Experimental Study of the Two-Dimensional Ising AntiferromagnetRb2CoF4

The critical behavior of ${\mathrm{Rb}}_2$Co${\mathrm{F}}_4$ has been studied by means of neutron scattering. In its long-range order dependence below $T_{\mathrm{N}}=103\mathrm{}$ K the material was found to behave as an almost ideal two-dimensional Ising system, with critical parameters $B=1.16\mathrm{}\pm 0.03$ and $\beta =0.119\mathrm{}\pm 0.008$. A novel phenomenon was discovered below $T_{\mathrm{N}}$: The three-dimensional stacking of the antiferromagnetic planes is sensitive to the heat treatment given at $T_{\mathrm{N}}$. Above $T_{\mathrm{N}}$ the inverse correlation length and the staggered susceptibilities were found to obey power laws with exponents $\nu =0.89\mathrm{}\pm 0.1$ and $\gamma =1.34\mathrm{}\pm 0.22$.