Mechanically activated synthesis and magnetoresistive behavior of double perovskite sr2femoo6

MECHANICALLY ACTIVATED SYNTHESIS AND MAGNETORESISTIVE BEHAVIOR OF DOUBLE PEROVSKITE Sr2FeMoO6 CHEN LI ( M.Eng., HUST) A THESIS SUBMITTED FOR THE DEGREE OF MASTER OF SCIENCE DEPARTMENT OF MATERIALS SCIENCE NATIONAL UNIVERSITY OF SINGAPORE 2006 ACKNOWLEDGEMENTS I would like to express my sincere appreciation to my supervisor, Associate Professor John Wang, for his constant guidance and support during the entire course of this project I would also like to thank Dr Xue Junmin for his invaluable advice and suggestions on my research work I would like to acknowledge all my colleagues in the Advanced Ceramics Lab, Anthony, Xingsen, Herman, Hwee Ping, David, Li Fang, Zhang Yu, Chow Hong and Fransiska for their discussions and assistance I am especially grateful to Dr Yuan Cailei for his help and cooperation I also appreciate the kind support and assistance from Mr Chan, Chen Qun, Agnes and Jiabao Finally, a special word of appreciation goes to my parents, my brother and my girl friend Fuxiao for their understanding and encouragement Chen Li NUS, Singapore January, 2006 I Table of Contents TABLE OF CONTENTS ACKNOWLEDGEMENTS I TABLE OF CONTENTS III SUMMARY V LIST OF TABLES VII LIST OF FIGURES VIII PUBLICATIONS XII CHAPTER INTRODUCTION 1.1 MAGNETORESISTANCE 1.1.1 Anisotropic Magnetoresistance (AMR) 1.1.2 Giant Magnetoresistance (GMR) 1.1.3 Tunneling Magnetoresistance (TMR) 1.1.4 Colossal Magnetoresistance (CMR) 1.2 LIMITATIONS OF CMR MANGANITES .7 1.3 DOUBLE PEROVSKITE SR2FEMOO6 1.3.1 Crystal Structure and Electronic Structure 10 1.3.2 Magnetic Structure 16 1.3.3 Electro-transport Properties 18 1.3.4 Magnetoresistive Properties 19 1.4 SYNTHESIS ROUTES .22 1.4.1 Conventional Synthesis Routes 22 1.4.2 Mechanical Activation 24 1.5 MOTIVATION AND RESEARCH OBJECTIVES 27 CHAPTER EXPERIMENTAL PROCEDURES 29 2.1 INTRODUCTION 29 2.2 CHEMICALS 31 2.3 EXPERIMENTAL PROCEDURES 32 2.3.1 MoO3-based Sr2FeMoO6 .32 2.3.2 MoO2-based Sr2FeMoO6 .33 2.3.3 Ni doped Sr2FeMoO6 .34 2.4 CHARACTERIZATION TECHNIQUES .35 2.4.1 X-ray Diffraction (XRD) 35 2.4.2 Scanning Electron Microscope (SEM) 36 2.4.3 Vibrating Sample Magnetometer (VSM) 38 2.4.4 Four-point Probe Technique 40 CHAPTER THE RIETVELD METHOD .43 III Table of Contents 3.1 INTRODUCTION 43 3.2 MATHEMATICAL BASIS 44 3.3 RIETVELD REFINEMENT IN PRACTICE 48 CHAPTER PHASE FORMATION AND MAGNETORESISTANCE OF MOO3-BASED SR2FEMOO6 53 4.1 MECHANICAL ACTIVATION 55 4.2 SINTERING BEHAVIORS 57 4.3 MAGNETIC PROPERTIES 60 4.4 ELECTRO-TRANSPORT AND MAGNETORESISTIVE PROPERTIES .62 4.5 REMARKS 65 CHAPTER MECHANICALLY ACTIVATED SYNTHESIS AND MAGNETORESISTANCE OF MOO2-BASED SR2FEMOO6 66 5.1 EFFECTS OF MECHANICAL ACTIVATION ON THE PHASE FORMATION AND MAGNETORESISTIVE BEHAVIORS OF SR2FEMOO6 68 5.1.1 Mechanical Activation 68 5.1.2 Sintering Behaviors and Microstructures 70 5.1.3 Magnetic Properties 74 5.1.4 Electro-transport and Magnetoresistive Properties 76 5.2 EFFECTS OF SINTERING TEMPERATURE ON THE B-SITE ORDERING AND MAGNETORESISTIVE BEHAVIORS OF SR2FEMOO6 80 5.2.1 Phase Formation and Microstructures 80 5.2.2 Rietveld Refinement and B-site Ordering 84 5.2.3 Magnetic Properties 87 5.2.4 Magnetoresistive Properties 90 5.3 REMARKS 92 CHAPTER B-SITE ORDERING AND MAGNETIC BEHAVIORS IN NI-DOPED SR2FEMOO6 94 6.1 PHASE FORMATION AND MAGNETIZATION .96 6.2 MICROSTRUCTURES AND B-SITE ORDERING 100 6.3 MAGNETIC PROPERTIES .104 6.4 RIETVELD REFINEMENT 110 6.4.1 B-site Long-range Order 110 6.4.2 Structural Parameters 114 6.4.3 Dependence of Magnetic Properties on B-site Ordering 116 6.5 