Nanoparticles*

We present a theory of the mechanisms that are responsible for the observation of enhanced ferromagnetism and the destruction of the cycloid in nanoparticles of the room temperature magnetoelectric multiferroic bismuth ferrite (BiFeO₃). We consider the role of uncompensated spins at the surface, the impact of surface-induced magnetic anisotropy, and the RKKY interaction mediated by the surface charge density in the ferroelectric nanoparticles. The competing interactions give rise to rich magnetic phase diagrams as a function of size and chemical doping, opening up many opportunities for engineering the magnetic state of bismuth ferrite.