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Mesoscopic phenomena in hybrid superconductor/ferromagnet structures
Stockholm University, Faculty of Science, Department of Physics. (Experimental Condensed Matter Physics)
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: urn:nbn:se:su:diva-56629ISBN: 978-91-7447-282-0OAI: oai:DiVA.org:su-56629DiVA: diva2:411885
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
List of papers
1. Anticorrelation between temperature and fluctuations of the switching current in moderately damped Josephson junctions
Open this publication in new window or tab >>Anticorrelation between temperature and fluctuations of the switching current in moderately damped Josephson junctions
2007 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 76, no 22, 224517- p.Article in journal (Refereed) Published
Abstract [en]

Presented are the results of calculations suggesting that the quasi-one-dimensional organic superconductors (TMTSF)2X (where TMTSF represents tetramethyltetraselenafulvalene and X is PF6 AsF6, ClO4, etc.) may show a substantial increase in their superconducting and spin-density-wave ordering temperatures when the Fermi level is raised through application of an electrostatic gating voltage. A rich behavior is observed, strongly dependent on the form of the superconducting order parameter, as the Fermi level approaches the Van Hove singularity at ka=0. Included are predictions for the behavior of these materials under zero and moderate applied pressure. It is found that TSDW as high as 50 K and superconducting Tc as high as 20 K may be achieved at optimal gate voltages of approximately 100 mV.

National Category
Natural Sciences
Identifiers
urn:nbn:se:su:diva-12493 (URN)10.1103/PhysRevB.76.224518 (DOI)000251986400087 ()
Available from: 2008-01-15 Created: 2008-01-15 Last updated: 2011-04-20Bibliographically approved
2. Field- and current controlled switching between vortex states in a mesoscopic superconductor
Open this publication in new window or tab >>Field- and current controlled switching between vortex states in a mesoscopic superconductor
2009 (English)In: Journal of Physics, Conference Series, ISSN 1742-6588, E-ISSN 1742-6596, Vol. 153, no 1, 012027- p.Article in journal (Refereed) Published
Abstract [en]

We study the controllable manipulation of vortices in a mesoscopic, superconducting "island" of Nb, using an integrated Josephson junction as a field-sensitive vortex detector. The island, divided by a single Josephson junction and suspended by Nb microbridges, was fabricated from a Nb/P11-xNix/Nb tri-layer using a focused ion beam. We find that the system at select magnetic fields behaves as a vortex memory cell, where current pulses can be used to switch the vortex configuration between metastable states of distinctly different junction critical currents. Non-destructive read-out of a state is then easily done with an intermediate current. Furthermore, we show that the Josephson junction displays a strong magnetoresistive effect at current bias well above the junction critical current but below the onset of flux flow. This enables the junction to be used as a quantitative probe of magnetic field with better than single flux quantum resolution.

National Category
Condensed Matter Physics
Research subject
Physics
Identifiers
urn:nbn:se:su:diva-33659 (URN)10.1088/1742-6596/153/1/012027 (DOI)
Available from: 2009-12-23 Created: 2009-12-23 Last updated: 2011-04-20Bibliographically approved
3. Nb-PtNi-Nb Josephson junctions made by 3D FIB nano-sculpturing
Open this publication in new window or tab >>Nb-PtNi-Nb Josephson junctions made by 3D FIB nano-sculpturing
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.

