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Surface lattice dynamics of La0.65Ca0.35MnO3 across the Curie temperature
surface science ELSEVIER
Surface Science 393 (1997) 64-71
Surface lattice dynamics of Lao.65Cao.35MnO3 across the Curie temperature Jiandi Zhang a.,, E.W. Plummer a.b " Dept. of Physics and Astronomy, The University of Tennessee, Knoxville, TN 37996, USA b SolidState Division, MS-6057, Oak Ridge NationalLab, Oak Ridge, TN37831, USA Received 15 May 1997; accepted for publication 20 June 1997
Abstract
We report a temperature-dependent study of the surface optical phonons of Lao.65Cao.35MnO3 films using high-resolution electronenergy-loss spectroscopy. Three surface modes have been observed and correspond to the bulk modes which are associated with the internal vibrations of the MnO6 octahedron. However, analogous to the high-To superconducting cuperates, the surface phonons have higher energies than the bulk modes, indicating that the surface and the bulk have significantly different lattice dynamics. The small energy dispersion and large linewidth reflect a short phonon life time. The strong temperature dependence of the linewidth corresponds to coupled-random dynamic lattice distortions. A 2 4 meV shift in the phonon energy across the Curie temperature is associated with a static lattice distortion related to the magnetic phase transition. The temperature dependence of the surface phonon modes are compared with the current understanding of the bulk lattice dynamics, above and below the critical temperature for the structural, electronic, and magnetic phase transitions in the "colossal" magnetoresistance materials. © 1997 Elsevier Science B.V.
Keywords: Electron energy loss spectroscopy; Magnetic phenomena; Surface waves
1. Introduction
The perovskite manganites Rl_xAxMnO3 (R = rare earth, A=alkaline earth) have attracted renewed attention owing to the remarkable temperature and magnetic field dependence of their electronic and magnetic properties [1-3]. Of special interest are the compounds with electronic hole doping concentrations x~½ which exhibit a "colossal" magnetoresistance (CMR) [1]. These compounds have paramagnetic and nonmetallic behavior at high temperature, and, ferromagnetic and metallic behavior at low temperature [4]. Such * Corresponding author. Fax: (+ 1) 423 5768135; e-mail: [email protected] 0039-6028/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. Pll S0039-6028 ( 9 7 ) 0 0 5 5 9 - 1
coupled electronic and magnetic phase transitions are also associated with the static and dynamic lattice distortions in these systems [2,3,5-7]. Interest in CMR compounds has been expanding, due to the increasing capability to fabricate epitaxial thin films which have potential technological applications as magnetic sensors. Although thin films are quite stable and exhibit similar CMR behavior to that observed in related bulk materials, the physical properties of the surface or interface are still unknown; a change in the surface composition or lattice structure could lead to a dramatic change in the magnetic and electronic properties. In this paper, we report on measurements of the surface lattice dynamics from La0.65Cao.35MnO3 films across the Curie temperature with high-
J. Zhang, E. V~ Plummer / SurJitce Science 393 (1997) 64 71
resolution electron energy loss spectroscopy (HREELS), a highly surface-sensitive technique [8]. These results are compared with similar measurements from the bulk compound. Our expectation was that the surface measurements might shed light on the bulk lattice dynamics, Historically, a magnetic phase transition coupled with an unusual change in transport behavior in the bulk materials was explained using the concept of a double exchange (DE) interaction [9]. However, recent theories have suggested that, in addition to the DE interaction, a strong electron phonon interaction arising from the lattice Jahn-Teller (JT) distortion also plays a crucial role in electronic and magnetic phase transitions [10,11]. The basic physical picture is as follows: without a JT distortion, the parent compound, LaMnO3, in its ideal cubic structure would be metallic with the Fermi level lying in the middle of the eg bands; a static JT distortion does occur, however, in the MnO6 octahedron. The M n - O basal-plane oxygen atoms (0(2)) are displaced by at least 0. l A from their ideal positions, creating an orthorhombic lattice structure and an insulating gap in the JT split eg bands [12, 13]. By randomly replacing La with divalent atoms like Ca, holes can be doped and will gradually deplete the JT-split e~ band with increasing doping concentration x. By producing an effective random field, the doped holes will disturb the long-range order in the lattice structure such that the static JT distortion is diminished compared with that in the parent compound. On the other hand, doping also introduces a dynamic JT distortion, a local fluctuation of the oxygen atoms away from their original crystallographic positions. Around most of the Mn ions there is a local tetragonal distortion, whereas around a few of them there is a locally symmetric (breathing) distortion [ 11 ]. The dynamic JT distortion, which attempts to localize (trap) the conduction band electrons as polarons due to the electron-phonon interaction, competes against the DE interaction, creating a metallic-ferromagnetic phase. Such competition can be manipulated by either temperature or doping and leads to a metal insulator transition around the Curie temperature (T~). Recent neutron diffraction experiments on the
65
bulk Lal ~Ca~MnO3 compounds have provided direct evidence for both the static and dynamic JT distortions across the phase transitions in these systems [5-7]. We found that the Ca-doping and temperature dependence of both the static and dynamic lattice distortion are inherently coupled with the electronic and magnetic phase transitions in bulk La 1 xCaxMnO3, though the lattice structure keeps an orthorhombic symmetry across the transitions [5]. A static lattice distortion anomaly at Tc is associated with the ferromagnetic-to-paramagnetic phase transition; however, a dynamic lattice distortion anomaly at T~ is related to the metal-to-insulator transition. Using pair-distribution-function analysis, Billinge et al. [6] provided the direct observation of lattice polaron formation associated with local structure distortion.
