Thesis (Ph.D.)--University of Nebraska-Lincoln, 2008. / Title from title screen (site viewed Aug. 12, 2008). PDF text: vii, 123 p. : ill. ; 2 Mb. UMI publication number: AAT 3297588. Includes bibliographical references. Also available in microfilm and microfiche formats.

Thesis (Ph. D., Material Sciences and Engineering)--University of Idaho, January 2008. / Major professor: Keith A. Prisbrey. Includes bibliographical references (leaves 85-90). Also available online (PDF file) by subscription or by purchasing the individual file.

Thesis (Ph.D.)--University of Delaware, 2006. / Principal faculty advisor: Edmund R. Nowak, Dept. of Physics & Astronomy. Includes bibliographical references.

Magnetic, magnetocaloric, magnetotransport and magnetoelastic properties of Ni-Mn-X (X = In, and Ga) Heusler alloys and La-Fe-Si based rare earth compounds have been synthesized and investigated by x-ray diffraction, magnetization, strain, and electrical resistivity measurements. The phase transitions, magnetic, magnetocaloric, magnetotransport and magnetoelastic properties strongly depend on the composition of these systems. In Ni50Mn50-xInx with x = 13.5, magnetocaloric and magnetotransport properties associated with the paramagnetic martensitic to paramagnetic austenitic transformation were studied. It was shown that magnetic entropy changes (SM) and magnetoresistance (MR) associated with this transformation are larger and the hysteresis effect is significantly lower when compared to that associated with paramagnetic-ferromagnetic transitions or ferromagnetic-antiferromagnetic/paramagnetic transitions in other systems. The Hall resistivity and the Hall angle shows unusual behavior in the vicinity of the martensitic phase transition for Ni50Mn50-xInx with x = 15.2. The observed Hall resistivity and Hall angle are 50 μ*cm and , respectively. It was observed that the presence of Ge, Al and Si atoms on the In sites strongly affects the crystal structure, and the electric and magnetic behaviors of Ni50Mn35In15. It was found that the partial substitution of In atoms by Si in Ni50Mn35In15 results in an increase in the magnetocaloric effect, exchange bias and shape memory effect. In Ni50Mn35In15-xSix, the peak values of positive SM for magnetic field changes H = 5 T were found to depend on composition and vary from 82 Jkg-1K-1 for x = 1 (at T = 275 K) to 124 Jkg-1K-1 for x = 3 (at T = 239 K). The partial substitution of Ni by Co in Ni50Mn35In15 significantly improves the magnetocaloric effect and MR in the vicinity of martensitic transition. In addition, significantly large inverse SM and MR were observed at the inverse martensitic phase transitions of the Ga-based magnetic shape memory Heusler alloys Ni50-xCoxMn32-yFeyGa18. The phase transition temperatures and magnetic properties were found to be correlated with the degree of tetragonal distortion in these samples. In LaFe11.57Si1.43Bx the crystal cell parameters and Curie temperatures were found to increase linearly with increasing B concentration up to ~ 0.1 % and 9 %, respectively. It was found that the characteristics of the magnetocaloric effect of LaFe11.57Si1.43 can be adjusted by a change in B concentration in the LaFe11.57Si1.43Bx system. A study of the influence of a small substitution of Ni, Cu, Cr, and V for Fe in LaFe11.4Si1.6 revealed that the magnetic, magnetocaloric, and magnetovolume coupling constant is related to an increase in the average Fe-Fe interatomic distances, leading to a change in the d-d exchange interaction.

