Mapping magnetic fields of Fe₃O₄ nanosphere assemblies by electron holography

Crystalline Fe₃O₄ nanospheres with averaged diameters of 150 nm have been synthesized by a facile solvothermal method and characterized using transmission electron microscopy and electron holography. The nanospheres can self-assemble into either chain-like or ring-like shapes with sizes of a few micrometers, where large magnetic moments are found for individual particles at the remanent state and lead to strong fringing field in vicinity of the assemblies. Magnetic dipolar moments can be aligned both within and out of the sample plane, with a typical length scale on the order of 500 nm.