過渡金屬陶鐵磁體的電子結構與磁性的特性，在過去的數十年間，無論在工業以及學術研究上，都受到相當廣泛的應用與重視。過渡金屬氧化物中，由於它們具有較強的電子電荷與自旋間的交互作用，以及晶格結構上的轉變，產生了許多豐富的物理特性。而這些特性的產生可以藉由控制不同的樣品製備以及量測的條件，諸如溫度、參雜濃度、磁場大小、或者是其他物理上的變因等，來獲得。使用分子束磊晶(MBE)長晶技術，以氧化鎂MgO為基底，可以製作出一系列高品質的過渡金屬氧化物薄膜以及超晶格樣品。本篇論文將著重在這些新穎材料與塊材之間有所不同的物理特性之分析，主要是以它們結構中的陽離子分配、電子結構、晶格結構上的轉變以及磁性的變化..等，一系列的陳述。
論文中主要分為兩個部份：第一個部份是以研究過渡金屬氧化物陶鐵磁體 Fe-M-O (M 表示不同的過渡金屬元素，如: Mn、Co、Ni) 的電子結構、陽離子分配、晶格結構上的改變..等。第二部份描述了超晶格樣品Mn3O4/Fe3O4和Fe3O4/MgO 的電子結構與磁性的特性。
研究中，運用了台灣新竹國家同步輻射研究中心(NSRRC)的高強度且連續可調的第三代同步幅射光源，配合著X光吸收光譜(XAS)的分析技術，其中包含了X光吸收光譜鄰近邊緣結構(XANES)和X光磁圓偏振二向性(XMCD)，來探討這些樣品的物理性質。

The electronic and magnetic structures of transition metal ferrites have been of interest for more than a decade. The rich of physical properties are attributed to the strong correlations between charge and spin of the electrons and the wealth of lattice structures in transition metal oxides. These properties can be tuned by controlling the composition, temperature or other physical variables such as magnetic field. The epitaxial growth of ferrites on single crystal MgO substrate allows the preparation of high quality thin films and superlattices. We have investigated the cation distributions, electronic and magnetic variations, and structural changes on these novel materials. Significant differences from their bulk forms are found and the grounds of these variations are discussed. 
This thesis comprises of two parts. First part describes study of the electronic strucure, cation distribution and structural change of transition metal oxide ferrites Fe-M-O (M is transition metal Mn, Co, Ni) thin films. Second part illustrates the electronic and magnetic properties of superlattices of Fe3O4/MgO and Mn3O4/Fe3O4. In these studies, principally x-ray absorption spectroscopy (XAS) including the X-ray Absorption Near Edge Structure (XANES) and the X-ray magnetic circular dichroism (XMCD) techniques, the intense and continuously tunable x-rays produced at third generation synchrotron facilities (National Synchrotron Radiation Research Centre, Taiwan) were employed.

Table of Contents

Acknowledgement............................................i
Abstract..................................................iv
List of Figures.........................................viii
List of Tables...........................................xiv

1.  Introduction
   1-1. Ferrite Material...................................1
  1-1.1. Ni-Fe Mixed Oxide Thin Films......................8
  1-1.2. Co-Fe Mixed Oxide Thin Films .....................9
  1-1.3. Mn-Fe Mixed Oxide Thin Films.....................11
1-2. Superlattice ........................................13
  1-2.1. Mn3O4 / Fe3O4 Superlattices......................15
  1-2.2. Fe3O4 / MgO Superlattices........................19

2.  Experimental Techniques
2-1. Need for Spectroscopic Studies.......................20
2-2. Synchrotron Radiation................................21
2-3. Beamlines of Utilized................................23
2-4. X-ray Absorption Spectroscopy (XAS)..................28
 2-4.1. Measurement Procedure ............................33
2-5. X-ray Magnetic Circular Dichroism (XMCD).............34
 2-5.1. Measurement Procedure.............................38
 
3.  Results on Mixed Oxide Thin Films 
3-1. Ni Substituted System................................40
   3-2. Co Substituted System.............................55
   3-3. Mn Substituted System.............................67

4.  Superlattices Base on Fe3O4
4-1. Mn3O4 / Fe3O4 Superlattices..........................79
4-1.1. XANES Results......................................88
4-1.3. Temperature Dependent XMCD Results.................94
4-2. Fe3O4 / MgO Superlattices...........................108
  

5.	Summary
 5-1. Mixed Oxide Thin Films.............................115
5-2. Superlattices.......................................116
Appendix ................................................119
Bibliography.............................................125 



List of Figures:

Chapter 1
1-1.   (a) Standard spinel structure. (b)Local coordination of oxygen, A (tetrahedral) and B (octahedral) manganese atoms.....................................................4
1-2.   Spinel structure. Large circles represent oxygen; small circles metal cations. The large cube is the conventional unit cell. For clarity, some of atoms are not shown.....................................................5
1-3.   Verway metal-insulator transition. ................5
1-4.   NaCl rocksalt structure............................7
1-5.   Superlattice grown on MgO (110) or (100) using oxygen plasma assisted molecular beam epitaxy (MBE)......13

