Formation routes and structural details of the CaF1 layer on Si(111) from high-resolution noncontact atomic force microscopy data

We investigate the CaF1/Si(111) interface using a combination of high-resolution scanning tunnelling and noncontact atomic force microscopy operated at cryogenic temperature as well as x-ray photoelectron spectroscopy. Submonolayer CaF1 films grown at substrate temperatures between 550 and 600 ◦C on Si(111) surfaces reveal the existence of two island types that are distinguished by their edge topology, nucleation position, measured height, and inner defect structure. Our data suggest a growth model where the two island types are the result of two reaction pathways during CaF1 interface formation. A key difference between these two pathways is identified to arise from the excess species during the growth process, which can be either fluorine or silicon. Structural details as a result of this difference are identified by means of high-resolution noncontact atomic force microscopy and add insights into the growth mode of this heteroepitaxial insulator-on-semiconductor system

Enhanced magnetization of ultrathin NiFe2_2O4_4 films on SrTiO3_3(001) related to cation disorder and anomalous strain

NiFe$_2$O$_4$ thin films with varying thickness were grown on SrTiO$_3$(001) by reactive molecular beam epitaxy. Soft and hard x-ray photoelectron spectroscopy measurements reveal a homogeneous cation distribution throughout the whole film with stoichiometric Ni:Fe ratios of 1:2 independent of the film thickness. Low energy electron diffraction and high resolution (grazing incidence) x-ray diffraction in addition to x-ray reflectivity experiments were conducted to obtain information of the film surface and bulk structure, respectively. For ultrathin films up to 7.3 nm, lateral tensile and vertical compressive strain is observed, contradicting an adaption at the interface of NiFe$_2$O$_4$ film and substrate lattice. The applied strain is accompanied by an increased lateral defect density, which is decaying for relaxed thicker films and attributed to the growth of lateral grains. Determination of cationic site occupancies in the inverse spinel structure by analysis of site sensitive diffraction peaks reveals low tetrahedral occupancies for thin, strained NiFe$_2$O$_4$ films, resulting in partial presence of deficient rock salt like structures. These structures are assumed to be responsible for the enhanced magnetization of up to $\sim$250\% of the NiFe$_2$O$_4$ bulk magnetization as observed by superconducting quantum interference device magnetometry for ultrathin films below 7.3 nm thickness.Comment: 11 pages, 9 figure

Electronic and magnetic structure of epitaxial NiO/Fe3_3O4_4(001) heterostructures grown on MgO(001) and Nb-doped SrTiO3_3(001)

We study the underlying chemical, electronic and magnetic properties of a number of magnetite based thin films. The main focus is placed onto NiO/Fe$_3$O$_4$(001) bilayers grown on MgO(001) and Nb-SrTiO$_3$(001) substrates. We compare the results with those obtained on pure Fe$_3$O$_4$(001) thin films. It is found that the magnetite layers are oxidized and Fe$^{3+}$ dominates at the surfaces due to maghemite ($\gamma$-Fe$_2$O$_3$) formation, which decreases with increasing magnetite layer thickness. From a layer thickness of around 20 nm on the cationic distribution is close to that of stoichiometric Fe$_3$O$_4$. At the interface between NiO and Fe$_3$O$_4$ we find the Ni to be in a divalent valence state, with unambiguous spectral features in the Ni 2p core level x-ray photoelectron spectra typical for NiO. The formation of a significant NiFe$_2$O$_4$ interlayer can be excluded by means of XMCD. Magneto optical Kerr effect measurements reveal significant higher coercive fields compared to magnetite thin films grown on MgO(001), and a 45$^{\circ}$ rotated magnetic easy axis. We discuss the spin magnetic moments of the magnetite layers and find that the moment increases with increasing thin film thickness. At low thickness the NiO/Fe$_3$O$_4$ films grown on Nb-SrTiO$_3$ exhibits a significantly decreased spin magnetic moments. A thickness of 20 nm or above leads to spin magnetic moments close to that of bulk magnetite

Post deposition annealing of epitaxial Ce1-xPrxO2-δ films grown on Si(111)

