Spin-torque Effect Research Articles

Achievement of high diode sensitivity via spin torque-induced resonant expulsion in vortex magnetic tunnel junction

We investigated the spin-torque diode effect in a magnetic tunnel junction with FeB free layer. Vortex-core expulsion was observed near the boundary between vortex and uniform states. A high diode voltage of 24 mV was obtained with alternative input power of 0.3 µW, corresponding to huge diode sensitivity of 80,000 mV/mW. In the expulsion region, a broad peak in the high frequency region was observed, which is attributed to the weak excitation of uniform magnetization by thermal noise. The high diode sensitivity is of great importance for device applications such as telecommunications, radar detectors, and high-speed magnetic-field sensors.

Potential implementation of reservoir computing models based on magnetic skyrmions

Reservoir Computing is a type of recursive neural network commonly used for recognizing and predicting spatio-temporal events relying on a complex hierarchy of nested feedback loops to generate a memory functionality. The Reservoir Computing paradigm does not require any knowledge of the reservoir topology or node weights for training purposes and can therefore utilize naturally existing networks formed by a wide variety of physical processes. Most efforts to implement reservoir computing prior to this have focused on utilizing memristor techniques to implement recursive neural networks. This paper examines the potential of magnetic skyrmion fabrics and the complex current patterns which form in them as an attractive physical instantiation for Reservoir Computing. We argue that their nonlinear dynamical interplay resulting from anisotropic magnetoresistance and spin-torque effects allows for an effective and energy efficient nonlinear processing of spatial temporal events with the aim of event recognition and prediction.

Spin Transport and Dynamics in Multilayer Magnetic Nanostructures

The interconnection between the spin current and spin dynamics via the spin-dependent scattering and an accompanying by spin torque effect in ferromagnetic/normal metal based magnetic multilayer nanostructures is studied including a high fast out-of-equilibrium spin dynamics. Features of the spin transport through interfaces and its impact on spin dynamics are described on the base of the scattering matrix formalism for spin flows. The dependence of the spin torque effect on conductance character of the normal metal layers is considered. The exchange processes between the itinerant <i>s</i> and the localized <i>d</i> electrons are described by kinetic rate equations for electron-magnon spin-flop scattering. It is shown that the magnon distribution function remains nonthermalized on the relevant time scales of the demagnetization process, and the relaxation of the out-of-equilibrium spin accumulation among itinerant electrons provides the principal channel for dissipation of spin angular momentum from the combined electronic system.

Electric field driven magnetic switching in nanoscale multiferroic heterostructures

Recently, there has been a surge of research interest in the electric field control of magnetism due to its promising application in spintronic and memory devices, which has become a hot topic in the field of multiferroic research. In current spintronic technology, magnetic reversal is usually driven by a large electric current via current generated magnetic field or spin-torque effect to write/erase a magnetic bit, and thus producing large power consumption and heat dissipation. While using insulating multiferroic materials, the reversal of magnetization can be triggered by applying an electric field instead of current, hence dramatically reducing the energy consumption and heat dissipation. With the current miniature trend in microelectronic technology, it is very essential to explore the electric field driven magnetic reversal (EFMS) behaviours in a micro/nanometer scale. In this article we briefly review the new progress in the field of EFMS based on multiferroic heterostructures, including some new features arising from size reduction, as well as some recent experimental and theoretical advances towards nanoscale EFMS, e.g. strain-mediated coupling, or spin exchange coupling in BiFeO3-based heterostructures, and their associated mechanisms. Finally, some key challenges in developing future EFMS based magnetoelectric devices, and some prospects for future research are also discussed.

Damping Dependence of Spin-Torque Effects in Thermally Assisted Magnetization Reversal

Thermal fluctuations of nanomagnets driven by spin-polarized currents are treated via the Landau-Lifshitz-Gilbert equation as generalized to include both the random thermal noise field and Slonczewski spin-transfer torque terms. The magnetization reversal time of such a nanomagnet is then evaluated for wide ranges of damping by using a method which generalizes the solution of the so-called Kramers turnover problem for mechanical Brownian particles, thereby bridging the very low damping and intermediate damping Kramers escape rates, to the analogous magnetic turnover problem. The reversal time is then evaluated for a nanomagnet with the free energy density given in the standard form of superimposed easy-plane and in-plane easy-axis anisotropies with the dc bias field along the easy axis.

