Spin torque transfer structure with new spin switching configurations

Spin torque transfer structures with new spin switching configurations are proposed, fabricated and investigated in this paper. The non-uniform current-induced magnetization switching is implemented based on both GMR and MTJ nano devices. The proposed new spin transfer structure has a hybrid free layer that consists of a layer with conductive channels (magnetic) and non-conductive matrix (non-magnetic) and traditional free layer(s). Two mechanisms, a higher local current density by nano-current-channels and a non-uniform magnetization switching (reversal domain nucleation and growth) by a magnetic nanocomposite structure, contribute in reducing the switching current density. The critical switching current density for the new spin transfer structure is reduced to one third of the typical value for the normal structure. It can be expected to have one order of magnitude or more reduction for the critical current density if the optimization of materials and fabrication processes could be done further. Meanwhile, the thermal stability of this new spin transfer structure is not degraded, which may solve the long-standing scaling problem for magnetic random access memory (MRAM). This spin transfer structure, with the proposed and demonstrated new spin switching configurations, not only provides a solid approach for the practical application of spin transfer devices but also forms a unique platform for researchers to explore the non-uniform current-induced switching process.     

Reference layer exchange in spin transfer torque experiment using magnetic-coated nanometric point contacts

We investigate the importance of using nanotips on a point contact spin-transfer torque (STT) experiment. A systematic analysis comparing the STT in a magnetic thin film in current-perpendicular-to-plane (CPP) geometry sample for magnetic coated and uncoated tungsten nanotips is shown. The STT effect presents a reverse resistance to current behavior when using a magnetic coating layer on the nanotips. We demonstrate that the magnetic layer on the tip may assume the role of a polarizer layer. This effect opens up the possibility of exploiting simpler architectures in STT-based devices, such as STT-random access memory (STT-RAM).     

Temperature dependence of current-induced magnetization switching in spin valves with a ferrimagnetic CoGd free layer

The temperature dependence of current-induced magnetization switching of ferrimagnetic CoGd free layers in spin valves is explored. At temperatures well above and well below the magnetization compensation temperature (T(MC)) of CoGd, a current flowing from the free layer to the CoFe fixed layer aligns the moments of the two layers parallel, and a current flowing in the opposite direction aligns them antiparallel. However, for intermediate temperatures just above T(MC), the current-induced alignment of the moments is reversed. We attribute this effect to the different compensation temperatures of the net magnetization and angular momentum of CoGd.     

Analytic expression of the temperature increment in a spin transfer torque nanopillar structure

The temperature increment due to the Joule heating in a nanopillar spin transfer torque system is investigated. We obtain a time-dependent analytic solution of the heat conduction equation in nanopillar geometry by using the Green's function method after some simplifications of the problem. While Holm's equation is applicable only to steady states in metallic systems, our solution describes the time dependence and is also applicable to a nanopillar-shaped magnetic tunneling junction with an insulator barrier layer. The validity of the analytic solution is confirmed by numerical finite element method simulations and by the comparison with Holm's equation.     

Magnetization reversal dependence on magnetic properties of a spin torque oscillator with in-plane anisotropy free layer and orthogonal polarizer

The effect of magnetic properties on magnetization dynamics is studied for a spin torque oscillator (STO) composed of a free layer with an in-plane magnetic anisotropy and a reference layer with a fixed out-of plane magnetization. A transition from damped to uniform oscillations is observed for a critical value of saturation magnetization M_S). In the uniform oscillations regime, the frequency is inversely proportional to M_S. Similarly, the critical current for achieving uniform oscillations is investigated as a function of free layer intrinsic properties. In a second part of the study, the magnetostatic field (H_m) from the reference layer is considered and it is revealed that the out-of plane component of magnetization has a strong dependence on H_m. For a particular configuration, H_m could reduce the out-of plane component maximizing thus the out-put signal of the STO.     

Theoretical model of spin transfer torque in multilayers

We present a model of spin transport in a Co/Cu(1 1 1)/Co pseudo-spin-valve (PSV) structure where current is flowing in the current perpendicular-to-plane (CPP) geometry. The model considers ballistic spin-dependent transmission at the two Co–Cu interfaces, as well as diffusive spin relaxation within the Cu spacer and free Co layer. In the latter, the spin relaxation process is composed of the usual longitudinal spin relaxation due to spin flip scattering, as well as transverse spin relaxation due to spin precession. The resulting spin transfer torque exerted on the moments within the free Co layer is composed of two contributions, the main contribution coming from “absorbed” spins in the interfacial regions. The second contribution arises from the relaxation of spin accumulation within the free Co layer. The calculated critical current density for switching is estimated to be approximately between 3.3×10⁷ and 1.1×10⁸ A/cm², which is in agreement with available experimental results.     

