Ferromagnet-superconductor Heterostructure Research Articles

Manipulation of magnetic skyrmions by superconducting vortices in ferromagnet-superconductor heterostructures

Dynamics of magnetic skyrmions in hybrid ferromagnetic films harbors novel physical phenomena and holds promise for technological applications. In this work, we discuss the behavior of magnetic skyrmions when coupled to superconducting vortices in a ferromagnet-superconductor heterostructure. We use numerical simulations and analytic arguments to reveal broader possibilities for manipulating the skyrmion-vortex dynamic correlations in the hybrid system, that are not possible in its separated constituents. We explore the thresholds of particular dynamic phases, and quantify the phase diagram as a function of the relevant material parameters, applied current and induced magnetic torques. Finally, we demonstrate the broad and precise tunability of the skyrmion Hall-angle in presence of vortices, with respect to currents applied to either or both the superconductor and the ferromagnet within the heterostructure.

A Peculiar Solitary Re-entrant Superconductivity Induced by an External Magnetic Field in Ferromagnet-Superconductor Heterostructures

We solve the boundary value problem for the Usadel-like equations in the dirty limit case for thin-film three-layered ferromagnet (F) - superconductor (S) structures. Our theoretical approach taking into account of the asymmetry and triplet superconducting correlations is valid in the external magnetic field presence as in the real experimental setup for the FS spin valve explorations. Based on the recent experimental data obtained for the symmetrical (FSF) trilayer in an external magnetic field, we expand the theoretical description to the asymmetrical FSF and FFS trilayers. We also consider the peculiar re-entrant superconductivity and solitary superconductivity induced by the external magnetic field for the FFS trilayer.

Unanticipated Proximity Behavior in Ferromagnet-Superconductor Heterostructures with Controlled Magnetic Noncollinearity

Magnetization noncollinearity in ferromagnet-superconductor (F/S) heterostructures is expected to enhance the superconducting transition temperature (T(c)) according to the domain-wall superconductivity theory, or to suppress T(c) when spin-triplet Cooper pairs are explicitly considered. We study the proximity effect in F/S structures where the F layer is a Sm-Co/Py exchange-spring bilayer and the S layer is Nb. The exchange-spring contains a single, controllable and quantifiable domain wall in the Py layer. We observe an enhancement of superconductivity that is nonmonotonic as the Py domain wall is increasingly twisted via rotating a magnetic field, different from theoretical predictions. We have excluded magnetic fields and vortex motion as the source of the nonmonotonic behavior. This unanticipated proximity behavior suggests that new physics is yet to be captured in the theoretical treatments of F/S systems containing noncollinear magnetization.

Nonlocal thermoelectric effects and nonlocal Onsager relations in a three-terminal proximity-coupled superconductor-ferromagnet device.

We study thermal and charge transport in a three-terminal setup consisting of one superconducting and two ferromagnetic contacts. We predict that the simultaneous presence of spin filtering and of spin-dependent scattering phase shifts at each of the two interfaces will lead to very large nonlocal thermoelectric effects both in clean and in disordered systems. The symmetries of thermal and electric transport coefficients are related to fundamental thermodynamic principles by the Onsager reciprocity. Our results show that a nonlocal version of the Onsager relations for thermoelectric currents holds in a three-terminal quantum coherent ferromagnet-superconductor heterostructure including a spin-dependent crossed Andreev reflection and coherent electron transfer processes.

Nonlocal conductance via overlapping Andreev bound states in ferromagnet-superconductor heterostructures

In a setup where two ferromagnetic electrodes are attached to a superconductor, Andreev bound states are induced at both ferromagnet/superconductor (FM/SC) interfaces. We study how these states propagate through the SC and interact with each other. As a result of this interaction, the energetic positions of the Andreev states are not anymore determined solely by the magnetic properties of a single interface, but also depend on the interface distance and the relative magnetization orientation of the FM contacts. We discuss how these bound states show up as distinct peaks in the nonlocal conductance signal, and lead to marked asymmetries with respect to the applied voltage. We relate our results to nonlocal crossed Andreev and elastic co-tunneling processes.

