Date of Award

12-2008

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Physics

Major Professor

Pengcheng Dai

Committee Members

Elbio Dagotto, Norman Mannella, Takeshi Egami, Jaime A. Fernandez-Baca

Abstract

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.

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