Rozprawy doktorskie na temat „Layer magnetic oxide”

The influence of the physicochemical state of the particle surface on the interparticle magnetic interaction in a closepacked system of singledomain microcrystals of highly anisotropic hexaferrite BaFe12O19 has been studied. The efficiency of the used technique of treatment of the particle surface with acid and alkali solutions has been determined from the data on the Fe3+ ion concentration in the solution and on the change in the elemental composition in the nearsurface layer. It has been shown that, when the etched layer thickness is 2.5c (c is the lattice parameter of ferrite), the parameter of the resulting interparticle interaction changes qualitatively and quantitatively. The technologically accessible technique used allows the attenuation of the interparticle magnetic interaction in a system of closepacked particles by several times. When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/35188

Die vorliegende Arbeit befasst sich mit magnetischen Tunnelkontakten (magnetic tunnel junctions, MTJs) auf Basis des Oxids Zinkferrit (ZnxFe3-xO4). Dabei soll das Potential dieses Materials durch die Demonstration des Tunnelmagnetowiderstandes (tunnel magnetoresistance, TMR) in zinkferritbasierten Tunnelkontakten gezeigt werden. Dazu wurde ein Probendesign für MTJs auf Basis der „pseudo spin valve“-Geometrie entwickelt. Die Basis für dieseStrukturen ist ein Dünnfilmstapel aus MgO (Substrat) / TiN / ZnxFe3-xO4 / MgO / Co. Dieser ist mittels gepulster Laserabscheidung (pulsed laser deposition, PLD) hergestellt. Im Rahmen dieser Arbeit wurden die strukturellen, elektrischen und magnetischen Eigenschaften der Dünnfilme untersucht. Des weiteren wurden die fertig prozessierten MTJ-Bauelemente an einem im Rahmen dieser Arbeit entwickeltem und aufgebautem TMR-Messplatz vermessen. Dabei ist es gelungen einen TMR-Effekt von 0.5% in ZnxFe3-xO4-basierten MTJs nachzuweisen. Das erste Kapitel der Arbeit gibt eine Einführung in die spintronischen Effekte Riesenmagnetowiderstand (giant magnetoresistance, GMR) und Tunnelmagnetowiderstand (TMR). Deren technologische Anwendungen sowie die grundlegenden physikalischen Effekte und Modelle werden diskutiert. Das zweite Kapitel gibt eine Übersicht über die Materialklasse der spinellartigen Ferrite. Der Fokus liegt auf den Materialien Magnetit (Fe3O4) sowie Zinkferrit (ZnxFe3-xO4). Die physikalischen Modelle zur Beschreibung der strukturellen, magnetischen und elektrischen Eigenschaften dieser Materialien werden dargelegt sowie ein Literaturüberblick über experimentelle und theoretische Arbeiten gegeben. Im dritten Kapitel werden die im Rahmen dieser Arbeit verwendeten Probenpräparations- und Charakterisierungsmethoden vorgestellt und technische Details sowie physikalische Grundlagen erläutert. Die Entwicklung eines neuen Probendesigns zum Nachweis des TMR-Effekts in ZnxFe3-xO4-basierten MTJs ist Gegenstand des vierten Kapitels. Die Entwicklung des Probenaufbaus sowie die daraus resultierende Probenprozessierung werden beschrieben. Die beiden letzten Kapitel befassen sich mit der strukturellen, elektrischen und magnetischen Charakterisierung der mittels PLD abgeschiedenen Dünnfilme sowie der Tunnelkontaktstrukturen.

My dissertation research focuses on the investigation of the transport and magnetic properties of transition metal and rare earth doped oxides, particularly SnO2 and HfO2 thin films. Cr- and Fe-doped SnO2 films were deposited on Al2O3 substrates by pulsed-laser deposition. Xray- diffraction patterns (XRD) show that the films have rutile structure and grow epitaxially along the (101) plane. The diffraction peaks of Cr-doped samples exhibit a systematic shift toward higher angles with increasing Cr concentration. This indicates that Cr dissolves in SnO2. On the other hand, there is no obvious shift of the diffraction peaks of the Fe-doped samples. The magnetization curves indicate that the Cr-doped SnO2 films are paramagnetic at 300 and 5 K. The Fe-doped SnO2 samples exhibit ferromagnetic behaviour at 300 and 5 K. Zero-field-cooled and field-cooled curves indicate super paramagnetic behavior above the blocking temperature of 100 K, suggesting that it is possible that there are ferromagnetic particles in the Fe-doped films. It was found that a Sn0.98Cr0.02O2 film became ferromagnetic at room temperature after annealing in H2. We have calculated the activation energy and found it decreasing with the annealing, which is explained by the increased oxygen vacancies/defects due to the H2 treatment of the films. The ferromagnetism may be associated with the presence of oxygen vacancies although AMR was not observed in the samples. Pure HfO2 and Gd-doped HfO2 thin films have been grown on different single crystal substrates by pulsed laser deposition. XRD patterns show that the pure HfO2 thin films are of single monoclinic phase. Gd-doped HfO2 films have the same XRD patterns except that their diffraction peaks have a shift toward lower angles, which indicates that Gd dissolves in HfO2. Transmission electron microscopy images show a columnar growth of the films. Very weak ferromagnetism is observed in pure and Gd-doped HfO2 films on different substrates at 300 and 5 K, which is attributed to either impure target materials or signals from the substrates. The magnetic properties do not change significantly with post deposition annealing of the HfO2 films.

