Possible mechanisms

Yang, C.-H, J Seidel, S.Y Kim, P.B Rossen, P Yu, M Gajek, Y.H Chu, L.W Martin, M.B Holcomb, Q He, P Maksymovych, N Balke, S.V Kalinin, A.P Baddorf, S.R Basu, M.L Scullin, and Ramamoorthy Ramesh. "Electric modulation of conduction in multiferroic Ca-doped BiFeO 3 films." Nature Materials 8 (2009) 485–493.