多相複雜氧化物材料其奈米結構與物理性質間交互作用之研究

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研究生：	劉恒睿 Liu, Heng-Jui
論文名稱：	多相複雜氧化物材料其奈米結構與物理性質間交互作用之研究 Interplay of Nano-structure and Physical Properties in Multi-phases Complex Oxide Materials
指導教授：	林樹均 Lin, Su-Jien 朱英豪 Chu, Ying-Hao
口試委員:	莊振益 Juang, Jenh-Yih 張立 Chang, Li 陳宜君 Chen, Yi-Chun 羅志偉 Luo, Chih-Wei
學位類別：	博士 Doctor
系所名稱：	工學院 - 材料科學工程學系 Materials Science and Engineering
論文出版年：	2012
畢業學年度：	100
語文別：	英文
論文頁數：	125
中文關鍵詞：	複雜氧化物、自組裝奈米結構、鉍鐵氧、鍶釕氧、鈷鐵氧、單斜晶相變、極化旋轉、光磁耦合效應
外文關鍵詞：	Complex oxides, Self-assembled nanostructure, BiFeO3, SrRuO3, CoFe2O4, Monoclinic phase trasition, polarization rotation, Photo-magnetic coupling effect
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本論文係研究多相系統之材料如多鐵性材料或奈米複合材料其結構與物理性質間之關聯。本論文亦分成兩部份，第一部份探討多鐵性材料鉍鐵氧薄膜之相變化過程與鐵電極性間的響應；第二部份則為觀察具有光致伸縮特性之鈣鈦礦鍶釕氧(SrRuO3)與磁致伸縮特性之尖晶石鈷鐵氧(CoFe2O4)材料所組成之奈米結構薄膜之間的耦合效應。
首先，許多關於多鐵性鉍鐵氧薄膜的研究上指出其電性與磁性有序的耦合現象常與本身豐富的相結構變化有關。尤其是成長在不匹配性過大的單晶基板上如鑭鋁氧(LaAlO3)或是釔鋁氧(YAlO3)時，原本為菱方晶結構的鉍鐵氧會因在平行表面方向上承受的過大的壓應力而變成似長方晶的結構，同時其鐵電的極性也會因此結構的變化而產生改變。本實驗裡，我們採用X光倒置晶格圖與穿透性電子顯微鏡來研究隨著厚度變化的應力發展。在適中厚度的試片裡可發現在應變釋放的過程中存在著似長方晶與菱形晶的結構，即應變誘發的形態相界區域。在更深入的結構分析中顯示為了彌補似長方晶與菱形晶間的晶格不匹配性，還需要多生成兩個傾斜的單斜晶中間相來容忍此極大的側向應力，故可在表面上觀察到週期性的帶狀結構。此外我們將鉍鐵氧成長在釔鋁氧單晶基板上時，其形態相界區域會由於基板在不同方向上所造成應變量的差異，而出現不同週期性的排列帶狀特徵。而這兩個中間相也被發現與溫度變化有強烈的關聯。同時我們也在溫度相關的XRD圖譜中發現約在100oC~200oC附近會發生單斜晶間的相變化，其被觀察到為MC對稱性到MA對稱性的轉變過程。然後我們也用壓電力顯微鏡觀察到在相變化過程中鐵電極化方向的旋轉，並量測其壓電係數，其在相變溫度下呈現出最大的壓電反應。這些研究可使我們更加瞭解鐵電系統在承受應變下其結構改變與極化特性間的關連
另一部份自組裝奈米結構薄膜近年來亦受到廣泛的注目，其優勢為有非常大的界面對體積比率，並且可藉由選擇適當的材料組合來設計新的功能性。雖然現今大部分的研究仍然在強調使用磁場或電場來控制奈米結構，但在此部分實驗裡，我們企圖在此自組裝的系統中使用另一種外在的控制參數：光，來控制奈米結構。我們成功的在鍶鈦氧單晶基板上(SrTiO3)製作出嵌入在鍶釕氧母材裡的鈷鐵氧奈米柱，並且經由超快雷射、導電原子力顯微鏡，超導量子干涉儀等量測來確定各個組成都能保有原來的性質。同時此具有光致伸縮特性的鍶釕氧與磁致伸縮特性的鈷鐵氧結合成的自組裝結構也展現出另一種的交互作用機制：光磁耦合效應，即可利用光來誘發且導致鈷鐵氧奈米柱呈現非常快速的磁性變化。因此，此實驗闡述了一種新的奈米結構設計工程的概念並開啟了另一條不同於傳統方式的新功能性之探索。
最後，我們可發現，不管在單一組成的材料或是多種組成而成的複合材料，多相共存的系統雖然結構上較為複雜，但高度的界面比所帶來豐富及多變的性質卻是單相系統無法比擬的。研究多相共存的系統可提供我們更多值得發掘的新物理現象。

In this thesis, we investigated the correlations between structural variation and physical properties of the some multi-phases materials, such as multiferroic or nano-structured composite thin films. It will be discussed in two parts: one is the study of phase transition and corresponding ferroelectric response in multiferroic BiFeO3 thin films; and the other is the study of coupling effect in self-assembled nano-structured thin films composed of photostrictive perovskite SrRuO3 (SRO) and magnetostrictive spinel CoFe2O4 (CFO).
