Graham E. Rowlands

Nanosecond-Timescale Low Energy Switching of In-Plane Magnetic Tunnel Junctions through Dynamic Oersted-Field-Assisted Spin Hall Effect

We investigate fast-pulse switching of in-plane-magnetized magnetic tunnel junctions (MTJs) within 3-terminal devices in which spin-transfer torque is applied to the MTJ by the giant spin Hall effect. We measure reliable switching, with write error rates down to 10-5, using current pulses as short as just 2 ns in duration. This represents the fastest reliable switching reported to date for any spin-torque-driven magnetic memory geometry and corresponds to a characteristic time scale that is significantly shorter than predicted possible within a macrospin model for in-plane MTJs subject to thermal fluctuations at room temperature. Using micromagnetic simulations, we show that in the three-terminal spin-Hall devices the Oersted magnetic field generated by the pulse current strongly modifies the magnetic dynamics excited by the spin-Hall torque, enabling this unanticipated performance improvement. Our results suggest that in-plane MTJs controlled by Oersted-field-assisted spin-Hall torque are a promising candidate for both cache memory applications requiring high speed and for cryogenic memories requiring low write energies.

Macrospin modeling of sub-ns pulse switching of perpendicularly magnetized free layer via spin-orbit torques for cryogenic memory applications

We model, using the macrospin approximation, the magnetic reversal of a perpendicularly magnetized nanostructured free layer formed on a normal, heavy-metal nanostrip, subjected to spin-orbit torques (SOTs) generated by short (≤0.5 ns) current pulses applied to the nanostrip, to examine the potential for SOT-based fast, efficient cryogenic memory. Due to thermal fluctuations, if solely an anti-damping torque is applied, then, for a device with sufficiently low anisotropy ( Hanis0 ∼ 1 kOe) suitable for application in cryogenic memory, a high magnetic damping parameter ( α∼0.1−0.2) is required for reliable switching over a significant variation of pulse current. The additional presence of a substantial field-like torque improves switching reliability even for low damping ( α≤0.03).

Experimental Demonstration of Efficient Spin–Orbit Torque Switching of an MTJ With Sub-100 ns Pulses

Efficient generation of spin currents from charge currents is of high importance for memory and logic applications of spintronics. In particular, generation of spin currents from charge currents in high spin–orbit coupling metals has the potential to provide a scalable solution for embedded memory. We demonstrate a net reduction in the critical charge current for spin torque-driven magnetization reversal via using spin–orbit mediated spin current generation. We scaled the dimensions of the spin–orbit electrode to 400 nm and the nanomagnet to 270 nm

Compact Model for Spin–Orbit Magnetic Tunnel Junctions

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Nanosecond magnetization dynamics during spin Hall switching of in-plane magnetic tunnel junctions

nm in a three-terminal spin–orbit torque, magnetic tunnel junction (SOT-MTJ) geometry. Our estimated effective spin Hall angle is 0.15–0.20 using the ratio of zero-temperature critical current from spin Hall switching and estimated spin current density for switching the magnet. We show bidirectional transient switching using spin–orbit generated spin torque at 100 ns switching pulses reliably followed by transient read operations. We finally compare the static and dynamic response of the SOT-MTJ with transient spin circuit modeling showing the performance of scaled SOT-MTJs to enable nanosecond class non-volatile MTJs.

A critical analysis of the feasibility of pure strain-actuated giant magnetostrictive nanoscale memories

Electrical control of a magnetic tunnel junction (MTJ) through spin-orbit torques (SOTs) offers opportunities to introduce MTJs into high-performance, low energy applications. SOTs support a high-speed and energy-efficient three terminal MTJ with perpendicular-to-the-plane magnetization (PMTJ). The read path is separated from the write path, enhancing the reliability of the device. SOTs exhibit two coexisting contributions: 1) a damping-like torque and 2) a field-like torque. In this paper, a physics-based compact model for a three terminal PMTJ is presented, which accurately models the magnetic, electrical, and thermal behaviors of a PMTJ controlled through SOTs. The proposed compact model is validated with experimental data, exhibiting reasonable accuracy with an average error of <;5.4%. The integration capability of the proposed compact model with CMOS technology is also demonstrated.

GPU-accelerated micromagnetic simulations using cloud computing

We present a study of the magnetic dynamics associated with nanosecond scale magnetic switching driven by the spin Hall effect in 3-terminal nanoscale magnetic tunnel junctions (MTJs) with in-plane magnetization. Utilizing fast pulse measurements in a variety of material stacks and detailed micromagnetic simulations, we demonstrate that this unexpectedly fast and reliable magnetic reversal is facilitated by the self-generated Oersted field, and that the short-pulse energy efficiency can be substantially enhanced by spatial non-uniformity in the initial magnetization of the magnetic free layer. The sign of the Oersted field is essential for this enhancement—in simulations in which we artificially impose a field-like torque with a sign opposite to the effect of the Oersted field, the result is a much slower and stochastic switching process that is reminiscent of the so-called incubation delay in conventional 2-terminal spin-torque-switched MTJs.

Efficient switching of 3-terminal magnetic tunnel junctions by the giant spin Hall effect of Pt85Hf15 alloy

Concepts for memories based on the manipulation of giant magnetostrictivenanomagnets by stress pulses have garnered recent attention due to their potential for ultra-low energy operation in the high storage density limit. Here, we discuss the feasibility of making such memories in light of the fact that the Gilbert damping of such materials is typically quite high. We report the results of numerical simulations for several classes of toggle precessional and non-toggle dissipative magnetoelastic switching modes. Material candidates for each of the several classes are analyzed and forms for the anisotropy energy density and ranges of material parameters appropriate for each material class are employed. Our study indicates that the Gilbert damping as well as the anisotropy and demagnetization energies are all crucial for determining the feasibility of magnetoelastic toggle-mode precessional switching schemes. The roles of thermal stability and thermal fluctuations for stress-pulse switching of giant magnetostrictivenanomagnets are also discussed in detail and are shown to be important in the viability, design, and footprint of magnetostrictive switching schemes.

All-Spin-Orbit Switching of Perpendicular Magnetization

Abstract Highly parallel graphics processing units (GPUs) can improve the speed of micromagnetic simulations significantly as compared to conventional computing using central processing units (CPUs). We present a strategy for performing GPU-accelerated micromagnetic simulations by utilizing cost-effective GPU access offered by cloud computing services with an open-source Python-based program for running the MuMax3 micromagnetics code remotely. We analyze the scaling and cost benefits of using cloud computing for micromagnetics.

NANOSECOND-TIMESCALE LOW-ERROR SWITCHING OF 3-TERMINAL MAGNETIC TUNNEL JUNCTION CIRCUITS THROUGH DYNAMIC IN-PLANE-FIELD ASSISTED SPIN-HALL EFFECT

Recent research has indicated that introducing impurities that increase the resistivity of Pt can enhance the efficiency of the spin Hall torque it generates. Here, we directly demonstrate the usefulness of this strategy by fabricating prototype 3-terminal in-plane-magnetized magnetic tunnel junctions that utilize the spin Hall torque from a Pt85Hf15 alloy and measuring the critical currents for switching. We find that Pt85Hf15 reduces the switching current densities compared to pure Pt by approximately a factor of 2 for both quasi-static ramped current biases and nanosecond-scale current pulses, thereby proving the feasibility of this approach in assisting the development of efficient embedded magnetic memory technologies.