Crossover between strong- and weak-field critical adsorption and the determination of the universal exponent η⊥

Abstract

We report a temperature-dependent x-ray diffraction study of critical behavior of sublattice order in thin epitaxial FeCo(001) films grown on MgO(001). The quantitative analysis of the diffraction profiles reveals a crossover phenomenon between strong-field and weak-field critical adsorption which occurs at a reduced temperature t1=1.3×10−3. We demonstrate that this scenario gives a first experimental access to the universal critical exponent η⊥ which governs the spatial decay of critical fluctuations perpendicular to the surface. We find η⊥=0.86(5).