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International audience; Perpendicular shape anisotropy (PSA) offers a practical solution to downscale spin-transfer torque magnetoresistive random-access memory (STT-MRAM) beyond the sub-20 nm technology node while retaining thermal stability. However, our understanding of the thermomagnetic behavior of PSA-STT-MRAM is often indirect, relying on magnetoresistance measurements and micromagnetic modeling. Here, the magnetism of a NiFe PSA-STT-MRAM nanopillar is investigated using off-axis electron holography, providing spatially resolved magnetic information as a function of temperature. Magnetic induction maps reveal the micromagnetic configuration of the NiFe storage layer (∼60 nm high, ≤20 nm diameter), confirming the PSA induced by its 3:1 aspect ratio. In situ heating demonstrates that the PSA of the storage layer is maintained up to at least 250 °C, and direct quantitative measurements reveal a moderate decrease of magnetic induction. Hence, this study shows explicitly that PSA provides significant stability in STT-MRAM applications that require reliable performance over a range of operating temperatures.

International audience; To improve the read/write margin in perpendicular spin transfer torque magnetic random access memory (STT-MRAM), a concept of the double-magnetic tunnel junction (MTJ) MRAM cell is proposed in which the STT efficiency can be changed between read and write modes. In conventional double-MTJ stacks, the storage layer magnetization is submitted to two additive STT contributions, one from the reference layer below the bottom tunnel barrier and the other from an additional polarizing layer above the top tunnel barrier. In the proposed stack, the magnetization of the top polarizing layer can be switched between the read and write mode by domain wall propagation or spin orbit torque. This allows us to maximize the STT on the storage layer during write and minimize it during read. The associated advantages are a lower write current, reduced read disturb, maximal magnetoresistance amplitude during read, and faster read thanks to larger read current. We report here the experimental demonstration of this concept on perpendicular double-MTJ stacks.

International audience; Diabetes is a major global health threat. Both academics and industry are striving to develop effective treatments for this disease. In this work, we present a new approach to induce insulin release from β-islet pancreatic cells (INS-1E) by mechanical stimulation. Two types of experiments were carried out. First, a local stimulation was performed by dispersing anisotropic magnetic particles within the cell medium, which settled down almost immediately on cell plasma membranes. Application of a low frequency magnetic field (up to 40 Hz) generated by a custom-made magnetic device resulted in oscillations of these particles, which then exerted a mechanical constraint on the cell plasma membranes. The second type of experiment consisted of a global stimulation, where cells were grown on magneto-elastic membranes composed of a biocompatible polymer with embedded magnetic particles. Upon application of a rotating magnetic field, magnetic particles within the membrane were attracted towards the field source, resulting in the membrane's vibrations being transmitted to the cells grown on it. In both experiments, the cell response to these mechanical stimulations caused by application of the variable magnetic field was quantified via the measurement of insulin release in the growth medium. We demonstrated that the mechanical action induced by the motion of magnetic particles or by membrane vibrations was an efficient stimulus for insulin granule secretion from β-cells. This opens a wide range of possible applications including the design of a system which triggers insulin secretion by β-islet pancreatic cells on demand.

Circularly polarized light is a fundamental tool in magnetic studies, notably for magnetization dynamics. It is less common in magneto-optics to exploit the orbital angular momentum (OAM) of value ℓ carried by light beams possessing a helical wavefront. After finding many applications in the visible range, recently OAM pulses with ultra-short duration and XUV wavelengths became available, widening the range of experiments that can be envisaged. We modelled the interaction of an XUV OAM beam with non-uniform magnetic structures, showing that the far field scattered intensity profile encodes the symmetry of the magnetic structure in a way that depends on the sign and value of ℓ. In analogy with magnetic circular dichroism, this effect, named magnetic helicoidal dichroism (MHD), can be observed by inverting the sign of either the orbital momentum or of the magnetization. We obtained experimental evidence of MHD by measuring ℓ-dependent resonant scattering from a magnetic vortex. The results of recent complementary experiments match well the theoretical predictions, confirming the potential of the new toolset provided by MHD for studying the laser-triggered ultrafast dynamics of complex magnetic materials.

