics in thin Ni$_{79}$Fe$_{21}$ and Co films near room temperature. Effective magnetic fields measured in high-frequency ferromagnetic resonance (115-345 GHz) show an additional stiffening term which is quadratic in frequency and $\sim$ 80 mT at the high frequency limit of our experiment. Our results extend understanding of magnetization dynamics at sub-picosecond time scales., Comment: 11 pages, 3 figures

International audience; The influence of dynamic coupling in between magnetic layers of a standard spin torque nano-oscillator composed of a synthetic antiferromagnet (SyF) as a polarizer and an in-plane magnetized free layer has been investigated. Experiments on spin valve nanopillars reveal non-continuous features such as kinks in the frequency field dependence that cannot be explained without such interactions. Comparison of experiments to numerical macrospin simulations shows that this is due to non-linear interaction between the spin torque (STT) driven mode and a damped mode that is mediated via the third harmonics of the STT mode. It only occurs at large applied currents and thus at large excitation amplitudes of the STT mode. Under these conditions, a hybridized mode characterized by a strong reduction of the linewidth appears. The reduced linewidth can be explained by a reduction of the non-linear contribution to the linewidth via an enhanced effective damping. Interestingly, the effect depends also on the exchange interaction within the SyF. An enhancement of the current range of reduced linewidth by a factor of two and a reduction of the minimum linewidth by a factor of two are predicted from simulation when the exchange interaction strength is reduced by 30%. These results open directions to optimize the design and microwave performances of spin torque nano-oscillators taking advantage of the coupling mechanisms. (C) 2015 AIP Publishing LLC.

For practical applications of spin torque nano-oscillators (STNO), one of the most critical characteristics is the speed at which an STNO responds to variations of external control parameters, such as current or/and field. Theory predicts that this speed is limited by the amplitude relaxation rate Γp that determines the timescale over which the amplitude fluctuations are damped out. In this study, this limit is verified experimentally by analyzing the amplitude and frequency noise spectra of the output voltage signal when modulating an STNO by a microwave current. In particular, it is shown that due to the non-isochronous nature of the STNO the amplitude relaxation rate Γp determines not only the bandwidth of an amplitude modulation, but also the bandwidth of a frequency modulation. The presented experimental technique will be important for the optimisation of the STNO characteristics for applications in telecommunications or/and data storage and is applicable even in the case when the STNO output signal is only several times higher than noise.

An equilibrium stability analysis of an in-plane magnetized synthetic ferrimagnetic free layer (SyF) submitted to multiple spin-transfer-torque (STT) influences and to applied field has been carried out. Analytical expressions for the frequency and linewidth of the excitations were derived, in the case of strong coupling between the two layers compared to the anisotropy field. The expression of the critical current for the onset of excitations in the SyF was then calculated. Above the critical current, the destabilized mode, acoustic or optical, is found to depend on the applied field direction, but also on the asymmetry of the two layers composing the SyF. A strong thickness asymmetry reduces the critical current at zero field, and allows to select the destabilized mode. The critical current of an in-plane SyF with mutual spin torque between the layers and without reference layer was computed. For a symmetric SyF, the optical mode is the only mode that can be destabilized by STT.

International audience; Spintronics relies on the spin dependent transport properties of ferromagnets (Fs). Although antiferromagnets (AFs) are used for their magnetic properties only, some fundamental F-spintronics phenomena like spin transfer torque, domain wall motion, and tunnel anisotropic magnetoresistance also occur with AFs, thus making AF-spintronics attractive. Here, room temperature critical depths and absorption mechanisms of spin currents in Ir 20 Mn 80 and Fe 50 Mn 50 are determined by F-resonance and spin pumping. In particular, we find room temperature critical depths originating from different absorption mechanisms: dephasing for Ir 20 Mn 80 and spin flipping for Fe 50 Mn 50.

This work reports on advances in MRAM cells aiming at sub-nanosecond switching and for sub-20nm technology nodes. Ultrafast precessional spin-transfer switching in elliptical magnetic tunnel junction nanopillars is possible to obtain in samples integrating a perpendicular polarizer and a tunnel junction with in-plane magnetized electrodes. We show that spin transfer torque (STT) switching in less than 500ps can be achieved in these structures with corresponding write energy less than 100fJ. For high density integration and possibly sub-20nm diameter cells the use of a thermally assisted concept for perpendicular anisotropy cells, where the intrinsic heating is used to simultaneously achieve high thermal stability and low current switching.

