Kompetenznetz Funktionelle Nanostrukturen Teilprojekt C2

Description

Elektronentransport an Grenzflächen zwischen ferromagnetischen und unmagnetischen
Metallen bildet die Grundlage für funktionale Strukturen der Spinelektronik.
Magnetische Isolatoren sind vielversprechende Spinfilter-Materialien für Spintronik-
Anwendungen. Dabei spielt zunehmend phasenkohärenter Transport in epitaktischen
Strukturen eine Rolle. Eine experimentell besonders klare Signatur spinabhängigen
phasenkohärenten Transports ist das Auftreten gebundener Andreev-Zustände aufgrund
nanoskaliger Inhomogenitäten an der Grenzfläche zwischen Ferromagneten und
Supraleitern, deren Untersuchung das Ziel dieses Projekts ist. Ebenso von fundamentaler
Bedeutung wie anwendungsrelevant ist die Möglichkeit Supraströme über
größere Distanzen durch Ferromagneten führen zu können, weil sich hier möglicherweise
die Gewichtung von quantenmechanischer Verschränkung zu Spinpolarisation
maßschneidern lässt. Deshalb sollen hier zunächst diese Ursachen des langreichweitigen
Proximity-Effekts von Supraleitern und Ferromagneten an zwei komplementären
Modellsystemen studiert werden.

External participants
  • Beckmann, Detlef - Project head
  • von Löhneysen, Hilbert - Project head
Institutions
  • FB Physik
  • AG Belzig (Theoretische Physik mit SP Quantentransport)
Publications
    Kolenda, Stefan; Machon, Peter; Beckmann, Detlef; Belzig, Wolfgang (2016): Nonlinear thermoelectric effects in high-field superconductor-ferromagnet tunnel junctions Beilstein Journal of Nanotechnology ; 7 (2016). - S. 1579-1585. - eISSN 2190-4286

Nonlinear thermoelectric effects in high-field superconductor-ferromagnet tunnel junctions

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Background: Thermoelectric effects result from the coupling of charge and heat transport and can be used for thermometry, cooling and harvesting of thermal energy. The microscopic origin of thermoelectric effects is a broken electron–hole symmetry, which is usually quite small in metal structures. In addition, thermoelectric effects decrease towards low temperatures, which usually makes them vanishingly small in metal nanostructures in the sub-Kelvin regime.<br /><br />Results: We report on a combined experimental and theoretical investigation of thermoelectric effects in superconductor/ferromagnet hybrid structures. We investigate the dependence of thermoelectric currents on the thermal excitation, as well as on the presence of a dc bias voltage across the junction.<br /><br />Conclusion: Large thermoelectric effects are observed in superconductor/ferromagnet and superconductor/normal-metal hybrid structures. The spin-independent signals observed under finite voltage bias are shown to be reciprocal to the physics of superconductor/normal-metal microrefrigerators. The spin-dependent thermoelectric signals in the linear regime are due to the coupling of spin and heat transport, and can be used to design more efficient refrigerators.

Origin (projects)

    Wiedwald, Ulf; Häring, Felix; Nau, Stefan; Schulze, Carsten; Schletter, Herbert; Makarov, Denys; Plettl, Alfred; Küpper, Karsten; Albrecht, Manfred; Boneberg, Johannes; Ziemann, Paul (2012): Tuning the properties of magnetic thin films by interaction with periodic nanostructures Beilstein Journal of Nanotechnology ; 3 (2012). - S. 831-842. - eISSN 2190-4286

Tuning the properties of magnetic thin films by interaction with periodic nanostructures

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The most important limitation for a significant increase of the areal storage density in magnetic recording is the superparamagnetic effect. Below a critical grain size of the used CoCrPt exchange-decoupled granular films the information cannot be stored for a reasonable time (typically ten years) due to thermal fluctuations arbitrary flipping of the magnetization direction. An alternative approach that may provide higher storage densities is the use of so-called percolated media, in which defect structures are imprinted in an exchange-coupled magnetic film. Such percolated magnetic films are investigated in the present work. We employ preparation routes that are based on (i) self-assembly of Au nanoparticles and (ii) homogeneous size-reduction of self-assembled polystyrene particles. On such non-close-packed nanostructures thin Fe films or Co/Pt multilayers are grown with in-plane and out-of-plane easy axis of magnetization. The impact of the particles on the magnetic switching behavior is measured by both integral magnetometry and magnetic microscopy techniques. We observe enhanced coercive fields while the switching field distribution is broadened compared to thin-film reference samples. It appears possible to tailor the magnetic domain sizes down to the width of an unperturbed domain wall in a continuous film, and moreover, we observe pinning and nucleation at or close to the imprinted defect structures.

Origin (projects)

Funding sources
Name Project no. Description Period
Baden-Württemberg-Stiftung-no information
Further information
Period: 01.01.2012 – 31.12.2014