Low-T[sub c] Josephson junctions with tailored barrier (original) (raw)
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High quality ferromagnetic 0 and π Josephson tunnel junctions
Applied Physics Letters, 2006
We fabricated high quality Nb/Al2Ø3/Ni0.6Cu0.4/Nb superconductor-insulator-ferromagnetsuperconductor Josephson tunnel junctions. Depending on the thickness of the ferromagnetic Ni0.6Cu0.4 layer and on the ambient temperature, the junctions were in the 0 or π ground state. All junctions have homogeneous interfaces showing almost perfect Fraunhofer patterns. The Al2Ø3 tunnel barrier allows to achieve rather low damping, which is desired for many experiments especially in the quantum domain. The McCumber parameter βc increases exponentially with decreasing temperature and reaches βc ≈ 700 at T = 2.11 K. The critical current density in the π state was up to 5 A/cm 2 at T = 2.11 K, resulting in a Josephson penetration depth λJ as low as 160 µm. Experimentally determined junction parameters are well described by theory taking into account spin-flip scattering in the Ni0.6Cu0.4 layer and different transparencies of the interfaces.
Josephson tunnel junctions with a strong ferromagnetic interlayer
Physical Review B, 2009
The dependence of the critical current density j c on the ferromagnetic interlayer thickness d F was determined for Nb/ Al 2 O 3 / Cu/ Ni/ Nb Josephson tunnel junctions with ferromagnetic Ni interlayer thicknesses from very thin films ͑ϳ1 nm͒ upward and classified into F-layer thickness regimes showing a dead magnetic layer, exchange, exchange+ anisotropy and total suppression of j c. The Josephson coupling changes from 0 to as function of d F , and-very close to the crossover thickness-as function of temperature. The strong suppression of the supercurrent in comparison to nonmagnetic Nb/ Al 2 O 3 / Cu/ Nb junctions indicated that the insertion of a F layer leads to additional interface scattering. The transport inside the dead magnetic layer was in dirty limit. For the magnetically active regime fitting with both the clean and the dirty limit theories was carried out, indicating dirty limit condition, too. The results were discussed in the framework of literature.
Characteristics of strong ferromagnetic Josephson junctions with epitaxial barriers
Physical Review B, 2005
We present the measurement of superconductor / ferromagnetic Josephson junctions, based on an epitaxial Nb bottom electrode and epitaxial Fe20Ni80 barrier. Uniform junctions have been fabricated with a barrier thicknesses in the range 2 − 12 nm. The maximum critical current density ∼ 2.4 ± 0.2 × 10 9 Am −2 was found for a devices with a 3 nm thick barrier at 4.2 K, corresponding to an average characteristic voltage IC RN ∼ 16 µV. The ICRN showed a non-monotonic behavior with Fe20Ni80 thickness. The variation of the resistance of a unit area ARN , of the junctions with barrier thickness gave a Nb/Py specific interface resistance of 6.0 ± 0.5 fΩm 2 and Fe20Ni80 resistivity of 174 ± 50 nΩm, consistent with other studies in polycrystalline samples.
Applied Physics Letters, 2014
Nb-based Josephson junctions have been fabricated, which can select one of two states depending on the relative magnetization of their ferromagnetic barrier layers. To minimize the free-layer switching energy, while maintaining adequate thermal stability at 4.2 K, a dilute Cu-permalloy alloy [Cu 0.7 (Ni 80 Fe 20 ) 0.3 ] with a low magnetic saturation (M s $ 80 emu/cm 3 ) is used. The optimal thickness of the permalloy (Ni 80 Fe 20 ) fixed-layer is shown to be 2.4 nm. Such devices exhibit switching at magnetic fields as low as 5 Oe, demonstrating their potential use in low power non-volatile memory for superconductor digital circuits. V C 2014 AIP Publishing LLC.
