Critical Switching Voltage Research Articles

A Comprehensive Study of Temperature and Its Effects in SOT-MRAM Devices.

We employ a fully three-dimensional model coupling magnetization, charge, spin, and temperature dynamics to study temperature effects in spin-orbit torque (SOT) magnetoresistive random access memory (MRAM). SOTs are included by considering spin currents generated through the spin Hall effect. We scale the magnetization parameters with the temperature. Numerical experiments show several time scales for temperature dynamics. The relatively slow temperature increase, after a rapid initial temperature rise, introduces an incubation time to the switching. Such a behavior cannot be reproduced with a constant temperature model. Furthermore, the critical SOT switching voltage is significantly reduced by the increased temperature. We demonstrate this phenomenon for switching of field-free SOT-MRAM. In addition, with an external-field-assisted switching, the critical SOT voltage shows a parabolic decrease with respect to the voltage applied across the magnetic tunnel junction (MTJ) of the SOT-MRAM cell, in agreement with recent experimental data.

Pulse-width dependence of spin–orbit torque switching in Mn3Sn/Pt thin films

Spin–orbit torque (SOT) switching in chiral antiferromagnets, such as D019-Mn3Sn, has opened prospects in antiferromagnetic spintronics. In this paper, we investigated the critical switching voltage as a function of the voltage pulse width in order to explore the thermal stability factor Δ in the D019-Mn3Sn. Although the critical switching voltage is found to decrease with increasing the voltage pulse width as expected by the thermally activated switching process with Δ of 131.1 at 300 K, our detailed examinations reveal that the Joule heating due to the voltage pulse could induce the magnetic phase transition in the D019-Mn3Sn, which significantly alters the energy landscape relevant to the SOT switching. This assisting mechanism for the SOT switching inhibits an accurate determination of Δ but could deliver a smaller Δ than the actual. Our study provides cautionary note in estimating the thermal stability of the chiral antiferromagnets.

Probing Relaxation Dynamics and Stepped Domain Switching in Boron‐Alloyed VO2

AbstractThe characteristic metal–insulator phase transition (MIT) in vanadium dioxide results in nonlinear electrical transport behavior, allowing VO2 devices to imitate the complex functions of neurological behavior. Chemical doping is an established method for varying the properties of the MIT, and interstitial dopant boron has been shown to generate a unique dynamic relaxation effect in individual B‐VO2 particles. This paper describes the first demonstration of an electrically stimulated B‐VO2 proto‐device which manifests a time‐dependent critical transformation temperature and switching voltage derived from the coupling of dopant diffusion dynamics and the metal–insulator transition of VO2. During quasi‐steady current‐driven transitions, the electrical responses of B‐VO2 proto‐devices show a step‐by‐step progression through the phase transformation, evidencing domain transformations within individual particles. The dynamic relaxation effect is shown to increase the critical switching voltage by up to 41% (ΔVcrit = 0.13 V) and also to increase the resistivity of the M1 phase of B‐VO2 by 14%, imbuing a memristive response derived from intrinsic material properties. These observations demonstrate the dynamic relaxation effect in B‐VO2 proto‐devices whose electrical transport responses can be adjusted by electronic phase transitions triggered by temperature but also by time as a result of intrinsic dynamics of interstitial dopants.

Impact of ambient temperature on the switching of voltage-controlled perpendicular magnetic tunnel junction

Voltage control of the magnetic anisotropy (VCMA) effect enables a voltage-mediated magnetization switching mechanism for lower-power applications. In this work, we experimentally investigate the characteristics of VCMA-induced switching and we observe a clear decrease in the critical switching voltage (Vc) at elevated temperatures. A 50% reduction in Vc is quantified when increasing the ambient temperature (T) from 300 K to 360 K. Such a T-dependence of Vc is well explained with the variations of saturation magnetization (MS), interfacial anisotropy (Ki), and VCMA coefficient (ξ). In addition, the dependences of these properties on temperature are well fitted and explained with the power law of MS(T). Our findings on the T-dependent magnetic and switching characteristics of VCMA are of technological importance for implementing VCMA in magnetic random access memory (MRAM) applications.

