Triplets Proliferate

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Science  06 Apr 2012:
Vol. 336, Issue 6077, pp. 13
DOI: 10.1126/science.336.6077.13-a

If a normal material is sandwiched between two superconductors, Cooper pairs—the electron pairs of opposite spin that cause superconductivity—can tunnel through it, forming a supercurrent. If, however, the middle material is a ferromagnet, it will tend to align the spins of the electron pairs, drastically reducing the supercurrent. One way around this is to use an inhomogeneously magnetic material, which can help to create a triplet supercurrent, where the pairs have parallel spin—a phenomenon rarely observed in natural materials. Klose et al. show that by applying an in-plane magnetic field to a junction with two magnetic materials, Co and Ni, the value of the supercurrent can be increased up to 20 times. When the magnetizations of the junction materials are parallel, no triplet supercurrent should be observed; the more orthogonal they are, the larger the supercurrent. Polarized scattering and microscopy experiments showed that the in-plane field caused a spin-flop transition in the Co layers, enhancing orthogonality with the Ni layers and thus increasing the supercurrent, making it possible to control these junctions with external magnetic fields.

Phys. Rev. Lett. 108, 127002 (2012).

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