Interlayer coupling in Co/Cu/permalloy/Cu multilayers

The interlayer coupling, domain structure, and microstructure of ion beam sputtered multilayers, were investigated by magnetization, optical microscopy using magnetic colloids, small and wide angle x-ray scattering, and atomic force microscopy measurements. The study focused on multilayers that were composed of ten repeats of Co(1.5 nm)/Cu(t(Cu))/permalloy(2.6 nm)/Cu(t(Cu)) with t(Cu)=2.6 and 5.0 nm. The top surface rms roughness of the multilayers was 0.31 and 0.96 nm for samples with t(Cu)=2.6 and 5.0 nm, respectively. The maximum peak to peak roughness in a 1 mu m(2) scan was 3.0 and 8.9 nm for samples with t(Cu)=2.6 and 5.0 nm, respectively. The lateral feature size was 30-40 nm and 70 nm for samples with t(Cu)=2.6 and 5.0 nm, respectively. The multilayers with t(Cu)=2.6 had sharper, flatter interfaces than those with t(Cu)=5.0 nm. Magnetization measurements and the domain structure at room temperature showed that when t(Cu)=2.6 nm the Co and permalloy layers were ferromagnetically coupled, while if t(Cu)=5.0 nm, the Co and permalloy layers were largely uncoupled. Twin walls present for t(Cu)=2.6 nm clearly indicated locked-in domains of the ferromagnetically coupled Co and permalloy layers; for t(Cu)=5.0 nm, such coupled wall structures were almost nonexistent. Modeling the magnetization, it is estimated that the ferromagnetic coupling only has to be greater than 0.0047 mJ/m(2) for the Co and permalloy layer magnetizations to be nearly parallel at all field values. Since not much coupling is needed, the coupling in the samples with t(Cu)=2.6 nm could be due to the magnetostatic coupling which exists even in perfectly flat films or to the magnetostatic coupling of surface magnetic dipoles created by roughness. (C) 1998 American Institute of Physics.