Torque Magnetic Tunnel Junction Research Articles

Efficient solution strategy to couple micromagnetic simulations with ballistic transport in magnetic tunnel junctions

We present a computationally efficient strategy that allows us to simulate magnetization switching driven by spin-transfer torque in magnetic tunnel junctions within a micromagnetic model coupled with a matrix-based nonequilibrium Green's function algorithm. Exemplary simulations for a realistic set of parameters are carried out and show switching times below $4\phantom{\rule{4.pt}{0ex}}\text{ns}$ for voltages above $300\phantom{\rule{4.pt}{0ex}}\text{mV}$ or around $2\ifmmode\times\else\texttimes\fi{}{10}^{10}\text{A}\phantom{\rule{0.16em}{0ex}}{\text{m}}^{\ensuremath{-}2}$ for the $\text{P}\ensuremath{\rightarrow}\text{AP}$ (parallel-to-antiparallel) direction. For $\text{AP}\ensuremath{\rightarrow}\text{P}$ switching, a trend reversal in the switching time is seen, i.e., the time for magnetization reversal first decreases with increasing bias voltage but then starts to rise again.

Design and Evaluation of a Self Write-Terminated Hybrid MTJ/CMOS Full Adder Based on LIM Structure

In this paper, a power-efficient self write-terminated hybrid full-adder (SWTHFA) has been developed using the self write-terminated write driver and an improved version of the sense amplifier already reported in the literature. The SWTHFA is designed using hybrid spin transfer torque-magnetic tunnel junction (STT-MTJ)/CMOS circuit based on logic-in-memory architecture. The use of a modified sense amplifier improves the power dissipation and output response on one hand, whereas, on the other hand, self-write-terminated write driver cuts off the unnecessary flow of write current in the driver circuit, thereby eliminates power wastage in SWTHFA. Proposed SWTHFA shows improvement in power saving, output response, read and write power delay product by 38.87%, 26.45%, 40.86% and 36.53%, respectively, compared to conventional write hybrid full-adder (CWHFA). Further, we performed Monte-Carlo simulations by incorporating process and mismatch variations for CMOS and extracted parameters of MTJ to demonstrate the feasibility of SWTHFA in low-power VLSI circuits.

Amplification of spin-transfer torque in magnetic tunnel junctions with an antiferromagnetic barrier

We theoretically study spin-transfer torques in magnetic tunnel junctions (MTJs) with an antiferromagnetic insulator (AFI) as the tunnel barrier. When a finite voltage bias is applied to the MTJ, the energy relaxation of the tunnel electrons leads to asymmetric heating of two metallic layers. Consequently, there would be a magnon current flowing across the AFI layer, resulting a magnon transfer torque in addition to the electron spin-transfer torque. Comparing to MTJs with a nonmagnetic insulator which prohibits the magnon transmission, we find the magnon transfer torque with an AFI barrier could be several times larger than the conventional spin-transfer torque of the tunnel electrons. This study presents a potential method to realize more efficient switching in MTJs and provides a motivation of experimental search for AFI-based MTJs.

Influence of MgO barrier quality on spin-transfer torque in magnetic tunnel junctions

We studied the bias dependence of spin transfer torque in the MgO-based magnetic tunnel junction using a field-modulated spin torque ferromagnetic resonance measurement technique for three devices with tunneling magnetoresistances (MRs) of 60%, 67%, and 73%, respectively. The devices with a lower MR ratio showed the presence of multiple modes, while the device with higher MR (73%) showed a single resonance mode. We found a lower out-of-plane torkance in our devices compared to the in-plane torkance. The out-of-plane torque is linear with applied bias, while the bias dependence of in-plane torque shows a strong dependence on the MR ratio and hence the barrier quality.

Erratum: Enhancing the spin transfer torque in magnetic tunnel junctions by ac modulation [Phys. Rev. B 95 , 115417 (2017)

Erratum: Enhancing the spin transfer torque in magnetic tunnel junctions by ac modulation [Phys. Rev. B <b>95</b> , 115417 (2017)

Enhancing the spin transfer torque in magnetic tunnel junctions by ac modulation

The phenomenon of spin transfer torque (STT) has attracted a great deal of interests due to its promising prospects in practical spintronic devices. In this paper, we report a theoretical investigation of STT in a noncollinear magnetic tunnel junction under ac modulation based on the nonequilibrium Green's function formalism, and derive a closed-formulation for predicting the time-averaged STT. Using this formulation, the ac STT of a carbon-nanotube-based magnetic tunnel junction is analyzed. Under ac modulation, the low-bias linear (quadratic) dependence of the in-plane (out-of-plane) torque on bias still holds, and the $\sin\theta$ dependence on the noncollinear angle is maintained. By photon-assisted tunneling, the bias-induced components of the in-plane and out-of-plane torques can be enhanced significantly, about 12 and 75 times, respectively. Our analysis reveals the condition for achieving optimized STT enhancement and suggests that ac modulation is a very effective way for electrical manipulation of STT.

