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Spintronics is showing promising results in the search for new materials and effects to reduce energy consumption in information technology. Among these materials, ferrimagnets are of special interest, since they can produce large spin currents that trigger the magnetization dynamics of adjacent layers or even their own magnetization. Here, we present a study of the generation of spin current by GdFeCo in a GdFeCo/Cu/NiFe trilayer where the FeCo sublattice magnetization is dominant at room temperature. Magnetic properties such as the saturation magnetization are deduced from magnetometry measurements while damping constant is estimated from spin-torque ferromagnetic resonance (ST-FMR). We show that the overall damping-like (DL) and field-like (FL) effective fields as well as the associated spin Hall angles can be reliably obtained by performing the dependence of ST-FMR by an added dc current. The sum of the spin Hall angles for both the spin Hall effect (SHE) and the spin anomalous Hall effect (SAHE) symmetries are: $\theta_{DL}^{SAHE} + \theta_{DL}^{SHE}=-0.15 \pm 0.05$ and $\theta_{FL}^{SAHE} + \theta_{FL}^{SHE}=0.026 \pm 0.005$. From the symmetry of ST-FMR signals we find that $\theta_{DL}^{SHE}$ is positive and dominated by the negative $\theta_{DL}^{SAHE}$. The present study paves the way for tuning the different symmetries in spin conversion in highly efficient ferrimagnetic systems., Comment: 20 pages, 4 figures

The development of approaches that can efficiently control the magnetization of magnetic materials is central to the creation of fast and low-power spintronic devices. Spin transfer torque can be used to electrically manipulate magnetic order in devices, but is typically limited to nanosecond timescales. Alternatively, spin–orbit torque can be employed, and switching with current pulses down to ~200 ps has been demonstrated. However, the upper limit to magnetization switching speed remains unestablished. Here, we show that photoconductive switches can be used to apply 6-ps-wide electrical pulses and deterministically switch the out-of-plane magnetization of a common thin cobalt film via spin–orbit torque. We probe the ultrafast magnetization dynamics due to spin–orbit torques with sub-picosecond resolution using the time-resolved magneto-optical Kerr effect (MOKE). We also estimate that the magnetization switching consumes less than 50 pJ in micrometre-sized devices. The magnetization of a cobalt thin film can be reversed by spin–orbit torques using picosecond electrical pulses that are generated by photoconductive switches.

International audience; The family of Co-based Heusler compounds contains promising candidates for spintronic applications regarding their predicted Half-Metal-Magnetic nature, ultra-low magnetic damping coefficients, high curie temperatures and tunable electronic properties. Here we focused on the transport properties of Co2MnSi thin films with thickness in the range of 4-44 nm exhibiting magnetic damping in the 10-4 range. The goals of this study are to examine the impact of the peculiar electronic band structure on the transport properties, to identify the temperature-dependent scattering process, and to extract robust conduction parameters to exploit this material in magnetoelectric devices. In order to undoubtedly correlate all results, the full study has been performed on the same series of samples. Scanning transmission electron microscopy experiments were performed to check the chemically-ordered L21 phase in our films, and also allowed us to identify misfit dislocations generated at the interface with the substrate. The variation of the resistivity with film thickness was measured at different temperatures. The results are examined under the Fuchs and Sondheimer model which allowed us to extract the electron mean free path in Co2MnSi in the temperature range 5-300 K. Values for the residual resistivity, Debye temperature, and distance between the Fermi energy and the conduction band for minority spins were obtained from the fit of the resistivity versus temperatures curves. A negative AMR ratio was measured for all the samples which confirmed the Half-metallic nature of the Co2MnSi films. The determination of the ordinary Hall coefficient alloys allowed us to extract the carrier concentration and carrier mobility and their dependency on the temperature. Finally, scaling of the anomalous Hall coefficient with the longitudinal resistivity was performed indicating that skew scattering is the dominant temperature-dependent scattering mechanism in our films.

International audience; The low power manipulation of magnetization is currently a highly sought-after objective in spintronics. Non ferromagnetic large spin-orbit coupling heavy metal (NM) / ferromagnet (FM) heterostructures offer interesting elements of response to this issue, by granting the manipulation of the FM magnetization by the NM spin Hall effect (SHE) generated spin current. Additional functionalities, such as the electric field control of the spin current generation, can be offered using multifunctional ferromagnets. We have studied the spin current transfer processes between Pt and the multifunctional magnetoelectric Ga0.6Fe1.4O3 (GFO). In particular, via angular dependent magnetotransport measurements, we were able to differentiate between magnetic proximity effect (MPE)-induced anisotropic magnetoresistance (AMR) and spin Hall magnetoresistance (SMR). Our analysis shows that SMR is the dominant phenomenon at all temperatures and is the only one to be considered near room temperature, with a magnitude comparable to those observed in Pd/YIG or Pt/YIG heterostructures. These results indicate that magnetoelectric GFO thin films show promises for achieving an electric-field control of the spin current generation in NM/FM oxide-based heterostructures.

Switching of perpendicular magnetization via spin–orbit torque (SOT) is of particular interest in the development of non-volatile magnetic random access memory (MRAM) devices. We studied current-induced magnetization switching of Ir/GdFeCo/Cu/Pt heterostructures in a Hall cross geometry as a function of the in-plane applied magnetic field. Remarkably, magnetization switching is observed at zero applied field. This is shown to result from the competition between SOT, the Oersted field generated by the charge current, and the material's coercivity. Our results show a means of achieving zero-field switching that can impact the design of future spintronics devices, such as SOT-MRAM.