First, recent researches have shown that the multiferroic bismuth ferrite (BiFeO3, BFO) thin films exhibit high correlation between coupled electric and magnetic orders and abundant structure variation. Especially for those grown on the substrate with large lattice misfit such as LaAlO3 (LAO) or YAlO3 (YAO), it is found that BFO becomes tetragonal-like structure instead of original rhombohedral structure due to the large in-plane compressive strain, and its ferroelectric polarization also changes with the structural variation. In this thesis, we studied a series of the strain evolution on the structural nature of BFO thin films by X-ray reciprocal space mapping (RSM) and transmission electron microscopy (TEM). A strain-driven phase boundaries occur at medium thickness with coexistence of tetragonal-like and rhombohedral-like phases because of strain relaxation. The detailed structures reveals that two extra tilted monoclinic phases form to accommodate the large lattice mismatch from tetragonal-like structure to rhombohedral-like structure, resulting in the feature of periodic strips presented in topography. Besides, the BFO thin films grown on YAO substrate even show a discrepancy of arrayed stripe morphology due to the anisotropic strain in different direction of YAO substrate. We then focused on the intermediate phase at the boundaries between the tetragonal-like and rhombohedral-like phases, whose content and structure are strongly dependent on temperature. We also found a monoclinic phase transition at temperature around 100~200℃ in ambient condition. The observed transition is between an MC symmetry and an MA symmetry. Studies of the ferroelectric domains of the MC and MA phases clearly show that their ferroelectric polarizations rotate when the phase transition occurs. Piezoelectric response in the BiFeO3 thin films displays a substantial enhancement at the MC−MA transition temperature. These findings directly unveil the close correlations between structural changes, polarization rotation and high piezoelectricity in ferroelectrics.
On the other hand, self-assembled vertical nanostructures have also attracted extensive attentions recently due to their advantage of high interface-to-volume ratio. They could be used to design new functionalities by choosing proper combination of constituents. While most of the studies up to date have emphasized the functional controllability of the nanostructures using external electric or magnetic fields, we try to demonstrate a new coupling mechanism: the photo-magnetic coupling effect, which describes light (or photons) as the external control parameter in this study. We have successfully synthesized oxide nanostructures with CoFe2O4 (CFO) nanopillars embedded in SrRuO3 (SRO) matrix on (001) SrTiO3 substrate and confirmed that all constitutes still keep their original properties form measurements of ultra-fast laser, conducting atomic force microscopy (CAFM), and Superconducting Quantum Interference Device (SQUID). Combination of the photostrictive SRO and magnetostrictive CFO in the intimately assembled nanostructures can leads to a light-induced, ultrafast change in magnetization of the CFO nanopillars. Our work demonstrates a new concept on oxide nanostructure design and engineering and opens a pathway alternative to the traditional routes for the explorations of new fuctionalities.
At the end of this work, we can simply conclude that whether in the single-constitute materials or composite materials, the multi-phase systems always have more complicated structural characteristic and abundant properties than single-phase systems due to the high interface-to-volume ratio, so studying these kinds of system can provide us more new un-discovered physical phenomena.

