Using applied field, pressure, and light to control magnetic states of materials

Due to their low energy scales, flexible architectures, and unique exchange pathways, molecule-based multiferroics host a number of unique properties and phase transitions under external stimuli. In this dissertation, we reveal the magnetic- and pressure-driven transitions in [(CH₃)₂NH₂]Mn(HCOO)₃ and (NH₄)₂[FeCl₅(H₂O)], present a detailed investigation of these materials away from standard equilibrium phases, and develop rich two- and three-dimensional phase diagrams.

The first platform for exploring phase transitions is [(CH₃)₂NH₂]Mn(HCOO)₃. This type-I multiferroic contains Mn centers linked by formate ligands creating Mn-O-C-O-Mn superexchange pathways. Magnetization measurements reveal two transitions - a spin-flop and a transition to the fully polarized state - and the loss of long-range order above the Neel temperature. Extending to the high-pressure regime, we perform vibrational spectroscopy across the order-disorder transition and use a correlation group analysis to determine the high pressure space groups. The superexchange pathway plays a crucial role in triggering the structural crossover to lower symmetry. Despite having driving different space groups above/below the order-disorder temperature, compression lowers each symmetry to the polar space group P1. We develop the pressure - temperature - magnetic field phase diagram for [(CH₃)₂NH₂]Mn(HCOO)₃ and articulate the potential for enhanced polarization under compression.

The type-II multiferrroic (NH₄)₂[FeCl₅(H₂O)] is different. It hosts a unique set of exchange pathways mediated by through-space hydrogen- and halogen-bonding. Magnetization displays a series of transitions, including the spin-flop, transition to the fully saturated state, and many associated reorientation transitions. Extending to high-pressure studies, we employ infrared absorption and Raman scattering under compression to reveal an increase in hydrogen bonding and changes in the FeCl₅H₂O polyhedron that are unique to this regime. A space group analysis uncovers a sequence of space group changes that suggests it is driven to a polar space group. We generate the complete three-dimensional phase diagram, which displays the many competing structural and magnetic interactions.

Together, these findings uncover magnetically-driven quantum phase transitions and reduced symmetry under compression to likely polar space groups. This work motivates extended investigations of non-equilibrium phases under external stimuli in these and other molecule-based materials with low energy scales, flexible architectures and unique spin interactions.