Deterioration of exchange bias in CoO-Co bilayers by the roughness of the ZnO substrates

Abstract

The Exchange Bias (EB) effect is observed at the interface of Antiferromagnet/Ferromagnet (AF/FM) structures and depends on the interface roughness (IR). Until today, only low IR values, usually below 10 nm, have been investigated. Here we investigate an extended range of IR through controlling the surface roughness (SR) of the employed substrates. We employ CoO/Co bilayers (thickness within 10-60 nm), a classic AF/FM structure that exhibits intense EB. ZnO was employed as the substrate in both film and bulk forms, enabling us to vary the SR up to 840 nm. Our data reveal a strong relative decrease, ranging within 20-65%, of both the shift HshiftEB and coercive HcEB fields upon increase of SR (IR), for both parallel and normal magnetic field-sample configurations. For the explanation of these findings we propose that in thin AF/FM structures deposited on rough substrates the local magnetization, Mf of the FM is ‘locked’ mainly in-layer due to shape anisotropy, thus it is forced to follow the morphologically rough landscape of the substrate. This imposes misalignment between Mf, that is ‘directionally random’, and Hex, that is ‘directionally oriented’. This weakens the biasing potential of Hex on Mf and reduces the relative macroscopic parameters Hshift EB amd Hc EB.