Multilayer Ni 80 Nb 20 /MgO Mirrors Efficiently Reflect Water-Window X-Rays
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ed a switchable superconducting quantum interference device (SQUID) by using novel magnetoquenched superconductor–normal-metal–superconductor (S–N–S) Josephson junctions formed in a loop. This arrangement of superconductor and ferromagnetic films utilized Fe0.1Co0.9 and Pb on polished Si substrates. While the junction properties of most SQUIDs are determined at fabrication, mesoscopic magnetoquenched superconducting valve (MMSV) S–N–S junctions can be switched on and off simply by changing the direction of the stable magnetization state in the ferromagnetic film. The researchers took the MMSV, a bilayer structure that consists of a ferromagnetic film spanning a narrow superconducting bridge, one sizable step further when they formed two superconducting bridges into a loop with one MMSV in each arm. When the MMSV junctions were “on,” the researchers were able to observe and study superconducting quantum interference effects in the device by applying a weak, perpendicular magnetic field. By also switching the junctions “off,” the researchers demonstrated a switchable SQUID. As reported in the September 24 issue of Applied Physics Letters, Eom and Johnson fabricated samples on polished Si substrates with 100-nm-thick silicon nitride cap layers. Next, they thermally deposited 110 nm of Pb in a mostly evacuated environment and then patterned the superconducting bridge using optical lithography and an Ar ion mill. They used optical lithography to define and open a window for deposition of the ferromagnetic film after the surface of the Pb was oxidized by O2 plasma. A 170-nm ferromagnetic layer of Fe0.1Co0.9 was deposited by e-beam evaporation on top of the oxide surface. The completed devices consisted of films less than 300-nm thick. When asked about their device, Johnson said, “The key to our success was simply trying something that nobody thought would work.” Now that their design has worked, Eom and Johnson see it potentially reaching fruition in timedependent magnetization measurements or as nonvolatile circuit elements in digital superconducting electronics. PAMELA JOHNSON
versität Göttingen has made a multilayered mirror from alloy-ceramic oxide— Ni80Nb20-MgO—for water-window soft x-rays (2.2–4.4-nm wavelength range) by pulsed-laser deposition. As reported in the September 15 issue of Optics Letters, the alloy target was prepared from high-purity Ni and Nb powders, while for the MgO target, high-purity single-crystal substrates were used. The mirror was deposited on a silicon substrate, at room temperature, using an excimer laser with 245-nm wavelength, 30-ns pulse width, and 10-Hz pulse rate. A total of 40 bilayers were deposited with a deposition rate of 0.023 nm per pulse for the MgO and 0.0076 nm per pulse for Ni80Nb20. The method used to analyze the resulting multilayered mirror was grazing incidence x-ray scattering with Co-K α radiation.
From the two types of x-ray scattering scans performed (reflectivity and transverse), the researchers obtained information about the growth of the layers and structure of
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