Spin Transfer Torque Efficiency Research Articles

High-frequency spin torque oscillation in orthogonal magnetization disks with strong biquadratic magnetic coupling

In this study, we numerically investigate the spin transfer torque oscillation (STO) in a magnetic orthogonal configuration by introducing a strong biquadratic magnetic coupling. The orthogonal configuration consists of top and bottom layers with in-plane and perpendicular magnetic anisotropy sandwiching a nonmagnetic spacer. The advantage of an orthogonal configuration is the high efficiency of spin transfer torque leading a high STO frequency; however, maintaining the STO in a wide range of electric current is challenging. By introducing biquadratic magnetic coupling into the orthogonal structure of FePt/spacer/Co90Fe10, Ni80Fe20 or Ni, we were able to expand the electric current region in which the stable STO is realized, resulting in a relatively high STO frequency. For example, approximately 50 GHz can be achieved in an Ni layer at a current density of 5.5 × 107 A/cm2. In addition, we investigated two types of initial magnetic state: out-of-plane and in-plane magnetic saturation; this leads to a vortex and an in-plane magnetic domain structure after relaxation, respectively. The transient time before the stable STO was reduced to between 0.5 and 1.8 ns by changing the initial state from out-of-plane to in-plane.

Evaluation of spin-transfer-torque efficiency using magnetization reversal against a magnetic field: comparison of FeCr with negative spin polarization and NiFe

We propose and demonstrate an experimental method to evaluate spin-transfer-torque (STT) efficiency in current-perpendicular-to-plane giant magnetoresistance devices, which utilizes STT-induced magnetization reversal against a perpendicular magnetic field. Using this method, we estimated the STT efficiency of FeCr with negative spin polarization, which attracts attention in spintronics applications. In comparison with NiFe with positive spin polarization, the sign of the STT induced by FeCr was opposite, reflecting the negative spin polarization. The STT efficiency of FeCr was approximately half that of NiFe at room temperature and surpassed that of NiFe at 50 K.

Absorption of transverse spin current in ferromagnetic NiCu: Dominance of bulk dephasing over spin-flip scattering

In ferromagnetic metals, transverse spin currents are thought to be absorbed via dephasing—i.e., destructive interference of spins precessing about the strong exchange field. Yet, due to the ultrashort coherence length of ≈1 nm in typical ferromagnetic thin films, it is difficult to distinguish dephasing in the bulk from spin-flip scattering at the interface. Here, to assess which mechanism dominates, we examine transverse spin-current absorption in ferromagnetic NiCu alloy films with reduced exchange fields. We observe that the coherence length increases with decreasing Curie temperature, as weaker dephasing in the film bulk slows down spin absorption. Moreover, nonmagnetic Cu impurities do not diminish the efficiency of spin-transfer torque from the absorbed spin current. Our findings affirm that the transverse spin current is predominantly absorbed by dephasing inside the nanometer-thick ferromagnetic metals, even with high impurity contents.

Real time investigation of double magnetic tunnel junction with a switchable assistance layer for high efficiency STT-MRAM

This work reports experimental confirmation of the working principles of a double magnetic tunnel junction (DMTJ) to achieve highly efficient spin-transfer-torque (STT) switching. This concept uses a magnetically switchable assistance layer (ASL) acting as a top perpendicular spin polarizer. The STT-induced switching dynamics were described by macrospin simulations, while the magnetic and electrical properties of the devices were investigated in switching experiments. The reversal dynamics of the storage layer/ASL coupled system were validated by time-resolved measurements of the device resistance during write operation, confirming that the storage layer is subjected to additive STT contributions both from the reference layer and the ASL throughout its parallel-to-antiparallel and antiparallel-to-parallel transitions. The STT efficiency of the ASL-DMTJ was compared to that of single MTJ stacks comprising the same storage layer and no assistance layer. The figure of merit ∆/Ic (stability/critical current) was found to nearly double in devices of 80 and 100 nm diameter, with a smaller 30% increase obtained for 50 nm diameter cells.

