11B NMR study of the spin structure and dynamics in TbB 4 single crystals

Abstract

11B nuclear magnetic resonance (NMR) measurements have been performed on single crystals of TbB4 to investigate the 4f spin structure and dynamics. 11B NMR spectrum and shift, as well as the spin-lattice and the spin-spin relaxation rates, have been measured down to 5 K at 8 T perpendicular to the (110) plane. 11B 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 TN = 43 K, the single broad resonance peak of 11B 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 the dipolar fields at the 16 boron nuclear sites from the 4f spins of Tb. The results show that the various peaks of the 11B 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/T1 and 1/T 2, are independent of temperature above TN, but decrease tremendously below TN, indicating a huge suppression of the spin fluctuations.