Orbital frustration at the origin of the magnetic behavior in LiNiO2

We report on the ESR, magnetization, and magnetic susceptibility measuremen ts performed over a large temperature range, from 1.5 to 750 K, on high-qua lity stoichiometric LiNiO2. We find that this compound displays two distinc t temperature regions where its magnetic behavior is anomalous. With the he lp of a statistical model based on the Kugel'-Khomskii Hamiltonian, we show that below T-of similar or equal to 400 K, an orbitally frustrated state c haracteristic of the triangular lattice is established. This then gives a s olution to the long-standing controversial problem of the magnetic behavior in LiNiO2.