Spin dependent impurity effects in the 2D frustrated magnetism of NiGa${}_2$S${}_4$

Abstract

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

5 pages, 4 figures