(11)B NMR Study of the Spin Structures and the Dynamics in GdB(4) Single Crystals

Abstract

(11)B nuclear magnetic resonance (NMR) measurements were performed on a single crystal of GdB(4) to 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 to the c-axis. Above TN = 42 K, the (11)B NMR shift and linewidth were large due to the 4f moments of Gd. In addition, both were strongly temperature-dependent and proportional to the magnetic susceptibility. These facts confirm that the hyperfine field at the boron site originates from the 4f spins of Gd. Below TN, the single broad resonance peak of the (11)B NMR split into various peaks, 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 at the 16 boron nuclear sites from the 4f spins of Gd. The results show that the various peaks of the (11)B NMR. spectrum at 5 K are generally consistent with the calculation, suggesting that the noncollinear 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(N), and then decreased tremendously. The vast variation of the relaxation rates indicates a rapid spin fluctuation at temperatures above TN, and a huge suppression of the 4f spin fluctuation after the phase transition at TA.