Nonmonotonic behavior of the anisotropy coefficient in superconductor-ferromagnet-superconductor trilayers

Abstract

We have measured critical temperatures and upper critical magnetic fields as a function of the ferromagnetic layer thickness, d F , in two different superconductor͑S͒/ferromagnet͑F͒/superconductor͑S͒ triple layers: Nb/ Cu 0.41 Ni 0.59 / Nb and Nb/ Pd 0.81 Ni 0.19 / Nb. We vary d F from the 0-phase coupling to the -phase coupling regime and find strong nonmonotonic behavior of the anisotropy coefficient ␥ GL = H c2ʈ ͑0͒ / H c2Ќ ͑0͒ characterized by an initial increase, a peak, and a subsequent decrease. The peak is a manifestation of the small coupling which exists around the 0-transition and it is qualitatively in agreement with recent theoretical predictions ͓B. Krunavakarn and S. Yoksan, Physica C 440, 25 ͑2006͔͒ which includes the effect of the different interface transparencies of the two systems. The experimental results demonstrate that the occurrence of the phase strongly influences the transport properties of S/F/S systems in external fields. The proximity effect in superconductor ͑S͒/ferromagnet ͑F͒ hybrids has recently attracted a lot of interest due to the inhomogeneous nature of the superconducting order parameter in these structures.1-3 One of the most relevant consequences of the peculiar character of the order parameter is the nonmonotonic behavior of the superconducting critical temperature T c as a function of the thickness d F of the F layer which has been observed in many S/F heterostructures.4-6 Also, in the so-called S/F/S Josephson junctions negative critical currents have been measured. [7][8][9] What essentially happens is that the interaction of the Cooper pairs with the exchange field E ex causes the order parameter to oscillate on the F side of the interface over a distance F , the coherence length in the ferromagnet. On the other hand, on the S side, the order parameter is strongly suppressed over a distance of the order of the superconducting coherence length S , which, in conventional superconductors such as Nb, is usually of a few nanometers. In weak ferromagnetic alloys such as PdNi and CuNi, due to the smaller value of E ex , F is of the order of some nanometers. In the dirty limitwhere D F is the diffusion coefficient of the F metal. Another characteristic length introduced when studying S/F hybrids is F ‫ء‬ = ͱ បD F / 2k B T c which is a measure of the diffusive motion of the Cooper pairs in the ferromagnet and which will be needed in order to compare our data with theoretical calculations. However, the strength of the proximity effect between the S and F layers depends also on the quality of the interfaces. An important parameter in the theoretical description therefore is the interface transparency, T.10,11 Its influence on the behavior of the T c both as a function of the thickness d S of the S layer and of d F has been studied both in Nb/CuNi and Nb/PdNi bilayers 12,13 and also, more recently, the behavior of the parallel upper critical field in these systems has been considered.14 All these studies revealed a somehow higher value of the interface transparency in the Nb/PdN...

Superconducting and Structural Properties of Nb/PdNi/Nb Trilayers

“…The thickness of the samples has been carefully calibrated against the deposition rate. As expected, the critical temperature T c dropped with increasing d F [11], and we obtained T c = 7.5, 6.2, 5.1, 4.1 K in the samples with d F = 0 (pure Nb), 1, 2, 8 nm, respectively.…”

Non-monotonic behaviour of the superconducting order parameter in Nb/PdNi bilayers observed through point contact spectroscopy

“…The critical temperature rapidly decreases with increasing the thickness of the ferromagnetic layer, showing a minimum around d PdNi ≈ 5 nm. The value of the dip position scales reasonably well with the ones obtained for Nb/PdNi systems for different Ni concentrations of the alloy [19,20,24].…”

“…The S/F bilayers consist of a 40-nm thick Nb layer, and of a weakly ferromagnetic alloy, Pd 0.84 Ni 0.16 (=PdNi) layer, with variable thickness, d PdNi = 2-4-5-9 nm. This system was chosen since the existence of an inhomogeneous superconducting order parameter in F, peculiar characteristic of S/F hybrids [1,2,6,15,16], has been already demonstrated for Nb/Pd 1-x Ni x systems [19][20][21][22][24][25][26]. Moreover, PdNi is characterized by longer spin-flip scattering length compared to the widely used CuNi alloy [27].…”

Experimental and theoretical analysis of the upper critical field in ferromagnet–superconductor–ferromagnet trilayers

“…In the case of a coupling, a 2D-3D crossover occurs [34,38]. There are also studies of H c2 in ferromagnetic alloy -superconductor heterostructures [40,41,42], where 2D-3D crossover, the flux pinning mechanism and the anisotropy coefficient, γ GL =H c2 (0)/H difference in the phase of the superconductig order parameter between S-layers can vary what significantly affects on T c and complicates the experimental detection of oscillatory T c (d F ) behavior [39]. Therefore, to explore experimentally the evolution of the T c (d F ) dependence from oscillatory type to reentrant one under applied magnetic field we have chosen the system with sole superconducting layer.…”