100 research outputs found
Enhancement of Superconductivity by Exchange Bias
In this work we study the transport properties of hybrids that consist of
exchange biased ferromagnets (FMs) combined with a low-Tc superconductor (SC).
Not only different FMs but also various structural topologies have been
investigated: results for multilayers of La(1-x)CaxMnO3 combined with Nb in the
form of [La0.33Ca0.67MnO3/La0.60Ca0.40MnO3]15/Nb, and for more simple
Ni80Fe20/Nb/Ni80Fe20 trilayers and Ni80Fe20/Nb bilayers are presented. The
results obtained in all hybrid structures studied in this work clearly uncover
that the exchange bias mechanism promotes superconductivity. Our findings
assist the understanding of the contradictory results that have been reported
in the recent literature regarding the transport properties of relative
FM/SC/FM spin valves
Effective ferromagnetic coupling between a superconductor and a ferromagnet in LaCaMnO/Nb hybrids
In this work we present magnetization data on hybrids consisting of
multilayers (MLs) of man- ganites [La0.33Ca0.67MnO3/La0.60Ca0.40MnO3]15 in
contact with a low-Tc Nb superconductor (SC). Although a pure SC should behave
diamagnetically in respect to the external magnetic field in our ML-SC hybrids
we observed that the magnetization of the SC follows that of the ML. Our
intriguing experimental results show that the SC below its TSC c becomes
ferromagnetically coupled to the ML. As a result in the regime where
diamagnetic behaviour of the SC was expected its bulk magne- tization switches
only whenever the coercive field of the ML is exceeded. By employing specific
experiments where the ML was selectively biased or not we demonstrate that the
ML inflicts its magnetic properties on the whole hybrid. Possible explanations
are discussed in connection to recent theoretical proposals and experimental
findings that were obtained in relative hybrids.Comment: To appear in Phys. Rev.
Manipulating superconductivity through the domain structure of a ferromagnet: experimental aspects and theoretical implications
In the present work we study experimentally the influence that the domain
structure of a fer- romagnet (FM) has on the properties of a superconductor
(SC) in bilayers and multilayers of La0.60Ca0.40MnO3/Nb and FePt/Nb proximity
hybrids. Specific experimental protocols that were employed in the performed
magnetization measurements enabled us to directly uncover a generic property of
FM/SC hybrids: in the absence of an external magnetic field, the multidomain
struc- ture of the FM promotes the nucleation of superconductivity, while its
monodomain state strongly suppresses it. Our experimental findings support
recent theoretical studies proposing that when an inhomogeneous exchange field
is offered by the FM to the SC the superconducting pairs are not susceptible to
pair-breaking.Comment: 4 pages, 4 figure
Ferromagnetic-superconducting hybrid films and their possible applications: A direct study in a model combinatorial film
Model combinatorial films (CFs) which host a pure superconductor adjacent to
a ferromagneticsuperconducting hybrid film (HF) are manufactured for the study
of the influence of ferromagnetic nanoparticles (FNs) on the nucleation of
superconductivity. Careful resistance measurements were performed
simultaneously on two different sites of the CFs. Enhancement of
superconductivity and magnetic memory effects were observed only on the hybrid
site of the CFs but were absent on their purely superconducting part. Our
results give direct proof that the FNs modulate the superconducting order
parameter in an efficient and controlled way giving us the possibility of
miscellaneous practical applications.Comment: To appear in Physical Review
Relaxation measurements in the regime of the second magnetization peak in Nb films
We report on magnetic measurements as a function of field, temperature and
time (relaxation) in superconducting Nb films of critical temperature Tc = 9.25
K. The magnetic measurements as a function of field exhibited a second
magnetization peak (SMP) which in general is accompanied by thermomagnetic
instabilities (TMIs). The lines where the SMP occurs and where the first flux
jump in the virgin magnetization curves is observed, end at a characteristic
point (To,Ho)=(7.2 K,80 Oe). Relaxation measurements showed that for T<To=7.2 K
the activation energy Uo and the normalized relaxation rate S exhibit
non-monotonic behavior as a function either of temperature or field. The
extrema observed in Uo and S coincide with the onset and the maximum points of
the SMP. In the regime T>To=7.2 K both Uo and S present a conventional
monotonic behavior. These results indicate that the SMP behavior observed in
our Nb films is promoted by the anomalous relaxation of the magnetization.Comment: To appear in Physica
Stray-fields-based magnetoresistance mechanism in Ni80Fe20-Nb-Ni80Fe20 trilayers
We report on the transport and magnetic properties of hybrid trilayers and
bilayers that consist of low spin-polarized Ni80Fe20 exhibiting in-plane but no
uniaxial anisotropy and low-Tc Nb. We reveal a magnetoresistance effect that is
pronounced. In our trilayers the magnetoresistance exhibits an increase of two
orders of magnitude when the superconducting state is reached: from the
conventional normal-state values 0.6 % it goes up to 1000 % for temperatures
below Tc. In contrast, in the bilayers the effect is only minor since from 3%
in the normal state increases only to 70 % for temperatures below Tc.
Magnetization data of both the longitudinal and transverse magnetic components
are presented. Most importantly, we present data not only for the normal state
of Nb but also in its superconducting state. Strikingly, these data show that
below its Tc SC the Nb interlayer under the influence of the outer Ni80Fe20
layers attains a magnetization component transverse to the external field. By
comparing the transport and magnetization data we propose a candidate mechanism
that could motivate the pronounced magnetoresistance effect observed in the
trilayers. Adequate magnetostatic coupling of the outer Ni80Fe20 layers is
motivated by stray fields that emerge naturally in their whole surface due to
the multidomain magnetic structure that they attain near coercivity. Atomic
force microscopy is employed in order to examine the possibility that such
magnetostatic coupling could be promoted by interface roughness. Referring to
the bilayers, although out-of-plane rotation of the magnetization of the single
Ni80Fe20 layer is still observed, in these structures magnetostatic coupling
does not occur due to the absence of a second Ni80Fe20 one so that the observed
magnetoresistance peaks are only modest.Comment: 9 pages, 7 figure
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