| Summary: | Magnetic properties of maghemite (γ-Fe<sub>2</sub>O<sub>3</sub>) nanoparticles grown on activated multiwall carbon nanotubes have been studied by alternating current (AC) magnetic susceptibility experiments performed under different temperatures, frequencies, and applied magnetic fields. Transmission electron images have suggested that the γ-Fe<sub>2</sub>O<sub>3</sub> nanoparticles are not isolated and have an average size of 9 nm, but with a relatively broad size distribution. The activation energies of these 9 nm γ-Fe<sub>2</sub>O<sub>3</sub> nanoparticles, determined from the generalized Vogel–Fulcher relation, are reduced upon increasing the direct current (DC) field magnitude. The large activation energy values have indicated the formation of a superspinglass state in the γ-Fe<sub>2</sub>O<sub>3</sub> nanoparticle ensemble, which were not observed for pure γ-Fe<sub>2</sub>O<sub>3</sub> nanoparticles, concluding that the multiwall carbon nanotubes favored the appearance of highly concentrated magnetic regions and hence the formation of superspinglass state. Magnetic relaxation studies, using Argand diagrams recorded for DC probe fields (<20 kOe) below the magnetic blocking temperature at 100 and 10 K, have revealed the presence of more than one relaxation process. The behavior of the ensemble of γ-Fe<sub>2</sub>O<sub>3</sub> nanoparticles can be related to the superspinglass state and is also supported by Almeida–Thouless plots.
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