Superconducting spin valves based on epitaxial Fe/V superlattices (original) (raw)
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Superconducting Spin Valve Effect of a V Layer Coupled to an Antiferromagnetic [Fe/V] Superlattice
Physical Review Letters, 2005
We studied superconducting V layers deposited on an antiferromagnetically coupled Fe 2 V 11 20 superlattice. The parallel upper critical magnetic field exhibits an anomalous T dependence up to the ferromagnetic saturation field of the superlattice, indicating that the superconducting transition temperature T S decreases when rotating the relative sublattice magnetization directions of the superlattice from antiparallel to parallel. This proves that the pair breaking effect of a Fe 2 layer is reduced if at a distance of 1.5 nm a second Fe 2 layer with antiparallel spin orientation exists.
Superconducting spin valve hffect of a v layer coupled to an antiferromagnetic [Fe/V] superlattice
Physical Review Letters
We studied superconducting V layers deposited on an antiferromagnetically coupled Fe 2 V 11 20 superlattice. The parallel upper critical magnetic field exhibits an anomalous T dependence up to the ferromagnetic saturation field of the superlattice, indicating that the superconducting transition temperature T S decreases when rotating the relative sublattice magnetization directions of the superlattice from antiparallel to parallel. This proves that the pair breaking effect of a Fe 2 layer is reduced if at a distance of 1.5 nm a second Fe 2 layer with antiparallel spin orientation exists.
Basic Superconducting Spin Valves
NanoScience and Technology, 2018
© 2018, Springer International Publishing AG, part of Springer Nature. The short review is devoted to the state of the art of a booming field of research in spintronics-superconducting spintronics. The spin valve properties of hybrid structures consisting of alternating layers of superconductor (S) and ferromagnetic (F) of nanoscale thicknesses and superconducting due to the proximity effect are considered in detail. The experimental data for the weak and strong ferromagnetic materials are analyzed; the role of the domain structure of the ferromagnet and the scattering of electrons with spin flip at the SF interfaces in the magnitude of the valve effect is considered. Theoretical works describing the effects of the spin valve for diffusive and clean limits are analyzed. The necessity of consideration of the multiplicity of configurations of the superconducting order parameter in multilayer SF heterostructures is underlined.
Physical Review B, 2013
The theory of superconductor-ferromagnet (S-F) heterostructures with two ferromagnetic layers predicts the generation of a long-range, odd-in-frequency triplet pairing at non-collinear alignment (NCA) of the magnetizations of the F-layers. This triplet pairing has been detected in a Nb/Cu 41 Ni 59 /nc-Nb/Co/CoO x spin-valve type proximity effect heterostructure, in which a very thin Nb film between the F-layers serves as a normal conducting (nc) spacer. The resistance of the sample as a function of an external magnetic field shows that for not too high fields the system is superconducting at a collinear alignment of the Cu 41 Ni 59 and Co layer magnetic moments, but switches to the normal conducting state at a NCA configuration. This indicates that the superconducting transition temperature T c for NCA is lower than the fixed measuring temperature. The existence of a minimum T c , at the NCA regime below that one for parallel or antiparallel alignments of the F-layer magnetic moments, is consistent with the theoretical prediction of a singlet superconductivity suppression by the long-range triplet pairing generation.
Beilstein Journal of Nanotechnology, 2016
Background: In nanoscale layered S/F1/N/F2/AF heterostructures, the generation of a long-range, odd-in-frequency spin-projection one triplet component of superconductivity, arising at non-collinear alignment of the magnetizations of F1 and F2, exhausts the singlet state. This yields the possibility of a global minimum of the superconducting transition temperature T c, i.e., a superconducting triplet spin-valve effect, around mutually perpendicular alignment. Results: The superconducting triplet spin valve is realized with S = Nb a singlet superconductor, F1 = Cu41Ni59 and F2 = Co ferromagnetic metals, AF = CoO x an antiferromagnetic oxide, and N = nc-Nb a normal conducting (nc) non-magnetic metal, which serves to decouple F1 and F2. The non-collinear alignment of the magnetizations is obtained by applying an external magnetic field parallel to the layers of the heterostructure and exploiting the intrinsic perpendicular easy-axis of the magnetization of the Cu41Ni59 thin film in conj...