We present an experimental study of vortex dynamics in magnetic nanocontacts based on pseudo spin valves comprising the ${\mathrm{Co}}_{2}\mathrm{MnGe}$ Heusler compound. The films were grown by molecular beam epitaxy, where precise stoichiometry control and tailored stacking order allowed us to define the bottom ferromagnetic layer as the reference layer, with minimal coupling between the free and reference layers. 20-nm diameter nanocontacts were fabricated using a nanoindentation technique, leading to self-sustained gyration of the vortex generated by spin-transfer torques above a certain current threshold. By combining frequency- and time-domain measurements, we show that different types of spin-transfer induced dynamics related to different modes associated to the magnetic vortex configuration can be observed, such as mode hopping, mode coexistence, and mode extinction appearing in addition to the usual gyration mode.

Spintronics exploits spin-orbit coupling (SOC) to generate spin currents, spin torques, and, in the absence of inversion symmetry, Rashba, and Dzyaloshinskii-Moriya interactions (DMI). The widely used magnetic materials, based on 3d metals such as Fe and Co, possess a small SOC. To circumvent this shortcoming, the common practice has been to utilize the large SOC of nonmagnetic layers of 5d heavy metals (HMs), such as Pt, to generate spin currents by Spin Hall Effect (SHE) and, in turn, exert spin torques on the magnetic layers. Here, we introduce a new class of material architectures, excluding nonmagnetic 5d HMs, for high-performance spintronics operations. We demonstrate very strong current-induced torques exerted on single GdFeCo layers due to the combination of large SOC of the Gd 5d states, and inversion symmetry breaking mainly engineered by interfaces. These "self-torques" are enhanced around the magnetization compensation temperature (close to room temperature) and can be tuned by adjusting the spin absorption outside the GdFeCo layer. In other measurements, we determine the very large emission of spin current from GdFeCo. This material platform opens new perspectives to exert "self-torques" on single magnetic layers as well as to generate spin currents from a magnetic layer., Comment: 26 pages, 4 figures plus 5 pages of sup. information

International audience; Spin Hall and Rashba–Edelstein effects, which are spin-to-charge conversion phenomena due to spin-orbit coupling (SOC), are attractingincreasing interest as pathways to manage rapidly and at low consumption cost the storage and processing of a large amount of data inspintronic devices as well as more efficient energy harvesting by spin-caloritronics devices. Materials with large SOC, such as heavy metals(HMs), are traditionally employed to get large spin-to-charge conversion. More recently, the use of graphene (gr) in proximity with largeSOC layers has been proposed as an efficient and tunable spin transport channel. Here, we explore the role of a graphene monolayer betweenCo and a HM and its interfacial spin transport properties by means of thermo-spin measurements. The gr/HM (Pt and Ta) stacks have beenprepared on epitaxial Ir(111)/Co(111) structures grown on sapphire crystals, in which the spin detector (i.e., top HM) and the spin injector(i.e., Co) are all grown in situ under controlled conditions and present clean and sharp interfaces. We find that a gr monolayer retains the spincurrent injected into the HM from the bottom Co layer. This has been observed by detecting a net reduction in the sum of the spin Seebeckand interfacial contributions due to the presence of gr and independent from the spin Hall angle sign of the HM used.Invited Paper published as part of the special topic on Emerging Materials for Spin-Charge Interconversion

This work aims to present a multifunctional surface acoustic wave (SAW) device that is capable of simultaneously measuring magnetic field as well as temperature. This is achieved in three parts where a multilayered approach is used to first achieve a temperature compensated Love wave structure, followed by a micro-structuration of the sensitive layer to eliminate the effects of temperature on magnetic anisotropy and finally multiple resonances observed allow a multifunctionality in our device.

International audience; Magnetic Surface Acoustic Wave (MSAW) devices combine a magnetostrictive material with a piezoelectric material to enable the sensing/measurement of magnetic fields by electrically generated SAWs. In such systems, the response is strongly dependent on the field direction with respect to the magnetic anisotropy of the magnetic material, which could give rise to detrimental hysteretic behaviour. The challenge in the current study is to control the shape of MSAW responses and remove the hysteresis in the magneto-acoustic response. We report especially on the magnetic response of a MSAW device with perpendicular magnetic anisotropy, rarely reported in literature. On the basis of the MSAW response with in-plane magnetic anisotropy studied previously, the MSAW signal with out-of-plane magnetization could be understood for all the field configurations. The final response is hysteresis free, which is suitable for interrogation of out of the plane magnetic fields. Such a technology could therefore complete the "in-plane" sensing technology to build up 3D sensors.

A multifunctional sensor based on a surface-acoustic-wave (SAW) device is fabricated. It allows independent measurements of temperature and applied magnetic field. Optimization of the multilayered device structure leads to a temperature-coefficient frequency of the Love-wave resonance reduced to zero. The sensitivity to an applied magnetic field is obtained through magnetostriction of a $\mathrm{Co}$-$\mathrm{Fe}$-$\mathrm{B}$ layer. By use of shape anisotropy, the variation of intrinsic magnetic anisotropy with temperature is strongly reduced. On one hand, interrogating the device at the Love-wave-resonance frequency allows us to extract the applied magnetic field independently of the temperature in a [130--370 K] range. On the other hand, the Rayleigh or Leaky waves are less or not sensitive to applied fields but have a high temperature coefficient of frequency. So interrogating the device at the Rayleigh resonance frequency allows us to extract the temperature independently of the magnetic field. In addition, the used resonator geometry offers the possibility for future batteryless and wireless interrogation.