REMARKS 119 CHAPTER CONCLUSIONS AND SUGGESTIONS FOR FUTURE WORK 120 7.1 CONCLUSIONS .120 7.2 SUGGESTIONS FOR FUTURE WORK 123 CHAPTER REFERENCES 125 IV Summary SUMMARY Mechanical activation was successfully developed to synthesize double perovskite Sr2FeMoO6 The effects of mechanical activation and heat-treatment temperature on the phase formation, magnetic and magnetoresistive behaviors of Sr2FeMoO6 were investigated, by using both MoO3 and MoO2 as the starting materials The effects of Ni doping on the B-site ordering and magnetic properties of Sr2FeMoO6 were systematically studied Rietveld refinement method was used to perform quantitative analysis on the B-site order in double perovskite Sr2FeMoO6 Sr2FeMoO6 with minimal level of SrMoO4 impurity was synthesized by mechanical activation of SrO, Fe2O3 and MoO3 in a nitrogen atmosphere Double perovskite Sr2FeMoO6 of single phase was realized at 700 °C in flowing 5% H2/Ar, which is ∼200 °C lower than what is required in the conventional solid state reaction The polycrystalline Sr2FeMoO6 exhibited an average crystallite size in the range of 30 to 50 nm Magnetization of thus derived Sr2FeMoO6 increases when the temperature was raised from 700 °C to 900 °C Magnetoresistance of MoO3-based Sr2FeMoO6 also increases with the increase in heat-treatment temperature, which is attributed to the elimination of insulating SrMoO4 impurity and enhancement in B-site ordering By changing the starting material from MoO3 to MoO2, Sr2FeMoO6 of single phase V Summary was successfully synthesized in air by mechanical activation for the first time Due to the effective elimination of nonmagnetic SrMoO4 impurity, magnetization of MoO2-based Sr2FeMoO6 increases when increasing mechanical activation time Similarly, MR effect also increases with increasing mechanical activation time up to 25 hours, due to the elimination of SrMoO4 impurity phase and the refinement in grain size However, too long a mechanical activation time led to excess contamination by Fe and thus reduced the MR effect of Sr2FeMoO6 B-site order in MoO2-based Sr2FeMoO6 was systematically enhanced by increasing sintering temperature in the range of 800 °C to 1100 °C Consequently, the magnetization is significantly enhanced by high temperature sintering Magnetoresistance of MoO2-based Sr2FeMoO6 sintered at temperatures ranging from 800 °C to 1100 °C also increases with the increase in sintering temperature, which can be ascribed to the increase in B-site long-range order Polycrystalline Sr2(Fe1-xNix)MoO6 (0.0 ≤ x ≤ 0.02) of double perovskite structure was successfully synthesized via mechanical activation The long-range order parameter S among octahedral B sites is significantly enhanced by Ni doping, from S = 0.584 for x = to S = 0.932 for x = 0.20 The B-site ordering results in a reduction in the lattice dimensions as well as an increase in the lattice tetragonal distortion Ni-doped Sr2FeMoO6 exhibits a linearly increasing magnetization at room temperature with the increasing level of Ni doping and thus the degree of B-site ordering The Curie temperature is also raised significantly by the increasing level of Ni doping, from Tc = VI Summary 411 K for x = to Tc = 432 K for x = 0.20, which is attributed to the enhancement in B-site ordering and magnetic interactions VII List of Tables LIST OF TABLES Table 1.1 Crystal structure and magnetic properties of some double perovskite A2B’B”O6 compounds ……………………………………………………………….11 Table 2.1 Chemicals used in the project …………………………………………….31 Table 3.1 Coordinates of atoms (x, y, and z) and site occupancies (n) in the unit cell of Sr2FeMoO6 according to the initial crystal structure model …… 49 Table 6.1 Structural parameters and reliability factors from the Rietveld refinements and results from magnetic measurements for the Sr2(Fe1-xNix)MoO6 (0.0 ≤ x ≤ 0.02) compounds ………………………………………………………………….