The effects of post-deposition ion-beam bombardment with oxygen on the Co surface in modifying the magnetic properties of Co thin films

Ion beam bombardment is a useful surface modification method in tailoring the microstructure and magnetic properties of ferromagnetic thin films. In this paper, Co thin films were bombarded by Ar⁺ ion beam with oxygen and then capped with Al (to prevent oxidation). The oxygen content was altered from 0 to 41% to investigate the ion-beam bombardment effect. An oxide layer containing Al₂O₃, Co₃O₄ and CoO was formed after the ion-beam bombardment with oxygen, as characterized by x-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, selected area electron diffraction, and x-ray diffraction. Enhanced coercivities (H_c's) (from 4.14 kA/m to 7.96 kA/m) and loop squareness (M_r/M_s. from 0.5 to 0.8) was induced by pure Ar⁺ ion-beam bombardment that resulted in domain wall pinning due to bombardment-induced defects and Co spin reordering. H_c exhibited a strong dependence on the oxygen content in the ion beam, due to the nature of the cobalt oxide formation. Field cooling to 180 K resulted in increased squareness (M_r/M_s. = 0.9) and reduced H_c. The low-field magnetization measured during zero field and field cooling to further investigate the exchange bias magnetism identified an irreversibility temperature at 320 K in a film treated by ion-beam bombardment with an O₂/Ar ratio of 21%.