Effect of interface roughness on exchange coupling in polycrystalline Co/CoO bilayer structure: An in-situ investigation

Abstract

The effect of interface roughness on exchange-bias (EB) properties of polycrystalline Co/CoO bilayer structure has been studied in-situ. Isothermal annealing of a 135 Å thick Co layer under the partial pressure of pure oxygen at 573 K results in the formation of a 35 Å thick CoO layer, the surface roughness of which increases with the increasing annealing time. Bilayers were characterized in-situ using magneto-optic Kerr effect, reflection high energy electron diffraction, and x-ray reflectivity for their magnetic and structural properties during each stage of bilayer growth. Combined analysis revealed that the increase in the roughness from 7 ±0.5 Å to 13 ±Å causes the exchange bias field (HEB) to decrease from 171 Oe to 81 Oe, whereas coercivity (HC) increases up to 616 Oe. In contrast to some earlier studies on polycrystalline films, where HEB increased with roughness due to the increase in the uncompensated spins at ferromagnetic-antiferromagnetic (AFM) layer interface, in the present case, dependence of HEB and HC on the roughness is attributed to the disorder at the interface of AFM layer, which leads to a decrease in HEB due to weakening of the effective spin coupling at the interface. Present in-situ experiments make it possible to study the variations in EB properties with the interface roughness in a single sample, and thus avoiding the possibility of the sample to sample variation in the morphological properties along with the change in the interface roughness.