MICRO-RAMAN STUDY OF MECHANICALLY ACTIVATED FERROELECTRICS AND ADVANCED MAGNETIC MATERIALS YU TING NATIONAL UNIVERSITY OF SINGAPORE 2003 MICRO-RAMAN STUDY OF MECHANICALLY ACTIVATED FERROELECTRICS AND ADVANCED MAGNETIC MATERIALS YU TING (B Sc Jilin University) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF PHYSICS NATIONAL UNIVERSITY OF SINGAPORE 2003 MICRO-RAMAN STUDY OF FERROELECTRICS AND MAGNETIC MATERIALS YU TING 2003 ACKNOWLEDGMENTS This project is made possible with the great help of many people First of all, I would like to express my sincere gratitude and appreciation to my supervisor Assoc Prof Shen Ze Xiang for his unfailing guidance and support throughout my research project Working with him has not only opened my eyes to the multiple facets of physics, but also has matured me to be a better researcher Sincere appreciations are given to Assoc Prof J Wang and Dr J Ding from the Department of Material Science of NUS, for providing the precious samples and for helpful discussions Many thanks are due to my colleagues in Raman Spectroscopy Lab of the Department of Physics of NUS Special thanks for my beloved wife Yu Jianhua and my parents for their motivations and encouragement in course of the project i CONTENTS Acknowledgements i Table of contents ii Summary vi Publications viii Chapter Introduction 1.1 General introduction 1.2 Organization of the thesis 1.3 References Chapter Mechanical Activation 2.1 Background 12 2.2 Effects of mechanical activation 14 2.2.1 Refinement in crystallite size 14 2.2.2 Creation of structural defects 15 2.2.3 Phase transformation 16 2.2.4 Crystallization from amorphous state 16 2.3 Mechanical activation in synthesis of nanocrystals 17 2.4 References 19 Chapter Basics of Raman Scattering 3.1 Introduction 21 3.2 Phenomena of Raman scattering 21 3.3 Classical model of Raman scattering 22 ii 3.4 Quantum model of Raman scattering 25 3.5 Raman tensor and selection rule 27 3.6 References 30 Chapter Lead Titanate Ultrafine Particles from Amorphous PbTi-O Precursor by Mechanical Activation 4.1 Introduction 32 4.2 Experimental 35 4.3 Results and discussion 35 4.3.1 Mechanical activation process 35 4.3.2 Effect of particle size on the structure and unit cell volume 42 4.4 Conclusions 47 4.5 References 48 Chapter Mechanical Activation Induced Seeding Effect on Formation of Perovskite PbTiO3 5.1 Introduction 50 5.2 Experimental 51 5.3 Results and discussion 52 5.3.1 Metastable intermediate phase 52 5.3.2 Activation energy 58 5.4 Conclusions 66 5.5 References 67 Chapter Size Effect on Ferroelectric Phase Transition in SrBi2Ta2O9 Ultrafine Particles 6.1 Introduction 68 6.2 Experimental 69 6.3 Theoretical background 70 6.3.1 Ferroelectric phase transition 70 iii 6.3.2 Soft mode 6.4 71 Results and discussion 74 6.4.1 Size determination using Scherrer equation 74 6.4.2 High temperature Raman spectra of SrBi2Ta2O9 powder 76 6.5 Conclusions 82 6.6 References 83 Chapter Micro-Raman Investigation of Cation Migration and Magnetic Ordering in Spinel CoFe2O4 Powder 7.1 Introduction 85 7.2 Experimental 88 7.3 Results and discussion 89 7.3.1 Phase analysis 89 7.3.2 Raman mode assignments 91 7.3.3 High temperature Raman study on cation migration 93 7.3.4 Magnetic ordering induced anomalous softening of Raman mode 96 7.4 Conclusions 99 7.5 References 100 Chapter Phase Control and Magnetic Raman Scattering Study of Half-Metallic CrO2 Ultrafine Particles 8.1 Introduction 102 8.2 Experimental 105 8.3 Sample characterization 106 8.3.1 X-ray analysis 106 8.3.2 Morphology by SEM image 106 8.3.3 Magnetic properties 109 8.4 Magnetic Raman investigation of CrO2 109 8.4.1 Raman modes assignments 109 8.4.2 Spin-phonon coupling in CrO2 powder 111 iv 8.5 Phase control in selective microregions by laser annealing 117 8.7 Conclusions 120 8.7 References 123 v SUMMARY This thesis presents results of our micro-Raman studies of mechanically activated ferroelectrics, PbTiO3 & SrBi2Ta2O9, and advanced magnetic materials, CoFe2O4 & CrO2 The mechanical activation process of amorphous Pb-Ti-O precursor was studied The crystallization of PbTiO3 (PT) phase could be triggered by milling alone if the milling time is longer than 20 h These PT crystallites introduced by room-temperature mechanical activation process act as seeds, dramatically reducing the activation energy and enhance the crystallization kinetics, during the subsequent formation of perovskite PT by post-annealing Consequently, the PT phase formation temperature is lowered and the metastable phase which is often observed in the conventional solid state reaction is bypassed The size effect on the structural phase