Iron oxide nanoparticles in NaA zeolite cages

Abstract Zeolite NaA samples with varying concentration of Fe 3+ ions have been prepared by wet chemical method. Based on powder X-ray diffraction, 29 Si and 27 Al MAS NMR and Fe 3+ EPR investigations, the formation of nano-sized ferric oxide particles inside the larger α-cages of zeolite NaA has been established. Both Mossbauer effect and magnetization measurements carried out down to 4.5 K established the superparamagnetic behaviour of these Fe 2 O 3 particles with a blocking temperature of ≈20 K, where the magnetization values showed deviation for the zero field cooled and field cooled samples and the appearance of a very narrow magnetic hysteresis loop below this temperature. For all Fe 3+ containing samples the room temperature Mossbauer spectrum is a broad quadrupole doublet with chemical shift, δ  ≈ 0.33 mm/s and quadrupole splitting, Δ E q  ≈ 0.68 mm/s. Variable temperature 57 Fe Mossbauer effect measurements exhibited magnetic features below the blocking temperature and at 4.5 K, the observed spectrum is a broad magnetic sextet characterized by an internal hyperfine field value of ≈504 kOe along with a very weak central superparamagnetic quadrupole doublet.