The paper describes the possible use of a spintorque oscillator in the design of a radiofrequency macro-oscillator. The low consumption and the nanometric size of spintorque oscillators comparatively to LC circuits open new perspectives for these devices. Unfortunately a single spintorque oscillator cannot be used for radiofrequency applications due to a very low power and a high level of phase noise. In order to improve the performance of the output signal a common synchronization approach is applied to an array of forced spintorque oscillators. The main issues for practical realization are presented and possible ways for their solution are proposed.

International audience; The coherence of the precessional switching was compared in planar spin-valves comprising either an additional simple perpendicular polarizer or a synthetic antiferromagnet perpendicular polarizer. A significant improvement in the precession coherence was observed experimentally in the second type of samples. Micromagnetic simulations were performed to study the effect of the stray field from the perpendicular polarizer. They provide an explanation for the gradual loss of coherence of the precession in terms of vortex formation, which occurs much faster when a simple perpendicular polarizer is used.

We have measured the size effect in nonlocal Gilbert relaxation rate in FM(t$_{FM}$) / Cu (5nm) [/ Pt (2nm)] / Al(2nm) heterostructures, FM = \{ Ni$_{81}$Fe$_{19}$, Co$_{60}$Fe$_{20}$B$_{20}$, pure Co\}. Common behavior is observed for three FM layers, where the additional relaxation obeys both a strict inverse power law dependence $\Delta G =K \:t^{n}$, $n=-\textrm{1.04}\pm\textrm{0.06}$ and a similar magnitude $K=\textrm{224}\pm\textrm{40 Mhz}\cdot\textrm{nm}$. As the tested FM layers span an order of magnitude in spin diffusion length $\lambda_{SDL}$, the results are in support of spin diffusion, rather than nonlocal resistivity, as the origin of the effect.

International audience; The microwave emission linewidth of spin transfer torque nano-oscillators is closely related to their phase and amplitude noise that can be extracted from the magnetoresistive voltage signal V(t) using single shot time domain techniques. Here we report on phase and amplitude noise studies for MgO based magnetic tunnel junction oscillators. The analysis of the power spectral densities allows one to separate the linear and nonlinear contributions to the phase noise, the nonlinear contribution being due to the coupling between phase and amplitude. The coupling strength as well as the amplitude relaxation rate can be directly extracted.

We present magnetotransport measurements as well as micromagnetic simulations of magnetization reversal in magnetic nanowires having strong transverse magnetocrystalline anisotropy. The interplay between exchange, demagnetizing, and magnetocrystalline energies gives rise to a micromagnetic configuration involving vortices with alternative chirality along the wire. Despite the complexity of the field angle-dependent magnetization reversal process, a very good agreement is obtained between experiments and simulations. This provides evidence that the reported magnetization process appears generally in nanowires with strong transverse magnetocrystalline anisotropy. Moreover an analytical expression is established for the angular dependency of the core switching field. The simulations indicate the occurrence of Bloch points during the core reversal.

The influence of dynamic interactions on the spin-polarized current-induced steady state oscillations has been studied numerically for a spin torque oscillator that is composed of a free layer and a synthetic antiferromagnetic (SAF) pinned layer. Two types of dynamical interactions have been considered. The first is the exchange interaction between the two layers that constitute the SAF pinned layer. The second is a mutual spin torque interaction between the free layer and the pinned SAF layer. Taking these interactions into account, the Landau-Lifshitz-Gilbert equation including the spin transfer torque has been solved simultaneously for all three layers in a macrospin approach. Besides the dynamic state diagram for such a coupled spin torque oscillator, three major results have been obtained for the dynamic excitation states: (1) the dynamic states are not in all cases steady state periodic oscillations but can present nonperiodic trajectories reminiscent of chaos, (2) due to the mutual spin torque interaction frequency jumps can occur which are interpreted in terms of resonance excitations and frequency locking between the free layer and SAF magnetizations, and (3) the acoustic-type steady state oscillations of the SAF differ from those of a single layer in that the frequency vs current dependence of the in-plane precession modes can show both redshift as well as blueshift. This is interpreted as a general property of the large-angle nonlinear dynamics of a SAF.

