NMR near ferroelectric, magnetic, and quantum phase transitions

This dissertation details the use of solid state 1H nuclear magnetic resonance (NMR) to investigate three compounds exhibiting phase transitions. The metal-organic framework (MOF) [(CH3)2NH 2]Zn(HCOO)3 has a ferroelectric (FE) transition at T = 156 K. Spin-lattice relaxation time (T 1) measurements are used to probe the local environment of the DMA + cations. It is shown that the FE transition involves a smooth slowing-down of the three-fold hopping motion of the nitrogen in DMA+, rather than a complete freezing. The T1 data reveal that this compound exhibits several different local structures with close-lying ground states, which is a manifestation of glassy behavior. This is observed through the appearance of new and distinct metastable relaxation paths as the sample is thermally cycled. The MOF [(CH3)2NH 2]Mn(HCOO)3 is a multiferroic, exhibiting both a FE (T = 190 K) and an antiferromagnetic transition (AFM, TN=8.5 K). Spectra, T1, and spin-spin relaxation time (T2) measurements reveal that the electron-nucleus magnetic interactions play a dominant role at temperatures as high as the FE transition. The AFM transition is observed in all measurements at higher temperature than reported, indicating that the local magnetic structure around the hydrogens is already an almost fully ordered magnetic lattice at the NMR time scale. The transition occurs at different temperatures as the sample is thermally cycled, and the spectral shape in the ordered state is different each time, indicating that this material exhibits different magnetic structures, a sign of glassy behavior. The AFM Cr(C4H 13N3)(O2)2·H2O exhibits a quantum phase transition. T1 data reveal that the critical field appears to be Hc ≈ 12.50 T, in good agreement with previously reported value. Near the quantum critical point, the 2-D antiferromagnet behaves as a 3-D Ising ferromagnet.