ESR Study of the Triangular Lattice Antiferromagnets with S =1/2: NaTiO 2 and LiNiO 2

Recent experimental studies on NaTiO 2 and LiNiO 2 suggest that they might be rare compounds of 2D triangular lattice antiferromagnet with S =1/2, the former being Heisenberg-like and the latter being Ising-like. We have studied ESR from 1.5 K to 300 K. In NaTiO 2 a broad ESR signal was observed at 300 K. A sharp ESR signal grows below ∼250 K and continues to increase the intensity toward 0 K. In LiNiO 2 , we found clear dip in resonance width at 210 K corresponding to the para- to ferrimagnet-like transition but no change in resonance field on passing through it. Contrary to this, a remarkable divergence of the width was found at ∼19 K and a shift of resonance field starts below it, though no magnetic order has been detected by neutron scattering. Possible origin of these phenomena is discussed.