International audience; A temperature compensated magnetic field sensor based on the combination of CoFeB ferromagnetic thin films and Quartz/ZnO Love waveguide platform is developed and optimized. The Love wave is a shear horizontal guided wave and therefore provides an optimal interaction with magnetisation in the magneto-elastic thin film resulting in higher acoustic wave magneto-elastic coupling compared to the conventional Rayleigh wave based devices. ST-cut Quartz was chosen as substrate, ZnO as insulating layer for Love wave generation and temperature coefficient of frequency (TCF) compensation and CoFeB as the magnetostrictive layer sensitive to magnetic field. Experimental results show a magneto-acoustic sensitivity of 15.53 MHz/T with almost zero TCF.

Iridium is a very promising material for spintronic applications due to its interesting magnetic properties such as large RKKY exchange coupling as well as its large spin-orbit coupling value. Ir is for instance used as a spacer layer for perpendicular synthetic antiferromagnetic or ferrimagnet systems. However, only a few studies of the spintronic parameters of this material have been reported. In this paper, we present inverse spin Hall effect - spin pumping ferromagnetic resonance measurements on CoFeB/Ir based bilayers to estimate the values of the effective spin Hall angle, the spin diffusion length within iridium, and the spin mixing conductance in the CoFeB/Ir bilayer. In order to have reliable results, we performed the same experiments on CoFeB/Pt bilayers, which behavior is well known due to numerous reported studies. Our experimental results show that the spin diffusion length within iridium is 1.3 nm for resistivity of 250 n$\Omega$.m, the spin mixing conductance $g_{eff}^{\uparrow \downarrow}$ of the CoFeB/Ir interface is 30 nm$^{-2}$, and the spin Hall angle of iridium has the same sign than the one of platinum and is evaluated at 26% of the one of platinum. The value of the spin Hall angle found is 7.7% for Pt and 2% for Ir. These relevant parameters shall be useful to consider Ir in new concepts and devices combining spin-orbit torque and spin-transfer torque., Comment: 8 pages, 4 figures

Reducing energy dissipation while increasing speed in computation and memory is a long-standing challenge for spintronics research. In the last 20 years, femtosecond lasers have emerged as a tool to control the magnetization in specific magnetic materials at the picosecond timescale. However, the use of ultrafast optics in integrated circuits and memories would require a major paradigm shift. An ultrafast electrical control of the magnetization is far preferable for integrated systems. Here we demonstrate reliable and deterministic control of the out-of-plane magnetization of a 1 nm-thick Co layer with single 6 ps-wide electrical pulses that induce spin-orbit torques on the magnetization. We can monitor the ultrafast magnetization dynamics due to the spin-orbit torques on sub-picosecond timescales, thus far accessible only by numerical simulations. Due to the short duration of our pulses, we enter a counter-intuitive regime of switching where heat dissipation assists the reversal. Moreover, we estimate a low energy cost to switch the magnetization, projecting to below 1fJ for a (20 nm)^3 cell. These experiments prove that spintronic phenomena can be exploited on picosecond time-scales for full magnetic control and should launch a new regime of ultrafast spin torque studies and applications., Includes article + supplementary information. Latest version uses full name of the first author. Nature Electronics (2020)

International audience; A one-port surface acoustic wave (SAW) resonator based on Co40–Fe40–B20/SiO2/ZnO/quartz multilayer structure and exhibiting a dual mode, Rayleigh and Love wave modes, is investigated to achieve a multifunctional sensor measuring both temperatures and magnetic fields. The Rayleigh wave mode of the resonator is used for temperature measurement with a temperature sensitivity of −37.9 ppm/°C, and the Love wave mode is used for magnetic field measurement. Co40-Fe40-B20 is the magnetic sensitive layer, and quartz crystal is the piezoelectric substrate. ZnO film is also a piezoelectric but considered here in combination with SiO2 as insulating layers and serves to control impedance matching and temperature dependence of the sensor. ZnO and SiO2 thicknesses are selected to realize temperature compensation for the Love wave mode and making this mode highly sensitive to magnetic fields and insensitive to temperatures. The magnetic field sensitivities of −170.4 kHz m−1 T−1 and −621.6 kHz m−1 T−1 are obtained respectively for the fundamental and the third harmonic of the Love wave mode. The proposed structure is beneficial to design reliable hybrid SAW magnetic field and temperature sensors

International audience; The development of novel on-chip device techniques is attracting more and more interest because of the increasing demand for communication electronics, wearable devices and Internet of Things (IoT) with features of low power consumption, high frequency-response, small size, fast transmission rate, and low-costs in the lab-on-chip field. This letter presents the high frequency performance enhancement of on-chip inductors by the use of a back hollow structure filled with CoFeB/ZnO/CoFeB thin ferromagnetic layers. The magnetization dynamic response of this ferromagnetic stacks deposited by RF-magnetron sputtering were investigated. The inductance increases by 41.2%–70.6% between 0.1 and 8 GHz reaching 70.6% at 6.9 GHz. Q-factor increases also in a range of 3% to 18% between 0.1 and 3.8 GHz and reach 18% at 1.5 GHz. The equivalent circuit model and simplified physical model of the individual inductor were established and used to model and describe the parameters of inductor, such as the inductance and Q-factor as function of frequency. The results show the potential for application of the back hollow structure inductors with ferromagnetic thin film in RF circuits.

