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Magnetic relaxation phenomena and inter-particle interactions in nanosized γ-Fe2O3 systems

Samples of γ-Fe2O3 nano-particles with a mean size of 4.0(3) nm and with different hydration and surfactant degrees were prepared by sol–gel methods. Morphology and structural data were obtained by transmission electron microscopy and x-ray diffraction, whereas the surface effects and hyperfine interactions were analysed mainly by Mössbauer spectroscopy. The relative number of surface iron positions was found to be proportional to the amount of OH− and SO42− groups on the particle surface, which in turn is strictly dependent on the preparation conditions. Strong relaxation processes versus temperature were evidenced in the analysed systems. New criteria for the evaluation of the blocking temperature via Mössbauer measurements are proposed. The results are in good agreement with blocking temperatures obtained by magnetic measurements. Moreover, it was shown that the inter-particle magnetic interactions decrease with the number of iron surface states.