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Register a new userReduction of charge fluctuation energies in ultrathin NiO films on Ag(001)
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We investigate on-site Coulomb interaction energy between two 3p holes U(Ni 3p) of ultrathin NiO films on Ag(001) by both x-ray photoelectron spectroscopy and Auger electron spectroscopy. As the film becomes thin, U(Ni 3p) monotonically decreases, and the difference of U(Ni 3p) for 1 monolayer (ML) film from that of bulk-like thick film delta U(Ni 3p) reaches ~ -2.2 eV. The observed delta U(Ni 3p) for 1 ML film is well reproduced by the differences of both the image potential and polarization energies between 1 ML film and the bulk-like thick film. Hence, the present results provide an evidence for the picture originally proposed by Duffy et al. [J. Phys. C: Solid State Phys., 16, 4087 (1983)] and Altieri et al. [Phys. Rev. B 59, R2517 (1999)]
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The phase immiscibility and the excellent matching between Ag(001) and Fe(001) unit cells (mismatch 0.8 %) make Fe/Ag growth attractive in the field of low dimensionality magnetic systems. Intermixing could be drastically limited at deposition temperatures as low as 140-150 K. The film structural evolution induced by post-growth annealing presents many interesting aspects involving activated atomic exchange processes and affecting magnetic properties. Previous experiments, of He and low energy ion scattering on films deposited at 150 K, indicated the formation of a segregated Ag layer upon annealing at 550 K. Higher temperatures led to the embedding of Fe into the Ag matrix. In those experiments, information on sub-surface layers was attained by techniques mainly sensitive to the topmost layer. Here, systematic PED measurements, providing chemical selectivity and structural information for a depth of several layers, have been accompanied with a few XRD rod scans, yielding a better sensitivity to the buried interface and to the film long range order. The results of this paper allow a comparison with recent models enlightening the dissolution paths of an ultra thin metal film into a different metal, when both subsurface migration of the deposit and phase separation between substrate and deposit are favoured. The occurrence of a surfactant-like stage, in which a single layer of Ag covers the Fe film is demonstrated for films of 4-6 ML heated at 500-550 K. Evidence of a stage characterized by the formation of two Ag capping layers is also reported. As the annealing temperature was increased beyond 700 K, the surface layers closely resembled the structure of bare Ag(001) with the residual presence of subsurface Fe aggregates.
Cobalt ferrite ultrathin films with inverse spinel structure are among the best candidates for spin-filtering at room temperature. We have fabricated high-quality epitaxial ultrathin CoFe2O4 layers on Ag(001) following a three-step method: an ultrathin metallic CoFe2 alloy was first grown in coherent epitaxy on the substrate, and then treated twice with O2, first at RT and then during annealing. The epitaxial orientation, the surface, interface and film structure were resolved combining LEED, STM, Auger and in situ GIXRD. A slight tetragonal distortion was observed, that should drive the easy magnetization axis in plane due to the large magneto-elastic coupling of such a material. The so-called inversion parameter, i.e. the Co fraction occupying octahedral sites in the ferrite spinel structure, is a key element for its spin-dependent electronic gap. It was obtained through in-situ x-ray resonant diffraction measurements collected at both the Co and Fe K edges. The data analysis was performed using the FDMNES code and showed that Co ions are predominantly located at octahedral sites with an inversion parameter of 0.88 +- 0.05. Ex-situ XPS gave an estimation in accordance with the values obtained through diffraction analysis.
The nature of the magnetic transition of the half-filled triangular antiferromagnet Ag$_{2}$NiO$_2$ with $T_{rm N}$=56K was studied with positive muon-spin-rotation and relaxation ($mu^+$SR) spectroscopy. Zero field $mu^+$SR measurements indicate the existence of a static internal magnetic field at temperatures below $T_{rm N}$. Two components with slightly different precession frequencies and wide internal-field distributions suggest the formation of an incommensurate antiferromagnetic order below 56 K. This implies that the antifrerromagnetic interaction is predominant in the NiO$_2$ plane in contrast to the case of the related compound NaNiO$_2$. An additional transition was found at $sim$22 K by both $mu^+$SR and susceptibility measurements. It was also clarified that the transition at $sim$260 K observed in the susceptibility of Ag$_{2}$NiO$_{2}$ is induced by a purely structural transition.
Phonon dispersion of detwinned NiO is measured using inelastic x-ray scattering. It is found that, near the zone center, the energy of the transverse optical phonon mode polarized parallel to the antiferromagnetic order is ~1 meV lower than that of the mode polarized perpendicular to the order, at room temperature. This is explained via anisotropic polarization of the Ni and O atoms, as confirmed using a Berrys phase approach with first-principles calculations. Our explanation avoids an apparent contradiction in previous discussions focusing on Heisenberg interaction.
We explore the interplay of electron-electron correlations and surface effects in the prototypical correlated insulating material, NiO. In particular, we compute the electronic structure, magnetic properties, and surface energies of the $(001)$ and $(110)$ surfaces of paramagnetic NiO using a fully charge self-consistent DFT+DMFT method. Our results reveal a complex interplay between electronic correlations and surface effects in NiO, with the electronic structure of the $(001)$ and $(110)$ NiO surfaces being significantly different from that in bulk NiO. We obtain a sizeable reduction of the band gap at the surface of NiO, which is most significant for the $(110)$ NiO surface. This suggests a higher catalytic activity of the $(110)$ NiO surface than that of the $(001)$ NiO one. Our results reveal a charge-transfer character of the $(001)$ and $(110)$ surfaces of NiO. Most notably, for the $(110)$ NiO surface we observe a remarkable electronic state characterized by an alternating charge-transfer and Mott-Hubbard character of the band gap in the surface and subsurface NiO layers, respectively. This novel form of electronic order stabilized by strong correlations is not driven by lattice reconstructions but of purely electronic origin. We notice the importance of orbital-differentiation of the Ni $e_g$ states to characterize the Mott-Hubbard insulating state of the $(001)$ and $(110)$ NiO surfaces. The unoccupied Ni $e_g$ surface states are seen to split from the lower edge of the conduction band to form strongly localized states in the fundamental gap of bulk NiO. Our results for the surface energies of the $(001)$ and $(110)$ NiO surfaces show that the $(001)$ facet of NiO has significantly lower energy. This implies that the relative stability of different surfaces, at least from a purely energetic point of view, does not depend on the presence or absence of magnetic order in NiO.
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