Date of Award
Doctor of Philosophy
Physics and Astronomy
David C. Johnston
An Oxford Instruments Faraday magnetometer, capable of operating in the temperature range between 1.5 K and 1000 K, and magnetic field range between 0 T to 8 T, with a sensitivity of 3x10[superscript]-8 cm[superscript]3G, was assembled and calibrated. The pumping station which supported this magnetometer was designed and built. Software for reducing the raw data and generating calibration files was written. The magnetic properties of several oxides related to the high temperature cuprate superconductors BaCuO[subscript]2+x, La[subscript]2CuO[subscript]4, Sr[subscript]2IrO[subscript]4, Sr[subscript]2RhO[subscript]4, Sr[subscript]2VO[subscript]4, and Sr[subscript]2CuO[subscript]3 were measured using the Faraday magnetometer;AC and DC magnetization, neutron diffraction, and heat capacity measurements on polycrystalline BaCuO[subscript]2+x revealed a combination of magnetic behaviors. The Cu[subscript]6 ring clusters and Cu[subscript]18 sphere clusters in this compound were found to have ferromagnetic ground states with large spins 3 and 9, respectively. The Cu[subscript]6 rings ordered antiferromagnetically below the Neel temperature T[subscript] N = 15 ± 0.5 Kelvin, whereas the Cu[subscript]18 spheres remained paramagnetic down to 2 Kelvin. The ordered moment below T[subscript] N was 0.89(5) Bohr magnetons per Cu in the Cu[subscript]6 rings, demonstrating that quantum fluctuation effects are small in these atomic clusters. The Cu[subscript]18 clusters are predicted to exhibit ferromagnetic intercluster order below about 1 Kelvin. Heat capacity C[subscript] P data for BaCuO[subscript]2+x show a sharp second order transition at the Neel 2 temperature. The value of the discontinuity in the heat capacity at Neel is close to that expected for antiferromagnetic ordering of the ring clusters, according to the mean field theory. An additional maximum in C[subscript] P at about 0.8 K may correspond to the ferromagnetic ordering of the sphere clusters postulated above. Understanding the magnetic behavior of BaCuO[subscript]2+x is relevant to the pairing mechanism in high T[subscript] c cuprates;Magnetic susceptibility measurements for Sr[subscript]2CuO[subscript]3±[delta] were made from 2 K to 800 K, and a strong dependence upon oxygen content ([delta]) was observed. High temperature magnetic susceptibility measurements for the sample with the smallest Curie-Weiss-type term clearly show the increase with temperature expected from the Bonner-Fisher model for a spin-1/2 one dimensional (1-D) Heisenberg antiferromagnet. This is the first direct experimental observation of 1-D magnetic behavior in this system. Estimates of the interchain magnetic magnetic interaction and low temperature neutron and synchrotron x-ray powder diffraction studies of Sr[subscript]2CuO[subscript]3 indicate that this material may be the best (nearly ideal) realization of a 1-D spin 1/2 Heisenberg antiferromagnet reported to date.
Digital Repository @ Iowa State University, http://lib.dr.iastate.edu/
Wang, Zhaorong, "Magnetization studies of oxides related to the high temperature cuprate superconductors " (1994). Retrospective Theses and Dissertations. 10865.