Fluxon traps in cuprate superconductors fabricated by masked ion beam irradiation

Wolfgang Lang, Georg Zechner, Kristijan Luka Mletschnig, Florian Jausner, Bernd Aichner, M. Dosmailov, Marius Bodea, Radovan Vranik, Johannes D. Pedarnig

The interaction of vortices with artificial defects in a superconductor is a vibrant topic in fundamental experimental and theoretical research, but also important for its prospects of electronic applications in the field of so-called Fluxonics. In contrast to clean metallic superconductors that were often used to study regular artificial defects, the typical average distance of random intrinsic defects in a thin film of a prototypical cuprate superconductor, namely YBa2Cu3O7 (YBCO) is smaller than one µm. The advantage of a higher operation temperature in YBCO is opposed by the demand for advanced nanopatterning methods. To this end, YBCO thin films on MgO substrate are irradiated with 75 keV He+ ions by shadow projection through a Si stencil mask [1] to create a square array of columnar defect regions of typically 180 nm diameter and 300 nm lattice constant [2].
This masked ion beam structuring (MIBS) technology provides many advantages over other methods previously used for nanopatterning of cuprate superconductors: The desired pattern is fabricated in a single-step process, directly resulting in the as-required modification of specific portions of the material. The extent of this change can be controlled by the ion fluence, leading to superconducting (with reduced critical temperature Tc), normal conducting, or even insulating properties. The method does not require chemical treatment or etching, thus, preventing possible surface damage. It is a time-economic parallel method applicable to large areas and the mask can be reused many times allowing for scalability in industrial manufacturing. The surface remains essentially flat which permits the preparation of multi-layer structures and avoids deterioration of the film by out-diffusion of oxygen through open side faces.
The commensurate trapping of fluxons at the array of defect columns fabricated by MIBS is identified by peaks of the critical current as a function of the applied magnetic field. At the same magnetic fields, the magnetoresistance exhibits minima. In this situation, the major part of the magnetic flux penetrating the sample is trapped in the defect columns and only a few mobile interstitial vortices remain. In equilibrium arrangements, this “vortex matching” effect appears at integer multiples of a magnetic field value predefined by the applied magnetic field, the geometry of the defect lattice and the flux quantum.
However, an equilibrium fluxon arrangement carries little information and for practical applications fluxons need to be trapped in out-of-equilibrium patterns. We have achieved a major step forward in this direction with the demonstration of a stable critical state of fluxons in an array of about 180,000 artificial traps. Depending on the external magnetic field, the fluxons arrange themselves in terraced zones, in which each trap either captures no fluxon, exactly one, or several fluxons. Upon ramping an external magnetic field, a pronounced hysteresis and different positions of the critical current peaks in virgin and field-saturated down-sweep curves are observed, respectively. Interestingly, the distances of the various peaks in a sweep remain constant and correspond exactly to the matching field [3]. This unconventional critical state emerges from the coexistence of two pinning mechanisms: strong pinning of fluxons at the artificial defects created by MIBS and weaker pinning of interstitial vortices in the unirradiated areas [4].

[1] W. Lang, M. Dineva, M. Marksteiner, T. Enzenhofer, K. Siraj, M. Peruzzi, J. D. Pedarnig, D. Bäuerle, R. Korntner, E. Cekan, E. Platzgummer, H. Loeschner, Microelec. Eng. 83 (2006) 1495.
[2] J. D. Pedarnig, K. Siraj, M. A. Bodea, I. Puica, W. Lang, R. Kolarova, P. Bauer, K. Haselgrübler, C. Hasenfuss, I. Beinik, C. Teichert, Thin Solid Films 518 (2010) 7075.
[3] G. Zechner, F. Jausner, L.T. Haag, W. Lang, M. Dosmailov, M. A. Bodea, and J. D. Pedarnig, Phys. Rev. Appl. 8 (2017) 14021.
[4] G. Zechner, K. L. Mletschnig, W. Lang, M. Dosmailov, M. A. Bodea, J. D. Pedarnig, Supercond. Sci. Technol. 31 (2018) 044002.

Electronic Properties of Materials
Publication date
Publication status
Peer reviewed
Austrian Fields of Science 2012
210003 Nanoelectronics, 103033 Superconductivity
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