Charge-Lattice-Spin Interactions in Molecule-Based Magnets

Many of the most attractive properties of multifunctional materials can be traced to the competition between charge, structure, and magnetism. The discovery that these interactions can be tuned with various physical stimuli has accelerated interest in their behavior under extreme conditions. In this dissertation I present a spectroscopic investigation of several model molecule-based magnets under external stimuli of magnetic field and temperature. The compounds of interest include M^II[N(CN)₂]₂ (M=Mn, Co) and [Ru₂(O₂CMe)₄]₃[Cr(CN)₆]. These materials are attractive for their subtle interplay between electronic, magnetic and structural degrees of freedom offering both physical property tunability and models with which to carry out fundamental studies of coupling phenomena. The vibrational properties of Mn[N(CN)₂]₂ reveal the magnetoelastic coupling through the quantum critical transition at 30.4 T that drives the system from the canted antiferromagnetic to the fully polarized state. The local lattice distortions, manifested in systematic phonon frequency shifts, suggest a combined MnN₆ octahedra distortion + counter-rotation mechanism that reduces antiferromagnetic interactions and accommodates the developing field-induced state. Work on Co[N(CN)₂]₂ combines high field Raman and infrared spectroscopies to explore the effect of the chemical tuning on lattice dynamics and coupling processes in a ferromagnet. In addition to a large anisotropy, our studies uncover electron-phonon coupling as a field-driven avoided crossings of the low-lying Co²⁺ electronic excitation with the ligand phonons and a magnetoelastic effect that signals a flexible local CoN₆ environment under external field. Finally, we employ vibrational spectroscopies to probe spin-lattice interactions in the [Ru₂(O₂CMe)₄]₃[Cr(CN)₆] metamagnet. In applied field, correlation between the vibrational response, the displacement patterns, and local lattice distortions reveals magnetoelastically-active [Cr(CN)₆]³⁻ octahedral units and rigid [Ru₂(O₂CMe)₄]⁺ paddle wheel dimers as the system is driven away from the antiferromagnetic ground state. At the same time, variable temperature studies show pronounced changes in modes connected with the [Cr(CN)₆]³⁻ octahedra, demonstrating the overall softness of this moiety and its readiness to adapt to a new physical environment. These findings deepen our understanding of coupling in multifunctional materials and demonstrate the tunability of competing interactions under extreme conditions.

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