Domain Observations on FeCrFe Layered Structnres. Evidence for a Biquadratic Coupling Effect

The recently discovered oscillating exchange effect in iron–chromium multilayers can best be studied on an epitaxial iron sandwich with a wedge shaped chromium interlayer. Domain patterns are analyzed as well as magnetization curves on such samples using magnetooptical techniques. Unusual domains are found in zones around the transitions between ferromagnetic and antiferromagnetic exchange. Also the magnetization curves for larger chromium thickness show some peculiar features. The interpretation of these patterns invokes coupling between the iron layers which favours a right angle between the magnetization directions in both layers. Such a non-collinear interaction can be derived phenomenologi-cally from a biquadratic coupling as introduced earlier in the analysis of spin structures in oxides. A biquadratic coupling can be understood either as a second order Heisenberg or a second order Dzyaloshinskii interaction. In the course of these investigations domain observations proved to be a useful and sensitive method to detect locally the nature of interactions between ferromagnetic layers.