Low-Temperature Transitions in Tetramethylammonium Manganese Chloride

We have measured the proton nuclear magnetic resonance of tetramethylammonium manganese chloride, a linear chain antiferromagnet, in the temperature region 0.4-300 K. In addition, its ac magnetic susceptibility was measured in applied fields of 0 to 22 kG in the temperature range 0.3-4.2 K. When measured along the crystallographic $c$ axis, the zero-field susceptibility had an anomaly at 0.84 K. When the external field was applied perpendicularly to the $c$ axis below 0.8 K, a critical field of 11.5 kG was observed in $\frac{\mathrm{dM}}{\mathrm{dB}}$. The proton NMR did not indicate any cooperative transition to a magnetically ordered state of the ${\mathrm{Mn}}^{2+}$ spins. It did indicate, however, a gradual diminution of fluctuations in the crystallographic $\mathrm{ab}$ plane until a nonrandom order had been established between chains below approximately 0.8 K. We find that the tetramethylammonium groups cease all rotations below 39 K. In the region of 40 to 50 K, the tetramethylammonium groups not only undergo some hindered rotations, but their orientation is different from the published room-temperature x-ray diffraction results. Above 50 K only a single narrow NMR line is observed and, consequently, no additional information on the crystal structure could be obtained.