Operation of Josephson electronics usually requires determination of the Josephson critical current Ic, which is affected both by fluctuations and measurement noise. Lock-in measurements allow obviation of 1/f noise, and therefore, provide a major advantage in terms of noise and accuracy with respect to conventional dc measurements. In this work we show both theoretically and experimentally that the Ic can be accurately extracted using first and third harmonic lock-in measurements of junction resistance. We derived analytical expressions and verified them experimentally on nano-scale Nb-PtNi-Nb and Nb-CuNi-Nb Josephson junctions.

We study superconductor/ferromagnet/superconductor junctions with CuNi, PtNi, or Ni interlayers. Remarkably, we observe that supercurrents through Ni can be significantly larger than through diluted alloys. The phenomenon is attributed to the dirtiness of disordered alloys leading to a short coherence length despite a small exchange energy. To the contrary, pure Ni is clean resulting in a coherence length as long as in a normal metal. Analysis of temperature dependencies of critical currents reveals a crossover from short (dirty) to long (clean) range proximity effects in Pt_1−xNi_x with increasing Ni concentration. Our results point out that structural properties of a ferromagnet play a crucial role for the proximity effect and indicate that conventional strong-but-clean ferromagnets can be advantageously used in superconducting spintronic devices.

Employment of the non-trivial proximity effect in superconductor/ferromagnet (S/F) heterostructures for the creation of novel superconducting devices requires accurate control of magnetic states in complex thin-film multilayers. In this work, we study experimentally in-plane transport properties of microstructured Nb/Co multilayers. We apply various transport characterization techniques, including magnetoresistance, Hall effect, and the first-order-reversal-curves (FORC) analysis. We demonstrate how FORC can be used for detailed in situ characterization of magnetic states. It reveals that upon reduction of the external field, the magnetization in ferromagnetic layers first rotates in a coherent scissor-like manner, then switches abruptly into the antiparallel state and after that splits into the polydomain state, which gradually turns into the opposite parallel state. The polydomain state is manifested by a profound enhancement of resistance caused by a flux-flow phenomenon, triggered by domain stray fields. The scissor state represents the noncollinear magnetic state in which the unconventional odd-frequency spin-triplet order parameter should appear. The non-hysteretic nature of this state allows for reversible tuning of the magnetic orientation. Thus, we identify the range of parameters and the procedure for in situ control of devices based on S/F heterostructures.

Phase shifter is one of the key elements of quantum electronics. In order to facilitate operation and avoid decoherence, it has to be reconfigurable, persistent, and nondissipative. In this work, we demonstrate prototypes of such devices in which a Josephson phase shift is generated by coreless superconducting vortices. The smallness of the vortex allows a broad-range tunability by nanoscale manipulation of vortices in a micron-size array of vortex traps. We show that a phase shift in a device containing just a few vortex traps can be reconfigured between a large number of quantized states in a broad [−3π, +3π] range.

We study experimentally nanoscale Josephson junctions and Josephson spin valves containing strongly ferromagnetic Ni interlayers. We observe that in contrast to junctions, spin valves with the same geometry exhibit anomalous I_c(H) patterns with two peaks separated by a dip. We develop several techniques for in situ characterization of micromagnetic states in our nanodevices, including magnetoresistance, absolute Josephson fluxometry, and first-order-reversal-curves analysis. They reveal a clear correlation of the dip in supercurrent with the antiparallel state of a spin valve and the peaks with two noncollinear magnetic states, thus providing evidence for generation of spin-triplet superconductivity. A quantitative analysis, based on micromagnetic simulations, brings us to the conclusion that the triplet current in our Ni-based spin valves is approximately three times larger than the conventional spin-singlet supercurrent.

The spin-triplet superconducting state, predicted in superconductor /ferromagnet heterostructures remains one of the most exotic states of nature. It is expected that the triplet state can be switched on/off by changing the relative orientation of magnetization in multilayer Josephson spin-valve structures. This is interesting not only from a fundamental point but could also lead to the creation of novel transistor-like devices with controlled supercurrent. However, there are many experimental challenges. The key issue is in achieving detailed knowledge and control of the micromagnetic state. This requires methods for in-situ characterization of actual nano-devices.

In this thesis we study Superconductor/Ferromagnet/Superconductor (SFS) Josephson junctions with Nb superconducting electrodes and Ni interlayers with thicknesses 2-20 nm. Nano-scale SFS junctions are made by a 3D nanosculpturing technique by Focused Ion Beam. Small sizes are needed for achieving mono-domain remagnetization of Ni interlayer in the junctions. Ni is a strong ferromagnet with the exchange energy E_ex ~ 631 K much larger than the superconducting critical temperature of Nb, T_c ≈9 K. Therefore, it might be expected that spin-singlet Cooper pairs should be rapidly destroyed in Ni. However, we observe a significant supercurrent through Ni with thicknesses up to 20 nm. We attribute this counterintuitive result to the cleanliness of Ni films with a mean-free-path ~100 nm larger than the film thickness. For determination of the micromagnetic state of F-layers in our nano-scale junctions we develop a new in-situ characterization technique based on a combination of the Absolute Josephson Fluxometry and the First-Order-Reversal-Curves analysis. It is demonstrated that this is a very powerful technique facilitating detailed in-situ measurements of magnetization curves of F-interlayers even in very small junctions. Finally, we fabricate and study nano-scale Nb/Ni/Nb junctions with planar geometry and argue that such junctions can be employed as sensitive scanning-probe sensors. Thus, we demonstrate that Ni, despite being a strong ferromagnet, is a promising material for application in superconducting spintronics.

Complex oxides exhibit a variety of unusual physical properties, which can be used for designing novel electronic devices. Here we fabricate and study experimentally nanoscale superconductor/ferromagnet/ superconductor junctions with the high-T_c cuprate superconductors YBa₂Cu₃O_7−x and the colossal magnetoresistive (CMR) manganite ferromagnets La_2/3X_1/3MnO_3+δ(X=CaorSr). We demonstrate that in a broad temperature range the magnetization of a manganite nanoparticle, forming the junction interface, switches abruptly in a monodomain manner. The CMR phenomenon translates the magnetization loop into a hysteretic magnetoresistance loop. The latter facilitates a memory functionality of such a junction with just a single CMR ferromagnetic layer. The orientation of the magnetization (stored information) can be read out by simply measuring the junction resistance in a finite magnetic field. The CMR facilitates a large readout signal in a small applied field. We argue that such a simple single-layer CMR junction can operate as a memory cell both in the superconducting state at cryogenic temperatures and in the normal state up to room temperature.

We propose a magnetic scanning-probe sensor based on a single-planar Josephson junction with a magnetic barrier. The planar geometry together with the high magnetic permeability of the barrier facilitates a double flux-focusing effect, which helps to guide magnetic flux into the junction and thus enhances field sensitivity of the sensor. We fabricate and analyze experimentally sensor prototypes with a superparamagnetic Cu−Ni and a ferromagnetic Ni barrier. We demonstrate that the planar geometry allows easy miniaturization to nanometer scale and facilitates an effective utilization of the self-field phenomenon for amplification of sensitivity and a simple implementation of a control line for feedback operation over a broad dynamic range. We argue that the proposed sensor can outperform equally sized superconducting quantum-interference devices (SQUIDs) both in terms of magnetic-field sensitivity and spatial resolution, which makes it advantageous for scanning-probe microscopy.