Recent demonstration of magnetization manipulation has been focused on the utilization of pure spin current converted by charge current in nonmagnetic materials with strong spin-orbit coupling (SOC), which stimulates intensive studies on the exploration of materials with larger SOC, such as heavy metals and topological insulators. We suggest an alternative approach for enhancing the effective charge-to-spin conversion efficiency by applying strain in Ta/Fe/Pt films grown on flexible mica substrates. We experimentally demonstrate a large and tunable charge-to-spin conversion efficiency in Ta/Fe/Pt films by applying compressive strain within a flexible substrate, and over 50% enhancement of effective spin Hall angle eff SHE of up to approximately 0.2 is achieved in a 6.26‰ strained film. Our findings may spur further work on the integration of flexible electronics and SOC and may potentially lead to the innovation of alternative flexible spintronics devices.

International audience; Spin-polarization and magnetic damping are measured for several polycrystalline films with each of them being made of a different single Co$_2$ Mn-based Heusler compound. As several epitaxial Co$_2$ Mn-based Heusler compounds are shown to be half-metal magnetic materials with full spin-polarization and ultralow magnetic damping, we explore here these properties but in polycrystalline films. Co$_2$ MnSi, Co$_2$ MnGe, and Co$_2$ MnGa thin films were grown on glass substrates and analyzed in situ by electron diffraction and spin-resolved photoemission and ex situ by transmission electron microscopy and ferromagnetic resonance. Despite the polycrystalline state of the films, they still exhibit high spin polarizations and very low magnetic damping coefficients. The latter are at least of the same order as the best epitaxial films using regular ferromagnetic materials. The key point to achieve such properties is to control the Heusler stoichiometry as best as possible.

International audience; We investigate spin-wave resonance modes and their damping in insulating thin films of bismuth-substituted yttrium iron garnet by performing femtosecond magneto-optical pump-probe experiments. For large magnetic fields in the range below the magnetization saturation, we find that the damping of high-order standing spin-wave (SSW) modes is about 40 times lower than that for the fundamental one. The observed phenomenon can be explained by considering different features of magnetic anisotropy and exchange fields that, respectively, define the precession frequency for fundamental and high-order SSWs. These results provide further insight into SSWs in iron garnets and may be exploited in many new photomagnonic devices.

We present an experimental study of spin-torque driven vortex self-oscillations in magnetic nanocontacts. We find that above a certain threshold in applied currents, the vortex gyration around the nanocontact is modulated by relaxation oscillations, which involve periodic reversals of the vortex core. This modulation leads to the appearance of commensurate but also more interestingly here, incommensurate states, which are characterized by devil's staircases in the modulation frequency. We use frequency- and time-domain measurements together with advanced time-series analyses to provide experimental evidence of chaos in incommensurate states of vortex oscillations, in agreement with theoretical predictions., Comment: 5 pages, 4 figures

The reduction of magnetic damping is one of the biggest challenges in low-energy-consumption spintronics and magnonics, in the pursuit of $e.g.$ low switching current for spin-transfer-torque-based technology, and long-range spin-wave propagation. This experimental study highlights ultralow damping values in high-quality epitaxial Co${}_{2}$Mn$Z$ ($Z$ = Al, Si, Ga, Ge, Sn, Sb) Heusler half-metallic magnets. As predicted theoretically, these ultralow values are intrinsically coupled to the underlying electronic structure. The width of the spin gap, as well as the location of the Fermi energy within it, play key roles in the relaxation of precessing magnetization.