Influence of Total Ionizing Dose on Magnetic Tunnel Junctions With Perpendicular Anisotropy

Spin-transfer torque magnetic random access memory (STT-MRAM) is a promising solution for onboard, radiation-tolerant memory for space environments. This is because magnetic tunnel junctions (MTJs) have demonstrated outstanding resilience to ionizing radiation. In this study, we examine the effect of total ionizing dose (TID) on the MTJ's thermal stability factor, intrinsic critical switching voltage, and write energy. These parameters were calculated from switching probability distribution curves taken at three different pulsewidths. Additionally, dc current hysteresis plots were obtained to measure the MTJ's tunnel magnetoresistance ratio, its dependence on voltage, and the dc current required to switch states. Tests were carried out at two TID exposure levels up to 1 Mrad(Si) on a total of 24 MTJs of varying cross section dimensions in order to identify the influence of scaling as well as to obtain statistical variation for any changes observed. Results indicate that TID exposure had small effects on the MTJ's thermal stability and critical switching voltage, which affected the write energy, especially for switching from the antiparallel (AP) to parallel (P) direction. For switching from P to AP, these changes fluctuated, suggesting that TID has multiple effects, which have competing influences on the MTJ's switching properties. The work presented here shows that, while MTJs are highly resistant to ionizing radiation, a subtle influence on some properties for the STT-MRAM write operation should be considered in radiation environment assessments for both terrestrial and space applications.

Field-free switching of magnetic tunnel junctions driven by spin–orbit torques at sub-ns timescales

We report time-resolved measurements of magnetization switching by spin–orbit torques in the absence of an external magnetic field in perpendicularly magnetized magnetic tunnel junctions (MTJs). Field-free switching is enabled by the dipolar field of an in-plane magnetized layer integrated above the MTJ stack, the orientation of which determines the switching polarity. Real-time single-shot measurements provide direct evidence of magnetization reversal and switching distributions. Close to the critical switching voltage, we observe stochastic reversal events due to a finite incubation delay preceding the magnetization reversal. Upon increasing the pulse amplitude to twice the critical voltage, the reversal becomes quasi-deterministic, leading to reliable bipolar switching at sub-ns timescales in zero external field. We further investigate the switching probability as a function of dc bias of the MTJ and external magnetic field, providing insight into the parameters that determine the critical switching voltage.

Complementary Logic Implementation for Antiferromagnet Field-Effect Transistors

In this paper, a compact and complementary logic implementation is proposed for antiferromagnet field-effect transistor (AFMFET) devices. The implementation enables a complete set of Boolean operations based on complementary logic as well as majority-gate logic. The impacts of several key device-level design parameters are investigated, such as the channel resistance and critical switching voltage, and their optimal values that minimize the overall energy-delay product (EDP) of a 32-bit arithmetic logic unit are quantified. In addition, it is shown that one can potentially take advantage of the large domain size of some AFM materials such as chromium and build a compact majority-gate-based logic. The potential performance benefits of the majority-gate-based logic are also quantified. Compared to the conventional CMOS logic circuit, the one with AFMFET devices using majority gates can potentially achieve $10\times $ improvement in terms of the EDP.

Scaling Projections on Spin-Transfer Torque Magnetic Tunnel Junctions

We investigate scaling of technologically relevant magnetic tunnel junction devices in the trilayer and pentalayer configurations by varying the cross-sectional area along the transverse direction using the non-equilibrium Green's function spin transport formalism. We study the geometry dependence by considering square and circular cross-sections. As the transverse dimension in each case reduces, we demonstrate that the transverse mode energy profile plays a major role in the resistance-area product. Both types of devices show constant tunnel magnetoresistance at larger cross-sectional areas but achieve ultra-high magnetoresistance at small cross-sectional areas, while maintaining low resistance-area products. We notice that although the critical switching voltage for switching the magnetization of the free layer nanomagnet in the trilayer case remains constant at larger areas, it needs more energy to switch at smaller areas. In the pentalayer case, we observe an oscillatory behavior at smaller areas as a result of double barrier tunneling. We also describe how switching characteristics of both kinds of devices are affected by the scaling.

