Mutual synchronization of spin-torque oscillators within a ring array.

An array of spin torque nano-oscillators (STNOs), coupled by dipolar interaction and arranged on a ring, has been studied numerically and analytically. The phase patterns and locking ranges are extracted as a function of the number N, their separation, and the current density mismatch between selected subgroups of STNOs. If [Formula: see text] for identical current densities through all STNOs, two degenerated modes are identified an in-phase mode (all STNOs have the same phase) and a splay mode (the phase makes a 2[Formula: see text] turn along the ring). When inducing a current density mismatch between two subgroups, additional phase shifts occur. The locking range (maximum current density mismatch) of the in-phase mode is larger than the one for the splay mode and depends on the number N of STNOs on the ring as well as on the separation. These results can be used for the development of magnetic devices that are based on STNO arrays.

Spintronic memristors for neuromorphic circuits based on the angular variation of tunnel magnetoresistance.

In this study, a new type of compact magnetic memristor is demonstrated. It is based on the variation of the conductivity of a nano-sized magnetic tunnel junction as a function of the angle between the in-plane reference layer magnetization and a free layer exhibiting an isotropic in-plane coercivity. The free layer magnetization is rotated by two spin transfer torque contributions: one originating from the in-plane magnetized reference layer and the other one from an additional perpendicular polarizer integrated in the stack. Thanks to a proper tuning of the relative influence of these two torques, the magnetization of the free layer can be rotated step by step clockwise or anticlockwise in a range of angle between 0° (parallel configuration) and 180° (anti-parallel configuration) by sending pulses of current through the stack, of one or opposite polarity. The amplitude of the rotation steps and therefore of the conductance variations depends on the pulse amplitude and duration. In this way, we achieve monotonous variations of the resistance with the voltage polarity through the application of pulses in the ns range. We also retrieve the analytical expression of critical current density which is found to be in good agreement with the experimental results. The thermal stability of the intermediate resistance levels and the role of Joule heating are also discussed.

All-optical spin switching probability in [Tb/Co] multilayers.

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 [[Formula: see text]/[Formula: see text]] 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 [[Formula: see text]/[Formula: see text]] and [[Formula: see text]/[Formula: see text]] 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.

Magneto-mechanical treatment of human glioblastoma cells with engineered iron oxide powder microparticles for triggering apoptosis.

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.

Magneto-mechanically actuated microstructures to efficiently prevent bacterial biofilm formation.

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.

An electron holography study of perpendicular magnetic tunnel junctions nanostructured by deposition on pre-patterned conducting pillars.

The fabrication of multi-gigabit magnetic random access memory (MRAM) chips requires the patterning of magnetic tunnel junctions at very small dimensions (sub-30 nm) and a very dense pitch. This remains a challenge due to the difficulty in etching magnetic tunnel junction stacks. We previously proposed a strategy to circumvent this problem by depositing the magnetic tunnel junction material on prepatterned metallic pillars, resulting in the junction being naturally shaped during deposition. Upon electrical contact, the deposit on top of the pillars constitutes the magnetic storage element of the memory cell. However, in this process, the magnetic material is also deposited in the trenches between the pillars that might affect the memory cell behaviour. Here we study the magnetic interactions between the deposit on top of the pillars and in the trenches by electron holography, at room temperature and up to 325 °C. Supported by models, we show that the additional material in the trenches is not perturbing the working principle of the memory chip and can even play the role of a flux absorber which reduces the crosstalk between neighboring dots. Besides, in the studied sample, the magnetization of the 1.4 nm thick storage layer of the dots is found to switch from out-of-plane to an in-plane configuration above 125 °C, but gradually decreases with temperature. Electron holography is shown to constitute a very efficient tool for characterizing the micromagnetic configuration of the storage layer in MRAM cells.

Indium Tin Oxide optical access for magnetic tunnel junctions in hybrid spintronic-photonic circuits.

