  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 A-antiferromagnetic
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 uC/cm2, 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.
