Competing interactions in dispersions of superparamagnetic nanoparticles -art. no. 012408

We study the magnetic properties of dispersions of uniformly sized, chemica lly synthesized cobalt nanoparticles (NP's), observing a crossover from a b locked state to a superparamagnetic one with increasing temperature. By ana lyzing magnetization data, we determine the distributions of NP volumes and anisotropies, and establish that variations in the shapes of the magnetic cores of the NP's generate the anisotropy governing the crossover. We chara cterize the frustrated low-temperature state produced by the competition be tween dipolar interactions and anisotropy at sufficiently high NP density t hrough remanent magnetization measurements, and explain the results through analysis of a simple model.