Kevin Garello

This study investigates the enhancement of double magnetic tunnel junction (DMTJ) performance through the integration of an assistance layer with perpendicular magnetic anisotropy. The diameter of the device and the thickness of the storage layer are identified as the key parameters that define thermal stability and switching speed. Both macrospin-model simulations and experimental measurements confirm that an increase in effective anisotropy produces faster switching. Previous reports confirmed that dual-spin torque mechanisms can be achieved with a hard polarizer or by introducing a thinner assistance layer alternative, but this can reduce tunneling magnetoresistance. In this work, by tuning the capping magnesium oxide, higher tunneling-magneto resistance values are achieved. Quasi-static measurements reveal that the highest spin-transfer torque efficiency is obtained when the effective anisotropy is maximized. Write-error-rate measurements, performed as a function of pulse width down to 7 ns, are analyzed with a new unified model introduced in this work, which combines ballistic and thermally activated regimes. One main result of this work shows that reducing the thicknesses of storage and assistance layers decreases the characteristic switching time τsw and that τsw further exhibits a clear dependence on device diameter, consistent with domain-wall-mediated switching dynamics, thereby indicating enhanced switching speed in the ballistic regime. The unified model introduces a characteristic switching time associated with minimum reversal energy, enabling the definition of a new figure of merit based on this minimum energy. The analysis highlights a trade-off where enhancing the ratios of thermal stability to current leads to longer characteristic switching times and higher write energy. These findings provide a comprehensive understanding of the factors that influence DMTJ performance and offer strategies to optimize their design for low-energy magnetic memory applications.

Switching of a ferromagnet in a magnetic tunnel junction (MTJ) with current-induced spin–orbit torque (SOT) has recently garnered significant attention as a promising non-volatile memory technology. Nonetheless, a fundamental challenge in SOT-MTJs with perpendicular magnetization is the symmetry constraint imposed by the SOT-induced in-plane spin polarization, which inhibits deterministic switching. One viable approach to break symmetry and enable field-free switching (FFS) is to exploit electric current gradients in SOT-MTJs. We achieve this by introducing a curvature in the SOT current path in the SOT-MTJs; we refer to these as bending structures. This curvature, in turn, generates a spin-current gradient and an inhomogeneous spin polarization in the SOT channel, thereby facilitating fully current-driven deterministic switching. Here, we vary the curvature in the SOT channel to assess the roles of the spin-current gradient and the inhomogeneous spin polarization in the deterministic switching of the bending structures. Using both experimental methods and micro-magnetic simulations, we identify that inhomogeneous spin polarization plays a major role in symmetry-breaking and dictates the switching polarity in bending structures. Furthermore, by scaling the SOT channel, we achieve an additional 48% reduction in the switching current. Lastly, we demonstrate fully electrical-driven ultrafast FFS in the bending structure with 400 ps pulses. This concept of curvature in the SOT current path is not confined to binary memory applications; it can also be explored for innovative spintronic-based computing applications.

Hardware implementations of the Ising model offer promising solutions to large-scale optimization tasks. In the literature, various nanodevices have been shown to emulate the spin dynamics for such Ising machines with remarkable effectiveness. Other nanodevices have been shown to implement spin-spin coupling with compact footprint and minimal energy dissipation. However, an ideal Ising machine would associate both types of nanodevices, and they must operate synergistically to support annealing: a progressive reduction of machine stochasticity that allows it to settle to an energy minimum. Here, we report an Ising machine that combines two nanotechnologies: memristor crossbar – storing multi-level couplings – and stochastic magnetic tunnel junction (SMTJ), acting as thermally driven spins. Because the same read voltage that interrogates the crossbar also biases the SMTJs, increasing this voltage automatically lowers the effective temperature of the machine, providing an intrinsic, analog-native annealing technique. Operating at zero magnetic field, our prototype consistently reaches the global optimum of a 24-vertex weighted MAX-CUT and a 10-vertex, three-color graph-coloring problem using an externally implemented feedback loop. Given that both nanotechnologies in our demonstrator are CMOS-integrated, this approach is compatible with advanced 3D integration, offering a scalable pathway toward compact, fast, and energy-efficient large-scale Ising solvers.

This application note discusses the working principle of spin-transfer torque magnetoresistive random access memory (STT-MRAM) and the impact that magnetic fields can have on STT-MRAM operation. Sources of magnetic fields and the typical magnitudes of magnetic fields are given.

We briefly summarize more than fifteen years of intense research in 2D materials (2DM)-based spintronics, which has led to an in-depth understanding of fundamental spin transport mechanisms, novel functionalities in magnetic tunnel junctions and spin orbit torque devices, and the formidable and unprecedented capability of proximity effects to make graphene a spin active material. Although the portfolio of functional 2DM-based devices and related heterostructures is continuously increasing, we outline key technological challenges that are still impeding practical spintronic applications in spin-logics and non-volatile memory technologies. We conclude by mentioning current and future directions which will maintain the momentum of the field of ultracompact spintronics based on 2DM and van der Waals heterostructures.

2023 International Conference on Solid State Devices and Materials ,Hardware Implementation of Probabilistic Models through Nanodevice Variability

Spin-orbit torques (SOTs) have opened a novel way to manipulate the magnetization using in-plane current, with a great potential for the development of fast and low power information technologies. It has been recently shown that two-dimensional electron gases (2DEGs) appearing at oxide interfaces provide a highly efficient spin-to-charge current interconversion. The ability to manipulate 2DEGs using gate voltages could offer a degree of freedom lacking in the classical ferromagnetic/spin Hall effect bilayers for spin-orbitronics, in which the sign and amplitude of SOTs at a given current are fixed by the stack structure. Here, we report the non-volatile electric-field control of SOTs in an oxide-based Rashba-Edelstein 2DEG. We demonstrate that the 2DEG is controlled using a back-gate electric-field, providing two remanent and switchable states, with a large resistance contrast of 1064%. The SOTs can then be controlled electrically in a non-volatile way, both in amplitude and in sign. This achievement in a 2DEG-CoFeB/MgO heterostructures with large perpendicular magnetization further validates the compatibility of oxide 2DEGs for magnetic tunnel junction integration, paving the way to the advent of electrically reconfigurable SOT MRAMS circuits, SOT oscillators, skyrmion and domain-wall-based devices, and magnonic circuits.

Non-volatile magnetic random-access memories (MRAMs), such as spin-transfer torque MRAM and next-generation spin–orbit torque MRAM, are emerging as key to enabling low-power technologies, which are expected to spread over large markets from embedded memories to the Internet of Things. Concurrently, the development and performances of devices based on two-dimensional van der Waals heterostructures bring ultracompact multilayer compounds with unprecedented material-engineering capabilities. Here we provide an overview of the current developments and challenges in regard to MRAM, and then outline the opportunities that can arise by incorporating two-dimensional material technologies. We highlight the fundamental properties of atomically smooth interfaces, the reduced material intermixing, the crystal symmetries and the proximity effects as the key drivers for possible disruptive improvements for MRAM at advanced technology nodes.