Memory Effects in Ultra-Small CoFe $_{2}$ O $_{4}$ Nanoparticles

We have employed the Monte Carlo (MC) simulation technique to study the aging effect on the Zero-Field-Cooled (ZFC) magnetization curves of ultra-small CoFe₂O₄ nanoparticles (mean size ~3 nm) embedded in a Si matrix. We consider spherical nanoparticles consisting of an ordered ferrimagnetic core and a ferrimagnetic disordered surface. The spins in the particles interact with nearest neighbors Heisenberg exchange interaction. Our simulations show that the spin-glass like disorder at the surface affects the magnetic properties to the extent that they exhibit aging effect: the low temperature ZFC magnetization depends on the time (waiting time, t_W) spent before applying the magnetic field at a temperature at which most of the surface moments are frozen. The results of our MC simulations are in good agreement with the experimental findings confirming that the random freezing of surface spins is responsible for the aging effect.