International audience; We demonstrate that femtosecond laser pulses allow triggering high-frequency standing spin-wave modes in nanoscale thin films of a bismuth-substituted yttrium iron garnet. By varying the strength of the external magnetic field, we prove that two distinct branches of the dispersion relation are excited for all the modes. This is reflected in particular at a very weak magnetic field (∼33 mT) by a spin dynamics with a frequency up to 15 GHz, which is 15 times higher than the one associated with the ferromagnetic resonance mode. We argue that this phenomenon is triggered by ultrafast changes of the magnetic anisotropy via laser excitation of incoherent and coherent phonons. These findings open exciting prospects for ultrafast photo magnonics. The continuous demand for more energy efficient and faster data transport and processing devices has triggered intense research activity to carry information with other means than the electron charge, which is associated with an inevitable Joule heating in current semiconductor-based information technologies [1]. One of the most promising ways to achieve this goal is the use of the coherent collective excitation of spins in magnetic materials, commonly known as magnons or spin waves (SWs) [2-5]. These waves can propagate in both metallic and dielectric magnetic media without involving any charge transport, which avoids the Joule heating, and therefore reduces substantially the power consumption for data processing. Besides this energy efficiency, using SWs offers other important advantages for ultrafast nanoscale computation due to their broad high frequency range from GHz to THz and tunable wavelength down to the nanoscale [6]. From a fundamental point of view, SWs exhibit remarkably rich physics involving both dipolar and exchange interactions [6,7]. This is reflected by versatile dispersion relations containing magnetostatic and exchange modes, which can either be tuned by an external magnetic field (H ext) and/or the sample size [3,6]. In this context, intense research is being carried out to understand the generation, propagation, manipulation, and detection of SWs in a continuously growing field of modern magnetism called magnonics [2-5]. Because of its low magnetic damping constant, the yttrium iron garnet (YIG) has attracted a lot of attention in the field of magnonics [8]. Indeed, many important magnetostatic SWs based devices have been realized using YIG during the last decade [9-12]. However, due to the small saturation magnetization of YIG together with the restrictions imposed by the size of the microwave antennas on the excited SWs wavelength with the standard method of microwave magnetic fields, magnetostatic SWs frequencies in such devices did not exceed a few GHz at low field, which is a major limiting factor for ultrafast applications. On the other hand, due to the strong exchange interaction between spins, the exchange SWs mode can have higher frequencies compared to the magnetostatic one. The possibility of triggering such exchange modes requires a nonuniform dynamical field across the magnetic film thickness [13], which is very challenging to induce using microwave antennas [13-15]. Recently, femtosecond laser pulses have been used as an efficient stimulus to excite a coherent spin precession [16,17]. In particular, it was demonstrated that femtosecond laser pulses can excite exchange standing SWs (SSWs) in metallic [18,19] and semiconductor [20,21] ferromagnets via thermal processes. On the other hand, most studies on iron garnets have been dedicated to the excitation and control of the homogenous resonance mode (k ¼ 0, i.e., low frequency) via nonthermal excitation mechanisms [22-29]. An important question in this context concerns the possibility to take advantage of femtosecond laser pulses for triggering a high frequency SSW in dielectric thin films of iron garnets. In this Letter, we demonstrate femtosecond laser-excited high frequency even and odd SSWs in nanoscale films of Bi-substituted yttrium iron garnet (Bi-YIG). Bi-YIG materials have, in addition to a low damping [30], very large magneto-optical Faraday effects [31], which make them well adapted for new photo magnonics devices. By varying the strength of the external applied magnetic field H ext , we demonstrate two distinct branches of the dispersion relation PHYSICAL REVIEW LETTERS 123, 027202 (2019) 0031-9007=19=123(2)=027202(6) 027202-1

International audience; Magnetization dynamics is of great interest in the aim of using spins in nanoscale information technology, which ultimately should reach the atomic size. In the present work, we explore magnetization and spin dynamics in atomic ferromagnetic contacts both experimentally and theoretically. We demonstrate that domain walls induce a giant rectification effect as the DC voltages measured across the contacts are greatly enhanced by the presence of a domain wall. This effect is understood using multiscale dynamic simulations showing that the atomic sized walls oscillate, both in position and size, when submitted to the radio-frequency excitation. This leads to an increase by three orders of magnitude due to the large atomic scale spin excursion at resonance in the presence of an atomic sized domain wall. Beside the interesting amplified rectification, this could also be used as a unique tool to measure dynamical properties at the atomic scale.

International audience; Due to the difficulty of growing high-quality semiconductors on ferromagnetic metals, the study of spin diffusion transport in Si was limited to lateral geometry devices. In this work, by using an ultrahigh-vacuum wafer-bonding technique, we have successfully fabricated metal–semiconductor–metal CoFeB/MgO/Si/Pt vertical structures. We hereby demonstrate pure spin-current injection and transport in the perpendicular current flow geometry over a distance larger than 2 μm in n-type Si at room temperature. In those experiments, a pure propagating spin current is generated via ferromagnetic resonance spin pumping and converted into a measurable voltage by using the inverse spin Hall effect occurring in the top Pt layer. A systematic study varying both Si and MgO thicknesses reveals the important role played by the localized states at the MgO–Si interface for the spin-current generation. Proximity effects involving indirect exchange interactions between the ferromagnet and the MgO–Si interface states appears to be a prerequisite to establishing the necessary out-of-equilibrium spin population in Si under the spin-pumping action.

We have conducted experiments to probe how the dynamics of nanocontact vortex oscillators can be modulated by an external signal. We explore the phase-locking properties in both the commensurate and chaotic regimes, where chaos appears to impede phase-locking while a more standard behavior is seen in the commensurate phase. These different regimes correspond to how the periodicity of the vortex core reversal relates to the frequency of core gyration around the nanocontact; a commensurate phase appears when the reversal rate is an integer fraction of the gyration frequency, while a chaotic state appears when this ratio is irrational. External modulation where the power spectral density exhibits rich features, appears due to the modulation between the external source frequency, gyration frequency, and core reversal frequency. We explain these features with first- or second-order modulation between the three frequencies. Phase-locking is also visible between the external source frequency and internal vortex modes (gyration and core reversal modes)., Comment: 11 pages, 7 figures

