Anomalous low-energy phonons in nearly tetragonal BiFeO3 thin films
K.-Y. Choi, S. H. Do, P. Lemmens, D. Wulferding, C. S. Woo, J. H. Lee, K. Chu, C.-H. Yang
aa r X i v : . [ c ond - m a t . s t r- e l ] A ug Anomalous low-energy phonons in nearly tetragonal BiFeO thin films K.-Y. Choi, S. H. Do, P. Lemmens, D. Wulferding, C. S. Woo, J. H. Lee, K. Chu, and C.-H. Yang Department of Physics, Chung-Ang University, Seoul 156-756, Republic of Korea Institute for Condensed Matter Physics, TU Braunschweig, D-38106 Braunschweig, Germany Department of Physics, KAIST, Daejeon 305-701, Republic of Korea
We present evidence for a concomitant structural and ferroelectric transformation around T S ∼
360 K in multiferroic BiFeO /LaAlO thin films close to the tetragonal phase. Phonon excitationsare investigated by using Raman scattering as a function of temperature. The low-energy phononmodes at 180-260 cm − related to the FeO octahedron tilting show anomalous behaviors uponcooling through T S ; (i) a large hardening amounting to 15 cm − , (ii) an increase of intensity by oneorder of magnitude, and (iii) an appearance of a dozen new modes. In contrast, the high-frequencymodes exhibit only weak anomalies. This suggests an intimate coupling of octahedron tilting toferroelectricity leading to a simultaneous change of structural and ferroelectric properties. The multiferroic compound BiFeO (BFO) is the fo-cus of experimental and theoretical research directed to-wards room-temperature multifunctional devices . Thedistinct multiferroic properties of BFO rely on the highferroelectric ( T c ∼ T N ∼
640 K) transition temperature, as well as their mutualcouplings . Bulk BFO has a rhombohedral structurewith R3c symmetry. This low symmetry puts a practicalconstraint on implementing switching devices.The possibility of circumventing this difficulty is pur-sued through strain engineering. With increasing strainabove 4 %, an isosymmetric transition takes place froma rhombohedral(R)- to a tetragonal(T)-like structure.The relative fraction of the T and the R phase is con-trollable as functions of both a film thickness and anelectric field . It has been shown that the magnetictransition temperature hardly varies but the ferroelec-tric Curie temperature is strongly reduced with strain .Octahedron tilting is suggested to be responsible for theanomalous strain dependence of ferroelectricity. X-raydiffraction, M¨ossbauer spectroscopy, and piezoresponseforce microscopy studies indicate that the majority T-like phase concomitantly undergo structural, magnetic,and ferroelectric transitions around T S ∼
360 K .Raman spectroscopy can serve as a sensitive, localprobe of structural and multiferroic properties. Singlecrystalline and low-strain BFO film exhibited an excep-tional coupling of multimagnon, phonon, and electronicexcitations . Raman scattering on highly strainedBFO films showed monoclinic (Cc symmetry) distortionsfrom the T-phase . However, a detailed temperaturestudy of these effects is unfortunately missing.In this study, we report on enormous anomalies of low-frequency FeO octahedra rotation modes. The concomi-tant, drastic change of the phonon parameters through T S suggests the mutual coupling of structural and mul-tiferroic properties.Two BiFeO thin films of different thickness (40 nmand 100 nm) were grown on (001) LaAlO (LAO) sub-strates by using pulsed laser deposition at the growthtemperature of 650 ◦ C and at the oxygen partial pressure (cid:11)(cid:68)(cid:12) (cid:11)(cid:69)(cid:12) (cid:20) µ (cid:80) (cid:20) µ (cid:80) FIG. 1: (Color online) Surface morphologies taken by scan-ning probe microscopy for the BFO films with thickness of(a) 40 nm and (b) 100 nm. of 100 mTorr. The surface morphologies were investi-gated by scanning probe microscopy (Veeco MultimodeV) with Ti/Pt coated Si tips (MikroMasch). Clear stepterrace structures, as shown in Fig. 1, reveal that bothfilms were grown in a step flow mode, indicating goodcrystallinity. The thinner film mainly consists of the T-like BFO, while the thicker one contains mixed phaseareas (darker areas in the AFM) where the T-like phaseand the R phase alternate on nano-length scales. We con-clude a strain relaxation in the thicker film. The surfacemorphologies and the thickness dependence of the phaseevolution match well to previous results .Raman scattering experiments have been performedin backscattering geometry with the excitation line λ =532 nm of Nd:YAG solid-state Laser by using a micro-Raman spectrometer (Jobin Yvon LabRam HR). Thelight beam was focused to a few µm -diameter spot on thesurface of the BFO thin films using a 50 times magnifi-cation microscope objective. The temperature is variedbetween 10 and 390 K by using a helium cryostat.Figure 2 compares unpolarized Raman spectra ofBFO (40nm) and BFO (100nm) thin films measured atT=10 K and T=395 K. Due to the small thickness of the
100 200 300 400 500 600 700
LAO
10 K
BFO(100nm) I n t en s i t y ( a r b . un i t s ) Raman shift (cm -1 ) BFO(40nm) (b)
LAO
231 690 (uu) 390 K
BFO(100nm)
BFO(40nm) (a)
FIG. 2: (Color online) (a) Comparison of unpolarized Ramanspectra between BFO (40nm) and BFO (100nm) thin filmsat 390 K. The given numbers are the phonon frequencies ofthe BFO (100nm) film. For reference Raman spectrum of aLAO substrate is also shown. (b) Unpolarized Raman spectraof BFO (40nm), BFO (100nm), and LAO at 10 K. The barsdenote the peaks from the BFO (100nm) film. studied films, we observe Raman signals from both thesubstrate and the BFO film. The substrate peaks areidentified by measuring separately the LAO crystal. At390 K we observe 6 phonon modes at 221, 231, 275, 365,594 and 690 cm − .For the P4mm tetragonal structure, the factor groupanalysis yields 8 Raman-active modes of 3A +B +4E.In the backscattering geometry normal to the (001) sur-face, 3A +B modes of them are symmetry allowed. Incomparison to Ref. , the 221, 275, 365, and 690 cm − modes are assigned to the tetragonal phase. The two ex-tra modes might be due to either monoclinic distortionsor the R-phase. This indicates that for temperaturesabove T S ∼
360 K the BFO film ( t ≤
100 nm) has anearly tetragonal structure. At 10 K we identify a total of22 phonon modes for frequencies above 170 cm − , whichmatch well with 27 expected modes (Γ = 14 A ′ + 13 A ′′ )for the monoclinic (Cc) symmetry. This suggests that amore monoclinic-like phase is stabilized at low tempera-tures . Both the BFO(40 nm) and the BFO(100 nm)
200 300 600 700
10 K (b)
Raman shift(cm -1 ) BFO(100nm) (a)
LAO I n t en s i t y ( a r b . un i t s )
50 K90 K120 K150 K180 K210 K240 K270 K330 K300 K360 K390 K
Raman shift(cm -1 ) FIG. 3: (Color online)(a) Temperature dependence of the low-frequency Raman spectra of the BFO(100 nm) thin film. Thedashed vertical lines are guide to the evolution of phonon fre-quencies. (b) Temperature dependence of the high-frequencypart of Raman spectra. films show the same number of phonons and no notice-able shift in the phonon frequency. The distinct differ-ence is seen in the relative intensities of the 177, 227, 244,274, 365, 409, 516 and 598 cm − modes, which are en-hanced in the BFO(100 nm) film. It is noteworthy thatthe numbers and frequencies are very close to those ob-tained from a rhombohedral crystal structure (compareto Table I of Ref. ). Since the substrate strain is releasedwith increasing thickness, these modes are attributed tothe minority R-phase. This evidences the coexistence ofthe T-like and the R-like phase.Figures 3 (a) and (b) zoom into the low- and the high-energy part of the Raman spectra. The low-energy modesbelow 300 cm − show a drastic temperature dependencein their number, frequency, and intensity while the high-energy modes exhibit a moderate change. To investigatethe evolution of the phonon modes in detail we fit themto Lorentzian profiles [see Fig. 5(a)]. The resulting fre-quency, linewidth, and intensity are plotted as a functionof temperature for the two lines at 180 and 690 cm − inFig. 