11BNMR Study of the Spin Structures andthe Dynamics in GdB4 Single Crystals

摘要

11B nuclear magnetic resonance (NMR) measurements were performed on a single crystal of GdB_4to investigate the 4f spin structure and dynamics. The ~(11)B NMR spectrum, shift, linewidth, spin-lattice and spin-spin relaxation rates were measured down to 5 K at a field of 8 T perpendicular tothe c-axis. Above T_N =42 K, the ~(11)B NMR shift and linewidth were large due to the 4f momentsof Gd. In addition, both were strongly temperature-dependent and proportional to the magneticsusceptibility. These facts confirm that the hyperfine field at the boron site originates from the4f spins of Gd. Below T_N,the single broad resonance peak of the ~(11)B NMR split into variouspeaks, confirming that internal magnetic fields set in due to an antiferromagnetic phase transition.Assuming that the 4f spins of Gd are aligned antiferromagnetically and noncollinearly along the<110> direction in the basal plane perpendicular to the c-axis, we calculated the dipolar fields atthe 16 boron nuclear sites from the 4f spins of Gd. The results show that the various peaks ofthe ~(11)B NMR spectrum at 5 K are generally consistent with the calculation, suggesting that thenoncollinear spin structure is a correct spin configuration. The relaxation rates, 1/T_1 and 1/T_2,were independent of temperature for temperatures above T_Nand then decreased tremendously. Thevast variation of the relaxation rates indicates a rapid spin fluctuation at temperatures above T_Nand a huge suppression of the 4f spin fluctuation after the phase transition at T_N .