MAGNETIC-ANISOTROPY EFFECTS IN NANO-CLUSTER NICKEL PARTICLES

Nano-cluster Ni particles with average particle size from 12-100 nm we re successfully prepared by the evaporation technique. For samples wit h an average particle size below roughly 50 nm, we observed a nonsymme tric magnetic hysteresis loop and a deviation between the zero field c ooling (ZFC) and the field cooling (FC) magnetizations at temperatures below a critical temperature T-x. T-x is a function of the applied ma gnetic field and the size of the nickel particles which is in general inversely proportional to the saturation magnetization of the nickel p articles. This can be explained by the effect of exchange anisotropy i nteraction between the interfaces of the ferromagnetic region of Ni na no-clusters and the layers of antiferromagnetic NiO on the surface of the nano-cluster Ni particles. A slope variation in magnetization belo w a special temperature T-s (roughly 50 K) for nano-cluster Ni particl es with average particle sizes roughly below 50 nm has been observed. This is explained by a spin flip behaviour near the interfaces between the nano-cluster Ni particles and the layers of NiO on the surface of the Ni particles.