Stability phase diagram of a perpendicular magnetic tunnel junction in noncollinear geometry

Experimental measurements performed on MgO-based perpendicular magnetic tunnel junctions show a strong dependence of the stability voltage-field diagrams as a function of the direction of the magnetic field with respect to the plane of the sample. When the magnetic field is applied in-plane, systematic nonlinear phase boundaries are observed for various lateral sizes. The simulation results based on the phenomenological Landau-Lifshitz-Gilbert equation including the in-plane and out-of-plane spin transfer torques are consistent with the measurements if a second-order anisotropy contribution is considered. Furthermore, performing the stability analysis in linear approximation allowed us to analytically extract the critical switching voltage at zero temperature in the presence of an in-plane field. This study indicates that in the noncollinear geometry investigations are suitable to detect the presence of the second-order term in the anisotropy. Such higher order anisotropy term can yield an easy-cone anisotropy which reduces the thermal stability factor but allows for more reproducible spin transfer torque switching due to a reduced stochasticity of the switching. As a result, the energy per write event decreases much faster than the thermal stability factor as the second-order anisotropy becomes more negative. Easy-cone anisotropy can be useful for fast-switching spin transfer torque magnetic random access memories provided the thermal stability can be maintained above the required value for a given memory specification.

A novel microfluidic valve controlledby induced charge electro-osmotic flow

In this paper, a novel microfluidic valve by utilizing induced charge electro-osmotic flow (ICEOF) is proposed and analyzed. The key part of the microfluidic valve is a Y-shaped microchannel. A small metal plate is placed at each corner of the junction of the Y-shaped microchannel. When a DC electrical field is applied through the channels, electro-osmotic flows occur in the channels, and two vortices will be formed near each of the metal plates due to the ICEOF. The two vortices behave like virtual ‘blocking columns’ to restrain and direct the flow in the Y-channel. In this paper, effects of the length of the metal plates, the applied voltages, the width of the microchannel, the zeta potential of the non-metal microchannel wall, and the orientation of the branch channels on the flow switching between two outlet channels are numerically investigated. The results show that the flow switching between the two outlet channels can be flexibly achieved by adjusting the applied DC voltages. The critical switching voltage (CSV), under which one outlet channel is closed, decreases with the increase in the metal plate length and the orientation angle of the outlet channels. The CSV, however, increases with the increase in the inlet voltage, the width of the microchannel, and the absolute value of the zeta potential of the non-metal microchannel wall. Compared with other types of micro-valves, the proposed micro-valve is simple in structure without any moving parts. Only a DC power source is needed for its actuation, thus it can operate automatically by controlling the applied voltages.

Enhancement of Spin-transfer torque switching via resonant tunneling

We propose the use of resonant tunneling as a route to enhance the spin-transfer torque switching characteristics of magnetic tunnel junctions. The proposed device structure is a resonant tunneling magnetic tunnel junction based on a MgO-semiconductor heterostructure sandwiched between a fixed magnet and a free magnet. Using the non-equilibrium Green's function formalism coupled self consistently with the Landau-Lifshitz-Gilbert-Slonczewski equation, we demonstrate enhanced tunnel magneto-resistance characteristics as well as lower switching voltages in comparison with traditional trilayer devices. Two device designs based on MgO based heterostructures are presented, where the physics of resonant tunneling leads to an enhanced spin transfer torque thereby reducing the critical switching voltage by up to 44%. It is envisioned that the proof-of-concept presented here may lead to practical device designs via rigorous materials and interface studies.

