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Perpendicularly magnetized structures that are switchable using a spin current under field-free conditions can potentially be applied in spin-orbit torque magnetic random-access memory (SOT-MRAM). Several structures have been developed; however, new structures with a simple stack structure and MRAM compatibility are urgently needed. Herein, a typical structure in a perpendicular spin-transfer torque MRAM, the Pt/Co multilayer and its synthetic antiferromagnetic counterpart with perpendicular magnetic anisotropy, was observed to possess an intrinsic interlayer chiral interaction between neighboring magnetic layers, namely, the interlayer Dzyaloshinskii-Moriya interaction (DMI) effect. Furthermore, using a current parallel to the eigenvector of the interlayer DMI, we switched the perpendicular magnetization of both structures without a magnetic field, owing to the additional symmetry breaking introduced by the interlayer DMI. This SOT switching scheme realized in the Pt/Co multilayer and its synthetic antiferromagnet structure may open a new avenue toward practical perpendicular SOT-MRAM and other SOT devices.

Magnetoresistance effects are used in a variety of devices including hard disk drives and magnetic random access memories. In particular, giant magnetoresistance and tunnelling magnetoresistance can be used to create spin valves and tunnel junctions in which the resistance depends on the relative magnetization orientations of two ferromagnetic conducting layers. Here, we report a magnetoresistance effect that occurs in a platinum layer deposited on a magnon junction consisting of two insulating magnetic yttrium iron garnet (YIG) layers separated by an antiferromagnetic nickel oxide spacer layer. The resistance of the platinum layer is found to depend on the magnetization of the YIG layer in direct contact with it (an effect known as spin Hall magnetoresistance), but also the magnetization of the adjacent YIG layer in the junction. The resistance of the platinum layer is higher when the two YIG layers are aligned antiparallel than when parallel. We assign this behaviour to a magnonic nonlocal spin Hall magnetoresistance in which spin-carrying magnon propagation across the junction affects spin accumulation at the metal interface and hence modulates the spin Hall magnetoresistance. The effect could be used to develop spintronic and magnonic devices that have spin transport properties controlled by an all-insulating magnon junction and are thus free from Joule heating. A magnetoresistance effect that occurs in a platinum layer deposited on a magnon junction consisting of two insulating magnetic yttrium iron garnet layers separated by an antiferromagnetic nickel oxide spacer layer could be used to create spintronic and magnonic devices that are free from Joule heating.

Understanding and manipulating of the antiferromagnetic (AF) ultrafast spin dynamics in antiferromagnets (AFMs) is a crucial importance issue because of the promising applications in terahertz spintronic devices. In this study, an analytical theory extended from the classic coupled pendulum model has been developed to describe the intrinsic magnetic excitation of AFMs. The derived frequency dispersion of the AF resonances has been further checked by using the atomistic-level Landau–Lifshitz–Gilbert simulations. We show that the rutile crystalline AFM MnF2 possess two separate resonance modes at low magnetic fields: high frequency mode with right-handed polarization and low frequency mode with left-handed polarization. In the absence of magnetic field, these two resonance modes could degenerate into a single resonance state. When the applied magnetic field is higher than the spin-flip field, the system behaves a quasi-ferromagnetic mode. Both quantitative and qualitative agreement with atomistic simulation results confirm the theoretical picture of the AF resonance dynamics. This study provides a simple but physical understanding of the ultrafast dynamics of AF excitations.

High frequency magnetic precessions with strong intensity are strongly desired in material systems for high performance magnetic memory or nano-oscillator applications with ultrafast manipulation speed. Here, we demonstrate an exchange-coupled asymmetric composite film structure of Ta/Pd/[Pd/Co]5/Cu(tCu)/[Co/Ni]5/Ta with adjustable strong perpendicular magnetic anisotropy and interlayer coupling strength, in which the dynamic magnetic properties are systematically studied by using time-resolved magneto-optical Kerr effect spectroscopy. It is demonstrated that the in-phase precession frequency is between those of the single hard magnetic [Pd/Co]5 and soft [Co/Ni]5 multilayers, which can be significantly enhanced for the strongly coupled case at tCu < 1 nm. Moreover, in the weakly coupled samples with tCu = 1.0–3.0 nm, besides the common in-phase acoustic mode, an out-of-phase optical mode occurs simultaneously with a frequency even higher than that of the hard magnetic [Pd/Co]5 layer. The optical mode precession frequency and amplitude show an unusual non-monotonic variation trend with the increase of tCu, which has been theoretically analyzed and attributed to the co-effect of decreased coupling strength and increased magnetic anisotropy field difference between the two multilayer stacks. Moreover, by adjusting tCu and the [Co/Ni] repetition number N, an optical mode of strong intensity can be actively achieved, even reaching 80% as compared to the acoustic mode. These results provide effective control and better understanding of magnetic dynamics in perpendicular composite films, which are of key importance for developing ultrafast spintronics-based devices.