2. Experimental detail The La0.65Cao.35MnO3 film was ~ 2500 ,~ thick, grown on a (001) surface of LaAIO3 substrate by rf sputtering [14]. The film was fully annealed in an oxygen atmosphere to improve the compositional homogeneity and analyzed with SEM and X-ray diffraction, showing a single phase with a perovskite-like structure. The quality of the film was characterized by the measurements of its bulk properties such as the resistivity and magnetization as a function of temperature using a four point probe and a SQUID magnetometer respectively. As shown in Fig. 1, the film exhibits a transition from a ferromagnetic-metallic phase to a paramagnetic-insulating phase near 260 K (T~). The surfaces were cleaned in a UHV chamber with a base pressure of ~ l × 1 0 - 1 ° T o r r by annealing at 450 500C. Cleanness was checked by Auger electron spectroscopy and showed no obvious contamination. An LK-2000 spectrometer with an angular resolution of ~ 1~ and a system energy resolution of 5.5-6.0meV was used for the HREELS measurements. The energy loss spectra were taken with both specular and off-specular geometries where the scattering angle was varied. The incident electron beam energy was 3.13 eV and the incident angle Oo was fixed to 60 '~ from the surface normal direction. Such a set-up of
Z Zhang, E. W. Plummer / SurJhce Science 393 (1997) 64-71
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r (K) Fig. 1. The temperature dependence of the magnetization(0) and resistivity(A) of the Lao.6sCao.35MnO3thin film.The bulk unit cell with a distorted cubic perovskite (i.e. orthorhombic) structure is shown in the inset and the Curie temperature T~of the system is indicated. scattering geometry provides a rather surface-sensitive probe. The temperature was measured with a type-K thermocouple attached to the sample holder.
3. Results and discussion
The surface order and crystalline structure of the Lao.65Cao.35MnO3 film were inferred from the square (1 x 1) low energy electron diffraction (LEED) pattern observed. Referring to the distorted cubic perovskite structure in the bulk, as determined by neutron diffraction and shown in the inset of Fig. 1 [5], the approximate size of the surface unit cell derived from the LEED pattern is about 5.5_+0.1 A2. This is consistent with the surface being terminated such that the b-axis is normal to the surface. This is also the orientation derived from X-ray scattering from the film. There are still two possibilities for the (001) surface termination, i.e. either the M n - O plane (see the inset of Fig. 1) or the L a / C a - O plane. As determined from the analysis of the emission angle dependence of the core level spectra in X-ray photoelectron spectroscopy (XPS) [15], the surface is terminated by the M n - O plane. This is consistent with the photoemission results for the Lao.65Cao.35MnO3 [ 16 ] and Lao.65Bao.35MnO 3 [ 17]
films, in which the surface electronic structure is dominated by the M n - O bonding states• The surface composition was characterized by the relative Auger and angle-resolved X-ray photoelectron spectroscopy (ARXPS) intensity ratios of Ca and La, described in detailed elsewhere [15]. It was found that the Ca/La ratio at the surface is different from that in the bulk, indicating a substantial segregation of Ca to the surface region, with the surface Ca concentration corresponding to x=0.59. Another key to the interpretation of the surface measurements is the surface oxygen stoichiometry. We found that varying our sample cleaning procedure does not have any obvious effect on our measurements• In particular, exposing the sample to oxygen during the annealing (at 450-500°C) and at room temperature does not change either the LEED pattern or the HREELS spectra compared with no oxygen treatment. This observation only proves that there is a stable oxygen surface stoichiometry, not that it is the same as in the bulk. Fig. 2 displays the electron energy loss spectra as a function of scattering angle O away from the specular and along F to X direction of the surface Brillouin (see the inset of Fig. 3b) at 100 K. There are three clearly identifiable loss peaks in the spectra which are associated with three surface optical phonon modes, ~ol (26 meV), 402 (58 meV) and, ~o3 (84 meV). These surface phonons have substantially higher energy than those observed in the bulk. Recent IR spectra from Lao.TCao.3MnO3 [18] show three major bulk optical phonon modes at about 20, 50, and 72 meV. These phonons correspond to the three Flu optical modes in an ideal cubic perovskite structure [19], though a structure distortion could change the energy of these modes or even induce new modes. The inset of Fig. 3a depicts the motion of the atoms for each of the three modes. Since the surface modes are similar in energy to the bulk modes they can be identified according to the assignment published for the bulk [18,20]• The feature with the lowest loss energy (0~1) is associated with the external mode, representing the vibration of the La/Ca ions against the MnO 6 octahedron (i.e. external to the MnO6). The loss feature at 58 meV (0~2), is the M n - O - M n bending
J. Zhang, E. W. Plummet / Sur/dce Science 393 (1997) 64 71
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Energy Loss (meV) Fig. 2. HREELS spectra from the surface of the La0.,+sCa0.35MnO 3 film at 100 K as a function of the scattering angle offthe specular (and a l o n g / ' t o X) direction (S) as shown in the inset. The incident beam energy was 3.13 eV and the incident angle 60' from the surface normal direction.
mode in the M n - O surface plane. The highest energy mode at 84 meV (~3) is the stretching mode, associated with the vibration of the Mn ion against the oxygen octahedron. These assignments are in qualitative agreement with the behavior of the loss features with changing scattering geometry, The phonon intensity as a function of the in-plane momentum transfer at 100 K (the temperature for the metallic phase) is plotted in Fig. 3a. The measured two-dimensional dispersion of phonon energy along F to X of the surface Brillouin zone at both 100 K and 300 K (the temperature above the magnetic and metal-to-nonmetal transition) is shown in Fig. 3b. Only the intensity and the dispersion of ~o2 and +~ are shown in the figure since the co~ mode is too weak to perform a systematic characterization. The intensities are normalized to the intensity of the quasi-elastic peaks• As shown
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in Fig. 3a, the intensity of the loss feature (02 decreases much faster than that of ~')3 with increasing scattering angle away from specular direction. This indicates that the mode oJ2 exhibits a stronger dipole-active character, truly reflecting the M n - O ( 2 ) - M n bending vibration out of the surface plane. Both (D2 and co3 disperse in energy along F to X with the surface momentum-transfer, corroborating the surface ordering as observed from LEED. However, the dispersion is only about 2 to 4meV, consistent with a coupled-random lattice vibration picture. Figs. 4-6 address the temperature dependence
J. Zhang, E. VK Plummer / Surface Science 393 (1997) 64 71
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E n e r g y Loss ( m e V ) Fig. 4. The temperature dependence of normalized HREELS spectra from the surface of the Lao.65Cao.35MnO3 film. The spectra were taken in the specular direction. The incident beam energy was 3.13 eV and the incident angle 60 ° from the surface normal direction.
