Detailed magnetization, specific heat, and ${}^7\mathrm{Li}$ nuclear magnetic resonance (NMR) measurements on single crystals of the hyperhoneycomb Kitaev magnet $\beta -{\mathrm{Li}}_2{\mathrm{IrO}}_3$ are reported. At high temperatures, anisotropy of the magnetization is reflected by the different Curie-Weiss temperatures for different field directions, in agreement with the combination of a ferromagnetic Kitaev interaction ($K$) and a negative off-diagonal anisotropy ($\mathrm{\Gamma }$) as two leading terms in the spin Hamiltonian. At low temperatures, magnetic fields applied along $a$ or $c$ have only a weak effect on the system and reduce the Néel temperature from 38 K at 0 T to about 35.5 K at 14 T, with no field-induced transitions observed up to 58 T on a powder sample. In contrast, the field applied along $b$ causes a drastic reduction in the $T_N$ that vanishes around $H_c=2.8\mathrm{T}$, giving way to a crossover toward a quantum paramagnetic state. ${}^7\mathrm{Li}$ NMR measurements in this field-induced state reveal a gradual line broadening and a continuous evolution of the line shift with temperature, suggesting the development of local magnetic fields. The spin-lattice relaxation rate shows a peak around the crossover temperature 40 K and follows power-law behavior below this temperature.

• Received 11 February 2019
• Revised 21 June 2019

DOI:https://doi.org/10.1103/PhysRevMaterials.3.074408