Magnetooptics – Flux density distribution in ferromagnets and superconductors

Thermomagnetic instabilities

The penetration of magnetic flux into superconductors such as borides or borocarbides can happen in form of chaotic flux avalanches which occur due to local heating by dissipative flux line movement. These avalanches exhibit large velocities much larger than the phononic heat transport allows. Covering these superconducting films with metal layers it is possible to suppress the formation of these avalanches. It is unclear, why there are superconductor exhibiting avalanche behaviour and others (like YBCO) where no avalanches can be seen. Samples with a nonhomogeneous microstructure can shed light on possible processes wich are responsible for avalanche formation.

S. Treiber, C. Stahl, G. Schütz and J. Albrecht, Phys. Rev. B 84, 094533 (2011).

S. Treiber and J. Albrecht, New. J. Phys. 12, 093043 (2010).

S. Treiber, B. Stuhlhofer, H.-U. Habermeier and J. Albrecht, SUST 22, 045007 (2009).

Superconductor-ferromagnet heterostructures

The similar crystallographic structure allows to grow superconducting cuprates and ferromagnetic manganites as thin films epitaxially on top of each other. The generated interfaces can show an atomically flat structure. This high interface quality leads to a magnetic and an electronic coupling of both ordering phenomena, namely superconductinvity and ferromagnetism. The magnetic coupling leads to a hysteretic behaviour of the critical current density in these structures. The size of the effect is plotted vs. temperature for different heterostructures in the figure.

J. Albrecht,S. Soltan und H.-U. Habermeier, PRB 72, 092502 (2005).

Films on nanopatterned substrates

Thin films of high-temperature superconducting YBCO are grown by pulsed laser deposition on single-crystalline substrates with artificial surface structures. Here, films can be grown that exhibit defect structures with can act as effective pinning sites for flux lines. Spatially-resolved magnetooptical investigations show a substantial increase of the critical current density. The figure depicts a typical surface topography of the substrate (a) and the current density in a YBCO film grown on top (b).

S. Brück und J. Albrecht, PRB 71, 174508 (2005).

M. Djupmyr, G. Cristiani, H.-U Habermeier und J. Albrecht, PRB 72, 220507 (2005).