We report on results of computer micromodelling of anti-vortex states in asymmetrical cross-like ferromagnetic nanostructures and their practical realization. The arrays of cobalt crosses with 1 μm branches, 100 nm widths of the branches and 40 nm thicknesses were fabricated using e-beam lithography and ion etching. Each branch of the cross was tapered at one end and bulbous at the other. The stable formation of anti-vortex magnetic states in these nanostructures during magnetization reversal was demonstrated experimentally using magnetic force microscopy.
We experimentally study the influence of 1-40 GHz radiation on the resistance of normal (N) mesoscopic conductors coupled to superconducting (S) loops (Andreev interferometers). At low radio-frequency (RF) amplitudes we observe the usual h/2e superconducting phase periodic resistance oscillations as a function of applied magnetic flux. We find that the oscillations acquire a π-shift with increasing RF amplitude, and consistently with this result the resistance at fixed phase is an oscillating function of the RF amplitude. The results are explained qualitatively as a consequence of two processes. The first is the modulation of the phase difference between the N/S interfaces by the RF field, with the resistance adiabatically following the phase. The second process is the change in the electron temperature caused by the RF field. From the data, the response time of the Andreev interferometer is estimated to be τ(f) < 40 ps. However there are a number of experimental features which remain unexplained; these include the drastic difference in behaviour of the resistance at ϕ = π and 0 as a function of the RF frequency and amplitude, and the existence of a 'window of transparency' where heating effects are weak enough to allow for the π-shift. A microscopic theory describing the influence of RF radiation on Andreev interferometers is required.
We have developed a new type of magnetometer consisting of a Hybrid Quantum Interference Device (HyQUID) that is set in a bi-stable state. We demonstrate its operation in a latching mode that can be employed to measure small changes in the applied flux. The device can be used to probe the flux state of a superconducting circuit using straightforward electrical resistance measurements, making it suitable as a simple qubit readout with low back-action.
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