Shengjie Shi

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

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

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.

All-Spin-Orbit Switching of Perpendicular Magnetization

Devices with ferromagnetic layers possessing a perpendicular magnetic easy axis are of great interest due to miniaturization capability and thermal stability, retaining deeply scaled magnetic bits over long periods of time. While the tunneling magnetoresistance effect has significantly enhanced electrical reading of magnetic bits, fast and energy efficient writing of magnetic bits remains a challenge. Current-induced spin-orbit torques (SOTs) have been widely considered due to significant potential for fast and energy-efficient writing of magnetic bits. However, to deterministically switch the magnetization of a perpendicularly magnetized device using SOTs, the presence of a magnetic field is required, which offsets possible advantages and hampers applications. In this paper, a perpendicularly magnetized device is presented, which, without the need for a magnetic field, can be deterministically switched in both toggle and nontoggle modes using a damping-like SOT induced by an in-plane current pulse. This capability is realized by shaping the magnetic energy landscape. Present device does not require any materials other than those widely utilized in conventional spin-orbit devices. The device provides two orders of magnitude enhancement in switching energy-time product as compared with state-of-the-art perpendicularly magnetized devices operating on spin-transfer torques.

Fast, reliable spin-orbit-torque switching in three terminal magnetic tunnel junctions with Hf dusting layer

Since the discovery of the large spin Hall effect in certain heavy metals, there has been continuous interest in utilizing this spin-orbit torque (SOT) effect in constructing a non-volatile memory that can be switched by an electric current. The key to future application of this type of memory is achieving both a short write time and a low write current, which will lower the energy cost compared to existing and other emerging memory technologies. We demonstrate an efficient way of reducing the switching current in tungsten-based three terminal magnetic tunnel junctions (MTJs) with in-plane magnetization (IPM) using a sub-atomic layer of Hf dusting inserted between the free FeCoB layer and the MgO tunnel barrier. We show with a simple FeCoB-MgO-FeCoB MTJ structure that in addition to low write current, fast pulse switching can be achieved with pulses ≤ 1 ns. We also confirm that in an SAF balanced MTJ structure with a PtHf spin Hall channel that the nanosecond switching behavior is typical of the switching of IPM three terminal spin-orbit-torque devices. We report write error rate of these structures down to ~10-6 at for 1 ns pulses, demonstrating feasibility for high performance cache memory.