Browsing by Subject "Magnetism"
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Item Current driven magnetization dynamics in ferromagnets and antiferromagnets(2015-05) Wang, Cheng, doctor of physics; Tsoi, Maxim; Li, Xiaoqin; MacDonald, Allan H; Niu, Qian; Banerjee, Sanjay KThe development of spintronics and potential applications demands a thorough understanding of various novel phenomena in ferromagnets and antiferromagnets. Magnetotransport measurements, which have been implemented in current data storage and magnetoresistive sensing technology, provide convenient and powerful approach to the characterizations of magnetism. We conduct point-contact magnetotransport investigations in metallic magnetic multilayers and antiferromagnetic insulators, aiming at probing the electron transports associated with local magnetic properties in those different materials. For metallic exchange biased spin valves, both radiofrequency (rf) and dc currents are injected through point contacts and we detect the rectified electrical signals. Point contacts with contact sizes of the order of 10-100 nm allow to probe the spins in very local scale. It is found that both linear ferromagnetic resonance and nonlinear parametric resonance can be observed driven by oscillating currents. Particularly, the parametric excitation driven by ac spin transfer torque (STT) is a promising candidate of techniques for realizing fast magnetic switching in spin torque based devices. As for investigating the single crystals of antiferromagnetic Mott insulator Sr₂IrO₄ (SIO), a large anisotropic magnetoresistance (AMR) signal originated from the entanglement of orbital physics and magnetic moments was revealed, shedding lights into the unexplored physics in heavy transition metal oxides in presence of comparable magnitudes of electron correlations and spin-orbit coupling. The crystalline AMR found in SIO may point out a practical path to the sensing of antiferromagnetic order in future AFM-based devices. Furthermore, detailed point-contact study of the electron transport in SIO under high electric biases discovers an electrically tunable transport band gap in this iridate, suggesting a very interesting playground for developing functional devices based on transition metal oxides.Item Development of novel porous coordination polymers with interest in catalysis, structure directing agents, and magnetism(2016-12) Waggoner, Nolan Wayne; Humphrey, Simon M.; Anslyn, Eric V; Rose, Michael J; Milliron, Deila J; Wood, Paul TPorous coordination polymers (PCPs) have emerged as a novel and versatile class of crystalline materials since the late 1990’s due to their high porosity and tunable reactivity. Applications for these materials have spread to areas including gas storage and separations, sensing, magnetism, and more recently, catalysis. However, designing a PCP-based material for a specific application remains a struggle within this field due to their unpredictable self-assembly. In order to overcome this hurdle, linker design has become paramount to the process. The Humphrey Group has developed a new class of PCPs called Phosphine Coordination Materials (PCMs). These materials incorporate one or more phosphorous sites within the linker to act as a point of functionalization. The lone pair of each P(III) site can act as a tunable handle, which allows for access to increased chemical versatility. With this larger goal in mind, the research discussed herein has been focused on the development of novel materials on three fronts: catalytically active linkers, employment of structure directing agents, and examination of PCMs in magnetism. Several catalytically active organic linkers were developed for the purpose of taking known homogenous reactivity and applying that knowledge to a heterogeneous framework. The linker systems described herein include trans-RuCl2(1,2-C6H4-((P-C6H4-p-CO2H)2)2, the ferrocene backbone system Fe(C5H4)2-(P-(C6H4-p-CO2H)2)2, and the extended building block 1,2-C6H4-(P-(¬p-C6H4-p-C6H4-CO2H)2)2. The second study examined a [Me-P-(C6H4-p-CO2H)3]+ Cl- linker to create a series of frameworks through variation of only the alkali hydroxide added during synthesis. This resulted in five frameworks, four of which were non-isostructural. Designated PCMs 6-9, the frameworks demonstrated unusual pore topology as well as the highest cryogenic oxygen uptake to date for a saturated metal site material. Next, two isostructural materials termed Ln-PCM-21 were synthesized and their experimental bulk magnetic properties were studied. Afterwards, three theoretical models were considered in relation to this experimental data and their relatability was reported. The final study explored the magnetic behavior of a set of 1-dimensional coordination polymers that conversely employed thiolate groups; referred to as Thiolate Coordination Materials (TCMs). One material, Fe-TCM-1, was extensively studied and two isostructural materials were attempted (CoII, MnII), with interest in