  The topological electronic structure of crystalline materials often gives
rise to intriguing surface states, such as Dirac surface states in topological
insulators, Fermi arc surface states in Dirac semimetals, and topological
superconductivity in iron-based superconductors. Recently, rhombohedral
multilayer graphene has emerged as a promising platform for exploring exotic
surface states due to its hosting of topologically protected surface flat bands
at low energy, with the layer-dependent energy dispersion. These flat bands can
promote electron correlations, leading to a plethora of quantum phenomena,
including spontaneous symmetry breaking, superconductivity, ferromagnetism, and
topological Chern insulators. Nevertheless, the intricate connection between
the surface flat bands in rhombohedral multilayer graphene and the highly
dispersive high-energy bands hinders the exploration of correlated surface
states. Here, we present a method to isolate the surface flat bands of
rhombohedral heptalayer (7L) graphene by introducing moire superlattices. The
pronounced screening effects observed in the moire potential-modulated
rhombohedral 7L graphene indicate its essential three-dimensional (3D) nature.
The isolated surface flat bands favor correlated states on the surface in the
regions away from charge-neutrality points. Most notably, we observe tunable
surface ferromagnetism, manifested as an anomalous Hall effect with hysteresis
loops, which is achieved by polarizing surface states using finite displacement
fields. Our work establishes rhombohedral multilayer graphene moire
superlattice as a unique 3D system for exploring correlated surface states.
