Transport properties of superconducting - ferromagnetic nanowire devices*

The interplay between superconductivity and magnetism is interesting to basic science and applied physics. Here we propose to use the superconducting (SC) nanowires as an ultrafast probe of the magnetization dynamics, focusing on the vortex and antivortex spin state as a model system. While the vortex dynamics is well-studied and understood, the magnetic state described as an antivortex remains elusive and lacks experimental works owing to its metastability. In this work, we concidered elliptically shaped particles that can support single- and multi-vortex states. Using high-amplitude excitation at one of the eigenmode frequencies, we were able to induce a dynamic transition from a single vortex into vortex-antivortex-vortex (v-av-v) states. A newly formed state is stable in remanence and can be probed experimentally. To this end, we are developing SC nanowire devices integrated with magnetic particles. In our device, a lithographically defined sub-100nm NbN wire is biased close to its critical current Ic, where it loses superconductivity. An oscilating demagnetizing field of magnetic particle at its resonance interacts with Cooper pairs in the superconductor, exciting quasi-electrons that release their energy into the SC condensate through many inelastic collision processes. This results into the formation of a normal conducting hotspot roughly of the size of the coherence length, which diverts the supercurrent through the zero-resistance area around it. As the wire is biased close to Ic, the constricted area will exceed critical current densities and cannot persist in an SC state, which will cause a runaway annihilation of superconductivity until the whole wire cross-section turns normal. This process translates into an immediate spike in resistance with sub-10ps rise time, which triggers active or passive quenching that diverts the current from the wire and allows it to relax back into an equilibrium SC state. Once demonstrated experimentally, this approach can be used to characterize various nanomagnetic systems at low temperatures, including the v-av-v states as a function of the combination of the vortex core polarities and/or the excitation field direction.