Interlayer coupling in Co/Cu/permalloy/Cu multilayers

Abstract

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(tCu)/permalloy(2.6 nm)/Cu(tCu) with tCu=2.6 and 5.0 nm. The top surface rms roughness of the multilayers was 0.31 and 0.96 nm for samples with tCu=2.6 and 5.0 nm, respectively. The maximum peak to peak roughness in a 1 μm2 scan was 3.0 and 8.9 nm for samples with tCu=2.6 and 5.0 nm, respectively. The lateral feature size was 30–40 nm and 70 nm for samples with tCu=2.6 and 5.0 nm, respectively. The multilayers with tCu=2.6 had sharper, flatter interfaces than those with tCu=5.0 nm. Magnetization measurements and the domain structure at room temperature showed that when tCu=2.6 nm the Co and permalloy layers were ferromagnetically coupled, while if tCu=5.0 nm, the Co and permalloy layers were largely uncoupled. Twin walls present for tCu=2.6 nm clearly indicated locked-in domains of the ferromagnetically coupled Co and permalloy layers; for tCu=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/m2 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 tCu=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.