Gerard Lim

Nanyang Technological University, School of Physics and Applied Physics, Faculty Member

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Papers by Gerard Lim

The impact of oxygen vacancy defect density on the nonlinearity and short-term plasticity of TiO2-based exponential selector

Nanotechnology

The readout margin of the one selector-one RRAM crossbar array architecture is strongly dependent... more The readout margin of the one selector-one RRAM crossbar array architecture is strongly dependent on the nonlinearity of the selector device. In this work, we demonstrated that the nonlinearity of Pt/TiO2/Pt exponential selectors increases with decreasing oxygen vacancy defect density. The defect density is controlled by modulating the sputtering pressure in the oxide deposition process. Our results reveal that the dominant conduction mechanisms of the Pt/TiO2/Pt structure transit from Schottky emission to Poole–Frenkel emission with the increase of sputtering pressure. Such transition is attributed to the rise of oxygen vacancy concentration. In addition, the short-term plasticity feature of the Pt/TiO2/Pt selector is shown to be enhanced with a lower defect density. These results suggest that low defect density is necessary for improved exponential selector performances.

Crystallinity Control of the Topological-Insulator Surface Bi85Sb15(012) via Interfacial Engineering for Enhanced Spin-Orbit Torque

Physical Review Applied

Topological insulators demonstrate high charge-spin conversion efficiency due to their spin-momen... more Topological insulators demonstrate high charge-spin conversion efficiency due to their spin-momentum locking at the Dirac surface states. However, the surface states are sensitive to disruption caused by exchange coupling when interfaced with a ferromagnet. Here, we demonstrate the use of various nonmagnetic insertion layer materials, Ti, Cu, and Pt, at the Co/Bi-Sb(012) interface to preserve the topological surface state and promote spin-orbit-torque efficiency through the crystallinity control of Bi-Sb(012). For 20-nm-thick Bi-Sb, a spin Hall angle of up to 8.93 is observed with the use of a Pt insertion layer, while it is otherwise negligible for Co/Bi-Sb(012) interfaces. We further explore the enhancement of Bi-Sb(012) crystallinity with increasing Bi-Sb thickness, revealing a rapidly increasing spin-orbit-torque efficiency that gradually saturates above 30 nm. A clear correlation between spin-orbit-torque efficiency and Bi-Sb(012) crystalline size is identified using x-ray diffractometry, establishing the origin of the high spin-orbit efficiency to be the Bi-Sb(012) crystalline orientation. Our work demonstrates the spin-orbittorque origin in Bi-Sb experimentally and paves the way for the adaptation of topological insulators as a class of low-energy spin source material for spintronics applications.

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Enhancement of temperature-modulated NbO2-based relaxation oscillator via interfacial and bulk treatments

Nanotechnology

This work demonstrates oscillation frequency modulation in a NbO2-based relaxation oscillator dev... more This work demonstrates oscillation frequency modulation in a NbO2-based relaxation oscillator device, in which the oscillation frequency increases with operating temperature and source voltage, and decreases with load resistance. An annealing-induced oxygen diffusion at 373 K was carried out to optimize the stoichiometry of the bulk NbO2 to achieve consistent oscillation frequency shift with device temperature. The device exhibits stable self-sustained oscillation in which the frequency can be modulated between 2 and 33 MHz, and a wider operating voltage range can be obtained. An additional surface treatment step was employed during fabrication to reduce the surface roughness of the bottom electrode and to remove surface contaminants that affect the interfacial properties of the device. The device frequency tunability coupled with high oscillating frequency and high endurance capability of more than 1.5 × 108 cycles indicates that the Pt/NbO2/Pt device is particularly suitable for a...

Frequency-Dependent Synapse Weight Tuning in 1S1R with a Short-Term Plasticity TiOx-Based Exponential Selector

ACS Applied Materials & Interfaces

Impact of Stopping Voltage and Hopping Conduction on the Oxygen Vacancy Concentration of Multi-Level HfO2-Based Resistive Switching Devices

2020 IEEE Silicon Nanoelectronics Workshop (SNW)

A multi-level state HfO<inf>2</inf>-based resistive switching model is reported, wher... more A multi-level state HfO<inf>2</inf>-based resistive switching model is reported, where the increase in stopping voltage (V<inf>stop</inf>) and thus activation energy (E<inf>AC</inf>) is attributed to the depletion of oxygen vacancy (V<inf>o</inf>) concentration (n<inf>c</inf>) during reset. Hopping conduction fittings also indicated a depletion of n<inf>c</inf> due to an increase of V<inf>stop</inf> as shown by an increase of trap-to-trap distance (a) and trap energy (ϕ<inf>T</inf>).

