Supra-oscillatory critical temperature dependence of Nb-Ho bilayers
We investigate the critical temperature Tc of a thin s-wave superconductor (Nb) proximity coupled to a helical rare-earth ferromagnet (Ho). As a function of the Ho layer thickness, we observe multiple oscillations of Tc superimposed on a slow decay, that we attribute to the influence of the Ho on the Nb proximity effect. Because of the inhomogeneous magnetization in Ho, singlet and triplet pair correlations are present in the bilayers. We take both into consideration when solving the self-consistent Bogoliubov-de Gennes equations, and we find a reasonable agreement with the experiments. We also observe non-trivial transitions into the superconducting state, the zero resistance state being attained after two successive transitions which appear to be associated with the magnetic structure of Ho.