Nous présentons une étude des effets de contrainte induits par l’épitaxie dans des couches minces La1-xSrxMnO3 (LSMO) (001) (x = 0.33) pour 3 épaisseurs de films (50, 25 et 12 nm) déposés par Ablation Laser Pulsée (PLD) sur différents substrats tels que SrTiO3 (STO) (001), STO buffered MgO (001), NdGaO3 (NGO) (110) et (LaAlO3)0.3(Sr2AlTaO6)0.7 (LSAT) (001). L’étude est complétée par l’effet de la composition sur les propriétés magnétiques de couches minces de La1-xSrxMnO3 avec x=0,33 et 0,38 déposées par Epitaxie à Jets Moléculaires (MBE). Des caractérisations par diffraction de rayons X (XRD), et microscopie à force atomique (AFM), des mesures de résistivité électrique en quatre points en fonction de la température, d’aimantation par magnetometrie à SQUID (Superconducting Quantum Interference Device) et d’anisotropie magnétique par magnétométrie magnéto-optique Kerr vectorielle (MOKE) sont présentées. Les évolutions angulaires de l’anisotropie magnétique, de l’aimantation à rémanence, du champ coercitif et du champ de renversement d’aimantation ont ainsi pu être analysées pour des films épitaxiés LSMO de différentes épaisseurs. Des études en fonction de la température complètent les données. L’origine de l’anisotropie (magnétique, magnétocristalline, magnétostrictive ou liée aux effets de marches et d’angle de désorientation du substrat) est finalement discutée
We report a quantitative analysis of thickness dependent epitaxial strain-induced effects in La1-xSrxMnO3 (LSMO) (001) (x = 0.33) thin films of thicknesses (50, 25 and 12 nm) grown on various single crystal substrates such as SrTiO3 (STO) (001), STO buffered MgO (001), NdGaO3 (NGO) (110) and (LaAlO3)0.3(Sr2AlTaO6)0.7 (LSAT) (001) by Pulsed Laser Deposition (PLD) technique. We also report the composition dependent magnetic properties of LSMO thin films with x = 0.33 and 0.38 in particular grown onto LSAT (001) substrate by Molecular Beam Epitaxy (MBE). The study mainly includes measurements such as X-ray Diffraction (XRD), Atomic Force Microscopy (AFM), temperature dependent four-probe resistivity, magnetization properties by Superconducting Quantum Interference Device (SQUID), magnetic anisotropy by Magneto-Optical Kerr Magnetometry (MOKE). Our results highlight the detailed study of angular evolution and thickness dependent magnetic anisotropy, remanence, coercivity and switching field in epitaxial LSMO thin films. Temperature-dependent studies are also performed on few selected films. We will also discuss the cause of magnetic anisotropy in LSMO films i.e., magneto-crystalline and magnetostriction anisotropy and the effects of steps or substrate mis-cut induced anisotropy