摘要    I
Abstract    III
Acknoledgement(Chineses)    VI
Table of contents    VIII
List of Table and Figures    X
Chapter 1. Introduction    1
1.1    Fundamental of multiferroics and magnetoelectrics    1
1.1.1    Ferroelectricity    3
1.1.2    Magnetism    6
1.1.3    Magnetoelectric multifeorric bismuth ferrite (BiFeO3)    9
1.2    Multifunctional nanostructure composites    18
1.2.1    Design of perovskite-spinel nanostructure thin films    20
1.2.2    Basic properties of perovskite strontium Ruthenate (SrRuO3)[72]    22
1.2.3    Basic properties of spinel cobalt ferrite (CoFe2O4)    28
1.2.4    The Photomagnetic effect    32
Chapter 2. Experimental Methods    34
2.1    Sample fabrication – Pulse laser depoistion    34
2.2    Structural characterization    38
2.2.1    X-ray diffraction    38
2.2.2    Transmission electron microscopy    41
2.3    Techniques for measuring physical properties    43
2.3.1    Atomic Force microscopy    43
2.3.2    Superconducting Quantum Interference Device Megnetometer    52
2.3.3    Pump-probe measurement    54
Chapter 3. Multiple phases (morphotropic phase boundary) in single constituent multiferroic BiFeO3 thin films    58
3.1    Brief reviews of “morphotropic phase boundary”    58
3.2    Case of BiFeO3 thin films grown on LaAlO3 substrate    62
3.2.1    Dependence of thickness on phase development    62
3.2.2    Dependence of temperature on dominate phase    67
3.2.3    Correlation between structural variation and electrical properties    75
3.3    Case of BiFeO3 thin films grown on YAlO3 substrate    79
3.3.1    Analysis of dominated phase comparing to the case of LaAlO3 substrate    79
3.3.2    Structural characterization on thickness variation and substrate-induced anisotropic array of stripes    83
3.4    Conclusion    88
Chapter 4 Epitaxial Photostriction-Magnetostriction Coupled Self-Assembled Nanostructures    90
4.1    Brief reviews of vertical heteroepitaxial self-assembled nanostructure    90
4.2    Dual target system for growing nanostructure thin film    92
4.3    Structural characterization    94
4.4    Verification of photomagnetic coupling effect    97
4.4.1    Magnetic properties of CoFe2O4 nano-pillars    97
4.4.2    Photostriction effect of conductive SrRuO3 matrix    101
4.4.3    Photo-magnetic coupling    103
4.5    Conclusion    106
Chapter 5 Summary    108
References    112
 

List of Table and Figures
Table 1. Lattice parameters of monoclinic BFO phase grown on YAO and LAO substrates with different thickness………………………………………………… 89
Figure 1.1 (a) Relationship between multiferroic and magnetoelectric materials. Illustrates the requirements to achieve both in a material. (Adapted from Eerenstein, et al.[3]). (b) Different types of coupling present in materials. Much attention has been given to materials where electric and magnetic order is coupled. These materials are known as magnetoelectric materials. (These figure are re-drawn accordint to ref.[1])    2
Figure 1.2 The potential distribution along the z-direction and the corresponding distorted tetragonal structure.    4
Figure 1.3 The typical ferroelectric hysteresis and relative domain switching with electric field.    5
Figure 1.4 Time-reversal and spatial-inversion symmetry in ferroics. (a) Ferromagnets: the local magnetic moment m may be represented classically by a charge that dynamically traces an orbit, as indicated by the arrowheads. A spatial inversion produces no change, but time reversal switches the orbit and thus m. (b) Ferroelectrics: The local dipole moment p may be represented by a positive point charge that lies asymmetrically within a crystallographic unit cell that has no net charge. There is no net time dependence, but spatial inversion reverses p. (c) Multiferroics that are both ferromagnetic and ferroelectric possess neither symmetry.[3]    6
Figure 1.5 Schematically illustration of basic five types of magnetic orders.    7
Figure 1.6 (a)-(d) are four types of antiferromagnetic order which can occur on simple cubic lattices; and (e)-(f) are those can occur on body-center cubic lattices. The two possible spin states are marked + and –.    8
Figure 1.7 The schematics of the relationship between rhombohedral structure and pseudocubic structure of BFO.    10
