Abstract
Soft ferromagnetic Co80Fe20 nanoparticles embedded discontinuously in [Co80Fe20(tn)/Al2O3]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, tn 1.2 nm, superferromagnetic (SFM) domain wall relaxation is, e.g., evidenced by polarized neutron reflectivity. Thermoremanent logarithmic decay rates reveal a universal power law with exponents changing from n 1 for SFM in agreement with recent Monte Carlo simulations. At intermediate densities, a crossover occurs from n 1 as T Tc. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
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