………112 VII List of Figures LIST OF FIGURES Figure 1.1 Schematic illustration of giant magnetoresistance (GMR) effect …………3 Figure 1.2 Schematic of Sr2FeMoO6 structure [41] Only a few of oxygen atoms are shown for clarity, while the Sr atoms at the body-centre positions are not shown …………………………………………………….………………………… 12 Figure 1.3 The density of states (DOS) of double perovskite Sr2FeMoO6 (Kobayashi et al [20]) ………………………………… ……………………………………… 15 Figure 2.1 Experimental procedures for Sr2FeMoO6 derived from mechanical activation by using MoO3 as the starting material ………………………… ………32 Figure 2.2 Experimental procedures for Sr2FeMoO6 derived from mechanical activation by using MoO2 as the starting material ………………………………… 33 Figure 2.3 Experimental procedures for Sr2(Fe1-xNix)MoO6 (0 ≤ x ≤ 1) derived from mechanical activation by using MoO2 as the starting material …………………… 34 Figure 2.4 Schematic diagram of a scanning electron microscopy (SEM) [100] … 38 Figure 2.5 Schematic diagram of a vibrating sample magnetometer (VSM).… … 39 Figure 2.6 Schematic diagram of four-point probe measurement system …… …….42 Figure 3.1 A fragment of the XRD Rietveld profile for Sr2FeMoO6 derived from mechanical activation and then sintered at 1100 °C in Ar ………………………… 52 Figure 4.1 XRD patterns of the mixed oxides of SrO, Fe2O3 and MoO3 subjected to various hours of mechanical activation (*:Sr2FeMoO6, x: SrMoO4, F: Fe2O3) …….56 Figure 4.2 XRD patterns of Sr2FeMoO6 subjected to 25 hours of mechanical activation and then heat-treated in 5% H2/Ar at different temperatures for hours (*:Sr2FeMoO6, x: SrMoO4) …………………………… ……………… …………58 Figure 4.3 SEM micrographs for Sr2FeMoO6 derived from 25 hours of mechanical activation and then heat-treated in 5% H2/Ar at: (a) 700°C and (b) 900°C for hours ………………………………………………………… …………………….59 VIII List of Figures Figure 4.4 Hysteresis loops at 290 K for Sr2FeMoO6 derived from 25 hours of mechanical activation and then heat-treated in 5% H2/Ar at different temperatures for hours ……………………………………………………………………………….61 Figure 4.5 Temperature dependence of electrical resistivity for the Sr2FeMoO6 derived from different thermal treatment temperatures …………………………… 63 Figure 4.6 Isothermal magnetoresistance at (a) 290 K and (b) 78 K for the Sr2FeMoO6 derived from different thermal treatment temperatures …………………………… 64 Figure 5.1 XRD patterns of the mixed oxides of SrO, Fe2O3 and MoO2 subjected to various hours of mechanical activation (*:Sr2FeMoO6, M: MoO2, FO: Fe2O3, F: Fe) …………………………………………………………………………………69 Figure 5.2 XRD patterns of Sr2FeMoO6 subjected to various hours of mechanical activation and then sintered in Ar at 900°C for hours (*:Sr2FeMoO6, SM: SrMoO4, F: Fe) ……………………………………………………………………………… 72 Figure 5.3 SEM micrographs for Sr2FeMoO6 derived from (a) hours, (b) 25 hours, and (c) 45 hours of mechanical activation and then heat-treated in Ar at 900°C for hours ……………………………………………………………………………… 73 Figure 5.4 Hysteresis loops at (a) 290 K and (b) 78 K for Sr2FeMoO6 derived from various hours of mechanical activation and then sintered in Ar at 900°C for hours …………………………………………………………………………………75 Figure 5.5 Temperature dependence of electrical resistivity for Sr2FeMoO6 derived from 5, 25, and 45 hours of mechanical activation at zero field (solid line) and 3T (dot line) ……………………… ……………………………………………………… 77 Figure 5.6 Isothermal