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of the HREELS spectra from the Lao.65Cao.35MnO3 surface. There are several interesting phenomena observed with changing temperature for both the surface and bulk. First, all three phonon peaks shift to lower energies with increasing temperature, as seen in Figs. 4 and 5. The three modes are located at 22 meV, 55 meV and 82 meV at T= 300 K, according to our best fit. Relative shifts, [~o(100 K)-~o(300 K)]/~o(300 K), are about 3-5%. Similar shifts in energy have also been observed in the bulk phonon spectra (Fig. 5) [18]. Such shifts are consistent with the static lattice distortion which is associated with the magnetic phase transition, but inconsistent with the metal-to-insulator transition in the bulk [1,2]. A dramatic anisotropic volume expansion is observed in the bulk material and occurs near the Curie temperature [5], indicating a strong static lattice distortion in the system. Such a distortion should be responsible for softening the surface and bulk phonons with increasing temperature, and might inherently couple to the magnetic phase transition observed in the bulk [5]. We note that, in normal Debye theory, the derivative of the "root mean
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Temperature (K) Fig. 5. The temperature dependence of the observed surface phonon energies of Lao.65Cao.35MnO3 (O), compared with those of the bulk measured by Kim et al. ( 0 ) [18]. The lines are a guide for the eye.
square" (RMS) motion amplitude of atoms with temperature, d(u2)/dT, is inversely related to the Debye temperature, which is proportional to the vibration frequency. From the measurement of (u 2) for bulk La0.65Ca0.asMnO3 by neutron scattering [5], the Debye temperature in the metallic phase (at low temperature) is lower than that in the insulating phase (at high temperature). This means that, according to simple Debye theory, the phonon frequency in the metallic phase should be smaller than that in the insulating phase, which contradicts the experimental observations of the phonon shifts both in the bulk and on the surface. Therefore, from the temperature dependence of the phonon energy, we conclude that the static lattice distortion and magnetic phase transition dominate the lattice dynamics.
J. Zhang, E. W, Plummer / SurIitce Science 393 (1997) 64 71
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The temperature dependence of the quasi-elastic peak is related to surface roughness, lattice dynamics, surface free-carrier density, and surface conductivity [21]. For Lao.6sCa0.35MnO3 there is a metal-to-insulator transition with temperature, so the change in asymmetry of the quasi-elastic peak with temperature can be related directly to the change of the bulk transport properties. By fitting both sides of the quasi-elastic peak to Gaussian functions, we find that the asymmetry decreases when the temperature changes from 100 K to 300 K, and then increases from 300 K to 450 K (see Fig. 6a). This behavior can be explained qualitatively by the change of allowed excitations with temperature. At 100 K, the metallic character creates a large asymmetry due to the strong Drudelike excitations. At 300 K, the insulating behavior minimizes the low energy excitations, creating a more symmetric quasi-elastic peak. At 450 K, the
69
asymmetry increases again with temperature due to the thermal population of the conduction band. The trend of the lineshape of the quasi-elastic peak indicates that the surface dielectric response is similar to that in the bulk, i.e. a metal-to-insulator transition likely occurs at the surface, consistent with the recent observation from the photoemission measurements [14]. On the other hand, the change of linewidth of the quasi-elastic peak as a function of temperature cannot be simply explained by collective excitations [21]. The FWHM of the quasi-elastic peak from the surface is almost proportional to T 2, independent of the change in the bulk conductivity (Fig. 6a). It is possible to account for such a change in FWHM with temperature if the phonon modes are dominated by lattice dynamical effects due to the change in electron-phonon coupling. However, these dynamical effects must be strong enough to overcome the suppression of the Drude tail as the system ceases to be metallic. The large phonon widths indicate a short phonon life time. The linewidth AE of ~-')2and o)3 is typically from 17 to 26 meV and changes with temperature as shown in Fig. 6b, indicating a strong temperature dependence in lattice dynamics. The increase in phonon damping (i.e. the decrease in intensity) and linewidth with increasing temperature may relate directly to the increase in surface random dynamical lattice distortions observed in the bulk [5 7] and suggested from theoretical calculations [10,11]. This should be associated with the random Ca-doping distribution in the system. As has been measured by Ramirez et al. [22], the sound velocity Vg in kao.TCao.3MnO3 at room temperature is 4.3 x 105 cm s-1. The mean free path Al of phonons can be estimated roughly as only about 2 A, by applying our data of the phonon linewidth (which includes the instrumental broadening) and by using the equation AI = VgAI = 11Vg/(2nAE), where tl is the Planck constant. Even considering the instrumental broadening to the phonon peaks, the mean free path of these optical phonons is still quite small, comparable with the size of the unit cell volume in the compound. Such a small mean free path can also be obtained directly from the dispersion behavior. As shown in Fig. 3, the dispersion of these optical
70
J. Zhang, E. W. Plummet / Surface Science 393 (1997) 64-71
phonons is small, within 4-5 meV. If we simply apply the relation as AI~ (dE/dqO/dE, where qll is the in-plane momentum transfer, and then use the dispersion results from Fig. 3, we can also obtain a mean free path smaller than 2 A. With such a small mean free path, we can conclude that the vibrations in these systems are rather local and strongly coupled. It is interesting to compare the observation of the surface phonons in CMR compounds with that in high-To superconducting materials [23-26]. A similarity in the behavior of these phonon modes can be drawn. First, just as in the CMR case, three major vibrational modes have been observed with energies of 26, 50, and 80 meV at the surface of Bi2Sr2CaCuOs+o [24]. It is found that the surface modes are associated with three major bulk ones located near 20, 36 and 70 meV, observed in Raman and infrared reflectivity spectra [25]. Second, the surface optical phonons in both CMR and high-T~ superconducting materials have substantially higher energies than the corresponding bulk ones. Such a difference reflects the nature of the surface termination, which can cause a change in the local bonding by breaking the octahedral structure of MnO 6 in CMR and CuO6 in high-T~ superconducting compounds. The change in the composition between the surface and bulk can be also accounted for by the difference in the phonon energies. Another similarity is the temperature dependence of these phonon modes. In the surface of high-T~ superconductors like Bi2Sr2CaCuO 8+~, a small decrease in phonon energies with increasing temperature has also been observed [24]. Despite the fundamental differences in the physical properties between CMR and high-T~ superconducting materials, the similarity in these optical phonon behavior may be related to their similar origin, i.e. from the internal vibrations of the M O 6 ( M = Mn, Cu) octahedron, though the lattice distortions do affect individual vibrational modes in the phonon structure. In order to study the interplay between lattice dynamics and electronic or magnetic phase transitions at the surface of these CMR compounds, it is very important to identify the surface lattice dynamics responding to the metal-to-insulator transition and to magnetic phase transition sepa-
rately. One way to do that is to study the doping dependence of the surface static and dynamic lattice distortion, as that has been studied for the bulk materials [5]. We attempted to study the surface lattice dynamics of an Lao.9Ca0.1MnO3 film which exhibits only a magnetic phase transition at Tc=150 K in the bulk. However, the ARXPS studies indicate a much larger Ca surface segregation, with the Ca concentration at the surface as much as x=0.92, and different surface termination compared with the surface of La0.65Ca0.35MnO3 film [15], so that makes the comparison for different Ca-doping concentrations impossible. Nonetheless, a detailed theoretical study on phonon band structure will be important to understand the lattice dynamic effects on both the bulk and surface phonon structures. For example, the ~92 loss peak may include more than one phonon mode which can be associated with the lifting of degeneracy due to the lattice distortion on the surface. Understanding the physics involved needs not only more detailed experimental studies on temperature dependence, but also theoretical calculations of phonon modes based on different lattice distortions.
