Enhancement of Ferromagnetism in Nonmagnetic Metal Oxide Nanoparticles by Facet Engineering

Ferromagnetism in semiconducting metal oxide nanoparticles has been intensively investigated due to their potential applications in spintronics, information storage, and biomedicine. Ferromagnetism can be produced in nonmagnetic metal oxide nanoparticles by a variety of methods or factors, but the saturated magnetization is typically of the order of 10^-4 emu g^-1 and too small to be useful in practice. In this work, it is demonstrated theoretically and experimentally that stronger ferromagnetism can be achieved in undoped nonmagnetic metal oxide semiconductors by exposing some specific polar crystal facets with carvings of special bonds via the interaction with underlying vacancies. In₂ O₃ microcubes with completely enclosed {001} polar facets show two orders of magnitude enhancement at room temperature compared to nanoparticles with an irregular morphology. The surface magnetic domains on the {001} facets account for the significantly enhanced ferromagnetism. The technique and concept described here can be extended to other types of metal oxide nanostructures to spur their application to spintronics.