NEUTRON SCATTERING STUDIES OF THE FRUSTRATED ANTIFERROMAGNETIC PYROCHLORE SYSTEM Tb2Sn2-xTixO7

Abstract

<p>The following dissertation shows the results of a series of inelastic neutron scattering experiments on the geometrically frustrated pyrochlore system Tb2Sn2-xTixO7 for x=0, 0.1, 0.2, 0.5, 1, 1.5 and 2. Inelastic neutron scattering measurements were performed on the SEQUOIA direct geometry time-of-flight spectrometer at the Spallation Neutron Source of Oak Ridge National Laboratory. For the two end members, x=0, Tb2Sn2O7 and x=2, Tb2Ti2O7 , they display related, but different exotic ground states, with Tb2Sn2O7 displaying “soft” spin ice order below T_N~0.87K, while Tb2Ti2O7 enters a glassy antiferromagnetic spin ice state below T_g~0.2K.</p> <p>The first two chapters give a brief introduction to the physics of geometrically frustrated magnetism and neutron scattering. Chapter 3 studies the two end members Tb2Ti2O7 and Tb2Sn2O7 experimentally and theoretically. Inelastic neutron scattering measurements and appropriate crystal field calculations together probe the crystal field states associated with the J=6 states of Tb^3+ within the appropriate Fd3m pyrochlore environment. These crystal field states determine the size and anisotropy of the Tb^3+ magnetic moment in each material’s ground state, information that is an essential starting point for any description of the low temperature phase behavior and spin dynamics in Tb2Ti2O7 and Tb2Sn2O7. Chapter 4 treats neutron scattering, as well as accompanying AC magnetic susceptibility and muSR measurements performed by our collaborators on a series of solid solutions Tb2Sn2-xTixO7 showing a novel, dynamic spin liquid state for all x other than the end members x=0 and x=2. This state is the result of disorder in the low lying Tb^3+ crystal field environments which de-stabilizes the mechanism by which quantum fluctuations contribute to ground state selection in Tb2Sn2-xTixO7.</p>