Anomalous Behavior of Spin Dynamics in the Metal-Organic Framework (MOF) Dimethylammonium Manganese Formate (DMMnF)*

Dimethylammonium Manganese Formate(DMMnF) is of interest as a multiferroic metal-organic framework. For the first time, ¹H and ⁵⁵Mn NMR are used to study the known dielectric transition at T_FE ~ 184K and antiferromagnetic (AF) transition at T_N ~ 8.5K. A broad and strong zero-field ⁵⁵Mn NMR signal was found below T_N, with a calculated 8T internal field due to ordered Mn moments. This signal is suppressed above 0.3T, consistent with the known spin-flop transition. Dramatic changes in ¹H spectra correlate to the magnetic ordering with a critical exponent of β = 1/4, in contrast to the mean field prediction. The spin-lattice relaxation T₁ recovery exhibits a double exponential behavior, with the long and short components differing by 3 orders of magnitude. The long T₁ component dips near 150K and 8.5K, while the short component exhibits a huge enhancement approaching the critical regime followed by an exponential decay at low temperatures. This behavior is suggestive of an opening of a spin gap, Δ ~ 4.45K, about half of T_N. Implications of these results will be discussed.