The Main Magnetite Layer (MML) from the Northern, Eastern and Western lobes of the Bushveld Complex shows significant differences in textures and in mineral chemistry. The MML in the Eastern and Western lobes is massive, with rare, small and altered pyroxene inclusions. By contrast, the MML in the Northern Lobe is more heterogeneous, and it is made of anastomosed and sometimes imbricated, thin layers of magnetitite, magnetite-rich and silicate-rich rocks, where the inclusions in Ti-magnetite are more numerous and consist of mainly altered subhedral and anhedral plagioclase. The comparison of the maximum values of the oxides shows that the MML in the Northern Lobe has the highest content of V2O3 (1.97 wt%), TiO2 (22.49 wt%) and MgO (2.92 wt%), while the MML in the Eastern Lobe has the highest content of Cr2O3 (2.92 wt%) and Al2O3 (9.80 wt%), but lowest V2O3 (0.52 wt%). The lower TiO2 content and higher V2O3 content in the MML of the Northern and Western Lobes suggest lower oxidising conditions during the crystallization of oxides. The MML in all three studied lobes consists of two layers of magnetitite, suggesting that MML was formed by two separate magma influxes, probably on a diverse and complex type of magma chamber floor. The high TiO2 content in magnetite, together with the negative correlation between TiO2 and V2O3 suggest that the maximum V content should represent a “less evolved” and less oxidized melt. In this respect, higher concentrations V2O3 in magnetite can be expected in magnetitite layers with lower TiO2. It can be inferred that the Ti-magetite in the MML from the Eastern Lobe was formed from a more evolved (TiO2 and FeO enriched) and more oxidized (lower V2O3) melt, compared with the MML from the Northern and Western lobes. These findings can be used to illustrate: a) that high fO2 can be responsible for the relatively low V content in magnetite from Fe-Ti oxide ores and b) the vanadium in magnetite decreases significantly in more evolved cumulates, due to a decreasing fO2 with differentiation. Compositional profiles of Ti- magnetite along the stratigraphic height of the MML in the Eastern Lobe (composed of two layers, separated in the outcrop by a parting plane) depicts a cryptic variation with depth in each of the two layers, where each layer can be divided into four sublayers, labelled upwards as A, B, C (with C1, C2, C3 and C4) and D based on Cr, Mg, Ti, Al and V variation. Small scale reversals of the mentioned elements and the repetition of A, B, C and D sub-layers in each layer suggest that MML formed from two successive influxes of magma (indicated by relatively elevated values of MgO), which evolved by crystallization and cooling in a similar manner, to produce the A to D variation. Based on these observations and theoretical considerations, this study dismisses several models for the genesis of the MML: the immiscibility, the increased oxygen fugacity, the relative increase of H2O content of the melt, pressure variation within the magma chamber, magma mixing, and crustal rock contamination. The model proposed here for MML genesis involves the crystallization of both Ti-magnetite and ilmenite from a Fe-Ti-Ca-Al-rich melt (ferro-diorite) along its line of descent, and gravitational settling of oxides in a dynamic regime. The factor which triggered the crystallization of magnetite is a critical saturation of melt in magnetite (attaining saturation of magnetite and ilmenite in the melt after some silicates crystallized). The difference between the nature of silicate inclusions in magnetite and the nature of the magnetite floor, suggest that the Fe-rich magma was not in equilibrium with the cumulates from the present floor, but rather it was emplaced laterally on long distances, the melt being disrupted from its own cumulates. The absence of correlation between the Cr2O3 in magnetite and co-existing ilmenite can indicate than no in-situ fractional crystallization took place at the moment of magnetite accumulation, but rather that magnetite and ilmenite gravitationally accumulated and the grains mechanically mixed from a flowing magma. The model presented herein proposes a five stage model of MML formation: Stage 1 is represented by the intrusion of a Fe-T-Ca-Al-rich magma which expands laterally within a flat and thin magma chamber. Oxides start to crystallize within a dynamic regime of the magma. Stage 2 is given by the accumulation of oxides at the bottom of the new floor. Some plagioclase starts to crystallize (e.g. subhedral plagioclase in the MML of the Northern Lobe). Stage 3 is a short living static regime, where both plagioclase and magnetite crystallized, without fractionation, forming the thin magnetite-anorthosite layer separating the MML into two layers. Stage 4 is represented by a new influx of Fe-Ti-Ca-Al-rich magma which is emplaced above the magnetite-bearing anorthosite, flushing out the liquid which was in equilibrium with the anorthosite. The oxides started crystallizing in a dynamic regime, as in Stage 1. In stage 5, the accumulation of oxides produced the upper layer of the MML. Our interpretation is that the flow of the magma was more dynamic (probably more turbulent on long distances) in the MML of the Northern Lobe, compared to the MML in the Western and Eastern lobes, producing highly heterogeneous and imbricated thin layers of magnetitite and silicates. The presence of olivine corona around orthopyroxene suggests the incongruent melting of orthopyroxene, which points out towards a local re-heating of existing silicate layers, this being a strong argument for multiple injections in generation of MML. Massive crystallization of oxides produced the sulphur saturation of the magma and caused the precipitation of the igneous sulphides, which nucleated on the existing oxides. Later hydrothermal fluids (and/or late magmatic volatiles?) percolated the MML, producing chloritization of the included silicates, remobilization of igneous sulphides and precipitation of hydrothermal sulphides.

Multiferroic materials exhibit unique properties such as simultaneous existence of two or more of coupled ferroic order parameters (ferromagnetism, ferroelectricity, ferroelasticity or their anti-ferroic counterparts) in a single material. Recent years have seen a huge research interest in multiferroic materials for their potential application as high density non-volatile memory devices. However, the scarcity of these materials in single phase and the weak coupling of their ferroic components have directed the research towards multiferroic heterostructures. These systems operate by coupling the magnetic and electric properties of two materials, generally a ferromagnetic material and a ferroelectric material via strain. In this work, horizontal heterostructures of composite multiferroic materials were grown and characterized using pulsed laser ablation technique. Alternate magnetic and ferroelectric layers of cobalt ferrite and lead zirconium titanate, respectively, were fabricated and the coupling effect was studied by X-ray stress analysis. It was observed that the interfacial stress played an important role in the coupling effect between the phases. Doped zinc oxide (ZnO) heterostructures were also studied where the ferromagnetic phase was a layer of manganese doped ZnO and the ferroelectric phase was a layer of vanadium doped ZnO. For the first time, a clear evidence of possible room temperature magneto-elastic coupling was observed in these heterostructures. This work provides new insight into the stress mediated coupling mechanisms in composite multiferroics.

Le but de ce travail de thèse est de réaliser des vannes de spin " naturelles " constituées d'un matériau unique. Le matériau en question doit contenir une alternance de feuillets magnétiques et non magnétiques et présenter différents états magnétiques. Ce système modèle ne présenterait alors ni d'inter diffusion ni de rugosité aux interfaces magnétiques/non-magnétiques et pourrait constituer un système modèle pour les études des phénomènes de transport dépendant de spin. Le CoV2O6 est un oxyde polymorphe de basse dimensionnalité. Les deux phases (α et γ) présentent chacune plusieurs plateaux d'aimantation induits par un champ magnétique. Le premier objectif est de comprendre la structure cristalline et magnétique de ce composé, ce qui a été réalisé par des mesures d'aimantation, de diffraction des rayons X et de neutrons sur des poudres et monocristaux. Les résultats de la phase α sont appuyés par des calculs ab initio. Le second objectif est de déposer ce matériau en couche mince afin d'analyser ses propriétés de transport. Des films épitaxiés de γ CoV2O6 ont été obtenus sur TiO2(100) et TiO2/Pt(111) par ablation laser. Le dépôt sur l'électrode de Pt montre la présence de six variants entrainant l'observation de plateaux d'aimantation supplémentaires.

