Sommaire:

BiFeO3 is a multiferroic material featuring ferroelectricity and noncollinear antiferromagnetism. Dynamic and efficient control of the characteristic spin texture of BiFeO3 is attractive for emerging quantum devices. Crystal-field d → d excitations localised on Fe atomic sites in BiFeO3 induce a complex interplay among the spin, charge and lattice degrees of freedom, making them relevant for manipulation of the spin texture. In this work, ab initio methods based on the GW approximation and the Bethe-Salpeter equation are used to characterize localised spin-flip excitations within Fe-3d shell. These excitations are strongly bound and appear deep within the electronic gap. Their spin-content and strong localisation are protected by the d5 antiferromagnetic ordering. The underlying crystal symmetry gives rise to chiral spin-flip exciton states localized on distinct Fe centers which possess net angular momentum of ±h/2π. These chiral excitons couple selectively to light of a particular circular polarisation and are confined to a particular Fe magnetic sub-lattice. As a consequence, net spin-magnetisation can be achieved using circularly polarised light coupling with exciton of desired chirality, thereby modulating the antiferromagnetic texture and giving rise to transient ferrimagnetism.

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