International audience; Voltage noise measurements were performed in the 3–7 GHz frequency range on magnetic tunnel junctions biased with a dc current. Magnetic noise associated with ferromagnetic resonance excitations is either amplified or reduced depending on the direction of the bias current. This effect is interpreted as the influence of spin transfer torque on the magnetization fluctuations and described using Gilbert dynamics equation including spin transfer torque and effective field terms.

Antiferromagnetic materials can host spin-waves with polarizations ranging from circular to linear depending on their magnetic anisotropies. Until now, only easy-axis anisotropy antiferromagnets with circularly polarized spin-waves were reported to carry spin-information over long distances of micrometers. In this article, we report long-distance spin-transport in the easy-plane canted antiferromagnetic phase of hematite and at room temperature, where the linearly polarized magnons are not intuitively expected to carry spin. We demonstrate that the spin-transport signal decreases continuously through the easy-axis to easy-plane Morin transition, and persists in the easy-plane phase through current induced pairs of linearly polarized magnons with dephasing lengths in the micrometer range. We explain the long transport distance as a result of the low magnetic damping, which we measure to be ≤ 10−5 as in the best ferromagnets. All of this together demonstrates that long-distance transport can be achieved across a range of anisotropies and temperatures, up to room temperature, highlighting the promising potential of this insulating antiferromagnet for magnon-based devices., Hitherto, only circularly polarized antiferromagnetic (AFM) spin-waves (SWs) were expected to convey spin-information. Here, the authors present persistent spin-transport over long distances in the easy-plane AFM phase of hematite, α-Fe2O3, via linearly polarized SW pairs with ultra-low damping.

This work presents a spin torque nano-oscillator (STNO) based binary frequency shift keying (BFSK) modulation schema implemented on a printed circuit board (PCB). Maximal input data rate reaches 20 Mbit/s. Depending on the STNO used, carrier frequency can range from 1 to 10 GHz. Both DC and AC currents flowing through the STNO can be tuned between 0 to 4 mA. Using one magnetic tunnel junction (MTJ) STNO, a 380 MHz frequency shift around the center frequency 9 GHz was observed, when the modulated current was toggled between 1.06 mA and 1.2 mA at a rate of 20 Mbit/s. The PCB with a size of 50 × 55 mm consumes maximal 130 mA at a 5 V supply. This is the first work demonstrating that the STNOs are applicable for BFSK modulation on the wireless application level.

