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Nb-PtNi-Nb Josephson junctions made by 3D FIB nano-sculpturing
Stockholm University, Faculty of Science, Department of Physics. (Experimental Condensed Matter Physics)
(Chalmers University of Technology, MC2)
Stockholm University, Faculty of Science, Department of Physics. (Experimental Condensed Matter Physics)
2009 (English)In: Journal of Physics, Conference Series, ISSN 1742-6588, E-ISSN 1742-6596, Vol. 150, no 5, 052062- p.Article in journal (Refereed) Published
Abstract [en]

We use Focused Ion Beam (FIB) for fabrication of nano-scale Superconductor-Ferromagnet-Superconductor (SFS) Josephson junctions, aiming to achieve a uniform, mono-domain state in the F-layer within the junction. We employ a Pt1-xNix alloy, characterized by the perfect solubility of the two components, for obtaining a homogeneous diluted ferromagnet. We perform a systematic analysis of both chemical composition, and ferromagnetic properties of Pt1—xNix thin films for different Ni—concentrations. The nano-scale homogeneity of the Pt1—xNix films is confirmed by energy dispersive X-ray spectroscopy. The Curie temperature of Pt1—xNix films decreases in a non-linear manner with Ni concentration. We observe that the critical current density of NbPt1—xNixNb junctions decreases non-monotonously with increasing Ni-concentration: at x 30% it exhibits a minimum, which we attribute to switching into the π state as a function of Ni-concentration.

Place, publisher, year, edition, pages
2009. Vol. 150, no 5, 052062- p.
National Category
Condensed Matter Physics
Research subject
Physics
Identifiers
URN: urn:nbn:se:su:diva-56627DOI: 10.1088/1742-6596/150/5/052062OAI: oai:DiVA.org:su-56627DiVA: diva2:411873
Available from: 2011-04-19 Created: 2011-04-19 Last updated: 2017-12-11Bibliographically approved
In thesis
1. Mesoscopic phenomena in hybrid superconductor/ferromagnet structures
Open this publication in new window or tab >>Mesoscopic phenomena in hybrid superconductor/ferromagnet structures
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis explores peculiar effects of mesoscopic structures revealed at low temperatures. Three particular systems are studied experimentally: Ferromagnetic thin films made of diluted Pt1-xNix alloy, hybrid nanoscale Nb-Pt1-xNix-Nb Josephson junctions, and planar niobium Josephson junction with barrier layer made of Cu or Cu0.47Ni0.53 alloy.

A cost-effective way is applied to fabricate the sputtered NixPt1-x thin films with controllable Ni concentration. 3D Focused Ion Beam (FIB) sculpturing is used to fabricate Nb-Pt1-xNix-Nb Josephson junctions. The planar junctions are made by cutting Cu-Nb or CuNi-Nb double layer by FIB.

Magnetic properties of PtNi thin films are studied via the Hall effect. It is found that films with sub-critical Ni concentration are superparamagnetic at low temperatures and exhibit perpendicular magnetic anisotropy. Films with over-critical Ni concentration are ferromagnetic with parallel anisotropy. At the critical concentration the films demonstrate canted magnetization with the easy axis rotating as a function of temperature. The magnetism appears via two consecutive crossovers, going from paramagnetic to superparamagnetic to ferromagnetic, and the extraordinary Hall effect changes sign at low temperatures.

Detailed studies of superconductor-ferromagnet-superconductor Josephson junctions are carried out depending on the size of junction, thickness and composition of the ferromagnetic layer. The junction critical current density decreases non-monotonically with increasing Ni concentration. It has a minimum at ~ 40 at.% of Ni which indicates a switching into the π state.

The fabricated junctions are used as phase sensitive detectors for analysis of vortex states in mesoscopic superconductors. It is found that the vortex induces different flux shifts, in the measured Fraunhofer modulation of the Josephson critical current, depending on the position of the vortex. When the vortex is close to the junction it induces a flux shift equal to Φ0/2 leading to switching of the junction into the 0-π state. By changing the bias current at constant magnetic field the vortices can be manipulated and the system can be switched between two consecutive vortex states. A mesoscopic superconductor can thus act as a memory cell in which the junction is used both for reading and writing information (vortex).

Place, publisher, year, edition, pages
Stockholm: Department of Physics, Stockholm University, 2011. 117 p.
National Category
Condensed Matter Physics
Research subject
Physics
Identifiers
urn:nbn:se:su:diva-56629 (URN)978-91-7447-282-0 (ISBN)
Public defence
2011-05-26, FB51, AlbaNova Universitetscentrum, Roslagstullsbacken 21, Stockholm, 10:00 (English)
Opponent
Supervisors
Available from: 2011-05-04 Created: 2011-04-19 Last updated: 2011-05-26Bibliographically approved

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