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.
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...
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>).
Abstract: <i>Abstract: </i>Fe-Co-Ni magnetic thin films with continuously varying com... more Abstract: <i>Abstract: </i>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.<br/><br/><i>Introduction: </i>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...
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.
As-deposited [Co/Pt] multilayers show strong perpendicular magnetic anisotropy (PMA) and spin-orb... more As-deposited [Co/Pt] multilayers show strong perpendicular magnetic anisotropy (PMA) and spin-orbit torque (SOT). However, such structures require properly textured Pt (111) seed layers and optimized interlayer Pt thicknesses in order to maximise both PMA and SOT efficiency. In this work, the interplay of seed and interlayer Pt thickness on PMA and SOT efficiency in Ta/Pt/[Co/Pt]3/Co/Ta multilayers was studied. Using the harmonic lock-in method, the SOT damping- and field-like efficiencies were determined, with corrections for current shunting as well as the planar Hall effect. Our measurements show that the corrected effective SOT efficiencies are at least twice as large as the uncorrected values, with damping-like efficiency of up to ∼22 Oe per 1010 A m−2, which scales inversely with the Pt seed and interlayer thicknesses due to screening of the spin current originating from the high-resistivity bottom β -Ta layer. The choice of Pt seed and interlayer thicknesses is a compromise b...
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.
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.
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...
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.
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.
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.
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.
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...
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>).
Abstract: <i>Abstract: </i>Fe-Co-Ni magnetic thin films with continuously varying com... more Abstract: <i>Abstract: </i>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.<br/><br/><i>Introduction: </i>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...
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.
As-deposited [Co/Pt] multilayers show strong perpendicular magnetic anisotropy (PMA) and spin-orb... more As-deposited [Co/Pt] multilayers show strong perpendicular magnetic anisotropy (PMA) and spin-orbit torque (SOT). However, such structures require properly textured Pt (111) seed layers and optimized interlayer Pt thicknesses in order to maximise both PMA and SOT efficiency. In this work, the interplay of seed and interlayer Pt thickness on PMA and SOT efficiency in Ta/Pt/[Co/Pt]3/Co/Ta multilayers was studied. Using the harmonic lock-in method, the SOT damping- and field-like efficiencies were determined, with corrections for current shunting as well as the planar Hall effect. Our measurements show that the corrected effective SOT efficiencies are at least twice as large as the uncorrected values, with damping-like efficiency of up to ∼22 Oe per 1010 A m−2, which scales inversely with the Pt seed and interlayer thicknesses due to screening of the spin current originating from the high-resistivity bottom β -Ta layer. The choice of Pt seed and interlayer thicknesses is a compromise b...
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.
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.
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...
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.
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.
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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Papers by Gerard Lim