From calculations using ab initio density-functional methods, the authors show that analogous to the much-studied polar LaAlO3/SrTiO3 interface, there also forms a two-dimensional electron gas at the LaMnO3/SrMnO3 (100) interface due to the polar catastrophe. The electron gas originates from the accumulation of half an electron per cell to quench the polar catastrophe and from the leakage of Mn(e(g)) electrons from the LaMnO3 side to the SrMnO3 side. These electrons mediate a ferromagnetic double-exchange interaction between the Mn moments at the interface, causing the interface region to be ferromagnetic and in turn the electrons becoming spin-polarized owing to the Zeeman field of the Mn moments. (C) 2010 American Vacuum Society. [DOI: 10.1116/1.3454370]