Figure caption Fig. 1 Schematic helical spin configuration. φ is the angle between the projections of spins on xy-plane and x-axis. The angle between spins and z-axis is twice of θ$_{j,m}$ = − → q · − → R$_{j,m}$/ 2 defined in text. Fig. 2 (a) Bond angle α . (b) A two-dimensional lattice with bond-bending. The solid dots and circles denote the transition metal ions and oxygen atoms respectively. The atoms are labeled so as to facilitate later deduction. Fig. 3 Polarization P$_{z}$ versus φ and φ$_{q}$ where φ$_{q}$ is the angle between − → q and x-axis. Fig. 4 A spin current along the Mn-O-Mn bond (y-component of spins moving in x-direction.) It can be coupled to the z-component of electric field and induce polarization P$_{z}$ . 22