Figure 1.8 Polarization of BiFeO3 in (a) bulk single crystal[44] and (b) epitaxial thin film[43]. Both has a high polarization, which is close to the theoretical predction.    12
Figure 1.9 Schematic representation of the spin cycloid. The canted antiferromagnetic spins (blue and green arrows) give rise to a net magnetic moment (purple arrows) that is spacially averaged out to zero due to the cycloidal rotation. The spins are contained within the plane defined by the polarization vector (red) and the cycloidal propagation vector (black).[46, 57]    13
Figure 1.10 Magnetoelectric effect in BiFeO3 (a) at low fields, P is proportional to the square of magnetic field (H2), and above 20 Tesla, P is linearly dependent on H instead. The linear ME is forbidden at low magnetic field due to the presence of a cycloid, but the ME will appear at high field (above 20 T) by the destruction of cycloid. (b) As the cycloid is destroyed, the small canted magnetic moment is recovered. Extrapolation of the magnetization to zero field yields a small net magnetization of nearly 0.3 emu/g.[49,57]    14
Figure 1.11 (a) Schematics of the planes of spin rotations and cycloids vector for the two polarization domains separated by a domain wall (in light gray). (b) three types of ferroelectric domain walls. The domain walls are labeled according to the angle between the polarization vectors on either side. Note that in this simplified picture the 71o and 180o walls are not in their most stable configurations, since the head-to-head polarization perpendicular to the wall would lead to large electric fields at the interface. [46,57]    15
Figure 1.12 Shape of (a) ferroelectric polarization and (b) magnetism across a domain wall in BiFeO3. These figures imply that the domain walls have net magnetic moment, even though the domains themselves do not. [54]    16
Figure 1.13 (a) Photoemission electron microscopy (PEEM) images before electric field poling, and (b) after electric field poling. The arrows show the X-ray polarization direction during the measurements. (c) and (d) are the in-plane piezoresponse force microscopy images before and after electric field poling, respectively. The arrows show the direction of the in-plane component of ferroelectric polarization. Green and red circles correspond to 109o ferroelectric switching, whereas black and yellow circles correspond to 71o, and white circles to 180o switching, respectively. In regions of green and red circles, the PEEM contrast reverses after electrical poling. (e) A superposition of in-plane PFM scans shown in (c) and (d) used to identify the different switching mechanisms that appear with different colors and are labeled in the figure. (f) Schematic illustration of coupling between ferroelectricity and antiferromagnetism in BiFeO3. Upon electrically switching BiFeO3 by the appropriate ferroelastic switching events (i.e., 71o and 109o changes in polarization) a corresponding change in the nature of antiferromagnetism is observed.[58]    17
Figure 1.14 Schematic illustration of three common connectivity schemes: (a) 0–3 particulate or nanoparticle composite, (b) 2–2 laminate or multilayer composite, and (c) 1–3 fiber/rod or nano-pillar composite.    18
Figure 1.15 The interplay of external parameters and intrinsic physical properties: big circles represent the external parameters and small circles represent the intrinsic physical properties. Each lines indicates the functionality generated from the interaction between external parameters and intrinsic physical properties.    20
Figure 1.16 two possible models for perovskite and spinel interface.    22
Figure 1.17 The schematics of the relationship between orthorhombic structure and pseudocubic structure of SrRuO3.    23
Figure 1.18 Resistivity curves of SrRuO3 films 1000 Å thick: (a) and (b) are on miscut SrTiO3 substrates and the current is perpendicular to the c axis, (c) is on a regular SrTiO3 substrate, (d) is on a LaAlO3 substrate, and (e) is on a ytrria-stabilized zirconia (YSZ) substrate.[72]    25
