Dipolar structures in liquid colloidal dispersions comprising well-defined magnetite (Fe3O4) nanoparticles with a permanent magnetic dipole moment are analyzed on a single-particle level by in situ cryogenic transmission electron microscopy (2D). Compared to conventional ferrofluids, these dispersions are better suited for quantitatively studying the microstructure and thermodynamics of dipolar fluids because of their much lower polydispersity and their better defined shape In zero field, the dipolar particles self-assemble into linear chains, branched clusters and flux-closure rings. Analyzing the cluster-size distribution using a one-dimensional aggregation model yields a dipolar attraction energy that agrees well with the dipole moment found from independent magnetization measurements. The field-induced columnar phase has a liquid-like internal structure, distorted by lens-shaped defects, due to the weak inter-chain attraction relative to field-directed dipole-dipole attraction. Both dipolar coupling and the dipole concentration determine the dimensions and the spatial arrangement of the columns. Furthermore, conclusive experimental evidence is presented for the effect of dipolar chains on the field-dependent magnetization of ferrofluids. Magnetization curves are measured for concentration series of colloidal magnetite dispersions with three different average sizes. At low concentrations or with the smallest particles, the magnetization data obey the Langevin equation for non-interacting dipoles. Magnetization curves for the largest particles, however, strongly deviate from the Langevin equation but quantitatively agree with a recently developed mean-field model that incorporates the field-dependent formation and alignment of flexible dipolar chains. SANS data obtained for 3D magnetite dispersions also demonstrate hexagonal symmetry that results from the formation of magnetic sheets. For decreasing particle dipole moments, the hexagonal symmetry gradually becomes distorted. The SANS data show qualitative agreement with cryo-TEM results obtained in 2D. In addition to the physics of dipolar fluids the chemistry of preparing magnetic nanoparticles is addressed
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