Reliable in situ control of spin wave (SW) excitation between localized and propagating SW modes is of great interest for both fundamental and applied spintronics and magnonics. While spin-transfer-torque-generated SWs can typically be tuned directly via the driving current, the frequency of the highest intensity SWs, achieved in the strongly self-localized magnetic droplet soliton, is virtually current independent, as the droplet frequency is given by the intrinsic material properties. Here, we demonstrate, using micromagnetic simulations, how the droplet frequency can be efficiently tuned by an applied voltage through the effect of electric field (E-field)-dependent perpendicular magnetic anisotropy (PMA). It is found that as the PMA decreases, the droplet begins to distort and eventually collapses to give way to propagating SWs. However, due to the geometrically confined structures, the radially propagating SWs are reflected by the periphery boundary of the sample, and then the forward and backward SWs superpose to produce a series of standing SWs. The node number of the standing SWs strongly depends on the sample size as well as the applied E field. These findings provide a deeper understanding of magnetic excitation properties, which will be helpful for designing advanced spintronic devices.

Understanding spin dynamics in perpendicular exchange coupled ferrimagnetic materials is an issue of crucial importance for progress in information processing and recording technology. In this paper, the Landau-Lifshitz-Gilbert equation at atomistic-level is employed to model the perpendicular interfacial exchange effect in ferrimagnetic FeGd/FeTb soft/hard bilayer structures. Three types of magnetization switching features are identified by tuning the interfacial exchange strength to be weak coupling, partial coupling, and strong coupling. These features have been further verified by the dynamical simulations, showing the switched soft layer could also help the hard layer magnetization switching. Based on the coercivity difference of the soft layer and hard layer, the left- or right-shift perpendicular exchange bias effect has been theoretically modelled.

Electric-controlled magnetization precession introduces technologically relevant possibility for developing spin torque nano-oscillators (STNO) with potential applications in microwave emission. Using the perpendicularly magnetized magnetic tunnel junction (MTJ), we show that the magnetization oscillation frequency can be tuned by the co-action of electric field and spin polarized current. The dynamical phase diagram of MTJ-based STNO is analytically predicted through coordinate transformation from the laboratory frame to the rotation frame, by which the nonstationary out-of-plane magnetization precession process is therefore transformed into the stationary process in the rotation frame. Furthermore, using this STNO as a microwave source, we numerically demonstrate that the bit signal can be transmitted by a binary frequency shift keying (BFSK) modulation technique. The BFSK scheme shows good modulation features with no transient state.

Magnetic droplets are self-localized spin wave solitons that can be excited in spin valves with perpendicular magnetic anisotropy by using spin-transfer torque generated by a dc current through a nano-contact. Using micromagnetic simulations, we demonstrate that the droplet soliton mode could be switched to a localized spiral precession mode due to the resistance effect of current-induced Oersted fields whose rotation direction is opposite to the precession direction of droplet. This spiral mode can switch back to the droplet mode by increasing current. With an assistance of electric field, both the localized droplet and spiral modes can develop to the radially propagating spin waves. The frequencies of all these localized spiral and droplet as well as the propagating spin wave modes show linear increase relationship with a magnetic field applied in out-of-plane direction. These results are of fundamental interest in understanding the rich dynamics of magnetic excitation properties, which will be helpful for the droplet-based spintronic devices.

Interlayer exchange-coupled synthetic antiferromagnets (SAFs) have the combined advantages of both high frequency of antiferromagnets and easy detection of ferromagnets. Here, magnetic excitations are investigated by theoretical analysis and micromagnetic simulations in SAFs that consist of two identical ferromagnetic layers with perpendicular magnetic anisotropy. Different from the common in-phase acoustic mode and out-of-phase optic mode, linearly or circularly polarized spin wave modes can be excited at zero bias field by using different types of microwave magnetic fields. Once a bias magnetic field is applied along the easy-axis, left-handed (LH) and right-handed (RH) polarization modes are observed, and the resonance frequency of RH (LH) mode of the SAFs increases (decreases) linearly with the increase of bias magnetic fields until a critical spin-flop field is reached, which is in accordance with collinear antiferromagnets with easy-axis anisotropy. These simulation results agree with the theoretical derivation and provide fundamental insight into the nature of dynamic properties of the perpendicularly magnetized SAFs, which may provide new prospects for spintronic applications.

One key advantage of antiferromagnets over ferromagnets is the high magnetic resonance frequencies that enable ultrafast magnetization switching and oscillations. Among a variety of antiferromagnets, the synthetic antiferromagnet (SAF) is a promising candidate for high-speed spintronic devices design. In this paper, micromagnetic simulations are employed to study the resonance modes in an SAF structure consisting of two identical CoFeB ferromagnetic (FM) layers that are antiferromagnetically coupled via interlayer exchange coupling. When the external bias magnetic field is small enough to ensure the magnetizations of two FM sublayers remain antiparallel alignments, we find that there exist two resonance modes with different precession chirality, namelyy-component synchronized mode andz-component synchronized mode, respectively. These two resonance modes show different features from the conventional in-phase acoustic mode and out-of-phase optic mode. The simulation results are in good agreement with our theoretical analyses.

