Electric-field-assisted switching in magnetic tunnel junctions

Magnetization switching of magnetic tunnel junctions (MTJs) via spin-orbit torque (SOT) is gaining interest in cache memory applications due to its benefits in lowering power consumption. Although SOT switching of perpendicular MTJs offers better scalability, in practice the requirement of an external magnetic field to assist SOT switching is a fundamental bottleneck. Meanwhile, in-plane (IP) MTJs, fabricated in the form of an ellipse, have been demonstrated to switch by SOT without an external field. Here, the easy axis of the elliptical MTJ is perpendicular to the charge flow in the spin-hall channel, and the magnetization trajectory is argued to undergo several precessions before magnetization flip similar to spin-transfer torque switching. [1] However, in this work we experimentally show that these IP-SOT MTJs, can in fact switch as fast as 400 ps, shorter than the precessional trajectory. The IP MTJs, which implements Pt/CoFeB/MgO as the free layer, are fabricated on a 200 mm wafer using stepper lithography and ion-beam etching. Interestingly, by applying asymmetric SOT pulse segments at substantially small voltage steps, we found that a significant number of MTJs demonstrated multilevel resistance states. While multilevel states have been reported in antiferromagnet/ferromagnet structures, the underlying mechanism arising from variation in exchange bias is not applicable in our case. [2] Using micromagnetic simulations, we formulate that the existence of meta-stable domain states inside the MTJ free-layer is a likely explanation in our case. In addition, we also probe the role of CoFeB thickness, aspect ratio, and switching probabilities of these devices. Multilevel resistance states in SOT devices could find potential in neuromorphic circuits.

Magnetization switching in magnetic tunnel junctions using spin-transfer torque and spin–orbit torque is key to the development of future spintronic memories. However, both switching mechanisms suffer from intrinsic limitations. In particular, the switching current in spin-transfer torque devices needs to be lowered, whereas an external magnetic field is required for spin–orbit torque devices to achieve deterministic switching in perpendicular magnetic tunnel junctions. Here, we experimentally demonstrate field-free switching of three-terminal perpendicular-anisotropy magnetic tunnel junction devices through the interaction between spin–orbit and spin-transfer torques. We show that the threshold current density of spin–orbit torque switching can be reduced by increasing the spin-transfer torque current density, and thus an optimal point for low-power perpendicular magnetic tunnel junction switching can be found by tuning the two current densities. Furthermore, and due to this interplay, low-power switching in two-terminal perpendicular magnetic tunnel junctions without an external magnetic field is also achieved. The interplay between spin–orbit and spin-transfer torques can be used to develop a low-power route to magnetization switching of perpendicular magnetic tunnel junctions without an external magnetic field.

Spintronic devices can be operated by either a magnetic field or a spin polarized current; however, the former is not site-specific, and the latter suffers from large current density issues. In this work, we show that voltage-controlled spintronic devices offer many attributes. Although a metallic ferromagnet responds only very weakly to an electric field if at all, under special circumstances an electric field can have a profound impact on its magnetic properties. An electric field can alter the interfacial perpendicular magnetic anisotropy (PMA) of CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJs) in a prescribed manner. By exploiting the voltage dependence of the PMA we have accomplished voltage-controlled MTJ for which the high- and low-resistance states can be accessed reversibly and repeatedly by voltage pulses associated with very low current density in the range of 104 A cm−2. This development opens up a new avenue to achieve ultra-low power consumption and ultra-fast operation in next-generation spintronic devices.

In order to study the effect of the interlayer exchange coupling of synthetic ferrimagnetic (SyF)-free layer on the current induced magnetization switching (CIMS) in magnetic tunnel junctions (MTJs), in the thermal activation regime, we examine the magnetization dynamics of MTJs with the SyF-free layer based on a macrospin model, combining with the STT analysis. Compared with the MTJ with a single layer, the reduction of the critical current (Jc) is observed in the MTJ with the SyF-free layer. The considerable reduction of Jc is also observed in MTJs with the ferromagnetically coupled-free layer over that with the antiferromagnetically coupled free layer. In connection with CIMS in each SyF-free layer of the MTJ, we found two different types of CIMS dependent on Jex, that is, decoupled CIMS and coupled CIMS.

