ABSTRACT
In this chapter, the author shows properties of exactly solved frustrated twodimensional (2D) spin systems. He emphasizes the physics near a phase boundary where interesting phenomena can occur due to competing interactions of the two phases around the boundary. This chapter recalls important results found by solving exactly several frustrated 2D Ising systems which help understand recent results in other non-solvable models such as frustrated thin films. The main purpose of the review is to show some frustrated magnetic systems which present a number of common interesting features. These features are discovered by solving exactly some 2D Ising frustrated models. However, the reentrance phenomenon is a symmetry breaking alternative which allows one ordered phase to change into another incompatible ordered phase by going through an intermediate reentrant phase. The presence of a reentrance may also require a coordination number at a disordered site large enough but below a limit to influence the neighboring ordered sites.
