title: Ac-susceptibility investigations of superspin blocking and freezing in interacting magnetic nanoparticle ensembles.
abstract: We have investigated the effect of dipolar interactions on the superspin blocking and freezing of 9 nm average size Fe3O4 magnetic nanoparticle ensembles. Our dynamic susceptibility data reveals a two-regime behavior of the blocking temperature, T(B), upon diluting a Fe3O4/hexane magnetic fluid. As the nanoparticle volume ratio, Φ, is reduced from an as-prepared reference Φ = 1 to Φ = 1/96, the blocking temperature decreases from 46.1 K to 34.2 K, but higher values reenter upon further diluting the magnetic fluid to Φ = 1/384 (where T(B) = 42.5 K). We found evidence that cooling below T B within the higher concentration range (Φ > 1/48) leads to the collective freezing of the superspins, whereas individual superspin blocking occurs in the presence of weaker interactions (Φ < 1/96). The unexpected increase of the blocking temperature with the decrease of the inter-particle interactions observed at low nanoparticle concentrations is well described by the Mørup-Tronc model.
authors.affiliation: Department of Physics, University of Texas at El Paso, 500 W. University Avenue, El Paso, TX 79968, USA.
authors.forename: Cristian E
authors.initials: CE
authors.lastname: Botez
authors.forename: Joshua L
authors.initials: JL
authors.lastname: Morris