Consistent microscopic analysis of spin pumping effects

We present a consistent microscopic study of spin pumping effects for both metallic and insulating ferromagnets. As for metallic case, we present a simple quantum mechanical picture of the effect as due to the electron spin flip as a result of a nonadiabatic (off-diagonal) spin gauge field. The effect of interface spin-orbit interaction is briefly discussed. We also carry out field-theoretic calculation to discuss on the equal footing the spin current generation and torque effects such as enhanced Gilbert damping constant and shift of precession frequency both in metallic and insulating cases. For thick ferromagnetic metal, our study reproduces results of previous theories such as the correspondence between the dc component of the spin current and enhancement of the damping. For thin metal and insulator, the relation turns out to be modified. For the insulating case, driven locally by interface $sd$ exchange interaction due to magnetic proximity effect, physical mechanism is distinct from the metallic case. Further study of proximity effect and interface spin-orbit interaction would be crucial to interpret experimental results in particular for insulators.

Thermally Assisted Spin Torque Switching in Magnetic Random Access Memory

Magnetic random access memory (MRAM) consisting of magnetic tunnel junctions (MTJs) is a fascinating candidate for high-density nonvolatile memory. An MTJ has two ferromagnetic metals called free and pinned layers separated by nonmagnetic spacer. The free layer in the MTJ shows two energetically stable states corresponding to the parallel and antiparallel alignments of the magnetizations. These magnetization alignments are distinguished by tunnel magnetoresistance (TMR) effect, i.e., the resistance at the antiparallel alignment is higher than that at the parallel alignment. The free layer is switched between these two stable states by spin torque effect [1], where the transfer of the spin angular momentum from the conducting electrons to the free layer excites the spin torque and induces the magnetization dynamics. The observation of the high TMR ratio more than 100% in CoFeB/MgO-based MTJ [2] has motivated both experimental and theoretical researchers to develop practical MRAM. The free layer should stay one of the stable states to keep information in MRAM. Unfortunately, however, the thermal fluctuation induces probabilistic switching of MTJ. Therefore, high thermal stability is required to guarantee the MRAM performance. Thermal stability of the MTJ is defined by the energy barrier of the free layer separating the two stable states divided by the thermal energy, k B T. Since the retention time of MRAM is exponentially proportional to the thermal stability, a small error in the evaluation of the thermal stability affects the estimation of the retention time significantly. Thus, an accurate evaluation of the thermal stability is necessary. The thermal stability has been evaluated by analyzing the switching probability of the free layer’s magnetization in the thermally activated region assisted by the spin torque. The fitting function of the switching probability is P=1-exp[-ft exp[-Δ(1-I/I c) b ]], where f, Δ, I, and I c are the attempt frequency, thermal stability, current, and switching current at zero temperature, respectively. The switching exponent, b, is assumed to be one [3]. In this study, we study the thermally assisted spin torque switching in an in-plane magnetized MTJ theoretically. The theoretical formula of the switching probability is derived from the Fokker-Planck equation. We find that the switching exponent is larger than one and depends on the applied electric current. The assumption that the switching exponent is one used in previous works is approximately valid only near the low current region, while relatively large current is applied in experiments to obtain the switching probability efficiently. In the high current region, the switching exponent is almost two. We also perform the numerical simulation of the Landau-Lifshitz-Gilbert (LLG) equation with stochastic torque. Figure 1(a) shows the dependence of the probability density, dP/dt, on the current pulse time for several temperatures. As shown, the probability density shows a peak at certain time. We call this time the switching time. Figure 1(b) shows the temperature dependence of the switching time on the temperature. It should be noted that the curvature of the switching time reflects the switching exponent [4]. The nonlinear dependence of the switching time is another evidence that the switching exponent is larger than one. The fact that the switching exponent is larger than one indicates that the previous works have underestimated the thermal stability. The presented formula enables us to evaluate the thermal stability with high accuracy. We also notice that the switching current at zero temperature, I c, can also be evaluated from the switching probability and our formula. We will talk the details of the theoretical analysis of the switching probability and numerical simulation. A comparison with latest experiments will also be presented. (Figure caption) Figure 1: (a) The dependence of the switching probability density on the current pulse time for several temperatures. (b) The temperature dependence of the switching time. [1] J. C. Slonczewski, J. Magn. Magn. Mater. 159, L1 (1996). [2] S. Yuasa, T. Nagahama, A. Fukushima, Y. Suzuki, and K. Ando, Nat. Mater. 3, 868 (2004). S. S. P. Parkin, C. Kaiser, A. Panchula, P. M. Rice, B. Hughes, M. Samant, and S.-H. Yang, ibid 3, 862 (2004). [3] R. H. Koch, J. A. Katine, and J. Z. Sun, Phys. Rev. Lett. 92, 088302 (2004). [4] T. Taniguchi and H. Imamura, J. Nanosci. Nanotechnol. 12, 7520 (2012). T. Taniguchi, M. Shibata, M. Marthaler, Y. Utsumi, and H. Imamura, Appl. Phys. Express 5, 063009 (2012). Figure 1

