Magnetic properties of antiferromagnets with mobile vacancies

We analyze the magnetic fluctuations in itinerant antiferromagnets close to the metal-insulator transition. We concentrate on the Pauli-like contribution of the carriers, which we describe within two related models: the Hubbard model and the t-t ’-J model. When there is antiferromagnetic long-range order, we show that the carriers give very different contributions to the parallel and perpendicular uniform susceptibilities. On approaching the metal-insulator transition, the parallel response diverges (or at least becomes very large), while the perpendicular response goes to zero. If there is only short-range order, we argue that the susceptibility, which is isotropic, should behave like the perpendicular susceptibility of the long-range-order case. Within a simple phenomenology of high-${\mathit{T}}_{\mathit{c}}$ materials, this conclusion agrees with experimental data.