4. Summary In conclusion, we have observed three optical phonons on the Mn-O-terminated Lao.65Cao.35MnO3 thin film. These modes are assigned as the external, bending, and stretching modes of the MnO6 octahedron in agreement with the bulk measurements. The surface phonons have higher energies than the equivalent modes in the bulk, similar to what have been observed on high-T¢ superconductors. The modes decrease in energy with increasing temperature across the Curie temperature, indicating that the bulk lattice distortion associated with the magnetic phase transition is similar at the surface. The temperature dependence of the width and asymmetry of the quasi-elastic peak reveals the presence of a metalto-nonmetal transition at the surface. This preliminary study of the surface lattice dynamics of Lao.6sCao.35MnO3 indicates the need for detailed studies of the electronic, geometric,
J. Zhang, E. V~ Plummer / Surface Science 393 ( 19971 64 71
and magnetic properties of surfaces of these complex materials. The key will undoubtedly be knowing and/or controlling the surface stoichiometry. Better in situ growth procedures are required, as well as a surface-sensitive magnetic measurement such as spin-polarized electron energy loss spectroscopy.
[8] [9]
[10]
Acknowledgements We gratefully acknowledge the use of samples provided by F. Foong and S.-H. Liou of University of Nebraska-Lincoln. We also benefited from helpful discussions with P.A. Dowben, P. Dai, A.T. Hanbicki, and P. Hyldgaard. This work was supported by the JRCAT of Japan. Oak Ridge National Lab is supported by the US DOE under contract No. DE-AC05-96OR22464, managed by Lockheed Martin Energy Research Corp. References [1] R. Von Helmolt, J. Wecker, B. Holzapfel, L. Schultz, K. Samwer, Phys. Rev. Lett. 71 (1993) 2331. S. Jin, T.H. Tiefel, M. McCormack, R.A. Fastnacht, R. Ramesh, L.H. Chert, Science 264 (19941 413. [2] HY. Hwang, S.-W. Cheong, P.G. Radaelli, M. Marezio, B. Batlogg, Phys. Rev. Len. 75 (1995) 914. P. Schiffer, A.P. Ramirez, W. Bao, S.-W. Cheong, Phys. Rev. Lett. 75 (19951 3336. M.R. Ibarra, P.A. Algarabel, C. Marquina, J. Blasco, J. Garcia, Phys. Rev. Lett. 75 (19951 3541. J.M.D. Coey, M. Viret, L. Ranno, K. Ounadjela, Phys. Roy. Lett. 75 (1995) 3910. K. Khazeni, Y.X. Jia, L. Lu, V.H. Crespi, M.L. Cohen, A. Zettl, Phys. Rev. Len. 76 ( 19961 295. J. Fontcuberta, B. Martlnez, A. Seffar, S. Pifiol, J.k. Garcia-Mufioz, X. Obradors, Phys. Rev. Len. 76 (19961 [122. [3] A. Asamitsu, Y. Moritomo, Y. Tomioka, T. Arima, Y. Tokura. Nature 373 (19951 407. G. Zhao, K. Conder, H. Keller, K,A. Miiller, Nature 381 (1996) 676. J.-S. Zhou, W. Archibald, J.B. Goodenough, Nature 381 (1996) 770. [4] G.tt. Jonker, J.H. Van Santen, Physica 16 (1950) 337. E.O. Wollan, W.C. Koehler, Phys. Rev. 100 (1955) 675. C.W. Searle, S.T. Wang, Can. J. Phys. 47 (19701 2703. [5] P. Dai, J. Zhang, H.A. Mook, S.-H. Liou, P.A. Dowben, E.W. Plummer, Phys. Rev B 54 (19961 R3694. P. Dai, J. Zhang, H.A. Mook, S.-H. Liou, P.A. Dowben, E.W. Plummer, Solid State Commun. 100 (19961 865. [6] S.J.L. Billinge, R.G. DoFrancesco, H.H. Kwei, J.J. Neumeier, J.D. Thompson, Phys. Rev. Lett. 77 (1996) 715. [7] P.G. Radaelli, D.E. Cox, M. Marezio, S.-W. Cheong, P.E. Schiffer, A.P. Ramirez, Phys. Rev. Lett. 75 (1995) 4488.
