S/N and S/FM point contacts through silicon-nitride membranes

F. Perez-Willard1, C. Sürgers1, H. v. Löhneysen1,2 und P. Pfundstein3
2Forschungszentrum Karlsruhe, Institut für Festkörperphysik, D-76344 Karlsruhe
3Laboratorium für Elektronenmikroskopie, Universität Karlsruhe, D-76128 Karlsruhe
1Physikalisches Institut and Center for Functional Nanostructures, Universität Karlsruhe, D-76128 Karlsruhe

We report on low-temperature (20 mK - 2 K) measurements of the electrical differential resistance R = dU/dI vs. voltage U of superconductor (S)/normal metal (N) and S/ferromagnet (FM) point contacts. The samples consist of an insulating 50-nm thick silicon-nitride membrane in which a single nanohole of diameter d  £  50 nm has been patterned previously by means of e-beam lithography and reactive ion etching. 200 nm aluminum (S) and X nm (X = 0, 6, 12, 24, 52) cobalt (FM) plus (200-X) nm copper are deposited under ultra-high vacuum conditions on either side of the membrane. The R-U spectra for the S/N contacts can be described well for all temperatures T within the BTK-Theory [1]. For the S/FM contacts, X ¹ 0 causes a dramatic change in the shape of the spectrum compared to X = 0. By further increasing X the amplitude of the spectrum is slightly reduced. An analysis of similar S/FM spectra has been used before to estimate the degree of spin polarization in the ferromagnetic metal [2]. The applicability of this procedure and the magnetic-field dependence of the spectra will be discussed.