The ferromagnetic materials were chosen based on previous work in our group reported in (13, 19) and references therein. Among strong ferromagnetic materials, we and others have found that Ni supports the largest supercurrents. In addition, thin Ni layers have a very high coercive field, so it serves as a good “fixed magnetic layer” in the stack. Two disadvantages of Ni are that it requires a very large initialization field to magnetize it completely, and that for our junction sizes, the Ni nanomagnets are almost certainly not single domain. For that reason, after the initial magnetization at high field, we always keep the applied field low enough to avoid disturbing the domain structure of the Ni. For the free layer, Py (Ni80Fe20) is the best choice we have found to date that provides clean switching between magnetic states at relatively low fields, as seen in Fig. 2. The one disadvantage of Py is that it does not support as much supercurrent as Ni. Typical switching characteristics of elliptically shaped Py and Ni nanomagnets can be seen in Fig. 2 of (30), which shows critical current data of single Josephson junctions (not SQUIDs) containing only two magnetic layers. In the current work, only one of the two junctions in each SQUID is shaped as an ellipse; the other junction is a hexagon with high aspect ratio. The Py layers in the hex junctions generally do not switch as cleanly as those in the elliptical junctions, but, fortunately, they switch at higher field. By keeping Hset sufficiently small, we were able to avoid switching the hex junctions during the experiments. The [Co/Pd] multilayers that form the SAF with PMA were used for the first time in the work reported in (17).

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