single-chain magnetic behavior.Item Development of variable temperature NMR force microscopy : proton spin relaxation measurements in ammonium sulfate(2012-08) Manzanera Esteve, Isaac Vicente, 1977-; Markert, John T.; de Lozanne, Alejandro; Li, Xiaoqin; Yao, Zhen; Goodenough, JohnNuclear magnetic resonance force microscopy (NMRFM) of a micron size sample of ammonium sulfate was performed by measuring the cantilever deflection produced by coupling the magnetic force to a mechanical cantilever at its resonance frequency. Spin-lattice and spin-spin relaxation measurements were obtained with our newly developed NMRFM probe. A system with more advanced positioning, acquisition and analysis has been fabricated. A new device in which a semi-automatic system performs nanoposition control, spin manipulation, dynamical measurements, and data analysis has been demonstrated to be successful. The new system has proven to be an improvement with respect to other versions of NMRFM probes, thanks to its versatility for pulse sequence designs, faster data acquisition, and automatic analysis of the information. This thesis presents an explanation of the theoretical details of nuclear magnetic resonance force microscopy, and experiments are described in which dynamical measurements of proton spin interactions are obtained. Finally, relaxation time e ffects of the observed force signal are considered in detail. A novel spin manipulation technique which is being implemented for future measurements is described in detail, and magnet con figurations for larger magnetic field gradients and consequently larger signal-to-noise ratio, are also described.Item Electronic structure of dimetal bonded systems: ditungsten, dimolybdenum and diruthenium systems(Texas A&M University, 2007-04-25) Villagran Martinez, DinoThis dissertation investigates three topics in the field of multiple-bonded metal chemistry. The first topic concerns the synthetic and theoretical considerations of ditungsten formamidinates and guanidinates compounds. This work presents an enhanced synthetic path to the W2(hpp)4 molecule (Hhpp = 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine). The reflux of W(CO)6 with Hhpp in o-dichlorobenzene at 200 oC produces W2(hpp)4Cl2 in a one-pot reaction in 92% yield. This compound is stable and easily stored for further use, and it can be efficiently reduced in a one-step reaction to the most easily ionized compound W2(hpp)4. This work also examines the electronic structure and geometry of the intermediates W2(????-CO)2(????- hpp)2(????2-hpp)2 and W2(hpp)4Cl2. The second topic concerns the theoretical investigation by DFT of the electronic structure of [Mo2] units bridged by oxamidate ligands or bridging hydride ions ([Mo2] = (Mo2(DArF)3, where DArF is the anion of a diarylformamidine). It is shown that the effect of the gauche conformation of the ???? oxamidate isomers is due to steric interactions, and that the planar ???? oxamidate isomers have an electronic structure similar to that of naphthalene when it is doubly oxidized. The [Mo2](????-H)2[Mo2] compound shows interdimetal unit interactions between the ???? orbitals of the two [Mo2] units. These interactions are theoretically predicted and experimentally observed by a decrease in the [Mo2]---[Mo2] distance with a one-electron oxidation of [Mo2](????- H)2[Mo2]. The final topic concerns the magnetic and structural properties of two Ru2(DArF)4Cl compounds. The compounds with Ar = p-anisyl (para) and m-anisyl (meta) both show different temperature dependence of their molar magnetic susceptibility, ????. For the para compound, there is a Boltzmann distribution between a ????*3 ground state and a ????*2????* upper state, and this is confirmed by a temperature dependence of the Ru-Ru bond length: 2.4471(5) ???? at 23 K and 2.3968(5) ???? at 300 K. For the meta compound, a ????*2????*configuration persists over the range of 23-300 K as shown by an invariant Ru-Ru bond length and its molar magnetic susceptibility.Item Investigation of electronic transport and magnetic properties in Ca₂-₂xLa₂xRu₂-xCoxO₆(2007-08) Kim, Sangwon, 1978-; Goodenough, John B.Ca2-2xLa2xRu2-xCoxO6 ( 0.0 ≤ x ≤ 1.0 ) is prepared by a "Polymerizable-Complex Method." Electronic transport and magnetic properties are measured to show that the system becomes semiconducting at x ≈ 0.1 and goes through long-range antiferromagnetic ordering at x ≈ 0.4. The proximity in Néel temperature TN of La2RuCoO6 to that of La2TiCoO6 previously reported suggests that the magnetic moment of Ru4+ becomes quenched as temperature decreases. This suppression is explained by a theory that due to (1) the strong spin-orbit coupling ( large spin-orbit coupling constant [Greek small letter lambda] ) present in Ru4+ ions and (2) the sign inversion of [Greek small letter lambda] suggested by Kanamori, the Ruions favors the antiparallel alignment of L and S leading to J = 0. Based on (1) the similarity in the inverse magnetic susceptibility 1/[Greek small letter