Magnetization dependent effects in superconducting nanowires influenced by the ferromagnetic effect

This thesis presents the experimental study of superconducting nanowires under the in
uence
of ferromagnetic nano-structures. Placing superconducting and ferromagnetic materials
in contact with one another causes their long range orders to compete. This manifests
as the leakage of superconducting properties into the ferromagnet and the suppression of
superconductivity in the superconductor near the interface, known as the proximity and
inverse proximity eects, respectively. The experiments presented in this thesis aim to
show that the inverse proximity eect is sensitive to the magnetization of the ferromagnet,
specically that the suppression is weaker if the ferromagnet has an inhomogeneous magnetization.
To do this, the magnetic vortex state in sub-micron nickel disks and L-shape
domain wall traps were used as the inhomogeneous magnetizations. The magnetization in
the nickel disks and L-shape domain wall traps were investigated using magnetic force microscopy
(MFM), in situ MFM, magnetotransport, and modelling. Aluminium nanowires
were deposited over the ferromagnets and low temperature transport measurements of the
hybrid structures were performed. It is found that the superconductivity in the nanowire
above the disks is suppressed, creating an SNS junction. The critical current is shown to
be sensitive to the magnetic history of the disks. The critical current of the entire nanowire
is found to be dependent on the properties of the hybrid junction. This long-range in
uence
has a thermal origin due to Joule heating in the hybrid junction as demonstrated by
use of heat sink structures and Andreev loop interferometers. Replacing the disk with Lshape
domain wall traps shows that the suppression of superconductivity is weakest when
a domain wall is placed beneath the nanowire. Lastly, comparison to theory indicates
the junction length is proportional to temperature. The results presented demonstrate
previously unknown complexity in the behaviour of so-called proximity junctions and a
step toward magnetically controlled superconducting circuitry.