The magnetic skyrmions have received a lot of attention because they have potential applications in spin torque nano-oscillators. In this paper, we numerically and analytically study the dynamics of edge skyrmions in circular nano-pillar oscillators with perpendicular magnetic anisotropy (PMA) free layer and spin polarizer. Numerical simulations demonstrate that a steady edge skyrmion oscillation mode with GHz frequency can be driven by spin currents and magnetic fields. The oscillation frequencies in circular nano-pillars with different radius are analytically derived, which agrees well with the numerical results. In addition, the steady oscillations of edge skyrmions are also observed in elliptical nano-pillars, in which the oscillation frequencies strongly depend on the eccentricity and perimeter of the elliptical nanopillar. It is found that the oscillation frequency of this edge skyrmion can range from 1 GHz to 6.7 GHz. The line width is about 55 MHz.

Field-free magnetization switching by in-plane current injection via spin–orbit interactions in heavy-metal/ferromagnet bilayer systems is of great interest for applications of spin–orbit torque (SOT). Here, we demonstrate theoretically how to deterministically switch the magnetization of CoFeB with a tilted perpendicular magnetic anisotropy (PMA) by using the SOT effect from the viewpoint of energy landscape. It is found that magnetization switching is dependent both on the polar angle θ and the azimuthal angle φ of the tilted PMA. A tiny remnant magnetization component that deviates away from the film plane during the spin current pulse impulsion period is a key point for the subsequent switching. The underlying mechanism is attributed to the shift of PMA energy barrier with the magnetization angle, which dominates the state of the tiny magnetization component and thereafter the bipolar deterministic switching. Based on the simulation results, a phase diagram of deterministic switching regarding θ and φ is established. Moreover, the positive and negative critical switching current densities are demonstrated to exhibit an astroid-like characteristic as a function of θ. This study provides a fundamental insight into the nature of SOT-induced deterministic switching with the tilted PMA method.

Graphical abstract, Highlights • PVA/AgNPs nanofibers were grafted with 3,3′,4,4′-benzophenone tetracarboxylic acid. • The water vapor transmittance and mechanical properties were improved. • It retained >70% of its original OH⋅ radical and H2O2 charging capacity after 7 cycles. • It showed higher filtration efficiency and antibacterial activity., In this study, poly(vinyl alcohol) (PVA) membranes containing Ag nanoparticles (AgNPs) were prepared by electrospinning and grafted copolymerization with 3,3′,4,4′-benzophenone tetracarboxylic acid (BPTA) to provide better mechanical properties, lower water vapor transmittance, and higher antibacterial activity (against Staphylococcus aureus and Escherichia coli) than the PVA/AgNPs membrane. The PVA/AgNPs/BPTA membrane showed higher antibacterial activity than the other membranes, and it produced inhibition zones with diameters of 18.12 ± 0.08 and 16.41 ± 0.05 mm against S. aureus and E. coli, respectively. The PVA/AgNPs/BPTA membrane was found to be capable of promoting reactive oxygen species (ROS) formation under both light and dark conditions. Cycling experiments performed following ROS quenching showed that the best-performing composite membrane retained >70% of its original OH⋅ radical and H2O2 charging capacity after seven cycles. In the filtration test, the electrospun nanofibrous membranes showed high filtration efficiencies of 99.98% for sodium chloride (NaCl). In addition, these membranes maintained a relatively low pressure drop of 168 Pa with a basis weight of 2.1 g m−2. Thus, the PVA/AgNPs/BPTA membrane was concluded to be a promising medical protective material offering the benefits of structural stability and reusability.

The skyrmion-based spin-torque nano-oscillator is a potential next-generation nano microwave signal generator. In this paper, the self-sustained oscillation dynamics of magnetic skyrmions are investigated in a nanodisk with synthetic antiferromagnetic (SAF) multilayer structure, in which the skyrmion Hall effect can be effectively suppressed. An analytical model based on the Thiele equation is developed to describe the dynamics of a pair of skyrmions formed in the SAF nanodisks. Combining the analytical solutions with the micromagnetic simulations, we demonstrate that circular rotations with opposite directions for a skyrmion pair could be suppressed by increasing the antiferromagnetic (AF) coupling in a nanopillar with dual spin polarizers. However, a stable circular rotation can be achieved in a nanopillar with a single spin polarizer, in which one skyrmion plays as a master whose rotation is driven by spin torque, while the other skyrmion is a slaver whose motion is dragged by the AF coupling between the two free layers. Moreover, we found that the effective mass factor in the SAF structure rather than the gyrotropic torque plays the dominant role in the circular rotation of skyrmions. The rotation orbit radius and frequency gradually increase with the decrease of damping factor and increase of applied current strength.

