Field-free Deterministic Switching Research Articles

Extremely energy-efficient, magnetic field-free, skyrmion-based memristors for neuromorphic computing

The human brain can process information more efficiently than computers due to the dynamics of neurons and synapses. Mimicking such a system can lead to the practical implementation of artificial spiking neural networks. Spintronic devices have been shown to be an ideal solution for realizing the hardware required for neuromorphic computing. Skyrmions prove to be an effective candidate as information carriers owing to their topological protection and particle-like nature. Ferrimagnet and antiferromagnet-based spintronics have been employed previously to obtain an ultrafast simulation of artificial synapses and neurons. Here, we have proposed a ferromagnetic device of stack Ta3nmPt3nmCu0.65nmCo0.5nmPt1nm that is capable of ultrafast simulation of artificial neurons and synapses, owing to the high velocity of the stabilized skyrmions in the system. Electrical pulses of nanosecond pulse width were used to control the accumulation and dissipation of skyrmions in the system, analogous to the variations in the synaptic weights. Lateral structure inversion asymmetry is used to bring about a field-free switching in the system, leading to an energy-efficient switching process. Magnetic field-free deterministic switching and low pulse width current pulses drastically reduce energy consumption by 106 times compared to the existing ferromagnet-based neuromorphic devices. Artificial neuron, synapse, and memristor functionalities have been reproduced on the same device with characteristic time scales and field-free switching, better than any existing ferromagnet-based neuromorphic devices. The results recognize ferromagnet-based skyrmions as viable candidates for ultrafast neuromorphic spintronics capable of executing cognitive tasks with extremely high efficiency.

Spin-orbit torque magnetic tunnel junction based on 2-D materials: Impact of bias-layer on device performance

Two-dimensional (2D) material-based Spin-orbit torque (SOT) Magnetic tunnel junction (MTJ) has drawn a lot of research interest due to its potential for low power logic and memory applications. In this study, we employed a Bias-layer (BL) on top of the Topological insulator (TI) based SOT-MTJ structure to achieve field-free deterministic switching. A systematic micromagnetic simulation is carried out to examine the impact of variations in the BL-width (BW) and BL-distance (BLD) from the Free-layer (FL) on the performance parameters of the device. The stray field produced by the BL has a non-uniform profile; it declines with increase inBLD from the FL and improves with as BW increases. We demonstrate that by engineering the BLD and BW, the critical current density reduces to 3.9 × 109 A/m2, switching speed improves by 0.14 ns and power dissipation density reduces to 6.98 × 1014 Watt/m3as compared to the conventional SOT-MTJ with an external magnetic field of 100 mH.

Voltage control of magnetic order in RKKY coupled multilayers.

In the field of antiferromagnetic (AFM) spintronics, there is a substantial effort present to make AFMs viable active components for efficient and fast devices. Typically, this is done by manipulating the AFM Néel vector. Here, we establish a method of enabling AFM active components by directly controlling the magnetic order. We show that magneto-ionic gating of hydrogen enables dynamic control of the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction in solid-state synthetic AFM multilayer devices. Using a gate voltage, we tune the RKKY interaction to drive continuous transitions from AFM to FM and vice versa. The switching is submillisecond at room temperature and fully reversible. We validate the utility of this method by demonstrating that magneto-ionic gating of the RKKY interaction allows for 180° field-free deterministic switching. This dynamic method of controlling a fundamental exchange interaction can engender the manipulation of a broader array of spin textures, e.g., chiral domain walls and skyrmions.

Orbit-Transfer Torque Driven Field-Free Switching of Perpendicular Magnetization

The reversal of perpendicular magnetization (PM) by electric control is crucial for high-density integration of low-power magnetic random-access memory. Although the spin-transfer torque and spin-orbit torque technologies have been used to switch the magnetization of a free layer with perpendicular magnetic anisotropy, the former has limited endurance because of the high current density directly through the junction, while the latter requires an external magnetic field or unconventional configuration to break the symmetry. Here we propose and realize the orbit-transfer torque (OTT), that is, exerting torque on the magnetization using the orbital magnetic moments, and thus demonstrate a new strategy for current-driven PM reversal without external magnetic field. The perpendicular polarization of orbital magnetic moments is generated by a direct current in a few-layer WTe2 due to the existence of nonzero Berry curvature dipole, and the polarization direction can be switched by changing the current polarity. Guided by this principle, we construct the WTe2/Fe3GeTe2 heterostructures to achieve the OTT driven field-free deterministic switching of PM.

