The magnetic behavior of -Fe/Au nanoparticle (NP) assemblies is studied over a very wide range of dipolar interactions among -Fe NPs, by changing the volume density of the -Fe NP. The assembly whose -Fe NP density is lower than 0.1% exhibits typical superparamagnetic behavior. When Fe NP density exceeds 8.6% the magnetic dynamics changes to that resembling superspin glass. Moreover, NP assembly with highest Fe concentration (43%), whose dipolar interaction is enormously strong compared with previous studies, exhibits a two-stage magnetic transition, i.e., ferromagnetic and spin glass-like transitions at 385 K and around 150 K, respectively. Therefore, we first observed the reentrant spin glass-like magnetism at the limit of strong interaction in a close-packed NP assembly. Based on these observations, the magnetic phase diagram of the interacting -Fe NP assembly is determined over a very wide range of interaction.
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