The all-optical magnetization reversal of magnetic layers, by picosecond optical pulses, is of particular interest as it shows the potential for energy-efficient and fast magnetic tunnel junction (MTJ) elements. This approach requires memory elements that are optically and electronically accessible, for optical writing and electronic read-out. In this paper, we propose the integration of indium tin oxide (ITO) as a transparent conducting electrode for magnetic tunnel junctions in integrated spintronic-photonic circuits. To provide light with sufficient energy to the MTJ free layer and allow electrical read-out of the MTJ state, we successfully integrated indium tin oxide as a top transparent electrode. The study shows that ITO film deposition by physical vapor deposition with conditions such as high source power and low O2 flow achieves smooth and conductive thin films. Increase in grain size was associated with low resistivity. Deposition of 150 nm ITO at 300 W, O2 flow of 1 sccm and 8.10-3 mbar vacuum pressure results in 4.8 × 10-4 Ω.cm resistivity and up to 80% transmittance at 800 nm wavelength. The patterning of ITO using CH4/H2 chemistry in a reactive ion etch process was investigated showing almost vertical sidewalls for diameters down to 50 nm. The ITO based process flow was compared to a standard magnetic tunnel junctions fabrication process flow based on Ta hard mask. Electrical measurements validate that the proposed process based on ITO results in properties equivalent to the standard process. We also show electrical results of magnetic tunnel junctions having all-optical switching top electrode fabricated with ITO for optical access. The developed ITO process flow shows very promising initial results and provides a way to fabricate these new devices to integrate all-optical switching magnetic tunnel junctions with electronic and photonic elements.

Single-shot all-optical switching of magnetization in Tb/Co multilayer-based electrodes.

Ever since the first observation of all-optical switching of magnetization in the ferrimagnetic alloy GdFeCo using femtosecond laser pulses, there has been significant interest in exploiting this process for data-recording applications. In particular, the ultrafast speed of the magnetic reversal can enable the writing speeds associated with magnetic memory devices to be potentially pushed towards THz frequencies. This work reports the development of perpendicular magnetic tunnel junctions incorporating a stack of Tb/Co nanolayers whose magnetization can be all-optically controlled via helicity-independent single-shot switching. Toggling of the magnetization of the Tb/Co electrode was achieved using either 60 femtosecond-long or 5 picosecond-long laser pulses, with incident fluences down to 3.5 mJ/cm2, for Co-rich compositions of the stack either in isolation or coupled to a CoFeB-electrode/MgO-barrier tunnel-junction stack. Successful switching of the CoFeB-[Tb/Co] electrodes was obtained even after annealing at 250 °C. After integration of the [Tb/Co]-based electrodes within perpendicular magnetic tunnel junctions yielded a maximum tunneling magnetoresistance signal of 41% and RxA value of 150 Ωµm2 with current-in-plane measurements and ratios between 28% and 38% in nanopatterned pillars. These results represent a breakthrough for the development of perpendicular magnetic tunnel junctions controllable using single laser pulses, and offer a technologically-viable path towards the realization of hybrid spintronic-photonic systems featuring THz switching speeds.

Thermal robustness of magnetic tunnel junctions with perpendicular shape anisotropy.

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).

Optical response of magnetically actuated biocompatible membranes.

Biocompatible suspended magneto-elastic membranes were prepared. They consist of PDMS (polydimethylsiloxane) films, with embedded arrays of micrometric magnetic pillars made with lithography techniques. For visible light wavelengths, our membranes constitute magnetically tunable optical diffraction gratings, in transmission and reflection. The optical response has been quantitatively correlated with membrane structure and deformation, through optical and magneto-mechanical models. In contrast to the case of planar membranes, the diffraction patterns measured in reflection and transmission vary very differently upon magnetic field application. Indeed, the reflected beam is largely affected by the membrane bending, whereas the transmitted beam remains almost unchanged. In reflection, even weak membrane deformation can produce significant changes of the diffraction patterns. This field-controlled optical response may be used in adaptive optical applications, photonic devices, and for biological applications.