Effect of quantum confinement on spin transport and magnetization dynamics in dual barrier spin transfer torque magnetic tunnel junctions

Electronic transport and magnetization dynamics associated with the current induced spin torque effects in dual barrier magnetic tunnel junctions (MTJs) have been investigated using nonequilibrium Green’s Function equations solved self-consistently with Landau–Lifshitz–Gilbert–Slonczewski equation. In a dual barrier (pentalayer) MTJ, a set of geometry and band-structure parameters jointly determines the position of resonant peaks and valleys within the energy range of interest. The presence of nonmonotonic quantum well states inside the central ferromagnetic free layer significantly modifies the critical switching voltage across MTJ and tunneling magnetoresistance simultaneously depending on whether the resonant condition is satisfied. Proper choice of (i) free ferromagnetic layer thickness, (ii) tunneling barrier height, (iii) width of the tunneling barrier, and (iv) operational voltage has been found to increase both in-plane and out-of-plane spin torque efficiencies in pentalayer MTJs by approximately an order in magnitude as compared to the conventional single barrier trilayer structures. Transport simulation results are in a reasonable quantitative agreement with the existing set of trilayer MTJ experiments. Energy efficiency of pentalayer MTJ structures over that of trilayers during spin torque driven magnetization switching has also been reported quantitatively under resonance conditions.

Minimal switching voltage for magnetization reversals in asymmetric nanorings

Micromagnetic simulations based on the Landau–Lifshitz–Gilbert equation are presented to study spin-polarized current induced magnetization switching in asymmetric nanoring-shaped magnetic tunnel junctions. The results show that in a nanoring with an intermediate eccentric distance S, the critical switching voltage V C reaches the minimum whose magnitude is less than half of the voltage for a symmetric nanoring. In addition to the spin-transfer torque, the current induced Ampère field is found to play a crucial part in the switching process. Analysis is given to explain the existence of such an energy valley that leads to the minimal voltage.

Electro‐Mechano‐Optical (ElMO) Microstructures Fabricated from Reactive Mesogens: A Novel Light Modulating Material

AbstractWe have manufactured birefringent polymer balls by photo‐polymerizing suspensions of reactive mesogens in the nematic phase. The dielectric and optical anisotropy is captured by photo‐polymerization. The balls are Electro‐Mechano‐Optical (ElMO) in that they undergo a mechanical reorientation in the presence of an applied electric field, and therefore may be useful for display applications. We show that our spheres are suited for both conventional electrode arrangements and in‐plane switching modes. We present our modeling for the switching times and critical switching voltage and show the potential of ElMO materials in electro‐optic applications.

Switching Properties of Liquid Crystalline Polymers at Interfaces

Main chain liquid crystalline polymers (LCPs) in nematic solvents are predicted to undergo a family of phase transitions at the solid-nematic interface. These second order phase transitions occur between the distorted and undistorted states of the nematic solvent. These effects involve free LCPs subject to confinement, tethered LCPs, uniformly adsorbed LCPs, and LCPs incorporated into lamellar mesogels. In every case, the transitions are opposed by the distortion free energy of the nematic. The driving force is due to polymeric effects. Depending on the situation, the transition results either from the elasticity of the deformed chains or from their interaction free energy. The critical switching voltage of the Frederiks transition can be reduced arbitrarily when the LCP layer approaches criticality. Mesogels incorporating main-chain LCPs are predicted to exhibit a variety of electro-optic, mechano-optic, and electromechanic effects.

Electrical conduction at low temperatures in high energy silicon detectors before and after irradiation with neutrons

Electrical I- V characteristics, at temperatures T ≥ 10 K, were studied. A drastic reduction of free carriers (freeze-out) was observed, due to cooling. An abrupt increase of I f at V f ∼ 1.1 V appears for non irradiated and lowly irradiated samples ( Φ < 10 14 n/ cm 2). Unlike these samples ( Φ < 10 14 n/ cm 2), by irradiation with neutrons, at fluences Φ ≥ 10 14n/cm 2, a similar abrupt increase of current I f, at a critical switching voltage V s, is seen. This value V s increases drastically with fluence Φ and for a value Φ = 5.9 × 10 14n/cm 2 V s = 400–500 V. At T = 10 K and Φ = 5.9 × 10 14n/cm 2 oscillations of I f occurred at voltages around V s. For higher temperatures, oscillations do not appear. These phenomena can be explained on the basis of double injection controlled by partially filled traps.