Characterization of non-contact torque transfer and switching system for superconducting flywheel

Superconducting flywheel energy storage system can store the energy for a long duration, in that the main body of a flywheel is placed in a vacuum chamber to minimize rotational loss, and is separated from a generation motor. The superconducting flywheel device need a non-contact system which can transfer the rotational torque without contact. A combination of two permanent magnets can transmit the power without contact. We calculated the torque forces and the field distributions of two types of magnetic arrays; repulsive type and Halbach type. Both magnetic circuits have respective inner and outer diameters of 61.5 and 144 mm and consist of eight poles of Fe–Nd–B permanent magnets 30 mm in thickness. We also studied the effects of the number of poles and the size on the transferable torque forces and found that a practical torque transfer and switching systems can be constructed with a combination of permanent magnetic circuits.     

All-electric-controlled spin current switching in single-molecule magnet-tunnel junctions

A single-molecule magnet (SMM) coupled to two normal metallic electrodes can both switch spin-up and spin-down electronic currents within two different windows of SMM gate voltage. Such spin current switching in the SMM tunnel junction arises from spin-selected single electron resonant tunneling via the lowest unoccupied molecular orbit of the SMM. Since it is not magnetically controlled but all-electrically controlled,the proposed spin current switching effect may have potential applications in future spintronics.     

Critical temperature and critical current density of superconducting V-Ga-Al alloys with A 15 structure

V-Ga-Al alloys containing 68–82 at.-% vanadium can be easily worked mechanically in the A2 phase and transformed into the superconducting A15 phase by subsequent heat treatment. The preparation, critical temperature and critical current density of the A15 phase are described. The critical current density, which at 5 Tesla is in the 10⁴ to 10⁵ A/cm² range, is compared with values of technically used superconductors. Ternary V-Ga-Al alloys are suitable for superconducting shielding devices for high fields.     

Effective reduction of critical current for current-induced magnetization switching by a Ru layer insertion in an exchange-biased spin valve

Recently, it has been predicted that a spin-polarized electrical current perpendicular to plane directly flowing through a magnetic element can induce magnetization switching through spin-momentum transfer. In this Letter, the first observation of current-induced magnetization switching (CIMS) in exchange-biased spin valves (ESPVs) at room temperature is reported. The ESPVs show the CIMS behavior under a sweeping dc current with a very high critical current density. It is demonstrated that a thin ruthenium (Ru) layer inserted between a free layer and a top electrode effectively reduces the critical current densities for the CIMS. An "inverse" CIMS behavior is also observed when the thickness of the free layer increases.     

Faster vortex core switching with lower current density using three-nanocontact spin-polarized currents in a confined structure

We perform micromagnetic simulations on the switching of magnetic vortex core by using spin-polarized currents through a three-nanocontact geometry. Our simulation results show that the current combination with an appropriate current flow direction destroys the symmetry of the total effective energy of the system so that the vortex core can be easier to excite,resulting in less critical current density and a faster switching process. Besides its fundamental significance, our findings provide an additional route to incorporating magnetic vortex phenomena into data storage devices.     

CPP-GMR heads with a current screen layer for high areal density

We investigated the potential for high-areal-density recording in current-perpendicular-to-plane giant magnetoresistive (CPP-GMR) heads with a current screen layer. The current screen layer is a nano-oxide layer with confined current paths. We fabricated the current screen CPP-GMR heads with a narrow sensor width of 40–50 nm, a high MR ratio of 17%, and low-resistance-area (RA) product of 0.2 Ω μm². The fabricated heads showed a sufficiently high signal-to-noise ratio (SNR) of 30–40 dB. No extra noise, such as spin-torque noise, was observed. Linear density of 1360 kFCI from the head with the magnetic read width of 45 nm was obtained. Distribution of sensor resistance due to nano-hole area distribution in the current screen layer can be reduced with low-RA film. Spin-torque noise can be suppressed by reducing the current-induced field and controlling the shape anisotropy. Accordingly, the current screen CPP-GMR head is a promising candidate that has the potential for high-areal-density recording.     

Self-consistent treatment of spin and magnetization dynamic effect in spin transfer switching

The effect of itinerant spin moment (m) dynamic in spin transfer switching has been ignored in most previous theoretical studies of the magnetization (M) dynamics. Thus in this paper, we proposed a more refined micromagnetic model of spin transfer switching that takes into account in a self-consistent manner of the coupled m and M dynamics. The numerical results obtained from this model further shed insight on the switching profiles of m and M, both of which show particular sensitivity to parameters such as the anisotropy field, the spin torque field, and the initial deviation between m and M.     

