Abstract
We study an Ising antiferromagnet on a triangular lattice which has interactions along three different directions via two J-bonds and one anisotropic J'-bond with J' ⩾ J. Although its finite-temperature state has long been considered as simply disordered, we find systematic generation of ferromagnetic J'-bonds named "good defects" each carrying the energy 2δJ ≡2(J' − J). They exhibit eminent spacial correlation which is regarded as a soft lattice network. The specific heat shows a universal peak at T*∼δJ and the corresponding energy follows activation type of temperature dependence with an excitation gap 2δJ. Energetics and crossover between this particular two-dimensional state and the other disordered or one-dimensional states are discussed.