Unconventional Superfluidity in Yttrium Iron Garnet Films.

We argue that the magnon condensate in yttrium iron garnet may display experimentally observable superfluidity at room temperature despite the 100 times dominance of the normal density over superfluid ones. The superfluidity has a more complicated nature than in known superfluids since the U(1) symmetry of the global phase shift is violated by the dipolar interaction leading to the exchange of spin moment between the condensate and the crystal lattice. It produces periodic inhomogeneity in the stationary superfluid flow. We discuss the manner of observation and possible applications of magnon superfluidity. It may strongly enhance the spin-torque effects and reduce the energy consumption of the magnonic devices.

Giant spin-torque diode sensitivity in the absence of bias magnetic field.

Microwave detectors based on the spin-torque diode effect are among the key emerging spintronic devices. By utilizing the spin of electrons in addition to charge, they have the potential to overcome the theoretical performance limits of their semiconductor (Schottky) counterparts. However, so far, practical implementations of spin-diode microwave detectors have been limited by the necessity to apply a magnetic field. Here, we demonstrate nanoscale magnetic tunnel junction microwave detectors, exhibiting high-detection sensitivity of 75,400 mV mW−1 at room temperature without any external bias fields, and for low-input power (micro-Watts or lower). This sensitivity is significantly larger than both state-of-the-art Schottky diode detectors and existing spintronic diodes. Micromagnetic simulations and measurements reveal the essential role of injection locking to achieve this sensitivity performance. This mechanism may provide a pathway to enable further performance improvement of spin-torque diode microwave detectors.

Nanoconstrictions for investigating spin transfer torque effect in granular Ag–Co films

AbstractWe used lithographically fabricated nanoconstrictions made of Ag–Co granular thin films to achieve current densities above 10 A/cm during magnetotransport measurements. Systematically measured variations of the resistance with current under high applied magnetic fields did not reveal any dramatically large magnetoresistance effect, in contrast to the previous observations using mechanical point contact based techniques. Our experimental results on resistance switching by current injection in the Ag–Co nanoconstrictions reveal spin torque effects. The switching amplitudes are considerably smaller when compared to the results obtained for pillar shaped trilayers due to the large number of Co grains in the nanoconstrictions. We demonstrate that the use of point contact patterns with nanometer dimensions, where effects resulting from current induced mechanical distortions at high magnetic fields are negligible, has obvious advantages over the mechanical point contact technique for studying spin injection effects.

Spin-torque resonant expulsion of the vortex core for an efficient radiofrequency detection scheme.

It has been proposed that high-frequency detectors based on the so-called spin-torque diode effect in spin transfer oscillators could eventually replace conventional Schottky diodes due to their nanoscale size, frequency tunability and large output sensitivity. Although a promising candidate for information and communications technology applications, the output voltage generated from this effect has still to be improved and, more pertinently, reduces drastically with decreasing radiofrequency (RF) current. Here we present a scheme for a new type of spintronics-based high-frequency detector based on the expulsion of the vortex core in a magnetic tunnel junction (MTJ). The resonant expulsion of the core leads to a large and sharp change in resistance associated with the difference in magnetoresistance between the vortex ground state and the final C-state configuration. Interestingly, this reversible effect is independent of the incoming RF current amplitude, offering a fast real-time RF threshold detector.

