554 research outputs found
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
Manifestation of the odd-frequency spin-triplet pairing state in diffusive ferromagnet / superconductor junctions
Using the quasiclassical Green's function formalism, we study the influence
of the odd-frequency spin-triplet superconductivity on the local density of
states (LDOS) in a diffusive ferromagnet (DF) attached to a superconductor.
Various possible symmetry classes in a superconductor are considered which are
consistent with the Pauli's principle: even-frequency spin-singlet even-parity
(ESE) state, even-frequency spin-triplet odd-parity (ETO) state, odd-frequency
spin-triplet even-parity (OTE) state and odd-frequency spin-singlet odd-parity
(OSO) state. For each of these states, the pairing state in DF is studied.
Particular attention is paid to the study of spin-singlet s-wave and
spin-triplet p-wave superconductors as the examples of ESE and ETO
superconductors. For spin-singlet case the magnitude of the OTE component of
the pair amplitude is enhanced with the increase of the exchange field in DF.
When the OTE component is dominant at low energy, the resulting LDOS in DF has
a zero energy peak (ZEP). On the other hand, in DF / spin-triplet p-wave
superconductor junctions LDOS has a ZEP in the absence of the exchange field,
where only the OTE paring state exists. With the increase of the exchange
field, the ESE component of the pair amplitude induced in DF is enhanced. Then,
the resulting LDOS has a ZEP splitting. We demonstrate that the appearance of
the dominant OTE component of the pair amplitude is the physical reason of the
emergence of the ZEP of LDOS.Comment: 9 pages, 9 figure
Two-dimensional Ising model with competing interactions and its application to clusters and arrays of -rings and adiabatic quantum computing
We study planar clusters consisting of loops including a Josephson
-junction (-rings). Each -ring carries a persistent current and
behaves as a classical orbital moment. The type of particular state associated
with the orientation of orbital moments at the cluster depends on the
interaction between these orbital moments and can be easily controlled, i.e. by
a bias current or by other means. We show that these systems can be described
by the two-dimensional Ising model with competing nearest-neighbor and diagonal
interactions and investigate the phase diagram of this model. The
characteristic features of the model are analyzed based on the exact solutions
for small clusters such as a 5-site square plaquette as well as on a mean-field
type approach for the infinite square lattice of Ising spins. The results are
compared with spin patterns obtained by Monte Carlo simulations for the 100
100 square lattice and with experiment. We show that the -ring
clusters may be used as a new type of superconducting memory elements. The
obtained results may be verified in experiments and are applicable to adiabatic
quantum computing where the states are switched adiabatically with the slow
change of coupling constants.Comment: 32 pages, 22 figures, RevTe
Inhomogeneous superconductivity induced in a weak ferromagnet
Under certain conditions, the order parameter induced by a superconductor (S)
in a ferromagnet (F) can be inhomogeneous and oscillating, which results e.g.
in the so-called pi-coupling in S/F/S junctions. In principle, the
inhomogeneous state can be induced at T_c as function of the F-layer thickness
d_F in S/F bilayers and multilayers, which should result in a dip-like
characteristic of T_c(d_F). We show the results of measurements on the S/F
system Nb/Cu_{1-x}Ni_x, for Ni-concentrations in the range x = 0.5-0.7, where
such effects might be expected. We find that the critical thickness for the
occurrence of superconductivity is still relatively high, even for these weak
ferromagnets. The resulting dip then is intrinsically shallow and difficult to
observe, which explains the lack of a clear signature in the T_c(d_F) data.Comment: 4 pages, 4 figures. To be publishedin Physica C (proceedings of the
Second Euroconference on Vortex Matter in Superconductors, Crete, 2001
Depopulation of dense α-synuclein aggregates is associated with rescue of dopamine neuron dysfunction and death in a new Parkinson's disease model.
Parkinson's disease (PD) is characterized by the presence of α-synuclein aggregates known as Lewy bodies and Lewy neurites, whose formation is linked to disease development. The causal relation between α-synuclein aggregates and PD is not well understood. We generated a new transgenic mouse line (MI2) expressing human, aggregation-prone truncated 1-120 α-synuclein under the control of the tyrosine hydroxylase promoter. MI2 mice exhibit progressive aggregation of α-synuclein in dopaminergic neurons of the substantia nigra pars compacta and their striatal terminals. This is associated with a progressive reduction of striatal dopamine release, reduced striatal innervation and significant nigral dopaminergic nerve cell death starting from 6 and 12 months of age, respectively. In the MI2 mice, alterations in gait impairment can be detected by the DigiGait test from 9 months of age, while gross motor deficit was detected by rotarod test at 20 months of age when 50% of dopaminergic neurons in the substantia nigra pars compacta are lost. These changes were associated with an increase in the number and density of 20-500 nm α-synuclein species as shown by dSTORM. Treatment with the oligomer modulator anle138b, from 9 to 12 months of age, restored striatal dopamine release, prevented dopaminergic cell death and gait impairment. These effects were associated with a reduction of the inner density of large α-synuclein aggregates and an increase in dispersed small α-synuclein species as revealed by dSTORM. The MI2 mouse model recapitulates the progressive dopaminergic deficit observed in PD, showing that early synaptic dysfunction is associated to fine behavioral motor alterations, precedes dopaminergic axonal loss and neuronal death that become associated with a more consistent motor deficit upon reaching a certain threshold. Our data also provide new mechanistic insight for the effect of anle138b's function in vivo supporting that targeting α-synuclein aggregation is a promising therapeutic approach for PD
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