  Recent neutron scattering [1,2], and optical measurements [3,4] have detected
evidence in underdoped cuprate superconductors for a phase transition near the
pseudogap onset temperature T* to a time reversal-breaking state. The neutron
scattering indicates antiferromagnetic ordering, while it is often assumed that
optical polarization rotation requires at least a weak ferromagnetic component.
In this note we identify several antiferromagnetic structures, compatible with
neutron scattering data, that allow intrinsic polarization rotation through the
magnetoelectic effect.
