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Hybrid Superconductor/Ferromagnet junctions with a strongly ferromagnetic barrier
Stockholm University, Faculty of Science, Department of Physics.
2019 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

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.

Place, publisher, year, edition, pages
Stockholm: Department of Physics, Stockholm University , 2019.
Keywords [en]
Superconductivity, Josephson junctions, planar Josephson junctions, micro/nanoscale fabrication, micromagnetic state, in-situ characterization technique
National Category
Condensed Matter Physics
Research subject
Physics
Identifiers
URN: urn:nbn:se:su:diva-176283OAI: oai:DiVA.org:su-176283DiVA, id: diva2:1373700
Presentation
2019-12-19, FP22, hus 1, AlbaNova universitetcentrum, Roslagstullsbacken 21, Stockholm, 13:00 (English)
Opponent
Supervisors
Available from: 2019-12-09 Created: 2019-11-27 Last updated: 2019-12-09Bibliographically approved
List of papers
1. Planar Superconductor-Ferromagnet-Superconductor Josephson Junctions as Scanning-Probe Sensors
Open this publication in new window or tab >>Planar Superconductor-Ferromagnet-Superconductor Josephson Junctions as Scanning-Probe Sensors
2019 (English)In: Physical Review Applied, E-ISSN 2331-7019, Vol. 11, no 1, article id 014062Article in journal (Refereed) Published
Abstract [en]

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.

National Category
Physical Sciences
Identifiers
urn:nbn:se:su:diva-166785 (URN)10.1103/PhysRevApplied.11.014062 (DOI)000457141300003 ()
Available from: 2019-03-12 Created: 2019-03-12 Last updated: 2019-11-27Bibliographically approved

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