Half-Metallic Superconducting Triplet Spin Valve
Phys. Rev. B, 2016
We theoretically study a finite size SF1N F2 spin valve, where a normal metal (N) insert separates a thin standard ferromagnet (F1) and a thick half-metallic ferromagnet (F2). For sufficiently thin superconductor (S) widths close to the coherence length ξ0, we find that changes to the relative magnetization orientations in the ferromagnets can result in substantial variations in the transition temperature Tc, consistent with experiment [Singh et al., Phys. Rev. X 5, 021019 (2015)]. Our results demonstrate that, in good agreement with the experiment, the variations are largest in the case where F2 is in a half-metallic phase and thus supports only one spin direction. To pinpoint the origins of this strong spin-valve effect, both the equal-spin f1 and opposite-spin f0 triplet correlations are calculated using a self-consistent microscopic technique. We find that when the magnetization in F1 is tilted slightly out-of-plane, the f1 component can be the dominant triplet component in the superconductor. The coupling between the two ferromagnets is discussed in terms of the underlying spin currents present in the system. We go further and show that the zero energy peaks of the local density of states probed on the S side of the valve can be another signature of the presence of superconducting triplet correlations. Our findings reveal that for sufficiently thin S layers, the zero energy peak at the S side can be larger than its counterpart in the F2 side.
Spin-transfer torque and magnetoresistance in superconducting spin valves
Physical Review B, 2009
We study the spin-transfer torque and magnetoresistance of a ferromagnet|superconductor|ferromagnet spinvalve, allowing for an arbitrary magnetization misorientation and treating both s-wave and d-wave symmetries of the superconductor. We take fully into account Andreev reflection and also the spin-triplet correlations that are generated when the magnetizations are non-collinear. It is found that the torque and magnetoresistance are both strongly enhanced when topological zero-energy states are present at the interfaces, which is the case for dwave superconductors with a crystallographic orientation of [110] relative to the interface (dxy-wave symmetry). Moreover, we find that the magnetoresistance displays a strong oscillatory and non-monotonous behavior as a function of dS/ξ where dS and ξ are the interlayer width of the superconducting region and the superconducting coherence length, respectively. This feature is also attributed to the crossover from layers of size dS ∼ 2ξ to layers of size dS 2ξ, where the contribution to transport from zero-energy states gradually vanishes.
Spin current and spin transfer torque in ferromagnet/superconductor spin valves
Physical Review B
Using fully self consistent methods, we study spin transport in fabricable spin valve systems consisting of two magnetic layers, a superconducting layer, and a spacer normal layer between the ferromagnets. Our methods ensure that the proper relations between spin current gradients and spin transfer torques are satisfied. We present results as a function of geometrical parameters, interfacial barrier values, misalignment angle between the ferromagnets, and bias voltage. Our main results are for the spin current and spin accumulation as functions of position within the spin valve structure. We see precession of the spin current about the exchange fields within the ferromagnets, and penetration of the spin current into the superconductor for biases greater than the critical bias, defined in the text. The spin accumulation exhibits oscillating behavior in the normal metal, with a strong dependence on the physical parameters both as to the structure and formation of the peaks. We also study the bias dependence of the spatially averaged spin transfer torque and spin accumulation. We examine the critical bias effect of these quantities, and their dependence on the physical parameters. Our results are predictive of the outcome of future experiments, as they take into account imperfect interfaces and a realistic geometry.
Spin-split conductance and subgap peak in ferromagnet/superconductor spin valve heterostructures
Physical Review B
We consider the separate spin channel contributions to the charge conductance in superconducting/ferromagnetic spin valve F 1 /N/F 2 /S structures. We find that the up-and down-spin conductance contributions may have a very different behavior in the subgap bias region (i.e. there is a spin-split conductance). This leads to a subgap peak in the total conductance. This peak behavior, which can be prominent also in N/F/S systems, is strongly dependent, in a periodic way, on the thickness of the intermediate ferromagnetic layer. We study this phenomenon for the ballistic scattering regime using a numerical self consistent method, with additional insights gained from an approximate analytic calculation for an infinite N/F/S structure. We study also the angular dependence on the relative magnetization angle between F 1 and F 2 of both the spin-split and the total conductance. We do so for realistic material parameters, layer thicknesses and interface quality values relevant to previous [A. A. Jara et al, Phys. Rev. B 89, 184502 (2014)] experimental studies on such devices. We also find that the spin-split conductance is highly dependent on the interfacial scattering in these devices, and we carefully include these effects for realistic systems. A strong valve-effect is found for the angularly dependent subgap peak conductance that is largely independent on the scattering and may prove useful in actual realizations of a superconducting spin valve device.