Cobalt oxide films were prepared by oxidation of different amounts of cobalt deposited on Ir(100)-(1 × 1), where oxygen rich conditions were applied during deposition. The resulting oxide films with thicknesses of up to about 40 Å were investigated as regards their crystallographic structure and morphology, applying quantitative low energy electron diffraction (LEED) and scanning tunnelling microscopy (STM). It can be unequivocally shown that the spinel-type Co(3)O(4) phase develops, for which an excellent fit between measured and calculated LEED intensity spectra is achieved (Pendry R-factor R = 0.124). In spite of the quadratic unit cell of the substrate the oxide films are in the polar (111) orientation. Also, the native lattice parameter of the material is assumed, i.e. there is no pseudomorphic relation to the substrate. However, by means of orientational epitaxy, one of the unit-mesh vectors of the oxide and one of those of the substrate layer are aligned, leading to two mutually orthogonal domains in the oxide. The oxide is terminated by a sublayer of cobalt ions which in the bulk were tetrahedrally coordinated Co(2+) ions. There are drastic relaxations of layer spacings at and near the surface. As a consequence, the bond length between the surface terminating cobalt ions and oxygen ions below is considerably reduced, indicative of a substantial change of the ionicity of the cobalt and/or oxygen ions. This is interpreted as accounting for polarity compensation of the film, as surface reconstruction, oxygen vacancies and species adsorbed can be ruled out.
Cobalt oxide (CoO) films epitaxially grown on Ir(100) in (111) orientation were investigated by means of quantitative low-energy electron diffraction and scanning tunneling microscopy. We find with high crystallographic precision that in the bulk of the films the rocksalt structure prevails while near the surface there is a switch towards the wurtzite structure. As a consequence, nanosized CoO cannot be considered as a single structural phase. The film surfaces prove to be metallic, apparently connected with polarity compensation.
Cobalt oxides on the unreconstructed Ir(100) surface were prepared by reactive deposition of Co established by simultaneous oxygen flux at about 50 °C and subsequent annealing. The films were investigated by low-energy electron diffraction (LEED), scanning tunnelling microscopy (STM) and thermal desorption spectroscopy (TDS). We show that in spite of the quadratic unit mesh of the substrate, oxide films of (111) orientation develop. As long as oxygen-rich conditions are maintained they are of spinel-type Co(3)O(4)(111). They are non-pseudomorphic and transform to rocksalt-type CoO(111) when oxygen loss is induced by annealing at elevated temperatures. Thin films of CoO(111) are commensurate, and so, in order to realize that, they exhibit a slightly distorted unit cell when below a thickness equivalent to about seven cobalt monolayers. With increasing film thickness the uniaxial strain accompanied by the commensurability is gradually relieved by the insertion of dislocations so that eventually the film assumes ideal hexagonality. All CoO(111)-type surfaces are reconstructed at low sample temperatures equivalent to a [Formula: see text] superstructure. They reversibly transform into a (1 × 1) phase at about 50 °C.
We report on the self-organized growth of monatomic transition-metal oxide chains of (3×1) periodicity and unusual MO_{2} stoichiometry (M=Ni, Co, Fe, Mn) on Ir(100). We analyze their structural and magnetic properties by means of quantitative LEED, STM, and density functional theory (DFT) calculations. LEED analyses reveal a fascinating common atomic structure in which the transition-metal atoms sit above a missing-row structure of the surface and are coupled to the substrate only via oxygen atoms. This structure is confirmed by DFT calculations with structural parameters deviating by less than 1.7 pm. The DFT calculations predict that the NiO_{2} chains are nonmagnetic, CoO_{2} chains are ferromagnetic, while FeO_{2} and MnO_{2} are antiferromagnetic. All structures show only weak magnetic interchain coupling. Further, we demonstrate the growth of oxide chains of binary alloys of Co and Ni or Fe on Ir(100), which allows us to produce well-controlled ensembles of ferromagnetic chains of different lengths separated by nonmagnetic or antiferromagnetic segments.
We report on an unusual structural modulation of a single CoO͑111͒ bilayer grown on Ir͑100͒-͑1 ϫ 1͒ by oxidation of slightly less than one monolayer of Co deposited on the substrate. Quantitative low-energy electron diffraction and scanning tunneling microscopy in combination with standard x-ray photoelectron spectroscopy and thermal-desorption spectroscopy reveal a cobalt layer next to the substrate covered by an oxygen layer. Both layers' hexagonal atomic arrangements are, however, strongly distorted by the quadratic substrate and form a c͑10ϫ 2͒ superstructure on that. The Co layer's buckling amplitudes and atomic bond lengths to Ir atoms are consistent with the hard-sphere radius of metallic Co. The oxide's binding to the substrate appears to be further characterized by two types of oxygen ions. One of them is close to the expected rocksalt-type stacking with respect to the cobalt layer while the other type resides nearly on top of Ir atoms. Its hard-sphere radius is only 0.77 Å ͑in contrast to 1.25 Å in the CoO bulk͒ and it is by about 1 Å closer to the substrate than the other type. Being so almost coplanar with the Co layer, it locally forms a hexagonal boron-nitride-type oxide. The oxygen bond to Ir can be interpreted as local pinning of the oxide to the substrate so modulating the entire oxide bilayer.
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