MAGNETIC PROPERTIES OF NANOSIZED FERRITE POWDERS SYNTHESIZED BY MICROWAVE-HYDROTHERMAL METHOD

摘要

Nanosized powders of Fe_3O_4 and MgFe_2O_4 are synthesized by microwave-hydrothermal (MH) method. The ferrite powders thus obtained are characterized by powder X-ray diffraction, Moessbauer spectroscopy, scanning and transmission electron microscopies, vibrating sample magnetometer and AC magnetic susceptibility studies. Stoichiometric, nanosized (～34 nm) Fe_3O_4 particles, having cubic symmetry with a_0 ≈ 8.39 A, are obtained when molar ratio of Fe/NaOH used for the synthesis is 0.133. On the other hand, non-stoichiometric Fe_3O_4 is obtained when Fe/NaOH molar ratio is increased to 0.4. Though the powder XRD patterns of both Stoichiometric and non-stoichiometric Fe_3O_4 are identical, Mossbauer spectroscopic studies showed different features in terms of asymmetry, intensity, isomer shift, etc. Saturation magnetization, M_s = 70 emu/g and coercivity, H_c = 178 Oe, at room temperature, are obtained for Stoichiometric Fe_3O_4 powders. In the case of MgFe_2O_4, spherical, nanosized (～3 nm) powders with cubic symmetry (a_0 ≈ 8.30 A) and having very narrow size distribution are obtained under the MH conditions. The temperature and field dependent magnetic measurements on MgFe_2O_4 powders confirmed the existence of superparamagnetic state in this material with a superparamagnetic blocking temperature of 38 K below which typical magnetic hysteresis behaviour is observed.