Electronic structure studies of the spinel CoFe2O4 by X-ray photoelectron spectroscopy
Introduction
The cobalt ferrite CoFe2O4 is a very important magnetic materials, which has covered a wide range of applications including electronic devices, ferrofluids, magnetic delivery microwave devices and high density information storage due to its wealth of magnetic and electronic properties, such as cubic magnetocrystalline anisotropy, high coercivity, moderate saturation magnetization, high Curie temperature TC, photomagnetism, magnetostriction, high chemical stability, wear resistance and electrical insulation, etc. [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12].
In structure, the spinel cobalt ferrite CoFe2O4 crystallizes in a face-centered cubic structure with a large unit cell containing eight formula units. There are two kinds of lattices for cation occupancy, A and B sites have tetrahedral and octahedral coordination, respectively. In the normal spinel structure Co is a divalent atom, occupying tetrahedral A sites, while Fe is a trivalent atom, sitting on the octahedral B sites. When A sites being Fe3+ ions, while B sites equally populated by Co2+ and Fe3+ ions, the spinel structure is referred to as the inverse kind [13], [14]. Commonly, the CoFe2O4 material is considered to be mostly an inverse spinel compound with most divalent Co ions occupying octahedral sites [7], [15], [16]. It means that the Co and Fe cations distribute at both sites. Since the FeA3+–FeB3+superexchange interaction is normally different from the CoA2+–FeB3+ interaction, variation of the cation distribution over the A and B sites in the spinel leads to different magnetic properties of these oxides even though the chemical composition of the compound does not change [14].
The spinel cobalt ferrite CoFe2O4 materials have been synthesized by different methods [17], [18], [19]. In most cases, variation of the magnetic properties was obtained due to the different distribution of the Co and Fe cations over the A and B sites. Thus, investigations on the distribution of the Co and Fe cations over the A and B sites in the spinel cobalt ferrite CoFe2O4 are important [4], [7], [13], [14].
Quantitative X-ray photoelectron spectroscopy (XPS) gives not only the chemical composition, but also information on the chemical bonding and chemical state. This will help us to understand the distribution of the Co and Fe cations in the spinel cobalt ferrite CoFe2O4. Thus, in this paper, homogeneous CoFe2O4 powder and bulk were synthesized by combustion technique, and the XPS was taken to study on the valence of the elements and electronic configuration.
Section snippets
Experimental
CoFe2O4 powder and bulk samples in our experiment were synthesized by combustion technique. In brief description, analytical reagent cobalt (II) nitrate hexahydrate, iron (III) nitrate nonahydrate and urea were used. Stoichiometric amounts of the cobalt and iron nitrates were weighed out under dry conditions, intimately mixed in a widemouth vitreous silica basin and heated on a hot blanket inside a fume cupboard, under ventilation. With a rise in temperature, melting occurred and a dark liquid
Results and discussion
Fig. 1 is the X-ray powder diffraction patterns of the CoFe2O4 powder and bulk. It shows that the powder and bulk samples all have a single spinel phase and all peaks could be indexed according to the standard card of the spinel cobalt ferrite CoFe2O4 [JCPPS card No. 22-1086].
Fig. 2 gives the wide-scan XPS spectra of the CoFe2O4 bulk (curve 1) and powder (curve 2) samples in the binding energy of 0–1000 eV. As demonstrated in Fig. 2, the crystals contain Fe, Co, and O elements, and no other
Conclusion
In conclusion, powder and bulk CoFe2O4 samples have been fabricated by combustion reaction and analyzed by XPS technique. The specimens are found to be near-stoichiometric, although subsequent calcinations tend to reduce oxygen concentration. The results show that the Fe3+ cations occupy 60% tetrahedral sites and 70% octahedral sites both in the bulk and powder specimens, but some Fe2+ may be present in the powder sample. The cobalt cations are predominantly present as Co2+ cations. For the
Acknowledgments
This work was supported by the National Natural Science Foundation of China (Grant No. 10674105, No. 10474074 and No. 10534030) and 973 Program (2007CB607501).
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