Antiparallel State Research Articles

Back hopping in spin transfer torque switching of perpendicularly magnetized tunnel junctions

We analyse the phenomenon of back-hopping in spin-torque induced switching of the magnetization in perpendicularly magnetized tunnel junctions. The analysis is based on single-shot time-resolved conductance measurements of the pulse-induced back-hopping. Studying several material variants reveals that the back-hopping is a feature of the nominally fixed system of the tunnel junction. The back-hopping is found to proceed by two sequential switching events that lead to a final state P' of conductance close to --but distinct from-- that of the conventional parallel state. The P' state does not exist at remanence. It generally relaxes to the conventional antiparallel state if the current is removed. The P' state involves a switching of the sole spin-polarizing part of the fixed layers. The analysis of literature indicates that back-hopping occurs only when the spin-polarizing layer is too weakly coupled to the rest of the fixed system, which justifies a posteriori the mitigation strategies of back-hopping that were implemented empirically in spin-transfer-torque magnetic random access memories.

Tunable magnetic low-frequency noise in magnetic tunnel junctions: effect of shape anisotropy

The intrinsic magnetic low-frequency noise (LFN) is of fundamental scientific interest to the study of magnetic tunnel junctions (MTJs). To gain insight into its mechanism, the fluctuation-dissipation theorem, which describes the linear relation between magnetic LFN and magnetic sensitivity product, has been utilized. However, deviation from the linear correlation has been reported in some studies. To understand and effectively control the magnetic LFN, a more elaborate analytical description and further experimental validation are required. In this work, the magnetic LFN contributed from the magnetization fluctuation in the pinned layer of MTJs with various shape anisotropies was investigated. The MTJs with different shape anisotropies, achieved by altering their aspect ratios, possessed distinct demagnetizing factors. Large magnetic noise was correlated with the increase of magnetic phase loss of ferromagnetic layers during magnetization reversal at which magnetization fluctuation was enhanced. Upon increasing the shape anisotropy, a notable reduction of the magnetic phase loss in the antiparallel (AP) state was observed while it exhibited a slight decrease in the parallel (P) state, revealing that the increase of the shape anisotropy caused a more pronounced suppression of the equilibrium magnetization fluctuation in the AP state. These phenomena were computationally validated by constructing a macrospin model to describe the thermally-induced magnetization fluctuation in the pinned layer. This work reveals the physical relation between MTJ shape anisotropy and magnetic LFN. The effect of the shape anisotropy on the magnetic LFN can be extended to other types of in-plane uniaxial anisotropies.

Influence of rescattering effects on the helicity dependence of [formula omitted] near threshold and its implication to the E-asymmetry and the GDH sum rule

This work deals with investigation of the helicity dependence of the γ→d→→πNN reaction channels near threshold and its implication to the helicity E-asymmetry and the Gerasimov-Drell-Hearn sum rule. Our formalism based on the one loop corrections to the impulse approximation due to nucleon and pion rescattering, uses the realistic Paris NN potential model for the deuteron wave function and the unitary isobar MAID-2007 model for the elementary γN → πN amplitude. We present results for the doubly polarized differential and total cross sections for parallel and antiparallel helicity states, the deuteron spin asymmetry, the helicity E-asymmetry, and the deuteron GDH integral in the near-threshold region. The influence of NN and πN final-state rescattering on the estimated results has been discussed. It is shown that the inclusion of NN rescattering is important, and its inclusion strongly affected the estimated results in both π ± - and π0-production channels. The inclusion of πN rescattering is found to be insignificant in π ± -production, but it has a non- negligible contribution in the case of π0-production. A comparison with predictions of other theoretical models is given.

Computational Study of Magnetic Switching Mechanism of Nanoscale MRAM Cells

Investigation of magnetic switching of nanoscale single MRAM cells of different shapes and sizes is imperative for their applications in future magnetic memory devices. To this end, we have investigated the magnetic switching mechanism of nanoscale single MRAM cells of two different shapes with varying lateral aspect ratios by computational micromagnetic simulation. We have analysed how various parameters such as the coercive field, remanence and saturation field were affected by the variation in magnetic field. We have also analysed the change in shape of the hysteresis loops of the various samples. The magnetization reversal states were simulated to justify the spatial coherence of magnetization switching. As a result, the cells with higher aspect ratio show the Py and Fe layers forming antiparallel states in the plateau similar to synthetic antiferromagnets. As we reduce the aspect ratio, more complex quasi-uniform magnetic states are observed which are even more complicated for elliptical cells. The rectangular cell with the highest aspect ratio of 2.5 shows the most coherent and predictable switching behaviour, showing its suitability for the application of MRAM cells.

