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Title: Spin transport in few-layer graphene
Author: Yan, Wenjing
ISNI:       0000 0004 6057 9069
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2015
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Graphene spintronics refers to spin injection, spin transport and spin detection, using magnetic electrodes and single/multilayer graphene. Although it has been very actively researched since 2006, there is still disagreement regarding the spin diffusion length (lsf). This is partially because insufficient attention has been paid to the micromagnetics of the electrodes, particularly regarding the interpretation of local magnetoresistance (MR), where the spin-valve signal can be confused with tunnelling anisotropic magnetoresistance (TAMR) at the contacts. The magnetisation-reversal in unpatterned films of La0.67Sr0.33MnO3 (LSMO) on substrates of NdGaO3 (001), NdGaO3 (100) and SrTiO3 (110) were studied. And the study found nucleation-propagation along the easy axes at room temperature and 150 K. For LSMO on NdGaO3 (001) at 150 K, there was strong pinning to domain propagation. For LSMO on NdGaO3 (100) or SrTiO3 (110), there was sharp switching, with the switching field roughly equal to the nucleation field. The 150 K magnetic domain structures of patterned LSMO electrodes on NdGaO3 (001), NdGaO3 (100) and SrTiO3 (110) were imaged at remanence using photoemission electron microscopy with X-ray magnetic circular dichroism contrast (XMCD-PEEM). For the electrodes on NdGaO3 (001), intermediate magnetic fields yielded domains with opposing magnetisations that formed along the length of each electrode, and domain propagation was likely pinned due to defects introduced by patterning along the electrode edges. For the electrodes on NdGaO3 (100) or SrTiO3 (110), there were sharp switching at different fields. A magneto-optic Kerr effect microscopy (MOKE) study of the electrodes on NdGaO3 (100) and SrTiO3 (110), performed at room temperature and low temperature, showed that the electrodes have easy axes across their lengths, and sharp, well-defined switching, as desired for investigating spin transport through graphene, rather than TAMR. Electrical transport measurements were carried out on 15 devices, out of which only one device showed a low-field MR of 5.5%. This result was analysed using the drift diffusion model, which implied a 70 µm spin diffusion length and provided evidence for graphene to be used a channel for long distance spin transport.
Supervisor: Mathur, Neil Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
Keywords: Spintronics ; Graphene