chi] (T) between CaRuO3 and La2RuZnO6 and (2) the presence of features in CaRuO3 commonly associated with phase fluctuations, the suppression of magnetic moment in Ru4+ is proposed to be responsible for the absence of long-range magnetic ordering in CaRuO3 as well. Finally, the appearance of long-range ferromagnetic ordering in CaRu1-xMxO3 is explained by (1) the degree of hybridization of the Ru4+/Ru5+ redox couple with empty d orbitals and (2) the formation of ferrimagnetic clusters from t2g 3-O- t2g 3 interactions between dopants and localized Ru5+: t2g 3eg 0 configurations.Item Magnetic susceptibilities of titanium and vanadium in corundum structures(Texas Tech University, 1970-05) Arnold, Donald JeneNot availableItem Multi-trigger nechanism with shape memory polymer nanocomposite(2012-05) Carrell, John; Zhang, Hong-Chao; Wang, Shiren; Dai, Lenore L.; Rivero, Iris V.; Tate, DerrickShape memory polymers (SMPs) and their composites are set of smart materials that exhibit a special ability to recover a trained shape deformation upon activation of a single environmental trigger. This ability has made SMPs an emerging technology and has provided novel solutions for applications such as actively expandable vascular stents, deployable space structures, and releasable fasteners. However, the single trigger can be problematic in applications where an ambient field can accidentally trigger the SMP or where direct mechanical access is not available for training of SMP. This research has thus investigated a novel multi-trigger SMP nanocomposite. This SMP nanocomposite is sensitive to thermal and magnetic fields and requires both fields to be applied for shape deformation. SMP nanocomposites were manufactured using a commercially available SMP and magnetite nanoparticles at varying weights (5, 10, 15, 20, and 25 wt.% magnetite). Basic thermomechanical testing of the SMP nanocomposites at ambient conditions and transition conditions along with specially created thermomechanical-magnetic tests have been performed and have shown the multiple sensitivities of the SMP nanocomposites. Further, the varying addition of the magnetite nanoparticles and/or the applied magnetic field to the SMP nanocomposite shows results with higher magnetic sensitivity as well as larger shape deformations. Based on this special behavior, a constitutive model has been presented for the SMP nanocomposites. This constitutive model considers four specific phases of the SMP nanocomposites that are defined by the initial/final configurations of the SMP nanocomposite following and during the application of transition state environmental conditions. This model has been used within LS Dyna FEA to simulate the multi-trigger behavior of the SMP nanocomposite. The simulations have matched closely the actual test case scenario and have validated the developed constitutive model. Further, simulations have been performed to test the SMP nanocomposite in the application of active disassembly. From these simulations, a definite case can be made for the SMP nanocomposite over its SMP counterpart per reduced processing time. This research has exhibited novelty through the documentation of the multiple field sensitivities of the SMP nanocomposite smart material. Smart material research, especially with SMPs, has focused on shape memory actuation on application of a single trigger (i.e. heat, light, chemical, electrical, magnetic, etc.). This research looks to expand on this research by analyzing and understanding the special behavior of a developed multi-trigger SMP nanocomposite. This research documents the multiple field properties and actuation strategies for SMP nanocomposites. The development and study of these materials can create a transformative new dimension for smart material research that will provide additional avenues for manufacturers in smart material applications. Furthermore, this research can create a number of opportunities that extend beyond the technical contributions produced. The key will be in the use of the derived constitutive model as a design tool, which could be used in a number of fields.Item Spontaneous vortex phase and pinning in ferromagnetic-superconducting systems(Texas A&M University, 2004-09-30) Kayali, Mohammad AminHeterogeneous ferromagnetic-superconducting systems such as a regular array of ferromagnetic nano dots deposited on the top of a superconducting thin film have attracted many research teams both experimental and theoretical. The interest in these systems does not only stem from being good candidates for technological applications, but also because they represent a new class of physical systems where two competing order parameters can coexist. This work focuses on the theoretica laspects of these systems by studying the static and dynamics of few model systems. In the first part, the static properties of a superconducting thin film interacting with a ferromagnetic texture are considered within the London approximation. In particular, the ferromagnetic