NONCOLLINEAR AND COLLINEAR MAGNETIC-STRUCTURES IN EXCHANGE-COUPLED FECR(001) SUPERLATTICES/

The magnetic and structural properties of molecular beam epitaxy grown Fe/Cr(001) superlattices were studied as a function of the growth tem perature T-g using polarized neutron reflectometry (PNR) with polariza tion analysis, magneto-optic Kerr effect (h?OKE), and x-ray-scattering techniques. From MOKE and PNR as a function of external field we fmd strong noncollinear coupling between the Fe layers and a so far unexpe cted coupling angle of 50 degrees near remanence for a sample grown at T-g=250 degrees C. A detailed discussion of the domain structure of t he sample near remanence confirms the modeling. On the other hand, an otherwise equivalent sample grown at room temperature exhibits complet ely ferromagnetic or uncoupled behavior. Using diffuse x-ray-scatterin g methods these distinct differences in the magnetic structure are fou nd to be correlated with a growth temperature dependent length scale o f constant Cr interlayer thickness l(Cr). We find that l(Cr) increases significantly with T-g. These results are discussed in the framework of current theories of noncollinear exchange. It is demonstrated that the bilinear-biquadratic formalism used so far is inconsistent with th e data. The Cr specific proximity magnetism model is discussed which e xplains the occurrence of noncollinear coupling for systems with Cr in terlayer thickness fluctuations on the length scale observed here for T-g=250 degrees C. The model yields an exchange energy different from the bilinear-biquadratic formalism used so far, explaining the asympto tic approach to saturation observed by MOKE.