This report collects selected outstanding scientific and technological results obtained within the frame of the European project "FLASiC" (Flash LAmp Supported Deposition of 3C-SiC) but also other work performed in adjacent fields. Goal of the project was the production of large-area epitaxial 3C-SiC layers grown on Si, where in an early stage of SiC deposition the SiC/Si interface is rigorously improved by energetic electromagnetic radiation from purpose-built flash lamp equipment developed at Forschungszentrum Rossendorf. Background of this work is the challenging task for areas like microelectronics, biotechnology, or biomedicine to meet the growing demands for high-quality electronic sensors to work at high temperatures and under extreme environmental conditions. First results in continuation of the project work – for example, the deposition of the topical semiconductor material zinc oxide (ZnO) on epitaxial 3C-SiC/Si layers – are reported too.

This report collects selected outstanding scientific and technological results obtained within the frame of the European project "FLASiC" (Flash LAmp Supported Deposition of 3C-SiC) but also other work performed in adjacent fields. Goal of the project was the production of large-area epitaxial 3C-SiC layers grown on Si, where in an early stage of SiC deposition the SiC/Si interface is rigorously improved by energetic electromagnetic radiation from purpose-built flash lamp equipment developed at Forschungszentrum Rossendorf. Background of this work is the challenging task for areas like microelectronics, biotechnology, or biomedicine to meet the growing demands for high-quality electronic sensors to work at high temperatures and under extreme environmental conditions. First results in continuation of the project work – for example, the deposition of the topical semiconductor material zinc oxide (ZnO) on epitaxial 3C-SiC/Si layers – are reported too.

Ce travail de thèse porte sur la détermination de la structure atomique, électronique et magnétique de couches ultraminces ferromagnétique et antiferromagnétique pour une meilleure compréhension du mécanisme de couplage d'échange qui peut avoir lieu à leur interface. Le couplage d'échange, effet de l'interaction entre les deux matériaux, se manifeste par un décalage du cycle d'hystérésis et une augmentation de la coercivité en-dessous de la température de blocage. Nous avons porté notre attention sur les systèmes de CoO/FePt sur Pt(001), CoO/Fe et CoO/Fe3O4 sur Ag(001) et combiné des techniques expérimentales principalement utilisant le rayonnement synchrotron pour les caractériser. Dans un premier temps, nous avons optimisé l'élaboration de ces systèmes dans un environnement d'ultra-haut vide (UHV) par la recherche de surfaces adaptées, le contrôle fin des conditions de croissance et le suivi de la structure par diffraction de surface des rayons X in situ. Leur structure cristalline a ensuite été caractérisée avec précision. Dans un deuxième temps, nous avons étudié leurs structure et propriétés magnétiques ex situ via le dichroïsme magnétique circulaire et linéaire des rayons X et l'effet Kerr magnéto-optique. La relation entre le couplage d'échange et la structure de l'interface est discutée tout au long de ce manuscrit
This thesis deals with the determination of atomic, electronic and magnetic structure of ferromagnetic and antiferromagnetic ultrathin layers to better understand the mechanism of the exchange coupling which could takes place at their interface. Exchange coupling, expression of the interaction between the two materials, manifests itself by a shift of hysteresis loop and an increase in coercivity below the blocking temperature. We have paid attention to the systems of CoO/FePt on Pt(001), CoO/Fe and CoO/Fe3O4 on Ag(001). We combined experimental techniques mainly using synchrotron light to characterize them. As a first step, we optimized in a ultra-high vacuum (UHV) environment the elaboration of the systems looking for an appropriate surface, the high control of growth conditions and the supervision of the structure by in situ X-ray surface diffraction. The crystalline structure was precisely then detailed. As a second step, we studied the magnetic structure and properties ex situ by X-ray magnetic circular and linear dichroïsm and magneto-optic Kerr effect. The relation between exchange coupling and interface structure is discussed all along the manuscript

碩士
國立中興大學
材料科學與工程學系所
102
In this study, we investigated of two part, (i) the Co layer when its surface was bombarded using an End-Hall ion source with an ion beam made with different ratios of Ar ions and oxygen ions (8 - 41% O2/Ar), and (ii) the Fe layer coupled to different capping layers (Si and Si-oxide). First part, thin Co-oxide layers were formed due to intermixing at the interfaces between Co and CoO created via the energetic oxygen ion-beam bombardment. Results have shown that the structure is face-centered cubic Co phase and amorphous Al. In the oxygen ion-beam bombarded Co layers have little rock-salt CoO structures and cubic Co3O4. The magnetic properties of the films, Co layer bombarded with Argons for 10 minutes with a coercivity (Hc) ~ 105 Oe at room temperature. However, when the samples were field cooled (FC, Happ. = 12 kOe) to 180 K, the coercivity (Hc) down to 50 Oe. Further, no measurable exchange bias field (Hex) was present in these thin films. In the ZFC/FC results, the Co(21 %O2/Ar) layer the curves merge at ~ 320 K (irreversibility temperature, Tirr.), close to TN of CoO (~ 300 K). Second part, X-Ray Diffraction and Transmission electron microscopy have shown that the structure is body-centered cubic Fe phase and amorphous Si. The grain sizes ranging from 5-10 nm. The magnetic properties of the films, at room temperature or field cooled to 180 K the coercivity (Hc)6~9 Oe, and exchange bias field (Hex) 1~3 Oe. However, when the samples were field cooled to 10 K, the coercivity (Hc) enhanced 28~60 Oe in Si-oxide (8, 21, and 41%O2/Ar)/Fe bilayers. In the ZFC/FC results, the Si-oxide (8%O2/Ar)/Fe bilayer exhibited the presence of a maximum in the ZFC curve at ~ 70 K, irreversibility temperature, Tirr. ~ 390 K.

