$^{135,137}$Ba NMR study of Ba$_{3}$Mn$_{2}$O$_{8}$

We report results from $^{135,137}$Ba NMR spectroscopy and relaxation rate (1/$T_{1})$ measurements in single crystal Ba$_{3}$Mn$_{2}$O$_{8}$, an $S$ =1 dimer system with a singlet ground state. Thermodynamic measurements have shown it has multiple field-induced phase transitions for fields exceeding a critical field $H_{c1}\simeq $90 kOe and varying with field orientation. We have evaluated the hyperfine couplings and electric field gradients in the normal phase for one of the two inequivalent Ba sites, and find a significant anisotropic component to the hyperfine coupling. Measurements of 1/$T_{1}$ made at fixed fields down to temperatures $T<$0.4 K are consistent with critical behavior in the vicinity of $H_{c1}$. However, lower temperatures are needed to clarify the universality class. Goals for upcoming experiments include a determination of the spectrum in the low-symmetry phases and an evaluation of 1/$T_{1}$ for $T<$0.4 K. This work is supported at UCLA by NSF Grants 0520552 (SEB), DMR-00334869 (WGC), Stanford by DMR-0134613 (IRF), and NHMFL by 0084173 and the State of Florida.