  Nanomagnets with biaxial magnetocrystalline anisotropy have four stable
magnetization orientations that can encode 4-state logic bits (00), (01), (11)
and (10). Recently, a 4-state NOR gate derived from three such nanomagnets,
interacting via dipole interaction, was proposed. Here, we devise a Bennett
clocking scheme to propagate 4-state logic bits unidirectionally between such
gates. The nanomagnets are assumed to be made of 2-phase strain-coupled
magnetostrictive/piezoelectric multiferroic elements, such as nickel and lead
zirconate titanate (PZT). A small voltage of 200 mV applied across the
piezoelectric layer can generate enough mechanical stress in the
magnetostrictive layer to rotate its magnetization away from one of the four
stable orientations and implement Bennett clocking. We show that a particular
sequence of positive and negative voltages will propagate 4-state logic bits
unidirectionally down a chain of such multiferroic nanomagnets for logic flow.
