$^{11}$B NMR Study of Spin Structure and Dynamics in TbB$_{4}$ Single Crystal

$^{11}$B NMR measurements have been performed on the single crystal of TbB$_{4}$ to investigate $4f $spin structure and dynamics. $^{11}$B NMR spectrum, shift, $1/T_{1}$ and $1/T_{2}$ are measured down to 5 K at 8 T perpendicular to the (110) plane. $^{11}$B NMR shift and linewidth are huge and strongly temperature-dependent due to the $4f$ moments of Tb. In addition, both are proportional to the magnetic susceptibility, indicating that the hyperfine field at the boron site originates from the $4f$ spins of Tb. Below $T_{N}$ = 43 K, the single broad resonance peak of $^{11}$B NMR splits into several peaks reflecting the onset of internal magnetic fields due to the antiferromagnetic spin arrangements. Assuming that the $4f$ moments are aligned antiferromagnetically and collinearly in the basal plane perpendicular to the c-axis, we have calculated dipolar fields at the 16 boron nuclear sites from the $4f$ spins of Tb. The results show that the various peaks of $^{11}$B NMR spectrum at 5 K are generally consistent with the calculation confirming that the collinear spin arrangement is correct. The longitudinal and the transverse relaxation rates, $1/T_{1}$ and $1/T_{2}$, are independent of temperature above $T_{N}$ and then decreases tremendously indicating the huge suppression of spin fluctuations below $T_{N}$.