Enhanced magnetoresistance induced by spin transfer torque in granular films with a magnetic field

Spin-transfer torques (STT) provide a mechanism to alter the magnetic configurations of magnetic heterostructures, a result previously only achieved by an external magnetic field. In granular solids, we demonstrate a new form of STT effect that can be exploited to induce a large spin disorder when combined with a large magnetic field. We have obtained a very large magnetoresistance effect in excess of 400% at 4.2 K in a large magnetic field, the largest ever reported in any metallic systems. The STT characteristics of granular solids differ significantly from those of multilayers, showing no STT effect at low magnetic fields but prominent STT effects at high fields.     

Current-induced spin transfer torque in ferromagnet-marginal Fermi liquid double tunnel junctions

Current-induced spin transfer torque through a marginal Fermi liquid (MFL) which is connected to two noncollinearly aligned ferromagnets via tunnel junctions is discussed in terms of the nonequilibrium Green function method. It is found that in the absence of the spin–flip scattering, the magnitude of the torque increases with the polarization and the coupling constant λ of the MFL, whose maximum increases with λ linearly, showing that the interactions between electrons tend to enhance the spin torque. When the spin–flip scattering is included, an additional spin torque is induced. It is found that the spin–flip scattering enhances the spin torque and gives rise to a nonlinear angular shift.     

Consistency in formulation of spin current and torque associated with a variance of angular momentum

Based on the Noether's theorem, we develop systematically and rigorously the spin-dependent formulation of the conservation laws. The effect of the electronic polarization due to the spin-orbit coupling is included in the Maxwell equations. The polarization is related to the antisymmetric components of spin current, and it provides a possibility to measure the spin current directly. The variances of spin and orbit angular momentum currents imply a torque on the "electric dipole" associated with the moving electron. The dependencies of the torque on the polarization and the force on the motions of spin-polarized electrons in a two-dimensional electron gas with the Rashba spin-orbit coupling are discussed.     

Reduction in critical current density of current-induced magnetization switching

We have investigated the current-induced magnetization switching in an exchange-biased spin valve structure. By using an unpatterned antiferromagnetic layer to pin the fixed Co layer, we obtained a lower switching current density by a factor of 5 than a simple spin valve structure. For the application, it is important to know how to keep the spin polarization when the thicker layer is pinned by an antiferromagnet. The unpatterned pinned ferromagnetic lead can be a good solution for spin-transfer-torque-activated device. The effect of Cu buffer layer on the top of the thin Co and Ru buffer layer under the thick Co layer on the current-induced magnetization switching in cobalt-based trilayer spin valves was also investigated. The experimental results showed that the Ru buffer layer in combination with Cu buffer layer could induce a decrease in the critical switching current by 30%, and an increase in the absolute resistance change by 35%, which is caused by an improvement of a microstructure of a thicker Co polarizer.     

Mechanism of exchange switching of spin valves by an inverse current

The exchange switching of spin valves by an inverse current can be explained by the interaction of the charge carriers with the spin-injection effective magnetic field. Such an interaction gives rise to transverse spin components, which are transferred to the magnetic lattice and cause its instability and switching. The spin-injection field is produced by longitudinal spin components, but it opens up a channel for the transverse spin transfer to the lattice. The spin transfer to the lattice and the switching occur in the free layer of the spin valve.     

Domain wall dynamics driven by spin transfer torque and the spin-orbit field

We have studied current-driven dynamics of domain walls when an in-plane magnetic field is present in perpendicularly magnetized nanowires using an analytical model and micromagnetic simulations. We model an experimentally studied system, ultrathin magnetic nanowires with perpendicular anisotropy, where an effective in-plane magnetic field is developed when current is passed along the nanowire due to the Rashba-like spin-orbit coupling. Using a one-dimensional model of a domain wall together with micromagnetic simulations, we show that the existence of such in-plane magnetic fields can either lower or raise the threshold current needed to cause domain wall motion. In the presence of the in-plane field, the threshold current differs for positive and negative currents for a given wall chirality, and the wall motion becomes sensitive to out-of-plane magnetic fields. We show that large non-adiabatic spin torque can counteract the effect of the in-plane field.     

一种输出自旋流的装置--自旋池

文章作者设计了一种能够给未来的自旋电路提供驱动自旋流的动力装置，即自旋池．该自旋池有如下四个基本特点：(1)有两个极能使自旋流从一个极流入，从另一个极流出，从而建立一个闭合的自旋回路；(2)有一个能量源；(3)能保持自旋相干；(4)能输出不附加任何净电荷流的纯自旋流，值得注意的是，该自旋池能通过现有的技术手段实现。