4.With lowering temperature the 180-cm − mode under-goes a large hardening by 15 cm − , starting at around T S . Its linewidth decreases monotonically and its scat-tering intensity grows by one order of magnitude. As theRaman scattering intensity is proportional to the squareof the derivative of the dielectric function with respect tothe amplitude of the normal mode, the strong intensityincrease means that the low-energy modes are suscepti-ble to a change of ferroelectricity. Since the low-energy F r equen cy ( c m - ) L i ne w i d t h ( c m - ) N o r . I n t en s i t y T (K) F r equen cy ( c m - ) L i ne w i d t h ( c m - ) N o r . I n t en s i t y T (K)
FIG. 4: (Color online)(Left panel) Temperature dependenceof the frequency, the linewidth, and the normalized inten-sity of the low-energy 180 cm − mode for the BFO(100nm)film. (Right panel) Temperature dependence of the phononparameters of the high-energy 690 cm − mode. The shredbars indicate the structural transition temperature. modes are associated with the external vibrations of theFeO octahedra and polar cation displacements , theyprovide a measure of the tilting degree of the FeO octa-hedra . In this light, the concomitant change of inten-sity and frequency suggests that the increase of octahe-dral tilting accompanies the polar cation displacements.Exactly, this cross-coupling effect has been discussed asan origin of the reduction of the ferroelectric transitiontemperature under strain .The high-frequency modes are related to the internalvibrations of the FeO octahedra and are susceptible toa change in the Fe-O bond distance and angle. Thephonon parameters of the 690 cm − mode show onlysmall anomalies at about T S . Since the Fe-O bond angleand length determines a strength of superexchange inter-actions, we conclude that the magnetic interactions arerather weakly coupled to ferroelectricity.In Fig. 5 we focus on the evolution of the low-frequencyphonons with ∆ ω = 220 −
260 cm − . With decreas-ing temperature, several anomalies develop. For tem-peratures below T S , several extra modes appear at 231,244, and 248 cm − . Between T S and 150 K the phononfrequencies undergo a substantial hardening and thelinewidths strongly narrow. Noteworthy is the large en-hancement of the 222- and 234-cm − modes upon coolingthrough T S and the small drop of their intensity for tem-peratures below 170 - 240 K. The concomitant, drasticchange of the phonon number and intensity indicates acomplex nature of the phase transition. The increasednumber of the phonon modes is due to a transition from the more T-like phase to the monoclinic-like one. Thestructural change can also lead to the enhancements ofphonon scattering intensity. However, the observed huge R a m an s h i ft ( c m - ) Temperature (K) L i ne w i d t h ( c m - ) Temperature (K) I n t en s i t y ( a r b . un i t s ) Temperature (K) (a)(c) (b) I n t en s i t y ( a r b . un i t s ) Raman shift (cm -1 ) (d)
10 K
