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

We have performed $^{11}$B NMR measurements on a single crystal of GdB$_{4}$ to investigate $4f $spin structures 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 c-axis. $^{11}$B NMR shift and linewidth are huge and strongly temperature-dependent due to the $4f$ moments of Gd. In addition, both are proportional to the magnetic susceptibility, indicating that the hyperfine field at the boron site originates from the $4f$ spins of Gd. Below $T_{N}$ = 42 K, the single broad resonance peak of $^{11}$B NMR splits into various peaks reflecting the onset of internal magnetic fields due to the antiferromagnetic spin arrangements. Assuming that the $4f$ moments of are aligned noncollinearly along the $<$110$>$ direction 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 Gd. The results show that the various peaks of $^{11}$B NMR spectrum at 5 K are generally consistent with the calculation confirming that the noncollinear spin arrangement is correct. The 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 $T_{N }$.