We report on measurements of supercurrents through the half-metallic ferromagnet CrO 2 grown on hexagonal Al 2 O 3 ͑sapphire͒. The current was observed to flow over a distance of 700 nm between two superconducting amorphous Mo 70 Ge 30 electrodes which were deposited on the CrO 2 film. The critical current I c increases as function of decreasing temperature. Upon applying an in-plane magnetic field, I c goes through a maximum at the rather high field of 80 mT. We believe this to be a long-range proximity effect in the ferromagnet, carried by odd-frequency pairing correlations. DOI: 10.1103/PhysRevB.82.100501 PACS number͑s͒: 74.45.ϩc, 72.25.Mk, 74.50.ϩr, 75.70.Cn The proximity-effect arising when a ferromagnet ͑F͒ is brought into contact with a conventional superconductor ͑S͒, is generally assumed to be small. The superconducting pair correlations decay rapidly inside the magnet since the phase coherence between the two spins forming the singlet Cooper pair is broken up by the exchange field h ex . In the dirty limit, the decay length F ϰ 1 / ͱ h ex is no more than 10 nm even for a weak ferromagnet. To compare, in a normal ͑N͒ metal the dephasing is due to temperature fluctuations with a decay length N ϰ 1 / ͱ k B T, which can reach microns at low T. Longrange proximity ͑LRP͒ effects in ferromagnets would be possible with spin-triplet Cooper pairs, since they do not suffer decay through h ex , but the orbital p symmetry required by the Pauli principle makes such the pair strongly susceptible to potential scattering by defects in the material. However, under the principle of odd-frequency pairing, also s symmetry is possible, 1,2 and the existence of odd-frequency s-wave triplet pairs could lead to LRP effects in dirty ferromagnets. To produce such triplets in the magnet, the singlet Cooper pair on the S side of the interface needs to sample an inhomogeneous magnetization on the F side, 1,2 or in a variant, spin mixing and magnetic disorder at the interface.3 Fully spin-polarized magnets ͑also called half-metallic ferromagnet͒ are particularly interesting since in such materials triplet correlations cannot be broken by spin-flip scattering and the decay length is set by thermal dephasing only.Subsequently, experimental observations indicating LRP effects were made by Sosnin et al., 4 who found supercurrents flowing in ferromagnetic Ho wires with lengths up to 150 nm using an Andreev interferometer geometry; and by Keizer et al., 5 who found supercurrents induced in half-metallic ferromagnetic CrO 2 , when superconducting electrodes of NbTiN with separations up to 1 m were placed on unstructured films. Even for normal metals this can be considered a very long range. No other experiments were reported for quite some time but this is now rapidly changing. In the last few months, reports came out on Josephson junctions where thin PdNi layers 6 or Ho layers 7 ͑providing magnetic inhomogeneity͒ were combined with Co layers and where no decay of the value of the Josephson current was found up to a thickness of 3...

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We introduce a type of vortex entry edge barrier which controls the critical current in a perpendicular magnetic field in thin-film weak-pinning superconducting strips. Measurements of the critical current in thinfilm amorphous-MoGe strips show a linear decrease with increasing magnetic field strength at low magnetic fields, and a crossover at a well-defined threshold field to an inverse power-law decay that is independent of the strip width. This behavior has not been observed previously due to bulk pinning, which only becomes dominant in our MoGe samples at high magnetic fields. To describe our results, we present calculations of the current distribution in thin superconducting strips with a finite penetration depth and negligible bulk pinning, and show that the measured critical currents in our MoGe samples correspond to a current density at the strip edge which approaches the Ginzburg-Landau depairing limit. Shape variations and defects along the strip edges influence the vortex entry conditions, leading to deviations from the ideal behavior, including offsets in the critical current maximum with respect to zero field.

The proximity effect between a superconductor ͑S͒ and a weak ferromagnet ͑F͒ in sputtered Nb/ Pd 0.86 Ni 0.14 bilayers has been studied. The dependence of the critical temperature on the Sand F-layer thicknesses can be interpreted in the framework of recent theoretical models and yields reasonable numbers for the exchange energy of the ferromagnet and the interface transparency of the S/F barrier.

We have investigated the behavior of the depairing current J dp in ferromagnet/superconductor/ferromagnet (F/S/F) trilayers as function of the thickness d s of the superconducting layers. Theoretically, J dp depends on the superconducting order parameter or the pair-density function, which is not homogeneous across the film due to the proximity effect. We use a proximity-effect model with two parameters ͑proximity strength and interface transparency͒, which can also describe the dependence of the superconducting transition temperature T c on d s . We compare the computations with the experimentally determined zero-field critical current J c0 of small strips ͑typically 5-m wide͒ of Fe/Nb/Fe trilayers with varying thickness d Nb of the Nb layer. Near T c the temperature dependence J c0 (T) is in good agreement with the expected behavior, which allows extrapolation to Tϭ0. Both the absolute values of J c0 (0) and the dependence on d Nb agree with the expectations for the depairing current. We conclude that J dp is correctly determined, notwithstanding the fact that the strip width is larger than both the superconducting penetration depth and the superconducting coherence length, and that J dp (d s ) is correctly described by the model.