Layered ferromagnet/superconductor heterostructures: Nonequilibrium quasiparticle dynamics and photodetector applications

Abstract

Time-resolved optical pump-probe and photoimpedance studies of proximized ferromagnet/superconductor nanobilayers are presented. The weak ferromagnetic nature of an ultrathin ${\mathrm{Ni}}_{0.48}{\mathrm{Cu}}_{0.52}$ film makes it possible to observe the dynamics of the nonequilibrium superconductivity in $\mathrm{Ni}\mathrm{Cu}∕\mathrm{Nb}$ hybrids through time-resolved measurements of a near-surface optical reflectivity change, which is generated by femtosecond optical pump pulses and discussed within a nonequilibrium two-temperature electron-heating model. We observed that the NiCu overlay significantly reduced the slow bolometric contribution present in the photoresponse of a pure Nb film, resulting in a strong enhancement of the nonequilibrium kinetic-inductive component of the transient photoimpedance, measured as an $\ensuremath{\sim}700\text{\ensuremath{-}}\mathrm{ps}$-wide voltage waveform generated across an optically excited current-biased $\mathrm{Ni}\mathrm{Cu}∕\mathrm{Nb}$ bilayer microbridge. The sensitive picosecond photoresponse makes our $\mathrm{Ni}\mathrm{Cu}∕\mathrm{Nb}$ heterostructures suitable for ``engineered'' ultrafast superconducting photodetectors since the photoimpedance signals observed in plain Nb bridges were at least $10\phantom{\rule{0.3em}{0ex}}\mathrm{ns}$ long and were due to the light-induced simple-heating effect.