Detection of para–antiferromagnetic transition in Bi2Fe4O9 powders by means of microwave absorption measurements

doi:10.1016/j.jmmm.2013.08.014

Journal of Magnetism and Magnetic Materials

Volume 348, December 2013, Pages 17-21

https://doi.org/10.1016/j.jmmm.2013.08.014 Get rights and content

Highlights

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The changes in lineshape of the EPR spectra in Bi₂Fe₄O₉ powders are studied.
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The onset of the para–antiferromagnetic transition is detected.
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A weak ferromagnetism is also observed in this material.
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MAMMAS and LFMA techniques are used to give a further knowledge on the bismuth ferrite.

Abstract

An electron paramagnetic resonance (EPR) study of Bi₂Fe₄O₉ powders is carried out in X-band (8.8–9.8 GHz) and the 200–350 K temperature range. For all the temperatures, the EPR spectra show a single broad line attributable to Fe³⁺ (S=5/2) ions. The onset of the para–antiferromagnetic transition has been determined from the temperature dependence of the parameters deduced from EPR spectra: the peak-to-peak linewidth (ΔH_pp) and the resonant field (H_res); a weak ferromagnetism is also observed at low temperature and it is attributed to canting of Fe³⁺ ion sublattices in the antiferromagnetic matrix. The magnetically modulated microwave absorption spectroscopy (MAMMAS) and the low-field microwave absorption (LFMA) are used to give further information on this material. These techniques give evidence of the magnetic transition, suggesting a weak ferromagnetism at low temperature.

Introduction

Bi₂Fe₄O₉ compound is known to have an orthorhombic structure [1], [2], see Fig. 1, where the iron ions are evenly distributed between the tetrahedral and octahedral positions with the bismuth ions surrounded by eight oxygen atoms. But also, it can be described by columns of edge-sharing FeO₆ octahedra parallel to the c axis, bonded by corner-sharing FeO₄ tetrahedra and Bi atoms. This bismuth ferrite material is generally synthesized by a traditional solid-state reaction at temperature over 850 °C [2], [3]. However, because of some disadvantages of this method such as high temperature, the presence of impurities in the compounds and largesize of particles, other synthesis methods have also been employed; e.g. Park et al. [4] have used a molten salt technique for the synthesis of submicron-sized Bi₂Fe₄O₉ cubes using Bi₂O₃ and Fe₂O₃ as starting materials.

Additionally, concerning the application, it is important to say that Bi₂Fe₄O₉ compound is an important material that can be used for a semiconductor gas sensor and catalyst for ammonia oxidation to NO [5], [6]. This compound shows a para–antiferromagnetic transition around 240–265 K [3], [7], in which the magnetic ordering is due to superexchange interaction between the Fe ions mediated by the O atoms. Singh et al. [3] reported remarkable multiferroic effects in polycrystalline Bi₂Fe₄O₉ which exhibits an antiferromagnetic ordering and ferroelectric hysteresis. Therefore, Bi₂Fe₄O₉ is a promising multiferroic material.

The electron paramagnetic resonance (EPR) is the most powerful spectroscopic method available to unambiguously determine the valence state of paramagnetic ions [8], as well as to determine the local structural information and symmetry of paramagnetic ions incorporated in the structure [9], [10]. This technique also allows the investigation of the nature of magnetic phases in multiferroic materials at different temperatures [11], [12], [13], [14].

On the other hand, recently, the microwave absorption around zero magnetic field (low-field microwave absorption, LFMA) has also been used to study multiferroic materials [12], [13], [14], [15]. A complementary method particularly well adapted to study magnetic transitions is the magnetically modulated microwave absorption spectroscopy (MAMMAS) [12], [15], [16], [17]. The MAMMAS technique is based on the temperature variations of the modulated microwave absorption, and provides valuable information about the nature of magnetic ordering [17].

To our knowledge, however, studies on the polycrystalline Bi₂Fe₄O₉ with EPR, MAMMAS and LFMA techniques are scarce. Therefore, in this work, we present a detailed study of the changes in lineshape of the EPR spectra in Bi₂Fe₄O₉ powders; these changes are quantified by means of the following parameters: the peak-to-peak linewidth (ΔH_pp) and the resonant field (H_res) as a function of temperature, through para–antiferromagnetic transition. MAMMAS and LFMA techniques are used to give a further knowledge on the magnetic behavior of this material; these techniques give evidence of the magnetic transition and a weak ferromagnetism at low temperature.

Section snippets

Samples preparation and experimental techniques

The Bi₂Fe₄O₉ powders are prepared by molten salt technique [4]. Stoichiometric amounts of Bi₂O₃ (Aldrich, 99.99%) and Fe₂O₃ (Baker, 99.9%) were mixed (in molar ratio of 1:2) with acetone and ground in an agate mortar. To this oxides mixture is added a salts mixture in the molar ratio of 1:5, where the salts mixture has the composition of Na₂SO₄ (37.5% mol) and Li₂SO₄ (62.5% mol). The powders were placed in an alumina crucible, heated at a ramp rate of 10 °C/min up to an annealing temperature at 800

Results and discussion

XRD pattern of the polycrystalline sample of Bi₂Fe₄O₉ is shown in Fig. 2. All observed reflection lines are indexed with the orthorhombic structure of space group Pbam, in a good agreement with the standard data of the Bi₂Fe₄O₉ powders (JCPDS Card no.: 25-0090). Additionally, in Fig. 3(a), we show the scanning electron microscopy (SEM) image, revealing the morphology of the Bi₂Fe₄O₉ powders. It can be observed that the Bi₂Fe₄O₉ product mainly consists of particles clusters with cubic

Conclusions

The changes in ΔH_pp and H_res in the EPR spectra for Bi₂Fe₄O₉ powders, within the region between 350 K and 200 K, can be interpreted in terms of the para–antiferromagnetic transition. At low temperature a weak ferromagnetism is observed in this material, and it is attributed to canting of Fe³⁺ ion sublattices in the antiferromagnetic matrix. Additionally, MAMMAS and LFMA measurements also give evidence of the magnetic ordering and also suggest a weak ferromagnetism at low temperature, with a very

Acknowledgments

G. Alvarez acknowledges research support in the laboratory of magnetic mensurations and biophysics of ESFM-IPN-Mexico. Support from project PAPIIT-UNAM No. IN111111 is gratefully acknowledged.

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Detection of para–antiferromagnetic transition in Bi2Fe4O9 powders by means of microwave absorption measurements

Highlights

Abstract

Introduction

Section snippets

Samples preparation and experimental techniques

Results and discussion

Conclusions

Acknowledgments

Materials Chemistry and Physics

Sensors and Actuators B: Chemical

Physica B

Materials Science and Engineering B

Journal of Alloys and Compounds

Solid State Communications

Journal of Physics and Chemistry of Solids

Solid State Sciences

Journal of Alloys and Compounds

Journal of Magnetism and Magnetic Materials

Detection of para–antiferromagnetic transition in Bi₂Fe₄O₉ powders by means of microwave absorption measurements