|dc.description.abstract||Superconductivity (S) and ferromagnetism (F) are competing phases whose coexistence
is unlikely to occur. Exceptions require a non−uniform profile both for the pairing and the
magnetization, as in the case of the FFLO state predicted independently by Fulde and
Ferrell  and by Larkin and Ovchinnikov . The coexistence of a superconducting and
magnetic phase in a finite temperature range was first discovered in ternary rare earth
compounds [3, 4]. Later on, other examples of coexistence of S and magnetic ordering
were found [5–7] which motivated the investigation of alternative possibilities for the
interplay of ferromagnetic and homogeneous superconducting order [8, 9]. Differently
from the case of bulk systems, the coexistence may be easily achieved in artificial S/F
hybrids, where the two antagonistic orderings are confined in spatially separated layers
interacting via the proximity effect. Recently, these systems have been the subject of
intensive theoretical and experimental studies and new concepts have been developed
[10–14]. The improvement of the fabrication techniques has made possible the realization
of heterostructures consisting of very thin layers of different materials coupled through
high quality contacts. In this way, the reciprocal influence of the two opposite phases can
be tuned by changing the materials, the layer thicknesses, and their configuration and
topology. The analogy with the bulk situation is provided by the proximity effect: when a
superconductor and a ferromagnet are brought into contact, Cooper pairs enter the F side
and magnetic excitations leak into the S region across the S/F interface. As a result,
superconductivity is suppressed in the superconductor within a distance S (the coherence
length) from the interface, while S correlations are induced in the ferromagnet. The
presence of the exchange field Eex in F causes an energy shift between the electrons of the
pairs entering in the ferromagnet and this results in the creation of Cooper pairs with
non−zero momentum. Thus, the S order parameter does not simply decay in the F metal,
as it would happen in a normal one, but it also oscillates in the direction perpendicular to
the interface over a length scale given by F, the coherence length in F. This
inhomogeneity of the order parameter may be interpreted as a manifestation of a FFLO
phase in these structures [14–16].
In particular, in S/F hybrids, the inhomogeneous character of the S order
parameter, caused by the proximity to the F side, leads to a non-monotonic behavior of
all the physical quantities depending on the gap, as for instance for the transition
temperature as a function of the F layer thickness, dF . In addition, re−entrant
superconductivity has recently been experimentally observed as a function of dF in
Nb/CuNi bilayers [18, 19], as well as non-monotonic behavior of the anisotropy
coefficient in S/F/S trilayers , negative critical current and reversed density of states
in Josephson [21, 22] and tunnel  S/F/S −junctions. Some peculiarities of the shape
of the R(T) curves in S/F/S trilayers and multilayers have been analyzed and, in general,
transport properties of these systems have been studied [24–28]. Very recently,
experimental results were obtained on S/F structures when measuring the dynamic
instabilities of the vortex lattice at high driving currents. The role played on the nonequilibrium
properties of the hybrids by both the ferromagnetic and the superconducting
materials has been analyzed with a special focus on the values and the temperature
dependence of the quasiparticle relaxation times, E. Knowledge of the relaxation
mechanisms in these systems is extremely important in view of possible applications
since it can drive the optimal choice of both materials to realize, in particular, ultrafast
superconducting single photon detectors based on S/F hybrid structures .
Another area of special interest in the field of the S/F structures concerns the
investigation of spin triplet superconductivity . In systems where the magnetization is
spatially inhomogeneous, an equal spin pairing S component can be generated, that may
survive over very long distances − of the order of the normal coherence length − inside F.
In conventional spin−singlet S/F hybrids, superconductivity rapidly decays in the
ferromagnet over distances of order of tenth of nanometers. However, the removal of the
translational invariance due to the presence of interfaces leads, in clean systems, to a
different mixed parity pairing, which can be responsible for the generation and the
induction in the F side of a p−type spin−triplet component . Such component can be
induced in fully spin-polarized metals (half−metals) only in the presence of spin−active
interfaces . Recently it has been argued that in dirty systems even s−type spin−triplet
superconductivity can survive in F over much longer distances [12, 32, 33].
Inhomogeneous magnetization in the F side [32, 33] or spin−active interfaces  can be
responsible for the appearance of this triplet component. Some hints of the presence of
odd−frequency spin−triplet correlations have been observed in S/F systems with
half−metallic CrO2 [35, 36], metallic Co−PdNi−CuNi layers  and more exotic Ho
ferromagnet [38, 39].
In this work we will analyze the effect of different ferromagnets on the
superconducting transport properties of S/F hybrids. In particular, in chapter 1 and
chapter 2 weak ferromagnets such as PdNi and CuNi will be used in conjunction with Nb
to study the static and dynamic properties of the vortex lattice in Nb/PdNi and Nb/CuNi
bilayers to obtain information on the quasiparticles relaxation processes in these systems.
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