Spin-waves have been proposed as a possible technological path to overcome the hurdles encountered by the miniaturization of complementary metal-oxide-semiconductor (CMOS) into the nanometer range, demonstrated by recent developments in spin-wave-based logic devices. However the industrial appeal of these proofs-of-concept is conditional upon their scalable integration with CMOS technology. Here, we report on ultrathin Ta/CoFeB/MgO wires used as spin-wave waveguides. This system is chosen for its compability with CMOS processes, its perpendicular magnetic anisotropy and strong spin-orbit interactions. The latter are of interest for manipulating spin waves and are characterized via spin-torque ferromagnetic resonance where it is shown that the inverse spin Hall effect is responsible for the detection of magnetization dynamics. Following this, we use integrated nanometric coplanar waveguides to locally excite spin-waves in a broad range of wavevectors. Comparison of the measured spin-wave spectrum with analytical calculations show that the inverse spin Hall effect allows the wavevector-independent detection of spin-waves with wavelengths down to 150 nm. Complementary Brillouin light scattering experiments reveal that spin-waves in the ultrathin spin-wave waveguide with perpendicular magnetic anisotropy have unexpectedly high propagation lengths considering the relatively high damping in Ta/CoFeB/MgO systems. These findings pave the way for ultrathin CMOS-compatible spin-wave devices with excitation and detection techniques that are scalable into the nanometer range, with the prospect of controlling spin-waves via spin-orbit torques., Les ondes de spin sont une des voies technologiques proposées pour surmonter les obstacles que rencontre la miniaturisation des complementary metal-oxide-semiconductor (CMOS) dans la gamme du nanomètre, comme en témoignent les derniers développements en matière de dispositifs logiques à ondes de spin. Cependant, l'attrait industriel de ces preuves de concept est conditionné par leur intégration évolutive à la technologie CMOS. Ici, nous présentons des pistes ultrafines de Ta/CoFeB/MgO utilisées comme guides d'ondes de spin. Ce système a été choisi pour sa compatibilité avec les procédés CMOS, son anisotropie magnétique perpendiculaire et ses fortes interactions spin-orbite. Ces derniers sont intéressants pour manipuler les ondes de spin et ont été caractérisés par résonance ferromagnétique à couple de spin où il est démontré que l'effet Hall de spin inverse est responsable de la détection de la dynamique de magnétisation. Ensuite, nous utilisons des guides d'ondes coplanaires nanométriques intégrés pour exciter localement des ondes de spin dans une large gamme de vecteurs d'ondes. La comparaison du spectre d'ondes de spin mesuré avec les calculs analytiques montre que l'effet Hall de spin inverse permet la détection des ondes de spin indépendamment de leur vecteur d'onde avec des longueurs d'onde allant jusqu'à 150 nm. Des expériences complémentaires de diffusion de la lumière de Brillouin révèlent que les ondes de spin dans le guide d'ondes de spin ultra-mince à anisotropie magnétique perpendiculaire ont des longueurs de propagation étonnamment élevées compte tenu de l'amortissement relativement élevé des systèmes Ta/CoFeB/MgO. Ces résultats ouvrent la voie à des dispositifs à ondes de spin ultraminces compatibles CMOS avec des techniques d'excitation et de détection évolutives jusqu'à l'ordre du nanomètre, avec la perspective de contrôler les ondes de spin via des couples spin-orbite.

