Superconductivity induced in a ferromagnet

Four years ago, this paper caught my attention.  The authors had made a structure with superconducting NbTiN contacts on top of a CrO₂ film, with the intent of studying how superconductivity leaks into the chromium dioxide.  The "leakage" of superconductivity into a non-superconducting metal is called the proximity effect.  In a normal metal, the proximity effect extends over a spatial scale comparable to the coherence length, the distance that the electrons can travel before their quantum mechanical phases become scrambled due to inelastic processes (such as electron-electron scattering, or spin-flip scattering from magnetic impurities).  The coherence length in a normal metal can be quite long at low temperatures - say a micron in a clean normal metal at 1 K.  

Now, CrO₂ is not a normal metal.  Rather, it is a half-metal, an extreme limit of an itinerant ferromagnet, where all of the mobile charge carriers have the same spin polarization.  This is important, because ordinary ("s-wave") superconducting correlations rely on pairing up electrons with opposite spins and momenta.  If only one spin polarization is allowed, that should preclude any s-wave superconductivity.  Practically speaking, in a typical ferromagnet with some magnetic exchange characterized by an exchange energy U, one can define an exchange length (in a diffusive material, given by sqrt(\hbar D/U), where D is the diffusion constant for the electrons) over which these correlations should die.  For a strong ferromagnet, one finds that the exchange length is very short - a few nanometers or less.  Knowing this, one would not expect to see any proximity induced superconductivity in a ferromagnet over longer distances.  That's why this paper was surprising - the authors did see evidence of long-range (hundreds of nm) superconductivity in the ferromagnetic oxide.  This implies some kind of unusual superconductivity in the ferromagnet - either p-wave pairing (when each pair of electrons in the superconducting material has one quantum of orbital angular momentum), or some more exotic state ("odd-frequency pairing", for the experts).

Several years passed, and no one reproduced this result.  Until now.  The authors of this new paper see the same sort of thing, and they try to explain in detail why this has been so hard to reproduce.  The short version:  CrO₂ is a pain to work with.

Interestingly, there have been other signs of similar effects within the last year.  For example, the Birge group at Michigan State has reported long-ranged proximity superconductivity induced in cobalt layers, though careful engineering of the contacts was required.  Likewise, a Penn State collaboration has seen proximity superconductivity in Co nanowires hundreds of nm long.  It's nice to see so much progress in this area lately.