Monoclinic phase transition in stress-induced BiFeO3 epitaxial films

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(1)Monoclinic phase transition in stress-induced BiFeO3 epitaxial films. Yen-Chin Huang 1, Hsin-Hua Lee1, Yi-Chun Chen1, Ching Cheng1, Kuang-I Lin 1, Jenn-Shyong Hwang 1, Wen-I Liang2, Hsiang-Jung Chen2, and Ying-Hau Chu2 1Department of Physics, National Cheng Kung University, Tainan, Taiwan 2Department of Material Science and Engineering, National Chiao Tung University, Hsinchu, Taiwan Material system near morphotropic phase boundary usually attracted a lot of attention due to their unique physical properties. The key issue of the mechanism is to reveal the coupling between the multiple phases or the intermediate states during phase transformation. Recently, highly-strained multiferroic BiFeO3 (BFO) films had been reported to possess a particular isosymmetric boundary between tetragonal(T) and rhombohedral(R) phases. In this study, we investigated the as-grown state of mixed-phase BFO and the evolution of phases under external stimulus. Through first principle study and Raman measurement, we found the mixed phase BFO films at room temperature included two monoclinic phases, R-like MA (P1) and T-like MC (Pm). MC phase transformed gradually to T-like MA phase when the temperature was increased to about 448 K. The R-like phase disappeared at about 450 ℃ . However, the T-like phase lasted to higher temperatures. This result showed the possible path of transition near the morphotropic phase boundary.. BFO is rhombohedrally distorted perovskite. It is the multiferroism material that exhibits ferroelectricity (TC~1100 K) and G-type antiferromagnetism (TN~643 K) at room temperature. When growing on the substrate of compressive stress, the BFO structure coexists tetragonal-like and rhombohedral-like phase. (a) The mix phases are also showed in the topography. (b)(c)Through the RSM, these two phases belong to the monoclinic MC and MA structures respectively. (a). (b). (a). (b). Double monochromator microscope. (c) (c). Laser(532nm) scanning direction. TIP. 1 μm. TIP. (a) Topography with thickness : 90 nm. 110 nm. 160 nm. PC. mirror. Schematic diagram of the (a) Piezo- response force microscopy and (b) Raman instrument. (c) By rotating the sample, the orientation of distinct polarization is directly obtained by analyzing the in-plane signal with different contrast.. Sample rotated 45∘. 20 nm. CCD. R. (c) In-plane PFM :. (d) Simulated structures of T-like (Pm) and R-like (P1) phases :. P1 phase :. 1 μm. P. (b) Thickness-dependent Raman spectra : R3. R3. R3 R3 R3. R3 R3. R3 R3. 200 nm. P1/Pm. R3 200 nm. P parallel [110] a = 3.60 Å b = 4.01 Å c = 4.86 Å. P1/Pm P1/Pm. P1. 160 nm. 145 nm 125 nm 110 nm 90 nm. 145 nm. P. 125 nm 110 nm. a = 3.90 Å b = 3.67 Å c = 4.98 Å. P parallel [100]. 90 nm. Fig. (a)(b) The T-like phase sample transformed to the mixed (T-like and R-like) phase sample when 35 nm increasing thickness. (c) PFM also showed at least two phases with in-plane polarization along [100] and 20 nm [110], which may correspond to the MC and MA phases, separately. (b)(d) Raman spectrum and the first principle calculation showed the T-like and R-like phases belonged to Pm and P1 symmetry. The ratio of P1 200 250 300 350 400 to Pm phase increased with thickness. The in-plane polarization of Pm phase paralleled to the [100], and -1 the P1 phase along [110]. Raman shift (cm ). 35 nm 20 nm. 600. Pm phase :. 55 nm. 55 nm. 400. 160 nm Pm. Intensity (a.u.). P1/Pm. P1/Pm P1. Intensity (a.u.). Pm P1. 200. R3. 800 -1. Raman shift (cm ). (c) The topography changed as temperature of BFO film increased:. (a) Temperature-dependent Raman spectra : o. P. o. 600 C o. 450 C. 125 ℃. 25 ℃. 250 C. 175 ℃. 175 ℃. 150 ℃. 200 ℃. P. o. 175 C. o. 400 C o. o. 275 C o. 250 C o. 175 C o. 150 C. o. 125 C. o. 450 C o. 450 C o. 75 C. o. 75 C o. 25 C o. 25 C. 600. 800 -1. Raman shift (cm ). 600 C. 600 C. o. 400. o. o. 125 C. 200. (e). 200. 250. 300. 350. (d) Temperature-dependent X-ray diffraction :. 400. o. 400 C. o. 350 C. o. 325 C. o. Intensity (a.u.). 325 C. (b) In-plane polarization rotated from [100] to [110] :. Intensity (a.u.). o. o. 150 C. Intensity (a.u.). Intensity (a.u.). 350 C. 2.2 μm. 400 C. o. 350 C o. 325 C o. 275 C. o. -1. 275 C. Raman shift (cm ). o. 250 C. o. 250 C. •Piezo-response microscopy showed at least two phases with in-plane polarization along [100] and [110] direction in the strained BFO films. This result corresponded with the MC and MA phases shown in X-ray data. •Raman and first principle simulation results suggested the two phases in mixed-phase BFO were of Pm (T-like) and P1 (R-like) symmetry. •The in-plane polarization of T-like phase (Pm) rotated from [100] to [110] at about 175 ℃. •The ratio of the R-like (P1) phase decreased with increasing temperature, and totally disappeared at about 450 ℃.. 200. 250. 300. 350 -1. 400. o. 175 C. 700. 750. 800. 850. 900. 950 -1. Raman shift (cm ) Raman shift (cm ) Fig. (a) The peak about 360 cm-1 was disappeared about 175 ℃, which showed the Pm phase transformed to another phase. (b)The in-plane polarization also rotated from [100] to [110]. (c) The stripes of R-like (P1) phase became thinner with temperature. (d) It disappeared at about 450 ℃. (d)(e) At about 300 ℃, the peak of R-like phase split to two peaks, which corresponded with the appearance of new vibrational mode at 810 cm-1. Both PFM and Raman results showed that the Tlike phase lasted to at least 600 ℃. Supported by Ying-Hao Chu Lab.. This work was supported by National Science Council through project NSC 96-2112-M-006-018-MY3..

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