Non-volatile magnetic random-access memories (MRAMs), such as spin-transfer torque MRAM and next-generation spin–orbit torque MRAM, are emerging as key to enabling low-power technologies, which are expected to spread over large markets from embedded memories to the Internet of Things. Concurrently, the development and performances of devices based on two-dimensional van der Waals heterostructures bring ultracompact multilayer compounds with unprecedented material-engineering capabilities. Here we provide an overview of the current developments and challenges in regard to MRAM, and then outline the opportunities that can arise by incorporating two-dimensional material technologies. We highlight the fundamental properties of atomically smooth interfaces, the reduced material intermixing, the crystal symmetries and the proximity effects as the key drivers for possible disruptive improvements for MRAM at advanced technology nodes., M.C., S.C., B.D., A.F., K.G., M.-B. M., P.S., S.O.V. and S.R. acknowledge the European Union Horizon 2020 research and innovation programme for grant number 881603 (Graphene Flagship). The Catalan Institute of Nanoscience and Nanotechnology is supported by the Severo Ochoa Centres of Excellence programme, funded by the Spanish Research Agency (AEI, grant number SEV-2017-0706). S.O.V. thanks the European Research Council (ERC) under grant agreements 306652 SPINBOUND and 899896 SOTMEM, and the Spanish Research Agency (AEI), Ministry of Science and Innovation (PID2019-111773RB-I00/AEI/10.13039/501100011033). H.Y. is supported by SpOT-LITE programme (A*STAR grant, A18A6b0057) through RIE2020 funds, Singapore Ministry of Education (MOE) Tier 2 (R-263-000-E29-112), National Research Foundation (NRF) Singapore Investigatorship (NRFI06-2020-0015) and Samsung Electronics’ University R&D programme. S.C. and G.S.K. acknowledge IMEC’s Industrial Affiliation Program on MRAM devices. M.C. and M.J. acknowledge the French National Research Agency through the MAGICVALLEY project (ANR-18-CE24-0007). B. Dieny acknowledges ERC MAGICAL 669204. M.C. acknowledges H. X. Yang, A. Hallal and F. Ibrahim. P.S. and B. Dlubak acknowledge the French National Research Agency through the SoGraphMem project (ANR-18-CE24-0007).

Perpendicular shape anisotropy (PSA) and double magnetic tunnel junctions (DMTJ) offer practical solutions to downscale spin-transfer-torque Magnetic Random-Access Memory (STT-MRAM) beyond 20 nm technology nodes, whilst retaining their thermal stability and reducing critical currents applied. However, as these modern devices become smaller and three-dimensionally (3D) complex, our understanding of their functional magnetic behavior is often indirect, relying on magnetoresistance measurements and micromagnetic modelling. In this paper, we review recent work that was performed on these structures using a range of advanced electron microscopy techniques, focusing on aspects specific to the 3D and nanoscale nature of such elements. We present the methodology for the systematic transfer of individual SST-MRAM nano-pillars from large-scale arrays to image their magnetic configurations directly using off-axis electron holography. We show that improved phase sensitivity through stacking of electron holograms can be used to image subtle variations in DMTJs and the thermal stability of < 20 nm PSA-STT-MRAM nano-pillars during in-situ heating. The experimental practicalities, benefits and limits of using electron holography for analysis of MRAM devices are discussed, unlocking practical pathways for direct imaging of the functional magnetic performance of these systems with high spatial resolution and sensitivity., 22 pages, 7 figures

We demonstrate that a spin-torque nano-oscillator (STNO) rapidly sweep-tuned by a bias voltage can be used to perform an ultrafast time-resolved spectral analysis of frequency-manipulated microwave signals. The critical reduction in the time of the spectral analysis comes from the naturally small-time constants of a nanosized STNO (1-100 ns). The demonstration is performed on a vortex-state STNO generating in a frequency range around 300 MHz, when frequency down-conversion and matched filtering is used for signal processing. It is shown that this STNO-based spectrum analyzer can perform analysis of frequency-agile signals, having multiple rapidly changing frequency components with temporal resolution in a μs time scale and frequency resolution limited only by the "bandwidth" theorem. Our calculations show that using uniform magnetization state STNOs it would be possible to increase the operating frequency of a spectrum analyzer to tens of GHz.

International audience; Cancer treatment by magneto-mechanical effect of particles (TMMEP) is a growing field of research. The principle of this technique is to apply a mechanical force on cancer cells in order to destroy them thanks to magnetic particles vibrations. For this purpose, magnetic particles are injected in the tumor or exposed to cancer cells and a low-frequency alternating magnetic field is applied. This therapeutic approach is quite new and a wide range of treatment parameters are explored to date, as described in the literature. This review explains the principle of the technique, summarizes the parameters used by the different groups and reports the main in vitro and in vivo results.