Figure 1.19 (a) Change of the superlattice period ΔdSL/d0 as a function of delay time as derived from the angular positions of the Bragg peaks (d0: period of the unexcited SL). (b) Change of tetragonality Δη/η0 of the SRO and PZT layers after 1.5 ps (open symbols) and 200 ps (solid symbols). The solid lines are guides to the eye. Dashed lines: calculated changes of the SRO and PZT tetragonalities due to thermal expansion and compression.    27
Figure 1.20 The scheme of inverse spinel strucuture of CoFe2O4.    29
Figure 1.21 Three kinds of superexchange mechanism and Goodenough–Kanamori–Anderson (GKA) rules, illustrated for the magnetic exchange between the 3d orbital of two transition-metal ions via the 2pz orbital of an O2− ion. Dependent on the occupation with electrons, the resulting interaction can be antiferromagnetic (a) or ferromagnetic (b), (c).    31
Figure 2.1 (a) The molecular beam expitaxy (MBE) laser deposition system used in this study. (b) The schematic deposition process of MBE-PLD system.    35
Figure 2.2 (a) The typical scheme of double-axis diffractometer (left), and the corresponding motion for each angular motor in real space (right). (b) The high resolution triple-axis diffractometer (Huber 8 circle diffractometer) (left), and the scheme of the corresponding motion of each angular motor in real space (right).    40
Figure 2.3 (a) The detailed setup of a common TEM instrument. (b) the concept of the image mode in TEM observation. (c) The concept of the diffraction mode in TEM observation    42
Figure 2.4 (a) The insrumental depiction for AFM operation. (b) The correspondingly operative region of three typical modes of AFM in Lennard-Jones pair potential energy.    44
Figure 2.5 (a) Structural variations depend on the polarization sign in vertical PFM manipulations, which cause the deflection of laser signal in photodetector. (Figure courtesy of S. Jesse, ORNL.) (b) Two types of distortion of cantilever in in-plane PFM manipulations: torsion and buckling. The torsion makes the laser signal a horizontal shift in photodiode, whereas the buckling will make a similar feature to deflection.    48
Figure 2.6 Working Principle of Conductive AFM (C-AFM).    50
Figure 2.7 The operation of MFM usually is obtained in lift mode, which means the measurement needs 2 routes. The route 1 indicates the cantilever moves along surface topography on first trace and retrace. And then Cantilever ascends to lift scan height. The lifted cantilever also profiles topography while responding to magnetic influences on second trace and retrace. (Figure courtesy of Veeco Instruments Inc.)    52
Figure 2.8 Schematic representation of a Josephson device (or a simple SQUID magnetometer) interacted with a magnetic field. The change of magnetic flux will induce a variation in measured voltage. (Redrawn from the website http://hyperphysics.phy-astr.gsu.edu/hbase/solids/squid.html)    53
Figure 2.9 Schematic illustration of the principle for pump-probe technique. (Redrawn according to [126])    55
Figure 2.10 The experimental setup for dual-color pump-probe spectroscopy. Code: AOM: acousto-optic modulator. P: polarizer. PD: photodiode. The solid and dashed lines represent the laser beam path and the dotted lines stand for the electrical signal connection. The blue line after the BBO is the path of pump beam laser and the lower red line after the splitter is the path of probe beam. Nearly normal incident angles of pump beam and probe beam are 0o and 180o, respectively.    57
Figure 3.1 The schematics of each structure and the corresponding diffraction features: MC, with the shear orientation along [100] direction; MA, with the shear orientation along [110]. The shear angle will cause the peak splitting due to the four kinds of domains, and its direction will results in the reverse patterns in the (H0L) and (HHL) scattering zones.    59
Figure 3.2 Reciprocal space maps from BFO films around LAO (001) peak with thickness of (a) 20 nm, (b) 46 nm, (c) 77 nm, (d) 85 nm, (e) 165 nm, (f) 260 nm, (g) 500 nm.    63
Figure 3.3 (a) Height profile and topography (inset) of an 85-nm mixed-phase BFO. Data is taken from the line trace shown in the inset. (b) Reciprocal space map of the 001-diffraction peaks for a 88 nm thick BFO thin flim. (c) Cross-sectional TEM image of a mixed-phase BFO sample. (d) and (e) High-resolution images at the phase boundaries. The corresponding electron diffraction patterns of the boundaries are shown in (f) and (g).    64