Kyoto University (京都大学) / 0048 / 新制・課程博士 / 博士(工学) / 甲第14050号 / 工博第2962号 / 新制||工||1439(附属図書館) / 26329 / UT51-2008-F442 / 京都大学大学院工学研究科電子物性工学専攻 / (主査)教授 鈴木 実, 教授 髙岡 義寛, 教授 藤田 静雄 / 学位規則第4条第1項該当

Le manuscrit présente des travaux relatifs à la caractérisation structurale, électrique et magnétique des matériaux manganites poly-cristallin, les manganites qui présentent un intérêt industriel croissant compte tenu de ses nombreux domaines d'application et sa complexité du point de vu fondamentale; ces matériaux sont des systèmes dont les électrons sont fortement corrélés par la présence de plusieurs interactions en compétition. Les principaux objectifs de la thèse étaient de mieux comprendre l'effet de la substitution aléatoire sur le site A de la structure double pérovskite (famille Ruddlesden – Popper A2MnO4 dérivée de pérovskite AMnO3), principalement sur les propriétés magnétique. Cette étude a été effectuée sur toute la gamme de température, étude des phénomènes critique au pont de transition et l'étude de la susceptibilité magnétique a haut température. Enfin, une étude d'autres matériaux présentant des propriétés cristallographiques et physique très intéressantes de type RMX5. Les résultats ainsi obtenu permettent des progrès significatifs dans la compréhension de ces matériaux. / The manuscript presents the work on the structural, electrical and magnetic materials manganites poly-crystalline manganites that have a growing industrial interest due to its many applications and the complexity of fundamental point of view, these materials are systems which electrons are strongly correlated with the presence of several competing interactions. The main objectives of the thesis were to better understand the effect of random substitution on the A site of the perovskite structure twice (family Ruddlesden - Popper A2MnO4 derived from perovskite AMnO3), mainly on the magnetic properties. This study was conducted over the entire temperature range, ie, the study of phenomena Spin Glass at very low temperatures, the study of critical phenomena at the transition point and the study of the magnetic susceptibility at high temperature. Finally, a study of other materials having physical and crystallographic properties very interesting type RMX5.

Spintronics discusses about fundamental physics and material science in mostly nanometer size structures. Spintronics also delivers many promising technologies for now and the future. One of the interesting spintronic structures is called “Magnetic Tunnel junction” (MTJ). A typical MTJ consists of a thin (1-3nm) insulator layer sandwiched between two ferromagnetic layers. In this work, I present MTJ with perpendicular magnetic anisotropy (PMA) using an MgO tunnel barrier. The effect of different heavy metals (HMs) adjacent to the ferromagnets (FMs) on tunneling magnetoresistance (TMR) and PMA of the junctions are discussed. Namely, Ta, Mo, Ta/Mo, W, Ir, and Hf have been utilized in HM/FM/MgO structures, and magneto-transport properties are explored. It is shown that when Ta/Mo is employed, TMR values as high as 208%, and highly thermally stable PMA can be obtained. Some physical explanation based on electronic band structure and thermochemical effects are discussed. In the last part of this work, the newly discovered tunneling anisotropic magnetoresistance (TAMR) effect in antiferromagnets is studied, and clear TAMR is demonstrated for NiFe/IrMn/MgO/Ta structures.

No description available.