Magnetic Modulation of Terahertz Waves via Spin-Polarized Electron Tunneling Based on Magnetic Tunnel Junctions

Magnetic tunnel junctions (MTJs) are a key technology in modern spintronics because they are the basis of read-heads of modern hard disk drives, nonvolatile magnetic random access memories, and sensor applications. In this paper, we demonstrate that tunneling magnetoresistance can influence terahertz (THz) wave propagation through a MTJ. In particular, various magnetic configurations between parallel state and antiparallel state of the magnetizations of the ferromagnetic layers in the MTJ have the effect of changing the conductivity, making a functional modulation of the propagating THz electromagnetic fields. Operating in the THz frequency range, a maximal modulation depth of 60% is reached for the parallel state of the MTJ with a thickness of 77.45 nm, using a magnetic field as low as 30 mT. The THz conductivity spectrum of the MTJ is governed by spin-dependent electron tunneling. It is anticipated that the MTJ device and its tunability scheme will have many potential applications in THz magnetic modulators, filtering, and sensing.

Influence of Current Redistribution in Switching Models for Perpendicular STT-MRAM

The outstanding improvement in the performance of modern integrated circuits is supported by the continuous down-scaling of semiconductor devices. However, this leads to a substantial increase in leakages, which results in growing stand-by power consumption. A viable path to mitigate these issues is the introduction of nonvolatility in integrated circuits. Spin-transfer torque (STT) magnetoresistive random access memory (MRAM) is a promising candidate. It is competitive with conventional nonvolatile flash memory as it combines high-speed, excellent endurance, and low costs. The range of potential STT-MRAM applications goes from automotive and Internet-of-Things to embedded memories on System-on-Chips and L3 caches [1]. The core of an STT-MRAM device is a magnetic tunnel junction, where the relative orientation of its magnetic layers provides a way of storing the binary information. Switching between the two possible configurations is achieved by passing an electric current through the structure. The electrons get spin-polarized by the reference layer and, when entering the free layer, act via the exchange interaction on its magnetization providing the torque. With a sufficiently large current density, the magnetization of the free layer can be flipped.The usual approach for micromagnetic simulations of STT switching is to assume a constant and uniform current density [2]. In circuits and applications, however, the voltage, rather than the current density, is fixed during the switching process. As the tunneling resistance in an MTJ depends on the relative magnetization orientation of the two layers, the current depends on time. Moreover, the magnetization of the free layer is nonuniform during switching, which results in a position (and time) dependent current density J(r,t) (Fig. 1). In order to evaluate the validity and the limits of the fixed current density approach in computing the switching time, we compare it with a model in which the total current is fixed, but redistributed according to the position-dependent resistance [3], determined by the local magnetization orientation. In addition, the fixed current and the fixed current density models are compared with a realistic approach in which the voltage during switching is kept constant. Herein, the value of the current in the fixed current/current density models is equal to the voltage in the fixed voltage model divided by the resistance in the initial parallel (P) or anti-parallel (AP) state. The free layer is perpendicularly magnetized. The switching time depends on the realization of the stochastic magnetic field which mimics the magnetization fluctuations at room temperature.It is demonstrated that, by slightly increasing the current from its initial value for AP to P switching and decreasing it for P to AP switching, the switching time distributions can be matched [4] for any value of the stray field induced by the pinned layer (Fig.2). Here, we systematically study the dependence of this current correction on system parameters. The dependence of the current correction on the tunneling magnetoresistance ratio (TMR) at room temperature is reported in Fig. 3. It is observed that the value of the correction increases with higher TMR. We then performed simulations at zero temperature, and in this case, the correction required to reproduce the fixed voltage results is lower than the one at room temperature (Fig. 4). However, the switching is also slower. This proves that the correction to the current is not universal and depends on the system parameters. To elaborate on the physical origin of this dependence we performed macrospin simulations. The initial magnetization orientation is slightly tilted from its perfect perpendicular orientation in order to reduce the incubation time of switching. By changing the initial tilting of the magnetization, we demonstrate that a faster switching requires a higher correction to the current value, which explains the difference between the room and zero temperature simulations (Fig. 5). Simulation results for the current correction as a function of the surface area of the structure at room temperature are shown in Fig. 6. The switching is faster for higher values of the area, and the current correction required is consequently higher, in agreement with the macrospin results. We conclude that it is possible to reproduce the switching times obtained within the realistic fixed voltage approach with the approximate fixed current density approach for a wide range of system parameters, provided that the dependence of the current correction on such parameters is taken into consideration.REFERENCES[1] S.Sakhare, M.Perumkunnil, T.H.Bao, S.Rao, W.Kim et al., Proc.IEDM, 420 (2018).[2] A.Makarov, T.Windbacher, V.Sverdlov, S.Selberherr, Semic.Sci.Tec. 31, 113006 (2016).[3] D.Aurelio, L.Torres, G.Finocchio, J.Magn.Magn.Mat. 321, 3913 (2009).[4] S.Fiorentini, R.Orio, W.Goes, J.Ender, V.Sverdlov, Proc.SISPAD, 57 (2019). Figure 1