IEEE Transactions on Applied Superconductivity, 1995
A unique normal-metal (N) layer construction was used to fabricate high temperature superconducting (S) YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// SNS Josephson junctions. The normal-metal included a gradient Pr-doped Y/sub 1-x/Pr/sub x/Ba/sub 2/Cu/sub 3/O/sub 7-/spl delta// layer which was composed of a light doping (x=0.1) next to both YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// electrodes, a slightly higher doping (x=0.3) towards the center, and a doping concentration of x=0.5 in the middle of the N-layer. A gradient of the doping profile of the N-layer instead of an abrupt one provides good thermal, structural, and chemical compatibility between adjacent regions. The multilayer configuration of the gradient Pr-doped N-layers on YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// showed good growth structure as confirmed by X-ray diffraction and Rutherford backscattering channeling tests. The SNS junctions fabricated in such a way showed resistively shunted junction current vs voltage characteristics above 77 K. Microwave induced Shapiro steps above 77 K and voltage modulation of dc SQUIDs at 77 K were both demonstrated with this technology.>
Josephson tunnel junctions with ferromagnetic Fe 0.75 Co 0.25 barriers
Journal of Physics D: Applied Physics, 2009
Josephson tunnel junctions with the strong ferromagnetic alloy Fe0.75Co0.25 as the barrier material were studied. The junctions were prepared with high quality down to a thickness range of a few monolayers of Fe-Co. An oscillation length of ξF 2 ≈ 0.79 nm between 0 and π-Josephson phase coupling and a very short decay length ξF 1 ≈ 0.22 nm for the amplitude of the superconducting pair wave function in the Fe-Co layer were determined. The rapid damping of the pair wave function inside the Fe-Co layer is caused by the strong ferromagnetic exchange field and additional magnetic pair breaking scattering. Josephson junctions with Fe-Co barriers show a significantly increased tendency towards magnetic remanence and flux trapping for larger thicknesses dF .
Fabrication and characterization of SNS Josephson junctions with an aluminum barrier
IEEE Transactions on Appiled Superconductivity, 2001
We report the current status of our SNS technology aimed at the development of programmable voltage standard devices. Using the simple Nb/Al/Nb trilayer process, with an A1 barrier thickness of the order of 100 nm, we have fabricated SNS Josephson junctions whose electrical properties can be changed by varying the morphology of the A1 film. The major role in determining the electrical behavior of the junctions is played by the roughness of the thick AI barrier. AFM analysis shows that the AI roughness is strongly reduced by increasing its deposition rate. The critical current density varies by two orders of magnitude, from lo3 A/cmZ up to LO5 A/cm2, with correspondingly normal resistances from 1 R down t o few m a . The magnetic field dependence of the critical current is also affected by the barrier structure, while all the junctions show regular Shapiro-like rf-induced steps at 70 GHz.
Hybrid Josephson Junctions with Iron-based and Conventional Superconductor Electrodes
Journal of Superconductivity and Novel Magnetism, 2014
We investigate the iron-based superconductor Ba(Fe 1−x Co x ) 2 As 2 (Ba-122) regarding its superconducting properties and possible applications. Therefore Ba-122 thin films are used as base electrode to prepare different kinds of hybrid Josephson junctions with a counter electrode of the conventional superconductor Pb. Additionally, we use both c-axis and ab-plane transport geometries and different kinds of barriers like interface-engineered surfaces, sputtered titanium oxide and gold layers. Temperature dependent I-V characteristics as well as magnetic field dependence and microwave response of the junctions are shown. The examined I-V characteristics and I c R n -T behaviours of each junction type are compared and described according to the electrical behaviour of the respective normal conducting or insulating barrier. While the I c R n product of the interface-engineered barrier junction was 12 µV and the planar junction with Au barrier showed 18 µV, we could increase the I c R n to 90 µV for planar TiO x barrier junctions.
Properties of tunnel Josephson junctions with a ferromagnetic interlayer
Physical Review B, 2008
We investigate superconductor/insulator/ferromagnet/superconductor (SIFS) tunnel Josephson junctions in the dirty limit, using the quasiclassical theory. We formulate a quantitative model describing the oscillations of critical current as a function of thickness of the ferromagnetic layer and use this model to fit recent experimental data. We also calculate quantitatively the density of states (DOS) in this type of junctions and compare DOS oscillations with those of the critical current.
High-Quality Ferromagnetic Josephson Junctions Based on Aluminum Electrodes
Nanomaterials
Aluminum Josephson junctions are the building blocks for the realization of superconducting quantum bits. Attention has been also paid to hybrid ferromagnetic Josephson junctions, which allow switching between different magnetic states, making them interesting for applications such as cryogenic memories, single-photon detectors, and spintronics. In this paper, we report on the fabrication and characterization of high-quality ferromagnetic Josephson junctions based on aluminum technology. We employed an innovative fabrication process inspired by niobium-based technology, allowing us to obtain very high-quality hybrid aluminum Josephson junctions; thus, supporting the use of ferromagnetic Josephson junctions in advanced quantum circuits. The fabrication process is described in detail and the main DC transport properties at low temperatures (current–voltage characteristic, critical current as a function of the temperature, and the external magnetic field) are reported. Here, we illustr...