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

Gradient of spin accumulation in spintronic devices such as lateral spin-valves allows to generate pure spin-current without charge-current. Spin accumulation is an out-of-equilibrium magnetization in which thermal fluctuations can occur. These fluctuations may constitute a source of noise in lateral spin-valve structures. In this study, the thermally induced fluctuations of the vector of spin-accumulation were investigated theoretically in diffusive regime. It is shown that paramagnetic resonance may arise in the spin-current carrying channel due to electron-electron interactions and exchange splitting induced by the spin-accumulation. This leads to an effect that was not previously considered: resonant increase of the magnetic susceptibility of the paramagnetic channel material and an associated decrease in signal-to-noise ratio around the resonance frequency. Frequency dependence of the magnetic susceptibility and signal-to-noise ratio were calculated analytically in the case of a specific T-shaped lateral spin-valve structures. It was shown however that this noise caused by thermally induced fluctuations in spin-accumulation is generally negligible in comparison to other sources of noise present in lateral spin-valves such as Johnson noise or thermal fluctuations of magnetization in the magnetic electrodes.

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

Researchers present a method of phase modulation and phase-shift keying for spin-torque nano-oscillators (STNOs), and demonstrate transmission of a voice signal (telephony) with their STNO-based gear. The key elements of the proposed method are the ability of an STNO to be synchronized by an external signal, and to shift its phase relative to the source's phase according to an additional control signal. These properties allow not only control of the phase, but also reduction of phase noise, which is a main issue for all oscillators with nanoscale dimensions. The results comprise a significant advance for wireless data and signal communication with nano-oscillators.

International audience; In nanomedicine, treatments based on physical mechanisms are more and more investigated and are promising alternatives for challenging tumor therapy. One of these approaches, called magneto-mechanical treatment, consists in triggering cell death via the vibration of anisotropic magnetic particles, under a low frequency magnetic field. In this work, we introduce a new type of easily accessible magnetic microparticles (MMPs) and study the influence of their surface functionalization on their ability to induce such an effect, and its mechanism. We prepared anisotropic magnetite microparticles by liquid-phase ball milling of a magnetite powder. These particles are completely different from the often-used SPIONs: they are micron-size, ferromagnetic, with a closed-flux magnetic structure reminiscent of that of vortex particles. The magnetic particles were covered with a silica shell, and grafted with PEGylated ligands with various physicochemical properties. We investigated both bare and coated particles' in vitro cytotoxicity, and compared their efficiency to induce U87-MG human glioblastoma cell apoptosis under a low frequency rotating magnetic field (RMF). Our results indicated that (1) the magneto-mechanical treatment with bare MMPs induces a rapid decrease in cell viability whereas the effect is slower with PEGylated particles; (2) the number of apoptotic cells after magneto-mechanical treatment is higher with PEGylated particles; (3) a lower frequency of RMF (down to 2 Hz) favors the apoptosis. These results highlight a difference in the cell death mechanism according to the properties of particles used – the rapid cell death observed with the bare MMPs indicates a death pathway via necrosis, while PEGylated particles seem to favor apoptosis.

Heat-assisted magnetic recording (HAMR) is a new technology which uses temporary near-field heating of the media during write to increase hard disk drive storage density. By using a plasmonic antenna embedded in the write head, an extremely high thermal gradient is created in the recording media (up to 10 K/nm). State-of-the-art HAMR media consist of grains of L10-FePt exhibiting high perpendicular anisotropy separated by 1–2 nm-thick carbon segregant. Next to the plasmonic antenna, the difference of temperature between two nanosized FePt grains in the media can reach 80 K across the 2 nm-thick grain boundary. This represents a gigantic local thermal gradient of 40 K/nm across a carbon tunnel barrier. In the field of spincaloritronics, much weaker thermal gradients of ~1 K/nm were shown to cause a thermal spin-transfer torque (TST) capable of inducing magnetization switching in magnetic tunnel junctions (MTJs). Considering on the one hand, two neighboring grains separated by an insulating grain boundary in an HAMR media can be viewed as an MTJ, and on the other hand, the thermal gradients in HAMR are 1–2 orders of magnitude larger than those used in the conventional spincaloritronic experiments; one may expect a strong impact from these TSTs on magnetization switching dynamics in HAMR recording. This issue has been totally overlooked in the previous investigations on the development of the HAMR technology. This paper combines theory, experiments aiming at determining the polarization of tunneling electrons across the media grain boundaries, and micromagnetic simulations of the recording process taking into account these thermal gradients. It is shown that the thermal in-plane torque can have a detrimental impact on the recording performances by favoring antiparallel magnetic alignment between neighboring grains during the media cooling. Implications on media design are discussed in order to overcome the influence of these thermal torques. Suggestions of spincaloritronic experiments taking advantage of these huge thermal gradients produced by plasmonic antenna are also given.