A comparison of two different mechanisms for deterministic spin orbit torque magnetization switching

In this article we analyze by modeling two possible mechanisms for magnetization switching using spin orbit torques, which have been reported to cause field-free deterministic switching in experiments. Here we compare the field-free magnetization switching due to a tilt of the anisotropy direction against the use of an antiferromagnetic bias field. Simple results obtained analytically show that a bias field not only causes the magnetization reversal but also reduces the corresponding energy barrier. The critical current required for magnetization switching is analyzed on the basis of a macrospin model. It is shown that although the field-free deterministic switching caused by a tilt of the anisotropy is more robust than the bias field in the development of memory elements, a compromise between requirements has to be adopted when selecting the parameters for specific applications.

Spin-orbit Torque Switching of Perpendicular Magnetization in Ferromagnetic Trilayers

In ferromagnetic trilayers, a spin-orbit-induced spin current can have a spin polarization of which direction is deviated from that for the spin Hall effect. Recently, magnetization switching in ferromagnetic trilayers has been proposed and confirmed by the experiments. In this work, we theoretically and numerically investigate the switching current required for perpendicular magnetization switching in ferromagnetic trilayers. We confirm that the tilted spin polarization enables field-free deterministic switching at a lower current than conventional spin-orbit torque or spin-transfer torque switching, offering a possibility for high-density and low-power spin-orbit torque devices. Moreover, we provide analytical expressions of the switching current for an arbitrary spin polarization direction, which will be useful to design spin-orbit torque devices and to interpret spin-orbit torque switching experiments.

Field-free spin-orbit torque switching through domain wall motion

Deterministic current-induced spin-orbit torque (SOT) switching of magnetization in a heavy transition metal/ferromagnetic metal/oxide magnetic heterostructure with the ferromagnetic layer being perpendicularly-magnetized typically requires an externally-applied in-plane field to break the switching symmetry. We show that by inserting an in-plane magnetized ferromagnetic layer CoFeB underneath the conventional W/CoFeB/MgO SOT heterostructure, deterministic SOT switching of the perpendicularly-magnetized top CoFeB layer can be realized without the need of in-plane bias field. Kerr imaging study further unveils that the observed switching is mainly dominated by domain nucleation and domain wall motion, which might limit the potentiality of using this type of multilayer stack design for nanoscale SOT-MRAM application. Comparison of the experimental switching behavior with micromagnetic simulations reveals that the deterministic switching in our devices cannot be explained by the stray field contribution of the in-plane magnetized layer, and the roughness-caused N\'eel coupling effect might play a more important role in achieving the observed field-free deterministic switching.

Field-free spin–orbit-torque switching of perpendicular magnetization aided by uniaxial shape anisotropy

It has been demonstrated that the switching of perpendicular magnetization can be achieved with spin–orbit torque (SOT) at an ultrafast speed and low energy consumption. However, to make the switching deterministic, an undesirable magnetic field or unconventional device geometry is required to break the structure inverse symmetry. Here we propose a novel scheme for SOT-induced field-free deterministic switching of perpendicular magnetization. The proposed scheme can be implemented in a simple magnetic tunnel junction (MTJ)/heavy-metal system, without the need of complicated device structure. The perpendicular-anisotropy MTJ is patterned into elliptical shape and misaligned with the axis of the heavy metal, so that the uniaxial shape anisotropy aids the magnetization switching. Furthermore, unlike the conventional switching scheme where the switched final magnetization state is dependent on the direction of the applied current, in our scheme the bipolar switching is implemented by choosing different current paths, which offers a new freedom for developing novel spintronics memories or logic devices. Through the macrospin simulation, we show that a wide operation window of the applied current pulse can be obtained in the proposed scheme. The precise control of pulse amplitude or pulse duration is not required. The influences of key parameters such as damping constant and field-like torque strength are discussed as well.