Magnetic properties of the lightly dopedt-Jmodel: A study through high-temperature expansions

We develop high-temperature expansions for the complete q-dependent static structure factor of the two-dimensional t-J model for arbitrary parameter ratio t/J and particle density {ρ}. The analysis of the series reveals several different types of magnetic behavior. The spin-spin correlations are peaked at the antiferromagnetic vector ({π},{π}) at half filling ({ρ}=1). For small t/J, these correlations stay commensurate even with moderate doping down to as low a temperature as is accessible to our extrapolations; they become incommensurate at large dopings at temperatures below J. The antiferromagnetic correlation length decreases rapidly with doping. The uniform susceptibility {χ} shows a temperature and doping dependence similar to experiments on doped cuprates. The maximum in {χ} as a function of T shifts to lower temperatures with doping. Furthermore, at low temperatures the susceptibility increases with doping. The enhancement of the susceptibility with respect to the bare tight-binding value is also in the experimental range for high-Tc materials. At large t/J values the spin correlations become ferromagnetic.