Spin Transfer Nano-Oscillators (STNOs) are a novel type of Radio Frequency (RF) oscillators that make use of the Spin Transfer Torque (STT) effect in a magnetic tunnel junction (MTJ) device to produce high-frequency auto-oscillations. STNOs are attractive for applications in wireless communications due to their nanometric size and their frequency tuning capabilities via either a dc current or an applied field. This frequency tuning permits to encode the information via frequency shift keying (FSK) by digital modulation of the current or applied field between two discrete values without the need of an external RF mixer, leading to potentially less complex RF components. In this thesis, the feasibility of the digital frequency modulation (frequency shift keying (FSK)) using in-plane magnetized MTJ STNOs has been studied. For this, the maximum modulation rate, up to which a signal can be modulated or the frequency can be shifted between two discrete values, is an important aspect that need to be characterized.The characterization of the maximum modulation rate for in-plane magnetized MTJ STNOs has been studied via numerical macrospin simulation for different modulation configurations, i.e. modulation by a sinusoidal RF current and a sinusoidal RF field. It revealed that the maximum modulation rate under RF current modulation is given by the amplitude relaxation frequency fp of the STNO. Under RF field modulation, i.e. an RF field applied parallel to the easy axis, an enhanced modulation rate above fp can be achieved since the frequency is modulated directly via the field and not via the amplitude. This suggests an important strategy for the design of STNO-based wireless communications and to achieve high data rates. Besides numerical simulation, experimental studies of frequency shift keying (FSK) by current modulation in STNOs have been also demonstrated. The first demonstration is the FSK in standalone STNOs. The analysis confirmed that the FSK was successfully observed with a frequency shift around 200MHz (the frequency shift between ≈8.9 GHz and ≈9.1 GHz) at the modulation rate of 10Mbps. This modulation rate is however less than the upper limit, which is given by the relaxation frequency fp of the STNO as predicted in the numerical simulation, because of the relatively high phase noise of the device measured. In order to test the feasibility of the STNO within microwave systems, the FSK modulation of STNOs was performed on a printed circuit board (PCB) emitter. FSK with a frequency shift around 300MHz (the frequency shift between ≈9 GHz and ≈9.3 GHz) was observed with a modulation rate of 20 Mbps. The data rate here was limited by characteristics of the PCB emitter and not intrinsic to the STNO. The simulation and experiment studies of frequency modulation of STNOs demonstrate that the data rate of is adequate for wireless communication used in WSN. However, further improvements in materials and nanofabrication of STNOs are required to enhance the output power and improve the spectral characteristics of the oscillations to push the data rates to higher values with large frequency shift., Les Oscillateurs Spintronique (STNO) sont un nouveau type d'oscillateurs à fréquence radio (RF) qui utilisent l'effet « Spin Transfer Torque (STT) » dans un dispositif de jonction tunnel magnétique (MTJ) pour produire des oscillations entretenues à haute fréquence. Les STNO fournissent des solutions compactes pour la communication sans fil utilisées dans « wireless sensor network (WSN) » car leur fréquence peut être réglée via un courant continu. Ce réglage de fréquence permet de coder l'information via « Frequency shift keying (FSK) » par modulation numérique entre deux valeurs discrètes sans besoin d'un RF mixer, ce qui conduit à des composants RF potentiellement moins complexes. Dans cette thèse, la faisabilité de FSK a été étudiée pour des STNO MTJ à aimantation dans le plan en vue des communications sans fil utilisées dans les WSN. Les paramètres abordés dans cette étude sont le décalage de fréquence et le taux de modulation maximum, auquel la fréquence peut être décalée entre deux valeurs discrètes.Pour caractériser le taux de modulation maximum, des simulations macrospin et des études expérimentales ont été réalisées. Les simulations révèlent que le taux de modulation maximum pour FSK par courant est limité par la fréquence de relaxation du STNO, qui est de l'ordre de quelques centaines de MHz pour les STNO à aimantation dans le plan. Cela signifie que le taux de modulation maximum est limité à quelques centaines de Mbps, ce qui est ciblé ici pour une communication sans fil à débit de données modéré utilisées dans les WSN. Des études expérimentales du FSK par modulation de courant dans les STNO ont été effectuées pour des STNO autonomes et pour des STNO intégrés dans des systèmes hyperfréquences. Le FSK sur les STNO autonomes montre un décalage de fréquence autour de 200 MHz (le décalage de fréquence entre ≈ 8,9 GHz et ≈9,1 GHz) au taux de modulation de 10Mbps. Ce taux de modulation est inférieur à la limite supérieure donnée par la fréquence de relaxation du STNO comme prévu dans la simulation numérique en raison du bruit de phase relativement élevé du dispositif mesuré. Afin de tester la faisabilité du STNO dans les systèmes hyperfréquences, la modulation FSK des STNO a été effectuée sur un émetteur de carte de circuit imprimé (PCB). L'émetteur de PCB a été réalisé et développé par le partenaire du projet Mosaic FP7, TUD University. L'analyse confirme qu'un changement de fréquence autour de 300 MHz (le décalage de fréquence entre ≈9 GHz et ≈9,3 GHz) a été observé avec un taux de modulation de 20 Mbps. Le taux de données est limité par les caractéristiques de l'émetteur de PCB et non intrinsèque au STNO. Les études de simulation et d'expérience de la modulation de fréquence des STNO démontrent que le débit de données est adéquat pour la communication sans fil utilisée dans WSN.Cependant, d'autres améliorations dans les matériaux et la nanofabrication de STNO sont nécessaires pour améliorer la puissance de sortie et améliorer les caractéristiques spectrales des oscillations pour pousser les débits de données à des valeurs plus élevées avec un grand décalage de fréquence

