We studied the proximity effect between a superconductor (Nb) and a diluted ferromagnetic alloy (CuNi) in a bilayer geometry. We measured the local density of states on top of the ferromagnetic layer, which thickness varies on each sample, with a very low temperature Scanning Tunneling Microscope. The measured spectra display a very high homogeneity. The analysis of the experimental data shows the need to take into account an additional scattering mechanism. By including in the Usadel equations the effect of the spin relaxation in the ferromagnetic alloy, we obtain a good description of the experimental data.

We investigated the local electronic density of states in superconductor-normal metal (Nb-Au) bilayers using a very low temperature (60 mK) STM. High resolution tunneling spectra measured on the normal metal (Au) surface show a clear proximity effect with an energy gap of reduced amplitude compared to the bulk superconductor (Nb) gap. Within this mini-gap, the density of states does not reach zero and shows clear sub-gap features. We show that the experimental spectra cannot be described with the well-established Usadel equations from the quasi-classical theory.At the contact with a superconductor (S), the Andreev reflection of the electrons from a Normal metal (N) locally modifies the N metal electronic properties, including the local density of states (LDOS) [1]. The precise LDOS spectra depend on the N-S structure geometry, in particular the N metal length, and on the electron transport regime. In a diffusive N-S junction with a N metal shorter than the phase coherence length, one expects a fully opened mini-gap, which remains smaller than the superconductor's energy gap ∆ [2]. In a larger N metal, the LDOS shows, within a pseudo-gap, a linear evolution with the energy, reaching zero precisely at the Fermi level. In the ballistic regime, the same distinction holds between a chaotic (mini-gap) and an integrable (pseudogap) cavity [3]. In general terms, a mini-gap shows up if every electronic state at the Fermi level can couple to the S interface while maintaining quantum phase coherence. The order of magnitude of the energy gap E g is h/τ AR , where τ AR is the characteristic diffusion time before an electron experiences an Andreev reflection. Here, we shall consider the case of diffusive N and S metals brought in contact through a highly transparent interface. Then τ AR ≃ L 2 n /D n and the predicted gap E g is about the Thouless energyhD n /L n 2 , where L n and D n are the length and the diffusion constant of the normal metal.Recently, the LDOS of lateral S-N (Nb-Au) structures was probed with solid tunnel junctions [4] and (very) low temperature STM [5,6]. These studies focused on the pseudo-gap regime in long N metals. Also, a mini-gap was observed in a very thin (20 nm) Au layer on top of a Nb dot [5]. A good agreement with the quasi-classical theory based on the Usadel equations [2] and with the Bogoliubov-de Gennes equations [7] was obtained. In the mini-gap regime, a NbSe 2 crystal covered with a varying thickness of Au was studied by STM at 2.5 K [8] but the temperature did not allow the observation of a fully open mini-gap. Therefore, the mini-gap regime remains to be investigated, in particular its evolution with the normal metal size and the possible presence of states within the gap. A clear distinction between a fully open gap and a pseudo-gap requires a high energy resolution k B T ≪ ∆, which can be achieved only at very low temperatures (T ≪ 1K) and with a large gap ∆.In this paper, we present measurements of the local density of states at the N metal surface of S-N bilayers with a varying N m...

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