Spin Transfer Torque Oscillation in Orthogonal Magnetization Disks

We numerically demonstrated the spin transfer torque oscillation (STO) in a magnetic orthogonal configuration by introducing a 90° magnetic coupling. In our model, the multilayer consists of a top layer with in-plane magnetic anisotropy, a nonmagnetic spacer, and a bottom layer with perpendicular magnetic anisotropy. The advantage of utilizing the orthogonal configuration is the high efficiency of spin transfer torque but it is difficult to stabilize STO. By introducing 90° magnetic coupling into the trilayer of FePt/spacer/Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">90</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sub> , or Ni <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">80</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">20</sub> or Ni, a stable STO in an orthogonal configuration was realized. When the top layer was Ni with low levels of saturation magnetization, a small damping constant, and a small anisotropy, it exhibited high frequency and low amplitude. This indicates the possibility of controlling the STO frequency. Moreover, the STO frequency was increased by enhancing the 90° coupling. The combination of the orthogonal magnetization structure and 90° magnetic coupling is an effective approach to obtain a stable spin transfer torque oscillator with high frequency.

High-resolution tunneling spin transport characteristics of topologically distinct magnetic skyrmionic textures from theoretical calculations

High-resolution tunneling electron spin transport properties (longitudinal spin current (LSC) and spin transfer torque (STT) maps) of topologically distinct real-space magnetic skyrmionic textures are reported by employing a 3D-WKB combined scalar charge and vector spin transport theory in the framework of spin-polarized scanning tunneling microscopy (SP-STM). For our theoretical investigation metastable skyrmionic spin structures with various topological charges (Q=-3,-2,-1,0,1,2) in the (Pt0.95Ir0.05)/Fe/Pd(111) ultrathin magnetic film are considered. Using an out-of-plane magnetized SP-STM tip it is found that the maps of the LSC vectors acting on the spins of the magnetic textures and all STT vector components exhibit the same topology as the skyrmionic objects. In contrast, an in-plane magnetized tip generally does not result in spin transport vector maps that are topologically equivalent to the underlying spin structure, except for the LSC vectors acting on the spins of the skyrmionic textures at a specific relation between the spin polarizations of the sample and the tip. The magnitudes of the spin transport vector quantities exhibit close relations to charge current SP-STM images irrespectively of the skyrmionic topologies. Moreover, we find that the STT efficiency (torque/current ratio) acting on the spins of the skyrmions can reach large values up to ~25 meV/μA (~0.97 h/e) above the rim of the magnetic objects, but it considerably varies between large and small values depending on the lateral position of the SP-STM tip above the topological spin textures. A simple expression for the STT efficiency is introduced to explain its variation. Our calculated spin transport vectors can be used for the investigation of spin-polarized tunneling-current-induced spin dynamics of topologically distinct surface magnetic skyrmionic textures.

Ultrathin perpendicular free layers for lowering the switching current in STT-MRAM

The critical current density Jc0 required for switching the magnetization of the free layer (FL) in a spin-transfer torque magnetic random access memory (MRAM) cell is proportional to the product of the damping parameter, saturation magnetization, and thickness of the free layer, αMStF. Conventional FLs have the structure CoFeB/nonmagnetic spacer/CoFeB. By reducing the spacer thickness, W in our case, and also splitting the single W layer into two layers of a sub-monolayer thickness, we have reduced tF while minimizing α and maximizing MS, ultimately leading to lower Jc0 while maintaining high thermal stability. Bottom-pinned MRAM cells with a device diameter in the range of 55–130 nm were fabricated, and Jc0 is the lowest for the thinnest (1.2 nm) FLs, down to 4MA/cm2 for 65 nm devices, ∼30% lower than 1.7 nm FLs. The thermal stability factor Δdw, as high as 150 for the smallest device size, was determined using a domain wall reversal model from field-switching probability measurements. With high Δdw and the lowest Jc0, the thinnest FLs have the highest spin-transfer torque efficiency.

