In conventional superconductors, the opposite-spin electrons are bound together to form Cooper pairs (spin-singlet), whereas the electrons in magnetic materials align parallel to each other in the same spin band. When a superconductor is placed in contact with aferromagnet, researchers have learned that the two electrons from the spin-singlet Cooper pair enter different spin bands and rapidly lose phase coherence. The coherence length for the spin-singlet correlation in ferromagnetic materials is around a few nm. On the other hand, some theorists proposed that one could obtain long-ranged spin-triplet correlation in some well engineered superconductor/ferromagnet (S/F) hybrid structures, where the correlation length could be around a few ìm.Indeed, during the last few years, several groups around the world have confirmed the long-range spin-triplet correlation in different systems, including our group. In our case, we made Josephson junctions containing a ferromagnetic multilayer, which can carry spin-triplet supercurrent under certain conditions. At the same time, theorists predicted that the phase of these junctions can be controlled by the magnetizations of the magnetic layers. They predicted the existence of 0 and π junctions. Since our large-area junctions contain multiple domains, we expected to have a random distribution of 0 or π coupling regions across the junction surface, whereas magnetized samples should have uniquely π coupling everywhere according to theories. Indeed we observed the enhancement of the critical current of our Josephson junctions after magnetizing our samples, which indirectly indicated the mixture of 0 and π coupling in the virgin state. According to a random walk model, we would expect that the critical current in the as-grown state would be proportional to the square root of the area of the junction, whereas in magnetized samples it should be proportional to the area. We have measured the area dependence of the critical current in such junctions, and confirm that the critical current scales linearly with area in magnetized junctions. For as-grown (multi-domain) samples, the results are mixed. Samples grown on a thick Nb base exhibit critical currents that scale sub-linearly with area, while samples grown on a smoother Nb/Al multilayer base exhibit critical currents that scale linearly with area. The latter results are consistent with a theoretical picture due to Zyuzin and Spivak that predicts that the as-grown samples should have global π/2 coupling. We even attempted to test the Zyuzin-Spivak prediction by making spin-triplet superconducting quantum interferencedevices (SQUIDs). If the SQUID is made of the two π/2-state junctions, we would expect the flux periodicity to be half compared with traditional SQUIDs. Yet, we have not observedany half quantum flux periodicity in our Nb/Al multilayer based SQUID. Further research will be needed to solve this mystery.
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