Proximity effects, exchange bias and magnetic relaxation in γ-Fe₂O₃ nanoparticles

Carbon-encapsulated γ-Fe₂O₃ nanoparticles (NPs) with emerging proximity effects were synthesized using a single-step solid-state pyrolysis at 750 °C. The morphology and size distribution of the NPs were investigated using high-resolution transmission and scanning electron microscopies revealing that the γ-Fe₂O₃ NPs, with an average diameter of 9 nm, are embedded in the amorphous porous carbon matrix. In addition, other trace phases (Fe₃C and metallic-Fe) were also detected through X-ray absorption spectroscopy and Mössbauer spectrometry. Moreover, the dynamics of the system was explored by means of AC susceptibility, magnetic memory and relaxation measurements, together with low-temperature exchange bias, suggesting the emergence of a spin-glass-like state that remains robust under magnetic cooling fields up to 140 kOe. These findings point to a strong exchange coupling between the spins in the ordered core and those on the disordered surface of the maghemite nanoparticles. Furthermore, the identification of robust spin-glass behaviour under high cooling fields and the demonstration of coexisting blocked and superparamagnetic phases at room temperature provide valuable insights for applications in magnetic data storage and spintronics.