Raman study of BiFeO3 with different excitation wavelengths
Introduction
Multiferroics are crystals that exhibit ferromagnetic/antiferromagnetic and ferroelectric order parameters in the same phase [1], [2], [3]. BiFeO3 is a typical multiferroic material exhibiting antiferromagnetic and ferroelectric properties at room temperature. It has a rhombohedrally distorted ferroelectric phase (space group R3c) with the Curie temperature of about 1100 K (TC) and simultaneously shows G-type antiferromagnetism below 640 K (TN) [4]. This material has attracted much attention because of its basic physical properties and potential applications in the future [3], [4], [5], [6]. Many theoretical and experimental researches have been performed recently in order to characterize the properties of BiFeO3 [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19]. Raman spectroscopy has been proved to be a convenient method to study the elementary excitations and their interactions in solid-state materials [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30]. In the past Raman studies of BiFeO3, most of the fundamental Raman-active modes have been assigned [15], [16], [17], [18], with three peaks at 958, 1050 and 1250 cm−1 identified as two-phonon modes [19]. However, to the best of our knowledge there is no report about how the Raman characters of BiFeO3 is affected by different excited wavelengths so far.
In this work, Raman spectra of BiFeO3 excited at different wavelengths were performed in the range of 100–1500 cm−1. The two-phonon peaks showed prominent resonance under the excitation of 532 nm. Raman spectra of BiFeO3 have been performed over a wide temperature range from 77 to 678 K excited at 532 nm. At the vicinity of TN, beyond the A1 mode at low frequency, the two-phonon modes at 958, 1050 and 1250 cm−1 showed pronounced changes in peak position, bandwidth and intensity. All these results may provide useful information for further understanding the relationship between magnetic properties and structure of BiFeO3.
Section snippets
Experimental methods
The BiFeO3 powders of the single-phase rhombohedral R3c structure were prepared by a molten salt synthesis (MSS) reported in details elsewhere [31]. High-resolution micro-Raman spectrometer (Jobin Yvon HR800) was used in this study. The laser lines (related photon energies) with 473 nm (2.62 eV), 488 nm (2.54 eV), 532 nm (2.33 eV), 633 nm (1.96 eV) and 785 nm (1.59 eV) were used as the excitations. Raman spectra of BiFeO3 were recorded in a temperature range from 77 to 678 K by using a Linkman TS600 stage
Experimental results and discussion
At room temperature, BiFeO3 belongs to rhombohedral R3c space group with two formulas in one primitive cell [7]. According to the group theory, it has 18 optical phonon modes: 4A1+5A2+9E. The A1 and E modes are both Raman and IR-active modes, whereas the A2 modes are Raman and IR-inactive modes. The degenerate E modes should split into LO–TO doublets due to the long-range electric fields of the phonons.
Fig. 1 presents the Raman spectra of the BiFeO3 sample excited with different excitation
Conclusion
The Raman spectra of BiFeO3 excited at different wavelengths have been systematically studied. The resonant behavior of the two-phonon peaks under the 532 nm excitation was attributed to the exchange mechanism between the nearest Fe3+ ions in BiFeO3. The Raman spectra of BiFeO3 excited at 532 nm were collected over the temperature range from 77 to 678 K. With increasing temperature, the dramatic changes in the frequency, the bandwidth and the intensity of the two-phonon at 1250 cm−1 as well as the A
Acknowledgments
This work was supported by the National Nature Science Foundation of China under Grant nos. 10674171 and 10874236.
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