The concept of Perpendicular Shape Anisotropy (PSA) spin transfer torque (STT) MRAM has been recently proposed as a solution to achieve downsize scalability of MRAM below sub-10 nm technology nodes, down to 3–4 nm cell size lateral dimensions. In conventional p-STT-MRAM, at sub-20 nm diameters, the perpendicular anisotropy arising from the MgO/CoFeB interface becomes too weak to ensure sufficient stability of the storage layer magnetization. In addition, this interfacial anisotropy decreases rapidly with increasing temperature, resulting in a significant drawback for applications having to operate on a wide temperature range. Here, we combine both coercivity and electron holography measurements as function of temperature to show that in a PSA storage layer, the source of anisotropy is much more robust versus temperature compared to the interfacial anisotropy of conventional STT-MRAM stacks. This allows to considerably reduce the temperature dependence of coercivity. This property is quite beneficial for applications having to operate on an extended temperature range, such as automotive (−40°C to 150°C), or to fulfill solder reflow compliance requiring 1 minute retention at 260°C.

Since the first experimental observation of all-optical switching phenomena, intensive research has been focused on finding suitable magnetic systems that can be integrated as storage elements within spintronic devices and whose magnetization can be controlled through ultra-short single laser pulses. We report here atomistic spin simulations of all-optical switching in multilayered structures alternating n monolayers of Tb and m monolayers of Co. By using a two temperature model, we numerically calculate the thermal variation of the magnetization of each sublattice as well as the magnetization dynamics of [Tbn/Com] multilayers upon incidence of a single laser pulse. In particular, the condition to observe thermally-induced magnetization switching is investigated upon varying systematically both the composition of the sample (n,m) and the laser fluence. The samples with one monolayer of Tb as [Tb1/Co2] and [Tb1/Co3] are showing thermally induced magnetization switching above a fluence threshold. The reversal mechanism is mediated by the residual magnetization of the Tb lattice while the Co is fully demagnetized in agreement with the models developed for ferrimagnetic alloys. The switching is however not fully deterministic but the error rate can be tuned by the damping parameter. Increasing the number of monolayers the switching becomes completely stochastic. The intermixing at the Tb/Co interfaces appears to be a promising way to reduce the stochasticity. These results predict for the first time the possibility of TIMS in [Tb/Co] multilayers and suggest the occurrence of sub-picosecond magnetization reversal using single laser pulses., 9 pages, 4 figures

Magnetic tunnel junctions with perpendicular anisotropy form the basis of the spin-transfer torque magnetic random-access memory (STT MRAM), which is nonvolatile, fast, dense, and has quasi-infinite write endurance and low power consumption. Based on density-functional-theory (DFT) calculations, we propose an alternative design of magnetic tunnel junctions comprising $\mathrm{Fe}(n)\mathrm{Co}(m)\mathrm{Fe}(n)|\mathrm{Mg}\mathrm{O}$ storage layers [n and m denote the number of monolayers (ML)] with greatly enhanced perpendicular magnetic anisotropy (PMA) up to several mJ/m${}^{2}$, leveraging the interfacial perpendicular anisotropy of $\mathrm{Fe}|\mathrm{Mg}\mathrm{O}$ along with a strain-induced bulk PMA discovered within bcc $\mathrm{Co}$. This giant enhancement dominates the demagnetizing energy when increasing the film thickness. The tunneling magnetoresistance (TMR) estimated from the Julliere model is comparable with that of the pure $\mathrm{Fe}|\mathrm{Mg}\mathrm{O}$ case. We discuss the advantages and pitfalls of a real-life fabrication of the structure and propose the $\mathrm{Fe}(3\mathrm{ML})\mathrm{Co}(4\mathrm{ML})\mathrm{Fe}(3\mathrm{ML})$ as a storage layer for $\mathrm{MgO}$-based STT MRAM cells. The large PMA in strained bcc $\mathrm{Co}$ is explained in the framework of second-order perturbation theory by the $\mathrm{MgO}$-imposed strain and consequent changes in the energies of ${d}_{yz}$ and ${d}_{{z}^{2}}$ minority-spin bands.

