Li-NMR study of Spin liquid state due to orbital fluctuation in LiNio ₂

We report magnetic field (H) effect on the unconventional magnetism in LiNiO₂ that consists of a spin-1/2 triangular (TRI) lattice through the ⁷Li nuclear spin lattice relaxation (1/T₁) and NMR intensity measurements. Instead of a ferromagnetic (FM) ground state expected from the FM-Weiss constant, a marginal ordered state emerges below T_M sim 20 K, evidenced from the wipe-out effect of ⁷Li-NMR signal and the divergent behavior of 1/T₁ towards T_M- The existence of the strong antiferromagnetic (AF) interaction along with the FM one is concluded to lead to a magnetically frustrated state at low temperatures and its AF exchange constant J_AF is estimated to be about -20 K. This marginal ordered state due to the frustration of the antiferro-orbital order is found to be suppressed by applying H exceeding 10 T, revealing an orbital exchange constant J° ∼ -10 K. In the paramagnetic regime at high temperatures, the system behaves like an assembly of local moments with an exchange frequency w_ex sim 11 K, consistent with the energy scale of \J°;\∼10 K. The present experiments have revealed that | J°\ ∼ 10 K is lower than spin exchange constants J_FM ∼ 30 K, \J _AF\ ∼ 20 K.