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P.G. Radaelli, M. Marezio, H.Y. Hwang, S.-W. Cheong, B. Batlogg, Phys. Rev. B 54 (1995) 8992. H. Ibach, D.L. Mills, Electron Energy Loss Spectroscopy and Surface Vibrations, Academic Press, New York, 1982. C. Zener, Phys. Rev. 82 ( 1951 ) 403. P.W. Anderson, H. Hasegawa, Phys. Rev. 100 (19551 675. P.G. DeGennes, Phys. Rev. 118 (1960) 141. K. Kubo, N. Ohata, J. Phys. Soc. Jpn. 33 (1971) 21. A.J. Millis, P.B. Littlewood, B.I. Shraiman, Phys. Rev. Lett. 74 (1995) 5144. H. R6der, J. Zang, A.R. Bishop, Phys. Rev. Lett. 76 (19961 1356. A.J. Millis, Phys. Rev. B 53 ( 19961 8434. A.J. Millis, B.I. Shraiman, R. Mueller, Phys. Rev. Lett. 77 (1996) 175. A.J. Millis, R. Mueller, B.I. Shraiman, Phys. Rcv. B 54 (1996) 5389; 5405. D.D. Sarma, N. Shanthi, S.R. Barman, N. Hamada, H. Sawada, K. Terakura, Phys. Rev. Lett. 75 ( 19951 1126. I. Solovyev, N. Hamada, K. Terakura, Phys. Rev. Lett. 76 ( 19961 4825. S. Satpathy, Z.S. Popovic, F.R. Vukajlovic, Phys. Rev. Lett. 76 (19961 960. W.E. Pickett, D.J. Singh, Europhys. Lett. 32 (1995) 759; Phys. Rev. B 53 (1996) 1146. D.N. Mcllroy, J. Zhang, S.-H. Liou, P.A. Dowben, Phys. Lett. 207 (19951 367. J. Choi, S.-H. Liou, P.A. Dowben, J. Zhang. E.W. Plummer, in preparation. J. Zhang, D.N. Mcllroy, P.A. Dowben, S.-H. Liou, R. Sabirianov. S.S. Jaswal, Solid State Commun. 97 (1996) 39. D.N. McIlroy, C. Waldfried, J. Zhang, S.-H. Liou, P.A. Dowben, Phys. Rev. B 54 t1996) 17483. C. Waldfried, D.N. Mcllroy, S.-H. Liou, R. Sabirianov, S.S. Jaswal, P.A. Dowben, J. Phys.: Condens. Matter 9 (19971 1031. K.H. Kim, J.Y. Gu, H.S. Choi, G.W. Park, T.W. Nob, Phys. Rev. Lett. 77 ( 19961 1877. M.D. Fontana. G. Mdtra, J.L. Servoin. F. Gervais. J. Phys. C: 16 ( 19841 483. Y. Okimoto, T. Katsufuji, T. lshikawa, A. Urushibara, T. Arima, Y. Tokurm Phys. Rev. Lett. 75 (1995) 109. B.N.J. Persson. J.E. Demuth, Phys. Rev. B 30 (1984) 5968. A.P. Ramirez, P. Schiffer, S.-W. Cheong, C.H. Chert, T.T.M. Palstra, P.L. Gammel, D.J. Bishop, B. Zegarski, Phys. Rev. Lett. 76 (1996) 3188. A.P. Ramirez, personal communications. J.E. Demuth, B.N.J. Persson, F. Holtzberg, C.V. Chandrasekhar, Phys. Rev. Lett. 64 603 (1990). B.N.J. Persson, J.E. Demuth, Phys. Rev. B 42 ( 19901 8057. R.B. Phelps, P. Akavoor, L.L. Kesmodel, J.E. Demuth, D.B. Mitzi, Phys. Rev. B 48 (1993) 12936. R.B. Phelps, P. Akavoor, L.L. Kesmodel. A.L. Barr, J.T. Markert, J. Ma, R.J. Kelley, M. Onellion, Phys. Rev. B 50 ( 19941 6526. A. Zibold, M. Darrler, A. Gaymann, H.P. Geserich, N. Nt~cker, V.M. Burlakov, P. Miiller, Physica C 193 ( 19921 171. R. Liu, M.V. Klein, P.D. Ham D.A. Payne. Phys. Rev. B 45 (1992) 7392. D.L. Mills, R.B. Phelps, L.L. Kesmodel, Phys. Rev. B 50 ( 19941 6394.
Surface Science 393 (1997) 64-71
Surface lattice dynamics of Lao.65Cao.35MnO3 across the Curie temperature Jiandi Zhang a.,, E.W. Plummer a.b " Dept. of Physics and Astronomy, The University of Tennessee, Knoxville, TN 37996, USA b SolidState Division, MS-6057, Oak Ridge NationalLab, Oak Ridge, TN37831, USA Received 15 May 1997; accepted for publication 20 June 1997
Abstract
We report a temperature-dependent study of the surface optical phonons of Lao.65Cao.35MnO3 films using high-resolution electronenergy-loss spectroscopy. Three surface modes have been observed and correspond to the bulk modes which are associated with the internal vibrations of the MnO6 octahedron. However, analogous to the high-To superconducting cuperates, the surface phonons have higher energies than the bulk modes, indicating that the surface and the bulk have significantly different lattice dynamics. The small energy dispersion and large linewidth reflect a short phonon life time. The strong temperature dependence of the linewidth corresponds to coupled-random dynamic lattice distortions. A 2 4 meV shift in the phonon energy across the Curie temperature is associated with a static lattice distortion related to the magnetic phase transition. The temperature dependence of the surface phonon modes are compared with the current understanding of the bulk lattice dynamics, above and below the critical temperature for the structural, electronic, and magnetic phase transitions in the "colossal" magnetoresistance materials. © 1997 Elsevier Science B.V.