transition is also discussed The size effect on the ferroelectric phase in the SrBi2Ta2O9 (SBT) nanoparticles was studied by high temperature micro-Raman scattering The SBT ultrafine particles were formed by mechanical activation of mixed oxides followed by post-calcination The results show that the phase transition temperatures of SBT nanoparticles decrease with the reduction of particle sizes A critical size of 2.6 nm, below which ferroelectricity disappears, was obtained from an empirical expression This small critical size implies that SBT is a potential candidate for formation of ferroelectric devices of untrafine size The micro-Raman investigation of cation migration and magnetic ordering in spinel CoFe2O4 powder were carried out A marked increase of line widths of T-site and O-site Raman peaks was observed when the ambient temperature was at around 390 K Considering the cation migration model and previous work, we attribute this anomaly to vi Chapter Phase Control and Magnetic Raman Scattering Study of HalfMetallic CrO2 Ultrafine Particles in which four optic modes are Raman active ( A1g + B1g + B2 g + E g ) and four are infrared active ( A2u + 3Eu ) [8.29] The atomic motions corresponding to the Raman active modes are shown in Fig.8.4 8.4.2 Spin-phonon coupling in CrO2 powder The room-temperature Raman spectra of half-metallic CrO2 powder at zerofield (ZF) and at a low-magnetic field up to 500 mT were shown in Fig 8.5 It is seen that the room-temperature ZF Raman scattering spectrum is mainly characterized by two peaks at 455.4 cm-1 and 573.9 cm-1 Iliev et al [8.17] performed the first detailed Raman scattering study of half-metallic CrO2 Four peaks at 570, 149, 682, and 458 cm-1 were observed and assigned to A1g, B1g, B2g, and Eg mode, respectively It is obvious that the two peaks presented in our spectra correspond to the vibrational modes of Eg and A1g symmetries, respectively These two Raman-active modes are internal phonon modes, involving only vibrations of the oxygen atoms which form the sub-lattice CrO6 octahedron (see Fig 8.4) Upon applying a magnetic field, the Raman phonon (Eg mode) parameters: frequency, intensity and linewidth show pronounced anomalies In order to study the variation of the Eg mode in the presence of an external magnetic field shown in Fig 8.6, a least-squares fit with Lorentzian line-shape was used to fit the Raman peak of the Eg mode Figure 8.7 shows the fitted full-width-athalf-maximum (FWHM), peak intensity and position of the Eg mode as a function of the applied magnetic field In the presence of a relatively low-magnetic field (H < 250 111 Chapter Phase Control and Magnetic Raman Scattering Study of HalfMetallic CrO2 Ultrafine Particles B1g A1g B2g Eg(1) Eg(2) Fig 8.4 Main atomic motions in the Raman modes of CrO2 (space group P42/mnm) 112 Chapter Phase Control and Magnetic Raman Scattering Study of HalfMetallic CrO2 Ultrafine Particles CrO2(Eg) 500 mT CrO2(A1g) 400 mT Intensity (a.u.) 250 mT 150 mT 100 mT 50 mT ZF 200 400 600 -1 Raman shift (cm ) Fig 8.5 Magnetic-field dependence of the room-temperature Raman spectra of half-metallic CrO2 powder 113 Chapter Phase Control and Magnetic Raman Scattering Study of HalfMetallic CrO2 Ultrafine Particles 500 mT Intensity (a.u.) 400 mT 250 mT 150 mT 100 mT 50 mT ZF 420 430 440 450 460 470 480 -1 Raman shift (cm ) Fig 8.6 Detailed Raman spectra of the Eg mode under a magnetic field The openedsquares are experimental data and the lines are fitted data using Lorentzian line shapes 114 Chapter Phase Control and Magnetic Raman Scattering Study of HalfMetallic CrO2 Ultrafine Particles mT), the frequency and peak intensity of the Eg mode show an obviously monotonic increase with increase in the strength of magnetic field while the linewidth decreases monotonously When the applied magnetic field was within the range of 250~500 mT, where the CrO2 powder entered the saturation state (see Fig 8.3), no pronounced change of the Raman phonon parameters was observed Three possible physical mechanisms may lead to measurable anomalies of Raman phonon parameters or even activation of new Raman mode in a magnetic ordering system The first mechanism is a structure transition A typical material of this kind is cupric oxide CuO [8.30] Due to the folded-zone from the Z´ point of the Brillouin-zone boundary, this material exhibits new Raman-active lines upon entering the magnetically ordered state The second mechanism is the anharmonic effect which is involved in all temperature induced magnetic ordering systems The last one is spin-phonon coupling The first two mechanisms are not active in the present case, since