Improvement of the thermal stability of nanomaghemite by functionalization with type 5A zeolite and magnetic properties studied by in-field ⁵⁷Fe Mössbauer measurements

Maghemite nanoparticles functionalized with type 5A zeolite were prepared by co-precipitation to produce three different nanohybrid materials with potential for application in water remediation process. Raman and in-field ⁵⁷Fe Mössbauer spectroscopies were used to bring information about structural and magnetic properties of these materials, while their magnetic properties were obtained from Direct Current magnetization experiments. The Raman data suggested the maghemite to hematite phase transition when two of the samples were exposed to laser power above 4.14 mW. For the two zeolite-based samples that exhibited a secondary goethite phase, the temperature-dependent Mössbauer results indicated low magnetic anisotropy for this phase, an effect associated with its low crystallinity. The spin structure of maghemite nanoparticles is described using the core-shell model, where ferrimagnetic core grains are easily polarized by external magnetic fields and are surrounded by non-collinear spins (canted spins) originating from the particle surface. No strong magnetic interaction between the maghemite nanoparticles and the goethite phase that would yield the exchange bias effect was observed; this may be due to the low magnetic anisotropy of the antiferromagnetic goethite, as suggested by the low crystallinity measured by Raman spectroscopy. Minor loop effects that occurred for in field-cooling loops are recorded in low scan fields due to the large magnetic anisotropy of the superspinglass state of the maghemite nanoparticles established by dipolar magnetic interactions.