A highly thermally stable sub-20 nm magnetic random-access memory based on perpendicular shape anisotropy.

A new approach to increase the downsize scalability of perpendicular STT-MRAM is presented. It consists of significantly increasing the thickness of the storage layer in out-of-plane magnetized tunnel junctions (pMTJ) as compared to conventional pMTJ in order to induce a perpendicular shape anisotropy (PSA) in this layer. This PSA is obtained by depositing a thick ferromagnetic (FM) layer on top of an MgO/FeCoB based magnetic tunnel junction (MTJ) so that the thickness of the storage layer is of the order of or larger than the diameter of the MTJ pillar. In contrast to conventional spin transfer torque magnetic random access memory (STT-MRAM) wherein the demagnetizing energy opposes the interfacial perpendicular magnetic anisotropy (iPMA), in these novel memory cells, both PSA and iPMA contributions favor the out-of-plane orientation of the storage layer magnetization. Using thicker storage layers in these PSA-STT-MRAMs has several advantages. Due to the PSA, very high and easily tunable thermal stability factors can be achieved, even down to sub-10 nm diameters. Moreover, a low damping material can be used for the thick FM material thus leading to a reduction of the write current. The paper describes this new PSA-STT-MRAM concept, practical realization of such memory arrays, magnetic characterization demonstrating thermal stability factor above 200 for MTJs as small as 8 nm in diameter and possibility to maintain the thermal stability factor above 60 down to 4 nm diameter.

Second order anisotropy contribution in perpendicular magnetic tunnel junctions.

Hard-axis magnetoresistance loops were measured on perpendicular magnetic tunnel junction pillars of diameter ranging from 50 to 150 nm. By fitting these loops to an analytical model, the effective anisotropy fields in both free and reference layers were derived and their variations in temperature range between 340 K and 5 K were determined. It is found that a second-order anisotropy term of the form -K2cos(4)θ must be added to the conventional uniaxial -K1cos(2)θ term to explain the experimental data. This higher order contribution exists both in the free and reference layers. At T = 300 K, the estimated -K2/K1 ratios are 0.1 and 0.24 for the free and reference layers, respectively. The ratio is more than doubled at low temperatures changing the ground state of the reference layer from "easy-axis" to "easy-cone" regime. The easy-cone regime has clear signatures in the shape of the hard-axis magnetoresistance loops. The existence of this higher order anisotropy was also confirmed by ferromagnetic resonance experiments on FeCoB/MgO sheet films. It is of interfacial nature and is believed to be due to spatial fluctuations at the nanoscale of the first order anisotropy parameter at the FeCoB/MgO interface.

Spontaneous anomalous and spin Hall effects due to spin-orbit scattering of evanescent wave functions in magnetic tunnel junctions.

We theoretically investigated the anomalous Hall effect (AHE) and spin Hall effect (SHE) transversal to the insulating spacer I, in magnetic tunnel junctions of the form F/I/F where the F's are ferromagnetic layers and I represents a tunnel barrier. We considered the case of purely ballistic (quantum mechanical) transport. These effects arise because of the asymmetric scattering of evanescent wave functions due to the spin-orbit interaction in the tunnel barrier. The AHE and SHE we investigated have a surface nature due to the proximity effect. Their amplitude is of first order in the scattering potential. This contrasts with ferromagnetic metals wherein these effects are of second (side-jump scattering) and third (skew scattering) order in these potentials. The value of the AHE current in the insulating spacer may be much larger than that in metallic ferromagnetic electrodes. For the antiparallel orientation of the magnetizations in the two F electrodes, a spontaneous Hall current exists even at zero applied voltage.

Temporal coherence of MgO based magnetic tunnel junction spin torque oscillators.

