Research articles
Influence of Fe substitution on structural and magnetic features of BiMn2O5 nanostructures

https://doi.org/10.1016/j.jmmm.2017.11.101Get rights and content

Highlights

  • Nanostructures of complex oxides [BiFexMn2−xO5 (x = 0, 1, 2)] were designed via co-precipitation route.

  • The observed structural changes via tuning B-site were correlated with magnetic performance.

  • Magnetic stability of prepared nanostructures enhanced as compared to bulk.

Abstract

Nanostructures of complex oxides [BiFexMn2−xO5 (x = 0, 1, 2)] have been designed to study their structural, optical and magnetic behaviour. X-ray diffraction data (XRD) revealed orthorhombic phase with Pbam space group. Noticeable expansion in unit cell parameters has been found from BiMn2O5 (x = 0) to BiFe2O4.5 (x = 2). The observed structural changes via tuning of B-site (x = 0–2) played an important role in overall magnetic properties. Transmission electron microscopic images confirm that the average particle size of all the materials are in nano domain range with different morphologies. From optical studies, it has been found that the observed energy band gap values are strongly related to 3d electron numbers. These values appear to be larger than that reported for bulk. Isothermal magnetization plots (at 5 K) show increase in coercivity (Hc) from x = 0 to x = 2. Temperature dependent magnetization studies implied anti-ferromagnetic interactions for BiMn2O5, frustrated magnet for BiFeMnO5 and ferromagnetic behaviour for BiFe2O4.5. Ferromagnetic state of nanostructured BiFe2O4.5 is in contrast with its bulk counterparts.

Introduction

Significant research is currently devoted to multiferroic materials due to their fundamental aspects and potential applications (sensors, memories, transducers) [1]. Most of the known multiferroic compounds shows weak magneto-electric (ME) output [2], [3]. Continuous efforts has been given to search out new class of multiferroics in which multiferroicity/ferroelectricity can be induced by charge ordering that is required for high ME coefficient [3], [4], [5]. Recent studies demonstrated that RMn2O5 (R = rare earth, Y or Bi) based oxides are most promising for magnetic and electrical properties. RMn2O5 are type II multiferroics, in which ferroelectricity occurs only in magnetically ordered state [6]. In RMn2O5, Mn has mixed valence state (+3, +4) which decides the magnetic behaviour via superexchange interaction through oxygen [7], [8]. Among RMn2O5 family, BiMn2O5 (BMO) is capable of showing significant polarization due to presence of highly polarisable Bi3+ ions [9]. BMO crystallizes in orthorhombic structure (space group: Pbam). It shows commensurate antiferromagnetic (AFM) ordering below 40 K [10], [11], [12], [13]. In BMO, Mn3+ and Mn4+ are coupled through AFM superexchange interactions, but due to odd number of spins, all spins cannot be compensated leading to uncompensated magnetic structure [10]. The magnetic structure of BMO can be tuned by replacing Mn3+ and Mn4+ by Fe3+. It is interesting to study the structural and magnetic aspects when Mn3+ sites are partially or fully substituted by Fe3+, since such substitution might induce ferromagnetism in the system [2]. In the present work, we have designed Mn-rich (BiMn2O5), Fe-rich (BiFe2O4.5), and Fe-Mn (BiFeMnO5) materials at nanoscale and investigated the influence of size as well as atomic substitution on their structural, optical and magnetic properties.

Section snippets

Experimental details

Complex metal oxides can be synthesized by several routes [1], [14], [15], [16], [17], [18]. Nanoparticles of these oxides can be synthesized also by low temperature route and details may be found elsewhere [19]. Nanostructured complex oxide systems [BiFexMn2−xO5 (x = 0, 1, 2)] were prepared by co-precipitation route. For preparation of BiMn2O5 (BMO) nanostructures; 0.1 M bismuth nitrate [Bi(NO3)3·5H2O, Aldrich, 99.999%], and 0.2 M manganese chloride [MnCl2·4H2O, Himedia, 97%] were dissolved in

Results and discussion

Raw X-ray diffraction patterns of BMO, BFMO and BFO were refined with Rietveld method using initial structural model [20]. Rietveld refined XRD data (Fig. 1) indicate single phase orthorhombic (Pbam) structure. Refined XRD patterns are indexed with orthorhombic phase which shows good agreement with standard powder diffraction files; PDF-00-027-0048 (for BMO and BFMO) and PDF-00-025-0090 (for BFO). Using Scherrer formula, average crystallite sizes were calculated as 40 ± 3, 34 ± 4, and 24 ± 2 nm

Conclusion

In summary, single phase nanostructured oxides [BiFexMn2−xO5 (x = 0, 1, 2)] were designed via simple co-precipitation route. Overall expansion in unit cell was found from BMO (x = 0) to BFO (x = 2). The band gap increases with increasing number of d-electron in the system. The band gap estimated for these nanostructures appear to be larger than that reported for their bulk. In contrast to bulk, an antiferro to ferromagnetic transition through frustrated magnetic state is observed as we go from

Acknowledgments

The authors acknowledge DST-SERB, India for financial support through research grant [File no. EMR/2015/001716]. VMG and SG wants to acknowledge INST, Mohali for providing financial assistance through Post Doctoral and Doctoral Fellowship, respectively.

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