Mesenchymal stem cells (MSCs) have been widely used for regenerative therapy. In most current clinical applications, MSCs are delivered by injection but face significant issues with cell viability and penetration into the target tissue due to a limited migration capacity. Some therapies have attempted to improve MSC stability by their encapsulation within biomaterials; however, these treatments still require an enormous number of cells to achieve therapeutic efficacy due to low efficiency. Additionally, while local injection allows for targeted delivery, injections with conventional syringes are highly invasive. Due to the challenges associated with stem cell delivery, a local and minimally invasive approach with high efficiency and improved cell viability is highly desired. In this study, we present a detachable hybrid microneedle depot (d-HMND) for cell delivery. Our system consists of an array of microneedles with an outer poly(lactic-co-glycolic) acid (PLGA) shell and an internal gelatin methacryloyl (GelMA)-MSC mixture (GMM). The GMM was characterized and optimized for cell viability and mechanical strength of the d-HMND required to penetrate mouse skin tissue was also determined. MSC viability and function within the d-HMND was characterized in vitro and the regenerative efficacy of the d-HMND was demonstrated in vivo using a mouse skin wound model.

The effective manipulation of skyrmion motion by magnetic field gradients introduces the technological possibility with skyrmion as information carriers for spintronic applications. Recent studies indicate that the skyrmion Hall effect can be suppressed in a synthetic antiferromagnetic (SAF) multilayer structure driven by a spin polarized current. In this paper, an analytical model based on Thiele equation is developed to describe the properties of a skyrmion pair in a SAF structure driven by a magnetic field gradient. Combining the analytical solutions with the micromagnetic simulation results, we demonstrate the essential dynamic features of the skyrmion pair as a function of AF coupling and magnetic field gradients, revealing that the skyrmion pair motion could be suppressed by increasing the interlayer antiferromagnetic coupling.

The ferromagnetic resonance induced by spin-transfer torque (ST-FMR) is important for microwave detectors, as it can convert an ac microwave signal directly into a dc signal that can be easily measured. However, so far only one resonance mode could be detected for a given external magnetic field, resulting in a relatively low efficiency. This study considers ST-FMR in geometrically confined nanoring magnetic tunnel junctions (NR-MTJs), in which $t\phantom{\rule{0}{0ex}}w\phantom{\rule{0}{0ex}}o$ resonance modes are observed at a given magnetic field. This result might significantly improve the efficiency of such microwave detectors, by means of a dual-mode detection configuration.

We theoretically investigate the effect of stray field on the current-induced magnetization dynamics in a nanopillar with a perpendicular spin polarizer. The threshold current to excite out-of-plane (OP) magnetization procession is analytically calculated based on a simple Newton-like dynamic equation. The numerical calculations of this analytical expression are confirmed by macrospin simulations, revealing that the critical current linearly depends on the stray field. The stray field versus current phase diagram of magnetization oscillation states in such a device is presented, showing three regions of magnetization dynamics including parallel, antiparallel, and OP precession states. The physical insight of the phase transitions between the three states can be explained by calculating the energy profiles as well as the phase portraits of the free layer.

We report a comprehensive study of annealing treatment modulation on the laser-induced ultrafast magnetic behaviors in antiferromagnetically exchange-coupled [Ni/Co]4/Ru/[Co/Ni]3 multilayers with perpendicular magnetic anisotropy (PMA). Magnetic hysteresis loops indicate that the uniaxial PMA field Hkeff decreases monotonously with the increase in annealing temperature Ta, but the variation of interlayer coupling field Hex is rather complicated. Time-resolved magnetic-optical Kerr effect (MOKE) measurements demonstrate that the laser-excited demagnetization and precession process relies significantly on Ta. Upon laser impulsion, the MOKE signal immediately shows a nonchanging transient increase and decrease with H increasing for low Ta, but only the ultrafast decreasing behavior for high Ta. From the subsequent dynamic precession spectra, the optical and acoustic precession modes are identified. By fitting the field-dependent frequency curves via the deduced dispersion relations, both Hkeff and Hex are determined and their variation trends agree well with the results from the static magnetic measurement. Moreover, it is found that the critical field where the ultrafast signal decrease occurs is dependent on the co-effect of Hkeff and Hex, whereas the maximum field at which the optical mode precession disappears shares the same trend as Hex. The magnetic damping of acoustic mode is seen to increase with Ta due to the increased inhomogeneities. Our findings provide a deep understanding of the magnetic properties in synthetic antiferromagnetic multilayers with high annealing temperatures, which will be helpful for designing advanced spintronic devices.

