Saima Siddiqui

Room temperature spin-orbit torque switching induced by a topological insulator

The strongly spin-momentum coupled electronic states in topological insulators (TI) have been extensively pursued to realize efficient magnetic switching. However, previous studies show a large discrepancy of the charge-spin conversion efficiency. Moreover, current-induced magnetic switching with TI can only be observed at cryogenic temperatures. We report spin-orbit torque switching in a TI-ferrimagnet heterostructure with perpendicular magnetic anisotropy at room temperature. The obtained effective spin Hall angle of TI is substantially larger than the previously studied heavy metals. Our results demonstrate robust charge-spin conversion in TI and provide a direct avenue towards applicable TI-based spintronic devices.

Logic circuit prototypes for three-terminal magnetic tunnel junctions with mobile domain walls

Spintronic computing promises superior energy efficiency and nonvolatility compared to conventional field-effect transistor logic. But, it has proven difficult to realize spintronic circuits with a versatile, scalable device design that is adaptable to emerging material physics. Here we present prototypes of a logic device that encode information in the position of a magnetic domain wall in a ferromagnetic wire. We show that a single three-terminal device can perform inverter and buffer operations. We demonstrate one device can drive two subsequent gates and logic propagation in a circuit of three inverters. This prototype demonstration shows that magnetic domain wall logic devices have the necessary characteristics for future computing, including nonlinearity, gain, cascadability, and room temperature operation.

Polymethyl methacrylate/hydrogen silsesquioxane bilayer resist electron beam lithography process for etching 25 nm wide magnetic wires

A method of patterning magnetic metallic thin films is presented using a bilayer polymethyl methacrylate and hydrogen silsesquioxane electron beam lithography resist mask combined with ion beam etching. The bilayer resist process allows for the combination of a high-resolution resist mask with easy postprocess removal of the mask without damage to the magnetic quality of the film. Co60Fe20B20 and Co/Ni multilayer films were patterned with electron beam lithography at 10–125 keV down to 25 nm wide features with 2 nm average root-mean square edge roughness. Both the in-plane and out-of-plane magnetic anisotropies of the respective film types were preserved after patterning.

Edge-modulated perpendicular magnetic anisotropy in [Co/Pd]n and L10-FePt thin film wires

Thickness modulation at the edges of nanostructured magnetic thin films is shown to have important effects on their perpendicular magnetic anisotropy. Thin film wires with tapered edges were made from [Co/Pd]20 multilayers or L10-FePt films using liftoff with a double layer resist. The effect of edge taper on the reversal process was studied using magnetic force microscopy and micromagnetic modeling. In [Co/Pd]20 the anisotropy was lower in the tapered edge regions which switched at a lower reverse field compared to the center of the wire. The L10-FePt wires showed opposite behavior with the tapered regions exhibiting higher anisotropy.

Depinning of Domain Walls by Magnetic Fields and Current Pulses in Tapered Nanowires With Anti-Notches

The influence of the size of anti-notches on the domain wall propagation in Permalloy nanowires with edge taper is investigated. The critical magnetic fields and current pulses required for a domain wall to pass a symmetrical circular anti-notch obstacle were estimated by high-resolution in-situ Kerr microscopy experiments and by micromagnetic simulations. The nanowires, made using electron beam lithography and ion beam etching, had an average width of 220 nm and the anti-notches consisted of circular features that increased the wire width from 5% to 35%. The critical magnetic flux densities for domain walls to pass the obstacles increased with anti-notch diameter, from 0.6 mT to 3.4 mT in the simulations and 0.3 mT to 1.5 mT in the experiment. The critical current densities ranged from 0.5 × 10¹² A/m² to 10 × 10¹² A/m² in the simulations, with a strong dependence on the domain wall type, but the experiment yielded higher critical current densities of 6 × 10¹² A/m² to 25 × 10¹² A/m².

Micromagnetic modeling of domain wall motion in sub-100-nm-wide wires with individual and periodic edge defects

Reducing the switching energy of devices that rely on magnetic domain wall motion requires scaling the devices to widths well below 100 nm, where the nanowire line edge roughness (LER) is an inherent source of domain wall pinning. We investigate the effects of periodic and isolated rectangular notches, triangular notches, changes in anisotropy, and roughness measured from images of fabricated wires, in sub-100-nm-wide nanowires with in-plane and perpendicular magnetic anisotropy using micromagnetic modeling. Pinning fields calculated for a model based on discretized images of physical wires are compared to experimental measurements. When the width of the domain wall is smaller than the notch period, the domain wall velocity is modulated as the domain wall propagates along the wire. We find that in sub-30-nm-wide wires, edge defects determine the operating threshold and domain wall dynamics.

The Spatial Resolution Limit for an Individual Domain Wall in Magnetic Nanowires

Magnetic nanowires are the foundation of several promising nonvolatile computing devices, most notably magnetic racetrack memory and domain wall logic. Here, we determine the analog information capacity in these technologies, analyzing a magnetic nanowire containing a single domain wall. Although wires can be deliberately patterned with notches to define discrete positions for domain walls, the line edge roughness of the wire can also trap domain walls at dimensions below the resolution of the fabrication process, determining the fundamental resolution limit for the placement of a domain wall. Using a fractal model for the edge roughness, we show theoretically and experimentally that the analog information capacity for wires is limited by the self-affine statistics of the wire edge roughness, a relevant result for domain wall devices scaled to regimes where edge roughness dominates the energy landscape in which the walls move.

Current-induced domain wall motion in compensated ferrimagnet

Owing to the difficulty in detecting and manipulating the magnetic states of antiferromagnetic materials, studying their switching dynamics using electrical methods remains a challenging task. By employing heavy-metal-rare-earth-transition-metal alloy bilayers, we experimentally study current-induced domain wall dynamics in an antiferromagnetically coupled system. We show that the current-induced domain wall mobility reaches a maximum at the angular momentum compensation point. With experiment and modeling, we further reveal the internal structures of domain walls and the underlying mechanisms for their fast motion. We show that the chirality of the ferrimagnetic domain walls remains the same across the compensation points, suggesting that spin orientations of specific sublattices rather than net magnetization determine Dzyaloshinskii-Moriya interaction in heavy-metal-ferrimagnet bilayers. The high current-induced domain wall mobility and the robust domain wall chirality in compensated ferrimagnetic material opens new opportunities for high-speed spintronic devices.

Effect of Magnetostatic Interactions on Stochastic Domain Wall Motion in Sub-100 nm Wide Nanowires

Magnetic field-driven domain wall motion is investigated in closely spaced sub-100 nm wide Co nanowires. Anticorrelations appear in the spatial distribution of pinning sites within weakly interacting nanowires, with a reduced probability of pinning adjacent to another pinning site over a mean correlation length similar to the line width roughness. In contrast, strong magnetostatic interactions between domain walls in adjacent nanowires eliminate the correlations, reducing the domain wall propagation distance for a given applied magnetic field.

A logic-in-memory design with 3-terminal magnetic tunnel junction function evaluators for convolutional neural networks

Analog implementations of neuromorphic circuits within digital systems are increasingly becoming attractive due to the high throughput and low energy per operation they offer. Magnetic logic devices based on spin-orbit torque offer a pathway to low-power resistive analog circuits. We present the magnetic tunnel junction (MTJ) function evaluator, a design for a logic device that evaluates nonlinear and linear functions for neural networks. We model the device and extend it into a functional design implementation in a logic-in-memory architecture in a hybrid process with magnetic device layers and 45 nm CMOS.