Theory of spin-orbit enhanced electric-field control of magnetism in multiferroic BiFeO$_3$*

We present a microscopic theory that shows the importance of spin-orbit coupling in multiferroic compounds with heavy ions. In BiFeO$_3$ (BFO) the spin-orbit coupling at the bismuth ion sites results in a special kind of magnetic anisotropy that is linear in the applied $E$-field. We show how this interaction is capable of disrupting the magnetic cycloid state of bulk BFO, leading to a remarkable level of $E$-field control of magnetism. R. de Sousa, M. Allen, and M. Cazayous, arXiv:1209.6612.