We present an overview in the understanding of spin-transfer torque (STT) induced magnetization dynamics in spin-torque nano-oscillator (STNO) devices. The STNO contains an in-plane (IP) magnetized free layer and an out-of-plane (OP) magnetized spin polarizing layer. After a brief introduction, we first use mesoscopic micromagnetic simulations, which are based on the Landau–Lifshitz–Gilbert equation including the STT effect, to specify how a spin-torque term may tune the magnetization precession orbits of the free layer, showing that the oscillator frequency is proportional to the current density and the z-component of the free layer magnetization. Next, we propose a pendulum-like model within the macrospin approximation to describe the dynamic properties in such type of STNOs. After that, we further show the procession dynamics of the STNOs excited by IP and OP dual spin-polarizers. Both the numerical simulations and analytical theory indicate that the precession frequency is linearly proportional to the spin-torque of the OP polarizer only and is irrelevant to the spin-torque of the IP polarizer. Finally, a promising approach of coordinate transformation from the laboratory frame to the rotation frame is introduced, by which the nonstationary OP magnetization precession process is therefore transformed into the stationary process in the rotation frame. Through this method, a promising digital frequency shift-key modulation technique is presented, in which the magnetization precession can be well controlled at a given orbit as well as its precession frequency can be tuned with the co-action of spin polarized current and magnetic field (or electric field) pulses.

Ultrafast demagnetization excited by laser pulses introduces the technological possibility for controlling the magnetic properties of materials on sub-picosecond regime. In this paper, the demagnetization dynamics of amorphous ferrimagnetic TbFeCo alloy is modeled by using the stochastic Landau-Lifshitz-Gilbert equation on an atomistic level. The transition metal (Fe, Co) and rare earth (Tb) atoms are constructed by two different sub-lattice systems. Laser-heating induced demagnetization processes are clearly demonstrated by a series of snapshots. Our simulations show that full demagnetization on the sub-picosecond time scale requires a temperature over the Curie point, and the demagnetization time increases with the Tb content.

Recent experiments performed in current-driven nano-contacts with strong perpendicular anisotropy have shown that spin-transfer torque can drive self-localized spin waves [1, 2] that above a certain threshold intensity can condense into a highly nonlinear magnetodynamic and nano-sized state known as a magnetic droplet soliton [3]. Here we demonstrate analytically, numerically, and experimentally that at sufficiently large driving currents, and for a spin polarization that is tilted away from the film normal, the circular droplet soliton can become unstable to periodic excitations of its perimeter. We furthermore show that these perimeter excitation modes (PEMs) are parametrically excited when the fundamental droplet soliton precession frequency is close to twice the frequency of one or more of the PEMs. As a consequence, for increasing applied fields, progressively higher PEMs can be excited. Quantitative agreement with experiment confirms this picture., Submitted to PRB

A spin-torque nanopillar oscillator (STNO) that combines a perpendicular-to-plane polarizer (PERP) with an in-plane magnetized free layer is a good candidate for phase locking, which opens a potential approach to enhancement of the output power of STNOs. In this paper, the magnetic dipolar coupling effect is used as the driving force to synchronize two STNOs. We develop an approximation theory for synchronizing two identical and nonidentical pairs of PERP STNOs, by which the critical current of synchronization, dipolar coupling strength, phase-locking transient time, and frequency can be analytically predicted. These predictions are further confirmed by macrospin and micromagnetic simulations. Finally, we show the phase diagrams of the phase locking as a function of applied current and separation between two STNOs.

We demonstrate how magnetic droplet soliton pairs, nucleated by two separated nano-contact (NC) spin torque oscillators, can merge into a single droplet soliton. A detailed description of the magnetization dynamics of this merger process is obtained by micromagnetic simulations: A droplet pair with a steady-state in-phase spin precession is generated through the spin-transfer torque effect underneath two separate NCs, followed by a gradual expansion of the droplets volume and the out phase of magnetization on the inner side of the two droplets, resulting in the droplets merging into a larger droplet. This merger occurs only when the NC separation is smaller than a critical value. A transient breathing mode is observed before the merged droplet stabilizes into a steady precession state. The precession frequency of the merged droplet is lower than that of the droplet pair, consistent with its larger size. Merged droplets can again break up into droplet pairs at high enough magnetic field with a strong hysteretic response., Comment: accepted by Physical Review B

Effective manipulation of magnetization orientation driven by electric field in a perpendicularly magnetized tunnel junction introduces technologically relevant possibility for developing low power magnetic memories. However, the bipolar orientation characteristic of toggle-like magnetization switching possesses intrinsic difficulties for practical applications. By including both the in-plane (T//) and field-like (T⊥) spin-transfer torque terms in the Landau-Lifshitz-Gilbert simulation, reliable and deterministic magnetization reversal can be achieved at a significantly reduced current density of 5×109 A/m2 under the co-action of electric field and spin-polarized current, provided that the electric-field pulse duration exceeds a certain critical value τc. The required critical τc decreases with the increase of T⊥ strength because stronger T⊥ can make the finally stabilized out-of-plane component of magnetization stay in a larger negative value. The power consumption for such kind of deterministic magnetization switching is found to be two orders of magnitude lower than that of the switching driven by current only.

Magnetization orientation of a nanoscale ferromagnet can be manipulated by an electric current via spin-transfer torque (STT) effect, which holds great promise in the applications of non-volatile magnetic random access memory (MRAM) and spintorque oscillators. We review the fundamental mechanism and experimental progress of the STT effect. Then, different formula of STT torque has been classified, which can be added to the conventional Landau-Lifshitz-Gilbert equation. After that, we show some simulation results that mainly concern the STT-driven vortex dynamics, magnetization oscillations excited by a perpendicular polarizer, and the detail dynamics by in-plane and out-of-plane dual spin polarizers.

