Tuesday, March 30, 2010

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.


cccwind said...

The competition between magnetism and superconductivity is interesting, and the large proximity length in half metal is very impressive. I wonder whether there are ways to know the distribution (similar to type 2 superconductivity in a magnetic field?) and transport types (drift,diffusion,tunneling, or any other?) of these electron pairs in magnet.

Uncle Al said...

"Are the surfaces of CrO2 metallic?" doi:10.1088/0953-8984/19/31/315207

"the formation of a surface may be causing a transition from metallic to semiconducting behaviour."

More studies are needed.

CrO2 tetragonal P42/mnm (#136), Z = 2
a=b = 4.421 Å, c = 2.916 Å

TiO2 tetragonal P42/mnm (#136), Z = 2
a=b = 4.5937 Å, c = 2.9618 Å

Corundum hex-R R-3c(#167), Z=6
a=b = 4.7591 c = 12.9894

cccwind said...

The surface layer may be responsible for lost polarization in half metals.

However, does "half metal" mean a metallic conduction? Take Fe3O4 (a well known half metal) for example, it seems to be not true.

Dr. Muhammad Shahbaz Anwar said...

A group of Prof. dr. M.G. Blamire from Cambridge University also observed the supercurrent through a Josephson junction fabricated using Co/Ho as ferromagnetic multilayer and Nb as superconductor. It has been claimed that supercurrent is triplet in nature that is penetrating over a distance of 15nm, half of the Khaire’s observations.
Recently some interesting observations have been done with CrO2 as a half metallic ferromagnet and MoGe superconductor. The supercurrnet is observed over 700nm lateral junction length (http://arxiv.org/abs/1003.4446).
These measurements along with a recent one with Cu2MnAl (http://arxiv.org/abs/1003.2082), we can confidently say that this year 2010 is the year of triplet superconductivity.

fashion accessories said...

Although the origin of the ‘critical peak’ remains mysterious, our analysis strongly points against charge or spin imbalance as its underlying cause.

Dr. Muhammad Shahbaz Anwar said...

Well, it is getting interesting in my case. The critical peak in R(T) at Tc is present only for CrO2 not for any other ferromagnet or normal metal. If I do the cleaning to remove the Cr2O3 at the surface of CrO2 then only huge up jump in resistance otherwise it goes down normally at Tc and starts to increase again (remains two time less than the normal resistance of the junctions). Ref. Anwar et al. SUST 24, 024016 (2011). (http://iopscience.iop.org/0953-2048/24/2/024016).