National Category
Condensed Matter Physics
Research subject
Physics
Identifiers
urn:nbn:se:su:diva-56627 (URN)10.1088/1742-6596/150/5/052062 (DOI)
Available from: 2011-04-19 Created: 2011-04-19 Last updated: 2011-04-20Bibliographically approved
4. Application of nano-scale Josephson junction as phase sensitive detector for analysis of vortex states in mesoscopic superconductors
Open this publication in new window or tab >>Application of nano-scale Josephson junction as phase sensitive detector for analysis of vortex states in mesoscopic superconductors
2010 (English)In: Physica. C, Superconductivity, ISSN 0921-4534, Vol. 570, no 19, 890-892 p.Article in journal (Refereed) Published
Abstract [en]

We study phase shifts in a Josephson junction induced by vortices in superconducting mesoscopic electrodes. The position of the vortices are controlled by suitable geometry of a nano-scale Nb–Pt1−xNix–Nb junction of the overlap type made by Focused Ion Beam (FIB) sculpturing. The vortex is kept outside the junction, parallel to the junction plane. From the measured Fraunhofer characteristics the entrance and exit of vortices are detected. By changing the bias current through the junction at constant magnetic field the vortices can be manipulated and the system can be switched between two consecutive vortex states which are characterized by different critical currents of the junction. A mesoscopic superconductor thus acts as a non-volatile memory cell in which the junction is used both for reading and writing information (vortex). Furthermore, we observe that the critical current density of Nb–Pt1−xNix–Nb junctions decreases non-monotonously with increasing Ni concentration. It exhibits a minimum at 40 at.% Ni, which is an indication of switching into the π state.

National Category
Natural Sciences
Identifiers
urn:nbn:se:su:diva-48326 (URN)10.1016/j.physc.2010.02.039 (DOI)
Available from: 2010-12-07 Created: 2010-12-07 Last updated: 2011-04-20Bibliographically approved
5. Detection of the Phase Shift from a Single Abrikosov Vortex
Open this publication in new window or tab >>Detection of the Phase Shift from a Single Abrikosov Vortex
2010 (English)In: Physical Review D. Particles and fields, ISSN 0556-2821, E-ISSN 1089-4918, Vol. 104, 227003- p.Article in journal (Refereed) Published
Abstract [en]

We probe a quantum mechanical phase rotation induced by a single Abrikosov vortex in a superconducting lead, using a Josephson junction, made at the edge of the lead, as a phase-sensitive detector. We observe that the vortex induces a Josephson phase shift equal to the polar angle of the vortex within the junction length. When the vortex is close to the junction it induces a π step in the Josephson phase difference, leading to a controllable and reversible switching of the junction into the 0-π state. This in turn results in an unusual Φ0/2 quantization of the flux in the junction. The vortex may hence act as a tunable “phase battery” for quantum electronics.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:su:diva-48324 (URN)10.1103/PhysRevLett.104.227003 (DOI)000278477500008 ()
Note
authorCount :3Available from: 2010-12-07 Created: 2010-12-07 Last updated: 2011-05-26Bibliographically approved
6. Anomalous Hall effect in NiPt thin films
Open this publication in new window or tab >>Anomalous Hall effect in NiPt thin films
2011 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 110, no 3, 033909- p.Article in journal (Refereed) Published
Abstract [en]

We study Hall effect in sputtered NixPt1-x thin films with different Ni concentrations. Temperature, magnetic field andangular dependencies are analyzed and the phase diagram of NiPt thin films is obtained. It is found that films with sub-critical Ni concentration exhibit cluster-glass behavior at low temperatures with a perpendicular magnetic anisotropy below the freezing temperature. Films with super-critical Ni concentration are ferromagnetic with parallel anisotropy. At the critical concentration the state of the film is strongly frustrated. Such films demonstrate canted magnetization with the easy axis rotating as a function of temperature. The magnetism appears via consecutive paramagnetic - cluster glass - ferromagnetic transitions, rather than a single second-order phase transition. But most remarkably, the extraordinary Hall effect changes sign at the critical concentration. We suggest that this is associated with a reconstruction of the electronic structure of the alloy at the normal metal - ferromagnet quantum phase transition.

National Category
Condensed Matter Physics
Research subject
Physics
Identifiers
urn:nbn:se:su:diva-56668 (URN)10.1063/1.3615959 (DOI)000293956600097 ()
Available from: 2011-04-20 Created: 2011-04-20 Last updated: 2012-07-10Bibliographically approved

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