Spin-torque excited ferromagnetic resonance (ST-FMR) performed in nanoscale devices can be used to characterize material properties and optimize spintronic devices for applications. In this paper, micromagnetic simulation is employed to study the FMR property in a nano-ring shaped magnetic tunnel junction. The simulation predictions are confirmed by experiments, revealing that two resonance states (mode-1 and mode-2) with different resonance frequencies are excited by a microwave frequency current or magnetic field. The relatively weak response signal of FMR at the negative field can be attributed to the intrinsic asymmetric structure of magnetic tunnel junctions, which leads to a bias-dependent asymmetric spin transfer torque. Besides, high-order harmonics with two and three times the fundamental frequencies of the mode-1 and mode-2 are also observed.

Laser-induced magnetization switching in GdFe ferrimagnets strongly depends on the pulse duration. To gain insight into the ferrimagnetic switching process, Landau–Lifshitz–Gilbert (LLG) spin dynamic modeling is performed at an atomistic-level with two sub-lattice systems. The switching dynamics triggered by laser pulses can be considered as two stages: Demagnetization stage by the laser pulse heating and re-magnetization at the cooling down stage after the pulse impulsion. By tuning the laser pulse duration, the intensity of demagnetization can be well controlled, by which the subsequent magnetization switching during the cooling process can be obtained. The simulation results indicate that the occurrence of the transient ferromagnetic-like (TFL) state is necessary during the magnetization switching process. Furthermore, the thermal temperature and heating time play an important role for the ferrimagnetic switching. In the overheating region, the random switching behavior appears because the TFL state is uncontrollable.

Synchronization of spin-torque nano-oscillators (STNOs) by magnetic dipolar interactions in a parallel electrical connection (PC) introduces the technological possibility for an enhancement of output signal. In this study, we consider two identical or nonidentical STNOs in serial electrical connection (SC). By using a pendulum-like approximation model, we analytically predict the dipolar coupling strength, critical current and frequency of phase-locking precession. The numerical calculations of those analytical expressions are confirmed by macrospin simulations, revealing that the dipolar coupling strength responsible for the phase-locking in SC case is three times larger than that in the PC case for the two STNOs with the same separation, which results in the enlarged and symmetric phase-locking window of frequency versus current. These results suggest that the SC of multiple STNOs could be more promising for the enhancement of output power.

The ferromagnetic thin-film elements can be prepared to form a ?diamond? remanent state that contains two vortices with opposite circulation and with the same or opposite core polarities. The vortex polarity could influence their normal modes through the magnetostatic interactions between two vortices. In this paper, micromagnetic simulations, combined with Fourier techniques, are employed to study the spin-wave (SW) modes excited in a submicron Permalloy ellipse with a vortex pair configuration. After the application of an inplane Gaussian field pulse, it is found that a series of azimuthal SW modes is excited and the high-order harmonics of SW modes are strongly dependent of the core polarities due to the dynamic magnetic interactions between the two vortices.

Reliable manipulation of magnetic droplets is of immense importance for their applications in spin torque oscillators. Using micromagnetic simulations, we find that the antiphase precession state, which originates in the dynamic dipolar interaction effect, is a favorable stable state for two magnetic droplets nucleated at two identical nano-contacts. A magnetic field pulse can be used to destroy their stability and merge them into a big droplet. The merging process strongly depends on the pulse width as well as the pulse strength.

Magnetic fields generated by current lines are the standard way of switching between resistance (R) states in magnetic random access memories. A less common but technologically more interesting alternative to achieve R-switching is to use an electrical current crossing the tunnel barrier. Such current induced magnetization switching (CIMS) or current induced switching (CIS) effects were recently observed in thin magnetic tunnel junctions, and attributed to spin transfer (CIMS) or electromigration of atoms into the insulator (CIS). In this work, electromigration-driven resistance changes (resistive switching) are superimposed with thermally induced pinned layer reversal (magnetic switching), producing a reproducible, three-state memory device. The tunnel junctions under study show a tunnel magnetoresistance of 14% with a RA product of 50 �µ m 2 . The reversible electromigration-driven resistance changes amount to 7–8% of the full resistance change and more than 10 4 R-switching events can be current induced without significant damage to the tunnel junction. Typical critical current densities are of the order of 2 × 10 6 Ac m −2 .

