Ion irradiation effects in structural and magnetic properties of Co/Cu multilayers

400 keV Ar ion (the Ar ion) and 50 keV He ion (the He ion) irradiations were performed in order to elucidate roles of Co/Cu interfacial structures in physical origins of giant magnetoresistance (GMR) in the [Co (2 nm)/Cu (2 nm)]30 multilayers (MLs). The magnetoresistance (MR) ratio after the Ar ion irradiation decreases abruptly with increasing Ar ion fluence. On the other hand, the MR ratio after the He ion irradiation decreases slowly with increasing He ion fluence. The Ar ion irradiation induces the decrease in the difference (Rmax − Rsat) between the maximum resistance (Rmax) and the saturated resistance (Rsat) under in-plane magnetic field and the increase in the Rsat, although the effect of the He ion irradiation is not remarkable. The decrease in the (Rmax − Rsat) rather than the increase in the Rsat seems to be effective for the decrease in the MR ratios after the Ar ion and the He ion irradiation. The increase in the Rsat implies the mixing of Co atoms in Cu layers. The antiferromagnetic coupling fraction (AFF) estimated from the magnetization curves after the Ar ion and the He ion irradiation shows the similar behavior with the MR ratio as a function of ion fluence. Therefore, although the degrees of the irradiation effects by the Ar ion and the He ions are different, we suggest the relation between the GMR and the AFF affected by the ion-induced interfacial structures accompanied with the atomic mixing in the interfacial region.