the ambient temperature is fixed at 300 K, and to our knowledge, no data on low-magnetic field (H ≤ 500 mT) variation of the structure available for cold-pressed powder compact CrO2 Thus, it is reasonable to attribute the anomalies of Raman mode observed in this work to the spin-phonon coupling Similar anomalies of Raman phonon in half-metallic CrO2 were reported by Iliev et al [8.17], where anomalous broadening of phonon lines, in addition to the broadening induced by the normal phonon–phonon scattering (anharmonic decay), was observed when the temperature was increased to Tc This additional broadening was attributed to collective spin fluctuations near Tc, which results in spin disorder scattering In the present work, the spin ordering was controlled by an external magnetic field and the degree of spin 115 Position (cm-1) Peak Intensity (a.u.) FWHM (cm-1) Chapter Phase Control and Magnetic Raman Scattering Study of HalfMetallic CrO2 Ultrafine Particles 14 12 10 2500 2000 458 457 456 455 100 200 300 400 500 H (mT) Fig 8.7 The parameters of the Eg mode as a function of magnetic field 116 Chapter Phase Control and Magnetic Raman Scattering Study of HalfMetallic CrO2 Ultrafine Particles ordering was measured using Raman spectroscopy via spin–phonon interaction A higher external magnetic field results in a more ordered spin state that in turn gives rise to sharper Raman bands Hence the Raman linewidths will decrease with increasing magnetic field As the temperature was fixed at room temperature in our study, our results are free from the anharmonic broadening effect, making spectral interpretation simpler and more definitive Our study complements that of [8.17] The temperature was slightly above Tc in [8.17] and no definite conclusion was drawn about the effect of spin on the phonon frequencies However, in their study of another “bad” metal SrRuO3 [8.31], the spin-phonon coupling was investigated in detail According to the above studies, the observed anomalies of the Eg mode in this work could be explained as follows Upon applying an external magnetic field, the magnetic ordering of CrO2 is enhanced due to higher degree of spin alignment This magnetic ordering subsequently increases the interaction among the neighboring Cr ions Due to the high sensitivity to the interaction among Cr ions, the Cr-O bond length and bond angles in the CrO6 octahedra also change, resulting in change of the frequency of Raman modes (Eg mode in this work) The narrowing of the linewidth and the increase in peak intensity is direct evidence of increased ordering In our previous magnetic Raman scattering study on spinel CoFe2O4 [8.32], the magnetismlattice interaction was demonstrated by the red-shift of the Raman modes 8.4 Phase control in selective microregions by laser annealing Figure 8.1 inset shows the detailed XRD patterns of laser-annealed samples for various laser irradiation times from to 90 s A weak and broad peak centered at 117 Chapter Phase Control and Magnetic Raman Scattering Study of HalfMetallic CrO2 Ultrafine Particles 2θ angle of about 33.4o was observed in the XRD trace of the s laser-annealed sample This peak grew stronger with increase in anneal duration, and was assigned to the (104) plane of Cr2O3 [8.33] Our micro-Raman scattering study discussed below strongly supported this assignment and also identified the second phase as Cr2O3 When the original CrO2 sample was subjected to a laser annealing for 30 s, the broad diffraction peak was greatly strengthened and became obvious The appearance of the Cr2O3 diffraction peak demonstrates that the decomposition of half-metallic CrO2 into insulating Cr2O3 can be triggered by laser annealing Upon laser annealing for 90 s, the Cr2O3 diffraction peak was well established Estimation of the normalized intensity of the XRD peaks [8.33] shown in Fig 8.1 inset indicates that increasing the laser irradiation time under a fixed laser power density can dramatically increase the relative fraction of the Cr2O3 phase In comparison with the XRD results, the Raman spectrum (Fig 8.8) of s laser-annealed sample shows a sharp peak centered at 549 cm-1, which corresponds to the A1g mode of Cr2O3 [8.34] Since the A1g Raman peak of Cr2O3 is strong, Raman spectroscopy provides higher sensitivity compared with XRD [8.35] With a spatial resolution of about 0.5 micrometer, Raman spectroscopy is also advantageous in characterizing patterned samples with small features Another Raman mode, the Eg mode [8.34] of Cr2O3 at 303 cm-1 appeared after 30 s laser annealing Upon 90 s laser annealing, both the Eg and A1g modes belonging to the Cr2O3 were further