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Temperature and frequency dependence of the AC magnetic susceptibilities are measured for 13 nm magnetite in aqueous and in dried samples. Two thermal activation type relaxation peaks were observed in the same sample. Quantitative analyses using the Néel-Arrhenius and the Vogel-Fulcher models show that a single particle relaxation from isolated nanoparticles embedded in a frozen carrier fluid and a relaxation from clustered particles coexist. Particles in the clusters experience an additional energy barrier represented by a simple shift T0 ~ 61 K in imaginary part of susceptibility due to the inter-particle dipolar interaction.