Double MgO-Based Perpendicular Magnetic Tunnel Junction for Artificial Neuron

A perpendicular spin transfer torque (p-STT)-based neuron was developed for a spiking neural network (SNN). It demonstrated the integration behavior of a typical neuron in an SNN; in particular, the integration behavior corresponding to magnetic resistance change gradually increased with the input spike number. This behavior occurred when the spin electron directions between double Co2Fe6B2 free and pinned layers in the p-STT-based neuron were switched from parallel to antiparallel states. In addition, a neuron circuit for integrate-and-fire operation was proposed. Finally, pattern-recognition simulation was performed for a single-layer SNN.

Magnetization dynamics in synthetic antiferromagnets: Role of dynamical energy and mutual spin pumping

We investigate magnetization dynamics in asymmetric interlayer exchange coupled Py/Ru/Py trilayers using both vector network analyzer-based and electrically detected ferromagnetic resonance techniques. Two different ferromagnetic resonance modes, in-phase and out-of-phase, are observed across all three regimes of the static magnetization configurations, through antiparallel alignment at low fields, the spin-flop transition at intermediate fields, and parallel alignment at high fields. The nonmonotonic behavior of the modes as a function of the external field is explained in detail by analyzing the interlayer exchange and Zeeman energies and is found to be solely governed by the interplay of their dynamical components. In addition, the linewidths of both modes were determined across the three regimes and the different behaviors of the linewidths versus external magnetic field are attributed to mutual spin pumping induced in the samples. Interestingly, the difference between the linewidths of the out-of-phase and in-phase modes decreases at the spin-flop transition and is reversed between the antiparallel and parallel aligned magnetization states.

Systematic Study of Medium States in Spin-Transfer Torque Magnetoresistance Random Access Memory and Their Implication for the Bit Error Rate

Medium states are common anomalies in spin-transfer torque magnetoresistance random access memory (STT-MRAM). In this work, these medium states are systematically studied using an ultrafast characterization system and compared with the normal parallel (P) and antiparallel (AP) states. The medium states that exist in the switching processes both from the AP state to the P state (AP2P) and from the P state to the AP state (P2AP) are presented. To obtain a deeper understanding of the medium states, the temperature and voltage dependences of the state rates are investigated. It is found that the medium states are more persistent at low temperatures and have a state rate with a symmetric voltage dependence. Consequently, it is confirmed that medium states should be one of the sources of bifurcated switching in STT-MRAMs.

Resonantly Driven Singlet-Triplet Spin Qubit in Silicon.

We report implementation of a resonantly driven singlet-triplet spin qubit in silicon. The qubit is defined by the two-electron antiparallel spin states and universal quantum control is provided through a resonant drive of the exchange interaction at the qubit frequency. The qubit exhibits long T_{2}^{*} exceeding 1 μs that is limited by dephasing due to the ^{29}Si nuclei rather than charge noise thanks to the symmetric operation and a large micromagnet Zeeman field gradient. The randomized benchmarking shows 99.6% single gate fidelity which is the highest reported for singlet-triplet qubits.

Observation of the dominant spin-triplet supercurrent in Josephson spin valves with strong Ni ferromagnets

We study experimentally nanoscale Josephson junctions and Josephson spin-valves containing strong Ni ferromagnets. We observe that in contrast to junctions, spin valves with the same geometry exhibit anomalous Ic(H) patterns with two peaks separated by a dip. We develop several techniques for in-situ characterization of micromagnetic states in our nano-devices, including magnetoresistance, absolute Josephson fluxometry and First-Order-Reversal-Curves analysis. They reveal a clear correlation of the dip in supercurrent with the antiparallel state of a spin-valve and the peaks with two noncollinear magnetic states, thus providing evidence for generation of spin-triplet superconductivity. A quantitative analysis brings us to a conclusion that the triplet current in out Ni-based spin-valves is approximately three times larger than the conventional singlet supercurrent.