Spintronics, also known as spin electronics, or magnetoelectronics, refers to the study of the role that electron and (less frequently) nuclear spins play in solid state physics, and a group of devices that specifically exploit both the intrinsic spin of the electron and its associated magnetic moment, in addition to its fundamental electronic charge. As a principal type of spintronic device, a spin-valve is a device that uses ferromagnetic electrodes to polarize and analyze the electronic spins. The electrical resistance of the device depends sensitively on the relative magnetization of its two ferromagnetic electrodes, a phenomenon referred to as Giant Magnetoresistance (GMR). Having been successfully applied in the field of data storage, GMR also shows potential for future logic devices. Organic semiconductors possess many advantages in electronic device applications. Therefore, using organic semiconductors in spintronics is very interesting and promising, in part, because of their exceptionally long spin-decoherence times. This thesis concerns itself with the scientific study of magnetic field and spin effects in organic spin valves (OSV) and organic light emitting diodes (OLED). Three projects were finished, achieving a better understanding of the transportation of charge and spin carriers inside organic films, and paving the way to enhancing the spin diffusion length and the organic magnetoresistance (OMAR) effect. Firstly, C60 films were used as the spin-transport layer of OSV devices, because of its low hyperfine coupling and high mobility, which prior work suggested to be beneficial. Subsequently we studied the spin injection and transport properties by measuring the devices' magnetoresistance (MR) response at various biasing voltages, V, temperatures, T and different C60 film thickness. But we do not observe a significantly increased spin-diffusion length compared to OSV devices based on other organic semiconductors. We propose conductivity mismatch as a likely cause of the loss of spin-valve signal with increasing C60 layer thickness. There exists some disagreement in the scientific literature regarding whether OSV operate in the so-called tunneling regime or the so-called injection regime. To shed light on this question, we fabricated spin-valve devices made of organic semiconductor thin films of rubrene sandwiched between ferromagnetic cobalt and iron electrodes. Current-voltage (I-V) characteristics in Co/AlOx/rubrene/Fe junctions with a rubrene layer thickness, d, ranging from 5-50 nm, were measured, and we found two different modes of conductivity. The first mode, tunneling, occurs in relatively thin junctions, d < 15 nm, and decays exponentially with increasing rubrene thickness. We determined the tunneling decay length to be 1 nm. The tunneling mode is also characterized by a weak temperature dependence and a nearly parabolic differential conductance. The second mode, injection followed by hopping, occurs in relatively thick devices, d ≥ 15 nm, and can be identified by strongly temperature dependent, highly non-linear I-V traces that are similar to those commonly measured in organic injection devices such as OLEDs. We observed MR in devices with a rubrene thickness of 5 nm and 10 nm. Those devices are clearly in the tunneling regime. For the 15 nm device, for which the tunneling current is just barely measurable we could not observe MR. In the third project, we show that the performance of both OMAR and OSV devices very sensitively depends on whether the metallic layers are deposited by thermal evaporation or electron-beam evaporation. A strongly reduced spin diffusion length and an enhanced OMAR response can be achieved in devices fabricated by electron-beam evaporation. Then we showed that the difference must be attributed to the generation of traps resulting from the exposure of the organic layer to X-ray bremsstrahlung that is generated during the e-beam evaporation process. We also used the thermally stimulated current technique (TSC) to characterize these traps.

This thesis concerns itself with the scientific study of the recently discovered organic magnetoresistance (OMAR) whose underlying mechanism is currently not known with certainty. As an introduction, we briefly review the major findings from prior work done by my colleagues. They found that OMAR can be as large as ~10% magnetoresistance at 10 mT magnetic fields at room temperature. Both OMAR and other kinds of magnetic field effect data in organics can be fitted using the empirical laws B^2/(B^2+B_0^2) or B^2/(|B|+B_0)^2, dependent on material. The fitting parameter B_0 is a measure of the characteristic magnetic field strength of OMAR. We explore the dependence of B_0 on material parameters to clarify the origin of OMAR. Various pi-conjugated semiconductor OMAR devices were studied to explore the possibility that hyperfine interaction causes OMAR. For a quantitative analysis of the experiments, we developed a theoretical fitting formula to relate B_0 to the hyperfine coupling strength. In addition, organic materials with different spin-orbit coupling strengths were also measured. Fluorescence and phosphorescence spectroscopies were used to estimate the spin-orbit coupling strength from the measured spectra. For analyzing our measurements, we developed a fitting formula from the time-dependent Schrodinger equation that takes into account the combined effect of hyperfine and spin-orbit coupling on spin-dynamics. We found that in the case of strong spin-orbit coupling, it dominates the behavior, resulting in magnetic field effect traces that are much wider than those in ordinary organics. However, a small cone remains at zero field with a width equal to the hyperfine coupling strength. We find qualitative agreement between the experimental results and the model. We also investigated the question whether OMAR is related to an excitonic effect, or is primarily a transport effect. We measured the magnetic field effects on current, photocurrent and electroluminescence to address this question. By varying the injection efficiency of the minority carriers, we show that OMAR most likely is not an excitonic effect. Our results provide strong evidence in support of the claim that OMAR is caused by spin-dynamics. However, further study is required to study the mechanism connecting spin-dynamics and conductivity.