Abstract:
Between two pieces of bread is a world of unlimited possibilities to discover exciting
new flavours. As scientists with access to advanced layering technologies, we procure
our own sandwiches with atomic-level precision, creating thin-film multilayers from two
chemically dissimilar solids. This allows us to combine mutually exclusive phenomenon
such as superconductivity and magnetism so as to study their interplay and grow new
functional materials.
Superconductors carry electrical current with zero resistance (i.e. no energy loss) when
cooled below its critical temperature (Tc). Generally, however, superconductivity is degraded by large magnetic fields or electric currents – a key performance limitation that
creates a bottleneck for upcoming superconductor technologies. Recently, however, the
combination of the high temperature cuprate superconductor YBa2Cu3O7 –x (YBCO)
and the magnetic manganite Pr0.5La0.2Ca0.3MnO3 (PLCMO) or Nd1 –x (Ca1 –ySry )xMnO3,
(x = 0.35, y = 0.2-0.4) found these ’superconducting sandwiches’ to host an exotic magnetic field driven insulating-to-superconducting (IST) transition. The enhancement of
superconductivity by a magnetic field in these sandwiches is contrary to the near all
other superconductors and is unique to the cuprate superconductors. The motivation
at the heart of this work is to gain a better understanding of these thin-film multilayers
and their novel behaviour from a materials chemistry point of view. This thesis explores
the growth, chemistry, and magnetic ordering in superconducting sandwiches comprised
of YBCO and the manganite (NCSMO, x = 0.35, y = 0.3).
The first part focuses on how such thin-film multilayer materials can be grown. It
follows the pulsed laser deposition (PLD) process from the very beginning at the preparation of a bulk NCSMO target through to the final in-situ oxygen annealing step to
set the oxygen stoichiometry. Differing growth modalities of YBCO and NCSMO on
(001)-oriented La0.3Sr0.7Al0.65Ta0.35O3 (LSAT) substrates are highlighted. Interface regions were chemically intermixed with clear signatures of orbital reconstruction effects
observed by polarized X-ray absorption analysis resonant to the interfacial Cu and O
ions.
Whether or not the stoichiometry of the bulk material is transferred to the film is a
key issue that is not always addressed. Here, X-ray photoelectron spectroscopy (XPS)
and near-edge X-ray absorption fine structure (NEXAFS) were used to investigate this
question. Analyses over the Mn L-edge of the confirmed the top NCSMO layer indeed
had a mixed Mn3+/4+ valence state and a composition close to that of the bulk, target
NCSMO material. That is, a hole doping of x = 0.35 and Sr substitution for Ca of y = 0.3. Tuning to the Cu L-edge probed the doping states of YBCO which found that the
interface was underdoped (∼0.07 holes/Cu) while the central bulk YBCO layers had a
doping of around 0.13 holes/Cu to support superconductivity.
These studies were then used to address what impact the core and interface chemical
structure has on the IST in these YBCO-NCSMO sandwiches. By careful characterization of properties such as doping states, interface roughness, layer thicknesses and
lattice parameters, we found that the thickness of YBCO layer itself is a key parameter
determining whether or not the IST is observed in magneto-resistance measurements. In
turn, this shows that the electronic properties of these sandwiches are ‘emergent’ in the
sense that the electronic response of the sandwiches are distinct to that of the materials
in their constituent layers.
The final body of work in this thesis addresses the role of magnetic structure in the IST.
Magnetization and neutron scattering studies of NCSMO in its bulk polycrystalline
form, films, and as sandwiches were carried out. The bulk showed an evolution of a
pseudo-charge exchange (CE)-type antiferromagnetic (AFM) structure below the Ne´el
temperature of ∼120 K. Signatures of AFM order in the single layer film manifested as
a half-order AFM peak in the in-plane [h00] direction. This peak is further modified in
the presence of YBCO where the onset occurs at a far lower temperature of 80 K and
persists in high fields up to 9 T. Depth-resolved layer magnetization modeled nanometer
scale interfaces to host magnetic order in the superconductor sandwiches. These models
resolve the magnitude and orientation of the NCMSO net magnetic moment and was
affected by the layer thickness and proximity to YBCO.
In summary, this thesis provides clear, novel evidence of emergent properties in YBCONCSMO superconducting sandwiches. These include electronic, magnetic and orbital
properties that were shown to result from NCSMO-YBCO coupling. The coupling mechanisms that describe and predict these properties, unfortunately, are unclear in many
cases and it remains a significant theoretical challenge to develop an understanding of
those. The results in this thesis will help to inform and constrain the development of
a theory of the IST in superconducting sandwiches, and more generally of coupling between layers of materials in sandwiches that have strongly correlated electrons and host
exotic ground states such as superconductivity, magnetism and charge order.