Spin transfer torque in the presence of Andreev reflections

A first-principles method is developed to study the spin transfer torques in magnetic noncollinear textured ferromagnet-superconductor heterostructure. We apply the method to study the long-range spin-triplet pairing $({S}_{z}=\ifmmode\pm\else\textpm\fi{}1)$ induced by the spin-flip scattering near the ferromagnet-superconductor interface. The long-range spin-triplet Andreev reflection will saturate to the same value as usual Andreev reflection with strong enough spin-flip scattering. We find that the giant magnetoresistance effect of a spin valve with a superconductor contact can be restored by a small amount of interfacial spin-flip scattering.

Spin-polarized quasiparticles injection effects in the normal state of YBCO thin films

We report detailed transport studies on ferromagnet-superconductor heterostructures. Epitaxial heterostructures of half-metal colossal magnetoresistive La 2/3Ca 1/3MnO 3 (HM-CMR) and high- T c superconducting YBa 2Cu 3O 7− δ (HTSC) are grown on SrTiO 3 (1 0 0) single crystal substrates using pulsed laser deposition. Using the HM-CMR layer as source for spin-polarized quasiparticles, we show the effect of injection of spin-polarized quasiparticles into the ab-plane and along the c-axis in the normal state of YBCO. The results show a drop in the ab-plane resistance R ab ( T) in the case of injection along the c-axis that is discussed to be related to the opening of a spin-gap in the superconductor.

Origin of spontaneous currents in a superconductor–ferromagnetic proximity system

We have previously shown that a ferromagnet–superconductor heterostructure may possess a spontaneous current circulation parallel to the interface. This current is caused by Andreev bound states in the thin ferromagnetic layer, and can be fully spin-polarized. Here we investigate the total energy of the system in cases where the current either does or does not flow. We show that the current is a true quantum ground state effect, and examine the effect of the current on the different contributions to the total energy.

Spin-polarized quasiparticle injection effects in YBCO thin films

We report detailed transport studies on ferromagnet-superconductor heterostructures. Epitaxial heterostructures of half-metal colossal magnetoresistive La 2/3Ca 1/3MnO 3 (HM-CMR) and high T c superconducting YBa 2Cu 3O 7 (YBCO) are grown on SrTiO 3 (100) single crystal substrates by pulsed laser deposition. Using the HM–CMR layer as source for spin-polarized quasiparticles, we show the effect of injection of spin-polarized quasiparticles into the ab-plane and along the c-axis of YBCO. The results show a drop in the ab-plane resistance R ab ( T) in the case of injection along the c-axis that is discussed to be related to the opening of a pseudogap.

Current-carrying Andreev bound states in a superconductor-ferromagnet proximity system

We study the ground-state properties of a ferromagnet-superconductor heterostructure on the basis of a quasiclassical theory. We have solved the Eilenberger equations together with Maxwell's equation fully self-consistently and found that due to the proximity effect a Fulde-Ferrel-Larkin-Ovchinnikov (FFLO)-like state is realized in such a system. Moreover, this state has oscillations of the pairing amplitude in either one or two directions, depending on the exchange splitting and thickness of the ferromagnet. In particular, using semiclassical arguments (Bohr-Sommerfeld quantization rule) we show that owing to the presence of the Andreev bound states in the ferromagnet, a spontaneous current in the ground state is generated as a hallmark of the FFLO state in the direction parallel to the interface. We also discuss the effects of the elastic disorder and finite transparency of the interface on the properties of the FFLO state in the system.

Andreev bound states in ferromagnet-superconductor nanostructures

We discuss the properties of a ferromagnet-superconductor heterostructure on the basis of a Hubbard model featuring exchange splitting in the ferromagnet and electron–electron attraction in the superconductor. We have solved the spin-polarized Hartree–Fock–Gorkov equations together with the Maxwell’s equation (Ampere’s law) fully self-consistently. We have found that a proximity effect––Fulde–Ferrell–Larkin–Ovchinnikov state is realized in such a heterostructure. It manifests itself in an oscillatory behavior of the pairing amplitude in the ferromagnet and spontaneously generated spin-polarized current in the ground state. We argue that it is built up from the Andreev bound states, whose energy can be tuned by the exchange splitting and hence can coincide with the Fermi energy giving rise to a current carrying π-state. We also suggest experiments to verify these predictions.