Magnetic skyrmions are topologically protected nanoscale objects, which are promising building blocks for novel magnetic and spintronic devices. Here, we investigate the dynamics of a skyrmion driven by a spin wave in a magnetic nanowire. It is found that (i) the skyrmion is first accelerated and then decelerated exponentially; (ii) it can turn L-corners with both right and left turns; and (iii) it always turns left (right) when the skyrmion number is positive (negative) in the T- and Y-junctions. Our results will be the basis of skyrmionic devices driven by a spin wave., Comment: 10 pages, 7 figures

Recently, reversible resistance (R) changes were observed in thin tunnel junctions (TJ) when a critical electrical current was applied. These changes are called current-induced switching (CIS) and are attributed to electromigration in nanoconstrictions in the insulating barrier. Here, we study the CIS effect on a thin TJ prepared by IBD, displaying a 3.4% R change when a CIS cycle is performed at room temperature. After complete (or half) CIS cycles with adequate maximum currents, we monitored R as a function of time. In both cases a non-monotonic relaxation occurs with two distinct relaxation times, τ 1 ∼ 10 min , τ 2 ∼ 1 0 2 min . First R increases (decreases) rapidly, but then a slow relaxation dominates, reducing (increasing) R. These opposite relaxation processes suggest two independent physical mechanisms acting simultaneously inside the TJ. The physical origin of these effects is discussed.

The effective manipulation of skyrmion motion introduces the technologically relevant possibility of skyrmion-based spintronics. Here we show how a magnetic skyrmion can be captured by a radially spatial gradient of the magnetic field. A theoretical model governed by the Thiele equation is employed to study the skyrmion motion. The analytical predictions are compared to micromagnetic simulations based on the Landau–Lifshitz–Gilbert equation, showing that the dynamic behavior of skyrmions strongly depends on the gradient strength of magnetic fields as well as the radius of the skyrmion and the Gilbert damping factor. Moreover, we find that the skyrmions can also be dragged by a moving magnetic tip.

Manipulating dissipative magnetic droplet is of great interest for both the fundamental and technological reasons due to its potential applications in the high frequency spin-torque nano-oscillators. In this paper, a magnetic droplet pair localized in two identical or non-identical nano-contacts in a magnetic thin film with perpendicular anisotropy can phase-lock into a single resonance state by using an oscillating microwave magnetic field. This resonance state is a little away from the intrinsic precession frequency of the magnetic droplets. We found that the phase-locking frequency range increases with the increase of the microwave field strength. Furthermore, multiple droplets with a random initial phase can also be synchronized by a microwave field.

An analytical pendulum-like model is developed to describe the dynamics of spin torque nano-oscillators that contain a perpendicular spin current polarizer combined with an in-plane magnetized free layer. Both the in-plane localized oscillation mode and out-of-plane (OP) large angle precession mode have been observed in the absence of damping effect, which can be attributed to the competition between the in-plane uniaxial anisotropic and demagnetization energies. These two types of dynamical oscillation modes correspond to the spin-transfer torque effect acting as a conservative or non-conservative driving force, respectively. We theoretically predict the critical currents for the excitation of the OP procession mode, which are well consistent with the macrospin simulations based on the Landau-Lifshitz-Gilbert equation. Moreover, we show the phase portrait of magnetization precession dynamics at different current and magnetic fields.

Devices displaying resistive-switching are being researched as possible candidates for non-volatile Random Access Memories. Here we study resistance ( R ) switching of non-magnetic origin on CoFe/AlO x /CoFe tunnel junctions. We observe R -switching to a low state for negative currents (top to bottom lead) only, here associated with electromigration (EM) of ions from the bottom metallic electrode into the insulator. EM in the reverse sense occurs only for positive current. We thus show the existence of asymmetric electromigration between the two electrode/barrier interfaces, likely reflecting their structural differences.

We present a micromagnetic simulation for detection of magnetic labels modulated by an ac magnetic field. As the sensing element, spin valves with synthetic ferrimagnetic (SF) free and pinned layers are optimized. At lower fields, the sensors with SF free layers show linear transfer curves, demonstrating a coherent switching due to the strong antiferromagnetic coupling inside the SF free layers. The sensor offset field can be reduced by tuning the effective thickness of SF free and pinned layers. Magnetic simulation for the detection of 250-nm magnetic labels with this sensor shows a linear giant magnetoresistive signal dependence on the number of particles.

Current induced switching (CIS) was recently observed in thin magnetic tunnel junctions and attributed to electromigration of metallic atoms in nanoconstrictions in the insulating barrier. Here we study the CIS effect in tunnel junctions with a Ta layer deposited just below the insulating barrier. We observe enhanced resistance switching with increasing maximum applied current. Anomalous voltage–current characteristics with negative derivative were observed and this effect is attributed to heating in the tunnel junction. We also observe that, after repeatedly applying large current pulses, the electrical resistance decreases sharply through irreversible steps when ions are displaced into the barrier.

