Structural evolution and magnetization enhancement of Bi1−xTbxFeO3

https://doi.org/10.1016/j.jpcs.2013.01.030Get rights and content

Abstract

The structural, vibrational, and magnetic properties of well prepared Bi1−xTbxFeO3 (x=0–0.2) powders are investigated by combining the X-ray diffraction, Raman scattering, and magnetometry measurements. It is found that partial substitution of Tb (10–12.5%) in Bi site results in a structural transformation from the rhombohedral R3c symmetry to the orthorhombic Pnma phase, accompanied by a ferroelectric–paraelectric phase transition. The magnetization of Bi1−xTbxFeO3 first gradually increases before reaching maximum at the polar–nonpolar phase boundary and then decreases for x≥0.125 until x=0.20. The enhancement of the remnant magnetization is attributed to the suppression of the space-modulated spin structure in the rhombohedral R3c phase. Meanwhile, the reduction of the remnant magnetization in the orthorhombic Pnma phase is suggested to result from the establishment of well parallel antiferromagnetic ordering.

Highlights

► Tb subsitutes at the Bi site in BiFeO3–Bi1−xTbxFeO3 (x=0–0.2) powders. ► Ferroelectric–paraelectric phase transition occurs at x=0.10–0.125. ► The highest remnant magnetization is at the phase boundary. ► The destruction of spin cycloid structure leads to the magnetization enhancement.

Introduction

Multiferroic materials are characterized by simultaneous existence of ferromagnetic, ferroelectric and/or ferroelastic ordering, holding promises of advanced devices that exploit the magnetoelectric (ME) effect and memory with dual read–write mechanism [1]. As an interesting candidate of multiferroic materials, single-phase BiFeO3 possesses a high ferroelectric Curie temperature of 830 °C and a high antiferromagnetic Néel temperature of 370 °C [2], which brings hope to the community that ME effect may be observed at room temperature [3]. Moreover, BiFeO3 almost achieved the commercial application for memory and logic device, such as ferroelectric capacitive memories [4] and switchable diode [5] by using the combinatorial thin film strategy. However, a superimposed spiral spin structure with an incommensurate long-wavelength period of ∼620 Å cancels the macroscopic magnetization and also inhibits the observation of the linear ME effect in bulk BiFeO3 [6]. In addition, due to the existence of a large number of charge centers caused by oxygen deficiency and bismuth evaporation during conventional solid-state synthesis process [7], the bulk BiFeO3 is characterized by serious current leakage problems, making it difficult to attain high resistivity.

In order to overcome these problems, much effort has been devoted, which include: (i) doping rare-earth or transition metal elements into Bi or Fe sites to increase the phase stability of BiFeO3 [8]; (ii) synthesizing BiFeO3 nanoparticle with grain size below 620 Å to achieve ferromagnetism by destroying spiral structure [9]. Among these approaches, it has been proved doping of BiFeO3 with a foreign atom at either A- or B-site of the ABO3 lattice has been shown to result in a remarkable improvement of its ferromagnetic properties [10], [11], [12] and ferroelectric properties [13], [14], [15]. It seems logical to attribute, in general, the changes in multiferroic properties on Bi substitution by a rare-earth to the cation size effect. If so, the hypothesis should be valid for the substitution of Bi (ionic radius=1.36 Å) by a rare-earth ion Tb having an ionic radius (ionic radius=1.25 Å) comparable to those of the reported rare earth substituents. Among the reports about the Tb substituted BiFeO3 system [16], [17], [18], Wang and Nan [16] thought that Tb substitution does not lead to the modification of the inhomogeneous spin modulated magnetic structure in Bi1−xTbxFeO3 (x=0–0.15) thin films rather than the enhancement of the remnant and saturation polarization. Palkar et al. [17] reported that Bi0.9−xTbxLa0.1FeO3 (0≤x≤0.3) powders have the same crystalline structure as the parent compound, and the magnetoelectric coupling was observed in the representative sample Bi0.825Tb0.075La0.1FeO3 at room temperature. Recently, Saxin and Knee [18] discovered a structural transformation from R3c phase to Pnma symmetry in Bi1−xTbxFeO3 (x=0.05–0.25). The crystal structure is bi-phasic coexistence of the R3c and Pnma phases for x=0.10 and 0.15 but becomes the Pnma phase for x=0.20. In their experiments, the impurity phases of Bi2Fe4O9 for x=0.05 and Tb3Fe5O12 for x≥0.20 are detected. The appearance of impurity phases suggests the ratio of Bi/Fe is not balance [19], and these will influence the magnetization and Néel temperature (TN) irregularly [20]. Detailed studies of the interplay among the structural, vibrational, and magnetic properties in single-phase Tb-substituted BiFeO3 with low doped concentration are highly desirable.

In this work, we report the investigations of the structure, vibrational, and magnetic properties of Tb-substituted BiFeO3 powders, i.e., Bi1−xTbxFeO3 within the 0≤x≤0.20 concentration range. We find that the samples keep the ferroelectricity in the low Tb substitution concentration less than 0.10. There is a structural phase transition from the rhombohedral R3c to orthorhombic Pnma at about x=0.10 at which ferroelectric–paraferroelectric phase transition takes place. There is a giant magnetization across the ferroelectric–paraferroelectric phase boundary.

Section snippets

Experiments

Bi1−xTbxFeO3 powders were synthesized in terms of a sol-gel route. Stoichiometric amounts of Bi(NO3)3·5H2O, Tb2O3, and Fe(NO3)3·9H2O were dissolved in dilute nitric acid, and calculated amounts of tartaric acid were added as a complexion agent. The resultant solution was evaporated and dried at 150 °C with stirring to obtain xerogel powders. Then the xerogel powders were grinded in an agate mortar. The obtained powders were preheated to 300 °C for 1 h in order to remove excess hydrocarbons and NOx

Results and discussion

Structural information was obtained through the powder XRD experiments. Fig. 1 shows the XRD patterns for sintered samples for Bi1−xTbxFeO3 (x=0.03, 0.09, and 0.175). The analysis is based on the Rietveld method using the FULLPROF program. The compound BiFeO3 in the rhombohedrally distorted perovskite structure at room temperature and the samples are consistent with the data from previous structural investigations [9]. Moreover, all the samples have the pure phases confirmed by the XRD results,

Conclusions

In summary, we have studied the structural, vibrational, and magnetic properties of Bi1−xTbxFeO3 (x=0–0.2). We found the structural phase transition from the rhombohedral R3c symmetry to orthorhombic Pnma phase between x=0.10 and 0.125. This structural behavior corresponds to the transition between the ferroelectric and paraferroelectric phases. Moreover, this phase structure transition accounts for the significantly enhanced magnetic properties of Bi0.9Tb0.1FeO3 due to the disappearance the

Acknowledgements

This work was supported by the Cultivation Fund of the Key Scientific and Technical Innovation Project, Ministry of Education of China (Grant No. 708070), the National Natural Science Foundation of China (Grant Nos. 10874046 and 11104081), and the Fundamental Research Funds for the Central Universities SCUT (No.2012zz0078).

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