JA-05 - Effect of process parameters on the properties of compositionally graded FeCoNi films: A High Throughput Strategy

Abstract: Abstract: Fe-Co-Ni magnetic thin films with continuously varying com... more Abstract: Abstract: Fe-Co-Ni magnetic thin films with continuously varying compositions have been fabricated by the magnetron co-sputtering method. The films were characterized by high-throughput techniques. The results reveal the correlation between a range of compositions and the electrical properties of the films fabricated at various process parameters. Introduction: Soft magnetic materials are essential elements of electro-magnetic energy transformation technologies and they are widely used in various distribution, conversion, generation devices, such as transformers, motors, converters, generators, actuators, sensors etc. However, it usually takes long time for a new material before its acceptance for commercial application. Therefore, it is very crucial to speed up the discovery of new materials and incorporate them into various systems. High-throughput methods are efficient research methods [1-3] to explore this imme...

Direct spin accumulation quantification in ferromagnetic heterostructures using DC bias harmonic Hall measurement

Physical Review B, 2021

We have shown that, by applying a DC bias during harmonic Hall measurement in Ta/Co/Pt structure ... more We have shown that, by applying a DC bias during harmonic Hall measurement in Ta/Co/Pt structure with in-plane magnetic anisotropy, a net spin accumulation that results in an amplitude offset of the measured first harmonic Hall resistance in the Co layer, can be electrically quantified. A difference of the harmonic Hall resistance amplitudes when at opposite magnetisation states under a fixed DC bias was obtained, which gave direct measurements of spin accumulation up to 0.29% per kV/m of the local magnetization. The strength of both spin accumulation and damping-like efficiency was found to increase with larger Ta thicknesses, further verifying our experimental measurement that provides direct allelectrical quantification of spin accumulation.

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Hopping Conduction Temperature Investigations on High Retention Electrochemical Metallization MgO-based Resistive Switching Devices in the Low Resistance State

2019 Electron Devices Technology and Manufacturing Conference (EDTM), 2019

We report hopping conduction in Pt/MgO/Cu resistive switching memory (RSM) devices predominantly ... more We report hopping conduction in Pt/MgO/Cu resistive switching memory (RSM) devices predominantly in the low resistance state. Current-voltage measurements of Pt/MgO/Cu RSM devices exhibited good cycle-to-cycle variability. Promising DC endurance exceeding 2000 cycles and retention exceeding 10 years at 125°C were obtained at 103-104ON/OFF ratio within a consistent range of SET and RSET voltages. Multi-level state properties were also exhibited, while TEM and EDX studies suggested the possibility of filament in Pt/MgO/Cu RSM devices.

Spintronics for Neuromorphic Engineering

Enhanced Spin-Orbit Torques in Rare-Earth Pt/[Co/Ni]2/Co/Tb Systems

Physical Review Applied, 2019

We report the enhanced spin-orbit torque (SOT) effect due to the rare-earth metal Tb in a Pt/[Co/... more We report the enhanced spin-orbit torque (SOT) effect due to the rare-earth metal Tb in a Pt/[Co/Ni] 2 / Co/Tb multilayer with perpendicular magnetic anisotropy (PMA). Due to the large spin-orbit coupling in Tb, the effective dampinglike efficiency, ξ eff DL , is determined to be 0.55 for 9-nm Tb as compared to ξ eff DL = 0.18 for the reference Pt/[Co/Ni] 2 /Co/Ta stack. A relatively large effective spin Hall efficiency of Tb, θ eff SH,Tb , is determined to be −0.480 for Tb with a 9-nm thickness through a H z-biased harmonic measurement. Enhanced magnetization switching efficiency is observed with increasing Tb thickness, affirming the sizable antidamping torque responsible for deterministic switching. Furthermore, due to a more substantial interfacial coupling in Co/Tb interface than in Co/Ta interface, strong angular dependence of the dampinglike and fieldlike terms is, but diminishes with increasing Tb thickness. These results affirm that Tb plays a significant role in tuning the SOT efficiency in these structures as the increased Tb concentration leads to an enhancement of the effective spin Hall angle and switching efficiency.

