Long-range supercurrents through half-metallic ferromagneticCrO2

Abstract

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...

Experimental observation of the triplet spin-valve effect in a superconductor-ferromagnet heterostructure

“…Intense activities followed to formulate optimal conditions and realize experimental schemes for the generation and detection of this odd-triplet pairing utilizing the Josephson effect [5][6][7][8][9][10][11][12][13][14]. The observation of a current crossing a weak link of ferromagnetic material with a thickness much exceeding the penetration length for singlet-paired electrons [5][6][7][8][9][10][11] indicated a triplet contribution to the Josephson current.…”

“…At least two ferromagnetic layers (F 1 ,F 2 ) with a non-collinear alignment of their magnetizations, are required to couple the conventional opposite-spin singlet s-wave pairing channel with the unconventional, odd-triplet s-wave pairing channel. The latter one is of extraordinary long range in F layers [1,2,4], because the magnetized conduction band of a ferromagnetic metal serves as an eigenmedia supporting the equal-spin pairing.Intense activities followed to formulate optimal conditions and realize experimental schemes for the generation and detection of this odd-triplet pairing utilizing the Josephson effect [5][6][7][8][9][10][11][12][13][14]. …”

“…Intense activities followed to formulate optimal conditions and realize experimental schemes for the generation and detection of this odd-triplet pairing utilizing the Josephson effect [5][6][7][8][9][10][11][12][13][14].…”

Evidence for spin mixing in holmium thin film and crystal samples

“…Evidence for the LRSTPE has also been observed in other systems, including: Pd 0.88 Ni 0.12 and Pd 0.987 Fe 0.013 coupled to Co in S/F1/N/F2/N/F1/S-type junctions 2 ; Co nanowires with W superconducting leads 10 ; and CrO 2 , a half-metallic oxide coupled to NbTi or MoGe superconducting leads 11,12 . In these systems the origin of LRSTPE is more likely to be related to artificially created inhomogeneity, either through the difference in spin scattering generated by the choice of thin films in the multilayer stack or though the particular arrangement that a polycrystalline random alignment of grains may present to the superconducting interface.…”

Phase-dependent heat transport through magnetic Josephson tunnel junctions

“…22 Once generated, triplet correlations can penetrate over long distances into ferromagnets as observed in experiments on SFS Josephson junctions. [23][24][25][26] These experiments suggest the possibility of using SF hybrids in spintronic circuits with the aim of lowering the dissipation. 4 On the other hand, the study of heat transport in nanoscale devices, i.e., caloritronics, also attracts the attention of researchers working on nanodevies [27][28][29] containing, for example, normal metal, ferromagnets 30,31 and superconductors.…”