  Gallium arsenide (GaAs) is the widest used second generation semiconductor
with a direct band gap and increasingly used as nanofilms. However, the
magnetic properties of GaAs nanofilms have never been studied. Here we find by
comprehensive density functional theory calculations that GaAs nanofilms
cleaved along the <111> and <100> directions become intrinsically metallic
films with strong surface magnetism and magnetoelectric (ME) effect. The
surface magnetism and electrical conductivity are realized via a combined
effect of transferring charge induced by spontaneous electric-polarization
through the film thickness and spin-polarized surface states. The surface
magnetism of <111> nanofilms can be significantly and linearly tuned by
vertically applied electric field, endowing the nanofilms unexpectedly high ME
coefficients, which are tens of times higher than those of ferromagnetic metals
and transition metal oxides.