Collective spin waves in arrays of asymmetric and symmetric width nanowires: effect of the film layering sequence

We have studied, experimentally and theoretically, the effect of a layering sequence on the magnonic band structure in dense arrays of both asymmetric- and symmetric cross-section tri-layered Py/Cu/Fe and Fe/Cu/Py nanowires. The spin-wave dispersion for these artificial crystals has been measured with Brillouin light scattering (BLS) spectroscopy. We also carried out numerical simulations of the dispersion using an original model employing a 2D Green’s function description of the dynamic dipole field of the precessing magnetization. The presence of the Cu spacer exchange-decouples the two magnetic layers, which stabilizes two equilibrium states of static magnetization. These are the parallel and antiparallel states, for which the static magnetization vectors for the layers are either co-aligned or anti-aligned to each other, respectively. These states are stable in a range of applied fields that depend on the layer width and their ordering in the stack. The magnetization configurations and layers sequence, as well as the presence of acoustic (in-phase) and optic (out-of-phase) spin-wave modes, have a significant impact on the magnonic band structure both in terms of the frequency positions of the dispersive and stationary modes and on their spatial profiles.

Biophysical characterization of p53 core domain aggregates.

Aggregation is the cause of numerous protein conformation diseases. A common facet of these maladies is the transition of a protein from its functional native state into higher order forms, such as oligomers and amyloid fibrils. p53 is an essential tumor suppressor that is prone to such conformational transitions, resulting in its compromised ability to avert cancer. This work explores the biophysical properties of early-, mid-, and late-stage p53 core domain (p53C) aggregates. Atomistic and coarse-grained molecular dynamics (MD) simulations suggest that early- and mid-stage p53C aggregates have a polymorphic topology of antiparallel and parallel β-sheets that localize to the core amyloidogenic sequence. Both topologies involve similar extents of interstrand mainchain hydrogen bonding, while sidechain interactions could play a role in regulating strand orientation. The free energy difference between the antiparallel and parallel states was within statistical uncertainty. Negative stain electron microscopy of mature fibrils shows a wide distribution of fiber widths, indicating that polymorphism may extend to the quaternary structure level. Circular dichroism of the fibrils was indicative of β-sheet rich structures in atypical conformations. The Raman spectrum of aggregated p53C was consistent with a mixture of arranged β-sheets and heterogeneous structural elements, which is compatible with the MD findings of an ordered β-sheet nucleus flanked by disordered structure. Structural polymorphism is a common property of amyloids; however, because certain polymorphs of the same protein can be more harmful than others, going forward it will be pertinent to establish correlations between p53C aggregate structure and pathology.

Manipulation of free-layer bias field in giant-magnetoresistance spin valve by controlling pinned-layer thickness

The manipulation of the bias field of the free-layer in giant magnetoresistance spin-valves is of great importance in sensor applications because this feature dominantly affects the low-field sensitivity of magnetoresistance. In this study, it is demonstrated that the bias field of the free-layer can be manipulated by controlling the thickness of the pinned-layer deposited afterward. The key to success is the utilization of the magnetostatic interactions between the free-poles formed on the Néel walls in both free- and pinned-layers. Magnetostatic interactions play a role in stabilizing the antiparallel magnetization state and hence in suppressing the magnetization switching of the free-layer from an antiparallel to a parallel state. A nearly zero bias field is achieved for a Ta-buffered sample with a pinned-layer thickness of 1.75 nm, where a very high low-field sensitivity of 7.7 mV/mA·Oe is obtained.

Optical method to determine the composition of lithium niobate crystals by digital holography.

Because the physical properties of lithium niobate (LiNbO3) strongly depend on composition, accurate and convenient methods for the determination of the composition are of great significance. Although several optical methods, including the measurement of UV absorption edge, the birefringence, and the second-harmonic generation, have been proved to be convenient for an accurate and fast standard determination of composition in LiNbO3 single crystals, their research and commercial applications are limited by the doping component and the complex nonlinear relationships. Based on preliminary work, a novel optical method to determine the composition of LiNbO3 crystals by digital holography is proposed. This method is based on the static internal field, which is obtained by means of the three-dimensional (3D) static measurement of the phase difference between antiparallel poling states without applying external voltage by digital holography. In order to investigate the influences of composition and doping on the static internal field in LiNbO3 crystals, the measured static internal fields from various LiNbO3 samples with different stoichiometry, doping type, and doping level are summarized and compared. Excluding the influence of dopant, the composition has been proved to be a unique key influencing factor on the static internal field in LiNbO3 crystals. A systematic measurement based on the static internal field from various sources with compositions ranging from 48.5 to 49.9 mol.% (Li/[Li+Nb] ratio) has been carried out. The approximate linear fit between the static internal field and composition can provide an easy, reliable, and sensitive determination of the composition in undoped and doped LiNbO3 samples.

