Coexistence of gap-less and gapped excitations in NH₄CuCl ₃

A low-temperature ESR investigation is performed on a low-dimensional compound NH₄CuCl₃ with magnetization plateaux. In the phase below the 1/4-plateau, a coexistence of gapped two triplets and a gap-less mode is found. Below 1.3 K at which a part of spins show some magnetic ordering, the higher energy triplet mode exhibit no change. In addition to the previously reported gapped modes at 1/4- and 3/4- plateaux, a new low energy mode is found at the half of the saturation field. This mode may be caused by an 'irrelevant instability' of 1/2-plateau.