Preparation and characterization of Nickel-and cobalt-doped magnetites

Abstract

Nickel-and cobalt-doped magnetites were prepared by a co-precipitation method and studied in some detail, in an effort to identify some effects of the doping cations on the magnetic, crystallographic and morphological properties of the resulting spinel. The synthetic samples were characterized by conventional chemical analysis, powder X-ray diffractometry, Mössbauer spectroscopy, saturation magnetization and scanning electron microscopy. From chemical analysis , the continuous increase of Ni 2+ or Co 2+ is accompanied by a simultaneous decrease of the Fe 2+ contents, in the spinel structure. The magnetization values also decrease continuously with increasing doping cation contents. Mössbauer parameters are characteristic of substituted magnetites and indicate the presence of a single phase only. Based on the inverted intensities of the lines 1 (leftmost, on the negative Doppler velocity scale) and 2 of Mössbauer spectra of doped samples, relatively to the pure magnetite, it was assumed that the isomorphical substitution occurs preferentially on octahedral coordination sites of the spinel structure. The coercive field of these ferrites decrease steadily with Ni 2+ but increases with Co 2+ contents, reaching a maximum at x = 0.38, in the general formula Co x Fe 3-x O 4. of the doped-magnetite may vary, depending on the synthesis conditions 6,7. The replacement of Fe 2+ by Co 2+ or Ni 2+ does not change essentially the nature of crystallographic structure but its unit cell dimension. The cation distribution in spinels has long been a topic of interest as it affects their magnetic, electric and thermodynamic propertiesb 8-10. In addition, it has been found that ferrite particles of similar composition differ on their magnetic properties depending on the preparation method. One reason for such a behavior is believed to be differences in particle size. Decreasing the particle sizes leads to an increase of non-magnetic species on the particle surface 6. Various preparation procedures, including hydrothermal, co-precipition, sol-gel methods and mechanical alloying have been reportedly used to produce ferrites 11. Following the chemical via, some coarse particles may be formed due to agglomeration during the dehydration step.