Spin transfer oscillators (STOs) are promising nanometer scaled oscillators (~100nm) for radiofrequency applications. They rely on the steady precession of the magnetization of a thin magnetic layer induced by spin-transfer torque (STT). A STO device based on a magnetic tunnel junction (MTJ) will typically generate an electrical signal with a frequency of the order of ten GHz and an output power of several nW. Compared to voltage controlled oscillators (VCO) used today for microwave generation, STOs have the advantage of having an important frequency tunability with current. However, criteria in terms of the quality of the output signal are not yet fulfilled for STO to be competitive.To enhance the STO signal properties, two suggestions are proposed: (i) optimization of the magnetic stack within a single STO device and (ii) synchronization of several STOs. The second approach was examined during this thesis: here we look at the electrical synchronization of a STO to a stabilized RF current source, or “injection-locking”. The case of a STO with homogenous magnetization of in-plane precession (IPP) type is investigated.Interestingly, synchronization of a STO at 2f, i.e. when the frequency of the injected current is close to twice the generation frequency of the STO, is favored compared to synchronization at f. The experimental results from several groups have shown both enhanced synchronization range and a more pronounced linewidth reduction at 2f.This singular behavior is examined first through an analytical study of magnetization dynamics along with numerical macrospin simulations, in order to identify synchronization mechanisms taking effect in the system.Indeed, current models (in particular the KTS auto-oscillator formalism) describe synchronization of a STO with making a clear distinction between synchronization at f and 2f, and the resulting predictions turn out to be insufficient at 2f. Here, by extension of the KTS formalism, the keys to the synchronization process at 2f are presented: frequency adjustment by adjustment of the oscillation amplitude via the STO non-linearity and modification of the anti-damping term through the phase-difference.The experimental characterization of the synchronized regime in a MTJ-based STO is also detailed in the manuscript. Utilizing experimental signal processing techniques in both frequency and temporal domain, we extract characteristic quantities for synchronization such as the locking-range and the phase-difference, and we compare these quantities with the analytical predictions. Finally, the effects of current injection on the coherence of the output signal are also discussed., Les oscillateurs à transfert de spin (STO) sont des oscillateurs nanométriques (~100nm) prometteurs pour les applications radiofréquence. Ils reposent sur la précession de l'aimantation d'une couche magnétique mince induite par transfert de spin (STT). Un dispositif STO basé sur jonction tunnel magnétique (MTJ) fournira typiquement un signal électrique de l'ordre d'une dizaine de GHz et d'une puissance de plusieurs nW. Comparés aux oscillateurs contrôlés en tension (VCO) utilisés actuellement pour la génération de microondes, les STO ont l'avantage d'être hautement accordables en fréquence. Malgré cela, les critères requis en termes de qualité de signal ne sont pas encore remplis par les STO pour être compétitifs.Deux approches existent pour améliorer la qualité du signal de sortie : (i) optimisation de l'empilement magnétique d'un dispositif STO unique et (ii) synchronisation de plusieurs STOs. C'est la deuxième approche qui a été retenue dans le cadre de cette thèse : ici nous nous intéressons à la synchronisation électrique d'un STO à une source de courant RF stabilisée, dit « injection-locking ». Le cas d'un STO à aimantation homogène, de type précession dans le plan (IPP) est étudié.En particulier, la synchronisation d'un STO à 2f, c'est-à-dire lorsque la fréquence du courant injecté est proche du double de la fréquence de génération du STO, est favorisée par rapport à la synchronisation à f. Les résultats expérimentaux obtenus par plusieurs groupes montrent à la fois une gamme de synchronisation et une réduction de largeur de raie plus prononcées à 2f qu'à f.Ce comportement singulier est examiné dans un premier temps par une étude analytique de la dynamique de l'aimantation couplée aux simulations numériques macrospin dans le but d'identifier les mécanismes de synchronisation qui prennent effet au sein du système.En effet, les modèles actuels (formalisme auto-oscillateur KTS) décrivent la synchronisation d'un STO à un courant RF sans faire de distinction entre la synchronisation à f et 2f, et les prédictions qui en découlent s'avèrent être insuffisantes pour la synchronisation à 2f. Pour combler à cela, nous mettons en évidence par extension du formalisme existant les clés du processus de synchronisation à 2f : l'ajustement de fréquence par ajustement de l'amplitude d'oscillation via la non-linéarité, ainsi que la modification du terme d'anti-damping se faisant par l'intermédiaire de la différence de phase.La caractérisation expérimentale du régime synchronisé pour un STO basé sur jonction tunnel magnétique est également détaillée dans le manuscrit. Grâce aux techniques de mesures en domaine temporel et fréquentiel développées spécialement, les grandeurs caractéristiques (gamme de synchronisation et différence de phase) du système sont extraites et comparées aux prédictions théoriques. Enfin, les effets de l'injection du courant RF sur la cohérence du signal de sortie sont discutés.