Origin of the Resistance-Area-Product Dependence of Spin-Transfer-Torque Switching in Perpendicular Magnetic Random-Access Memory Cells

We report on an experimental study of current-induced switching in perpendicular magnetic random-access memory (MRAM) cells with variable resistance-area products (RAs). Our results show that in addition to spin-transfer torque (STT), current-induced self-heating and voltage-controlled magnetic anisotropy also contribute to switching and can explain the RA dependencies of the switching-current density and STT efficiency. Our findings suggest that thermal optimization of perpendicular MRAM cells can result in significant reduction of switching currents.

Perpendicular Magnetized Magnetic Random-Access Memory Cells Utilizing the Precessional Spin-Current Structure: Benefits for Modern Memory Applications

The various memory applications such as embedded nonvolatile memory, embedded static random-access memory, and standalone dynamic random-access memory have different requirements for the performance of the perpendicular magnetic tunnel junction (pMTJ). However, in conventional pMTJ structures, the write current and stability of the free layer are directly coupled, leading to undesirable tradeoffs in performance specifications such as endurance and data retention. With the introduction of the precessional spin-current (PSC) structure, we demonstrate a significant gain in the spin transfer torque (STT) efficiency. The PSC structure shows similar critical current values as the conventional pMTJ structure. But we observe a significant increase in thermal stability, which is largely responsible for the enhancement in the STT efficiency. Smaller device dimensions show higher efficiency increase. We confirm the higher data retention time in a thermally activated switching experiment at 180 °C. The PSC structure exhibits more than two decades lower thermal decay rates within 100 h of wait over the conventional pMTJ. These results demonstrate that it is possible to achieve the necessary retention times at high-temperature operation without increasing the power consumption or higher current in normal operation. Furthermore, we demonstrate that even for short write pulsewidth, the switching currents of the PSC structure remain similar to the pMTJ structure while still having better retention.

Insertion Layer Thickness Dependence of Magnetic and Electrical Properties for Double-CoFeB/MgO-Interface Magnetic Tunnel Junctions

We have studied the magnetic and electrical properties of double CoFeB–MgO interface magnetic tunnel junctions (MTJs) with various W insertion layer thicknesses $t_{\mathrm {W}}$ annealed at 400 °C and evaluated $t_{\mathrm {W}}$ dependence of thermal stability factor $\Delta $ , intrinsic critical current $I_{\mathrm {C0}}$ and damping constant $\alpha $ . It was found that spin-transfer torque efficiency ( $\Delta /I_{\mathrm {C0}}$ ) values increased with decreasing $t_{\mathrm {W}}$ as compared with the same MTJ size $D$ . $\alpha $ measured by ferromagnetic resonance increased with an increase in $t_{\mathrm {W}}$ , which is consistent with those calculated from $\Delta /I_{\mathrm {C0}}$ . This result indicates that lower damping constant of MTJ with thin W insertion layer contributes to higher $\Delta /I_{\mathrm {C0}}$ .

Spin-transfer torque in ferromagnetic bilayers generated by anomalous Hall effect and anisotropic magnetoresistance.

We propose an experimental scheme to determine the spin-transfer torque efficiency excited by the spin-orbit interaction in ferromagnetic bilayers from the measurement of the longitudinal magnetoresistace. Solving a diffusive spin-transport theory with appropriate boundary conditions gives an analytical formula of the longitudinal charge current density. The longitudinal charge current has a term that is proportional to the square of the spin-transfer torque efficiency and that also depends on the ratio of the film thickness to the spin diffusion length of the ferromagnet. Extracting this contribution from measurements of the longitudinal resistivity as a function of the thickness can give the spin-transfer torque efficiency.

