Magnetic enhancement across a ferroelectric–paraelectric phase boundary in Bi1−xSmxFeO3
Introduction
Multiferroic materials, as a new class of materials in which electric and magnetic orderings coexist in a single phase, have attracted a lot of attentions. BiFeO3 is one of such materials having an antiferromagnetic (AFM) behavior with a Néel temperature (TN) of ∼370 °C and a ferroelectric order with a high Curie temperature (TC) of ∼830 °C [1], [2]. The crystal structure of the polar phase of BiFeO3 is described as a rhombohedrally distorted perovskite structure with space group of R3c, which allows antiphase octahedral tilting and ionic displacements off the centrosymmetric position along the (0 0 1)H direction (H denotes hexagonal setting). Even though the R3c symmetry permits existence of a weak ferromagnetic moment [3], the antiferromagnetic BiFeO3 has a spiral modulated spin structure with the period of 62 nm [4], resulting in no observation of any net magnetization. Although BiFeO3 is being examined as a possible environmentally friendly alternative to the current lead-based piezoelectrics, it suffers from high-leakage current and large coercive fields, as well as small electromechanical coefficient. It has been shown [5] that some of such shortcomings can be overcame by A- and/or B-site substitution of BiFeO3 perovskite cell. The subtle modifications in atomic composition can induce lattice distortion of the perovskite cell and, hence, functional behavior of a parent ferroelectric and magnetic perovskite can be tuned.
Recently, it has been demonstrated [5] that the use of rare-earth ions in A-site substitution for the Bi3+ ion in BiFeO3 composition can effectively modulate the crystal structure and improve the multiferroric properties. Among the rare-earth ion substituted BiFeO3, the Sm substituted system has unique properties. Sm3+ has a much smaller ionic size than that of La3+ or Nd3+. The Goldschmidt tolerance factor for Sm3+ substituted BiFeO3 is less than 1 assuming a high spin state for the Fe3+ ion. There have been many reports of a lead-free morphotropic phase boundary which exhibits a ferroelectric to antiferroelectric transition at approximately Bi0.86Sm0.14FeO3 with a simple perovskite structure using the combinatorial thin film strategy [6], [7], [8]. A substantial enhancement of the electromechanical properties was also observed to accompany at the phase boundary [9]. Moreover, the Sm substituted BiFeO3 films also show enhanced ferroelectricity and reduced leakage current density [10]. In fact, the high-temperature paraelectric phase of BiFeO3 was found to be in the Pnma space group [12]. However, the presence of various impurities [13], [14], [15], mainly comprising Bi2Fe4O9 and Bi25FeO39, in Sm-substituted samples in the concentration range of x<0.1 make the accurate structure identification and magnetization characteristic unrealistic. These impurities could lead to high leakage current and poor electric behavior. Detailed studies of the interplay among the structural, vibrational, and magnetic properties in single-phase Sm-substituted BiFeO3 are highly desirable.
In this work, we report an experimental study of structural, vibrational, and magnetic properties of Bi1−xSmxFeO3 within the concentration range of 0≤x≤0.20. By using an improved synthesis method, we obtain the pure phase samples. The high-quality samples enable us to establish the common regularities of changes of structural, vibrational, and magnetic properties of BiFeO3 upon samarium substitution. We find a clear evolution of the ferroelectric–paraelectric phase transition through Sm substitution. Such structural modifications eventually enhance the magnetization at ferroelectric–paraelectric boundary. These results are important for designing and engineering multiferroric materials with excellent magnetoelectric performance.
Section snippets
Experimental technique
Bi1−xSmxFeO3 powders were synthesized in terms of a sol–gel route. Stoichiometric amounts of Bi(NO3)35 H2O, Sm(NO3)36 H2O, and Fe(NO3)39 H2O were dissolved in dilute nitric acid, and calculated amounts of tartaric acid were added as a complexing agent. The resultant solution was evaporated and dried at 150 °C with stirring to obtain xerogel powders. Then the xerogel powders were ground in an agate mortar. The obtained powders were preheated to 300 °C for 1 h to remove excess hydrocarbons and NOx
Results and discussion
Structural information was obtained through x-ray diffraction. Fig. 1 shows the XRD patterns for two selected samples for Bi1−xSmxFeO3 (x=0.03 and 0.15). The analysis is based on the Rietveld method using the FULLPROF program [16]. The compound BiFeO3 in the rhombohedrally distorted perovskite structure at room temperature and the samples are consistent with the data from previous structural investigations [17]. Moreover, all the samples have the pure phase confirmed by the XRD results,
Conclusion
In summary, we have studied structural, vibrational, and magnetic properties of Bi1−xSmxFeO3 (0≤x≤0.20). We find the structural phase transitions from the rhombohedral R3c to orthorhombic Pnma with Sm substitution. This structural behavior corresponds with the transition from ferroelectric to paraelectric phase at x∼0.15. A significant enhancement in magnetization across ferroelectric–paraelectric boundary of this Sm-substituted system stems from the combined effects of complete removal of the
Acknowledgments
This work was supported by the Cultivation Fund of the Key Scientific and Technical Innovation Project, Ministry of Education of China (No. 708070), the National Natural Science Foundation of China (No. 10874046 and 11104081), and the Fundamental Research Funds for the Central Universities SCUT (No.2012zz0078).
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