Spin effects induced by thermal perturbation in a normal metal/magnetic insulator system

Using one of the methods of quantum nonequilibrium statistical physics, we have investigated the spin transport transverse to the normal metal/ferromagnetic insulator interface in hybrid nanostructures. An approximation of the effective parameters, when each of the interacting subsystems (electron spin, magnon, and phonon) is characterized by its own effective temperature, has been considered. The generalized Bloch equations which describe the spin-wave current propagation in the dielectric have been derived. Finally, two sides of the spin transport ``coin'' have been revealed: the diffusive nature of the magnon motion and magnon relaxation processes, responsible for the spin pumping, and the spin-torque effect.

Microwave detectors based on the spin-torque diode effect

The spin-transfer torque (STT) effect provides a new method of manipulation of magnetization in nanoscale objects. The STT effect manifests itself as a transfer of spin angular momentum between the parallel magnetic layers separated by a nonmagnetic spacer and traversed by a dc electric current. The transfer of the spin angular momentum from one layer to another could result in the excitation of the microwave-frequency magnetization dynamics in one of the magnetic layers. On the other hand, when a magnetization dynamics is excited in a magnetic layered structure by an external microwave signal both the structure electrical resistance and current through the structure will acquire microwave components resulting in the appearance of a rectified dc voltage on the magnetic structure. This “spin-torque diode effect” can be used for the development of ultra-sensitive spin-torque microwave detectors (STMD). Below we present a brief review of our recent work on the general properties of STMDs, analyze the performance of the “resonance-type” and “threshold-type STMD” and consider the possible applications for such microwave detectors.

Effect of stripe height on the critical current density of spin-torque noise in a tunneling magnetoresistive read head with a low resistance area product below 1.0 Ω μm2

To understand the spin-torque effect on the noise in tunneling magnetoresistive (TMR) read heads, the GHz range noise spectra of TMR read heads with a narrow track width (w = 36 nm), and various stripe heights (h) are investigated as a function of the external magnetic field (Hex) and dc bias current density (j). The strong noise peak intensity depends on both Hex and j, indicating that the spin-torque affects the thermal mag-noise under a positive (negative) j for a positive (negative) Hex, regardless of h in the TMR heads. Due to the increased shape anisotropy, the critical current density (jc), where the non-thermal fluctuation noise originates from the spin-torque, increases markedly as the head dimension is reduced, and the maximum value of jc is approximately +1.5 × 1012 A/m2 for a head with w = 36 nm and h = 15 nm. These results demonstrate that the non-thermal fluctuation noise originating from the spin-torque in the TMR head can be suppressed in the current density range below 1012 A/m2, as the head dimension is reduced and the shape anisotropy is increased.

Nanowire spin torque oscillator driven by spin orbit torques.

Spin torque from spin current applied to a nanoscale region of a ferromagnet can act as negative magnetic damping and thereby excite self-oscillations of its magnetization. In contrast, spin torque uniformly applied to the magnetization of an extended ferromagnetic film does not generate self-oscillatory magnetic dynamics but leads to reduction of the saturation magnetization. Here we report studies of the effect of spin torque on a system of intermediate dimensionality--a ferromagnetic nanowire. We observe coherent self-oscillations of magnetization in a ferromagnetic nanowire serving as the active region of a spin torque oscillator driven by spin orbit torques. Our work demonstrates that magnetization self-oscillations can be excited in a one-dimensional magnetic system and that dimensions of the active region of spin torque oscillators can be extended beyond the nanometre length scale.

Time-domain detection of current controlled magnetization damping in Pt/Ni81Fe19 bilayer and determination of Pt spin Hall angle

The effect of spin torque from the spin Hall effect in Pt/Ni81Fe19 rectangular bilayer film was investigated using time-resolved magneto-optical Kerr microscopy. Current flow through the stack resulted in a linear variation of effective damping up to ±7%, attributed to spin current injection from the Pt into the Ni81Fe19. The spin Hall angle of Pt was estimated as 0.11 ± 0.03. The modulation of the damping depended on the angle between the current and the bias magnetic field. These results demonstrate the importance of optical detection of precessional magnetization dynamics for studying spin transfer torque due to spin Hall effect.

