Short communication
Enhancement of magnetic and ferroelectric properties of BiFeO3 by Er and transition element (Mn, Co) co-doping

https://doi.org/10.1016/j.mseb.2014.05.012Get rights and content

Highlights

  • BiFeO3, Bi0.8Er0.2FeO3, Bi0.8Er0.2Fe0.9Mn0.1O3 and Bi0.8Er0.2Fe0.9Co0.1O3 nanoparticles were prepared by sol–gel method.

  • The introduction of Er and Mn, Co into BiFeO3 leads into a phase transition with reduced grain size.

  • The phase transformation combined with size reduction has significantly increased saturated polarization (Ps), remanent polarization (Pr) and saturated magnetization (Ms), remanent magnetization (Mr) behaviors of the doped samples with the same variation trend.

  • The formation of dipolar defect complexes (DDCs) in the doped samples may also contribute to the improved ferroelectric property.

  • Bi0.8Er0.2Fe0.9Mn0.1O3 exhibits significantly improved ferroelectric and ferromagnetic properties.

Abstract

BiFeO3(BFO), Bi0.8Er0.2FeO3 (BEFO), Bi0.8Er0.2Fe0.9Mn0.1O3 (BEFMO) and Bi0.8Er0.2Fe0.9Co0.1O3(BEFCO) nanoparticles were prepared by sol–gel method having an average size of 200 nm for BFO, under100 nm for BEFO and under 60 nm for BEFMO and BEFCO. Phase transition from a rhombohedral symmetry (R3c) for BFO to an orthorhombic symmetry (Ibmm) for BEFO, BEFMO and BEFCO has been observed. The phase transformation combined with size reduction has significantly improved both ferroelectric and ferromagnetic behaviors of the doped samples in a similar way. The formation of dipolar defect complexes (DDCs) in the doped samples also contributes to the improved ferroelectric property with saturated polarization (Ps) of 0.375 μC/cm2 and remanent polarization (Pr) of 0.244 μC/cm2 for BEFMO. Size effect may also impact the simultaneously developed Pr for BEFMO and BEFCO. Owning to the interactions between the ferromagnetic and antiferromagnetic microdomains, improved saturated magnetization (Ms) and remanent magnetization (Mr) are also observed in BEFMO.

Introduction

BiFeO3 (BFO) is a well-known single-phase multiferroic material with ABO3 structure and G-type antiferromagnetic behavior below Neel temperature TN  643 K and ferroelectric behavior below the Curie temperature TC  1103 K [1]. In general the ferroic phenomena can arise from different ions or bonds, such as the ferroelectric Bi ions and magnetic Fe ions in BiFeO3 [2]. BFO has attracted much attention as a promising candidate material for various fields including data storage, spintronics, detectors, photovoltaic devices and so on [3], [4], [5], [6]. The modulated AFM spin spiral structure with a wavelength of λ  62 nm provides no net magnetization in bulk BFO, which restricts multiferroic property [7], [8]. Inducing the lattice distortion can improve the magnetic properties (magnetization or TN) of BFO by enhancing the canting of the antiferromagnetic arranged neighboring spins as well as ferroelectric properties (polarization, TN or anti-fatigue property) by altering structural symmetry [9], [10]. A-site substitution by rare earth (RE) ions and B-site by transition metal ions could enhance the multiferroic behavior due to the structural distortion as well as ferromagnetic (FM) coupling existed between Fe3+ and the doped cation [11], [12], [13], [14], [15], [16], [17]. Numerous studies indicate that A-site substitution with RE elements in BFO can give rise to improved magnetic properties of these doped BFO compounds. Only minor attention has been paid to Er-doped BFO so far, while Er3+ ion is magnetically active and its coupling to Fe3+ ion is expected to have a giant impact on the magnetization of BFO even at the light doping level [18], [19]. The structural characterization, and transport properties of a new solid solution of the general formula LaxBi1−xFe0.5Cr0.5O3 (0.4 < x < 1) have been reported [20]. It is worth noting that the magnetic behavior is striking because, for all materials studied, zero-field-cooling curves appear above field cooling ones. Belik et al. have achieved BiFe0.75Mn0.25O3 in space group Pnma at high pressure. The (FeMn)O6 octahedra are distorted, with one longer metal oxygen bond that can be attributed to a compensation for covalent Bisingle bondO bonding, which results in the localization of the lone electron pair on Bi3+ cations. The HP phase undergo an antiferromagnetic ordering at 520 K, respectively, and develop a weak net magnetic moment at low temperatures [21]. Bi0.99Eu0.01Fe0.99Co0.01O3 has also been prepared by a simple sol–gel route. The successful co-doping of Eu and Co dramatically enhances the saturation magnetization (Ms) and coercivity (Hc) [22]. Regarding the magnetism, doping on the B-site directly affects magnetic couplings, several attempts to prepare BiFe1−xMnxO3 and BiFe1−xCoxO3 solid solutions has been carried out [14], where Mn and Co are chosen due to their similar size but different electronic structure and tendency toward ferromagnetic couplings.

For this paper, the lattice distortion and ferromagnetic (FM) coupling through doping on A-site and B-site have been combined to improve the properties of BFO. A relatively high doping level of 0.2 was adopted for Er to achieve a structural change. For B-site, neighboring transition elements of Mn and Co were doped on B-site to investigate the influence of the electronic structure and ions radius on BFO. The aim was to prepare new materials of BFO, Bi0.8Er0.2FeO3 (BEFO), Bi0.8Er0.2Fe0.9Mn0.1O3 (BEFMO) and Bi0.8Er0.2Fe0.9Co0.1O3 (BEFCO) to explore the structural, electrical, and magnetic properties.

Section snippets

Experimental

Polycrystalline samples of BFO, BEFO, BEFMO and BEFCO were synthesized via sol–gel method. Appropriate proportions of Bi2O3 (analytical pure), Er2O3 (high purity reagent), Mn(NO3)2 (analytical pure), Co(NO3)2·6H2O(≥99.0%) and Fe(NO3)3·9H2O(≥98.5%) were used as starting materials and dissolved in dilute nitric(analytical pure) acid and distilled water. Tartaric acid was put into the mixture as organic complex. After complete dissolution of the reactants, the solution was stirred for several

Results and discussion

Fig. 1 shows the XRD patterns of the four samples. Analysis of the XRD patterns is performed by MDI Jade software and listed in Table 1. BFO is in the rhombohedral symmetry (R3c) in a hexagonal setting with lattice parameters of a = b = 5.603 Å and c = 13.926Å, which is in agreement with the JCPDS card No. 71-2494. Although minor impurity of Bi4O7 could be detected (marked asterisk in Fig. 1), it is not ferromagnetic in nature at room temperature. The phase transition from trigonal (R3c) to

Conclusion

BFO, BEFO, BEFMO and BEFCO multiferroics have been successfully prepared via sol–gel method. The introduction of Er and Mn, Co leads into a phase transition from rhombohedral phase (R3c) to orthorhombic phase (Ibmm) combined with reduced grain size. This transformation is favorable for a similar increase in saturated polarization (Ps) remanent polarization (Pr) and saturated magnetization (Ms) and remanent magnetization (Mr). The formation of dipolar defect complexes (DDCs) in the doped samples

Acknowledgments

We acknowledge the Key Project of National High Technology Research of China (2011AA050526), the Ministry of Education of China (No. IRT1148), the Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) and the National Natural Science Foundation of China (51172110, 51372119, 61377019).

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