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

The occurence of spin-polarization at ZrO$_{2}$, Al$_{2}$O$_{3}$ and MgO surfaces is proved by means of \textit{ab-initio} calculations within the density functional theory. Large spin moments, as high as 1.56 $\mu_B$, 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

Ferromagnetism in substituted zinc oxide

Room-temperature ferromagnetism is observed in (110) oriented ZnO films containing 5 at % of Sc, Ti, V, Fe, Co or Ni, but not Cr, Mn or Cu ions. There are large moments, 1.9 and 0.5 muB/atom for Co- and Ti-substituted oxides, respectively. Sc-substituted ZnO shows also a moment of 0.3 muB/Sc. Magnetization is very anisotropic, with variations of up to a factor three depending on the orientation of the applied field relative to the R-cut sapphire substrates. Results are interpreted in terms of a spin-split donor impurity band model, which can account for ferromagnetism in insulating or conducting high-k oxides with concentrations of magnetic ions that lie far below the percolation threshold. The variation of the ferromagnetism with oxygen pressure used during film growth is evidence of a link between ferromagnetism and defect concentration.Comment: 15 pages, 4 figure

Magneto-optic Kerr effect in a spin-polarized zero-moment ferrimagnet

The magneto-optical Kerr effect (MOKE) is often assumed to be proportional to the magnetisation of a magnetically ordered metallic sample; in metallic ferrimagnets with chemically distinct sublattices, such as rare-earth transition-metal alloys, it depends on the difference between the sublattice contributions. Here we show that in a highly spin polarized, fully compensated ferrimagnet, where the sublattices are chemically similar, MOKE is observed even when the net moment is strictly zero. We analyse the spectral ellipsometry and MOKE of Mn 2 Ru x Ga, and show that this behaviour is due to a highly spin-polarized conduction band dominated by one of the two manganese sublattices which creates helicity-dependent reflectivity determined by a broad Drude tail. Our findings open new prospects for studying spin dynamics in the infra-red.Comment: 7 pages, 7 figure

Contact-induced spin polarization in carbon nanotubes

Motivated by the possibility of combining spintronics with molecular structures, we investigate the conditions for the appearance of spin-polarization in low-dimensional tubular systems by contacting them to a magnetic substrate. We derive a set of general expressions describing the charge transfer between the tube and the substrate and the relative energy costs. The mean-field solution of the general expressions provides an insightful formula for the induced spin-polarization. Using a tight-binding model for the electronic structure we are able to estimate the magnitude and the stability of the induced moment. This indicates that a significant magnetic moment in carbon nanotubes can be observed.Comment: To appear in Phys. Rev. B (2003

Vanishing Fe 3d orbital moments in single-crystalline magnetite

We show detailed magnetic absorption spectroscopy results of an in situ cleaved high quality single crystal of magnetite. In addition the experimental setup was carefully optimized to reduce drift, self absorption, and offset phenomena as far as possible. In strong contradiction to recently published data, our observed orbital moments are nearly vanishing and the spin moments are quite close to the integer values proposed by theory. This very important issue supports the half metallic full spin polarized picture of magnetite.Comment: 7 pages, 4 figure

Oxygen Electromigration and Energy Band Reconstruction Induced by Electrolyte Field Effect at Oxide Interfaces

Electrolyte gating is a powerful means for tuning the carrier density and exploring the resultant modulation of novel properties on solid surfaces. However, the mechanism, especially its effect on the oxygen migration and electrostatic charging at the oxide heterostructures, is still unclear. Here we explore the electrolyte gating on oxygen-deficient interfaces between SrTiO3 (STO) crystals and LaAlO3 (LAO) overlayer through the measurements of electrical transport, X-ray absorption spectroscopy (XAS) and photoluminescence (PL) spectra. We found that oxygen vacancies (Ovac) were filled selectively and irreversibly after gating due to oxygen electromigration at the amorphous LAO/STO interface, resulting in a reconstruction of its interfacial band structure. Because of the filling of Ovac, the amorphous interface also showed an enhanced electron mobility and quantum oscillation of the conductance. Further, the filling effect could be controlled by the degree of the crystallinity of the LAO overlayer by varying the growth temperatures. Our results reveal the different effects induced by electrolyte gating, providing further clues to understand the mechanism of electrolyte gating on buried interfaces and also opening a new avenue for constructing high-mobility oxide interfaces.Comment: 5 figures; Supplementary materials included at the end of the main tex

Cinética de sorção de fósforo em alguns solos de Minas Gerais.

Os processos cineticos e termodinamicos envolvidos nos mecanismos de troca de fosforo no sistema fase solida-solucao do solo sao ainda pouco conhecidos. O proposito do presente trabalho foi investigar o comportamento dinamico do fosforo em sistemas multifasicos. Como fase solida, foram utilizadas as fracoes argila de onze amostras de duas pedossequencias do Estado de Minas Gerais: uma com litologia variando em proporcoes de rochas peliticas e de tufito; outra de arenito e de basalto. E proposto um modelo cinetico para descrever a dinamica de sorcao do fosforo. Estimaram-se as constantes de taxa de reacao (ki, i = + ou - 1, + ou - 2) de primeira ordem, para as duas primeiras etapas consecutivas do mecanismo multiplo de reacao. Na maior parte das amostras, metade do fosforo adicionado (2 g de P kg -1 de solo) foi adsorvida entre 30 e 90 minutos de reacao. Como tendencia geral, a constante de equilibrio da primeira etapa do processo foi menor do que 1, indicando que a reacao reversa (dessorcao e termodinamicamente favorecida no equilibrio (variacao da energia livre padrao de Gibbs, delta G1 > 0), exceto para uma amostra. Nao houve correlacao significativa entre k e magnetizacao, o que reflete a influencia de rocha mafica no material de origem

ZnO:Co Diluted Magnetic Semiconductor or Hybrid Nanostructure for Spintronics?

We have studied the influence of intrinsic and extrinsic defects in the magnetic and electrical transport properties of Co-doped ZnO thin films. X ray absorption measurements show that Co substitute Zn in the ZnO structure and it is in the 2+ oxidation state. Magnetization (M) measurements show that doped samples are mainly paramagnetic. From M vs. H loops measured at 5 K we found that the values of the orbital L and spin S numbers are between 1 and 1.3 for L and S = 3/2, in agreement with the representative values for isolated Co 2+. The obtained negative values of the Curie-Weiss temperatures indicate the existence of antiferromagnetic interactions between transition metal atoms.Comment: To be published in Journal of Materials Scienc