Figure 3.4 RSM results of BFO/LAO (001) taken at (a) 25ºC, (b) 200ºC, (c) 400ºC and (d) 600ºC. Markers beside the XRD peaks indicate the presence of LAO, R-phase BFO, MI-phase BFO, MII-phase BFO and MII,tilt-phase BFO. (e) and (f) are the rocking curves of MI and MII phases from the heating to cooling process, respectively.    67
Figure 3.5 AFM topography of BFO thin films with mixed phases at various temperatures. Samples were imaged at (a) RT, (b) 200ºC, (c) 300ºC, (d) 385ºC, (e) 295ºC, (f) 200ºC, (g) 100ºC, (h) RT. Images from (a) to (d) were taken during the heating process, while images from (e) to (h) were taken during the cooling process. All images were taken at the same area of the sample and have the same scan size.    69
Figure 3.6 (a) X-ray normal scan of a BFO thin film with reference to LAO (001) peaks at various temperatures. Dashed line is used as a guide to visualize the shifts of the MII peaks. The arrow marks the R peaks. (b) c-axis lattice parameters of BFO (001) and LAO (001) from RT to 350°C.    70
Figure 3.7 (a) Schematics of expected XRD peaks of MA and MC phases in the (H0L) and (HHL) scattering zones of a RSM. (b) and (c) show experimental RSMs of (103) and (113) of our BFO thin film at RT. (d) and (e) are the RSMs of (103) and (113) of the BFO thin film at 150°C. (f) and (g) are the RSMs of (103) and (113) of the BFO thin film at 200°C. (h) Schematics the lattice structures of the MC (blue) and the MA (red) phases of the BFO thin films at the phase transition temperature.    72
Figure 3.8 Thickness-temperature phase diagram of compressively strained BFO thin films.    75
Figure 3.9 (a) Schematics of the ferroelectric polarizations in MC, which shows four in-plane polarization variants on the {100} planes. Three contrasts (dark, light, and brown) are expected from PFM measurements when the cantilever is aligned to [100]. (b) Schematics of the ferroelectric polarizations in MA. The polarization of MA lies on {110}, also resulting in four in-plane polarization variants. However, when conducting PFM measurements with the cantilever aligned to [100], two contrasts (dark, light) are expected. (c) AFM, (d) PFM phase, and (e) PFM amplitude images of a BFO sample at RT with the cantilever aligned to [100]. (f) AFM, (g) PFM phase and (h) PFM amplitude images of the same scanning area as shown in (c), (d), (e) but at 150oC. The cantilever is also aligned to [100]. The white black and brown arrows in (d), (e), (g), (h) indicate the directions of the in-plane polarization.    77
Figure 3.10 (a) Piezoelectric response loops of the BFO thin films at RT, 150°C and 175°C. (b) Variation of d33 against temperatures.    79
Figure 3.11 (a) X-ray normal scan of a BFO thin film with reference to YAO (001)pc peaks at various temperatures. Dashed line is used as a guide to visualize the shifts of the MII peaks. (b) c-axis lattice parameters of BFO (001) and YAO (001)pc from RT to 400°C. (c) and (d) show experimental RSMs of (103) and (113) of our BFO thin film at RT. (e) and (f) are the RSMs of (103) and (113) of the BFO thin film at 150°C. (g) and (h) are the RSMs of (103) and (113) of the BFO thin film at 275°C. These RSMs unveiled the phase transition of MC-MA-T.    80
Figure 3.12 (a) AFM and PFM phase images of a BFO sample at RT with the cantilever aligned to [100]. (b) AFM and PFM phase images of the same scanning area and direction as shown in (a) but at 150oC. The colored arrows in phase images of (a) and (b) indicate the directions of the in-plane polarization variants illustrated in Figure 3.9(a) and 3.9(b). The combinations of these polarization variants would cause the stripe-like and puddle-like domain features in MC and MA, respectively.    82
Figure 3.13 (a)-(d) X-ray normal reciprocal space maps (RSM) of thin films with various thicknesses. MII, MII,tilt, MI, and R phase are marked near the position of relevant peaks. (e) and (f) are the rocking curves of MI and MII phases with the in-plane vector along (100)pc and (010)pc with different thickness.    83