Current-induced magnetization switching (CIMS) in low-resistance magnetic tunnel junctions was shown at average critical current densities (Jc=1.33×106 A/cm2). When large vertical currents pass through the junctions, spin-transfer torque, and vortex fields can rotate the magnetization of the free layer from the initial parallel state to a vortex state, resulting in 10.8% CIMS resistance change at zero-bias current, which is about half of the resistance change (22%) induced when switching is created by an external field. A micromagnetic simulation including the spin-transfer torque and the vortex field correctly predicts the critical negative-current-inducing switching from the parallel state into the vortex state, but fails to explain the reverse switching from the vortex state into the parallel state at an approximately symmetric positive critical current. Lead fields were analyzed and found to be not the cause of the observed switching. The very small dependence of the switching currents on an external magnetic field suggests the existence of hot-spots where local current densities may be much larger.

Substantial reduction of write-error rate for voltage-controlled magnetoresistive random access memory by in-plane demagnetizing field and voltage-induced negative out-of-plane anisotropy field

We propose a mechanism to substantially enhance spin transfer torque induced switching in perpendicular anisotropy magnetic tunnel junctions. Our method is based on injecting an additional assisting DC current with circular spin polarization into a magnetic free layer at frequencies that are resonant with its normal ferromagnetic resonance frequencies. We observe 80 % reduction in switching delays at constant switching currents and 2× improvement in critical switching current density. Spin current polarization chirality and spin polarization efficiency dependencies are investigated. Further, a device structure to experimentally realize the mechanism proposed in this letter is presented.

The detailed characteristics of magnetization switching in MgO-based magnetic tunnel junctions (MTJs) of diameters in the range of 21–57 nm with perpendicular magnetization are investigated by using pulses of durations in the range of 1–30 ns. Periodic fluctuations in the switching probability are observed at room temperature as a function of the biasing voltage. The period of the fluctuations is about 20–80 mV. The periods are in the same energy range as magnon excitations observed in tunnel spectroscopy. These fluctuations may also occur if there is a preferential trajectory in the free layer magnetization when it transverses the equator, as has been noted for in-plane magnetized MTJs. This kind of detailed observation of the switching probability may provide novel information about the switching dynamics.

Temperature plays an important role in spin-torque switching of magnetic tunnel junctions, causing magnetization fluctuations that decrease the switching voltage but also introduce switching errors. Here we present a systematic study of the temperature dependence of the spin-torque-switching probability of state-of-the-art perpendicular-magnetic-tunnel-junction nanopillars (40--60 nm in diameter) from room temperature down to 4 K, sampling up to a million switching events. The junction temperature at the switching voltage---obtained from the thermally assisted spin-torque-switching model---saturates at temperatures below about 75 K, showing that junction heating is significant below this temperature and that spin-torque switching remains highly stochastic down to 4 K. A model of heat flow in a nanopillar junction shows this effect is associated with the reduced thermal conductivity and heat capacity of the metals in the junction.

The magnetoelectric effect is technologically appealing because of its ability to manipulate magnetism using an electric field rather than magnetic field or current, thus providing a promising solution for the development of energy‐efficient spintronics. Although 180° magnetization switching is vital to spintronic devices, the achievement of 180° magnetization switching via magnetoelectric coupling is still a fundamental challenge. Herein, voltage‐driven full resistance switching of a magnetic tunnel junction (MTJ) with dipole interaction on a ferroelectric substrate through switchable parallel/antiparallel magnetization alignment is demonstrated. Parallel magnetization alignment along the y direction is obtained under a bias magnetic field. By rotating the magnetic easy axis via strain‐mediated magnetoelectric coupling, the parallel magnetizations in the MTJ reorient to the x axis with opposite paths because of dipole interaction, thus resulting in antiparallel alignment. Moreover, this voltage switching of MTJs is nonvolatile owing to variations in dipole interaction and can be well understood via phase field simulations. The results provide an avenue to realize electrical switching of MTJs and are significant for exploring energy‐efficient spintronic devices.

