Ordered magnetic nanostructures: fabrication and properties

The new class of phenomena described in this review is based on the interaction between spatially separated, but closely located ferromagnets and superconductors, the so-called ferromagnet–superconductor hybrids (FSH). Typical FSH are: coupled uniform and textured ferromagnetic and superconducting films, magnetic dots over a superconducting film, magnetic nanowires in a superconducting matrix, etc. The interaction is provided by the magnetic field generated by magnetic textures and supercurrents. The magnetic flux from magnetic structures or topological defects can pin vortices or create them, changing the transport properties and transition temperature of the superconductor. On the other hand, the magnetic field from supercurrents (vortices) strongly interacts with the magnetic subsystem, leading to formation of coupled magnetic–superconducting topological defects.  Each time the Nambu and spin matrices are written together we mean the direct product. The proximity of ferromagnetic layer dramatically cha...

Ferromagnet–superconductor hybrids

The theoretical and experimental results concerning the thermodynamical and low-frequency transport properties of hybrid structures, consisting of spatially separated conventional low-temperature superconductors (S) and ferromagnets (F), are reviewed. Since the superconducting and ferromagnetic parts are assumed to be electrically insulated, no proximity effect is present and thus the interaction between both subsystems is through their respective magnetic stray fields. Depending on the temperature range and the value of the external field Hext, different behavior of such S/F hybrids is anticipated. Rather close to the superconducting phase transition line, when the superconducting state is only weakly developed, the magnetization of the ferromagnet is solely determined by the magnetic history of the system and it is not influenced by the field generated by the supercurrents. In contrast to that, the nonuniform magnetic field pattern, induced by the ferromagnet, strongly affects the nucleation of superconductivity, leading to an exotic dependence of the critical temperature Tc on Hext. Deeper in the superconducting state the effect of the screening currents cannot be neglected anymore. In this region of the phase diagram T–Hext various aspects of the interaction between vortices and magnetic inhomogeneities are discussed. In the last section we briefly summarize the physics of S/F hybrids when the magnetization of the ferromagnet is no longer fixed but can change under the influence of the superconducting currents. As a consequence, the superconductor and ferromagnet become truly coupled and the equilibrium configuration of this 'soft' S/F hybrid requires rearrangements of both superconducting and ferromagnetic characteristics, as compared with 'hard' S/F structures.

https://iopscience.iop.org/article/10.1088/0953-2048/22/5/053001/pdf

Nucleation of superconductivity and vortex matter in superconductor–ferromagnet hybrids

Superconducting vortex pinning with artificial magnetic nanostructures

On the basis of a phenomenological Ginzburg-Landau approach we investigate the problem of order parameter nucleation in hybrid superconductor-ferromagnet systems with a domain structure in an applied external magnetic field Both isolated domain boundaries and periodic domain structures in ferromagnetic layers are considered We study the interplay between superconductivity localized at the domain walls and far from the walls and show that such an interplay determines a peculiar field dependence of the critical temperature ${T}_{c}$ For a periodic domain structure the behavior of the upper critical field of superconductivity nucleation near ${T}_{c}$ is strongly influenced by the overlapping of the superconducting nuclei localized over different domains

Domain-wall superconductivity in hybrid superconductor-ferromagnet structures

The problem of the giant vortex state around a magnetic dot which is embedded in a superconducting film is investigated. The full nonlinear, self-consistent Ginzburg-Landau equations are solved numerically in order to calculate the free energy, the order parameter of the host superconductor, the internal magnetic field due to the supercurrents, the corresponding current density, the magnetization probed in the vicinity of the dot, and the normal electron density as a function of the various parameters of the system. We find that, as we increase the magnetic moment of the dot, higher flux quanta vortex states become energetically more favorable, as they can better compete with the external magnetic field via the Meissner effect. In addition to that, they progressively become closer to each other in energy with direct experimental consequences, i.e., physical quantities like magnetization may fluctuate when measured, for example, as a function of a uniform external magnetic field. \textcopyright{} 1996 The American Physical Society.

Vortex structure around a magnetic dot in planar superconductors

The electronic structure, binding energy, and correlation functions of three different spatial configurations of ${\mathrm{D}}^{\mathrm{\ensuremath{-}}}$ centers in GaAs-based semiconductor structures are studied using a numerical technique that solves the two-electron Schr\"odinger equation for such systems. We compare our results for the quantum well ${\mathrm{D}}_{\mathrm{w}}^{\mathrm{\ensuremath{-}}}$ with experimental data and variational and diffusion Monte Carlo calculations. We demonstrate the existence of the spatially separated ${\mathrm{D}}_{\mathrm{s}}^{\mathrm{\ensuremath{-}}}$ center as a bound state, even in the regime of very low magnetic fields, and find interesting anticrossings among the energy levels in its electronic structure, which underlie a spectrum composition of a ${\mathrm{D}}^{0}$ donor center and a free electron in a magnetic field. For the remote ${\mathrm{D}}_{\mathrm{r}}^{\mathrm{\ensuremath{-}}}$ center we find that strong electron-electron correlations can give rise to magnetic-field-induced angular-momentum transitions, and as the magnetic field increases, these correlations weaken, while eventually the system magnetically evaporates approaching the classical unbound regime.

Remote and spatially separated D − centers in quasi-two-dimensional semiconductor structures

A donor-related resonance is observed in double-barrier resonant tunneling devices with Si donors incorporated in the quantum well. In high magnetic fields the resonance becomes dominant over the Is resonance associated with the ground state of a single donor. The bias position of the donor resonance, its magnetic field dependence, and large amplitude indicate unambiguously that the resonance is due to tunneling through the ground state of a shallow donor with two bound electrons (D- level).

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D- centers probed by resonant tunneling spectroscopy

https://repository.ubn.ru.nl/bitstream/handle/2066/27708/27708.pdf?sequence=1

I. K. Marmorkos

Papers

Vortex structure around a magnetic dot in planar superconductors

Remote and spatially separated D − centers in quasi-two-dimensional semiconductor structures

D- centers probed by resonant tunneling spectroscopy

Resonant tunnelling through D- states