Magnetic relaxation of diluted and self-assembled cobalt nanocrystals

We have studied the magnetic relaxation of monodispersed 4nm cubic ε-cobalt nanocrystals in both randomly oriented and pre-aligned assemblies. The blocking temperature T_B, for the closely packed Co nanocrystal assemblies, is 30% higher than that of the highly diluted and well-dispersed Co nanocrystal-organic composites. This increase is attributed to the strong magnetic dipole interaction induced from the close packing of the nanocrystals. It is found that the frequency-dependent susceptibility data, obtained from the diluted samples, can be fitted to the half-circle Argand Diagrams, indicating a single barrier (or very narrow energy distribution) of the nanocrystals. This agrees well with the physical observation from TEM that the nanocrystals are monodispersed. The long time magnetic relaxation measurements reveal that energy barrier distribution in a pre-aligned nanocrystal assembly is significantly different from that in a randomly oriented one.