New partially disordered phases with commensurate spin density wave in frustrated triangular lattice

We have studied magnetic ordering of frustrated classical Ising spins with S=1/2 on the antiferromagnetic triangular lattice taking into account of the 2nd and the 3rd neighbor interactions by means of Monte Carlo simulations and the mean field approximation. The antiferromagnetic second and third neighbor interactions give rise to the first order phase transitions from the ground state to partially disorder states characterized by quasi-long range order with Kosterlitz-Thouless character due to the spin frustration. These partially disordered phases with 5- and 9-sublattices have commensurate sinusoidal spin density waves and related to 10- and 18-clock XY spin models, respectively.