Spin-torque diode radio-frequency detector with voltage tuned resonance

We report on a voltage-tunable radio-frequency (RF) detector based on a magnetic tunnel junction (MTJ). The spin-torque diode effect is used to excite and/or detect RF oscillations in the magnetic free layer of the MTJ. In order to reduce the overall in-plane magnetic anisotropy of the free layer, we take advantage of the perpendicular magnetic anisotropy at the interface between ferromagnetic and insulating layers. The applied bias voltage is shown to have a significant influence on the magnetic anisotropy, and thus on the resonance frequency of the device. This influence also depends on the voltage polarity. The obtained results are accounted for in terms of the interplay of spin-transfer-torque and voltage-controlled magnetic anisotropy effects.

Vector spin modeling for magnetic tunnel junctions with voltage dependent effects

Integration and co-design of CMOS and spin transfer devices requires accurate vector spin conduction modeling of magnetic tunnel junction (MTJ) devices. A physically realistic model of the MTJ should comprehend the spin torque dynamics of nanomagnet interacting with an injected vector spin current and the voltage dependent spin torque. Vector spin modeling allows for calculation of 3 component spin currents and potentials along with the charge currents/potentials in non-collinear magnetic systems. Here, we show 4-component vector spin conduction modeling of magnetic tunnel junction devices coupled with spin transfer torque in the nanomagnet. Nanomagnet dynamics, voltage dependent spin transport, and thermal noise are comprehended in a self-consistent fashion. We show comparison of the model with experimental magnetoresistance (MR) of MTJs and voltage degradation of MR with voltage. Proposed model enables MTJ circuit design that comprehends voltage dependent spin torque effects, switching error rates, spin degradation, and back hopping effects.

Dynamic exchange via spin currents in acoustic and optical modes of ferromagnetic resonance in spin-valve structures

Two ferromagnetic layers magnetically decoupled by a thick normal metal spacer layer can be, nevertheless, dynamically coupled via spin currents emitted by the spin-pump and absorbed through the spin-torque effects at the neighboring interfaces. A decrease of damping in both layers due to a partial compensation of the angular momentum leakage in each layer was previously observed at the coincidence of the two ferromagnetic resonances. In case of non-zero magnetic coupling, such a dynamic exchange will depend on the mutual precession of the magnetic moments in the layers. A difference in the linewidth of the resonance peaks is expected for the acoustic and optical regimes of precession. However, the interlayer coupling hybridizes the resonance responses of the layers and therefore can also change their linewidths. The interplay between the two mechanisms has never been considered before. In the present work, the joint influence of the hybridization and non-local damping on the linewidth has been studied in weakly coupled NiFe/CoFe/Cu/CoFe/MnIr spin-valve multilayers. It has been found that the dynamic exchange by spin currents is different in the optical and acoustic modes, and this difference is dependent on the interlayer coupling strength. In contrast to the acoustic precession mode, the dynamic exchange in the optical mode works as an additional damping source. A simulation in the framework of the Landau-Lifshitz-Gilbert formalism for two ferromagnetic layers coupled magnetically and by spin currents has been done to separate the effects of the non-local damping from the resonance modes hybridization. In our samples both mechanisms bring about linewidth changes of the same order of magnitude, but lead to a distinctly different angular behavior. The obtained results are relevant for a broad class of coupled magnetic multilayers with ballistic regime of the spin transport.

Damping parameter and interfacial perpendicular magnetic anisotropy of FeB nanopillar sandwiched between MgO barrier and cap layers in magnetic tunnel junctions

We systematically investigated the Gilbert damping (α) and interfacial perpendicular magnetic anisotropy (Ki) in magnetic tunnel junctions with a MgO-barrier/FeB/MgO-cap layered structure using the spin-torque diode effect. By increasing the MgO cap thickness, α decreased, whereas Ki increased monotonically. Values down to 0.0054 for α and up to 3.3 erg/cm2 for Ki were obtained for a MgO cap thickness of 0.6 nm. The small α and large Ki suggest that MgO-capped FeB is a suitable free layer for spintronics devices such as spin-torque oscillators and spin-torque magnetic random access memories.