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Courant supraconducteur au travers d'un métal ferromagnétique : étude de la jonction pi

Hermann Sellier
Abstract : This work studies different aspects of the proximity effect between a superconductor (S) and a ferromagnetic metal (F). In a normal metal confined between two superconducting electrodes, bound states appear and electron pairs (with opposite spins) can be transferred in a coherent way. The supercurrent carried by these states depends on the phase difference $\phi$ between the two superconductors. In an S/F/S junction, the ferromagnetic exchange energy modifies the bound states spectrum and can reverse the direction of the supercurrent (compared to the S/N/S case). The ground state phase difference is then $\phi=\pi$ (instead of $\phi=0$) and the junction is called a $\pi$-junction. The 0-$\pi$ crossover can be observed as a function of the ferromagnetic thickness, but also as a function of the temperature if the exchange energy is not much larger than the superconducting gap. The 0-$\pi$ crossover is revealed by a non-monotonic dependence of the critical current versus temperature, as we experimentally observed in Nb/Cu$_{52}$Ni$_{48}$/Nb junctions. To obtain this result, we used a very weakly ferromagnetic copper-nickel alloy near the super-paramagnetic limit. The critical current vanishes as a function of the temperature at a value $T^*$ (less than $T_c$): below $T^*$ the junction is in the $\pi$-state, above $T^*$ it is in the 0-state. This behavior is independent of the magnetic field that produces diffraction patterns always centered at zero field. The alternative Josephson effect studied on both sides of the 0-$\pi$ crossover does not provide any difference between the two states. The dependence of the critical current with the ferromagnetic thickness and with temperature can be understood using the Usadel equations. This analysis suggests that a spin-flip diffusion process strongly reduces the critical current amplitude. S/F bilayers also have bound states whose spectrum depends on the ferromagnetic exchange energy and thickness. The superconducting transition temperature exhibits oscillations as a function of these two parameters, because it is sensitive to the energy of these states via the inverse proximity effect. We measured a small signature of this effect in Nb/CuNi bilayers. In F$_1$/S/F$_2$ trilayers, similar to spin-valves, the transition temperature should theoretically depend on the relative direction of the ferromagnetic magnetizations. However we did not measure any difference neither in NiO/Co/Nb/Co nor in NiO/CuNi/Nb/CuNi multilayers, because the niobium thickness below which the superconductivity disappears is several times larger than the coherence length. This saturation of the critical thickness is attributed to a spin-flip process in this very dilute alloy.
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Submitted on : Friday, February 14, 2003 - 11:30:14 AM
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Hermann Sellier. Courant supraconducteur au travers d'un métal ferromagnétique : étude de la jonction pi. Matière Condensée [cond-mat]. Université Joseph-Fourier - Grenoble I, 2002. Français. ⟨tel-00002089⟩

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