Deterministic Spin-Orbit Torque Induced Magnetization Reversal In Pt/[Co/Ni]_n/Co/Ta Multilayer Hall Bars

Spin-orbit torque (SOT) induced by electric current has attracted extensive attention as an efficient method of controlling the magnetization in nanomagnetic structures. SOT-induced magnetization reversal is usually achieved with the aid of an in-plane bias magnetic field. In this paper, we show that by selecting a film stack with weak out-of-plane magnetic anisotropy, field-free SOT-induced switching can be achieved in micron sized multilayers. Using direct current, deterministic bipolar magnetization reversal is obtained in Pt/[Co/Ni]2/Co/Ta structures. Kerr imaging reveals that the SOT-induced magnetization switching process is completed via the nucleation of reverse domain and propagation of domain wall in the system

Skyrmion dynamics in magnetic thin films

Magnetic skyrmions are nanoscopic magnetization textures that have intrigued the spintronics community for more than a decade now due to their potential as a next-generation information carrier. Their highly sought-after characteristics, such as high current-induced transport speeds, small sizes, and topological stability allows them to simultaneously fulfil the function of both random-access memory and high capacity storage media. However, several challenges should first be addressed, such as the skyrmion Hall effect (SkHE), skyrmion transport efficiency, and the lack of suitable injection methods. In this thesis, the dynamics of skyrmions under the influence of spin-orbit torques and magnetostatic field gradients were investigated by using a combination of theoretical and numerical modelling methods. Our model on spin-orbit torque reveals that the skyrmion speed scales linearly with its size, leading to a tradeoff between skyrmion density and speed. A mechanism exploiting the transverse repulsive forces from the nanowire edges was revealed, where the skyrmion speed was shown to be increased many times. A similar mechanism was found in antiferromagnetically-coupled skyrmions, that allows the negation of the SkHE while passively increasing their speed. Furthermore, the difficulty in nucleating these type skyrmions was also tackled; a combination of DMI-induced edge tilting and spin-orbit torque allows single skyrmions to be injected on-demand efficiently. For the development of energy-efficient skyrmion memory devices, a model was developed to describe the skyrmion motion under a voltage-controlled magnetic anisotropy (VCMA) gradient. As no electric currents are required, the VCMA-based devices consume several orders of magnitude lesser power. A VCMA-based device architecture was proposed using multiplexed discrete gate electrodes. A maximum speed of 70 ms-1 was achieved, similar to current-induced speeds. However, VCMA-based devices are clear winners in terms of design flexibility; a recirculating skyrmion track was demonstrated, where skyrmions could be shifted and cycled in a loop. To overcome the SkHE in such devices, a transverse driving scheme was devised such that the net skyrmion motion is directed parallel to the device axis. Finally, a hybrid drive combining both the transverse and longitudinal scheme was also demonstrated, resulting in high velocity skyrmion motion with low SkHE. While only a few types of skyrmion devices were discussed, the model developed in this thesis serves as a platform for the design of novel VMCA devices.Doctor of Philosoph

Effect of seed and interlayer Pt thickness on spin-orbit torque efficiency in Co/Pt multilayer with perpendicular magnetic anisotropy

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 between PMA and SOT efficiency of the device.Agency for Science, Technology and Research (A*STAR)National Research Foundation (NRF)The work was supported by the Singapore National Research Foundation, under a Competitive Research Programme (Non-volatile Magnetic Logic and Memory Integrated Circuit Devices, NRF-CRP9-2011-01), and an Industry-IHL Partnership Program (NRF2015- IIP001-001). The support from an RIE2020 ASTAR AME IAF-ICP Grant (No. I1801E0030) is also acknowledged

Bilayer skyrmion dynamics on a magnetic anisotropy gradient

Magnetic skyrmion transport has been primarily based on the use of spin torques which require high current densities and face performance deterioration associated with Joule heating. In this work, we derive an analytical model for energy efficient skyrmion propagation in an antiferromagnetically-coupled bilayer structure using a magnetic anisotropy gradient. The interlayer skyrmion coupling provides a strong restoring force between the skyrmions, which not only prevents annihilation but also increases their forward velocity up to the order of km s–1. For materials with low Gilbert damping parameter, the interlayer skyrmion coupling force can be amplified up to ten times, with a corresponding increase in velocity. Furthermore, the analytical model also provides insights into the dynamics of skyrmion pinning and relaxation of asymmetric skyrmion pairs in bilayer-coupled skyrmion systems.NRF (Natl Research Foundation, S’pore)Published versio

Spin-orbit torque induced magnetization anisotropy modulation in Pt/(Co/Ni)4/Co/IrMn heterostructure

In this work, we show that domain wall (DW) dynamics within a system provide an alternative platform to characterizing spin-orbit torque (SOT) effective fields. In perpendicularly magnetized wires with a Pt/(Co/Ni)4/Co/IrMn stack structure, differential Kerr imaging shows that the magnetization switching process is via the nucleation of the embryo state followed by domain wall propagation. By probing the current induced DW motion in the presence of in-plane field, the SOT effective fields are obtained using the harmonic Hall voltage scheme. The effective anisotropy field of the structure decreases by 12% due to the SOT effective fields, as the in-plane current in the wire is increased.NRF (Natl Research Foundation, S’pore)MOE (Min. of Education, S’pore)Published versio

