When a conventional superconductor (S) is placed in contact with a ferromagnet (F), the decay length of the pair correlations in the ferromagnet is very short, on the order of a nm in a strong ferromagnet such as Co or Fe. This is due to the spin-polarized nature of the ferromagnet, whereas the spins of Cooper pairs in a conventional superconductor are anti-aligned in a spin-singlet state. However, in 2001, theorists predicted that long-range pair correlations in a spin-triplet state could be generated through magnetic inhomogeneity. With parallel spins, the decay length of these correlations extends in principle to that of a superconductor-normal metal system, which can be on the order of a micron at sufficiently low temperature.This effect has been observed experimentally by several groups, commonly through the use of extrinsic magnetic inhomogeneity in samples with multiple magnetic layers. Josephson junction measurements have demonstrated critical currents orders of magnitude larger in samples with this inhomogeneity compared to samples without. However, the ability to reliably control the spin-state of the pair correlations in a single sample has yet to be realized.The goal of this work is to perform measurements on Josephson junctions in which the inhomogeneity can be manipulated. Our approach is to fabricate S/F'/F/F"/S Josephson junctions where we can control the relative magnetization orientations of all three ferromagnetic layers. In order to realize this control, we first had to perform studies to characterize various magnetic materials, most notably a NiFe alloy similar to Permalloy and Co/Ru/Co, a synthetic antiferromagnet. Studies of the NiFe have demonstrated its ability to be used as a spin-triplet generator. Measurements have also been taken of NiFe films to determine how easily its magnetization direction can be rotated in an external field. We have also measured the magnetic hardness of Co/Ru/Co synthetic antiferromagnets as a function of the Co thickness. By keeping the Co thin, we can minimize the rotation of this layer under the influence of small applied magnetic fields.Using these results, we demonstrate amplitude modulation of the supercurrent in S/F'/F/F"/S Josephson junctions which is dependent on the magnetization direction of NiFe. Through the use of an external field, the magnetization of the NiFe F" layer can be rotated with respect to the magnetization of the Co/Ru/Co F layer. By rotating into and out of a non-collinear state, we have demonstrated the ability to tune the supercurrent from a spin-triplet to a spin-singlet state, effectively turning the supercurrent in these junctions "on" and "off."
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