We study the influence of a second order magnetic anisotropy on magnetization reversal by spin transfer torque in perpendicularly magnetized magnetic tunnel junctions (pMTJs). Using a macrospin model to describe the dynamics of the free layer, analytical solutions for the switching voltage and the voltage threshold for precession are determined as a function of the first and second order magnetic anisotropies. To compare the spin-transfer-torque energy efficiency to that of a classical pMTJ, a junction without the second order anisotropy term, we compare these cases at a fixed energy barrier to thermally activated reversal. We show that the critical voltage for switching can be reduced by a factor 0.7 when the ratio of the second to the first order magnetic anisotropy is 1/3. Importantly, the switching time can be reduced by nearly a factor of two for this magnetic anisotropy ratio. These results highlight an important and practical method to increase the spin-torque efficiency, while reducing the energy dissipation and switching time in magnetic random access memory devices.We study the influence of a second order magnetic anisotropy on magnetization reversal by spin transfer torque in perpendicularly magnetized magnetic tunnel junctions (pMTJs). Using a macrospin model to describe the dynamics of the free layer, analytical solutions for the switching voltage and the voltage threshold for precession are determined as a function of the first and second order magnetic anisotropies. To compare the spin-transfer-torque energy efficiency to that of a classical pMTJ, a junction without the second order anisotropy term, we compare these cases at a fixed energy barrier to thermally activated reversal. We show that the critical voltage for switching can be reduced by a factor 0.7 when the ratio of the second to the first order magnetic anisotropy is 1/3. Importantly, the switching time can be reduced by nearly a factor of two for this magnetic anisotropy ratio. These results highlight an important and practical method to increase the spin-torque efficiency, while reducing the energy ...

Harnessing chaos or intrinsic nonlinear behaviours from dynamical systems is a promising avenue for the development of unconventional information processing technologies. However, the exploitation of such features in spintronic devices has not been attempted despite the many theoretical and experimental evidence of nonlinear behaviour of the magnetization dynamics in nanomagnetic systems. Here, we propose a first step in that direction by unveiling and characterizing the patterns and symbolic dynamics originating from the nonlinear chaotic time-resolved electrical signals generated experimentally by a nanocontact vortex oscillator (NCVO). We use advanced filtering methods to dissociate nonlinear deterministic patterns from thermal fluctuations and show that the emergence of chaos results in the unpredictable alternation of simple oscillatory patterns controlled by the NCVO's core-polarity switching. With phase-space reconstruction techniques, we perform a symbolic analysis of the time series to assess the level of complexity and entropy generated in the chaotic regime. We find that at the centre of its incommensurate region, it can exhibit maximal entropy and complexity. This suggests that NCVOs are promising nonlinear nanoscale source of entropy that could be harnessed for information processing., Comment: 16 pages, 10 figures

In the above article [1] , by Yang et al. , published in IEEE Sensors Journal, volume 20, issue 19, pp. 11292–11301, in 2020, we would like to correct the coauthor Daniel Lacour’s biography on page 11300.

Heusler magnetic alloys offer a wide variety of electronic properties very promising for spintronics and magnonics. Some alloys exhibit a spin gap in their band structure at the Fermi energy, the so-called half-metal magnetic (HMM) behavior. This particular property leads to two very interesting properties for spintronics, i.e., fully polarized current together with ultra-low magnetic damping, two key points for spin-transfer-torque based devices. This Tutorial gives experimental details to grow and characterize Heusler Co2MnZ compounds in thin films (Z = Al, Si, Ga, Ge, Sn, Sb) by using molecular beam epitaxy in order to get the proper predicted electronic properties. A first part of this Tutorial is dedicated to control the stoichiometry as best as possible with some methods to test it. The chemical ordering within the lattice was examined by using electron diffraction during growth, regular x-ray diffraction, and scanning transmission electron microscopy. In particular, standard x-ray diffraction is carefully analyzed depending on the chemical ordering in the cubic cell and shown to be inefficient to distinguish several possible phases, on the contrary to electron microscopy. The electronic properties, i.e., magnetic moment, spin polarization, and magnetic damping were reviewed and discussed according to the stoichiometry of the films and also theoretical predictions. Polycrystalline films were also analyzed, and we show that the peculiar HMM properties are not destroyed, a good news for applications. A clear correlation between the spin polarization and the magnetic damping is experimentally demonstrated. At least, our study highlights the major role of stoichiometry on the expected properties.

Engineering of magnetic materials for developing better spintronic applications relies on the control of two key parameters: the spin polarization and the Gilbert damping, responsible for the spin angular momentum dissipation. Both of them are expected to affect the ultrafast magnetization dynamics occurring on the femtosecond timescale. Here, engineered Co2 MnAlx Si1- x Heusler compounds are used to adjust the degree of spin polarization at the Fermi energy, P, from 60% to 100% and to investigate how they correlate with the damping. It is experimentally demonstrated that the damping decreases when increasing the spin polarization from 1.1 × 10-3 for Co2 MnAl with 63% spin polarization to an ultralow value of 4.6 × 10-4 for the half-metallic ferromagnet Co2 MnSi. This allows the investigation of the relation between these two parameters and the ultrafast demagnetization time characterizing the loss of magnetization occurring after femtosecond laser pulse excitation. The demagnetization time is observed to be inversely proportional to 1 - P and, as a consequence, to the magnetic damping, which can be attributed to the similarity of the spin angular momentum dissipation processes responsible for these two effects. Altogether, the high-quality Heusler compounds allow control over the band structure and therefore the channel for spin angular momentum dissipation.

