臺灣博碩士論文加值系統

鑭鍶錳氧化物因為具有優越的磁/電特性，例如：高居里溫度(Tc）、巨磁阻（CMR）、相分離（Phase separation）等，所以特別引人注目。本論文特針對結晶構造、成分組態、鐵磁性、導電性及表面奈米特性加以研究。其結果分述如下：
利用R.F.磁控濺鍍製作x~0.1之(La1-x,Srx)MnO3(LSMO)薄膜，結果發現以不同的Ar/O2比例可鍍製出具有及不具有(100)優選指向的薄膜。三種薄膜皆呈現鐵磁與順磁之相轉變，其居里溫度約為250K，此值大大高於塊材的居里溫度。不具(100)優選指向的薄膜，其飽和磁化量及導電率皆明顯高於具有(100)優選指向的薄膜，推究其原因為熱應力致使Jahn-Teller distortion，此機制使薄膜產生了不同的鐵磁與電阻特性，並且可能存在著金屬與絕緣相共存的現象。
為了確定應力對鐵磁、導電特性之影響，我們選定具有(100)優選指向（Ar/O2=2/1），及不具有(100)優選指向（Ar/O2=1/1）二種薄膜，並利用超高真空系統之導電性原子力顯微術（C-AFM in UHV system）加以觀測其導電機制，結果顯示：在室溫下，的確有金屬/絕緣二相共存之情況，且具有(100)優選指向的薄膜其導電性在同一晶粒上分佈不均，但不具(100)優選指向的薄膜其導電性則是跟晶粒分佈成正相關。使用C-AFM所得到之金屬-絕緣相轉變溫度(TM-I)幾乎與變溫電阻量測的結果一致。
接著，同樣以具有(100) 和不具(100)優選指向的兩種薄膜，改變其厚度（~350nm, ~160nm, 及~ 50nm），探討膜厚對鐵磁性與電導特性之影響。實驗顯示：二種不同優選指向的薄膜隨著厚度增加，其鐵磁特性/居里溫度與電導度/金屬-絕緣相轉變溫度亦隨著提升。其原因是隨著厚度增加，其應力鬆弛，即應力相對變小，導致於鐵磁性及電導特性變好。
在LSMO薄膜中埋入白金奈米粒子，結果發現：其居里溫度會下降。原因為：添加白金奈米粒子在其界面產生了磁性不均勻或自旋失序等狀況，導致居里溫度降低。而白金奈米粒子之添加確實會改變在垂直膜面方向的自旋狀態；且此種設計也提供電子一種更便捷的路徑進行傳導，所以導致薄膜的導電度變好。

The manganites La1-xSrxMnO3 (LSMO), exhibiting a high Curie temperature (Tc), a good canonical double exchange system, and a good colossal magnetoresistance (CMR), have attracted much attention for their magnetic, electrical, and structural properties. The crystal structure, chemical configuration, ferromagnetic property, electrical transport, and nanoscale characterization are investigated in this study.
The LSMO thin films were deposited on SiO2(200 nm)/Si(100) substrate by R.F. magnetron sputtering at 6000C in an atmosphere of 5 mtorr with Ar/O2 ratio ranging from 4/1, 2/1 to 1/1, which is fabricated to different preferred orientation. The diversity of ferromagnetotransport is attributed to the alleviation of Jahn-Teller distortion induced by the thermal stress in films having different preferred orientations. And the polycrystalline films of (La0.91,Sr0.09)MnO3 with and without (100)-preferred orientation have spatial inhomogeneities with conducting and insulating domains coexisted in submicrometer scale, which is observed by conductive atomic force microscopy (CAFM). The domains undergo a percolative metal-insulator transition, and the transition temperature (TM-I) observed from CAFM is consistent to the result of magnetoresistance measurement. Different preferred orientation of LSMO thin films with various thickness were also studied. The ferromagnetic property and electrical transport increase with increasing film thickness. It is considered as the result of small deformation of the Mn-O bond length/bond angle by the relaxation of thermal strain. We fabricate the metallic Pt nano-crystals embeded in the LSMO matrix for the enhancement of ferromagnetotransport. The downshift of Curie temperature with LSMO/Pt(NP)/LSMO thin films is due to the magnetic inhomogeneity or spin disorder, which is caused by the addition of Pt nano-crystals in the films. The spin alignment in the normal direction of the films is significantly affected by the insertion of the ultrathin layers of Pt nanocrystals. The sub-layer of Pt nanocrystals provides an easier pathway for electron transport between LSMO grains, and thus the resistivity becomes significantly reduced.