The theme of this thesis is Raman spectroscopic study of a variety of exotic states of matter under extreme conditions, such as, hydrostatic pressure as high as 25 GPa and a wide temperature range from 77 K to 390 K. Raman spectroscopy and in some cases, X-ray diffraction studies were performed on systems such as; Topological Crystalline Insulator (TCI) SnTe, normal semiconductor SnSe, natural van der Waals heterostructures from the (SnTe)m(Bi2Te3)n (with m = 1 and n = 1, 2) homologous family, which are also predicted to be Topological Insulators (TI), then Excitonic Insulator (EI) Ta2NiSe5, and its S-counterpart, a normal semiconductor Ta2NiS5. In addition, a Ru2O6-layer honeycomb lattice compound, the silver ruthenium oxide AgRuO3 was also studied. The work presented in this thesis is divided into three parts. (1) Pressure dependent Raman studies were performed to look for signatures of topological phase transitions in SnTe, and a comparative study with a normal semiconductor SnSe. High-pressure Raman studies were also performed on SnBi2Te4 and SnBi4Te7 to look for electronic topological transitions as well as structural phase transitions. For structural characterization as a function of pressure, X-ray diffraction measurements using synchrotron source have been pursued. (2) Pressure and temperature dependent Raman studies were performed to look for signatures of stability of the excitonic insulating phase in Ta2NiSe5 and a comparative temperature dependent study on the S-counterpart, Ta2NiS5, to look for signatures of any phase transition. (3) Lastly, our temperature dependent Raman studies on AgRuO3 reveal a signature of subtle phase transition in addition to an antiferromagnetic transition. For pressure-dependent structural characterization, Raman and X-ray diffraction measurements have been pursued.
Department of Science and Technology (DST) India, Fellowship

碩士
國立臺灣師範大學
物理學系
103
Pulsed-laser deposition (PLD) was applied to grow erbium(Er)-doped ZnO thin films on c-sapphire substrate under two different oxygen pressures : 3×〖10〗^(-3) and 3×〖10〗^(-1) mbar by using shadow mask or silver glue. XPS analysis showed Er/Zn ratio of target was close to thin film which was calculated by using Er ion area. X-ray diffraction spectrum showed decreasing crystal quality of ZnO and increasing c-axis lattice constant with increasing Er doping concentration in two oxygen pressures for both methods. The overall crystal quality of Er-doped ZnO films in high oxygen pressure were better than in low oxygen pressure for shadow mask method. Raman spectroscopy only observed ZnO vibrational modes E_2(high) and E_2(low) in low oxygen pressure for shadow mask method. All films have E_2(low) and ZnO have stronger E_2(high) for two method in high oxygen pressure, however above Er 3at.%, fluorescent effect was observed for all films. PL spectra show near band gap(NBE) emission intensity decreased and defect luminescence emission(DLE) intensity increased with Er concentration increased . PL spectra emission consisted of near band gap(NBE), zinc vacancies and zinc interstitials in low oxygen pressure, however whose consisted of near band gap(NBE), zinc vacancies, zinc interstitials, oxygen vacancies, oxygen interstitials in high oxygen pressure. Magnetic investigations by SQUID showed no magnetism for pure ZnO thin film and paramagnetism for Er-doped ZnO thin films at T=300K and T=5K.

碩士
國立臺灣師範大學
物理學系
101
In this thesis, Gadolinium (Gd)-doped ZnO (Zn1-xGdxO) thin films were grown on c-sapphire by pulsed-laser deposition. The nominal Gd concentration is between 0% and 20%. The XRD patterns show that there is no secondary phase, and c-lattice constant decreases with increasing Gd density. Only one weak ZnO oscillation mode was observed from Raman spectroscopy because the thickness of the thin films is sm-aller than 300nm. Photoluminescence spectroscopy at different temperatures showed a slight decrease in band gap and increase in defect emissions as Gd concentration increases. The major defects are zinc vacancy and interstitial zinc. The room temperature m-H curves show that there is only hysteresis loop for 5.1% Gd, the rest show only paramagnetism.

碩士
國立臺灣師範大學
物理學系
102
Pulsed-laser deposition (PLD) was applied to grow holmium-doped ZnO (Ho:ZnO) thin films on c-sapphire substrate with different holmium(Ho) concentrations. The Ho doping concentration ranges between 3% and 30%. The oxygen pressure of crystal growth is 3×10-2 mbar and the substrate temperature is 750℃.Half of the samples are thermally annealed treatment at 650℃ for an hour. The Raman-scattering spectroscopy reveals that there is no secondary phase except Zn0.95Ho0.05O after annealing. Photoluminescence spectra of most of the samples consist of zine vacancy、zine interstitial and antisite zine defect emissions, and the overall intensity increases monotonically with Ho density. The m-H curves show that all samples are paramagnetic at 5K and 300K.