FIG. 5: (Color online) (a) Fit of the low-energy Raman spec-trum to Lorentzian profiles at 10 K. (b), (c), and (d) Tempera-ture dependence of the optical phonon frequencies, linewidthsand intensities. anomalies invoke the strong change of electronic polar-izabilities beyond the structural transformation. As dis-cussed in the 180 cm − rotation mode, the octahedraltiltings are intimately coupled to ferroelectricity. Theincrease of the octahedral tilting degree seems to beclosely related to the stabilization of the monoclinic-likephase. Therefore, the intensity-polarizability enhance-ments of the low-energy modes are evidence for the cou-pled structural-ferroelectric transition. Actually, Infante et al., showed that a hard to a soft ferroelectric transi-tion accompanies the structural and magnetic phase tran-sitions.In summary, we have presented a Raman scatteringstudy of nearly tetragonal BFO thin films in the temper-ature range of 10 - 390 K. We observe large anomalies ofthe internal phonon modes of 180 − − , which aresusceptible to the tiltings of the FeO octahedron. Themost salient feature is the strong enhancement of phononintensity and the increase of a phonon number througha structural phase transition. Our study suggests thata structural and ferroelectric transition is closely tiedby cross-coupling effects between octahedron tiltings andpolar cation displacements.We acknowledge financial support from DFG, theAlexander-von-Humboldt Foundation and Korea NRFGrants No. 2009-0093817 and No. 2010-0013528. J. Wang, J. B. Neaton, H. Zheng, V. Nagarajan, S. B.Ogale, B. Liu, D. Viehland, V. Vaithyanathan, D. G.Schlom, U. V. Waghmare, N. A. Spaldin, K. M. Rabe,M. Wutting, R. Ramesh, Science , 1719 (2003). R. J. Zeches, M. D. Rossell, J. X. Zhang, A. J. Hatt, Q.He, C.-H. Yang, A. Kumar, C. H. Wang, A. Melville, C.Adamo, G. Sheng, Y.-H. Chu, J. F. Ihlefeld, R. Erni, C.Ederer, V. Gopalan, L. Q. Chen, D. G. Schlom, N. A.Spaldin, L. W. Martin, and R. Ramesh, Science , 977(2009). H. Bea, B. Dupe, S. Fusil, R. Mattana, E. Jacquet, B.Warot-Fonrose, F. Wilhelm, A. Rogalev, S. Petit, V. Cros,A. Anane, F. Petroff, K. Bouzehouane, G. Geneste, B.Dkhil, S. Lisenkov, I. Ponomareva, L. Bellaihe, M. Bibes,A. Barthelemy, Phys. Rev. Lett. , 217603 (2009). I. C. Infante, S. Lisenkov, B. Dup´e, M. Bibes, S. Fusil, E.Jacquet, G. Geneste, S. Petit, A. Courtial, J. Juraszek, L.Bellaiche, A. Barth´el´emy, and B. Dkhil, Phys. Rev. Lett. , 057601 (2010). Y. E. Roginska, Y. Y. Tomashpo, Y. N. Venevtse, V. M.Petrov, and G. S. Zhdanov, Sov. Phys. JETP , 47-51(1966). S. V. Kiselev, R. P. Ozerov, and G. S. Zhdanov, Sov. Phys.Dokl. , 742-744 (1963). D. Mazumdar, V. Shelke, M. Iliev, S. Jesse, A. Kumar, S.V. Kalinin, A. P. Baddorf, and A. Gupta, Nano Lett. ,2555 (2010). I. C. Infante, J. Juraszek, S. Fusil, B. Dup´e, P. Gemeiner,O. Di´eguez, F. Paillouz, S. Jouen, E. Jacquet, G. Gen-este, J. Pacaud, J. ´Iniguez, L. Bellaiche, A. Barth´el´emy,B. Dkhil, and M. Bibes, arXiv: 1105.6016 (2011), unpub- lshed. M. O. Ramirez, M. Krishnamurthi, S. Denev, A. Kumar,S-Y. Yang, Y-H. Chu, E. Saiz, J. Seidel, A. P. Pyatakov,A. Bush, D. Viehland, J. Orenstein, R. Ramesh, and V.Gopalan, App. Phys. Lett. , 022511 (2008). M. O. Ramirez, A. Kumar, S. A. Denev, Y. H. Chu, J.Seidel, L. W. Martin S.-Y. Yang, R. C. Rai, X. S. Xue,J. F. Ihlefeld, N. J. Podraza, E. Saiz, S. Lee, J. Klug, S.W. Cheong, M. J. Bedzyk, O. Auciello, D. G. Schlom, J.Orenstein, R. Ramesh, J. L. Musfeldt, A. P. Litvinchuk,and V. Gopalan, App. Phys. Lett. , 161905 (2009). M. N. Iliev, M. V. Abrashev, D. Mazumdar, V. Shelke, andA. Gupta, Phys. Rev. B , 014107 (2010). G. Xu, H. Hiraka, G. Shirane, J. Li, J. Wang, and D.Viehland, Appl. Phys. Lett. , 182905 (2005). A. J. Hatt, N. A. Spaldin, and C. Ederer, Phys. Rev. B , 054109 (2010). R. Palai, H. Schmid, J. F. Scott, and R. S. Katiyar, Phys.Rev. B , 064110 (2010). Although Fe ions of the T-like phase is thought tobe coordinated in FeO pyramids, isomer shift values ofM¨ossbauer spectroscopy indicate that Fe ions of boththe T- and R-like phases are in octahedral enviroments . P. Rovillain, M. Cazayous, Y. Gallais, A. Sacuto, R. P. S.M. Lobo, D. Lebeugle, and D. Colson, Phys. Rev. B ,180411(R) (2009). K.-Y. Choi, P. Lemmens, T. Sahaoui, G. Guntherodt, Yu.G. Pashkevich, V. P. Gnezdilov, P. Reutler, L. Pinsard-Gaudart, B. Buchner, and A. Revcolevschi, Phys. Rev. B71