The sample fabrication procedure was described in detail in (17); here, we summarize the main steps. The bottom leads consist of a multilayer of the form [Nb(25)/Al(2.4)]3/Nb(20)/Au(2)/Cu(2)/Py(1.25)/Cu(4)/[Pd(0.9)/Co(0.3)]n/Ru(0.95)/[Co(0.3)/Pd(0.9)]n/Cu(4)/Ni(1.6)/Cu(7)/Au(2), where all thicknesses are in nanometers. The [Nb/Al] multilayer base was used in place of pure Nb because it has a smoother surface and leads to better magnetic switching behavior of the free Py layer [see (17) and references therein]. The [Pd/Co] multilayers have strong PMA, and the Ru(0.95) spacer couples the two PMA multilayers into a SAF. The Cu(4) spacers decouple adjacent magnetic layers, while the bottom Cu(2) spacer facilitates growth of face-centered cubic Py on top of body-centered cubic Nb. The multilayer was sputtered in a seven-gun high-vacuum sputtering system with a base pressure below 2 × 10−8 torr, with the substrate maintained at a temperature between −30° and −15°C. Because the multilayer contains nine different materials while the system contains only seven guns, the sputtering was interrupted after the first Au(2) layer, and the system was opened to replace the Nb and Al guns with the Co and Pd guns. During the gun exchange, the samples were enclosed in a plastic bag filled with flowing N2 gas to limit contamination. After the system was closed, it was pumped down overnight, and the Au(2) was ion-milled away before deposition of the remainder of the stack. The entire bottom multilayer described above was deposited through a photolithographic stencil (with an S1813 photoresist) to define the dimensions of the bottom leads using the lift-off process.

Following lift-off, e-beam lithography and Ar ion milling were used to define the junction areas, using the negative e-beam resist ma-N2401 as the ion mill protective mask. Immediately following ion milling, 50 nm of SiOx was deposited by thermal evaporation for electrical isolation. Then, the samples were ion-milled at glancing angle from two directions to break through the SiOx sidewalls around the junctions. Subsequent lift-off of the ma-N2401 was performed in warm Remover PG with the aid of gentle wiping with a cotton swab. Following lift-off, the sample was subjected to an O2 plasma “descum” process to ensure complete removal of the ma-N2401 from the tops of the junctions. Finally, the top lead pattern was defined with another photolithography step. The protective Au(2) layer was ion-milled away immediately before sputtering the top Nb(150)/Au(10) electrode. Sample fabrication was completed by lift-off of the photoresist.

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