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First-principles electronic structure calculations are performed to elucidate magnetic properties of LaAlO3 /SrTiO3 heterostructures. We find that TiO2-terminated interfaces are n-type conducting which is consistent with experimental observations. In a (LaAlO3 ) 3 / (SrTiO3)3 superlattice, this interface is magnetic with a magnetic moment on the Ti3+ atom of 0.2 μB, as revealed by the spin-polarized calculations within the local density approximation (LDA). For thicker SrTiO3 layers the magnetic moment decreases and eventually disappears because the electron gas spreads over more than one unit cell, making the electron delocalized across the superlattice and violating the Stoner criterion for magnetism. Thus, magnetization in these superlattices is due to geometric confinement of the electron gas. The inclusion of electron correlations via the LDA+U approximation with U=5 eV on the Ti atoms makes the two-dimensional electron gas half-metallic and enhances and stabilizes the interface magnetization.