Impurity effects on the triangular antiferromagnets ${\mathrm{Ni}}_{1-x}{M}_x{\mathrm{Ga}}_2{\mathrm{S}}_4$ ($M=\mathrm{Mn}$, Fe, Co and Zn) are studied. The 2D frozen spin-disordered state of ${\mathrm{NiGa}}_2{\mathrm{S}}_4$ is stable against the substitution of ${\mathrm{Zn}}^{2+}$ ($S=0$) and Heisenberg ${\mathrm{Fe}}^{2+}$ ($S=2$) spins, and exhibits a $T^2$-dependent magnetic specific heat, scaled by the Weiss temperature. In contrast, the substitutions with ${\mathrm{Co}}^{2+}$ ($S=3/2$) spin with Ising-like anisotropy and Heisenberg ${\mathrm{Mn}}^{2+}$ ($S=5/2$) spin induce a conventional spin glass phase below 1 K. From these comparisons, it is suggested that the integer size of the Heisenberg spins is important to stabilize the 2D coherent behavior observed in the frozen spin-disordered state.

• Received 10 February 2008

DOI:https://doi.org/10.1103/PhysRevLett.101.207204