Origin of ferromagnetism in BaTiO3 nanoparticles

Based on a microscopic approach we demonstrate that the unexpected ferromagnetic properties of BaTiO3 (BTO) or PTO observed recently at room temperature are due to oxygen vacancies at the surface of the nanocrystalline materials. Such vacancies lead to the appearance of Ti3+ or Ti2+ ions with nonzero net spin. The resulting different valence states composed of Ti3+ or Ti2+ offer a nonzero magnetization which decreases with increasing particle size. The system shows a multiferroic behavior below a critical size of the nanoparticles and the related polarization tends to a saturation value when the particle size is enhanced. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim