Complex magnetic internal order in structurally disordered Ni nanoparticles.

A detailed study of the magnetic properties of colloidal Ni nanoparticles (∼5.2 nm) that do not present long range order is presented. Two magnetic contributions were observed at high temperatures (>25 K), a dominating paramagnetic one and a ferromagnetic contribution, detected by ferromagnetic resonance and low field magnetic measurements. Interestingly, the ferromagnetic behavior does not follow a classical superparamagnetic description. The effective anisotropy increases at low temperatures due to interactions among the ferromagnetic clusters, which leads the system to a frustrated state. [ABSTRACT FROM AUTHOR]