Magnetism and half-metallicity at the O surfaces of ceramic oxides

Abstract

The occurence of spin-polarization at ZrO2, Al2O3 and MgO surfaces is proved by means of abinitio calculations within the density functional theory. Large spin moments, as high as 1.56 µB, develop at O-ended polar terminations, transforming the non-magnetic insulator into a half-metal. The magnetic moments mainly reside in the surface oxygen atoms and their origin is related to the existence of 2p holes of well-defined spin polarization at the valence band of the ionic oxide. The direct relation between magnetization and local loss of donor charge makes possible to extend the magnetization mechanism beyond surface properties.PACS numbers: 75.70. Rf, 73.20.At, When dimensions are reduced to the nanoscale, we are faced to a new understanding of the physical properties of matter: bulk insulators and semiconductors exhibit metallic surfaces [1], non-magnetic materials get spin polarization when forming nanoparticles [2], unstable bulk structures exist in ultrathin film form [3,4], etc. At the origin of these phenomena are the reduced dimensionality and the enhanced role of the surfaces or boundaries in the final properties of the system. Together with its inherent fundamental interest, this has important technological consequences, auspicating the birth of new technologies [5,6]. Special attention is devoted to magnetic low dimensional structures, in particular as sources of spin current in the emerging field of spintronics [5].In this letter, we report on the existence of large magnetic moments and half-metallicity at the O-rich surfaces of ceramic oxides, focusing on ZrO 2 , Al 2 O 3 and MgO. These are non-magnetic ionic insulators widely applied in bulk and thick film form, that have also been grown as ultrathin films and nanometric grains [7]. Their electronic structure can be roughly described as a valence band formed by the filled O 2p orbitals and a conduction band formed by the empty metal levels. When a M x O y unit -M being the metal donor and x,y accounting for the particular metal to oxygen ratio-is broken to form the surface, the loss of coordination of the surface O atoms originates 2p holes in the valence band of the oxide. Our results show that this generates high spin moments at the topmost O layer, which induce magnetization at the adjacent planes and, remarkably, alter the electronic structure of the oxide from insulating to half-metallic.Very recently unexpected ferromagnetism has been measured in thin films of undoped non-magnetic oxides, like HfO 2 and ZrO 2 , possibly assigned to the presence of lattice defects concentrated at the film interface. [8,9]. Here we prove the existence of a magnetization mechanism rooted in the loss of donor charge of the O atoms, something that can also occur in films with cation vacancies. This provides an explanation for the origin of the magnetic moments of these so called 'd-zero' ferromagnets. Furthermore, we predict that this kind of magnetism can be extended to a wider class of non-magnetic oxides, like MgO and Al 2 O 3 , where the cation is not necessarily...

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