  We study antiferromagnetism and single-particle properties in the
two-dimensional half-filled Hubbard model at low temperature. Collective spin
fluctuations are governed by a non-linear sigma model that we derive from the
Hubbard model for any value of the Coulomb repulsion. As the Coulomb repulsion
increases, the ground state progressively evolves from a Slater to a
Mott-Heisenberg antiferromagnet. At finite temperature, we find a
metal-insulator transition between a pseudogap phase at weak coupling and a
Mott-Hubbard insulator at strong coupling.