Chapter 2
2-1.   The electromagnetic spectrum. ....................22
2-2.  Schematics diagram showing the basic production of synchrotron radiation....................................22
2-3.   HSGM beamlime and experiment apparatus. ..........24
2-4.   Wiggler-C beamlime and end-station................24
2-5.   EPU beamlime......................................25 
2-6.   Dragom beamline and accessories...................25
2-7.   (a) The top view and (b) storage ring, beamlines and end-stations of National synchrotron radiation research center (NSRRC) in Taiwan.................................26
2-8.   Photon Spectra of NSRRC...........................26
2-9state above the Fermi level...........................28
2-10.  X-ray absorption spectrum of Fe3O4 at Fe K-edge, as an example which corresponds to excitation of a Fe 1s electron into empty p state. The spectrum is divided into XANES and EXAFS......................................... 29
2-11.  (a) EXAFS, Pictorial view of photoelectron scattering processes in the single-scattering regime, and (b) in the multiple-scattering regime....................30
2-12.  A picture, which shows the origin of EXAFS oscillation..............................................31
2-13.  Schematic view of an x-ray absorption spectrometer.............................................32
2-14.  Two decay mechanisms for the core hole left after the absorption of an x-ray photon........................32
2-15.  Illustration of XMCD in the 2p-3d excitation......37
2-16.  The spin resolved density of states and x-ray absorption selection rules of Fe3O4 are shown on the left.  On the upper right is the XAS spectra acquired with each of the light polarizations and on the lower right is the "difference" spectrum, the XMCD......................37
2-17.  The two sketch of basic MCD measurements (a) varying circular polarized light direction (b) varying applied magnetic fields for the electromagnet current............38
2-18.  Temperature calibration: Two silicon diodes, a diode set in the end of cryostat probe and b mount on sample holder, shown in right chart. The left diagram is the temperature relation of a and b..........................39

Chapter 3
3-1.   The relationship of d-spacing vs Ni concentration ratio for Fe-Ni-O thin films............................41
3-2.   Fe L2,3-edge XANES spectra of a series of Fe-Ni-O thin films and standard oxides Fe2O3 and Fe3O4. All these spectra were normalized to the peak of highest intensity..............................................43
3-3.   Ni L2,3-edge XANES spectra of a series of Fe-Ni-O thin films and NiO. All these spectra were normalized to the peak of highest intensity.. .......................44
3-4.   FWHM of Ni L3-edge as a function of the Ni concentration x........................................45 
3-5.  The normalized XANES spectra of various Fe-Ni-O thin films at O K-edges.....................................47
3-6.   (a) The absorption pre-peak spectra of O K-edge at high concentration, (b) Fitting with the different ratio for NiO and Fe2O3  ....................................49
3-7.   Normalized Fe K-edge XANES spectra at different x values along with Fe3O4 films..........................51
3-8.   Normalized Ni K-edge XANES spectra at different x values along with NiO films............................52
3-9.   The radii model of Fe-Ni-O cation distribution...........................................54
3-10.  The relationship of d-spacing vs Co concentration ratio for Fe-Co-O thin films...........................56
3-11.  Fe L2,3 edge XANES spectra of a series of Fe-Co-O thin films, FeO, Fe2O3, and Fe3O4. These spectra were normalized to the peak of highest intensity............58
3-12.  Co L3 edge XANES spectra of a series of Fe-Co-O thin films, CoO, Co3O4, and Co2O3. These spectra were normalized to the peak of highest intensity......................59
3-13.  O K-edge XANES spectra of a series of Fe-Co-O thin films. All of the spectra were normalized to the peak E3 (~540eV)..............................................61
3-14.  Normalized Fe K-edge XANES spectra at different x values along with Fe3O4 films and FeO bulk............62
3-15.  Normalized Co K-edge XANES spectra at different x values along with Co3O4 bulk and CoO film.............63
3-16.  (a)Valence of Fe ion and (b) valence of Co ion with various x value.......................................64
3-17.  The d-space with various Fe/Mn ratio...........68
3-18.  Fe L2,3-edge XANES spectra of a series of Fe-Mn-O thin films and standard oxides MnFe2O3 and Fe3O4. All these spectra are normalized to the peak of highest intensity.............................................70
3-19.  Mn L2,3-edge XANES spectra of a series of Fe-Mn-O thin films and standard oxides MnFe2O4 and Mn3O4. All these spectra were normalized to the peak of highest intensity.............................................71
3-20.  O K-edge XANES spectra of a series of Fe-Mn-O thin films. All of the spectra were normalized to the peak G4 (~544eV)..............................................72  
3-21.  Normalized Mn K-edge XANES spectra at different x values along with Mn3O4 film and MnO, Mn2O3 ,MnO2, and MnFe2O4 bulks.........................................74
3-22.  Normalized Fe K-edge XANES spectra at different x values along with Fe3O4 film..........................75
3-23.  (a) Valence of Mn ion with various x value. (b) Valence of Fe ion with various x value................76

