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- Kapran, Olena, 1993-
(författare)
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Hybrid Superconductor/Ferromagnet junctions with a strongly ferromagnetic barrier
- 2019
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- 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 Eex ~ 631 K much larger than the superconducting critical temperature of Nb, Tc ≈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.
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| 2. |
- Kapran, Olena M., 1993-, et al.
(författare)
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Observation of the dominant spin-triplet supercurrent in Josephson spin valves with strong Ni ferromagnets
- 2020
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Ingår i: Physical Review Research. - : American Physical Society (APS). - 2643-1564. ; 2:1
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Tidskriftsartikel (refereegranskat)abstract
- 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 Ic(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.
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