  A partial disorder on a layered triangular lattice is theoretically
investigated from a viewpoint of toroidal ordering and magnetoelectric effects.
We consider an extended periodic Anderson model including a site-dependent
antisymmetric spin-orbit coupling between conduction and localized electrons.
We show that, by the mean-field approximation, the model exhibits a coplanar
vortex-lattice-type magnetic order as observed in a hexagonal uranium compound
UNi4B, in the parameter region with intermediate hybridization and electron
correlation. This peculiar state accommodates a toroidal order, which leads to
the linear magnetoelectric effect. We discuss the implications of our results
to UNi4B, focusing on the possible source of the site-dependent antisymmetric
spin-orbit coupling.
