Magnetic anisotropy effects in nano-cluster nickel particles

Nano-cluster Ni particles with average particle size from 12-100 nm were successfully prepared by the evaporation technique. For samples with an average particle size below roughly 50 nm, we observed a nonsymmetric magnetic hysteresis loop and a deviation between the zero field cooling (ZFC) and the field cooling (FC) magnetizations at temperatures below a critical temperature T-x. T-x is a function of the applied magnetic field and the size of the nickel particles which is in general inversely proportional to the saturation magnetization of the nickel particles. This can be explained by the effect of exchange anisotropy interaction between the interfaces of the ferromagnetic region of Ni nano-clusters and the layers of antiferromagnetic NiO on the surface of the nano-cluster Ni particles. A slope variation in magnetization below a special temperature T-s (roughly 50 K) for nano-cluster Ni particles 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.