Keywords: Electron energy loss spectroscopy; Magnetic phenomena; Surface waves
1. Introduction
The perovskite manganites Rl_xAxMnO3 (R = rare earth, A=alkaline earth) have attracted renewed attention owing to the remarkable temperature and magnetic field dependence of their electronic and magnetic properties [1-3]. Of special interest are the compounds with electronic hole doping concentrations x~½ which exhibit a "colossal" magnetoresistance (CMR) [1]. These compounds have paramagnetic and nonmetallic behavior at high temperature, and, ferromagnetic and metallic behavior at low temperature [4]. Such * Corresponding author. Fax: (+ 1) 423 5768135; e-mail: [email protected] 0039-6028/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. Pll S0039-6028 ( 9 7 ) 0 0 5 5 9 - 1
coupled electronic and magnetic phase transitions are also associated with the static and dynamic lattice distortions in these systems [2,3,5-7]. Interest in CMR compounds has been expanding, due to the increasing capability to fabricate epitaxial thin films which have potential technological applications as magnetic sensors. Although thin films are quite stable and exhibit similar CMR behavior to that observed in related bulk materials, the physical properties of the surface or interface are still unknown; a change in the surface composition or lattice structure could lead to a dramatic change in the magnetic and electronic properties. In this paper, we report on measurements of the surface lattice dynamics from La0.65Cao.35MnO3 films across the Curie temperature with high-
J. Zhang, E. V~ Plummer / SurJitce Science 393 (1997) 64 71
resolution electron energy loss spectroscopy (HREELS), a highly surface-sensitive technique [8]. These results are compared with similar measurements from the bulk compound. Our expectation was that the surface measurements might shed light on the bulk lattice dynamics, Historically, a magnetic phase transition coupled with an unusual change in transport behavior in the bulk materials was explained using the concept of a double exchange (DE) interaction [9]. However, recent theories have suggested that, in addition to the DE interaction, a strong electron phonon interaction arising from the lattice Jahn-Teller (JT) distortion also plays a crucial role in electronic and magnetic phase transitions [10,11]. The basic physical picture is as follows: without a JT distortion, the parent compound, LaMnO3, in its ideal cubic structure would be metallic with the Fermi level lying in the middle of the eg bands; a static JT distortion does occur, however, in the MnO6 octahedron. The M n - O basal-plane oxygen atoms (0(2)) are displaced by at least 0. l A from their ideal positions, creating an orthorhombic lattice structure and an insulating gap in the JT split eg bands [12, 13]. By randomly replacing La with divalent atoms like Ca, holes can be doped and will gradually deplete the JT-split e~ band with increasing doping concentration x. By producing an effective random field, the doped holes will disturb the long-range order in the lattice structure such that the static JT distortion is diminished compared with that in the parent compound. On the other hand, doping also introduces a dynamic JT distortion, a local fluctuation of the oxygen atoms away from their original crystallographic positions. Around most of the Mn ions there is a local tetragonal distortion, whereas around a few of them there is a locally symmetric (breathing) distortion [ 11 ]. The dynamic JT distortion, which attempts to localize (trap) the conduction band electrons as polarons due to the electron-phonon interaction, competes against the DE interaction, creating a metallic-ferromagnetic phase. Such competition can be manipulated by either temperature or doping and leads to a metal insulator transition around the Curie temperature (T~). Recent neutron diffraction experiments on the
65
bulk Lal ~Ca~MnO3 compounds have provided direct evidence for both the static and dynamic JT distortions across the phase transitions in these systems [5-7]. We found that the Ca-doping and temperature dependence of both the static and dynamic lattice distortion are inherently coupled with the electronic and magnetic phase transitions in bulk La 1 xCaxMnO3, though the lattice structure keeps an orthorhombic symmetry across the transitions [5]. A static lattice distortion anomaly at Tc is associated with the ferromagnetic-to-paramagnetic phase transition; however, a dynamic lattice distortion anomaly at T~ is related to the metal-to-insulator transition. Using pair-distribution-function analysis, Billinge et al. [6] provided the direct observation of lattice polaron formation associated with local structure distortion.
2. Experimental detail The La0.65Cao.35MnO3 film was ~ 2500 ,~ thick, grown on a (001) surface of LaAIO3 substrate by rf sputtering [14]. The film was fully annealed in an oxygen atmosphere to improve the compositional homogeneity and analyzed with SEM and X-ray diffraction, showing a single phase with a perovskite-like structure. The quality of the film was characterized by the measurements of its bulk properties such as the resistivity and magnetization as a function of temperature using a four point probe and a SQUID magnetometer respectively. As shown in Fig. 1, the film exhibits a transition from a ferromagnetic-metallic phase to a paramagnetic-insulating phase near 260 K (T~). The surfaces were cleaned in a UHV chamber with a base pressure of ~ l × 1 0 - 1 ° T o r r by annealing at 450 500C. Cleanness was checked by Auger electron spectroscopy and showed no obvious contamination. An LK-2000 spectrometer with an angular resolution of ~ 1~ and a system energy resolution of 5.5-6.0meV was used for the HREELS measurements. The energy loss spectra were taken with both specular and off-specular geometries where the scattering angle was varied. The incident electron beam energy was 3.13 eV and the incident angle Oo was fixed to 60 '~ from the surface normal direction. Such a set-up of
Z Zhang, E. W. Plummer / SurJhce Science 393 (1997) 64-71
66
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r (K) Fig. 1. The temperature dependence of the magnetization(0) and resistivity(A) of the Lao.6sCao.35MnO3thin film.The bulk unit cell with a distorted cubic perovskite (i.e. orthorhombic) structure is shown in the inset and the Curie temperature T~of the system is indicated. scattering geometry provides a rather surface-sensitive probe. The temperature was measured with a type-K thermocouple attached to the sample holder.
3. Results and discussion
The surface order and crystalline structure of the Lao.65Cao.35MnO3 film were inferred from the square (1 x 1) low energy electron diffraction (LEED) pattern observed. Referring to the distorted cubic perovskite structure in the bulk, as determined by neutron diffraction and shown in the inset of Fig. 1 [5], the approximate size of the surface unit cell derived from the LEED pattern is about 5.5_+0.1 A2. This is consistent with the surface being terminated such that the b-axis is normal to the surface. This is also the orientation derived from X-ray scattering from the film. There are still two possibilities for the (001) surface termination, i.e. either the M n - O plane (see the inset of Fig. 1) or the L a / C a - O plane. As determined from the analysis of the emission angle dependence of the core level spectra in X-ray photoelectron spectroscopy (XPS) [15], the surface is terminated by the M n - O plane. This is consistent with the photoemission results for the Lao.65Cao.35MnO3 [ 16 ] and Lao.65Bao.35MnO 3 [ 17]
films, in which the surface electronic structure is dominated by the M n - O bonding states• The surface composition was characterized by the relative Auger and angle-resolved X-ray photoelectron spectroscopy (ARXPS) intensity ratios of Ca and La, described in detailed elsewhere [15]. It was found that the Ca/La ratio at the surface is different from that in the bulk, indicating a substantial segregation of Ca to the surface region, with the surface Ca concentration corresponding to x=0.59. Another key to the interpretation of the surface measurements is the surface oxygen stoichiometry. We found that varying our sample cleaning procedure does not have any obvious effect on our measurements• In particular, exposing the sample to oxygen during the annealing (at 450-500°C) and at room temperature does not change either the LEED pattern or the HREELS spectra compared with no oxygen treatment. This observation only proves that there is a stable oxygen surface stoichiometry, not that it is the same as in the bulk. Fig. 2 displays the electron energy loss spectra as a function of scattering angle O away from the specular and along F to X direction of the surface Brillouin (see the inset of Fig. 3b) at 100 K. There are three clearly identifiable loss peaks in the spectra which are associated with three surface optical phonon modes, ~ol (26 meV), 402 (58 meV) and, ~o3 (84 meV). These surface phonons have substantially higher energy than those observed in the bulk. Recent IR spectra from Lao.TCao.3MnO3 [18] show three major bulk optical phonon modes at about 20, 50, and 72 meV. These phonons correspond to the three Flu optical modes in an ideal cubic perovskite structure [19], though a structure distortion could change the energy of these modes or even induce new modes. The inset of Fig. 3a depicts the motion of the atoms for each of the three modes. Since the surface modes are similar in energy to the bulk modes they can be identified according to the assignment published for the bulk [18,20]• The feature with the lowest loss energy (0~1) is associated with the external mode, representing the vibration of the La/Ca ions against the MnO 6 octahedron (i.e. external to the MnO6). The loss feature at 58 meV (0~2), is the M n - O - M n bending
J. Zhang, E. W. Plummet / Sur/dce Science 393 (1997) 64 71
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Energy Loss (meV) Fig. 2. HREELS spectra from the surface of the La0.,+sCa0.35MnO 3 film at 100 K as a function of the scattering angle offthe specular (and a l o n g / ' t o X) direction (S) as shown in the inset. The incident beam energy was 3.13 eV and the incident angle 60' from the surface normal direction.
