High quality ferromagnetic 0 and pi Josephson tunnel junctions

M. Weides (1) , M. Kemmler (2) , E. Goldobin (2) , D. Koelle (2) , R. Kleiner (2) , H. Kohlstedt (1, 3, 4) , Alexandre I. Buzdin (5, 6)

1 CNI - Center of Nanoelectronic Systems for Information Technology
2 Physikalisches Institut II-Experimentalphysik II
3 Department of Physics [Berkeley]
4 Department of Materials Science and Engineering [Berkeley]
5 CPMOH - Centre de physique moléculaire optique et hertzienne
6 IUF - Institut universitaire de France

We fabricated high quality $\Nb/\Al_2\O_3/\Ni_{0.6}\Cu_{0.4}/\Nb$ superconductor-insulator-ferromagnet-superconductor Josephson tunnel junctions. Depending on the thickness of the ferromagnetic $\Ni_{0.6}\Cu_{0.4}$ layer and on the ambient temperature, the junctions were in the 0 or $\pi$ ground state. All junctions have homogeneous interfaces showing almost perfect Fraunhofer patterns. The $\Al_2\O_3$ tunnel barrier allows to achieve rather low damping, which is desired for many experiments especially in the quantum domain. The McCumber parameter $\beta_c$ increases exponentially with decreasing temperature and reaches $\beta_c\approx700$ at $T=2.1 {\rm K}$. The critical current density in the $\pi$ state was up to $5\:\rm{A/cm^2}$ at $T=2.1 {\rm K}$, resulting in a Josephson penetration depth $\lambda_J$ as low as $160\:\rm{\mu m}$. Experimentally determined junction parameters are well described by theory taking into account spin-flip scattering in the $\Ni_{0.6}\Cu_{0.4}$ layer and different transparencies of the interfaces.

Domaines: Supraconductivité [cond-mat.supr-con]

M. Weides, M. Kemmler, E. Goldobin, D. Koelle, R. Kleiner, et al.. High quality ferromagnetic 0 and pi Josephson tunnel junctions. Applied Physics Letters, 2006, 89, pp.122511 (1-3). ⟨10.1063/1.2356104⟩. ⟨hal-00401261⟩