PhD (Physics) / Department of Physics / Understanding how and why superconductivity (SC) occurs in a given material has been
very challenging for physicists for more than a hundred years, notwithstanding the major
milestones, such as the London theory, the Landau-Ginzburg theory, and the BCS theory.
The extreme challenge to predict the occurrence of SC is symbolized by the long string
of unanticipated but breathtaking advances, i.e., the unexpected discoveries of cuprates
and Fe-pnictides being the dramatic modern examples. Because of their incompatibility,
the nucleation of SC near a ferromagnet is di cult and has never been realized except for
the case that another superconductor provides proximity-boosted Cooper pairs.
This perceived necessity to start with another superconductor is engrained in the exten-
sive study of the proximity e ect in superconductor/ferromagnet (S/F) powder sample,
where all the structures involve a superconductor with either stable or metastable struc-
ture.
Compounding the di culty, it is also generally recognized that SC with substantial
Tc is favourable in low dimensionality because of strong quantum
uctuation. In this
thesis, we report a serendipitous nding of SC that emerges under the most implausible
circumstances in low eld microwave absorption measurement. This new revelation may
lead to unconventional avenues to explore novel SC for applications in superconducting
spintronics.
By means of a varienty of techniques, including EPR, SEM, FTIR, PPMS/VSM and
XRD, nanonickel incorporated YBCO in di erent weighting factors have been studied.
With its complex chemical structure and magnetic properties, Ni-YBCO is far from well
understood and the magentic behavior of the system under di erent conditions is investi-
gated. From the dilute mixture of nanonickel particles, it is found that groups of normal
Josephson junctions (JJs) and JJs due to YBCO-nickel-YBCO interparticle weaklinks
form as nickel is ferromagnetic. We experimentally show, for the rst time multiple phase
reversals in the non-resonant microwave absorption (NRMA) spectra from Ni-YBCO pos-
sibly, due to the formation of JJs. We also show that these multiple phase reversals then
vii
depend on microwave power and temperature. We argue that microwave power induced
coherence among some groups of JJs and breaking of some of the weaker JJs can then
lead to the disappearance of multiple phase reversals at higher microwave power levels.
Further, we also report a role of pair breaking e ects that shall give a linear eld de-
pendence of the derivative microwave absorption signal, which is essentially the NRMA
signal. This pair-breaking e ect dominates at temperatures closer to Tc as expected
thermodynamically.
The presence of two peaks in the system, results in high permeability ferromagnet
which acts as a magnetic short circuit for magnetic
ux density and creates low reluctance
path. A transition from normal to anomalous does not occur in this work, because of the
possibility of junction in the sample.
As predictable at the region around the origin where the weaklinks are supposed to
be very strong for a very low doping or low nanonickel addition ( 0.5 % wt), not much
e ect was observed. However, when the nanonickel addition is increased to 2 % and 3%
we see a signi cant change in the magnetization and the associated hysteresis, indicating
ux pinning. / NRF
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:univen/oai:univendspace.univen.ac.za:11602/1242 |
| Date | 21 September 2018 |
| Creators | Nemangwele, Fhulufhelo |
| Contributors | Vallabhapurapu, Vijaya Srinvasu, Maluta, Nnditsheni Eric, Kirui, Joseph Kiprono |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | 1 online resource ( xv, 142 leaves, illustration) |
| Rights | University of Venda |
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