Les couches minces d'oxyde présentent un large éventail de phénomènes physiques et riches diagrammes de phase, ajustables par l'ingénierie des déformations et des interfaces. Le CaMnO3, en particulier, est extrêmement sensible au dopage et aux contraintes d’épitaxie et, lorsqu'il est élaboré sous compression, il passe d'un état isolant et antiferromagnétique à un état métallique et faiblement ferromagnétique à seulement 2% de dopage au Ce.Nous avons utilisé une combinaison de spectroscopie de photoémission résolue en angle, de magnétotransport et de théorie de la fonctionnelle de la densité pour étudier les propriétés électroniques de ce matériau. Nous avons observé l'existence de deux porteurs de charge distincts, les électrons légers et les polarons lourds, dont la nature diffère en raison de leur couplage radicalement différent aux phonons. Nous attribuons ces différences à un remplissage relatif différent de la bande en raison des corrélations, qui améliorent considérablement le couplage aux phonons de la bande des polarons lourds. Les expériences de magnétotransport révèlent que la bande polaire domine le transport malgré sa mobilité réduite.La compression épitaxiale donne également lieu à une forte anisotropie magnétique qui stabilise des bulles magnétiques et s’accompagnent d’un effet Hall topologique. Cela suggère que ces bulles ont un caractère topologique. L'effet Hall topologique diverge lorsque la manganite s'approche de la transition métal-isolant à faible dopage. Nous avons utilisé une théorie récemment développée pour interpréter ce comportement, et nous concluons que des corrélations peuvent entrer en jeu, augmentant la masse effective des porteurs et par conséquent l'effet Hall topologique.Comme cette manganite est très sensible aux changements de densités de porteurs, nous avons développé des transistors à effet de champ ferroélectriques BiFeO3/(Ca,Ce)MnO3. Lors de la commutation de la polarisation ferroélectrique de la couche supérieure de BiFeO3, nous n'avons pu observer de changement notable dans les propriétés des couches de manganite sous-jacentes. Nous avons utilisé la microscopie électronique en transmission pour étudier les propriétés de ces bicouches avec une résolution atomique, et nous avons observé que l'épinglage de polarisation à l’interface BiFeO3/(Ca,Ce)MnO3 empêche une commutation complète de la polarisation et réduit ainsi la capacité opérationnelle de ces dispositifs. Néanmoins, nous avons pu détecter l’influence de la polarisation ferroélectrique sur les propriétés électroniques de la manganite à l’échelle atomique. / Oxide thin films feature a wide range of physical phenomena and rich phase diagrams tunable by strain and interface engineering. CaMnO3, in particular, is extremely sensitive to both doping and strain and, when grown with compressive strain, transitions from an insulating and antiferromagnetic state to a metallic and weakly ferromagnetic state at only 2% Ce doping.We used a combination of angle-resolved photoemission spectroscopy, magnetotransport, and density functional theory to study the electronic properties of this material. We observed the existence of two separate charge carriers, light electrons and heavy polarons, whose physical nature differs because of drastically different couplings to phonons. We ascribe these differences to a different relative band filling due to correlations, which enhance greatly the coupling to phonons of the heavy polarons band. Magnetotransport experiments reveal that the polaron band dominates transport despite its lower mobility.Compressive strain also gives rise to a strong magnetic anisotropy which stabilizes magnetic bubbles that accompany a topological Hall effect. This suggests that these bubbles have topological character, i.e. are skyrmion bubbles. The topological Hall effect diverges as the manganite approaches the metal-insulator transition at low dopings. We used a recently developed theory in order to interpret this behavior, and we conclude that correlations may come into play, enhancing the effective mass of the carriers, and in turn the topological Hall effect.As this manganite is highly sensitive to changes in doping and carrier density, we grew BiFeO3/(Ca,Ce)MnO3 ferroelectric field-effect transistors. Upon switching the ferroelectric polarization of the BiFeO3 top layer, we could not observe any sizable changes in the properties of the underlying manganite layers. We used transmission electron microscopy to study the properties of these bilayers with an atomic resolution, and we observed that polarization pinning at the BiFeO3/(Ca,Ce)MnO3 impedes a complete switch of the polarization and so reduces the operational capabilities of these devices. Nevertheless, we could detect modifications of the electronic properties of the manganite induced by polarization reversal at the atomic scale.