magnetoresistance for Sr2FeMoO6 derived from 5, 25 and 45 hours of mechanical activation (a) at 290 K, and (b) at 78 K ……………………….79 Figure 5.7 XRD patterns of Sr2FeMoO6 subjected to 25 hours of mechanical activation and then heat-treated in Ar at different temperatures for hours ……… 81 Figure 5.8 SEM micrographs for Sr2FeMoO6 derived from 25 hours of mechanical activation and then heat-treated in Ar at: (a) 800 °C, (b) 900 °C, (c) 1000 °C, and (d) 1100 °C for hours ………………………………………………………………….83 Figure 5.9 XRD Rietveld profile for Sr2FeMoO6 sintered at 800 °C using the space group I4/m Observed (black cross signs) and calculated (red solid line) intensities are shown together with their difference (green curve at the bottom) The blue vertical bars indicate the expected Bragg reflection positions ……… …………… …… 85 IX Chapter Conclusions and Suggestions for Future Work Chapter Conclusions and Suggestions for Future Work 7.1 Conclusions In this project, a novel mechanical activation route was developed to synthesize double perovskite Sr2FeMoO6 First of all, the effects of mechanical activation on the phase formation and magnetoresistance of MoO3-based Sr2FeMoO6 were investigated Sr2FeMoO6 with minimal level of SrMoO4 impurity was realized by 25 hours of mechanical activation of SrO, Fe2O3 and MoO3 in a nitrogen atmosphere A single phase of double perovskite Sr2FeMoO6 was obtained at 700 °C in flowing 5% H2/Ar, which is about 200 °C lower than what is required in the conventional solid state reaction The polycrystalline Sr2FeMoO6 exhibited an average crystallite size in the range of 30 to 50nm, increasing with an increase in temperature in the range of 600 °C to 900 °C The magnetization of thus derived Sr2FeMoO6 increases when the temperature was raised from 700 °C to 900 °C, due to the enhancement in crystallinity and grain size as well as B-site ordering Magnetoresistance of MoO3-based Sr2FeMoO6 also increases with an increase in thermal treatment temperature, which is attributed to the elimination of insulating SrMoO4 impurity and enhancement in B-site ordering 120 Chapter Conclusions and Suggestions for Future Work Secondly, the effects of both mechanical activation and sintering temperature on the phase formation and magnetoresistive behaviors of MoO2-based Sr2FeMoO6 were studied By changing the starting material from MoO3 to MoO2, a single phase of Sr2FeMoO6 with double perovskite structure was successfully synthesized in air via mechanical activation Polycrystalline Sr2FeMoO6 with grain sizes in the nanometer range was then obtained by sintering the mechanically activated composition in Ar Due to the effective elimination of nonmagnetic SrMoO4 impurity, the magnetization of MoO2-based Sr2FeMoO6 increases when increasing mechanical activation time Similarly, the MR effect also increases with increasing mechanical activation time up to 25 hours, due to the elimination of SrMoO4 impurity phase and the refinement in grain size However, too long a mechanical activation time led to excess contamination by Fe and thus reduced the MR effect of Sr2FeMoO6 Further, a single phase of MoO2-based Sr2FeMoO6 derived from mechanical activation was achieved in Ar in the temperature range of 800 °C to 1100 °C The B-site ordering in Sr2FeMoO6 was systematically enhanced by increasing sintering temperature in the range of 800 °C to 1100 °C Consequently, the magnetization of Sr2FeMoO6 is significantly enhanced by high temperature sintering Magnetoresistance of MoO2-based Sr2FeMoO6 sintered at different temperatures ranging from 800 °C to 1100 °C also increases with the increase in sintering