Observation of thermally driven field-like spin torque in magnetic tunnel junctions

We report the thermally driven giant field-like spin-torque in magnetic tunnel junctions (MTJ) on application of heat current from top to bottom. The field-like term is detected by the shift of the magneto-resistance hysteresis loop applying temperature gradient. We observed that the field-like term depends on the magnetic symmetry of the MTJ. In asymmetric structures, with different ferromagnetic materials for free and fixed layers, the field-like term is greatly enhanced. Our results show that a pure spin current density of the order of 109 A/m2 can be produced by creating a 120 mK temperature difference across 0.9 nm thick MgO tunnelling barrier. Our results will be useful for writing MTJ and domain wall-based memories using thermally driven spin torque.

Anomalous Tunnel Magnetoresistance and Spin Transfer Torque in Magnetic Tunnel Junctions with Embedded Nanoparticles.

The tunnel magnetoresistance (TMR) in the magnetic tunnel junction (MTJ) with embedded nanoparticles (NPs) was calculated in range of the quantum-ballistic model. The simulation was performed for electron tunneling through the insulating layer with embedded magnetic and non-magnetic NPs within the approach of the double barrier subsystem connected in parallel to the single barrier one. This model can be applied for both MTJs with in-plane magnetization and perpendicular one. We also calculated the in-plane component of the spin transfer torque (STT) versus the applied voltage in MTJs with magnetic NPs and determined that its value can be much larger than in single barrier system (SBS) for the same tunneling thickness. The reported simulation reproduces experimental data of the TMR suppression and peak-like TMR anomalies at low voltages available in leterature.

Thermal spin-transfer torque in magnetic tunnel junctions (invited)

The thermal spin-transfer torque (TSTT) is an effect to switch the magnetic free layer in a magnetic tunnel junction by a temperature gradient only. We present ab initio calculations of the TSTT. In particular, we discuss the influence of magnetic layer composition by considering FexCo1–x alloys. Further, we compare the TSTT to the bias voltage driven STT and discuss the requirements for a possible thermal switching. For example, only for very thin barriers of 3 monolayers MgO, a thermal switching is imaginable. However, even for such a thin barrier, the TSTT is still too small for switching at the moment and further optimization is needed. In particular, the TSTT strongly depends on the composition of the ferromagnetic layer. In our current study, it turns out that at the chosen thickness of the ferromagnetic layer, pure Fe gives the highest thermal spin-transfer torque.

Spin torque in magnetic tunnel junctions with asymmetric barriers

We derive expressions for both parallel and perpendicular components of spin transfer torque (STT) in magnetic tunnel junctions (MTJs), which have several important advantages over the currently available expressions: First they are derived in a more realistic approximation, resulting in excellent agreement with exact results even in the presence of resonant tunneling. Second, we show that they can be expressed in terms of the scattering matrix elements, which gives them a clear physical interpretation. Third, they are given entirely in terms of collinear quantities, which are readily available in existing transport codes. We use these expressions to investigate STT behavior in MTJs with asymmetric barriers at finite bias. The results show that lowering the barrier height in the bulk does not qualitatively change the behavior of STT. The absolute STT increases on account of the overall increase of the barrier transparency; however, the STT efficiency remains in the same range. At the same time, modifications of the interfaces can qualitatively change STT behavior. Thus, interface engineering can be used to control the bias dependence of STT and optimize the performance of STT-based devices.

Influence of MgO tunnel barrier thickness on spin-transfer ferromagnetic resonance and torque in magnetic tunnel junctions

Spin-transfer ferromagnetic resonance (ST-FMR) in symmetric magnetic tunnel junctions (MTJs) with a varied thickness of the MgO tunnel barrier ($0.75\phantom{\rule{0.222222em}{0ex}}\text{nm}&lt;{t}_{\mathrm{MgO}}&lt;1.05\phantom{\rule{0.222222em}{0ex}}\text{nm}$) is studied using the spin-torque diode effect. The application of an rf current into nanosized MTJs generates a dc mixing voltage across the device when the frequency is in resonance with the resistance oscillations arising from the spin-transfer torque. Magnetization precession in the free and reference layers of the MTJs is analyzed by comparing ST-FMR signals with macrospin and micromagnetic simulations. From ST-FMR spectra at different dc bias voltage, the in-plane and perpendicular torkances are derived. The experiments and free electron model calculations show that the absolute torque values are independent of tunnel barrier thickness. The influence of coupling between the free and reference layer of the MTJs on the ST-FMR signals and the derived torkances are discussed.

Crucial role of interfacial alloying on spin-transfer torque in magnetic tunnel junctions

We demonstrate that interfacial disorder in magnetic tunnel junctions (MTJs) has a dramatic effect on the bias behavior of both spin-transfer and field-like spin torques, leading to strong enhancement, sign reversal, and bias asymmetry. The underlying mechanism lies on the impurity-induced resonance energies, which can be selectively controlled with alloying and/or bias. The predictions explain for the first time the experimental results in ``dirty'' junctions and have important implications on exploiting further the alloying effect to optimize the current-induced magnetization switching.

