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

We have measured critical temperatures and upper critical magnetic fields as a function of the ferromagnetic layer thickness, ${d}_{F}$, in two different superconductor(S)/ferromagnet(F)/superconductor(S) triple layers: $\text{Nb}/{\text{Cu}}_{0.41}{\text{Ni}}_{0.59}/\text{Nb}$ and $\text{Nb}/{\text{Pd}}_{0.81}{\text{Ni}}_{0.19}/\text{Nb}$. We vary ${d}_{F}$ from the 0-phase coupling to the $\ensuremath{\pi}$-phase coupling regime and find strong nonmonotonic behavior of the anisotropy coefficient ${\ensuremath{\gamma}}_{GL}={H}_{c2\ensuremath{\parallel}}(0)/{H}_{c2\ensuremath{\perp}}(0)$ characterized by an initial increase, a peak, and a subsequent decrease. The peak is a manifestation of the small coupling which exists around the $0\text{\ensuremath{-}}\ensuremath{\pi}$ transition and it is qualitatively in agreement with recent theoretical predictions [B. Krunavakarn and S. Yoksan, Physica C 440, 25 (2006)] which includes the effect of the different interface transparencies of the two systems. The experimental results demonstrate that the occurrence of the $\ensuremath{\pi}$ phase strongly influences the transport properties of S/F/S systems in external fields.