Spin-transfer-induced spin-wave excitations in spin-valve nanopillars with perpendicular polarizers have been investigated by micromagnetic simulations. Two coexistent steady states with in-plane magnetizations are observed at small current. With the increased current, periodic oscillating spin waves with frequency from several to tens of gigahertz can be tuned by current alone. High-order harmonics with odd times of fundamental frequency only are observed in these periodic oscillations. The fundamental frequency decreases with the increased thickness of the free layer. Large current could excite chaotic spin waves

A micromagnetic simulation for the detection of magnetic nanobeads by planar Hall sensors is presented. The calculated dipole field created by the polarized nanobeads shows the strong dependence on the distance from beads to the sense plane. In this simulation, nonuniform dipole field, random configuration of beads, and experimental parameters of device were used. Micromagnetic calculation shows the linear signal response to the bead number. This linear behavior is insensitive to the random distribution of beads, but is sensitive to the bead number, indicating the possible application to quantitative biomolecular recognition.

Using first-principles density-functional theory calculations, we systematically investigate the magnetic anisotropy of the multilayer system Cu/(FePt)n/MgO, a promising spintronics structure. Particularly, we have studied the influence of the epitaxial strain, thickness of the ferromagnetic layer, and different interfaces on the magnetic anisotropy energy (MAE) of the system. It is found that the thickness of FePt has slight influence on the MAE, while the increase of the in-plane lattice constant a, or tensile strain, can significantly reduce and even change the sign of the MAE. The calculated density of states shows that the occupation number of the minority spin channel of Fe dx(2)-y(2) orbital decreases with the increase of a, which leads to the reduction of the orbital moment anisotropy of the Fe atom and therefore the decrease of MAE. We also consider the influence of the Cu/FePt and FePt/MgO interfaces on the MAE, and find that both interfaces can reduce the MAE. Especially, the effect of the Cu/FePt interface is more pronounced due to the increased occupation number of the minority spin channel of Fe dz(2) orbital.

Equipe 101 : Nanomagnétisme et électronique de spin; International audience; Special spin-transfer-driven magnetic behaviors can take place in a spin valve nanopillar with perpendicular anisotropy due to reduced symmetry of easy axis. Micromagnetic simulation demonstrates a dip in the average magnetization curve, which corresponds to the experimentally observed undulation of dc resistance. The dip is a signature of spin-transfer-driven reversable magnetic process with slightly tilted easy axis in the free layer. This featured magnetization dynamics includes non-uniform magnetization precession and multi-mode propagating spin waves, which are attributed to the competition among tilted magnetic anisotropy, magnetic field, and spin transfer torque. (C) 2013 American Institute of Physics.

In this paper, we performed spin simulations at atomistic level to study the temperature dependent properties of perpendicularly magnetized TbFe thin films. The crystallographically amorphous feature of TbFe ferrimagnetic alloys is modeled by using a lattice system with disordered site occupation of rare earth (RE) and transition metal (TM) spins. The simulated Curie temperature ([Formula: see text]) is consistent well with the mean-field approximation theory. With the increase of Tb concentration, the [Formula: see text] decreases almost linearly, whereas the magnetization compensation temperature ([Formula: see text]) increases gradually until the [Formula: see text] value is reached. The inter-sublattice exchange coupling strength [Formula: see text] between the RE and TM atoms can significantly affect [Formula: see text], but has less impact on [Formula: see text]. With the increase of Tb concentration, the TbFe sample of high [Formula: see text] exhibits a much faster increase in [Formula: see text] than the sample with low [Formula: see text]. Moreover, we have tested the simulation code to model the laser pulse induced ultrafast nonequilibrium spin dynamics. As an example, the femto-second pulse laser induced demagnetization and recovery process is clearly reproduced. These features are in a good agreement with the experiments, indicating that the simulation model can capture the basic physics in describing the high temperature dependent magnetic property as well as the ultrafast spin dynamics.

Experimental results on the current-induced resistance change in low-resistance (10-60 /spl Omega//spl middot//spl mu/m/sup 2/) magnetic tunnel junctions with AlO/sub x/ barriers are presented. The observed current-induced resistance change remains unaltered even when the ferromagnetic electrodes are saturated by an external field of 8.5 kOe and, therefore, is not related to the relative magnetization configuration of both electrodes. The critical switching current increases with decreasing barrier thickness (increasing hot-spot density), as well as with increasing junction area. Repeated switching (4000 pulses) leads to junction resistance decrease by 3%-4% and a decrease of the relative current-induced resistance change of about 20%, indicating an increase of hot spots or pinhole density upon current stressing. Critical current increases as current pulsewidth decreases.

The occurrence of pinholes in thin barrier low-resistance junctions degrades the TMR signal and increases the coupling field between pinned and free layers. The tunnel junction coupling field (Hf), junction resistance and TMR signal dependence on the barrier thickness was studied for various types of barriers (HfOx,HfAlOx,ZrAlOx,AlOx). Micromagnetic simulation was employed to simulate the coupling field versus pinhole density. From the coupling field results, HfOx makes the thinnest continuous barriers, followed by doped HfAlOx and ZrAlOx, and then AlOx. HfAlOx and ZrAlOx offer the best compromise between low resistance (1–5 Ω μm2) and reasonable TMR (12%–14%). Pure HfOx can be made with R×A products of 0.4 Ω μm2 but the TMR does not exceed 5.5%.