We conducted a thorough experimental and numerical study of the micromagnetic properties of Permalloy (Ni$_{80}$Fe$_{20}$) microdisks exhibiting target domain structures at remanence. Vortex configurations are quite common in such microdisks and correspond to an in-plane flux closure configuration of cylindrical symmetry with an out-of-plane magnetized core. In contrast, target domain configuration are observed in thicker microdisks and are characterized by a vortex configuration of the in-plane component of the magnetization superposed to an out-of-plane component of magnetization which oscillates as a function of the distance to the microdisk center resulting in the formation of concentric domains. The ratio of the out-of-plane oscillatory component of the magnetization to the in-plane vortex one increases with the thickness of the microdisk. Hysteresis loops were measured under in-plane and out-of-plane field. The results at remanence and under magnetic field could be interpreted by micromagnetic simulations in which the microdisks were described as an assembly of partially coupled columns representing the granular nanostructure of the films from which the microdisks were patterned. Quite original magnetization processes take place in these microdisks exhibiting target domain remanent configuration. These include in particular entire flipping of the domain configuration and annihilation/creation of ring domains., Comment: 17 pages, 11 figures

The advantage of an ultra-fast frequency-tunability of spin-torque nano-oscillators (STNOs) that have large (> 100MHz) relaxation frequency of amplitude fluctuations is exploited to realize ultra-fast wide-band time-resolved spectral analysis at nanosecond time scale with the frequency resolution limited only by the 'bandwidth' theorem. The demonstration is performed with an STNO generating in the 9 GHz frequency range, and comprised of a perpendicular polarizer and a perpendicularly and uniformly magnetized 'free' layer. It is shown that such a uniform-state STNO-based spectrum analyzer can efficiently perform spectral analysis of frequency-agile signals with rapidly varying frequency components.

Biofilm colonisation of surfaces is of critical importance in various areas ranging from indwelling medical devices to industrial setups. Of particular importance is the reduced susceptibility of bacteria embedded in a biofilm to existing antimicrobial agents. In this paper, we demonstrate that remotely actuated magnetic cantilevers grafted on a substrate act efficiently in preventing bacterial biofilm formation. When exposed to an alternating magnetic field, the flexible magnetic cantilevers vertically deflect from their initial position periodically, with an extremely low frequency (0.16 Hz). The cantilevers’ beating prevents the initial stage of bacterial adhesion to the substrate surface and the subsequent biofilm growth. Our experimental data on E. coli liquid cultures demonstrate up to a 70% reduction in biofilm formation. A theoretical model has been developed to predict the amplitude of the cantilevers vertical deflection. Our results demonstrate proof-of-concept for a device that can magneto-mechanically prevent the first stage in bacterial biofilm formation, acting as on-demand fouling release active surfaces.

International audience; The concept of a spintronic memristor based on angular variation of Tunnel Magnetoresistance relies on the development of an isotropically coercive free layer able to stabilize the magnetization in any in-plane direction. In this work, we first demonstrate those properties in a composite ferromagnet/antiferromagnet/ferromagnet trilayer by performing hysteresis loops at different in-plane angles and rotating field experiments. In the second part, we describe the material composition of the total stack and the fabrication process to create the final device. In the last part, the hysteresis loops at different in-plane angles and the rotating field experiments are performed at device level and compared with the ones performed at full sheet film, confirming the magnetically isotropic nature of the free layer.

The concept of Perpendicular Shape Anisotropy STT-MRAM (PSA-STT-MRAM) has been recently proposed as a solution to enable the downsize scalability of STT-MRAM devices beyond the sub-20 nm technology node. For conventional p-STT-MRAM devices with sub-20 nm diameters, the perpendicular anisotropy arising from the MgO/CoFeB interface becomes too weak to ensure thermal stability of the storage layer. In addition, this interfacial anisotropy rapidly decreases with increasing temperature which constitutes a drawback in applications with a large range of operating temperatures. Here, we show that by using a PSA based storage layer, the source of anisotropy is much more robust against thermal fluctuations than the interfacial anisotropy, which allows considerable reduction of the temperature dependence of the coercivity. From a practical point of view, this is very interesting for applications having to operate on a wide range of temperatures (e.g. automotive -40 °C/+150 °C).