Modulation of magnetization in BiFeO₃ using circularly polarized light

BiFeO₃ is a multiferroic material featuring ferroelectricity and noncollinear antiferromagnetism, the latter manifested as a cycloid of spin density. Definitive and efficient control of the characteristic spin texture of BiFeO₃ is attractive for emerging quantum devices. In this regard, crystal-field d→ d excitations localized on Fe atomic sites in BiFeO₃ provide an avenue for manipulation of the spin texture as they induce a complex interplay among the spin, charge and lattice degrees of freedom. In this work, the ab initio GW-BSE method is used to characterize these excitations within the exciton picture. We find that the $d-d$ transitions appear as strongly bound chiral spin-flip excitons deep within the electronic band gap as a result of the intricate competition between the lattice potential, the antiferromagnetic ordering, the spin-orbit coupling, and the electron-hole interaction. Most crucially, these excitons are composed of electron-hole pairs with opposite spins constituting their ± angular momentum. Excitons with a specific angular momentum can be selectively excited using circularly polarized light, consequently modulating the local magnetic moment to give rise to transient ferrimagnetism.