LIST OF TABLES............................................Ⅲ
LIST OF FIGURES...........................................Ⅳ

Chapter 1 Introduction.....................................1
1-1 Prelude................................................1
1-2 Motivation.............................................3
1-3 Outline of the dissertation ...........................5

Chapter 2 The background study of CMR manganites ..........7
2-1 Magnetism of materials.................................7
2-1-1 Magnetic source of materials.........................7
2-1-2 Magnetic classification of materials.................8
2-1-3 Direct exchange .....................................9
2-1-4 Super-exchange .....................................11
2-2 Mechanism of CMR materials............................13
2-2-1 Introduction to CMR ................................13
2-2-2 Double exchange ....................................15
2-2-3 Jahn-Teller distortion .............................18
2-2-4 Phase separation....................................19
2-3 The property of perovskite manganites ...............................................21

Chapter 3 Experimental procedures.........................34
3-1 Substrate.............................................34
3-2 Fabrication of LSMO thin films .......................34
3-3 Characterization......................................34
3-3-1 Structural analysis.................................34
A. X-ray diffraction (XRD)................................34
B. Scanning electron microscope (SEM) ...................35
C. Transmission electron microscope (TEM).................35
3-3-2 Chemical analysis...................................35
A. Inductively coupled plasma-mass spectroscopy...........35
B. Ruthford backscattering spectrometry (RBS).............36
C. X-ray photoelectron spectroscopy (XPS).................36
3-3-3 Stress measurement..................................37
3-3-4 Magnetic measurement ...............................37
3-3-5 Transport measurement...............................37
3-3-6 Topography and current distribution.................37

Chapter 4 Preferred-orientation effects on the ferromagnetic properties of La1-xSrxMnO3 films deposited on Si substrates.............................................42
4-1 Introduction .........................................42
4-2 Experiment................................................43
4-3 Results and discussion................................44
4-4 Conclusion............................................49

Chapter 5 Conductive atomic force microscopy of percolative metal-insulator transition in polycrystalline (La0.91Sr0.09)MnO3 thin films deposited on Si substrate.................56
5-1 Introduction .........................................56
5-2 Experiment............................................58
5-3 Results and discussion................................59
5-4 Conclusion............................................66

Chapter 6 Effects of thickness-dependent Strain on the magnetotransport properties and phase separation in La1-xSrxMnO3 thin films......................................72
6-1 Introduction.........................................72
6-2 Experiment...........................................73
6-3 Results and discussion...............................74
6-4 Conclusion...........................................79

Chapter 7 The ferromagnetotransport of the La0.9Sr0.1MnO3 thin films embedded with Pt nano-particles................90
7-1 Introduction..........................................90
7-2 Experiment............................................91
7-3 Results and discussion................................92
7-4 Conclusion............................................96

Chapter 8 Conclusions..............................................105

Reference................................................107



LIST OF TABLES


Table 2-1 Some typical magnetoresistance values...........25

Table 3-1 Sputtering conditions of LSMO thin films........39

Table 4-1 The composition, stress, transition temperatures, activation energy and hopping barrier of conduction in the LSMO films grown on SiO2/Si in atmosphere of different oxygen content............................................50

Table 6-1 The composition, structure, Tc, and TM-I in the LSMO thin films having Ar/O2=2/1 and 1/1 grown on SiO2/Si substrate with different hickness.........................80

Table 7-1 The ratio of I(100)/{I(100)+I(110)+I(111)} and I(111)/{I(100)+I(110)+I(111)} for the LSMO-Pt(NP)-LSMO thin films recived from the XRD patterns.......................97



LIST OF FIGURES


Fig. 1-1 Phase diagram of La1-xSrxMnO3 in the plane of doping concentration x and temperature. Abbreviations: PI, paramagnetic insulating; PM, paramagnetic metallic; CI, spin-canted insulating; FI, ferromagnetic insulating; FM, ferromagnetic metallic.....................................6

Fig. 2-1 Ordering of the magnetic dipoles in magnetic materials.................................................26

Fig. 2-2 Direct exchange for (a) non-degenerate orbitals. (b) The energy of configuration is decreased due to virtual hopping of electrons to the neighbor site.................27

Fig. 2-3 Antiferromagnetic super-exchange interaction in a linear metal-oxygen-metal (M-O-M) system..................27