Chapter 4
4-1.   A comparison of the XANES spectra of Fe L2.3-edge (a) for superlattice at different thickness of bilayers and (b) for various mixed oxide films, with Fe3O4, and MnFe2O4 as the reference.......................................81
4-2.   A comparison of the XANES spectra of Mn L2.3-edge (a) for superlattice at different thickness of bilayers and (b) for various mixed oxide films, with Mn3O4, and MnFe2O4 as the reference.......................................82
4-3.   A comparison of the XANES spectra of oxygen K-edge (a) for superlattice at different thickness of bilayers and (b) for various mixed oxide films, with Fe3O4, Mn3O4, and MnFe2O4 as the reference...............................83
4-4.   The XANES spectra of superlattices at (a) Fe K-edge and (b) Mn K-edge absorption spectra with MnFe2O4 bulk, Fe3O4 and Mn3O4 thin films as reference samples........85 
4-5.   The valence variation of Fe (upper label with star symbol)and Mn (down label with circular symbol) with bilayer thickness of the superlattices and reference samples................................................86  
4-6.   Fe (a) and Mn (b) L2,3-edge XAS spectra for the 68Å/68Å superlattice. The solid line (μ+) and the dotted (μ-) line are the absorption intensities taken with the projection of the photon spin parallel and antiparallel to the sample magnetization direction, respectively. XMCD, the difference μ+-μ-, is plotted below the XAS curves................................................89
4-7.   Fe (a) and Mn (b) L2,3-edge XAS spectra for bulk sample of MnFe2O4. The solid line (μ+) and the dotted (μ-) line are the absorption intensities taken with the projection of the photon spin parallel and antiparallel to the sample magnetization direction, respectively. XMCD, the difference μ+-μ-, is plotted below the XAS curves................................................90
4-8.   Fe (a) and Mn (b) L2,3-edge XAS spectra for the 17Å/17Å superlattice. The solid line (μ+) and the dotted (μ-) line are the absorption intensities taken with the projection of the photon spin parallel and antiparallel to the sample magnetization direction, respectively. XMCD, the difference μ+-μ-, is plotted below the XAS curves.................................................92
4-9.   Fe L2,3-edge XMCD of superlattice samples of different layer thickness as indicated.................93
4-10.  XMCD Fe L2,3-edge spectra of Fe3O4/MgO(100) single layer standard with various temperature................96
4-11.  XMCD Fe L2,3-edge (left) and Mn L2,3-edge (right) spectra of MnFe2O4 bulk standard with various temperature............................................97
4-12.  Mn3O4¬¬/MgO(100) single layer standard (a) XMCD Mn L2,3-edge  spectra with various temperature (b) XAS and XMCD Mn L2,3-edge spectra at 35K ......................98
4-13.  The experimentally observed temperature dependent MCD of 17Å/17Å superlattice L2,3-edge spectra for (a)Fe (b)Mn (upper). ..........................................100
4-14.  The experimentally observed temperature dependent MCD of  34Å/34Å superlattice L2,3-edge spectra for (a)Fe (b)Mn.................................................102
4-15.  The experimentally observed temperature dependent MCD of 68Å/68Å superlattice L2,3-edge spectra for (a)Fe (b)Mn....................................................103
4-16.  Comparison of the Mn L2,3-edge spectrum contribution for superlattices, standard Mn3O4 and MnFe2O4 bulk at 35K...................................................104
4-17.  Comparison of Fe and Mn L3-edge spectral areas of the Mn3O4/Fe3O4 superlattices, standard Fe3O4 and Mn3O4 single layer films at different temperatures..........106
4-18.  RHEED images of a clean MgO (001) substrate (a) , after a thin base layer is ~50 Å of MgO was deposited (b), a MgO terminated surface after 30 Fe3O4/MgO bilayer repeats (c), and a Fe3O4 terminated surface after 32 Fe3O4/MgO bilayer repeats (d)...................................109
4-19.  XRD results for the superlattices of structures as indicated, for instance, (19/82)30.5 indicates 30 layers of MgO (19 Å)/Fe3O4 (82 Å) and an additional MgO layer.................................................111
4-20.  Fe L2,3 -edge XAS of Fe3O4 thin film and superlattice samples of structures as indicated, all spectra are normalized to the highest peaks...........112
4-21.  Fe L3-edge XAS spectral difference taken by subtracting Fe3O4 spectrum from the spectra in Fig. 4-3.....................................................113

Chapter 5
5-1.   The cation valences as functions of the relative concentration.........................................118
 


List of Tables:


1-1	Cation distributions in spinel ferrite...........3

1-2	Basic physical properties of Fe3O4 and Mn3O4....12

2-1.	The beamline operation in NSRRC, where the marked beamlines are used for our experiments...................27

3-1.	Represent the Ni replacing the Fe at different site with a relative ratio for Fe-Ni-O mixed oxide thin films....................................................54

3-2.	Represent the Co3+ and Co2+ replacing the Fe at different site with a relative ratio for Fe-Co-O mixed oxide thin films.........................................66

3-3.	Represent the Mn3+ and Mn2+ replacing the Fe at different site with a relative ratio for Fe-Mn-O mixed oxide thin films.........................................78

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