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In situ Kerr and harmonic measurement in determining current-induced effective fields in MgO/CoFeB/Ta

Journal of Physics D: Applied Physics, 2018

A combination of the harmonic measurement and in situ Kerr imaging was used to experimentally det... more A combination of the harmonic measurement and in situ Kerr imaging was used to experimentally determine the spin–orbit (SO) effective fields in a MgO/CoFeB/Ta structure. Here, we evaluate the SO effective fields through an analytical energy approach by transforming the anomalous Hall effect and planar Hall effect (PHE) voltage into a field dependency while imaging the magnetisation behaviour by differential Kerr microscopy. The analytical fitting to the measurement data indicates the significant coexistence of both a transverse field, HT, and longitudinal field, HL, in the longitudinal (HL = –12 Oe, HT = 8 Oe per 106 A cm−2) and transverse (HL = –12 Oe, HT = –17 Oe per 106 A cm−2) measurement schemes, respectively, due to the PHE. Additionally, dendritic-like domains, indicating the influence of the interfacial Dzyaloshinskii–Moriya interaction (DMI) at the CoFeB/Ta interface, were observed by in situ Kerr imaging. Micromagnetic simulations confirm the dendritic domain formation and edge tilting of the magnetisation, as being due to the DMI.

Electric field control on gated Pt/Co/SiO2 heterostructure with insulating polymer

Journal of Physics D: Applied Physics, 2018

View the article online for updates and enhancements. Related content Control of magnetism in Co ... more View the article online for updates and enhancements. Related content Control of magnetism in Co by an electric field D Chiba and T Ono-The 2014 Magnetism Roadmap Robert L Stamps, Stephan Breitkreutz, Johan Åkerman et al.-Electric-field control of magnetism via strain transfer across ferromagnetic/ferroelectric interfaces Tomoyasu Taniyama

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Tuning the spin-orbit torque effective fields by varying Pt insertion layer thickness in perpendicularly magnetized Pt/Co/Pt(t)/Ta structures

Journal of Magnetism and Magnetic Materials, 2019

We experimentally determine that a Pt layer insertion between the Co/Ta interface can effectively... more We experimentally determine that a Pt layer insertion between the Co/Ta interface can effectively negate the field-like spin-orbit torque (SOT) in perpendicularly magnetized Pt/Co/Pt(t)/Ta structures, as the Rashba effects at the Pt/Co and Co/Pt interfaces counteract each other. The damping-like term can be tuned by varying the thickness of the Pt insertion layer to change both the sign and magnitude of the SOT effective fields. An asymmetric SOT contribution from the Pt/Co and Co/Pt interfaces was found, leading to zero damping-like SOT being obtained at different inserted Pt and bottom Pt layer thickness. The ability to change the sign of damping-like SOT allows control over the magnetic switching direction in SOT devices.

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Current-induced spin-orbit effective field modulations in synthetic antiferromagnetic structures

Journal of Magnetism and Magnetic Materials, 2018

We demonstrate the modulation of spin-orbit torque (SOT) fields in synthetic antiferromagnetic (S... more We demonstrate the modulation of spin-orbit torque (SOT) fields in synthetic antiferromagnetic (SAF) structures sandwiched between two heavy metals of opposite spin Hall angles. The SOTs fields that are measured by using harmonic Hall voltage technique, increase with net areal magnetization of the Pt/SAF/Ta structures. The result suggests an antiparallel orientation of the SOT fields in the two ferromagnetic layers. The antiparallel alignment of the SOT fields switches the magnetization of SAF structures with a current density as low as 2.3 × 10 11 A/m 2 .

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Current-induced three-dimensional domain wall propagation in cylindrical NiFe nanowires

Journal of Applied Physics, 2016

We report on the magnetization configurations in single NiFe cylindrical nanowires grown by templ... more We report on the magnetization configurations in single NiFe cylindrical nanowires grown by template-assisted electrodeposition. Angular anisotropic magnetoresistance measurements reveal that a three-dimensional helical domain wall is formed naturally upon relaxation from a saturated state. Micromagnetic simulations support the helical domain wall properties and its reversal process, which involves a splitting of the clockwise and anticlockwise vortices. When a pulsed current is applied to the nanowire, the helical domain wall propagation is observed with a minimum current density needed to overcome its intrinsic pinning.

