The maximum supercurrent (I.e. the dissipationless current) that can flow through a Josephson junction made of two superconductors separated by ferromagnetic material, strongly decreases with the thickness of this latter material. We had theoretically predicted that the stacking of appropriate ferromagnetic layers would enable the transport of a supercurrent in relatively thick junctions. Three years later, our prediction has been confirmed by two experiments.
A superconductor can transmit its properties to a normal metal through the exchange of quantum coherent electron pairs: this is called the proximity effect. When an electron pair in the singlet quantum state is injected into a ferromagnetic metal, each of the electron spins is submitted to an exchange field of opposite directions. This field rapidly dephases them, destroying the pair’s coherence. The proximity effect is therefore limited to a few nanometers. A much longer range proximity effect (a few tens of nanometers) can be observed with multilayers having non-collinear magnetisation : a singlet state electron pair in a layer is converted into an equal spin electron pair, called triplet state, in the next layer. The electrons then feel the same exchange field and the pair can survive.
In 2007, in collaboration with a colleague from the University of Bordeaux, we had predicted that this long range proximity effect could be checked from the measurement of a supercurrent through a ferromagnetic trilayer connected to two superconducting electrodes. Using the very same geometry, the effect was experimentally verified in 2010. A group from Cambridge University used a trilayer of Ho/Co/Ho. Another one from Michigan State University considered a multilayer of PdNi/Co. The measurements of the charge current in these structures are consistent with the predictions of a spin-polarised current. We are currently studying whether it is possible to directly probe the pair spin state.
Further reading: Robinson JWA et al., Science 329 (2010) 59; Khaire TS et al., Phys. Rev. Lett. 104 (2010) 137002; Houzet M and Buzdin AI, Phys. Rev. B 76 (2007) 060504
Maj : 17/02/2014 (915)