Nanoparticle clusters of varying morphology (spherical crystal superlattices, spherical amorphous clusters, toroidal clusters, coated polymer beads and Janus nanoparticles) were prepared by the controlled evaporation of the internal phase of an O/W emulsion. These ~100 nm nanoclusters respond to applied magnetic fields to form structured chains, in accord with theoretical predictions. In a second approach, polymer-free magnetic fibers were synthesized by exploiting the dynamic phase separation of a magnetite nanoparticle suspension in a binary solvent mixture with changes in temperature. The phase diagram consists of two different coexistence regions, one due to the simple binary mixture of the solvents, in which the nanoparticle dispersion is stable in one of the two phases, and the other due to particle-particle interactions. The phase-separating domains in which the magnetic nanoparticles were concentrated were elongated into extremely long filaments under a weak external magnetic field near the phase transition temperatures. Rapid quenching, either by lowering the temperature or by the addition excess external phase solvent, stabilized these structures.