Over last decades, a great attention has been devoted to iron oxide-based nanoparticles coated by dielectric organic or inorganic materials for the potential contributions in biomedical applications and microwave absorbers industry. The present Ph.D. study focuses on the fabrication of iron-based oxide nanoparticles and coating the particles within the silica matrix in the form of a core-shell structure exhibiting interesting magnetic and electrical properties for application in the biomedical and microwave technology areas. The combined study of electrical and magnetic properties of the silica-iron oxide composites allowed us to examine the requisites for application in a new generation of the high-efficient microwave absorbents and also for the possibility of hyperthermia and drug delivery agents. In this context, four synthesis methods: sol-gel route, laser floating zone (LFZ), auto-combustion, and Pechini method were employed to prepare the iron oxide-based/silica composites. Several studies, namely, structural, morphological, electrical and magnetic characterizations, were performed on the prepared samples. The utilization of several experimental characterizing techniques not only provide us a comprehensive knowledge of the physical properties of these materials but also look over the future employment of the prepared composites in the biomedical application and the microwave absorbing materials. It was prepared a series of glasses with the composition, xFe2O3-(100-x)SiO2 (x=1, 2, 10, 20 in mol%) by the sol-gel route. The effect of the heat treatment conditions and the Fe/Si ratio on the morphology, electrical and magnetic properties of the glass ceramics were investigated. Fibers with the composition of 10Fe2O3-90SiO2 were processed by LFZ technique. Their morphology, structure, magnetic properties and redox state were studied in function of the pulling rate. An exhautive study on the Raman spectroscopy and the local magnetic microstructure of the fibers were performed to support for a deeper understanding of the magnetic properties of the fibers. A series of manganese ferrite-silica nanocomposites with the composition xMnFe2O4-(100-x)SiO2 (x=100, 20, 15 and 10 in mol%) were prepared by the auto combustion method. The physical parameters obtained from this study showed the great potential of the silica-manganese ferrite nanocomposites for the proposed applications as well other suggested future works. In addition, a preliminary study on the synthesis by the Pechini method and characterization of iron oxide was carried out. The structure, the magnetic properties and the electric conduction mechanism of the prepared iron oxide polycrystallites were investigated leaving open other possible applications industries.
Nas últimas décadas grande atenção tem sido dedicada ao estudo de nanopartículas à base de óxido de ferro revestidas por materiais dielétricos orgânicos ou inorgânicos com potencial para aplicações biomédicas e na indústria de absorventes de microondas. O presente trabalho de doutoramento centra-se na fabricação de nanopartículas de óxido de ferro e no seu revestimento com sílica na forma de uma estrutura núcleo-casca com o objectivo de obter propriedades magnéticas e elétricas interessantes para aplicações práticas na área biomédica e na indústria de microondas. O estudo combinado das propriedades elétricas e magnéticas dos compósitos de óxido de ferro-sílica permitiu estudar as características para aplicações em nova geração de absorventes de micro-ondas de alta eficiência e também agentes com possíveis utilizações em hipertermia e libertação de drogas. Neste contexto foram utilizados, quatro métodos de síntese: método de sol-gel, técnica fusão de zona flutuante com laser (LFZ), autocombustão e método de Pechini na preparação dos compósitos à base de óxido de ferro / sílica. Vários estudos como caracterização estrutural, morfológica, elétrica e magnética, foram realizados nas amostras preparadas. A utilização de várias técnicas de caracterização experimental fornece um conhecimento abrangente das propriedades físicas destes materiais e permite perspectivar o possível emprego destes compósitos para aplicações biomédicas e como materiais absorventes de micro-ondas no futuro. Foi preparada a série de vidros xFe2O3- (100-x) SiO2 (x = 1, 2, 10, 20 em mol%) pelo método de rota sol-gel. O efeito das condições de tratamento térmico e da relação Fe / Si na morfologia, propriedades elétricas e magnéticas das cerâmicas vítreas foi investigado. Fibras com a composição 10Fe2O3-90SiO2 foram processadas pela técnica de LFZ. A sua morfologia, estrutura, propriedades magnéticas e estado de redução foram estudadas em função da taxa de extração da preparação das amostras. Foi realizada uma análise exaustiva dos resultados da espectroscopia de Raman e uma pesquisa da microestrutura magnética local para melhor interpretar as características magnéticas das fibras. Uma série de nanocompósitos de silício-ferrite de manganês com a composição xMnFe2O4- (100-x) SiO2 (x = 100, 20, 15 e 10 em % molar) foi preparada pelo método de auto-combustão. Foi realizada investigação das propriedades estruturais, morfológicas, elétricas e magnéticas destas amostras. Os parâmetros físicos obtidos a partir desse estudo mostraram o grande potencial dos nanocompósitos de silício-ferrita de manganês para as aplicações propostas assim como para o trabalho futuro sugerido. Além disso, também foi realizado um estudo preliminar sobre a síntese e caracterização do óxido de ferro pelo método de Pechini.A estrutura, as propriedades magnéticas, o mecanismo de condução elétrica das policristalitas de óxido de ferro preparados foram realizadas deixando em aberto outras possíveis aplicações industriais.
Programa Doutoral em Engenharia Física

