Cation distribution and magnetic properties of CoAlxFe2−xO4/SiO2 nanocomposites

doi:10.1016/j.physb.2013.04.020

Physica B: Condensed Matter

Volume 421, 15 July 2013, Pages 8-12

https://doi.org/10.1016/j.physb.2013.04.020 Get rights and content

Abstract

CoAl_xFe_2−xO₄/SiO₂ (x=0.0–1.0) nanocomposites have been prepared by the sol–gel method. Cation distribution and magnetic properties are studied by X-ray diffraction (XRD), Mössbauer spectroscopy (MS) and vibrating sample magnetometer (VSM). The morphology and size are measured by transmission electron microscope (TEM). Cation distribution obtained indicates that Al³⁺ besides preferring octahedral (B) sites, also have a small tendency to occupy tetragonal (A) sites. And the ratio of Al³⁺(B)/Al³⁺(A) is not a constant, but a variable with increasing Al content. Mössbauer spectra show that the samples transfer from the completely magnetic order to mixed state of superparamagnetic and magnetic order with increasing x. Otherwise, both calculated from MS and VSM values of the magnetic moment per formula unit decrease with increasing Al content, indicating a collinear ferrimagnetic structure.

Introduction

Cubic spinel ferrite, crystallizing with space group Fd3m, has two sublattices, tetrahedral (A) sites and octahedral (B) sites. The important structural, electrical and magnetic properties of spinels, responsible for their application in various fields, are found to depend on the distribution of cations among sites. Therefore the estimation of cation distribution turns out to be important [1], [2], [3]. In addition, in order to understand the magnetic interaction behavior, it is necessary to perform non-magnetic cation (Al³⁺, Y³⁺) doping [4], [5]. But when Al³⁺ substitutes for Fe³⁺ in CoFe₂O₄ ferrite, the problem of the distribution of Al³⁺ between A- and B-sites is often been oversimplified in the literatures. Thanki et al. regarded Al³⁺ to only occupy B-site for Zn_xCo_1−xAl_xFe_2−xO₄ [6]. Singhal et al. guessed that Al³⁺ entered into the A- and B-sites in the ∼2:3 ratio for CoAl_xFe_2−xO₄ [7]. Mane et al. [8] and More et al. [9] obtained the ratio as 1:4 for CoAl_xCr_xFe_2−2xO₄. The reason for the oversimplification is that they only employed the X-ray diffraction to obtain cation distribution. It is well known that the intensities of (2 2 0), (2 2 2), (4 2 2) and (5 1 1) reflections of ferrite are mostly sensitive to cation distribution [10]. It is proposed to use the X-ray diffraction to investigate cation distribution of ZnFe₂O₄, MgFe₂O₄ and CuFe₂O₄. But for CoFe₂O₄ ferrite, it is difficult to do for the atomic number of Co being approximate to Fe. The Mössbauer technique is known to be a useful tool for investigation of the distribution of Fe³⁺ in A- and B-sites for ferrite.

In this paper, we study the structural and magnetic properties of CoAl_xFe_2−xO₄/SiO₂ (x=0.0–1.0) nanocomposites by X-ray diffraction (XRD), transmission electron microscope (TEM), Mössbauer spectroscopy (MS) and vibrating sample magnetometer (VSM). The main purpose is to obtain cation distribution of CoAl_xFe_2−xO₄/SiO₂ nanocomposites by combining the results of MS and XRD. MS is to study the distribution of Fe³⁺ and XRD is to investigate the assignation of Co²⁺ and Al³⁺.

Section snippets

Experiments

CoAl_xFe_2−xO₄/SiO₂ (x=0–1.0) nanocomposites were prepared by the sol–gel method using a precursor solution obtained from the mixture of Co(NO₃)₂·6H₂O, Fe(NO₃)₃·9H₂O, Al(NO₃)₃·9H₂O, tetraethylorthosilicate (TEOS), methoxyethanol, glycol, glacial acetic acid and deionized water. The mass ratio of CoAl_xFe₂O₄ and SiO₂ was 7:3. The mixtures were magnetically stirred for 5 h. Black brown sols were obtained after complete dissolution, which were evaporated on a water bath at 60 °C to remove the excess

Results and discussion

Fig. 1 shows the XRD patterns of the samples. It can be seen that there are only the diffraction peaks for cubic spinel ferrite. The SiO₂ matrix should remain amorphous in the samples. With increasing Al content x, the diffraction peaks became broader and shifted to a higher 2θ position, indicating the decrease in grain size and lattice constant for CoAl_xFe₂O₄ in the nanocomposites. Table 1 shows the values of lattice constant, average grain size, and X-ray intensity ratios of the samples. The

Conclusion

CoAl_xFe_2−xO₄/SiO₂ (x=0.0–1.0) nanocomposites have been prepared successfully by the sol–gel method. The cations distribution is determined combining the results obtained by fitting the Mössbauer spectra and the integrated intensity of XRD peaks. The results indicate that Al³⁺ besides preferring B-site, have also a small tendency to occupy A-site. The ratio of Al³⁺(B)/Al³⁺(A) increases with increasing x, which indicates that the preference of Al³⁺ to occupy B-site increases with increasing Al

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Cation distribution and magnetic properties of CoAlxFe2−xO4/SiO2 nanocomposites

Abstract

Introduction

Section snippets

Experiments

Results and discussion

Conclusion

J. Magn. Magn. Mater.

J. Solid State Chem.

J. Magn. Magn. Mater.

J. Magn. Magn. Mater.

Mater. Lett.

J. Magn. Magn. Mater.

Mater. Lett.

J. Alloy. Compd.

Cation distribution and magnetic properties of CoAl_xFe_2−xO₄/SiO₂ nanocomposites