In this work the structural and morphological changes of Ce1−xPrxO2−δ (x = 0.20, 0.35 and 0.75) films grown on Si(111) due to post deposition annealing are investigated by low energy electron diffraction combined with a spot profile analysis. The surface of the oxide films exhibit mosaics with large terraces separated by monoatomic steps. It is shown that the Ce/Pr ratio and post deposition annealing temperature can be used to tune the mosaic spread, terrace size and step height of the grains. The morphological changes are accompanied by a phase transition from a fluorite type lattice to a bixbyite structure. Furthermore, at high PDA temperatures a silicate formation via a polycrystalline intermediate state is observed

Step and kink correlations on vicinal Ge(100) surfaces investigated by electron diffraction

Using spot profile analysis in low-energy electron diffraction, we have investigated vicinal Ge(100) surfaces, which were miscut by 2.7° and 5.4°, respectively, in [011] direction with respect to the surface normal. Within the kinematic approximation the morphology was evaluated quantitatively both perpendicular and parallel to the step edge direction. In contrast to vicinal Si(100) surfaces with similar miscut angles, the Ge(100) surfaces still show an alternating configuration of (2×1) and (1×2) reconstructed (100) terraces, which are separated by steps of single atomic height. From the spot profiles and their energy dependence we extracted the morphological parameters such as the average terrace width, the variance of the terrace size distribution, and the average kink separation. Furthermore, step energies on the vicinal Ge(100) surfaces were estimated. These turn out to be significantly lower than for Si(100) and lead to the formation of the observed double domain structure. © 2002 The American Physical SocietyDFGK+S Grupp

Determination of step--edge barriers to interlayer transport from surface morphology during the initial stages of homoepitaxial growth

We use analytic formulae obtained from a simple model of crystal growth by molecular--beam epitaxy to determine step--edge barriers to interlayer transport. The method is based on information about the surface morphology at the onset of nucleation on top of first--layer islands in the submonolayer coverage regime of homoepitaxial growth. The formulae are tested using kinetic Monte Carlo simulations of a solid--on--solid model and applied to estimate step--edge barriers from scanning--tunneling microscopy data on initial stages of Fe(001), Pt(111), and Ag(111) homoepitaxy.Comment: 4 pages, a Postscript file, uuencoded and compressed. Physical Review B, Rapid Communications, in press

Particle currents and the distribution of terrace sizes in unstable epitaxial growth

A solid-on-solid model of epitaxial growth in 1+1 dimensions is investigated in which slope dependent upward and downward particle currents compete on the surface. The microscopic mechanisms which give rise to these currents are the smoothening incorporation of particles upon deposition and an Ehrlich-Schwoebel barrier which hinders inter-layer transport at step edges. We calculate the distribution of terrace sizes and the resulting currents on a stepped surface with a given inclination angle. The cancellation of the competing effects leads to the selection of a stable magic slope. Simulation results are in very good agreement with the theoretical findings.Comment: 4 pages, including 3 figure

Role of NiO in the nonlocal spin transport through thin NiO films on Y3Fe5 O12

In spin-transport experiments with spin currents propagating through an antiferromagnetic (AFM) material, the antiferromagnet is mainly treated as a passive spin conductor not generating nor adding any spin current to the system. The spin current transmissivity of the AFM NiO is affected by magnetic fluctuations, peaking at the Néel temperature and decreasing by lowering the temperature. To study the role of antiferromagnetism in local and nonlocal spin-transport experiments, we send spin currents through NiO of various thicknesses placed on Y3Fe5O12. The spin currents are injected either electrically or by thermal gradients and measured at a wide range of temperatures and magnetic field strengths. The transmissive role is reflected in the sign change of the local electrically injected signals and the decrease in signal strength of all other signals by lowering the temperature. The thermally generated signals, however, show an additional upturn below 100K that is unaffected by an increased NiO thickness. A change in the thermal conductivity could affect these signals. The temperature and magnetic field dependence are similar to those for bulk NiO, indicating that NiO itself contributes to thermally induced spin currents