Programmable spin–orbit-torque logic device with integrated bipolar bias field for chirality control

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.Agency for Science, Technology and Research (A*STAR)National Research Foundation (NRF)The work was supported by the Singapore National Research Foundation, Prime Minister’s Office under a Competitive Research Programme (Non-volatile Magnetic Logic and Memory Integrated Circuit Devices, NRF-CRP9-2011-01) and an Industry-IHL Partnership Program (NRF2015-IIP001-001). The support from a RIE2020 ASTAR AME IAF-ICP Grant (No. I1801E0030) and an ASTAR AME Programmatic Grant (No. A1687b0033) is also acknowledged. W.S.L. is a member of the SG-SPIN Consortium

Temperature-modulated magnetic skyrmion phases and transformations analysis from first-order reversal curve study

We performed a temperature-modulated first-order reversal curve (FORC) study on a Pt/Co/Fe/Ir magnetic stack that exhibited a magnetic phase transition from isolated skyrmions to skyrmion lattice with increasing temperature. Using in situ magneto-optical Kerr imaging, a generalized description of domain transformations associated with the FORC distribution peaks at both their reversal and sweeping field are derived to allow for direct analysis from the FORC diagram. The sweeping field of the peak, which is commonly ignored in analysis, is identified as the process of domain propagation or nucleation towards terminal domain separation. This process is found to be essential in inducing magnetization irreversibility to reveal domain transformations. In addition, a model characterized by the FORC distribution peaks was developed to describe the transition from the isolated skyrmion to skyrmion lattice phase as well as to identify important field ranges for the transformations. This study establishes an intuitive form of analysis for the otherwise abstract data of FORC distribution for the characterization of magnetic skyrmions in the active field of skyrmionics.Agency for Science, Technology and Research (A*STAR)National Research Foundation (NRF)Published versionThis work was supported by the Singapore National Research Foundation, Prime Minister's Office under a Competitive Research Programme (Non-volatile Magnetic Logic and Memory Integrated Circuit Devices, NRF-CRP9-2011- 01), and an Industry-IHL Partnership Program (NRF2015- IIP001-001). The support from a RIE2020 ASTAR AME IAF-ICP Grant (No. I1801E0030) is also acknowledged

Electrical control of skyrmion density via skyrmion-stripe transformation

A comprehensive understanding of numerous electrical current-induced magnetic texture transformations is necessary to ensure the reliability of skyrmionic devices during operation. Here, we present an experimental study of unipolar current-induced skyrmion-stripe transformation in a Pt/Co/Fe/Ir magnetic bilayer. High current density pulses induce a densely packed skyrmion state, as commonly reported in many other studies, and skyrmion nucleation is expected to lessen with diminishing current density. However, at a lower current density where pinning effects become significant, a regime where current-induced skyrmion annihilation and skyrmion-to-stripe transformation is observed. Kerr imaging reveals that, under a low current pulse, the rapidly expanding stripes crowd out and annihilate the skyrmions before quickly decaying and leaving behind a sparse skyrmion population. Our findings establish an additional requirement of a minimum operating current density in the design of skyrmionic devices to avoid unintended skyrmion deletion. On the other hand, this skyrmion annihilation can also be strategically employed as a technique for skyrmion density control using solely current modulation in future skyrmionic devices.Agency for Science, Technology and Research (A*STAR)National Research Foundation (NRF)Published versionThis work is supported by the Singapore National Research Foundation, Prime Minister’s Office under a Competitive Research Programme (Non-volatile Magnetic Logic and Memory Integrated Circuit Devices, NRF-CRP9-2011- 01), and an Industry-IHL Partnership Program (NRF2015- IIP001-001). The support from a RIE2020 ASTAR AME IAF-ICP Grant (No.I1801E0030) is also acknowledged. W.S.L. is a member of the SG-SPIN Consortium

Emergent geometric frustration of artificial magnetic skyrmion crystals

Magnetic skyrmions have been receiving growing attention as potential information storage and magnetic logic devices since an increasing number of materials have been identified that support skyrmion phases. Explorations of artificial frustrated systems have led to new insights into controlling and engineering new emergent frustration phenomena in frustrated and disordered systems. Here, we propose a skyrmion spin ice, giving a unifying framework for the study of geometric frustration of skyrmion crystals (SCs) in a nonfrustrated artificial geometrical lattice as a consequence of the structural confinement of skyrmions in magnetic potential wells. The emergent ice rules from the geometrically frustrated SCs highlight a novel phenomenon in this skyrmion system: emergent geometrical frustration. We demonstrate how SC topology transitions between a nonfrustrated periodic configuration and a frustrated icelike ordering can also be realized reversibly. The proposed artificial frustrated skyrmion systems can be annealed into different ice phases with an applied current-induced spin-transfer torque, including a long-range ordered ice rule obeying ground state, as-relaxed random state, biased state, and monopole state. The spin-torque reconfigurability of the artificial skyrmion ice states, difficult to achieve in other artificial spin ice systems, is compatible with standard spintronic device fabrication technology, which makes the semiconductor industrial integration straightforward.NRF (Natl Research Foundation, S’pore)MOE (Min. of Education, S’pore)Published versio