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In-plane current induced domain wall nucleation and its stochasticity in perpendicular magnetic anisotropy Hall cross structures

Applied Physics Letters, 2015

Hall cross structures in magnetic nanowires are commonly used for electrical detection of magneti... more Hall cross structures in magnetic nanowires are commonly used for electrical detection of magnetization reversal in which a domain wall (DW) is conventionally nucleated by a local Oersted field. In this letter, we demonstrate DW nucleation in Co/Ni perpendicular magnetic anisotropy nanowire at the magnetic Hall cross junction. The DWs are nucleated by applying an in-plane pulsed current through the nanowire without the need of a local Oersted field. The change in Hall resistance, detected using anomalous Hall effect, is governed by the magnetic volume switched at the Hall junction, which can be tuned by varying the magnitude of the applied current density and pulse width. The nucleated DWs are driven simultaneously under the spin transfer torque effect when the applied current density is above a threshold. The possibility of multiple DW generation and variation in magnetic volume switched makes nucleation process stochastic in nature. The in-plane current induced stochastic nature o...

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Characterizing the spin orbit torque field-like term in in-plane magnetic system using transverse field

Journal of Applied Physics, 2016

In this work, we present an efficient method for characterizing the spin orbit torque field-like ... more In this work, we present an efficient method for characterizing the spin orbit torque field-like term in an in-plane magnetized system using the harmonic measurement technique. This method does not require a priori knowledge of the planar and anomalous hall resistances and is insensitive to non-uniformity in magnetization, as opposed to the conventional harmonic technique. We theoretically and experimentally demonstrate that the field-like term in the Ta/Co/Pt film stack with in-plane magnetic anisotropy can be obtained by an in-plane transverse field sweep as expected, and magnetization non-uniformity is prevented by the application of fixed magnetic field. The experimental results are in agreement with the analytical calculations.

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Low field domain wall dynamics in artificial spin-ice basis structure

Journal of Applied Physics, 2015

Artificial magnetic spin-ice nanostructures provide an ideal platform for the observation of magn... more Artificial magnetic spin-ice nanostructures provide an ideal platform for the observation of magnetic monopoles. The formation of a magnetic monopole is governed by the motion of a magnetic charge carrier via the propagation of domain walls (DWs) in a lattice. To date, most experiments have been on the static visualization of DW propagation in the lattice. In this paper, we report on the low field dynamics of DW in a unit spin-ice structure measured by magnetoresistance changes. Our results show that reversible DW propagation can be initiated within the spin-ice basis. The initial magnetization configuration of the unit structure strongly influences the direction of DW motion in the branches. Single or multiple domain wall nucleation can be induced in the respective branches of the unit spin ice by the direction of the applied field.

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Coupled Néel domain wall motion in sandwiched perpendicular magnetic anisotropy nanowires

Scientific reports, 2015

The operating performance of a domain wall-based magnetic device relies on the controlled motion ... more The operating performance of a domain wall-based magnetic device relies on the controlled motion of the domain walls within the ferromagnetic nanowires. Here, we report on the dynamics of coupled Néel domain wall in perpendicular magnetic anisotropy (PMA) nanowires via micromagnetic simulations. The coupled Néel domain wall is obtained in a sandwich structure, where two PMA nanowires that are separated by an insulating layer are stacked vertically. Under the application of high current density, we found that the Walker breakdown phenomenon is suppressed in the sandwich structure. Consequently, the coupled Néel domain wall of the sandwich structure is able to move faster as compared to individual domain walls in a single PMA nanowire.

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Programmable Spin–Orbit‐Torque Logic Device with Integrated Bipolar Bias Field for Chirality Control

Advanced Electronic Materials, 2020

Driven by the need to address both the von Neumann bottleneck and scaling limits predicted by Moo... more Driven by the need to address both the von Neumann bottleneck and scaling limits predicted by Moore's law, spintronic devices have been shown to be strong contenders for logic‐in‐memory applications. While several field‐free spin–orbit torque (SOT)‐driven logic devices have been proposed, their operation typically requires additional initialization or reset pulses, the exchange‐coupled canted spins reduce both anomalous Hall sign‐to‐noise ratio as well as thermal stability of the ferromagnetic layer, and device‐to‐device variation in exchange coupling strength is expected. A reconfigurable SOT‐driven logic device using a double Hall cross structure with an integrated bias field line for the generation of a local bias field is experimentally demonstrated. The on‐chip bipolar bias field can be toggled to flip the SOT‐induced switching chirality, and to assist with deterministic SOT magnetization switching, thereby enabling on‐the‐fly reconfigurability of the logic device to function as one of the several possible logic gates, e.g., AND, NOR, XNOR, XOR, NIMP, and converse NIMP. It is then shown through compact‐modeling and circuit simulation that the applications of such reconfigurable logic devices can be further expanded to build half‐adders.

Gerard Lim

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