Domain state model for exchange bias. II. Experiments

The exchange bias coupling at ferro-/antiferromagnetic interfaces of epitaxially grown Co/CoO bilayers can be intentionally enhanced and controlled by diluting the antiferromagnetic CoO layer, i.e., by introducing (i) nonmagnetic substitutions $({\mathrm{Co}}_{1\ensuremath{-}x}{\mathrm{Mg}}_{x}\mathrm{O})$ or (ii) Co deficiencies $({\mathrm{Co}}_{1\ensuremath{-}y}\mathrm{O}).$ All intentional nonmagnetic cations or defects were placed away from the interface throughout the whole volume part of the antiferromagnetic layer. This way the roughness at the Co/CoO interface was kept practically the same. For both types of defects, the exchange bias field can be increased by a factor of 3 to 4. Hence, exchange bias is primarily not due to roughness at the interface but rather can be controlled by the defects in the volume part of the antiferromagnetic layer. We systematically investigate the dilution dependence of various phenomena of exchange bias, such as the vertical magnetization shift of the hysteresis loop, temperature dependence, training effect, cooling field dependence, and antiferromagnetic layer thickness dependence. All these phenomena are directly compared to results from Monte Carlo simulations and are shown to be consistently described by the domain state model for exchange bias. The combined experimental and theoretical findings suggest that the origin of exchange bias in Co/CoO results from a domain state in the volume part of the antiferromagnet stabilized by the defects.