Memory cells based on electric charge storage, such as flash memory, are rapidly approaching the physical limits of scalability. The spin transfer torque random access memory (STTRAM) is one of the promising candidates for future universal memory [1-6]. The reduction of the current density required for switching and the increase of the switching speed are among the most important challenges in this area. Measurements performed in [4] showed a decrease in the critical current density for the penta-layer magnetic tunnel junction (MTJ) compared with the tri-layer MTJ. To achieve symmetric switching in asymmetric MTJs an external in-plane compensating magnetic field has to be introduced [4]. By numerically investigating the dynamics of the switching process in a MTJ composed of five layers we present the methodology on how to achieve symmetric switching without an external magnetic field by properly engineering the nanopillar geometry. 2. Model Description Our micromagnetic simulations are based on the magnetization dynamics described by the Landau-Lifschitz-Gilbert equation:

We investigated the dynamics of current-pulse-induced magnetization switching in magnetic tunnel junctions (MTJs) with antiferromagnetically and ferromagnetically coupled synthetic free layers through micromagnetic simulations. We found that a magnetic vortex is formed in thick upper ferromagnetic layers and plays an important role in magnetization switching in both types of synthetic free layers. Furthermore, higher thermal stability is observed in an MTJ with the ferromagnetically coupled free layer at an annealing temperature of 250 °C.

We investigated the write error rate (WER) for voltage-driven dynamic switching in magnetic tunnel junctions with perpendicular magnetization. We observed a clear oscillatory behavior of the switching probability with respect to the duration of pulse voltage, which reveals the precessional motion of magnetization during voltage application. We experimentally demonstrated WER as low as 4 × 10−3 at the pulse duration corresponding to a half precession period (∼1 ns). The comparison between the results of the experiment and simulation based on a macrospin model shows a possibility of ultralow WER (<10−15) under optimum conditions. This study provides a guideline for developing practical voltage-driven spintronic devices.

We propose and demonstrate a scheme for magnetization switching in magnetic tunnel junctions, in which two successive voltage pulses are applied to utilize both spin-transfer torque and electric field effect. Under this switching scheme, a CoFeB/MgO magnetic tunnel junction with perpendicular magnetic easy axis is shown to switch faster than by spin-transfer torque alone and more reliably than that by electric fields alone.

The free layer in the current driven magnetic tunnel junction (MTJ) can be switched by injecting spin-polarized current from an adjacent spin injector. A nonmagnetic efficient spin injector, a converter from charge current to spin current, has long been and is still being quested in the field of spintronics. The first discovered nonmagnetic spin injector was the heavy spin Hall metals (HMs) such as Pt and β-W. The HMs can only convert 2%–10% of the charge current to spin current. The rest of the charge current is wasted and has no contribution in MTJ switching. The waste of charge current during MTJ switching is one of the major sources of energy loss in MTJ operation. Later, it has been found that topological insulators (TIs) such as Bi2Se3 can convert around 37% charge current to spin current. Nevertheless, the topological insulator has low conductivity compared with the free layer of an MTJ, which results in a large amount of shunting charge current loss through the free layer. Topological semimetals (TMs) such as Na3Bi provide us with a trade-off point between HM and TI as a nonmagnetic spin injector. TMs have higher charge current to spin current conversion efficiency than HMs and higher electrical conductivity than TIs. In this work, we first calculated the density functional theory band structure of Na3Bi and then modeled and matched the near-Fermi-level band structure with the 8 band k⋅p model. We have used the k⋅p Hamiltonian in quantum transport (nonequilibrium Green’s function) formalism to determine the charge current to spin current conversion efficiency in Na3Bi. We have found that Na3Bi can convert around 27.33% of charge current to spin current, and its conductivity is ∼12.5 times more than that of Bi2Se3. A CoFeB (fixed layer)-MgO (tunneling barrier)-CoFeB (free layer)-Na3Bi (spin injector) MTJ consumes almost 9.09× and 655.57× less electrical power during isospeed write operation compared with CoFeB-MgO-CoFeB-Pt and CoFeB-MgO-CoFeB-Bi2Se3 MTJs, respectively. Application of isowrite voltage of 1V shows that CoFeB-MgO-CoFeB-Na3Bi MTJ switches 4.3× faster than CoFeB-MgO-CoFeB-Pt MTJ, while CoFeB-MgO-CoFeB-Bi2Se3 MTJ fails to switch and continues to oscillate.