Fig. 2-4 Possible magnetic structures of the perovskite manganites. The circles represent manganese ions, and the sign indicates the direction of the projection of the spin along the z -axis.........................................28

Fig. 2-5 Magnetic-field effects on the resistivity of La1−xSrxMnO3 crystal. (a) Temperature dependence of resistivity, and (b) isothermal magnetoresistance….......28

Fig. 2-6 (a) Sketch of the Double Exchange mechanism which involves two Mn ions and one O ion. (b) The mobility of eg electrons improves if the localized spins are polarized. (c) Spin-canted state which appears as the interpolation between FM and AFM states in some mean-field approximations. ..........................................29

Fig. 2-7 Field splitting of five-fold degenerate atomic 3d levels into lower t2g and higher eg levels. Jahn-Teller distortion of MnO6 octahedron further lifts each degeneracy as shown in the figure....................................29

Fig. 2-8 Dark-field images for La5/8-yPryCa3/8MnO3 obtained by using a super-lattice peak caused by CO. Panel a shows the coexistence of charge-ordered (insulating) and charge-ordered (FM metallic) domains at 20 K for y = 0.375. The charge-disordered domain (dark area) is highlighted with dotted lines for clarity. The curved dark lines present in CO regions are antiphase boundaries, frequently observed in dark-field images for the commensurate CO states of La0.5Ca0.5MnO3. Panels b and c, obtained from the same area for y = 0.4 at 17 K and 120 K, respectively, show the development of nanoscale charge-disordered domains at T > Tc. The curved lines in a, b, c signify the presence of antiphase boundaries of the domains.......................30

Fig. 2-9 Topographic and spectroscopic atomic scale signatures of phase separation into metallic and insulating regions in the paramagnetic phase of Bi0.24Ca0.76MnO3 at 299 K. (a) 3.5*3.5 nm2 STM image of a grain boundary (twisted-line) between an insulating √2a0* √2a0 charge-ordered region (upper right) and a more metallic homogeneous cubic region (lower left). (b) Intensity profile extracted along the orange line in (a). Note the larger amplitude modulation in the ordered region owing to charge ordering. (c) Charge ordered regions with the √2a0* √2a0 lattice (subfuse-color) yield insulating dI/dV(V) characteristics, while the disordered cubic regions (light-color) are characterized by more metallic dI/dV(V) characteristics (numerical derivatives normalized to the metallic junction resistance R=V/I at 0.7 V). The low-bias part of the corresponding I(V) data are shown in the inset. The spectra were taken at the yellow crosses on the 3.7*2.9 nm2 STM images (white squares, cubic unit cell)...........31

Fig. 2-10 Schematic crystal structure of the manganite perovskite................................................32

Fig. 2-11 The lattice structure of (La1-x,Srx)MnO3 crystal at room temperature.......................................32

Fig. 2-12 The t2g (down) and eg (upper) orbitals..........33

Fig. 2-13 The structure types of perovskite manganites....33

Fig. 3-1 Schematic diagram of the dual guns R.F. sputtering system....................................................40

Fig. 3-2 Schematic diagram of the VT Beam Deflection AFM 25 and CAFM..................................................41

Fig. 4-1 The RBS results of LSMO thin films deposited in atmosphere of different Ar/O2 ratio.......................51

Fig. 4-2 Field-emission scanning electron microscopy of the LSMO films deposited on SiO2/Si substrate in atmosphere of different Ar/O2 ratio..................................52

Fig. 4-3 X-ray diffraction patterns of LSMO films deposited on SiO2/Si substrate in atmosphere of different Ar/O2 ratio...............................................53

Fig. 4-4 Relation of magnetization against change of temperature under different magnetic field for LSMO films deposited in atmosphere of different Ar/O2 ratio..........53

Fig. 4-5 Temperature dependence of resistivity measured under different magnetic field for LSMO films deposited in Ar/O2= (a) 4/1, (b) 2/1 and (c) 1/1. The insert shows the corresponding magnetoresistance (MR) of the specimens.....54

Fig. 4-6 Arrhenius plots of the temperature-dependent resistivity of LSMO films deposited in Ar/O2= (a) 4/1, (b) 2/1 and (c) 1/1. The inserts exhibit the linear section of log(1/σ) vs 1/T and log(1/σ) vs 1/T0.5...................55

Fig. 5-1 X-ray diffraction patterns of LSMO films deposited on highly N-type doped (100)-Si substrates in atmosphere of Ar/O2=2/1, and 1/1..........................67

