Metamagents and frustration.

Ising metamagnets and their ferroelectric analogs are excellent systems in which to study the effects of frustration. We demonstrate that even very small further neighbor interactions lead to new modulated phases, instead of phase separation. If the spins lie in the ferromagnetic planes, dipolar coupling is an example of such an interaction. We show how, if the spins are perpendicular to the planes macroscopic arrays of alternating paramagnetic and antiferromagnetic regions occur. These are similar to the domain patterns found in dipolar-coupled, uniaxial ferromagnets.