B-11 pulsed nuclear magnetic resonance study of HoB4

Abstract

B-11 pulsed nuclear magnetic resonance (NMR) measurements were performed on a single crystal of HoB4 to investigate disorder-induced effects on the 4f spin structures and dynamics. The B-11 NMR spectrum, shift, linewidth, and spin-lattice and spinespin relaxation rates (1/T-1 and 1/T-2, respectively) were measured down to 3.5 K at a field of 8 T perpendicular to the c-axis. Above the Ne el temperature, T-N - 5.7 K, the B-11 NMR linewidth is very large and the shift is also large and with negative sign. In addition, both are strongly temperature-dependent and the absolute shift value and linewidth increase at lower temperatures. This fact confirms that the hyperfine field at the boron sites originates mainly from the Ho 4f spins, as this is also the case for the susceptibility. Below T-N, the B-11 NMR spectrum displays a single, broad and featureless shape with an extremely large linewidth. This behavior is an unexpected result compared with typical NMR spectra in the magnetically ordered state for single crystal specimens, and is significantly different to that of other RB4 substances. Considering frustration and disorder effects on the static and dynamic NMR data, we conclude that this behavior originates from the magnetic frustration and quadrupole moment disorder effects on the NMR static data, since the Ho ions form a Shastry-Sutherland (SS)-type lattice in the ab plane. Above T-N, both relaxation rates (1/T-1 and 1/T-2) are very large as a result of the rapid and random fluctuation of the Ho 4f moments, and they then increase toward the Ne el transition. Below T-N, both rates decrease significantly since the 4f spin fluctuations and dynamics are frozen as a result of the antiferromagnetic ordering. (C) 2015 Elsevier B.V. All rights reserved.