Synthesis and characterization of magnetic iron oxide nanoparticles.

Three high-purity cubic spinel-type crystalline magnetic iron oxides i.e. Fe3O4, CoFe2O4, and NiFe2O4 nanoparticles were successfully synthesized by co-precipitation method. X-ray diffraction (XRD) showed the formation of stoichiometric phases with average particle size of 11.7 nm, 23.6 nm, and 16.4 nm for the as-prepared Fe3O4, CoFe2O4, and NiFe2O4 nanoparticles, respectively. Transmission electron microscopy (TEM) observation for all three samples revealed spherical morphology with single magnetic domain structure. From high resolution TEM (HR-TEM) imaging, lattice fringes with d-spacing of 0.473 nm and 0.248 nm corresponding to (111) and (311) reflections planes, were observed for both the Co-doped and Ni-doped samples. Energy-dispersive x-ray spectroscopy (EDX) analysis showed the presence and homogeneous distribution of main elements Fe, O, Co, and Ni in the samples. Quantitative EDX results confirmed the formation of stoichiometric CoFe2O4 and NiFe2O4 phases with the experimentally measured weight wt% of the samples closely equal to the theoretical calculated wt% values i.e. Fe = 46.35 wt%, O = 26.79 wt%, and Co = 26.87 wt% for CoFe2O4, and Fe = 47.02 wt%, O = 27.27 wt%, and Ni = 24.75 wt% for NiFe2O4. The magnetic properties of these nanoparticles were investigated by 57-Fe Mossbauer spectroscopy (MS) and Vibrating Sample Magnetometer (VSM) techniques. Room temperature MS spectrum for the pure Fe3O4 phase consist of two superimposed sextets with isomer shifts (0.321, 0.463) mm/s and hyperfine field (57.3, 43.4) T attributed to tetrahedral (A-sites) and octahedral (B-sites). The CoFe2O4 and NiFe2O4 samples both showed room temperature MS spectra consisting of two sextets and a single central paramagnetic doublet. The two sextets in each sample had almost equal isomer shifts for both A- and B-sites i.e. 0.2956 & 0.3247 mm/s and 0.3784 & 0.2761 mm/s for each of the sites of the CoFe2O4 and NiFe2O4 sample, respectively. The paramagnetic doublet was fitted with isomer shift of 0.3272 mm/s for the CoFe2O4 sample and 0.3249 mm/s for the NiFe2O4 sample. Temperature dependence M-T magnetization curves measured at H = 500 Oe inthe zero-field-cooled (ZFC) and field-cooled (FC) conditions showed the superparamagnetic nature of all three particles. The MZFC magnetization curve showed a maximum (cusp) at 225 K, 300 K, and 228 K corresponding to blocking temperature (TB), for Fe3O4, CoFe2O4, and NiFe2O4, respectively. For the CoFe2O4 sample the irreversibility temperature (Tirr) was equal to the blocking temperature (TB). While measured Tirr for Fe3O4 and NiFe2O4 was 300 K for both samples. The M-H magnetization curves at 300 K for all three samples revealed the coexistence of ferrimagnetic and superparamagnetic behaviour of the nanoparticles. At 300 K all three samples exhibit symmetrical and almost "closed" hysteresis loops with coercivity approximately 36, 70, and 117 Oe and remanence magnetization of approximately 5, 3, and 4 emu/g, for Fe3O4, NFe2O4, and CoFe2O4, respectively. Furthermore, M-H measurements at 300 K showed a high saturation magnetization of 89 emu/g for the Fe3O4 sample compared to 37 emu/g and 26 emu/g for the CoFe2O4, and NiFe2O4, respectively. M-H measurements recorded at low temperatures showed rather "opened" hysteresis loops compared to loops measured at 300 K. In contrast to saturated magnetization M-H curves for the Fe3O4 and NiFe2O4 nanoparticles, unsaturated M-H loops were observed for CoFe2O4 sample in the temperature range 10 - 100 K. A significant increase in coercivity to 102 Oe, 391 Oe, and 2.4 kOe was observed for Fe3O4, NiFe2O4, and CoFe2O4, respectively, when the temperature was reduced from 300 K to 10 K. For the CoFe2O4 sample, a highest coercivity of 2.7 kOe was measured at 100 K. And finally, M-H data at 10 K showed high saturation magnetization of 100 emu/g, 51 emu/g, and 31 emu/g, for the pure magnetite, CoFe2O4, and NiFe2O4 samples, respectively.