The low temperature dynamic magnetic properties of two dense magnetic nanoparticle assemblies with similar superspin glass transition temperatures $T_g\sim 140$ K are compared. The two samples are made from batches of 6 and 8 nm monodisperse $\gamma \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$ nanoparticles, respectively. The properties of the individual particles are extracted from measurements on reference samples where the particles have been covered with a thick silica coating. The blocking temperatures of these dilute assemblies are found at 12.5 K for the 6 nm particles and at 35 K for the 8 nm particles, which implies different anisotropy energy barriers of the individual particles and vastly different temperature evolution of their relaxation times. The results of the measurements on the concentrated particle assemblies suggest a strong influence of the particle energy barrier on the details of the aging dynamics, memory behavior, and apparent superspin dimensionality of the particles.

• Received 21 December 2015

DOI:https://doi.org/10.1103/PhysRevB.93.054407