Magnetic anisotropy in carbon encapsulated Co/CoO "lines" with large exchange

Carbon encapsulated cobalt nanoparticles were prepared by Co(II) reduction and post-annealing in inert argon gas. Cobalt nanoparticles form parallel "lines" with approximately equal space of 35 mum on the single-crystal silica substrate in the presence of a magnetic field of 3 kGs. Magnetic measurement results indicate that this system exhibits a strong magnetic anisotropy. Along the line direction, it was ferromagnetic with coercive force of 85 Oe, while perpendicular to the substrate plane, only a narrow hysteresis loop was observed. The samples were oxidized at 573 K for various times under air atmosphere. Coercivity was enhanced due to the exchange coupling between ferromagnetic Co and antiferromagnetic CoO. Compared with the random dispersed Co/CoO nanoparticles that was prepared under the same annealing and oxidation condition, a larger exchange bias was obtained in the magnetic aligned specimen. The mechanism of coercivity enhancement and larger exchange bias was discussed. (C) 2002 Elsevier Science B.V. All rights reserved.