Single-shot, time-resolved measurements are presented to investigate the temporal coherence of the microwave emission for MgO based magnetic tunnel junction spin torque oscillators. The time-domain data reveal that the steady state regime obtained from frequency-domain analysis can be subdivided into two regimes as a function of spin polarized current amplitude. According to these two regimes, two mechanisms that limit the temporal coherence are identified. At low current, extinctions of the steady state oscillations lead to a very short coherence time on the order of a few nanoseconds, while at higher current, the extinctions vanish and the coherence time saturates around 40 ns. As an important result it is shown that the latter is limited by frequency fluctuations. Quenching these frequency fluctuations suggests an intrinsic linewidth that is by a factor of 20 below the one of the free running oscillator.

Magnetic logic using nanowires with perpendicular anisotropy.

In addition to a storage function through the magnetization of nanowires, domain wall propagation can be used to trigger magnetic logic functions. Here, we present a new way to realize a pure magnetic logic operation by using magnetic nanowires with perpendicular anisotropy. Emphasis is given on the generation of the logic function 'NOT' that is based on the dipolar interaction between two neighbouring magnetic wires, which favours the creation of a domain wall. This concept has been validated on several prototypes and the results fit well with the expectations.

Spin diode based on Fe/MgO double tunnel junction.

We demonstrate a spin diode consisting of a semiconductor-free nanoscale Fe/MgO-based double tunnel junction. The device exhibits a near perfect spin-valve effect combined with a strong diode effect. The mechanism consistent with our data is resonant tunneling through discrete states in the middle ferromagnetic layer sandwiched by tunnel barriers of different spin-dependent transparency. The observed magnetoresistance is a record high>1000%, essentially making the structure an on/off spin switch. This, combined with the strong diode effect, approximately 100, demonstrates a new device principle, promising for memory and reprogrammable logic applications.

Spin-torque oscillator using a perpendicular polarizer and a planar free layer.

Spintronics materials have recently been considered for radio-frequency devices such as oscillators by exploiting the transfer of spin angular momentum between a spin-polarized electrical current and the magnetic nanostructure it passes through. While previous spin-transfer oscillators (STOs) were based on in-plane magnetized structures, here we present the realization of an STO that contains a perpendicular spin current polarizer combined with an in-plane magnetized free layer. This device is characterized by high-frequency oscillations of the free-layer magnetization, consistent with out-of-plane steady-state precessions induced at the threshold current by a spin-transfer torque from perpendicularly polarized electrons. The results are summarized in static and dynamic current-field state diagrams and will be of importance for the design of STOs with enhanced output signals.

Spin-torque influence on the high-frequency magnetization fluctuations in magnetic tunnel junctions.

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.

Creep and flow regimes of magnetic domain-wall motion in ultrathin Pt/Co/Pt films with perpendicular anisotropy.

We report on magnetic domain-wall velocity measurements in ultrathin Pt/Co(0.5-0.8 nm)/Pt films with perpendicular anisotropy over a large range of applied magnetic fields. The complete velocity-field characteristics are obtained, enabling an examination of the transition between thermally activated creep and viscous flow: motion regimes predicted from general theories for driven elastic interfaces in weakly disordered media. The dissipation limited flow regime is found to be consistent with precessional domain-wall motion, analysis of which yields values for the damping parameter, alpha.

Imprinting vortices into antiferromagnets.

The effect of imprinting symmetric and displaced vortex structures into an antiferromagnetic material is investigated in micron-sized disks consisting of exchange coupled ferromagnetic-antiferromagnetic bilayers. The imprint of displaced vortices manifests itself by the occurrence of a new type of asymmetric hysteresis loops characterized by curved, reversible, central sections with nonzero remanent magnetization. Such an imprint is achieved by cooling the disks through the blocking temperature of the system in small fields. Micromagnetic simulations reveal that asymmetric vortexlike loops naturally result from the competition between the different energies involved in the system.