mode in the M n - O surface plane. The highest energy mode at 84 meV (~3) is the stretching mode, associated with the vibration of the Mn ion against the oxygen octahedron. These assignments are in qualitative agreement with the behavior of the loss features with changing scattering geometry, The phonon intensity as a function of the in-plane momentum transfer at 100 K (the temperature for the metallic phase) is plotted in Fig. 3a. The measured two-dimensional dispersion of phonon energy along F to X of the surface Brillouin zone at both 100 K and 300 K (the temperature above the magnetic and metal-to-nonmetal transition) is shown in Fig. 3b. Only the intensity and the dispersion of ~o2 and +~ are shown in the figure since the co~ mode is too weak to perform a systematic characterization. The intensities are normalized to the intensity of the quasi-elastic peaks• As shown
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in Fig. 3a, the intensity of the loss feature (02 decreases much faster than that of ~')3 with increasing scattering angle away from specular direction. This indicates that the mode oJ2 exhibits a stronger dipole-active character, truly reflecting the M n - O ( 2 ) - M n bending vibration out of the surface plane. Both (D2 and co3 disperse in energy along F to X with the surface momentum-transfer, corroborating the surface ordering as observed from LEED. However, the dispersion is only about 2 to 4meV, consistent with a coupled-random lattice vibration picture. Figs. 4-6 address the temperature dependence
J. Zhang, E. VK Plummer / Surface Science 393 (1997) 64 71
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E n e r g y Loss ( m e V ) Fig. 4. The temperature dependence of normalized HREELS spectra from the surface of the Lao.65Cao.35MnO3 film. The spectra were taken in the specular direction. The incident beam energy was 3.13 eV and the incident angle 60 ° from the surface normal direction.
70
i ,
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of the HREELS spectra from the Lao.65Cao.35MnO3 surface. There are several interesting phenomena observed with changing temperature for both the surface and bulk. First, all three phonon peaks shift to lower energies with increasing temperature, as seen in Figs. 4 and 5. The three modes are located at 22 meV, 55 meV and 82 meV at T= 300 K, according to our best fit. Relative shifts, [~o(100 K)-~o(300 K)]/~o(300 K), are about 3-5%. Similar shifts in energy have also been observed in the bulk phonon spectra (Fig. 5) [18]. Such shifts are consistent with the static lattice distortion which is associated with the magnetic phase transition, but inconsistent with the metal-to-insulator transition in the bulk [1,2]. A dramatic anisotropic volume expansion is observed in the bulk material and occurs near the Curie temperature [5], indicating a strong static lattice distortion in the system. Such a distortion should be responsible for softening the surface and bulk phonons with increasing temperature, and might inherently couple to the magnetic phase transition observed in the bulk [5]. We note that, in normal Debye theory, the derivative of the "root mean
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Temperature (K) Fig. 5. The temperature dependence of the observed surface phonon energies of Lao.65Cao.35MnO3 (O), compared with those of the bulk measured by Kim et al. ( 0 ) [18]. The lines are a guide for the eye.
square" (RMS) motion amplitude of atoms with temperature, d(u2)/dT, is inversely related to the Debye temperature, which is proportional to the vibration frequency. From the measurement of (u 2) for bulk La0.65Ca0.asMnO3 by neutron scattering [5], the Debye temperature in the metallic phase (at low temperature) is lower than that in the insulating phase (at high temperature). This means that, according to simple Debye theory, the phonon frequency in the metallic phase should be smaller than that in the insulating phase, which contradicts the experimental observations of the phonon shifts both in the bulk and on the surface. Therefore, from the temperature dependence of the phonon energy, we conclude that the static lattice distortion and magnetic phase transition dominate the lattice dynamics.
J. Zhang, E. W, Plummer / SurIitce Science 393 (1997) 64 71
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Temperature(K) Fig. 6. (a) The asymmetry ( 0 ) and F W H M ('~) of the elastic peak and (b) the intensity ("F2" for ~')2 and " l l " for ~o3) and linewidth ( " ± , " for "~2 and " A ' " for ~')3) of the surface optical phonons with temperature. The instrumental broadening is included in the linewidth. The lines are a guide for the eye.
