Effect of annealing temperature on the structural, photoluminescence and magnetic properties of sol–gel derived Magnetoplumbite-type (M-type) hexagonal strontium ferrite

doi:10.1016/j.jmmm.2011.04.014

Journal of Magnetism and Magnetic Materials

Volume 323, Issue 17, September 2011, Pages 2318-2322

https://doi.org/10.1016/j.jmmm.2011.04.014 Get rights and content

Abstract

Magnetoplumbite-type (M-type) hexagonal strontium ferrite particles were synthesized via sol–gel technique employing ethylene glycol as the gel precursor at two different calcination temperatures (800 and 1000 °C). Structural properties were systematically investigated via X-ray diffraction (XRD), field emission scanning electron microscopy, high resolution transmission electron microscopy (HRTEM), energy dispersive spectroscopy (EDS), thermogravimetric analysis (TGA), photoluminescence spectrophotometry and superconducting quantum interference device magnetometer. XRD results showed that the sample synthesized at 1000 °C was of single-phase with a space group of P6₃/mmc and lattice cell parameter values of a=5.882 Å and c=23.048 Å. EDS confirmed the composition of strontium ferrite calcined at 1000 °C being mainly of M-type SrFe₁₂O₁₉ with HRTEM micrographs confirming the ferrites exhibiting M-type long range ordering along the c-axis of the crystal structure. The photoluminescence (PL) property of strontium ferrite was examined at excitation wavelengths of 260 and 270 nm with significant PL emission peaks centered at 350 nm being detected. Strontium ferrite annealed at higher temperature (1000 °C) was found to have grown into larger particle size, having higher content of oxygen vacancies and exhibited 83–85% more intense PL. Both the as-prepared strontium ferrites exhibited significant oxygen vacancies defect structures, which were verified via TGA. Higher calcination temperature turned strontium ferrite into a softer ferrite.

Highlights

► High annealing temperature produced M-type ferrite with higher oxygen vacancies. ► Photoluminescence intensity is proportional to the existence of oxygen vacancies. ► XRD data showed cell contraction well suited to the change of oxygen vacancies. ► Shift in hysteresis loop due to defect-induced exchange bias was observed.

Introduction

Strontium ferrite is a hard ferrite with high coercivity due to its magnetocrystalline anisotropy with a single easy magnetization [1]. Hence, SrFe₁₂O₁₉ has been widely used in industrial applications as permanent magnets, microwave devices and magnetic recording media [2]. SrFe₁₂O₁₉ is isomorphous with the M-type BaFe₁₂O₁₉, having a hexagonal symmetry of space group of P6₃/mmc [3], [4]. The lengths of the c- and a-axes in BaFe₁₂O₁₉ are 23.17 and 5.876 Å, respectively, and for SrFe₁₂O₁₉ the corresponding values are 23.03 and 5.861 Å [3]. The M-type hexagonal structure can be described as layers of closely packed oxygen ions, which are placed upon each other along the hexagonal c-axis. The structure is stacked alternately by spinel (S=Fe₆O₈²⁺) and hexagonal (R=SrFe₆O₁₁²⁻) layers [5]. The O²⁻ ions exist as close-packed layers, with the Sr²⁺ substituting for an O²⁻ in every five hexagonal layers. The 24 Fe³⁺ ions are distributed in five interstitial crystallographic sites of the close-packed layers, namely, three octahedral (2a, 12k, 4f₂), one tetrahedral (4f₁) and one trigonal bypyramidal (2b) [6].

The sol–gel synthesis technique employing ethylene glycol as the gel precursor has been widely attempted on the M-type ferrites [6], [7]. The synthetic technique has been successful in producing homogeneous nanosized ferrite crystallites at a much lower calcination temperature [8], [9]. It is known that calcination temperature will affect the crystalline size and ultimately the properties of the produced ferrites [10]. It was reported that at 500 °C maghemite, γ-Fe₂O₃ was formed and at 600 °C, mixed products consisting of hematite, α-Fe₂O₃ and M-type SrFe₁₂O₁₉ were obtained. However, at 800 °C and above, only M-type SrFe₁₂O₁₉ phase was observed [11]. As the calcination temperature increased from 650 to 900 °C, there was an increase in the coercivity but this decreased when the temperature is increased beyond 900 °C. The increment in the coercivity was due to the development of phase as the temperature increased from 650 to 900 °C. The decrease in coercivity at above 900 °C was due to the increase in the particle size. For a ferrite with Fe/Sr ratio of 10.8, the saturation magnetization (M_s) was measured as 59.1 emu/g and remnant magnetization (M_r) as 33.6 emu/g [12].

In this work, an attempt is made to employ the ethylene glycol as the gel precursor to synthesize nanosized M-type strontium ferrite at two different calcination temperatures (800 and 1000 °C). The produced ferrites were then characterized to determine their structural, magnetic and PL properties.

Section snippets

Experimental

The synthesis of SrFe₁₂O₁₉ (Sr:Fe=1:11.0) was performed using sol–gel route. Ethylene glycol (C₂H₆O₂) was used as the solvent and gel precursor where stoichiometric weight of Sr(NO₃)₂ [Sigma-Aldrich] and Fe(NO₃)₃ 9 H₂O [R & M Chemicals] was dissolved at a temperature of 50 °C for 2 h with continuous stirring using a magnetic stirring bar. The homogeneous solution was then placed on top of a hotplate with the magnetic stirring bar removed for dehydration to occur as this enhanced the gelation

Results and discussion

Fig. 1 showed the XRD patterns for SF800 and SF1000. All the reflections were indexed on the basis of the M-type structure retaining the P6₃/mmc space group (No. 194), with the initial cell parameters of a=5.886 Å, c=23.037 Å, and the JCPDS Files (43-0002, 40-1047 and 33-1340) had been referred to for all the peak position identification. (1 1 1) and (2 0 0) reflections from iron oxide were detected in SF800 and not in SF1000. The impurity iron oxide phase is preferentially formed when strontium

Conclusions

Higher calcination temperature at 1000 °C produced strontium ferrite with larger particle sizes having higher content of oxygen vacancies in its structure. Based on the calculated particle size by applying the Scherrer's equation on the (1 1 4) reflection peak of the ferrites, it showed that SF1000 was 15% larger in size compared to SF800. Rapid growth of particle sizes at higher calcination temperatures could have induced the formation of more oxygen vacancies in the M-type structures. Due to the

Acknowledgments

The authors would like to acknowledge the financial support of Tunku Abdul Rahman College, Malaysia, Swinburne University of Technology, Australia and MacDiarmid Institute of Advanced Materials and Nanotechnology, Victoria University of Wellington, New Zealand. The authors are also grateful to Dr. FL Lo for helpful discussion and providing constructive comments to the manuscript.

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Effect of annealing temperature on the structural, photoluminescence and magnetic properties of sol–gel derived Magnetoplumbite-type (M-type) hexagonal strontium ferrite

Abstract

Highlights

Introduction

Section snippets

Experimental

Results and discussion

Conclusions

Acknowledgments

J. Magn. Magn. Mater.

J. Magn. Magn. Mater.

Mater. Chem. Phys.

J. Alloys Compd.

J. Magn. Magn. Mater.

Alloys Compd.

J. Solid State Chem.

Phys. Rep.