Enhancement of the spin transfer torque efficiency in magnetic STM junctions

We introduce a method for a combined calculation of charge and vector spin transport of elastically tunneling electrons in magnetic scanning tunneling microscopy (STM). The method is based on the three-dimensional Wentzel-Kramers-Brillouin (3D-WKB) approach combined with electronic structure calculations using first principles density functional theory. As an application, we analyze the STM contrast inversion of the charge current above the Fe/W(110) surface depending on the bias voltage, tip-sample distance and relative magnetization orientation between the sample and an iron tip. For the spin transfer torque (STT) vector we find that its in-plane component is generally larger than the out-of-plane component, and we identify a longitudinal spin current component, which, however, does not contribute to the torque. Our results suggest that the torque-current relationship in magnetic STM junctions follows the power law rather than a linear function. Consequently, we show that the ratio between the STT and the spin-polarized charge current is not constant, and more importantly, it can be tuned by the bias voltage, tip-sample distance and magnetization rotation. We find that the STT efficiency can be enhanced by about a factor of seven by selecting a proper bias voltage. Thus, we demonstrate the possible enhancement of the STT efficiency in magnetic STM junctions, which can be exploited in technological applications. We discuss our results in view of the indirect measurement of the STT above the Fe/W(110) surface reported by Krause et al. in Phys. Rev. Lett. 107, 186601 (2011).

Thermally Robust Perpendicular STT-MRAM Free Layer Films Through Capping Layer Engineering

In order to enable the spin-transfer torque (STT) magnetoresistive random access memory technologies, it is essential to control the thermal budget, perpendicular magnetic anisotropy, and magnetic damping parameter of perpendicular magnetic tunnel junction (pMTJ) thin films. This paper demonstrates the enhancement of pMTJ free layer (FL) properties through selective engineering of capping materials placed between a CoFeB FL and the top electrode. By introducing a capping layer of Mg or MgO, thermal budget and tunneling magnetoresistance (TMR) ratio are significantly improved over the conventional FL schemes due to reduced atomic intermixing at interfaces. In full-stack pMTJ films, thin Mg above the FL enables the TMR above 170% after 400 °C annealing. Capping via MgO increases the interface anisotropy, allowing for the use of thicker CoFeB FLs with magnetic damping constants as low as 0.003. We discuss the implications of these capping layer improvements and suggest that Mg and MgO show the potential for optimizing the FLs with high-thermal budget and good STT efficiency.

Modulation of spin transfer torque amplitude in double barrier magnetic tunnel junctions

Magnetization switching induced by spin transfer torque is used to write magnetic memories (Magnetic Random Access Memory, MRAM) but can be detrimental to the reading process. It would be quite convenient therefore to modulate the efficiency of spin transfer torque. A solution is adding an extra degree of freedom by using double barrier magnetic tunnel junctions with two spin-polarizers, with controllable relative magnetic alignment. We demonstrate, for these structures, that the amplitude of in-plane spin transfer torque on the middle free layer can be efficiently tuned via the magnetic configuration of the electrodes. Using the proposed design could thus pave the way towards more reliable read/write schemes for MRAM. Moreover, our results suggest an intriguing effect associated with the out-of-plane (field-like) spin transfer torque, which has to be further investigated.

Spin-transfer-torque efficiency enhanced by edge-damage of perpendicular magnetic random access memories

We numerically investigate the effect of magnetic and electrical damages at the edge of a perpendicular magnetic random access memory (MRAM) cell on the spin-transfer-torque (STT) efficiency that is defined by the ratio of thermal stability factor to switching current. We find that the switching mode of an edge-damaged cell is different from that of an undamaged cell, which results in a sizable reduction in the switching current. Together with a marginal reduction of the thermal stability factor of an edge-damaged cell, this feature makes the STT efficiency large. Our results suggest that a precise edge control is viable for the optimization of STT-MRAM.

