Coherent behavior and nonmagnetic impurity effects of spin disordered state in NiGa ₂S ₄

Nonmagnetic impurity effects of the spin disordered state in the triangular antiferromagnet NiGa ₂S ₄ were studied through magnetic and thermal measurements for Ni _1-xZn _xGa ₂S ₄ (0:0 ≤ x ≤ 0:3). Only 1% substitution is enough to strongly suppress the coherence observed in the spin disordered state. However, the suppression is not complete and the robust feature of the T ²- dependent specific heat and its scaling behavior with the Weiss temperature indicate the existence of a coherent Nambu-Goldstone mode. The absence of either conventional magnetic order or bulk spin freezing suggests a novel symmetry breaking of the ground state.