Exchange-coupled MBE-grown Co/Cu/Co(111) trilayers

It is shown that the high-field curvature in the in-plane and polar magnetization curves of exchange-coupled MBE-grown Co/Cu/Co(111) structures can be accurately fitted by assuming a sizeable biquadratic coupling in addition to the bilinear coupling, and the existence of a large areal fraction of ferromagnetic inclusions. Measurements on a sample with a single Co layer confirm that the curvature does not originate from the individual Co layers. Analysis of two Co/Cu/Co(111) trilayers with different Co-Cu interface qualities, as measured by NMR, shows significantly higher bilinear and biquadratic coupling strengths for the trilayer with the better defined Co-Cu interfaces. While NMR indicates reasonably good structural quality on a short lateral length scale, polar Kerr measurements as a function of interlayer thickness, and polarized neutron reflection measurements, indicate the existence of structural imperfections on a long lateral length scale.