Measurement of perpendicular spin torque at high bias via the pulsed switching phase diagram

We report an experimental method to estimate the bias dependence of the perpendicular spin torque in magnetic tunnel junctions. This method utilizes the pulse-width-dependent change in the switching phase diagram and addresses the perpendicular-torque-driven precessional switching at high-bias ranges where the in-plane spin torque acts as an additional damping. The bias dependence of the perpendicular spin torque at high bias was found to be linear for a voltage polarity. This method will be useful to address spin torque effects in magnetic tunnel junctions at high bias.

Spin-transfer torque in nanoscale magnetic devices

We discuss recent highlights from research at Cornell University, Ithaca, New York, regarding the use of spin-transfer torques to control magnetic moments in nanoscale ferromagnetic devices. We highlight progress on reducing the critical currents necessary to produce spin-torque-driven magnetic switching, quantitative measurements of the magnitude and direction of the spin torque in magnetic tunnel junctions, and single-shot measurements of the magnetic dynamics generated during thermally assisted spin-torque switching.

Effect of disorder on spin-transfer torque in magnetic tunnel junctions

We have generalized the nonequilibrium Green’s functions Keldysh formalism to study the effect of interfacial disorder on the average spin transfer torque, 〈T∥〉, in magnetic tunnel junctions (MTJs). We find a sinusoidal angular behavior of the average 〈T∥〉 as in ideal MTJs. We demonstrate for the first time that the general expression of the bias behavior of the average 〈T∥〉 in terms of the interplay of average spin current densities in collinear configurations is valid even in the presence of disorder. This explains the strong enhancement and sign reversal of 〈T∥〉 in the positive bias region, due to the disorder-induced resonance states at interface which selectively assist the transmission of right-coming electrons.

Time-resolved measurement of spin-transfer-driven ferromagnetic resonance and spin torque in magnetic tunnel junctions

Several experimental techniques have been introduced in recent years in attempts to measure spin transfer torque in magnetic tunnel junctions (MTJs). The dependence of spin torque on bias is important for understanding fundamental spin physics in magnetic devices and for applications. However, previous techniques have provided only indirect measures of the torque and their results to date for the bias dependence are qualitatively and quantitatively inconsistent. Here we demonstrate that spin torque in MTJs can be measured directly by using time-domain techniques to detect resonant magnetic precession in response to an oscillating spin torque. The technique is accurate in the high-bias regime relevant for applications, and because it detects directly small-angle linear-response magnetic dynamics caused by spin torque it is relatively immune to artifacts affecting competing techniques. At high bias we find that the spin torque vector differs markedly from the simple lowest-order Taylor series approximations commonly assumed.

Adiabatic quantum pumping, magnification effects, and quantum size effects of spin torque in magnetic tunnel junctions

We study the adiabatic quantum pumping and quantum size effects of spin-torque in a magnetic tunnel junction within a scattering matrix approach. Quantum size effects are predicted in the presence of a dc bias as a function of the thickness of the normal metal layer inserted between two magnetic layers and of the fixed magnetic layer. In the presence of ac voltages, the results for the spin-torque show a peculiar magnification effect and advantages of spin-torque pumping in actual devices are also discussed.

Influence of Band Parameters on Spin-Transfer Torque in Tunnel Junctions: Model Calculations

We study the in-plane spin-transfer torque in magnetic tunnel junctions for different band fillings and exchange splittings. The bias range over which the in-plane torque is linear depends strongly on these parameters. If the ferromagnetic layer is half-metallic with respect to the tunneling states, the linear bias range for the in-plane torque is significantly larger than if the behavior is metallic. For parameters that reproduce the important features of the Fe band structure, the results are in agreement with experimental data as well as with ab initio calculations.

Field-like spin torque in magnetic tunnel junctions

We show that the exchange splitting asymmetry between the left and right ferromagnetic leads in non-collinear magnetic tunnel junctions (MTJ) tunes the bias behavior of the field-like spin torque, T⊥. These results can be understood by our recently derived general expression, which relates the non-collinear T⊥ to the algebraic sum of four independent non-equilibrium interlayer exchange couplings (IEC) solely in collinear configurations.

Bias and angular dependence of spin-transfer torque in magnetic tunnel junctions

We use spin-transfer-driven ferromagnetic resonance (ST-FMR) to measure the spin-transfer torque vector $\mathbit{\ensuremath{\tau}}$ in MgO-based magnetic tunnel junctions as a function of the offset angle between the magnetic moments of the electrodes and as a function of bias, $V$. We explain the conflicting conclusions of two previous experiments by accounting for additional terms that contribute to the ST-FMR signal at large $|V|$. Including the additional terms gives us improved precision in the determination of $\mathbit{\ensuremath{\tau}}(V)$, allowing us to distinguish among competing predictions. We determine that the in-plane component of $d\mathbit{\ensuremath{\tau}}/dV$ has a weak but nonzero dependence on bias, varying by 30%--35% over the bias range where the measurements are accurate, and that the perpendicular component can be large enough to be technologically significant. We also make comparisons to other experimental techniques that have been used to try to measure $\mathbit{\ensuremath{\tau}}(V)$.