A new tetragonal Heusler compound Mn1.4PtSn is synthesized. Crystal growth techniques that require growth directly from melt, such as Bridgman method, always result in microtwinned crystals. To get microtwin free crystals, another technique, flux method is employed, where growth can be done far below the melting point and martensitic transition temperature. The flux method results in successful large microtwin free crystals of Mn1.4PtSn. The single-crystal diffraction is done on a small piece of single crystals of Mn1.4PtSn. From structural analysis, it is found out that the crystal structure of Mn1.4PtSn is the first tetragonal superstructure in the family of Heusler compounds. The superstructure reflections are clearly observed in the powder X-ray diffraction patterns. Direction-dependent magnetic properties are measured. The compound is found to undergo two magnetic transitions. First, at 392 K, which corresponds to Curie temperature and second, at 170 K, which corresponds to the spin-reorientation transition temperature. The saturation magnetic moment at 2 K is very large of 4.7 µB/f.u. The refinement of powder neutron diffraction reveals that in the temperature range of 170 to 392 K, the magnetic structure is collinear ferromagnet whereas below 170 K, it is a non-coplanar spin structure. The magnetic moment, obtained from refinement, is close to the saturation moment obtained from magnetization. The electric transport properties are studied along the different crystallographic directions of the compound. The longitudinal resistivity measurement indicates that the compound is metallic and reveals the magnetic transitions at the same temperature as seen in the magnetization. An overall negative magnetoresistance of 3 percent is found. The Hall resistivity measurements reveal the presence of a large topological Hall resistivity (THE) of 0.9 µΩ cm and -0.1 µΩ cm for the magnetic field applied along [100] and [001], respectively. Two types of contributions in the THE for the field along [100] are seen. One that follows the quadratic form of longitudinal resistivity and second, that is independent of longitudinal resistivity. Anomalous Hall conductivity is found to be 250 and 165 Ω-1cm-1 for the field along [100] and [001], respectively. This value is close to the value obtained from theoretical calculations. The topological Hall conductivity is found to be approximately the same as its anomalous analog. A new series of polycrystalline samples with iridium substitution at the place of platinum in Mn1.4PtSn are prepared. The structural characterization show the crystal structure of these compounds is the same as Mn1.4PtSn, therefore, they also possess the tetragonal superstructure form. Magnetic properties, along with powder neutron diffraction data, reveal that the magnetic structure changes from out-of-plane ferromagnet to in-plane ferrimagnet with Ir-substitution. All the compounds are found to have metallic character. A large anomalous Hall conductivity of 405 Ω-1cm-1 is found for compound Mn1.4Pt0.7Ir0.3Sn. Three new series of compounds are prepared as an attempt to fill the vacancies present in the crystal structure of Mn1.4PtSn with transition-metal elements cobalt, nickel, and copper. The tetragonal superstructure survives up to 0.2 cobalt addition, 0.4 nickel addition and 0.6 copper addition. Further addition of elements leads to transformation to the inverse cubic Heusler structure. The magnetic properties show that the compounds with tetragonal structure have spin-reorientation transition, which is absent in the compounds with cubic structure. A new compound Mn1.7Pt0.8In is discovered. The single crystals are prepared by flux-method. Upon structural analysis from single-crystal refinement, it is found that the crystal structure is 3 × 3 × 3 superstructure of a Heusler structure and is so far the largest discovered in the Heusler family of compounds. Two magnetic transitions are revealed in the magnetization measurements. First, at 330 K, which corresponds to Curie temperature and second, at 220 K, which corresponds to spin-reorientation transition. The magnetic moment is 0.4 µB/Mn at 2 K and 0.07 µB/Mn at 300 K. Such a low moment might be due to possible compensated ferrimagnetic structure. Therefore, the compound is a potential candidate for spintronics devices.