Perpendicular magnetic tunnel junctions with double barrier and single or synthetic antiferromagnetic storage layer

The magnetic properties of double tunnel junctions with perpendicular anisotropy were investigated. Two synthetic antiferromagnetic references are used, while the middle storage magnetic layer can be either a single ferromagnetic or a synthetic antiferromagnetic FeCoB-based layer, with a critical thickness as large as 3.0 nm. Among the different achievable magnetic configurations in zero field, those with either antiparallel references, and single ferromagnetic storage layer, or parallel references, and synthetic antiferromagnetic storage layer, are of particular interest since they allow increasing the efficiency of spin transfer torque writing and the thermal stability of the stored information as compared to single tunnel junctions. The latter configuration can be preferred when stray fields would favour a parallel orientation of the reference layers. In this case, the synthetic antiferromagnetic storage layer is also less sensitive to residual stray fields.

Multiple Reflection Effect on Spin-Transfer Torque Dynamics in Spin Valves with a Single or Dual Polarizer

In this paper, spin-dependent multiple reflection effect on spin-transfer torque (STT) has been theoretically and numerically studied in a spin valve nanopillar with a single or dual spin-polarizer. By using a scattering matrix method, we formulate an analytical expression of STT that contains the multiple interfacial reflection effect. It is found that the multiple reflections could enhance the STT efficiency and reduce the critical switching current. The STT efficiency depends on the spin polarization of both the free layer and polarizer. In the nanopillars with a dual spin polarizer, the multiple reflections would cause an asymmetric frequency dependence on the applied current, albeit exactly the same parameters are used in all three ferromagnetic layers, indicating that the frequency in the negative current varies much faster than that in the positive case.

Controlling the spin-torque efficiency with ferroelectric barriers

Nonequilibrium spin-dependent transport in magnetic tunnel junctions comprising a ferroelectric barrier is theoretically investigated. The exact solutions of the free electron Schr\"odinger equation for electron tunneling in the presence of interfacial screening are obtained by combining Bessel and Airy functions. We demonstrate that the spin transfer torque efficiency, and more generally the bias dependence of tunneling magneto- and electroresistance, can be controlled by switching the ferroelectric polarization of the barrier. In particular, the critical voltage at which the in-plane torque changes sign can be strongly enhanced or reduced depending on the direction of the ferroelectric polarization of the barrier. This effect provides a supplementary way to electrically control the current-driven dynamic states of the magnetization and related magnetic noise in spin transfer devices.

Landau–Lifshitz–Gilbert micromagnetic simulation on spin transfer torque efficiency of sub-30 nm perpendicular magnetic tunnel junctions with etching damage

The threshold current density Jc0, effective anisotropy field Heff, and the spin transfer torque (STT) efficiency of magnetic tunnel junctions with perpendicular anisotropy (p-MTJs) with the free layer diameter d from 10 to 30 nm was evaluated when etching degraded the saturation magnetization Ms and/or anisotropic energy Ku of the ferromagnetic layers with thickness of 1 to 3 nm around the pillar, using Landau–Lifshitz–Gilbert (LLG) micro-magnetic simulation. The STT efficiency for MTJs with only Ms reduction increased with a decrease in d, which reproduces the experimental trend. Jc0 and Heff for MTJs with only Ku reduction dramatically decreased even when the thickness of the damaged region is only 1 nm. The thickness of the damaged region had a large influence on Jc0 and Heff for MTJs with either Ms or Ku reduction.

Efficient spin transfer torque in La2/3Sr1/3MnO3 nanostructures

We carry out low temperature magnetotransport measurements on nanostructured La2/3Sr1/3MnO3 wires to study the interaction between spin-polarized current and magnetization in this half metallic material. We selectively position domain walls by applying external fields. The domain wall resistance is found to be positive, in contrast to conventional 3d metals. The depinning field is reduced when current pulses are injected into the wire. By comparing measurements for both current polarities, we can disentangle heating and spin transfer torque effects. The determined spin transfer torque efficiency is of the order of 4 × 10−14 Tm2/A, which is significantly higher than in permalloy.