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We study the magnetic ordering of an anisotropic Heisenberg ferromagnet on a triangular lattice with antiferromagnetic next-nearest-neighbor interactions by means of the mean field theory and a computer simulation. We find that, as the temperature is decreased, an incommensurate short-range order state and a ferromagnetic long-range phase appear successively. The phase transition between them is of the first order and the relaxation time of the transition is very long. Hence, we suggest that the low temperature phase realized in nature is a domain state in which ferromagnetic domains with various sizes are tangled with each other. We also suggest that the model would be applicable to the compound LiNiO 2 .

The effect of the transverse magnetic field h x on the spin dynamics of a one dimensional Ising-like S =1/2 antiferromagnet is discussed. Based on a picture of the propagating domain walls, we obtain dynamical spin correlation functions along the magnetic field S x x ( k ,ω) and perpendicular to it S y y ( k ,ω). A remarkable effect is seen in S y y ( k ,ω) for π/2 ≤ k ≤π, where the spectral weight moves toward the low energy side with the increase of h x . The theory is applied to the analysis of the spin dynamics of CsCoCl 3 , taking into account weak interchain interactions. Zeeman ladders occur and S y y ( k , ω) for k ∼π exhibits a novel feature under the existence of h x .

The magnetization process of a quasi-one-dimensional antiferromagnet CsCoCl 3 is analyzed by the Monte Carlo simulation combined with the diagonalization technique. We explained the step and the hysteresis loop of the M - H curve observed experimentally together with the magnetization process characteristic to a one-dimensional Ising-like antiferromagnet at higher magnetic fields. Although the interchain interaction J 1 is about 1% of the intrachain interaction J , it is essential for reproducing those features.