Complex-oxides have seen an enormous amount of attention in the realm of Condensed Matter Physics and Materials Science/Engineering over the last several decades. Their ability to host a wide variety of novel physical properties has even caused them to be exploited commercially as dielectric, metallic and magnetic materials. Indeed, since the discovery of high temperature superconductivity in the “Cuprates” in the late 1980’s there has been an explosion of activity involving complex-oxides. Further, as the experimental techniques and equipment for fabricating thin films and heterostructures of these materials has improved over the last several decades, the search for new and more exotic properties has intensified. These properties stem from the interfaces formed by depositing these materials onto one another. Whether it be interfacial strain induced by the mismatch between the crystal structures, modified exchange interactions, or some combination of these and other interactions, thin films and heterostuctures provide an invaluable tool the modern condensed matter community. Simply put, a “complex-oxide” is any compound that contains Oxygen and at least two other elements; or one atom in two different oxidation states. Transition Metal Oxides (TMO’s) are a subset of complex-oxides which are of particular interest because of their strong competition between their charge, spin and orbit degrees of freedom. As we progress down the periodic table from 3d to 4d to 5d transition metals, the crystal field, electron correlation and spin-orbit energies become more and more comparable. Therefore, TMO thin films and heterostructures are indispensable to the search for novel physical properties. KTaO3 (KTO) is a polar 5d TMO which has been investigated for its high-k dielectric properties. It is a band insulator with a cubic perovskite crystal structure which is isomorphic to SrTiO3 (STO). This is important because non-polar STO is famous for forming a highly mobile, 2-Dimensional Electron Gas (2DEG) at the hetero-interface with polar LaAlO3 (LAO) as a result of the so-called “polar catastrophe”. Here, I use this concept of polarity to ask an important question: “What happens at hetero-interfaces where two different polar complex oxides meet?” From this question we propose that a hetero-interface between two polar complex-oxides with opposite polarity (I-V/III-III) should be impossible because of the strong Coulomb repulsion between the adjacent layers. However, we find that despite this proposed conflict we are able to synthesize KTO thin films on (110) oriented GdScO3 (GSO) substrates and the conflict is avoided through atomic reconfiguration at the hetero-interface. SrRuO3 (SRO) is a 4d TMO, and an itinerant ferromagnet that is used extensively as an electrode material in capacitor and transistor geometries and proof-of-concept devices. However, in the thin film limit the ferromagnetic transition temperature, TC, and conductivity drop significantly and even become insulating and lose their ferromagnetic properties. Therefore, we ask “Are the transport properties of SRO thin films inherently inferior to single crystals, or is there a way to maintain and/or enhance the metallic properties in the thin film limit?” We have fabricated SRO thin films of various thickness on GSO substrates (tensile strain) and find that all of our samples have enhanced metallic properties and even match those of single crystals. Finally, we ask “Can these enhanced metallic properties in SRO thin films allow us to observe evidence of a topological phase without the complexity of off-stoichiometry and/or additional hetero-structural layers?” Recent reports of oxygen deficient EuO films as well as hetero-structures and superlattices of SRO mixed with SrIrO3 or La0.7Sr0.3MnO3 have suggested that a magnetic skyrmion phase may exist in these systems. By measuring the Hall resistivity, we are able to observer a topological Hall effect which is likely a result of a magnetic skyrmion. We find that of the THE exists in a narrow temperature range and the proposed magnetic skyrmions range in size from 20-120 nm. Therefore, the SRO/GSO system can provide a more viable means for investigating magnetic skyrmions and their fundamental interactions.

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