Abstract
A Co–CoO system was studied to obtain high coercive materials. Co–CoO thin films were sputtered at various oxygen gas flow rates by dual ion-beam sputtering. The Co was deposited onto glass substrates at room temperature and 650 K by argon ion-beam sputtering and simultaneously oxidized by oxygen gas from an assist ion gun. As the oxygen gas flow rate increases, there appear Co, the Co–CoO mixed phase, CoO, and Co2O3 in a stepwise manner. The coercive force increases sharply in the Co–CoO mixed phase region. However, the maximum coercive force at room temperature is small, about 600 Oe. At low temperatures, the addition of the CoO phase to Co causes a significant increase of the coercive force. The perpendicular coercive force of the Co–CoO film is larger than the in-plane coercive force. The perpendicular coercive force is about 17000 Oe at about 10 K. This is due to the magnetic anisotropy of the antiferromagnetic microcrystalline CoO below the Néel temperature of about 220 K. The CoO (111) plane is preferably oriented and the spin axis of CoO stands against the plane, which leads to a strong perpendicular magnetic anisotropy. There is a possibility that the coexistence of the ferromagnetic phase and the antiferromagnetic phase significantly increases the coercive force.