  Magnetization reversal of individual, isolated high-aspect-ratio Fe
nanoparticles with diameters comparable to the magnetic exchange length is
studied by high-sensitivity submicron Hall magnetometry. For a Fe nanoparticle
with diameter of 5 nm, the magnetization reversal is found to be an incoherent
process with localized nucleation assisted by thermal activation, even though
the particle has a single-domain static state. For a larger elongated Fe
nanoparticle with a diameter greater than 10 nm, the inhomogeneous magnetic
structure of the particle plays important role in the reversal process.
