We report on the design, fabrication, and characterization of superconducting coplanar waveguide resonators with nanoscopic constrictions. By reducing the size of the center line down to 50 nm, the radio frequency currents are concentrated and the magnetic field in its vicinity is increased. The device characteristics are only slightly modified by the constrictions, with changes in resonance frequency lower than 1% and internal quality factors of the same order of magnitude as the original ones. These devices could enable the achievement of higher couplings to small magnetic samples or even to single molecular spins and have applications in circuit quantum electrodynamics, quantum computing, and electron paramagnetic resonance.

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The magnetostrictive stress for a Ho6/Y6 superlattice has been determined at low temperatures by means of a capacitive cantilever technique. The magnetostrictive stress responsible for the basal plane distortion is found to be strongly enhanced with respect to bulk holmium. An explanation accounting for the unusual thermal dependence of that stress is offered. PACS numbers: 75.50.Rr, 75.80.+q Magnetic rare-earth superlattices are a subject of great current interest [1 -7]. Two features are remarkable in