temperature, which is mainly attributed to the increase in B-site long-range order 121 Chapter Conclusions and Suggestions for Future Work Thirdly, the effects of Ni doping on the B-site ordering and magnetic behaviors of double perovskite Sr2FeMoO6 were systematically investigated A high Ni doping level (> 0.3) results in the occurrence of impurity phase and decreases the magnetization in Sr2FeMoO6 A single phase of polycrystalline Sr2(Fe1-xNix)MoO6 (0.0 ≤ x ≤ 0.02) with double perovskite structure has been successfully synthesized via mechanical activation by using MoO2 as the starting material The long-range order parameter S among octahedral B sites is significantly enhanced by Ni doping, from S = 0.584 for x = to S = 0.932 for x = 0.20 The B-site ordering results in a reduction in the lattice dimensions and unit cell volume, as well as an increase in the lattice tetragonal distortion Consequently, magnetization of Ni-doped Sr2FeMoO6 is enhanced, due to the strengthening of antiferromagnetic coupling between B’ and B” sublattices Ni-doped Sr2FeMoO6 exhibits a linearly increasing magnetization at room temperature with the increasing level of Ni doping and thus the degree of B-site ordering The Curie temperature is also raised significantly by the increasing level of Ni doping, from Tc = 411 K for x = to Tc = 432 K for x = 0.20, which is due to the enhancement in B-site ordering and magnetic interactions 122 Chapter Conclusions and Suggestions for Future Work 7.2 Suggestions for Future Work Double perovskite Sr2FeMoO6 has been widely prepared via conventional solid state reaction by using MoO3 as the starting material in a reducing atmosphere In this research project, a single phase of double perovskite Sr2FeMoO6 has been successfully synthesized in air for the first time by using MoO2 as the starting material via mechanical activation It would be of interest to further investigate and compare the possible differences in crystal structure, valence state, saturation magnetization, and magnetoresistance between the Sr2FeMoO6 derived from mechanical activation with those formed by solid state reaction For this, low temperature magnetization and magnetoresistance measurement, Mossbauer spectroscopy, and detailed structure refinement analysis could be done to compare the structure and properties of the Sr2FeMoO6 formed from different routes Ni doping at octahedral B sites via mechanical activation was found to progressively raise both the room temperature magnetization and Curie 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Xue, and J Wang, Mechanically Activated Synthesis and Magnetoresistance of Nanocrystalline Sr2FeMoO6, ” J Am Ceram Soc., 88 [9] 2635-38 (2005) 2 L Chen, C L Yuan, J M Xue, and J Wang, “Phase Formation and Magnetoresistance of Double Perovskite Sr2FeMoO6, ” J Am Ceram Soc., 88 [11] 3279-82 (2005) 3 L Chen, C L Yuan, J M Xue, and J Wang, “B-site Ordering and Magnetic Behaviors in Ni Doped Double Perovskite. .. Ni Doped Double Perovskite Sr2FeMoO6, ” J Phys D: Appl Phys., 38, 4003-08 (2005) 4 L Chen, C L Yuan, J M Xue, and J Wang, “Enhancement of Magnetization and Curie Temperature in Sr2FeMoO6 by Ni Doping,” J Am Ceram Soc., 89 [2] 672-74 (2006) CONFERENCE PRESENTATIONS 1 L Chen, C L Yuan, J M Xue, and J Wang, “Phase Formation Behaviors and Magnetoresistance of Double perovskite Sr2FeMoO6, ” MRS-S National... Inspection of the figure quickly reveals the half-metallic nature of the ground state of this compound: the density of states for the down-spin band is present at the Fermi level, whereas the up-spin band forms a gap at the Fermi level The occupied