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Investigating inhomogeneous FM at SC/FM interfaces using point-contact Andreev spectroscopy

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Title: Investigating inhomogeneous FM at SC/FM interfaces using point-contact Andreev spectroscopy
Authors: Usman, Irham Tri Muharram
Item Type: Thesis or dissertation
Abstract: The unconventional long range (m=1) spin triplet proximity effect is theorised to be created at a Superconductor/Ferromagnet (SC/FM) interface that is magnetically inhomogeneous and provides the conditions for spin mixing. Many experimental and theoretical works have identified the existence of the m=1 spin triplet state using Josephson junctions to show how the critical current does not diminish significantly in a thick FM layer, (thickness being relative to the decay length of a Cooper pair in an FM). In this thesis the challenge was to examine single SC/FM, interfaces via point-contact Andreev Reflection spectroscopy (PCARS), to see if it were possible to find clear signatures of the spin mixing or spin scattering conditions that facilitates formation of the spin triplet state. PCARS measurements were performed on Ho single crystals and thin films as a function of contact resistance. The results show an anomalous decrease of the polarisation parameter and an increase in interface scattering parameter as the contact resistance decreases. This is interpreted in terms of the tip experiencing a region in which some form of spin mixing or spin scattering dominates. Measurements of Ni0.19Pd0.81 spectra were also taken as a function of contact resistance but the change of polarisation and scattering parameter that was observed in the Ho samples was not observed here. PCARS temperature measurements were also taken for Ni0.03Pd0.97 to investigate if spin fluctuations close to TC could provide the magnetic inhomogeneity required for spin-mixing. However the trend in the results proved inconclusive. This is likely due to broadening that result in the fitting procedure, and the extremely small polarisation in these samples. During the course of the thesis I contributed to the development of the spectral fitting routines and examined the challenges associated with three and four parameter fitting, degenerate fits and series resistance.
Issue Date: Feb-2013
Date Awarded: Jul-2013
URI: http://hdl.handle.net/10044/1/12254
DOI: https://doi.org/10.25560/12254
Supervisor: Cohen, Lesley
Department: Physics
Publisher: Imperial College London
Qualification Level: Doctoral
Qualification Name: Doctor of Philosophy (PhD)
Appears in Collections:Physics PhD theses

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