Unraveling the electromagnetic structure of the epitaxial graphene buffer layer

We have employed density functional theory to study the structural, electronic and magnetic properties of the first all-carbon layer grown epitaxially on 6H-SiC(0 0 0 1). Using VDW-DF, M06-L, LSDA, LSDA+U, PBE and PBE-D2 methods we have performed a comparative study of the preferable magnetic configuration of the system. In this work, for the first time, we report a stable antiferromagnetic (AF) ordering in the buffer layer caused by the presence of silicon dangling bonds in the SiC top layer. This state is nearly degenerated with the ferromagnetic state with a magnetic moment equal to the number of silicon dangling bonds. A net magnetic moment of 0.55 µb per Si dangling bond was found for both states. However, only for the ferromagnetic state the carbon atoms of the buffer layer exhibited a magnetic moment. The magnetic configuration is much more stable than the non-polarized one and might explain SQUID results and spin transport experiments with epitaxial graphene. Furthermore, we found that, as previously observed experimentally, the buffer layer is a true semiconductor.