Molybdenum disulfide has recently emerged as a promising two-dimensional semiconducting material for nano-electronic, opto-electronic and spintronic applications. However, the demonstration of an electron spin transport through a semiconducting MoS2 channel remains challenging. Here we show the evidence of the electrical spin injection and detection in the conduction band of a multilayer MoS2 semiconducting channel using a two-terminal spin-valve configuration geometry. A magnetoresistance around 1% has been observed through a 450 nm long, 6 monolayer thick MoS2 channel with a Co/MgO tunnelling spin injector and detector [1]. It is found that keeping a good balance between the interface resistance and channel resistance is mandatory for the observation of the two-terminal magnetoresistance. Moreover, the electron spin-relaxation is found to be greatly suppressed in the multilayer MoS2 channel with an in-plane spin polarization. The long spin diffusion length (approximately 235 nm) could open a new avenue for spintronic applications using multilayer transition metal dichalcogenides. In this talk, we will present the main issues of the spin-injection problem at ferromagnet/Tunnel barrier/MoS2 interfaces as well as the spin-propagation in multilayered channel like played by MoS2 involving Schottky depletion layers giving thus an extension to earlier modeling of spin-injection in ferromagnet/ssource/ferromagnet systems [2-3]. [1] Shiheng Liang et al., Nat. Comm. 8, 14947 (2017) [2] A. Fert, H. Jaffres, Phys Rev B64, 184420, 2001 [3] A. Fert, J.-M. George, H. Jaffres, R. Mattana, IEEE, 54, 921-932 (2007)

International audience; Aftereffect measurements have been performed on synthetic ferrimagnets showing strong perpendicular magnetic anisotropy, namely (Co/Pt)/Ir/(Co/Pt), by measuring the magnetization of the sample as a function of time for several different applied magnetic fields. Unexpected magnetic relaxation has been observed. Indeed, for some particular applied magnetic fields, the magnetization as a function of time first plummets and then increases. This nonmonotonic aftereffect can be understood by considering the possible magnetic states and the transitions between those states. This understanding has been confirmed and detailed using magneto-optical Kerr effect (MOKE) microscopy. Indeed, the measurements show nucleation and propagation of a transient metastable magnetic configuration. Furthermore, we were able to obtain a good qualitative agreement between a simple one-dimensional model and the experimental observation. We could strongly support the hypothesis that this peculiar nonmonotonic behavior could be a very general feature that should be observed in any antiferromagnetically coupled system (synthetic ferrimagnets, synthetic antiferromagnets) with perpendicular magnetic anisotropy.

International audience; The Nano-Contact Vortex Oscillator (NCVO) is a highly nonlinear spintronic device that can depict chaotic and nonchaotic behaviors according to the current flowing through it. The potential use of such a device in the future-generation computing systems requires the knowledge of a realistic model capable of describing its exact dynamics. In this paper, we firstly investigate the behavior of NCVO based on the power spectral analysis. Furthermore, we propose and demonstrate two efficient approaches of reservoir computing for the modeling of such a device. The performances of the proposed models are addressed in two ways. First, the generated time-varying signals are compared with the simulated magnetizations of the NCVO at different operating currents. Then, the power spectral analysis of one of the two models is carried out to examine its overall behavior over the complete DC current operating range and its ability to diagnose chaotic and non-chaotic regimes. The proposed models show quite promising results that can be counted on for further research.

This communication describes an experimental study of multilayer piezo-magnetic acoustic wave devices used as sensors of external magnetic fields applied in plane, parallel or perpendicular to the direction of the acoustic wave propagation direction. The considered device consists of a layered structure Mag/ZnO/IDT/LNY-128, Mag being the sensitive magnetoelastic layer. Two magnetoelastic films are investigated as sensitive layers: Ni and CoFeB. Their sensitivities to the magnetic field intensity and direction are measured and compared.

International audience; The state diagram of a magnetic tunnel junction with perpendicularly magnetized electrodes in the presence of spin-transfer torques is computed in a macrospin approximation using a power dissipation model. Starting from the macrospin's energy we determine the stability of energy extremum in terms of power received and dissipated, allowing the consideration of non-conservative torques associated with spin transfer and damping. The results are shown to be in agreement with those obtained by direct integration of the Landau-Lifshitz-Gilbert-Slonczewski equation. However, the power dissipation model approach is faster and shows the reason certain magnetic states are stable, such as states that are energy maxima but are stabilized by spin transfer torque. Breaking the axial system, such as by a tilted applied field or tilted anisotropy, is shown to dramatically affect the state diagrams. Finally, the influence of a higher order uniaxial anisotropy that can stabilize a canted magnetization state is considered and the results are compared to experimental data.

Molybdenum disulfide has recently emerged as a promising two-dimensional semiconducting material for nano-electronic, opto-electronic and spintronic applications. However, the demonstration of an electron spin transport through a semiconducting MoS2 channel remains challenging. Here we show the evidence of the electrical spin injection and detection in the conduction band of a multilayer MoS2 semiconducting channel using a two-terminal spin-valve configuration geometry. A magnetoresistance around 1% has been observed through a 450 nm long, 6 monolayer thick MoS2 channel with a Co/MgO tunnelling spin injector and detector. It is found that keeping a good balance between the interface resistance and channel resistance is mandatory for the observation of the two-terminal magnetoresistance. Moreover, the electron spin-relaxation is found to be greatly suppressed in the multilayer MoS2 channel with an in-plane spin polarization. The long spin diffusion length (approximately ∼235 nm) could open a new avenue for spintronic applications using multilayer transition metal dichalcogenides., MoS2 is a promising two-dimensional candidate for opto-electronic and spintronic applications. Here, the authors report electrical spin injection and detection in a few-layered MoS2 channel, demonstrating that the spin diffusion length is at least 235 nm in MoS2 conduction band.

