Double-perovskite A2BBO6 oxides with magnetic B and B ions and E*-type antiferromagnetic order (E*-AFM, i.e. the  structure) are believed to exhibit promising multiferroic properties, and Y2CoMnO6 (YCMO) is one candidate in this category. However, the microscopic origins for magnetically induced ferroelectricity in YCMO remain unclear. In this work, we perform detailed symmetry analysis on the exchange striction effect and lattice distortion, plus the first-principles calculations on YCMO. The E*-AFM state as the ground state with other competing states such as ferromagnetic and Aantiferromagnetic orders is confirmed. It is revealed that the ferroelectricity is generated by the exchange striction associated with the E*-AFM order and chemically rdered Mn/Co occupation. Both the lattice symmetry consideration and first-principles calculations predict that the electric polarization aligns along the b-axis. The calculated polarization reaches up to 0.4682 C/cm 2 , mainly from the ionic displacement contribution. The present work presents a comprehensive understanding of the multiferroic mechanisms in YCMO and is of general significance for predicting emergent multiferroicity in other double-perovskite magnetic oxides.

multiferroics. Despite the progress in the past decade, for most multiferroics their magnetoelectric performance remains poor due to the intrinsic exclusion between charge dipole and spin moment. As an alternative approach, the oxide heterostructures may evade the intrinsic limits in bulk materials and provide more attractive potential to realize the magnetoelectric functions. Here we perform a first-principles study on LaAlO 3 /PbTiO 3 superlattices. Although neither of the components is magnetic, magnetic moments emerge at the ferroelectric domain walls of PbTiO 3 in these superlattices. Such a twist between ferroelectric domain and local magnetic moment, not only manifests an interesting type of multiferroicity, but also is possible useful to pursuit the electricalcontrol of magnetism in nanoscale heterostructures.With the developments of modern thin film technologies, it becomes possible to fabricate and characterize oxide superlattices (SLs) and heterostructures (HSs) down to the atomic level 1-3 . Oxide SLs and HSs can display many curious properties different from the parent bulk materials. For example, those SLs constructed by charge-polar layers and charge-neutral layers, have attracted a lot of research interests for the two-dimensional electron gas (2DEG) due to the so-called polar catastrophe [4][5][6] . Recently, theoretical simulations predicted that if ferroelectrics are involved in the SLs with charge polar discontinuity, the ferroelectricity can modulate the interfacial 2DEG, namely the 2DEG can be switched from one interface to the opposite one by flipping the ferroelectric (FE) polarization [7][8][9][10][11] .It is the common sense that ferroelectricity (or antiferroelectricity) and ferromagnetism (or antiferromagnetism) are difficult to coexist in one system because in general FE dipoles favor empty d orbitals while magnetic moments need partially-occupied d orbitals. Therefore, despite the great progress in the past decade, the magnetoelectric performance of most multiferroic bulks remains poor. An alternative route to pursuit magnetoelectricity is to utilize oxide HSs, e.g. a strong FE material plus a strong ferromagnetic (FM) material. The direct advantage of such magnetoelectric HSs is the strong FE and FM signals, however, the magnetoelectric coupling between the FE and FM layers is indirect, which is usually via the strain effect or field effect. Very recently, Rogdakis et al. obtained the FE polarization in the NdMnO 3 /SrMnO 3 /LaMnO 3 SL 12 , in which the FE polarization was related to the antiferromagnetic (AFM) order of Mn. In this sense, the NdMnO 3 /SrMnO 3 /LaMnO 3 SL is somewhat similar to the so-called type-II multiferroics, in which the FE polarizations are directly driven by magnetic orders and thus the strong magnetoelectric coupling is intrinsically guaranteed 13,14 .In this work, another kind of oxide SLs consisting of LaAlO 3 (LAO) and PbTiO 3 (PTO) is investigated to pursuit the inverse property: the polarization driven magnetism. Both LAO and PTO are well studied perov...