Enhancement of magnetic coercivity and macroscopic quantum tunneling in monodispersed Co/CoO cluster assemblies

Magnetic properties have been measured for monodisperse-sized Co/CoO cluster assemblies prepared by a plasma-gas-condensation-type cluster beam deposition technique. The clear correlation obtained between exchange bias field and coercivity suggests the enhancement of uniaxial anisotropy owing to the exchange coupling between the ferromagnetic Co core and antiferromagnetic CoO shell, and magnetic disorder at the core-shell interface. A nonthermal magnetic relaxation observed below 8 K, being referred to as macroscopic quantum tunneling of the magnetization, is ascribed to the enhanced uniaxial anisotropy. (C) 1999 American Institute of Physics. [S0003-6951(99)02150-6].