In a context where sub-micrometric magnetic dots are foreseen to play an active role in various new breeds of electronics components such as magnetic memories, magnetic logics or bio-sensors, the use of micromagnetic simulations to optimize their shapes and spatial arrangement with respect to a chosen application has become unavoidable. Prior realizing experimentally magnetic dots presenting four stable magnetic states (4SMS), we performed a micromagnetic study to select a design providing not only four equivalent magnetic states in a single dot but also exhibiting mostly uniform magnetic states.

We have studied a magnetic tunnel transistor (MTT) structure based on a MgO tunnelling barrier emitter and a [Co/Ni]5/Cu multilayer base on a Si (0 0 1) substrate. Evident links between the Schottky barrier preparation techniques and the properties of perpendicular magnetic anisotropy (PMA) in the [Co/Ni] multilayer have been revealed by combined x-ray diffraction and magnetometry analyses. The Si surface treated by hydrofluoric acid (HF) is found to favour a Cu [1 0 0] texture growth which is detrimental to the [Co/Ni]5 PMA properties. However, a Ta layer insertion can restore the [1 1 1] texture required for the PMA appearance. By carefully engineering the base crystallographic texture structure, we obtain both a good quality of Schottky barrier and PMA property; a magneto-current ratio of 162% has been measured for MTTs with a spin-valve base composed of one magnetic layer having in-plane anisotropy and another one with out-of-plane anisotropy.

International audience; Remanent electrical spin injection into an InGaAs/GaAs based quantum well light emitting diode is realized by using a perpendicularly magnetized MgO/CoFeB/Ta/CoFeB/MgO spin injector. We demonstrate that the Ta interlayer plays an important role to establish the perpendicular magnetic anisotropy and the thickness of Ta interlayer determines the type of exchange coupling between the two adjacent CoFeB layers. They are ferromagnetically or antiferromagnetically coupled for a Ta thickness of 0.5 nm or 0.75 nm, respectively. A circular polarized electroluminescence (Pc) of about 10% is obtained at low temperature and at zero magnetic field. The direction of the electrically injected spins is determined only by the orientation of the magnetization of the bottom CoFeB layer which is adjacent to the MgO/GaAs interface. This work proves the critical role of the bottom CoFeB/MgO interface on the spin-injection and paves the way for the electrical control of spin injection via magnetic tunnel junction-type spin injector.

International audience; This study describes the numerical implementation of accurate and fully coupled physical models in order to investigate the sensitivity of Surface Acoustic Wave (SAW) devices using the magnetoelas-tic interaction with an external magnetic field. The model was first validated using experimental data previously published by Kadota et al. obtained with SAW resonators based on quartz substrates and nickel Inter Digital Transducers (IDTs). The model was then used to optimize the geometry of a new magnetostrictive-piezoelectric layered structure (Ni/ZnO/IDT/LiNbO 3), regarding its sensitivity to the magnetic field intensity. The optimized structure was designed and fabricated and experimental results show a good correlation with the numerical modeling. Simulations also show that if alumina is used instead of ZnO, the Ni/Al 2 O 3 /IDT/LiNbO 3 structure exhibits a sensitivity that is 9 times higher than the one based on ZnO.

Recently, the study of interactions between electron spins and heat currents has given rise to the field of “Spin Caloritronics”. Experimental studies of these interactions have shown a possibility to combine the use of heat and light to power magnetic tunnel junction (MTJ) devices. Here we present a careful study of an MTJ device on Si substrate that can be powered entirely by light. We analyze the influence of the material properties, device geometry, and laser characteristics on the electric response of the sample. We demonstrate that by engineering the MTJ and its electrical contact, a large photovoltage reaching 100 mV can be generated. This voltage originates from the Si substrate and depends on the MTJ magnetic configuration. Finally, we discuss the origin of the photo-voltage in terms of Seebeck and photovoltaic effects.

Magnetization manipulation in the absence of an external magnetic field is a topic of great interest, since many novel physical phenomena need to be understood and promising new applications can be imagined. Cutting-edge experiments have shown the capability to switch the magnetization of magnetic thin films using ultrashort polarized laser pulses. In 2007, it was first observed that the magnetization switching for GdFeCo alloy thin films was helicity-dependent and later helicity-independent switching was also demonstrated on the same material. Recently, all-optical switching has also been discovered for a much larger variety of magnetic materials (ferrimagnetic, ferromagnetic films and granular nanostructures), where the theoretical models explaining the switching in GdFeCo films do not appear to apply, thus questioning the uniqueness of the microscopic origin of all-optical switching. Here, we show that two different all-optical switching mechanisms can be distinguished; a "single pulse" switching and a "cumulative" switching process whose rich microscopic origin is discussed. We demonstrate that the latter is a two-step mechanism; a heat-driven demagnetization followed by a helicity-dependent remagnetization. This is achieved by an all-electrical and time-dependent investigation of the all-optical switching in ferrimagnetic and ferromagnetic Hall crosses via the anomalous Hall effect, enabling to probe the all-optical switching on different timescales., Comment: 1 page, LaTeX; classified reference numbers