strengthened and well established, implying the increased amount of Cr2O3 Our Raman results are in good agreement with the X-ray diffraction phase analysis 118 Chapter Phase Control and Magnetic Raman Scattering Study of HalfMetallic CrO2 Ultrafine Particles Cr2O3(A1g) Intensity (a.u.) 90 s Cr2O3(Eg) 30 s 5s 0s CrO2(Eg) CrO2(A1g) 300 400 500 600 700 -1 Raman shift (cm ) Fig 8.8 Raman spectra of the samples before and after pulsed-laser annealing 119 Chapter Phase Control and Magnetic Raman Scattering Study of HalfMetallic CrO2 Ultrafine Particles showing that the decomposition of the half-metallic CrO2 into the insulating Cr2O3 can be triggered by laser annealing in air ambient and the relative fraction of each phase depends on the laser irradiation time The decomposition of CrO2 into Cr2O3 in selective micro-region was studied by SEM and micro-Raman scattering in this work As shown in Fig 8.9a, upon scanning a CW laser of mW power, a line about 12 µm in width was clearly observed The obvious difference, the round grain boundary and the appearance of spherical particles shown in Fig 8.9b compared with the rod-shaped morphology of the original CrO2 powder (Fig 8.2) implies that a second phase may be formed under this condition After laser annealing at a higher power (10 mW), as shown in Fig 8.9c, the rod-shaped CrO2 particles were almost completely transformed into spherical particles expected for Cr2O3 [8.33] The Raman spectra of the corresponding selective micro-regions are similar to the Raman scattering results derived from pulsed-laser annealing in macro-regions, whereas no CrO2 phase was observed upon laser annealing with 10 mW laser power (SEM image shown in Fig 8.9c) The appearance, subsequent development and finally the establishment of pure Cr2O3 spectrum demonstrate the ability of laser annealing in controlling the fraction of CrO2/Cr2O3 phase 8.6 Conclusions An inelastic light-scattering study on the half-metallic CrO2 powder was performed in a low-magnetic field at a fixed temperature (300 K) The Eg mode of CrO2 powder showed pronounced dependence on the applied external magnetic field, and the changes of Raman phonon parameters were attributed to the spin-phonon 120 Chapter Phase Control and Magnetic Raman Scattering Study of HalfMetallic CrO2 Ultrafine Particles (a) (b) (c) Fig 8.9 SEM images of the selectively laser-annealed micro-region of CrO2 powder with laser power of (a), (b) mW and (c) 10 mW 121 Chapter Phase Control and Magnetic Raman Scattering Study of HalfMetallic CrO2 Ultrafine Particles coupling induced by magnetic field-enhanced magnetic ordering The results of this work demonstrate that Raman scattering, especially magnetic Raman scattering could be an effective tool for the understanding of the novel TMR material CrO2 Phase control of half-metallic CrO2 powder in macro and micro-regions has been successfully realized by laser annealing in air ambient The ability to control the relative fraction of CrO2 and Cr2O3 phases with the laser irradiation time and power indicates that optical lithography is a potential method to directly control the magnetotransport properties which are determined by the interface tunnel barrier, Cr2O3 in this work The study on laser-induced phase control of CrO2 polycrystalline film, in selective area of micro/nano in size, is in process This opens a new approach for designing useful MR properties for MR materials, especially for materials with TMR properties 122 Chapter Phase Control and Magnetic Raman Scattering Study of HalfMetallic CrO2 Ultrafine Particles 8.7 References [8.1] W Thomson, Proc R Soc 8, 546 (1857) [8.2] K Schwarz, J Phys F: Met Phys 16, L211 (1986) [8.3] K P Kamper, W Schmitt, G Gntherodt, R J Gambino, and R Ruf, Phys Rev Lett 59, 2788 (1987) [8.4] R Wiesendanper, H J Guntherodt, G Guntherodt, R J Gambino, and R Ruf, Phys Rev Lett 65, 247 (1990) [8.5] R J Soulen, J M Byers, M S Osofsky, B Nadgorny, T Ambrose, S F Cheng, P R Broussard, C T Tanaka, J Nowak, J S Moodera, A Berry, and J M D Coey, Science 282, 85 (1998) [8.6] J S Moodera, L R Kinder, T M Wong, and R Meservey, Phys Rev Lett 74, 3273 (1995) [8.7] H Y Hwang and S.-W Cheong, Science 278, 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presents results of our micro- Raman studies of mechanically activated ferroelectrics, PbTiO3 & SrBi2Ta2O9, and advanced magnetic materials, CoFe2O4 & CrO2 The mechanical activation process of amorphous... DEPARTMENT OF PHYSICS NATIONAL UNIVERSITY OF SINGAPORE 2003 MICRO- RAMAN STUDY OF FERROELECTRICS AND MAGNETIC MATERIALS YU TING 2003 ACKNOWLEDGMENTS This project is made possible with the great help of