Anisotropy and effective dimensionality crossover of the fluctuation conductivity of hybrid superconductor/ferromagnet structures

Abstract

We study the fluctuation conductivity of a superconducting film, which is placed to perpendicular nonuniform magnetic field with the amplitude H0 induced by the ferromagnet with domain structure. The conductivity tensor is shown to be essentially anisotropic. The magnitude of this anisotropy is governed by the temperature and the typical width of magnetic domains d. For d≪LH0=Φ0/H0, the difference between diagonal fluctuation conductivity components Δσ∥ along the domain walls and Δσ⊥ across them has the order of (d/LH0)4. In the opposite case, for d≫LH0, the fluctuation conductivity tensor reveals effective dimensionality crossover from standard two-dimensional (T−Tc)−1 behavior well above the critical temperature Tc, to the one-dimensional (T−Tc)−3/2 one close to Tc for Δσ∥, or to the (T−Tc)−1/2 dependence for Δσ⊥. In the intermediate case d≈LH0, for a fixed temperature shift from Tc, the dependence Δσ∥(H0) is shown to have a minimum at H0∼Φ0/d2 while Δσ⊥(H0) is a monotonically increasing function.

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