Fig. 5-2 Temperature dependence of resistivity measured under different magnetic field for LSMO films deposited in Ar/O2= (a) 2/1, and (b) 1/1. The insert shows the corresponding magnetoresistance (MR) of the specimens.....67

Fig. 5-3 The topography, current images, and I-V curve of AFM at 300K from a scan size of 200*200nm2 of LSMO thin films deposited in Ar/O2= (a) 2/1, and (b) 1/1............68

Fig. 5-4 TEM images of polycrystalline LSMO thin films deposited in Ar/O2= (a) 2/1, and (b) 1/1..................69

Fig. 5-5 The topography and current images of AFM from a scan size of 1000*1000nm2 of LSMO thin films deposited in Ar/O2= 2/1, measured at temperatures: (a) 300K, (b) 180K, (c) 140K, (d) 130K, (e) 120K, and (f) 100K................70

Fig. 5-6 The topography and current images of AFM from a scan size of 1000*1000nm2 of LSMO film deposited in Ar/O2= 1/1, measured at temperatures: (a) 300K, (b) 200K, (c) 185K, (d) 170K, (e) 150K, and (f) 100K....................71

Fig. 6-1 Different thicknesses of the LSMO thin films deposited in the atmosphere of (a) Ar/O2=2/1, and (b) Ar/O2=1/1.................................................81

Fig. 6-2(a) The RBS results of LSMO thin films deposited in the atmosphere of Ar/O2=2/1 with a thickness of (1) 45nm, (2)163nm, and (3) 358nm...................................82

Fig. 6-2(b) The RBS results of LSMO thin films deposited in the atmosphere of Ar/O2=1/1 with a thickness of (1) 48nm, (2) 161nm, and (3) 382nm..................................83

Fig. 6-3 X-ray diffraction patterns of LSMO films deposited on SiO2/Si substrate having different film thickness with (a) Ar/O2=2/1, and (b) Ar/O2=1/1..........................84

Fig. 6-4 Relation of magnetization against change of temperature under 10 Os and 1T for LSMO thin films with various thickness deposited in the atmosphere of (a) Ar/O2=2/1, and (b) Ar/O2=1/1..............................85

Fig. 6-5(a) The x-ray photoelectron spectroscopy measurement of Mn4+/Mn3+ ratio in LSMO films having Ar/O2=2/1 with layer thickness of (1) 45nm, (2) 163nm, and (3) 358nm.................................................86

Fig. 6-5(b) The x-ray photoelectron spectroscopy measurement of Mn4+/Mn3+ ratio in LSMO films having Ar/O2=1/1 with layer thickness of (1) 48nm, (2) 161nm, and (3) 382nm.................................................87

Fig. 6-6 Temperature dependence of resistivity measurement for LSMO thin films with different film thickness deposited in the atmosphere of (a) Ar/O2=2/1, and (b) Ar/O2=1/1.....88

Fig. 6-7 The topography and current images of CAFM at 300K from a scan size of 1000*1000nm2 of LSMO thin films with various thicknesses of (a) Ar/O2=2/1, and (b) Ar/O2=1/1...89

Fig. 7-1 SEM images of La0.9Sr0.1MnO3-Pt(NP)-La0.9Sr0.1MnO3 thin films with a total film thickness of (a) 50nm, (b) 160nm, and (c) 350nm......................................90

Fig. 7-2 X-ray diffraction patterns of La0.9Sr0.1MnO3-Pt(NP)-La0.9Sr0.1MnO3 deposited on SiO2/Si substrate having different fabrication of Pt nano-particles................99

Fig. 7-3 TEM measurement of Pt nano-particles embedded in the LSMO films with the (a) cross-sectional image of L/Pt(15s)/L/Pt(15s)/L, and (b) plane-view image of L/Pt(30s)/L.................................................100

Fig. 7-4 The X-ray photoelectron spectroscopy spectra of Pt 4f electrons from Pt nano-crystals......................101

Fig. 7-5 Relation of magnetization against change of temperature for LSMO thin films having different fabrication of Pt nano-particles under horizontal magnetic field of (a) 10 Os and (b) 1T............................102

Fig. 7-6 Relation of magnetization against change of temperature for LSMO thin films having different fabrication of Pt nano-particles under perpendicular magnetic field of (a) 10 Os and (b) 1T...................103

Fig. 7-7 Temperature dependence of resistivity measurement for LSMO thin films having different Pt nano-particles processing...............................................104

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