The idea of half-metallic ferromagnets was first introduced by de Groot et al. in 1983 based on their calculations. The density of state at the Fermi level for half-metallic ferromagnet is completely polarized, meaning that only one of the spin up or spin down channel exists and has metallic behaviour while the other spin channel behaves as a semiconductor or insulator. This unusual electronic structure can be seen in different materials including Sr2FeMoO6, CrO2 and Mn-based Heusler alloys. The high spin polarization degree of the half-metallic ferromagnets makes them a perfect candidate to be used as a spin-injector/detector in spin-based electronics device (spintronics). However, the degree of spin polarization of these materials, particularly in the multilayered structure spintronic devices, strongly depends on the surface/interface quality and the presence of defects, which was the subject of the present study. Pulsed laser deposition (PLD) has been used to grow two examples of the half-metallic ferromagnets, namely, Sr2FeMoO6 and CrO2. The effects of the growth conditions (deposition temperature, gas pressure, laser power, target-to-substrate distance, post-annealing) and of the substrate lattice mismatch and thickness evolution have been studied. By optimizing the growth conditions, nanocrystalline Sr2FeMoO6 films have been grown on a Si(100) substrate for the first time. This single-phase Sr2FeMoO6 film was obtained at a temperature as low as 600°C, and it exhibits a high saturation magnetic moment of 3.4 μB per formula unit at 77 K. By using glancing-incidence X-ray diffraction with different incident beam angles, the crystal structure of the film was sampled as a function of depth. Despite the lack of good lattice matching with the Si substrate, a preferential orientation of the nanocrystals in the film was observed for the as-grown Sr2FeMoO6 films thicker than 60 nm. Furthermore, effects of the deposition temperature on the epitaxial growth of the Sr2FeMoO6 films on MgO(001) have been studied by means of high-resolution X-ray diffraction. The film grown at 800°C was post-annealed in oxygen, producing epitaxial films of SrMoO4 on top of the Sr2FeMoO6 film. The corresponding magnetization data showed that the post-annealing treatment lowered the saturation magnetic moment from 3.4 µB per formula unit (or /f.u.) for the as-grown Sr2FeMoO6 film to 1.4 µB/f.u. after annealing. X-ray photoemission measurements as a function of sputtering time further revealed the presence of SrMoO4 on both the as-grown and annealed films, and their corresponding depth profiles indicated a thicker SrMoO4 overlayer on the annealed film. The intensity ratios of the 3d features of Mo4+, Mo5+, and Mo6+ for Sr2FeMoO6 remained unchanged with sputtering depth (after 160 s of sputtering), supporting the conclusion that the observed secondary phase (SrMoO4) was formed predominantly on the surface and not in the sub-grain boundaries of the as-grown Sr2FeMoO6 film. The epitaxial growth evolution of Sr2FeMoO6 films of different thickness on substrates of MgO(001), SrTiO3(100) and LaAlO3(100) have also been studied. For each thickness, surface morphology, grain size, film epitaxy, and crystal quality were determined by atomic force microscopy and X-ray diffraction (-2θ scan and reciprocal space mapping). For thicker films (~120 nm), high resolution X-ray diffraction studies revealed that SrMoO4 and other parasitic phases tend to forms on SrTiO3 and LaAlO3 substrates, but not on those grown on MgO substrates. As a second part of the project, single-phase CrO2 nanostructured thin films have been grown for the first time directly on MgO(001) by PLD from a metallic Cr target in an O2 environment. X-ray diffraction shows that these films are strained and consist of CrO2 crystallites with two possible epitaxial relationships to the substrate: either CrO2(110) or CrO2(200) is parallel to MgO(001). X-ray photoemission further confirms that the films are primarily CrO2 covered with a thin CrO3 overlayer, and indicates its complete synthesis without any residual metallic Cr.

