AC-susceptibility investigations of superspin blocking and freezing in interacting magnetic nanoparticle ensembles

We have investigated the effect of dipolar interactions on the superspin blocking and freezing of 10 nm average size Fe3O4 magnetic nanoparticle ensembles. Our dynamic susceptibility data reveals a two-regime behavior of the blocking temperature, TB, upon diluting a Fe 3O4/hexane magnetic nanoparticle fluid. As the nanoparticle volume ratio, Phi, is reduced from an as-prepared reference Phi = 1 to Phi = 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 Phi = 1/384 (where TB = 42.5 K). We show that cooling below TB within the higher concentration range (Phi \u3e 1/48) leads to the collective freezing of the superspins in a spin-glass-like fashion, whereas individual superspin blocking occurs in the presence of weaker dipolar interactions (Phi \u3c 1/96). The unexpected increase of the blocking temperature with the decrease of the interparticle interactions observed at low nanoparticle concentrations is well described by the Mørup-Tronc (MT) model