Collective magnetic states of ferromagnetic nanoparticles in the superspin limit

摘要

Soft ferromagnetic Co_(80)Fe_(20) nanoparticles embedded discontinuously in [Co_(80)Fe_(20)(t_n)/Al_2O_3]_m multilayers are treated as superspins with random size, position and anisotropy. Freezing into collective states rather than single particle blocking is encountered at low temperatures. At low particle density, t_n < 1.1 nm, one observes memory and aging of a superspin glass (SSG), while at higher densities, t_n > 1.2 nm, superferro-magnetic (SFM) domain wall relaxation is, e.g., evidenced by polarized neutron reflectivity. Thermore-manent logarithmic decay rates reveal a universal power law with exponents changing from n < 1 for SSG to n > 1 for SFM in agreement with recent Monte Carlo simulations. At intermediate densities, a crossover occurs from n < 1 with Tln(t/τ) scaling at low T to n > 1 as T → T_c.