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: 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 pi-phase coupling regime and find strong nonmonotonic behavior of the anisotropy coefficient gamma(GL)=H(c2 parallel to)(0)/H(c2 perpendicular to)(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-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 pi phase strongly influences the transport properties of S/F/S systems in external fields.