Neutron Scattering Studies of Unusual Spin Structure and Local Correlation in Complex Oxides
Complex correlated electron materials (CEMs), such as transition-metal oxides with exotic properties and novel functionalities, present immense opportunities and formidable challenges in condensed matter physics, materials science, and engineering. These systems are characterized by a multitude of competing ground states that result from the close coupling between charge, lattice, orbital, and spin degrees of freedom, which can be tuned by chemical substitution, strain induction, or by the application of external stimulus (e.g. pressure, temperature, electric, or magnetic fields). Interest in CEMs is fueled by the richness of their novel properties (e.g. Colossal Magnetoresistance (CMR), Quantum Criticality, and High Temperature Superconductivity), the complexity of the underlying physics, and the promise of technological applications. In this work, elastic and inelastic neutron scattering is used to study two prototypes of CEMs: ruthenates and manganites. Our work on ruthenates has revealed the magnetic structure and dimensionality of the order parameter in the bilayered ruthenate Sr3(Ru1-xMnx)2O7 for manganese (Mn) concentrations of 12.5 and 16%. Results indicate 1) an unusual E-type antiferromagnetic structure with moments aligned along the c-direction exhibiting only single-bilayer (5-6 Å) ferromagnetic correlations along the c-direction, and 2) that Mn concentration is responsible for the stabilization of the long-range E-type AFM ordering observed along the basal plane. The investigation on the evolution of long- and short-range charge-ordered (CO), ferromagnetic, and antiferromagnetic correlations in single crystals of Pr1−xCaxMnO3 for various hole-doping concentrations has 1) provided direct evidence of magnetic phase separation and 2) revealed a critical doping concentration close to (x = 0.35) that divides the inhomogeneous from homogenous CO state. Preliminary studies of spin and lattice excitations in the ferromagnetic insulating phase of La1-xCaxMnO3 indicate an anomalous softening and broadening of the magnons near the zone boundary, where a longitudinal optical phonon is present, indicating that magnon-phonon coupling could play a critical role in the behavior of the spin wave dispersion of these systems. The work presented in this thesis should be a forward step towards the understanding of the nature of the competing interactions present in these CEMs, which result in the emergence of exotic phenomena and novel functionalities.
neutron scattering, magnetism, complex interactions, correlated electron systems, spin-lattice coupling
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