A perfect triangular dysprosium single-molecule magnet with virtually antiparallel Ising-like anisotropy

A perfect triangular Dy3 single-molecule magnet was reported. Each Dy(iii) magnetic axis is oriented almost normal to the plane of Dy3, and the intramolecular magnetic dipole interaction gives rise to a virtually antiparallel Ising-like ground state.

Independent Control of Antiparallel- and Parallel-State Thermal Stability Factors in Magnetic Tunnel Junctions for Telegraphic Signals With Two Degrees of Tunability

Magnetic tunnel junctions (MTJs) with low thermal stability at room temperature have been proposed as read units in beyond CMOS computing architectures including stochastic computing unit and probabilistic-bit (p-bit). Networks of multiple interconnected MTJs may face challenges due to potential device-to-device variations in thermal stability from design targets. Recently, we generated tunable telegraphic signals using a thermally stable MTJ through proper control over an external bias field and a dc voltage bias, where we showed that the average dwell times in the antiparallel (AP) and parallel states could be tuned separately. The implication for this method for p-bit designs is that it allows for p-bits to be compatible with the state-of-the-art magnetoresistive random-access memory (MRAM) technology and introduces a second degree of tunability to the input–output characteristics of the device. In this article, we expand on this method in two important ways. First, we demonstrate the applicability of our method to p-bit designs by modeling the transfer function using the existing p-bit models. Our results indicate that the transfer function can be adjusted with slight modifications to the bias field, which allows for the possibility of p-bit circuits capable of on-chip corrections against device-to-device variations in their thermal stabilities. Second, we identify the physical mechanisms that allow for two degrees of tunability in the output signal, which is explained through the Neel–Brown model. This article provides both applicability and predictability to the dual-biasing method.

Nanoscale true random bit generator based on magnetic state transitions in magnetic tunnel junctions

We present an investigation into the in-plane field induced free layer state transitions found in magnetic tunnel junctions. By applying an ac current into an integrated field antenna, the magnetisation of the free layer can be switched between the magnetic vortex state and the quasi-uniform anti-parallel state. When in the magnetic vortex state, the vortex core gyrates a discrete number of times, and at certain frequencies there is a 50% chance of the core gyrating n or n − 1 times, leading to the proposal of a novel nanoscale continuous digital true random bit generator.

Anomalous dynamical evolution and nonadiabatic level crossing in exactly solvable time-dependent quantum systems

The anomalous dynamical evolution and the crossing of nonadiabatic energy levels are investigated for exactly solvable time-dependent quantum systems through a reverse-engineering scheme. By exploiting a typical driven model, we elucidate the peculiarities of its dynamics with anomalous behavior: the evolution of the adiabatic states and of the nonadiabatic ones exhibits opposite behavior with their representative vectors evolving from a parallel state to an antiparallel state: the nonadiabatic level crossing is identified as a necessary consequence since the crossing point corresponds exactly to the perpendicular point of the two vectors in the parametric space. In the light of these results, we show that various driven models with anomalous dynamical evolution can be designed and they offer alternative protocols for the quantum state control.

Rationally Designed High-Performance Spin Filter Based on Two-Dimensional Half-Metal Cr2NO2

Summary Half-metals are promising candidates for designing efficient spin filters owing to their unique electronic structures, which show the electrical conductivity for spin-up states and a band gap for spin-down states. Herein, designing excellent ultrathin spin filters by using half-metal two-dimensional Cr2NO2, which has a Curie temperature of 566 K, is demonstrated based on first-principles calculations. Our results reveal that the required energy to overturn the spin arrangement of bilayer Cr2NO2, from parallel to anti-parallel states, is quite low. The bilayer Cr2NO2 maintains its half-metal behavior while sandwiched within the Au/Cr2NO2/Au heterojunction. Owing to the half-metal characteristic, the total current of Au/Cr2NO2/Au can reach a value of 15 nA per primitive cell under a low voltage of 10 mV in parallel states. The magnetoresistance ratio is 9,333% at low voltage. The robust half-metal behavior, high Curie temperature, and two-dimensional structure make Cr2NO2 an ideal ultrathin spin-filtering material with high switching ratio and low energy consumption.