Supercurrents in half-metallic ferromagnets

Start date 01/01/2006
Left panel: Conversion between singlet and triplet supercurr
Left panel: Conversion between singlet and triplet supercurrents takes place in the superconductors within a coherence length from the interfaces. Right panel: The precession of a spin around the magnetization M when the electron wave packet penetrates into the classically forbidden interface region leads to spin-dependent phase shifts that play an important role in the conversion process.
Half-metallic ferromagnets have great potential in the field of spintronics as sources of spin-polarized electric currents. Depending on the direction of the electron spin, they show conducting or insulating behavior. Since only a few 100% spin polarized ferromagnets are known, among them La2/3Ca1/3MnO3 and CrO2, their characterization has attracted great attention.
Recently, a Josephson supercurrent was observed between two singlet superconductors (NbTiN) separated by a wide region of CrO2. In the half-metal, only equal spin Cooper pairs can carry the supercurrent since the other spin species is insulating. The mechanism of conversion from singlet Cooper pairs and equal spin Cooper pairs at the interfaces between the materials remained controversial. In a recent publication [1], we have suggested a conversion mechanism based on electron spin precession together with triplet-pair rotation at interfaces with broken spin-rotation symmetry. In the diffusive limit of short mean free path, the triplet supercurrent is dominated by inter-related odd-frequency s-wave and even-frequency p-wave pairs. In the crossover to the ballistic limit, further symmetry components become relevant.
[1] M. Eschrig and T. Löfwander, Nature Physics 4, 138 (2008)
M. Eschrig, University Karlsruhe

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