Theses and Dissertations at Montana State University (MSU)
Permanent URI for this collectionhttps://scholarworks.montana.edu/handle/1/733
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Item Magnetic phase diagram of the two-dimensional Heisenberg spin one-half canted antiferromagnet ethyl-ammonium tetrabromocuprate(II)(Montana State University - Bozeman, College of Letters & Science, 1988) Hassani, YadollahItem Magnetic properties of the pseudo one-dimensional Heisenberg spin one-half antiferromagnet acetamidinium tetrachlorocuprate(II)(Montana State University - Bozeman, College of Letters & Science, 1987) Landenburger, Lisa AnneItem Application of high-resolution dilatometry to the study of critical phenomena in antiferromagnetic systems(Montana State University - Bozeman, College of Letters & Science, 2010) White, Benjamin Dakotah; Chairperson, Graduate Committee: John J. NeumeierThermal expansion is an under-utilized physical property with enormous potential in its application to the study of classical critical phenomena. The Pippard relation scales the coefficient of volume thermal expansion multiplied by temperature with heat capacity in the vicinity of a continuous phase transition. This justifies the study of critical behavior, characterized by critical-exponent a, with the coefficient of volume thermal expansion instead of heat capacity. We evaluate potential advantages and disadvantages and develop strategies uniquely suited to the analysis of critical behavior exhibited by the coefficient of thermal expansion. The most notable disadvantages arise as a result of numerically differentiating thermal-expansion data to obtain its coefficient. In the course of assessing the detrimental effects of this procedure, we developed a critical expression for thermal expansion, with which we quantitatively evaluate the effect numerical differentiation has on the study of critical phenomena. Antiferromagnets are less susceptible than ferromagnets to long-range dipole contributions, which adjust critical behavior away from theoretical predictions. Therefore, three antiferromagnetic materials were selected to test the suitability of studying critical behavior with thermal expansion. Single crystals of CaMn ₂O ₄ and Bi ₂CuO ₄ were grown and characterized by methods described in careful detail. The coefficient of thermal expansion is studied along the principal crystallographic axes of each material demonstrating that the critical behavior exhibited along each axis is the same as that exhibited by the volume. Both transitions belong to the three-dimensional Ising universality class which settles a long-standing question by suggesting that Bi ₂CuO ₄ exhibits easy-axis anisotropy rather than easy-plane anisotropy. The thermal expansion of each material (as opposed to the coefficient of thermal expansion) is studied with our critical expression in order to increase the critical temperature range close to the Neel temperature. The limitations of studying the critical behavior of a polycrystalline sample are demonstrated when we investigate the antiferromagnetic transition of a-Mn. This element has a surprisingly complicated magnetic structure and our results constrain its transition to be in either the three-dimensional Heisenberg or n = 4 universality class.Item Size dependence of the magnetic properties of cobalt oxide nanoparticles mineralized in protein cages(Montana State University - Bozeman, College of Letters & Science, 2005) Resnick, Damon Aaron; Chairperson, Graduate Committee: Yves U. IdzerdaA major question in the physics of magnetic nanoparticles is how the size affects the magnetic properties in magnetic nanoparticle systems. In particular, the magnetic properties can be affected by finite-size effects or surface effects. It is this author's belief that surface effects and not finite-size effects play the dominate role. This study is a specific example of how to try to answer this question by looking at different sizes of Co 3O 4 nanoparticles. In order to answer this question as well as better understand this system, different antiferromagnetic Co 3O 4 nanoparticles of 4.35 nm and 6.3 nm in diameter were synthesized. These particles were determined to be relatively uniform and monodispersed. In this study, Transmission Electron Microscopy (TEM), electron diffraction (ED) and X-ray Absorption Spectroscopy (XAS) were used to study the physical and electronic structure of the nanoparticles. Alternating Current Magnetic Susceptibility (ACMS) was used to measure the magnetic anisotropy energy density of the different size nanoparticles. It was found that the anisotropy energy density increases with decreasing size, from 1.09 x10 4 J/m 3 for the 6.3 nm particles to 7.53x10 4 J/m 3 for the 4.35 nm particles, consistent with the importance of surface anisotropy with decreasing particle size. Vibrating Sample Magnetometry (VSM) was used to measure the Neel temperature and coercive field of the different particles. It was found that the Neel temperature decreases with decreasing size, from 40 K to 15 K, consistent with a simple surface approximation of the finite-size scaling theory, while the coercive field increased with decreasing particle size consistent with a surface model. The main conclusion of this work is that surface effects and not finite-size effects play a major role in the change of the magnetic properties with size in Co 3O 4 nanoparticles. The evidence also suggests that the increase in the anisotropy energy density is due to the creation of a surface anisotropy component normal to the surface of the particles.