Date of Award
Doctor of Philosophy
Elbio Dagotto, Norman Mannella, Takeshi Egami, Jaime A. Fernandez-Baca
We present neutron scattering, with triple-axis and time-of-flight spectrometers, to study the structural and magnetic properties of the layered manganites Pr1-xCa1+xMnO4 (PCMO), and to determine the crystalline electric field (CEF) levels in iron pnictides CeFeAsO1-xFx and filled skutterudite compound PrOs4As12.
For the single-layered manganites PCMO, four dopings (x=0.5, 0.45, 0.40 and 0.35) have been studied. At half-doping, the system first becomes charge- and orbital- ordered (CO/OO) near TCO = 300 K and then develops CE-type antiferromagnetic (AF) order below TN = 130 K. At temperatures TN < T < TCO, the appearance of short-range AF spin correlations suppresses the CO/OO induced orthorhombic strain. These results suggest that a strong spin-lattice coupling.
In less hole-doped PCMO (x < 0:5) compounds, the long-range, three-dimensional anti- ferromagnetic (AF) correlations at x = 0:5 are suppressed. Most remarkably, the magnetic correlations appear in the form of coexisting commensurate (CM) and incommensurate (ICM) fluctuations with strong temperature and doping dependence.
The spin-wave measurement on PCMO (x = 0:5) shows good agreement with a mag- netic interaction model based on the CE-type arrangement. The magnetic excitations are dominated by ferromagnetic exchange along the zig-zag chain, while the antiferromagnetic coupling between the chains is relatively weak. The INS measurement on the x = 0:45 system reveals both symmetric and asymmetric magnetic excitations about the CM peak positions: one dispersive spin wave indicative of the x = 0:5 system; and another localized mode at about 1 meV. This strongly suggests two types of spin dynamics originated from separated phases: the CE-type magnetic phase and an additional electronic phase caused by extra electrons introduced into the CE template.
Chi, Songxue, "Structural and Magnetic Properties of Manganites Pr1-xCa1+xMnO4, Oxypnictides CeFeAsO1-xFx, and Filled Skutterudite PrOs4As12. " PhD diss., University of Tennessee, 2008.