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

Institute of Advanced Materials, Physicochemical Processes, Nanotechnology and Microsystems, National Center for Scientific Research ‘Demokritos’, 15310, Aghia Paraskevi, Greece

^a Corresponding author: densta@ims.demokritos.gr

Published online: 3 July 2014

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 H_shift^EB and coercive H_c^EB 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, M_f 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 M_f, that is ‘directionally random’, and H_ex, that is ‘directionally oriented’. This weakens the biasing potential of H_ex on M_f and reduces the relative macroscopic parameters H_shift ^EB amd H_c ^EB.