up-spin band is mainly composed of Fe 3d electrons hybridized with oxygen 2p states and much less of the Mo 4d electrons The nominal Mo t2g and eg up-spin bands... light of possible application to electromagnetic devices 14 Chapter 1 Introduction Figure 1.3 The density of states (DOS) of double perovskite Sr2FeMoO6 (Kobayashi et al [20]) 15 Chapter 1 Introduction 1.3.2 Magnetic Structure Based on the band structure, it has been suggested [20] that Sr2FeMoO6 is a ferrimagnet and consists of Fe3+ 3d5 (S = 5/2) and Mo5+ 4d1 (S = 1/2) ions alternating on the perovskite. .. exists for one electron spin The value of P usually decreases with increasing temperature Extensive band structure calculations have been carried out to understand the electronic and magnetic structures of Sr2FeMoO6 [20, 41, 43] The results of a typical calculation of the density of states (DOS) with majority ‘up’ and minority ‘down’ spins as well as the local density of states for the elements, done by... Perovskite Sr2FeMoO6 Compounds of the formula A2B’B”O6 tend to adopt the perovskite structure when A is a large cation capable of 12-fold coordination with oxygen while B’ and B” are smaller cations suitable for octahedral coordination If the difference in charge of the B’ and B” cations is large, these ions assume an ordered arrangement in the perovskite lattice The family of ordered double perovskites... structure and magnetic properties of selected A2B’B”O6 compounds are summarized and compared in Table 1.1 Thus, an explanation of the magnetic structure of Sr2FeMoO6 must also be consistent with such diverse properties observed within double perovskite oxide systems There are several other issues concerning the electronic and magnetic structures of this compound that are still controversial and we will... Chapter 1 Introduction 1.4 Synthesis Routes Double perovskites have been extensively studied since the discovery of room temperature and low field MR in Sr2FeMoO6 in 1998 At this stage the research of Sr2FeMoO6 is mainly focused on the bulk form, in understanding its various fundamental issues Thus, the discussion is limited to the synthesis routes for polycrystalline bulk Sr2FeMoO6 There have been... availability of oxide precursors, low cost and the precise weighing of oxide precursors and reaction components In preparation of bulk Sr2FeMoO6 samples by conventional solid-state reaction processing, Sr2CO3, Fe2O3 and MoO3 are often used as the starting materials After stoichiometric proportions of the starting components are mixed and ground, the mixed powder is calcined in air or Ar at a temperature of 800-1000... of atoms 1.3.3 Electro-transport Properties The electrical resistivity of Sr2FeMoO6 is dependent on the synthesis conditions, due to the cation disorder, grain-boundary scattering and oxygen content [54] The carriers in Sr2FeMoO6 are believed to be electron-like with a density of about 1.1×10-22 cm-3, corresponding to nearly one electron-type carrier per pair of Fe and Mo [26] Both semiconducting and ... CHAPTER MECHANICALLY ACTIVATED SYNTHESIS AND MAGNETORESISTANCE OF MOO2-BASED SR2FEMOO6 66 5.1 EFFECTS OF MECHANICAL ACTIVATION ON THE PHASE FORMATION AND MAGNETORESISTIVE BEHAVIORS OF SR2FEMOO6. .. to synthesize double perovskite Sr2FeMoO6 The effects of mechanical activation and heat-treatment temperature on the phase formation, magnetic and magnetoresistive behaviors of Sr2FeMoO6 were... order in double perovskite Sr2FeMoO6 Sr2FeMoO6 with minimal level of SrMoO4 impurity was synthesized by mechanical activation of SrO, Fe2O3 and MoO3 in a nitrogen atmosphere Double perovskite Sr2FeMoO6