Magnetic behavior of bulk and nanostructured MnxTaS2

At its base, material science research aims to categorize specific materials by their various attributes, such as structure, integrity, electronic properties, magnetic properties, and others. By categorizing materials in this way, it becomes easier to generalize the application of a specific material to those within a broader category. The interest in materials that exhibit useful characteristics at small scales derives directly from the technology industry’s need for smaller and smaller devices. Two-dimensional materials are of great interest for this reason. Two-dimensional materials are comprised of many single layers, or planes, stacked together to create a crystal. Each layer may be composed of single or multiple elements. The layers interact weakly with one another; consequently, the properties of the material may be largely determined by the characteristics of the layers. The electronic properties of these materials were researched in detail within the last decades. The result of this research was the categorization of specific two-dimensional materials as insulators, semimetals, superconductors, metals, and semiconductors (Ajayn, Kim, & Banerjee, 2016). Two-dimensional materials that are chemically similar to any of the specific materials exhibiting these properties quickly become candidates for similar behavior. The research that produced the results detailed within this work was completed with the above results in mind. The material described in this work is composed of layers of tantalum sulfide between which manganese was deposited. The number of manganese atoms per one tantalum is called the concentration, x. In contrast to the research that led to the categorizations described above, the magnetic properties of this material were explored. Specifically, this project aimed to characterize the magnetic phase transitions of bulk and nanostructured samples of manganese intercalated tantalum disulfide (MnxTaS2) using several well documented analysis methods such as those used by Anthony Arrott and John Noakes (Arrott & Noakes, 1967). Determining and comparing these magnetic characteristics will provide both novel results and a basis for subsequent projects

Testing Models of Sheaths and Instabilities with Particle-in-cell Simulations

Sheaths and presheaths represent the response of a plasma to boundaries and are an instance of plasma self-organization. They are commonly utilized in plasma technologies and reduced models of plasmas across a range of gas pressures. This thesis leverages the particle-in-cell method to explain discrepancies between models and measurements of ion temperature at low pressures, test untested models of high pressure sheaths, and explore a novel electron plasma wave instability driven by an ambipolar electric field. Simulations reveal that ion-acoustic instabilities excited in presheaths can cause significant ion heating. Ion-acoustic instabilities are excited by the ion flow toward a sheath when the neutral pressure is small enough and the electron temperature is large enough. A series of 1D simulations were conducted in which electrons and ions were uniformly sourced with an ion temperature of 0.026 eV and different electron temperatures (0.1 - 50 eV). Ion heating was observed when the electron-to-ion temperature ratio exceeded the minimum value predicted by linear response theory to excite ion-acoustic instabilities at the sheath edge (T_e/T_i ~ 28). When this threshold was exceeded, the temperature equilibriation rate between ions and electrons increased near the sheath so that the local temperature ratio did not exceed the threshold for instability. This resulted in significant ion heating near the sheath edge, which also extended back into the bulk plasma because of wave reflection from the sheath. The instability heating was found to decrease for higher pressures, where ion-neutral collisions damp the waves and ion heating is instead dominated by inelastic collisions in the presheath. Simulations using the direct simulation Monte Carlo method were used to study how neutral pressure influences plasma properties at the sheath edge. The high rate of ion-neutral collisions at pressures above several mTorr were found to cause a decrease in the ion velocity at the sheath edge (collisional Bohm criterion), a decrease in the edge-to-center density ratio, and an increase in the sheath width and sheath potential drop. A comparison with existing analytic models generally indicates favorable agreement, but with some distinctions. One is that models for the edge-to-center density ratio need to be made consistent with the collisional Bohm criterion. With this and similar corrections, a comprehensive fluid-based model of the plasma boundary was constructed that compares well with the simulations. Ambipolar electric fields are commonplace in plasmas and affect transport by driving currents and in some cases instabilities. Simulations demonstrate that an instability, named the electron-field instability, can be driven by an ambipolar strength electric field. The instability excites waves of 30 Debye-lengths and has a growth-rate that is proportional to the electric field strength. Unlike other instabilities, the electron-field instability only requires that the electrons interact with the field and does not result from the relative drift between electron populations (beam instability) or electrons and ions (ion-acoustic instability). In fact, the instability occurs near the electron plasma frequency which is much higher than most drift instabilities. Low-temperature and space-based plasmas are found to be likely systems where the instability may be excited. We find that our simulations and linear theory agree until a non-linear state is reached in the simulations. These results demonstrate that low pressure sheaths are susceptible to instabilities that can significantly affect plasmas properties, while fluid model accurately capture collisional effects at higher pressures.PhDApplied PhysicsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/176525/1/lbeving_1.pd