We have embarked on a systematic study of novel charge states at oxide interfaces. We have performed pulsed laser deposition (PLD) growth of epitaxial oxide thin films on single crystal oxide substrates. We studied the effects of epitaxial strain and rare-earth composition of the metal oxide thin films. We have successfully created TiO₂ terminated SrTiO₃ (STO) substrates and have grown epitaxial thin films of LaAlO₃ (LAO), LaGaO₃ (LGO), and RAlO₃ on STO using a KrF pulsed excimer laser. Current work emphasizes the importance of understanding the effect of both epitaxial strain and R³+ magnetism on the interface between RAlO₃ and STO. We have demonstrated that the interfaces between LAO/STO and LGO/STO are metallic with carrier concentrations of 1.1 x 10¹⁴ cm[superscript -2] and 4.5 x 10¹⁴ cm[superscript −2], respectively. Rare-earth aluminate films, RAlO₃, with R = Ce, Pr, Nd, Sm, Eu, Gd, and Tb, were also grown on STO. Conducting interfaces were found for R = Pr, Nd and Gd, and the results indicate that for R [does not equal] La the magnetic nature of the R³+ ion causes increased scattering with decreasing temperature that is modeled by the Kondo effect. Epitaxial strain between the polar RAlO₃ films and STO appears to play a crucial role in the transport properties of the metallic interface, where a decrease in the R³+ ion size causes an increase in sheet resistance and an increase in the onset temperatures for increased scattering.
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Zur Präparation von hochwertigen Heterostrukturen aus Übergangsmetallperowskiten wurde eine Anlage zur metallorganischen Aerosol-Deposition mit Wachstumskontrolle durch in-situ Ellipsometrie aufgebaut. Durch numerische Anpassung der in-situ Ellipsometrie kann man den beim Wachstum stattfindenden Transfer der $ e_{g} $-Elektronen zwischen unterschiedlich dotierten Perowskit-Manganaten erfassen. Im Verlauf des Wachstums von Übergittern aus $ \mathrm{LaMnO_{3}} $ (LMO) und $ \mathrm{SrMnO_{3}} $ (SMO) variiert die Längenskala, über die die $ e_{g} $-Elektronen delokalisieren, in einem Bereich von $ 0.4\, \mathrm{nm} $ bis $ 1.4 \, \mathrm{nm} $. Mit der neu eingeführten Atomlagenepitaxie (ALE) kann man die Chemie von Perowskit-Grenzflächen vollständig definieren. Am Beispiel von gestapelten $ \mathrm{SrO-SrTiO_{3}} $ (STO) Ruddlesden-Popper-Strukturen und LMO/SMO-Übergittern wird dieses Verfahren erprobt. Mit der in-situ Ellipsometrie kann man Defekte in der SrO-STO-Abfolge vorhersagen, die in anschließenden strukturellen Untersuchungen mit Transmissionselektronenmikroskopie (TEM) erkennbar sind. Außerdem lässt sich die Sr/Ti-Stöchiometrie mit einer Genauigkeit von $ 1.5\, \mathrm{\%} $ festlegen. In Bezug auf das Wachstum von LMO und SMO kann man über die in-situ Ellipsometrie Veränderungen der Mn-Valenzen erkennen und so auf ein zweidimensionales Wachstum jeder halben Perowskitlage schließen. Die in dieser Arbeit etablierte Depositionstechnologie erlaubt das systematische Studium der magnetischen Eigenschaften von Heterostrukturen aus verschieden dotierten Lanthan-Strontium-Manganaten ($ \mathrm{La_{1-x}Sr_{x}MnO_{3}} $ (LSMO(x)). Zunächst wird das magnetische Verhalten von Doppellagen aus dem ferromagnetischen LSMO(0.3) und verschiedenen antiferromagnetischen LSMO(x)-Lagen ($ x=0.6-1.0 $) untersucht. Für $ x>0.6 $ beobachtet man ein erhöhtes Koerzitivfeld und eine Verschiebung der Hysterese entlang der Feldachse (exchange bias (EB)). Als Funktion der Sr-Dotierung zeigen die durch das STO-Substrat verspannten LSMO(x)-Filme einen Übergang vom Antiferromagneten des G-Typs ($ x>0.95 $) zum Antiferromagneten des A-Typs ($ x \leq 0.95 $). Dieser Übergang wird von einer Halbierung der EB-Amplitude begleitet. Für LSMO(0.3)/SMO-Strukturen registriert man eine Abnahme der EB-Amplitude, wenn die epitaktischen Verzerrungen mit zunehmender Dicke der SMO-Lagen relaxieren. Bei Änderungen der tetragonalen Verzerrung der Manganat-Filme kommt es zu einer Modifikation der Balance zwischen den magnetischen Kopplungen in der Filmebene und senkrecht dazu. Dadurch verändern sich die Eigenheiten des Spin-Glases an der Grenzfläche und damit das magnetische Verhalten der Heterostruktur. LMO/SMO-Übergitter auf STO (001) zeigen eine zuvor noch nicht beobachtete Phänomenologie. Wenn die LMO/SMO-Bilagendicke $ \Lambda $ 8 Monolagen übersteigt, bemerkt man zwei separate ferromagnetische Signaturen. Die Tieftemperaturphase (LTP) hat eine Curie-Temperatur $ T_{C}^{L}=180\, \mathrm{K}-300 \, \mathrm{K} $, die mit $ \Lambda $ kontinuierlich abfällt, die Hochtemperaturphase (HTP) hat eine Curie-Temperatur $ T_{C}^{H}=345 \, \mathrm{K} \pm 10 \, \mathrm{K} $, die keine systematische Abhängigkeit von $ \Lambda $ besitzt. LTP und HTP sind magnetisch entkoppelt. Das Skalierungsverhalten des magnetischen Momentes der HTP mit der Bilagendicke und der Zahl von Wiederholungen der LMO/SMO-Einheit deutet auf einen Grenzflächencharakter der HTP. Außerdem besitzt die HTP eine große magnetische Anisotropieenergie, die Literaturwerte für dünne Manganatfilme um zwei Größenordnungen übersteigt. Beim Wachstum auf $ \mathrm{(La_{0.3}Sr_{0.7}) (Al_{0.65}Ta_{0.35})O_{3}} $ (LSAT), das eine kleinere Gitterkonstante als STO besitzt, verschwindet die HTP. Zur Erklärung der HTP stellt man ein Modell vor, bei dem die HTP an der chemisch scharfen SMO/LMO-Grenzfläche lokalisiert ist. Das STO-Substrat führt zu epitaktischen Zugspannungen, die die $ e_{g} $-Elektronen in den an den SMO/LMO-Grenzflächen befindlichen $ \mathrm{MnO_{2}} $-Lagen auf die $ d_{x^{2}−y^{2}} $-Orbitale zwingen. Dadurch ist der Austausch zu den in c-Richtung benachbarten $ \mathrm{MnO_{2}} $-Lagen sehr schwach und der magnetische Austausch findet vorwiegend in der Ebene statt. Durch den Ladungstransfer liegt in der $ \mathrm{MnO_{2}} $-Lage an der SMO/LMO-Grenzfläche ein $ \mathrm{Mn^{4+}} $-Anteil von $ x \approx 0.4 $ vor. Durch den Doppelaustausch bildet sich in dieser Ebene dann eine zweidimensionale ferromagnetische Phase, die die HTP darstellt.