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Title: Tuning spin currents with ultrathin heavy metal microstructure
Author: Brennan, Spencer John
ISNI:       0000 0004 7968 4600
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2019
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All-metallic spintronic heterostructures have the ability to generate pure spin currents at low resistance-area products via spin-orbit coupling. Heavy metal/ferromagnet (HM/NM) bilayers are often used in this context to extract key spin-dependent parameters such as the spin Hall angle, θ, and spin diffusion length, λ, of the constituent materials. However, in the ultrathin film limit, morphology is highly sensitive to growth parameters, making consistent extraction of the spin-dependent properties challenging. Although much work has been done to study spin transport in nanodevices, a long-standing controversy in literature remains over quantifying the magnitudes of spin-dependent parameters in highly spin-orbit coupled HMs. Despite growing evidence that spin-dependent properties vary with HM resistivity, a concerted effort to understand the effect of HM microstructure morphology on spin transport in HM|FM bilayers has yet to be made. In this thesis, we investigate the role of ultrathin HM microstructure morphology on spin transport properties within HM|FM bilayers (FM = Co, CoFeB; HM = W, Ta, Ru, Pt). By seeding HM growth with thin metallic buffer layers including Ta and Ru, we are able to tune the growth mode and so morphology of the HM layer. X-ray diffractometry and transmission electron microscopy confirm the good control of HM growth modes through film wetting, continuity, texture and roughness. The subsequently altered electronic properties mediate the coupling of HM morphology to spin transport by impacting resistance and voltage readouts in common electrical spin injection/detection schemes. We probe spin reflection, transmission and accumulation at the HM|FM interface in three such measurement schemes with a systematic set of spin Hall magnetoresistance (SHMR), spin pumping and spin-orbit torque (SOT) effective field measurements on (un)buffered Pt|CoFeB bilayers. We experimentally demonstrate a large enhancement of generated spin currents with SHMR and spin pumping measurements in ultrathin buffered devices, which can be directly correlated to HM microstructure. After extending current magnetoelectronic circuit theory to include the seed layer, we find that spin transport in buffered Pt|CoFeB bilayers can only be well understood when considering HM film morphology through Elliot-Yafet-dominated spin relaxation and intrinsic spin scattering in the Pt layer. By modelling both SHMR and spin pumping data simultaneously following this methodology, the different dependence on θ in the schemes allows us to estimate a single set of θ and λ despite drastically different spin signals in buffered Pt|CoFeB layers. We further confirm the domination of intrinsic spin scattering in Pt by measuring near-constant normalised SOT effective fields across the (un)buffered bilayers. The demonstration of the significant effect of HM microstructure morphology on spin current generation in common measurement schemes indicates this work may potentially provide resolution to the widely varying values of θ and λ reported across the literature.
Supervisor: Cowburn, Russell ; O'Brien, Liam Sponsor: SpinCal ; National Science and Engineering Research Council of Canada
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
Keywords: spintronics ; magnetism ; nanomagnetism ; solid state physics ; microstructure ; thin films ; sputtering ; XRD ; XRR ; diffractometry ; magnetometry