Figure 3.14 The topography of (a) 18nm, (b) 60nm, (c) 180nm, and (d) 300nm BFO thin films grown on YAO substrate. (e) 120nm BFO grown on LAO substrate. (f) line-trace along the blue line and red line in (d), which shows different periodic arrangement of strips at [100]pc,YAO and [010]pc,YAO. Unlike the uniform arrayed stripes along both LAO [100] and [010] observed in (e), this arrayed stripe anisotropy should results from the lattice difference of a-axis and b-axis of the YAO substrate.    85
Figure 3.15 (a) the ratio of c/a and a/b are calculated from Table. I at different thickness, indicating a stably MII phase can survive at thickness above 300nm. (b) the variation of shear angle of MII phase gradually decreases as thickness increases.    87
Figure 4.1 RHEED patterns recorded during the CFO-SRO deposition. Patterns at the same row were recorded in the same alternation cycle. Cartoons of the corresponding RHEED patterns are used to picture the growth of the CFO-SRO nanostructures.    93
Figure 4.2 (a) XRD 2- scans around the STO(002) peak of the CFO-SRO nanostructures, pure CFO thin film and pure SRO thin films. (b) off-normal RSM scan around the STO(112) peak. The SRO(112) and CFO(224) can be observed. (c) In-plane epitaxy revealed by Φ scans of CFO (400), SRO (200) and STO (200) peaks. (d) Topology (AFM) of the CFO-SRO nanostructures.    95
Figure 4.3 (a) Cross-sectional TEM image of CFO-SRO nanostructures. CFO nanopillars are clearly shown with a darker contrast. Electron diffraction patterns from SRO and CFO are shown in the left and right insets, respectively, on the top of the image. (b) TEM images of the CFO/SRO interface, marked in the red area in (a), with atomic resolution.    96
Figure 4.4 SQUID measurements on the CFO-SRO nanostructures along the OOP and IP directions.    98
Figure 4.5 The XAS spectra of the atoms which may contribute magnetic signal. Black line and red line represent right and left circularly polarized light, respectively (left side). The corresponding XMCD spectra resulting from the polarization-averaged soft XAS show the magnetism only come from Co and Fe atoms and no contribution (right side).    100
Figure 4.6 (a) AFM topology (left) and C-AFM image (right) of the CFO-SRO nanostructures. (b) Results of the ultrafast ΔR/R measurements on CFO-SRO nanostructures and pure SRO thin films. Oscillations have been observed in both types of samples. The dashed line indicates the moment when the acoustic wave of SRO, excited by the laser pulse, hits the SRO/STO interface.    102
Figure 4.7 Kerr angle modulated by pump laser on/off in a CFO/SRO sample with various fluences of (a) 0.5, (b) 1.98, (c) 9.90, and (d) 12.98 mJ/cm2. (e) The pump fluence dependence of Kerr angle changes between the first on and the first off of pump laser. The MOKE measurements for probing the optical-modulation magnetic moment in CFO/SRO samples were carried out by using a HeNe laser (λ=632.8 nm) and simultaneously pumped by a regenerative amplifier (repetition rate of 5 KHz and wavelength of 800 nm).    103
Figure 4.8 (a) AFM topology and (b) MFM image at the same area of a nanostructured CFO-SRO sample, which has been magnetized by applying large out-of-plane magnetic field before being illuminated. (c) MFM image at the same area after being illuminated. We randomly select 100 nano-pillars to conveniently observe the change in magnetic direction. The red circles in (b) are the nanopillars in (a) with downward magnetization, and half these pillars flipped upward in (c) are presented in yellow circles, suggesting a “liberation” in magnetization during illumination. And (d)-(f) are the schematic illustration of the process that the magnetic domain flipping during the illumination by ultrafast Ti:sapphire laser pulses. (d) Applying a large out-of-plane magnetic field (7T) to magnetize all nano-pillars downward. (e) Shining a light on the nanostructured thin film to expand the lattice of SRO and release the vertical compressive strain of CFO. (f) Removing the light, and the magnetization of CFO nano-pillars will become either upward or downward for energetic favorite.    106



                                

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