Comparison of reversible and irreversible dipolar assemblies in a ferrofluid

摘要: Zero-field aggregation of magnetic nanoparticles in a ferrofluid can either be irreversible or result from dynamic equilibrium; the two cases distinguished by measurements complex susceptibility and cryogenic transmission electron microscopy (cryo-TEM). We demonstrate this comparing colloidal systems that show dipolar structure formation zero field. A dispersion iron is gradually oxidized to decrease moments, despite vanishing attractions, thermal motion does not break up structures into single particles. Instead, become chemically fixed during oxidation process, an example aggregation. In contrast, zero-field stable magnetite are found disintegrate upon dilution, indicating reversible equilibrium.