The temperature dependence of the quasi-elastic peak is related to surface roughness, lattice dynamics, surface free-carrier density, and surface conductivity [21]. For Lao.6sCa0.35MnO3 there is a metal-to-insulator transition with temperature, so the change in asymmetry of the quasi-elastic peak with temperature can be related directly to the change of the bulk transport properties. By fitting both sides of the quasi-elastic peak to Gaussian functions, we find that the asymmetry decreases when the temperature changes from 100 K to 300 K, and then increases from 300 K to 450 K (see Fig. 6a). This behavior can be explained qualitatively by the change of allowed excitations with temperature. At 100 K, the metallic character creates a large asymmetry due to the strong Drudelike excitations. At 300 K, the insulating behavior minimizes the low energy excitations, creating a more symmetric quasi-elastic peak. At 450 K, the
69
asymmetry increases again with temperature due to the thermal population of the conduction band. The trend of the lineshape of the quasi-elastic peak indicates that the surface dielectric response is similar to that in the bulk, i.e. a metal-to-insulator transition likely occurs at the surface, consistent with the recent observation from the photoemission measurements [14]. On the other hand, the change of linewidth of the quasi-elastic peak as a function of temperature cannot be simply explained by collective excitations [21]. The FWHM of the quasi-elastic peak from the surface is almost proportional to T 2, independent of the change in the bulk conductivity (Fig. 6a). It is possible to account for such a change in FWHM with temperature if the phonon modes are dominated by lattice dynamical effects due to the change in electron-phonon coupling. However, these dynamical effects must be strong enough to overcome the suppression of the Drude tail as the system ceases to be metallic. The large phonon widths indicate a short phonon life time. The linewidth AE of ~-')2and o)3 is typically from 17 to 26 meV and changes with temperature as shown in Fig. 6b, indicating a strong temperature dependence in lattice dynamics. The increase in phonon damping (i.e. the decrease in intensity) and linewidth with increasing temperature may relate directly to the increase in surface random dynamical lattice distortions observed in the bulk [5 7] and suggested from theoretical calculations [10,11]. This should be associated with the random Ca-doping distribution in the system. As has been measured by Ramirez et al. [22], the sound velocity Vg in kao.TCao.3MnO3 at room temperature is 4.3 x 105 cm s-1. The mean free path Al of phonons can be estimated roughly as only about 2 A, by applying our data of the phonon linewidth (which includes the instrumental broadening) and by using the equation AI = VgAI = 11Vg/(2nAE), where tl is the Planck constant. Even considering the instrumental broadening to the phonon peaks, the mean free path of these optical phonons is still quite small, comparable with the size of the unit cell volume in the compound. Such a small mean free path can also be obtained directly from the dispersion behavior. As shown in Fig. 3, the dispersion of these optical
70
J. Zhang, E. W. Plummet / Surface Science 393 (1997) 64-71
phonons is small, within 4-5 meV. If we simply apply the relation as AI~ (dE/dqO/dE, where qll is the in-plane momentum transfer, and then use the dispersion results from Fig. 3, we can also obtain a mean free path smaller than 2 A. With such a small mean free path, we can conclude that the vibrations in these systems are rather local and strongly coupled. It is interesting to compare the observation of the surface phonons in CMR compounds with that in high-To superconducting materials [23-26]. A similarity in the behavior of these phonon modes can be drawn. First, just as in the CMR case, three major vibrational modes have been observed with energies of 26, 50, and 80 meV at the surface of Bi2Sr2CaCuOs+o [24]. It is found that the surface modes are associated with three major bulk ones located near 20, 36 and 70 meV, observed in Raman and infrared reflectivity spectra [25]. Second, the surface optical phonons in both CMR and high-T~ superconducting materials have substantially higher energies than the corresponding bulk ones. Such a difference reflects the nature of the surface termination, which can cause a change in the local bonding by breaking the octahedral structure of MnO 6 in CMR and CuO6 in high-T~ superconducting compounds. The change in the composition between the surface and bulk can be also accounted for by the difference in the phonon energies. Another similarity is the temperature dependence of these phonon modes. In the surface of high-T~ superconductors like Bi2Sr2CaCuO 8+~, a small decrease in phonon energies with increasing temperature has also been observed [24]. Despite the fundamental differences in the physical properties between CMR and high-T~ superconducting materials, the similarity in these optical phonon behavior may be related to their similar origin, i.e. from the internal vibrations of the M O 6 ( M = Mn, Cu) octahedron, though the lattice distortions do affect individual vibrational modes in the phonon structure. In order to study the interplay between lattice dynamics and electronic or magnetic phase transitions at the surface of these CMR compounds, it is very important to identify the surface lattice dynamics responding to the metal-to-insulator transition and to magnetic phase transition sepa-
rately. One way to do that is to study the doping dependence of the surface static and dynamic lattice distortion, as that has been studied for the bulk materials [5]. We attempted to study the surface lattice dynamics of an Lao.9Ca0.1MnO3 film which exhibits only a magnetic phase transition at Tc=150 K in the bulk. However, the ARXPS studies indicate a much larger Ca surface segregation, with the Ca concentration at the surface as much as x=0.92, and different surface termination compared with the surface of La0.65Ca0.35MnO3 film [15], so that makes the comparison for different Ca-doping concentrations impossible. Nonetheless, a detailed theoretical study on phonon band structure will be important to understand the lattice dynamic effects on both the bulk and surface phonon structures. For example, the ~92 loss peak may include more than one phonon mode which can be associated with the lifting of degeneracy due to the lattice distortion on the surface. Understanding the physics involved needs not only more detailed experimental studies on temperature dependence, but also theoretical calculations of phonon modes based on different lattice distortions.
4. Summary In conclusion, we have observed three optical phonons on the Mn-O-terminated Lao.65Cao.35MnO3 thin film. These modes are assigned as the external, bending, and stretching modes of the MnO6 octahedron in agreement with the bulk measurements. The surface phonons have higher energies than the equivalent modes in the bulk, similar to what have been observed on high-T¢ superconductors. The modes decrease in energy with increasing temperature across the Curie temperature, indicating that the bulk lattice distortion associated with the magnetic phase transition is similar at the surface. The temperature dependence of the width and asymmetry of the quasi-elastic peak reveals the presence of a metalto-nonmetal transition at the surface. This preliminary study of the surface lattice dynamics of Lao.6sCao.35MnO3 indicates the need for detailed studies of the electronic, geometric,
J. Zhang, E. V~ Plummer / Surface Science 393 ( 19971 64 71
and magnetic properties of surfaces of these complex materials. The key will undoubtedly be knowing and/or controlling the surface stoichiometry. Better in situ growth procedures are required, as well as a surface-sensitive magnetic measurement such as spin-polarized electron energy loss spectroscopy.
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[10]
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