95 research outputs found
Effect of ferromagnetic film thickness on magnetoresistance of thin-film superconductor-ferromagnet hybrids
We study the influence of the thickness Df of the plain ferromagnetic (F)
film on the electrical resistance of the flux-coupled hybrids, consisting of
superconducting (S) Al film and multilayer [Co/Pt] F film with out-of-plain
magnetization. The behavior of such hybrids at high and low temperatures is
found to be different: the nucleation of superconductivity at high temperatures
is governed mainly by the typical lateral dimensions of the magnetic domains,
while low temperature properties are determined by topology of the magnetic
template. We show that an increase in the Df value leads to a broadening of the
field- and temperature intervals where non-monotonous dependence of the
superconducting critical temperature Tc on the applied magnetic field H is
observed (for demagnetized F films). Further increase in the Df value results
in a global suppression of superconductivity. Thus, we determined an optimal
thickness, when the non-monotonous dependence Tc(H) can be observed in rather
broad T and H range, what can be interesting for further studies of the
localized superconductivity in planar Al-based S/F hybrids and for development
of the devices which can exploit the localized superconductivity.Comment: 10 pages, 9 figure
Reverse-domain superconductivity in superconductor-ferromagnet hybrids: effect of a vortex-free channel on the symmetry of I-V characteristics
We demonstrate experimentally that the presence of a single domain wall in an
underlying ferromagnetic BaFe_{12}O_{19} substrate can induce a considerable
asymmetry in the current (I) - voltage (V) characteristics of a superconducting
Al bridge. The observed diode-like effect, i.e. polarity-dependent critical
current, is associated with the formation of a vortex-free channel inside the
superconducting area which increases the total current flowing through the
superconducting bridge without dissipation. The vortex-free region appears only
for a certain sign of the injected current and for a limited range of the
external magnetic field
Crossover between different regimes of inhomogeneous superconductivity in planar superconductor-ferromagnet hybrids
We studied experimentally the effect of a stripe-like domain structure in a
ferromagnetic BaFe_{12}O_{19} substrate on the magnetoresistance of a
superconducting Pb microbridge. The system was designed in such a way that the
bridge is oriented perpendicular to the domain walls. It is demonstrated that
depending on the ratio between the amplitude of the nonuniform magnetic field
B_0, induced by the ferromagnet, and the upper critical field H_{c2} of the
superconducting material, the regions of the reverse-domain superconductivity
in the H-T plane can be isolated or can overlap (H is the external magnetic
field, T is temperature). The latter case corresponds to the condition
B_0/H_{c2}<1 and results in the formation of superconductivity above the
magnetic domains of both polarities. We discovered the regime of edge-assisted
reverse-domain superconductivity, corresponding to localized superconductivity
near the edges of the bridge above the compensated magnetic domains. Direct
verification of the formation of inhomogeneous superconducting states and
external-field-controlled switching between normal state and inhomogeneous
superconductivity were obtained by low-temperature scanning laser microscopy.Comment: 11 pages, 12 figure
Magnetically induced splitting of a giant vortex state in a mesoscopic superconducting disk
The nucleation of superconductivity in a superconducting disk with a Co/Pt
magnetic triangle was studied. We demonstrate that when the applied magnetic
field is parallel to the magnetization of the triangle, the giant vortex state
of vorticity three splits into three individual F0-vortices, due to a
pronounced influence of the C3 symmetry of the magnetic triangle. As a result
of a strong pinning of the three vortices by the triangle, their configuration
remains stable in a broad range of applied magnetic fields. For sufficiently
high fields, F0-vortices merge and the nucleation occurs through the giant
vortex state. The theoretical analysis of this novel reentrant behaviour at the
phase boundary, obtained within the Ginzburg - Landau formalism, is in
excellent agreement with the experimental data.Comment: to be published in Phys. Rev. B - Rapid. Com
Magnetic confinement of the superconducting condensate in superconductor/ferromagnet hybrid composites
The influence of an inhomogeneous magnetic field on the magnetoresistance of
thin Al films, used in different superconductor/ferromagnet hybrids, has been
investigated. Two contrasting magnetic textures with out-of-plane magnetization
are explored, namely (i) a plain film in a multidomain state and (ii) an array
of micro-sized dots. The stray fields of the ferromagnetic structures confine
the superconducting condensate and, accordingly, modify the condition for the
nucleation of superconductivity. By switching between different magnetic states
of the ferromagnet, this confinement can be tuned at will, hereby reversibly
changing the dependence of the critical temperature Tc on an external magnetic
field H. In particular, the continuous evolution from a conventional linear
Tc(H) dependence with a single maximum to a reentrant superconducting phase
boundary with multiple Tc peaks has been demonstrated
Hysteretic ac losses in a superconductor strip between flat magnetic shields
Hysteretic ac losses in a thin, current-carrying superconductor strip located
between two flat magnetic shields of infinite permeability are calculated using
Bean's model of the critical state. For the shields oriented parallel to the
plane of the strip, penetration of the self-induced magnetic field is enhanced,
and the current dependence of the ac loss resembles that in an isolated
superconductor slab, whereas for the shields oriented perpendicular to the
plane of the strip, penetration of the self-induced magnetic field is impaired,
and the current dependence of the ac loss is similar to that in a
superconductor strip flanked by two parallel superconducting shields. Thus,
hysteretic ac losses can strongly augment or, respectively, wane when the
shields approach the strip.Comment: 9 pages, 5 figures, submitted to PR
Vorticity and magnetic shielding in a type-II superconductor
We study in detail, solving the Bogoliubov-de Gennes equations, the magnetic
field, supercurrent and order parameter profiles originated by a solenoid or
magnetic whisker inserted in a type-II superconductor. We consider solutions of
different vorticities, n, in the various cases. The results confirm the
connection between the vorticity, the internal currents and the boundstates in
a self-consistent way. The number of boundstates is given by the vorticity of
the phase of the gap function as in the case with no external solenoid. In the
limiting case of an infinitely thin solenoid, like a Dirac string, the solution
is qualitatively different. The quasiparticle spectrum and wave functions are a
function of n-n_ext, where n_ext is the vorticity of the solenoid. The flux is
in all cases determined by the vorticity of the gap function.Comment: revised version, 25 pages, LaTex, 10 figure
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