Synthesis and Assembly of Magnetite Nanocubes into Flux-Closure Rings

Single-crystalline magnetite nanoparticles with a relatively narrow size distribution around 48 nm were solvothermally synthesized in a water−alcohol mixed solvent solution at 230 °C using ferrocene ((C5H5)2Fe), polyvinylpyrrolidone (PVP), and hydrogen peroxide (H2O2) as starting materials. These magnetite nanoparticles exhibited almost standard cube-like shape and were coated with a thick PVP layer (∼8 nm) to form a magnetite−PVP core−shell structure. Driven by strong magnetic dipolar attractions, magnetite nanocubes could be assembled into flux-closure rings, which consisted of several to dozens of nanocubes. The rings had one-particle annular thickness, and individual nanocubes were spaced fully together. It was found that only nanocubes, the average size of which was close to 50 nm, could be assembled into rings, while slightly smaller particles were aligned in dipolar chains, suggesting that ring self-assembly could be produced by the degradation of dipolar chains that were a metastable structure with respect to rings, and only larger nanocubes with strong magnetic dipoles could overcome the potential barrier for the transformation from chains to rings.