ADSORPTION, DESORPTION AND PHOTOCHEMISTRY OF SMALL MOLECULES ON CLEAN AND CHEMICALLY MODIFIED RU(001) SURFACES

Ph.DDOCTOR OF PHILOSOPH

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

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 NbO2to 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 × 108cycles indicates that the Pt/NbO2/Pt device is particularly suitable for applications in an oscillatory neural network.Agency for Science, Technology and Research (A*STAR)This work was supported by RIE2020 ASTAR AME IAFICP Grant No. I1801E0030 and EDB-IPP (Grant No. RCA2019–1353)

Reversible strain-induced spin-orbit torque on flexible substrate

We propose the use of mechanical strain and mild annealing to achieve reversible modulation of spin-orbit torque (SOT) and Gilbert damping parameter. X-ray diffraction results show that the residual spin-orbit torque enhancement and Gilbert damping reduction, due to the post-mechanical strain treatment, can be reset using mild annealing to alleviate the internal strain. The spin Hall efficiency of the heat- and strain-treated Pt/Co bilayer was characterized through spin-torque ferromagnetic resonance, and it was found that the device could switch between the strain enhanced SOT and the pristine state. The Gilbert damping parameter behaves inversely with the spin Hall efficiency, and therefore, strain can be used to easily tune the device switching current density by a factor of ∼2 from its pristine state. Furthermore, the resonance frequency of the Pt/Co bilayer could be tuned using purely mechanical strain, and from the endurance test, the Pt/Co device can be reversibly manipulated over 104 cycles demonstrating its robustness as a flexible device.Agency for Science, Technology and Research (A*STAR)Economic Development Board (EDB)Published versionThis work was supported by an Industry-IHL Partnership Program (No. NRF2015-IIP001-001) and an EDB-IPP (Grant No. RCA-17/284). This work was also supported by the RIE2020 ASTAR AME IAF-ICP Grant No. I1801E0030

Strain-Mediated SpinOrbit Torque Enhancement in Pt/Co on Flexible Substrate

Current-induced magnetization switching by spin-orbit torque generated in heavy metals offers an enticing realm for energy-efficient memory and logic devices. The spin Hall efficiency is a key parameter in describing the generation of spin current. Recent findings have reported enhancement of spin Hall efficiency by mechanical strain, but its origin remains elusive. Here, we demonstrate a 45% increase in spin Hall efficiency in the platinum/cobalt (Pt/Co) bilayer, of which 78% of the enhancement was preserved even after the strain was removed. Spin transparency and X-ray magnetic circular dichroism revealed that the enhancement was attributed to a bulk effect in the Pt layer. This was further confirmed by the linear relationship between the spin Hall efficiency and resistivity, which indicates an increase in skew-scattering. These findings shed light on the origin of enhancement and are promising in shaping future utilization of mechanical strain for energy-efficient devices.Agency for Science, Technology and Research (A*STAR)Economic Development Board (EDB)National Research Foundation (NRF)Submitted/Accepted versionThis work is supported by an Industry-IHL Partnership Program (NRF2015-IIP001-001) and an EDB-IPP (RCA − 17/284) grant. This work is also supported by the RIE2020 ASTAR AME IAF-ICP grant (No. I1801E0030). W.Z. and P.K.J.W. acknowledge financial support by the Fundamental Research Funds for the Central Universities