Magnetic ZnFe2O4 nanoferrites studied by X-ray magnetic circular dichroism and Moessbauer spectroscopy

ZnFe2O4 nanoparticles (6 nm) were synthesized by hydrothermal methods. Subsequent mechanical treatment applied to the nanocrystalline material caused an increment of the grain size up to 13 nm. The samples were characterized by X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) at the Fe-K edge and 57Fe Moessbauer spectroscopy. The absorption pre-edge features indicate that Fe3+ ions occupy non-centrosymmetric sites. XAS data evidence the presence of Fe3+ tetrahedrally coordinated while XMCD spectra reflect the magnetic character of the compound The Moessbauer results show a superparamagnetic behaviour with blocking temperatures at 40 and 250 K for 6 and 13 nm samples, respectively. The 4.2 K Moessbauer spectra reflect that Fe3+ ions occupy both octahedral and tetrahedral spinel sites. All these results provide consistent evidence of the high degree of inversion achieved by these combined methods, which modifies the long-range ordering. We also demonstrated that, starting from a non-equilibrium state, an increment of the inversion activated by the milling occurs in spite of the augment of the grain size