English abstract
The interplay between the cation distributions and magnetic properties has been demonstrated by studying the effect of cation-doping on the structure and magnetic properties of the cobalt ferrite (CoFe2O4) nanocrystalline system. Single-phased nanocrystalline MexCo1-xFe2O4 materials with (x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0, and Me = Zn2+, Mn2+, Ni2+) have been prepared using sol-gel auto-combustion. The materials were synthesized and characterized using different structural and magnetic characterization techniques . The refinement analysis of X-ray diffraction patterns confirm all samples are crystallized in the (Fd3̅m) space group. The corresponding lattice constants increase with increasing Zn2+ and Mn2+ contents (x), while decrease with increasing Ni2+ content (x). The average crystallite size ranges between 19 and 28 nm for the samples doped with Zn2+ and Mn2+, but show somehow larger with Ni2+ up to 43 nm. Energy dispersive X-ray spectroscopy (EDXS) shows that the weight percentages of the Co and Fe atoms are consistent within 3% accuracy. Transmission electron microscopy (TEM) shows that as Me2+ doping increases, the average particle size decreases. The 57Fe Mössbauer measurements at 295 K show two magnetic sextets superimposed on a paramagnetic doublet for the samples with high Zn2+ and Mn2+ concentrations; which decreases with decreasing the temperature to 78 K. The existence of the doublet for the samples with high Zn concentrations at 78 K suggests that the blocking temperature is still below 78 K. The doublet isomer shifts values for both dopant series show similar values to that measured for the octahedral sub-spectra. The hyperfine fields values for the octahedral and tetrahedral sites are decreased by doping. Mössbauer spectra of the samples doped with Ni2+ did not show any paramagnetic double even at 295 K; suggesting the blocking temperature is above 295 K. The magnetization measurements for the samples doped with Zn2+ and Ni2+ show similar decreasing trend for the saturation magnetization (Ms); while it fluctuates with Mn2+ concentration. The reduction in particle sizes for the samples with high Zn2+ concentration may cause the sharp decrease in Ms. The behavior of other magnetic parameters of all series and their relation with the magnetic super- exchange interactions will be discussed in details.
