Reducing the environmental impact of surgery on a global scale: systematic review and co-prioritization with healthcare workers in 132 countries

Abstract Background Healthcare cannot achieve net-zero carbon without addressing operating theatres. The aim of this study was to prioritize feasible interventions to reduce the environmental impact of operating theatres. Methods This study adopted a four-phase Delphi consensus co-prioritization methodology. In phase 1, a systematic review of published interventions and global consultation of perioperative healthcare professionals were used to longlist interventions. In phase 2, iterative thematic analysis consolidated comparable interventions into a shortlist. In phase 3, the shortlist was co-prioritized based on patient and clinician views on acceptability, feasibility, and safety. In phase 4, ranked lists of interventions were presented by their relevance to high-income countries and low–middle-income countries. Results In phase 1, 43 interventions were identified, which had low uptake in practice according to 3042 professionals globally. In phase 2, a shortlist of 15 intervention domains was generated. In phase 3, interventions were deemed acceptable for more than 90 per cent of patients except for reducing general anaesthesia (84 per cent) and re-sterilization of ‘single-use’ consumables (86 per cent). In phase 4, the top three shortlisted interventions for high-income countries were: introducing recycling; reducing use of anaesthetic gases; and appropriate clinical waste processing. In phase 4, the top three shortlisted interventions for low–middle-income countries were: introducing reusable surgical devices; reducing use of consumables; and reducing the use of general anaesthesia. Conclusion This is a step toward environmentally sustainable operating environments with actionable interventions applicable to both high– and low–middle–income countries

Magnetic skyrmion stabilization, nucleation, and dynamics on magnetic multilayer thin films

Magnetic skyrmions are particle-like nanoscale magnetic structures currently surging in interest in the spintronics field. Their small size combined with topological stability and efficient manipulation by various electromagnetic excitations make them promising next-generation high-density information carriers. Beyond data storage and memory applications, the diverse magnetic skyrmion interactions and excitations can be further exploited for conventional logic computing and unconventional computing like neuromorphic, probabilistic, and Brownian computing. Skyrmion research is advancing on many fronts to bring these technological ideas into reality, including stabilisation in materials and conditions, reliable nucleation techniques, and precise control of magnetic skyrmions. In this thesis, the spin texture evolution between the labyrinth, stripe, skyrmion, and ferromagnetic states in magnetic multilayer [Pt/Co/Fe/Ir]2 under the first-order reversal curve (FORC) magnetic field sweeps was investigated. Temperature modulation was performed to tune skyrmion phases and acquire their corresponding FORC signatures. Using magneto-optical Kerr microscopy, an analysis technique based on the sweeping field was developed and applied in the derivation of the skyrmion phase. A technique for skyrmion nucleation and deletion by current density modulation independent of current polarity in magnetic multilayer [Pt/Co/Fe/Ir]2 was demonstrated. A high current density induces the nucleation of skyrmions via spin-orbit torque acting on defects. In contrast, the low current density causes a volatile skyrmion-stripe transformation at pinning sites that annihilates other skyrmions. A voltage-induced magnetic skyrmion motion based on voltage-controlled magnetic anisotropy gradients was investigated towards a more energy-efficient skyrmion propagation technique. The dynamics of synthetic antiferromagnetic skyrmions on a magnetic anisotropy gradient was investigated numerically, and an analytical model was developed to describe their motion accurately.Doctor of Philosoph

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

Strain-induced degradation and recovery of flexible NbOx-based threshold switching device

Abstract We investigate the functionality of NbOx-based selector devices on a flexible substrate. It was observed that the failure mechanism of cyclic tensile strain is from the disruption of atom arrangements, which essentially led to the crack formation of the film. When under cyclic compressive strain, buckling delamination of the film occurs as the compressed films have debonded from their neighboring layers. By implementing an annealing process after the strain-induced degradation, recovery of the device is observed with reduced threshold and hold voltages. The physical mechanism of the device is investigated through Poole–Frenkel mechanism fitting, which provides insights into the switching behavior after mechanical strain and annealing process. The result demonstrates the potential of the NbOx device in flexible electronics applications with a high endurance of up to 105 cycles of cyclic bending strain and the recovery of the device after degradation

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

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

A technique for spin accumulation quantification, specifically in the ferromagnetic layer of spin-orbit torque inducing heterostructures has been elusive. Here, we demonstrate an easy-to-implement technique to achieve this quantification by applying an additional DC bias during the harmonic Hall measurement in Si/SiO2/Ta/Co/Pt. The spin accumulation arising from the DC bias generates an amplitude offset detectable in the first harmonic Hall magnetoresistance. By performing the first harmonic Hall magnetoresistance measurement under a fixed DC bias for two oppositely magnetized states, spin accumulation polarity set by the DC bias enhances the magnetoresistance if it aligns with the magnetization, and vice versa. Thus, the difference in the magnetoresistance amplitude provides the quantitative magnitude of spin accumulation relative to the ferromagnet's saturation magnetization, measured to be up to 0.29% in Si/SiO2/Ta/Co/Pt. The strength of both spin accumulation and dampinglike efficiency increased with Ta thickness, further verifying our experimental technique.Agency for Science, Technology and Research (A*STAR)Economic Development Board (EDB)Submitted/Accepted versionThis work was supported by RIE2020 ASTAR AME IAFICP Grant No. I1801E0030 and EDB-IPP (Grant No. RCA2019-1376)

Enhancement of spin-orbit torque in Pt/Co/HfOx heterostructures with voltage-controlled oxygen ion migration

Spin-orbit torque (SOT) induced magnetization switching and SOT modulation by interfacial coupling exhibit good potential in spintronic devices. In this work, we report the enhancement of damping-like field and SOT efficiency of up to 60% and 23%, respectively, in perpendicularly magnetized Pt/Co/HfOx heterostructures over a Pt/Co system at an optimal thickness of 2 nm HfOx. The SOT improvement is primarily attributed to the interfacial oxidization of the Co layer, and the strength is tunable via voltage-induced oxygen ion migration at the Co/HfOx interface. Our measurement reveals that by controlling gate voltages, the Co oxidation can be increased, which leads to the SOT efficiency enhancement. Our work promotes the SOT enhancement and modulation by oxidation effects for energy-efficient spintronic devices.Agency for Science, Technology and Research (A*STAR)Economic Development Board (EDB)Published versionThis work was supported by RIE2020 ASTAR AME IAF-ICP (Grant No. I1801E0030) and EDB-IPP (Grant No. RCA-2019- 1376)

Tilted magnetisation for domain wall pinning in racetrack memory

The interest in spintronics devices based on domain wall (DW) motion has gained attention for many years. However, the stochastic behaviour of DW motion is still a fundamental issue for the practical implementation of DW devices. In this study, we demonstrate that effective domain wall pinning can be achieved by using exchange interaction between Co/Ni multilayer with perpendicular magnetic anisotropy (PMA) and Co layer with in-plane magnetic anisotropy (IMA) to create locally tilted magnetisation. The strength of exchange interaction is tuned by varying the thickness of spacer layer Pt between the PMA and IMA layers, thus forming different tilt angles. Micromagnetic simulations were performed to verify the relation between pinning field and magnetisation tilt angle. Polar Kerr microscopy shows the current-driven DW pinning and depinning in the Co/Ni multilayer device with Co crossbars, where the thickness of spacer layer Pt is 1 nm. The proposed approach can potentially be used in future DW memory device applications.NRF (Natl Research Foundation, S’pore)ASTAR (Agency for Sci., Tech. and Research, S’pore)MOE (Min. of Education, S’pore)Accepted versio