textures considered here are a circular dot of submicrometer size with in-plane magnetization, an elliptical dot magnetized in the direction perpendicular to the superconductor, and a ferromagnetic dot magnetized in the direction normal to the superconducting film and containing non magnetic cavities. I also consider the interaction of vortices in the superconductor with a ferromagnetic columnar defect which penetrates the supercondcting film. In each case the vector potential and magnetic field of the ferromagnet in the presence of the superconductor are calculated. Afterward the presence of vortices in the superconductor is assumed and the energy of vortex-texture system is found. The pinning potential and force supplied by the texture are then derived from the energy of interaction between the ferromagnet and superconductor. I show that if the magnetization of the ferromagnet exceeds a critical value then vortices are spontaneously created in the ground state of the system. Such spontaneous creation of vortices is possible mostly in a close vicinity of the superconducting transition temperature Ts. For every case, the threshold value of the magnetization at which vortices start to be spontaneously created in the SC is calculated as a function of the parameters of the texture geometry. The phase diagrams for transitions from vortexless regime to regimes with one or more vortices are determined for all cases. In the second problem, the transport properties of a ferromagnetic superconducting bilayer with alternating magnetization and vortex density are studied within a phenomenological model. I show that pinning forces do not appear for continuous distribution of vortices, so a discrete model for the bilayer system is constructed. Afterward, I calculate the pinning forces acting on vortices and antivortices resulting from highly inhomogeneous distribution of flux lines and prove that this system has strong transport anisotropy. In the absence of random pinning, the system displays a finite resistance for the current in the direction perpendicular to the domains while its resistance vanishes for the parallel current. The transport anisotropy strongly depends on temperature. I study this dependence and show that the ratio of parallel to perpendicular critical current is largest close to the superconducting transition temperature Ts and the vortex disappearance temperature Tv while it has a minimum in between them.Item Synthesis, structure and magnetic properties of lanthanide cluster compounds(2009-05-15) Sweet, Lucas EdwardThis dissertation focuses on the exploratory synthesis of compounds that contain R6ZI12 (R= Ce, Gd, Er; Z=Mn, Fe, Co, C2) clusters with the goal of finding magnetically interesting compounds. Several new compounds were made via high temperature, solid state methods and structurally characterized using x-ray diffraction. Compounds that contain isolated clusters were studied in order to understand the magnetic coupling between lanthanide atoms. The exploration of transition metal centered clusters resulted in the discovery of two new structure types, CsR(R6CoI12)2 (R=Gd and Er) and (CeI)0.26(Ce6MnI9)2. The xray crystal structure of CsEr(Er6CoI12)2 was solved in the Pa3 ? space group with the cell length 18.063(2) ? at 250K (Z = 4, R1 [I>2?(I)] = 0.0459). (CeI)0.26(Ce6MnI9)2 was made by combining KI, CeI3, MnI2 and Ce metal and heating to 850?C for 500 hrs. The single crystal x-ray structure for (CeI)0.26(Ce6MnI9)2 was solved in the trigonal, P3 ? space group with lattice parameters of a = 11.695(1) ? c = 10.8591(2) ? (Z = 2, R1 [I>2?(I)] = 0.0895). The magnetic susceptibilities of hexanuclear gadolinium clusters in the compounds Gd(Gd6ZI12) (Z = Co, Fe or Mn), CaxGd1-x(Gd6MnI12) and CsGd(Gd6CoI12)2 are reported. The single-crystal structure of Gd(Gd6CoI12) and CaxGd1-x(Gd6MnI12) are reported here as well. The compound with a closed shell of cluster bonding electrons, Gd(Gd6CoI12), exhibits the effects of antiferromagnetic coupling over the entire range of temperatures measured (4 - 300 K). Clusters with unpaired, delocalized cluster bonding electrons (CBEs) exhibit enhanced susceptibilities consistent with strong ferromagnetic coupling, except at lower temperatures (less than 30 K) where intercluster antiferromagnetic coupling suppresses the susceptibilities. Four new compounds containing Gd6C2 clusters have been found: Gd6C2I11, Gd(Gd6C2I12), CsGd(Gd6C2I12)2 and Cs(Gd6C2I12). Gd6C2I11 and Cs(Gd6C2I12) crystallized in the P?1 space group while Gd(Gd6C2I12) and CsGd(Gd6C2I12)2 crystallized in the R?3 and Pa?3 space groups respectively. The magnetic susceptibility data for Cs(Gd6C2I12) indicate strong intracluster ferromagnetic coupling, but antiferromagnetic coupling suppresses the susceptibility below 150 K. DFT calculations on CsGd6C2I12 and molecular models indicate that the magnetic coupling between the basal Gd atoms is stronger than the magnetic coupling involving the axial Gd atoms in the distorted clusters.