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 $5\mathrm{nm}$, 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 $10\mathrm{nm}$, the inhomogeneous magnetic structure of the particle plays important role in the reversal process.

• Received 6 September 2004

DOI:https://doi.org/10.1103/PhysRevB.71.214425