A. Mascaraque

Magnetism in nanometer-thick magnetite

The oldest known magnetic material, magnetite, is of current interest for use in spintronics as a thin film. An open question is how thin can magnetite films be and still retain the robust ferrimagnetism required for many applications. We have grown 1-nm-thick magnetite crystals and characterized them in situ by electron and photoelectron microscopies including selected-area x-ray circular dichroism. Well-defined magnetic patterns are observed in individual nanocrystals up to at least 520 K, establishing the retention of ferrimagnetism in magnetite two unit cells thick.

FeSi(111) interface formation studied by photoelectron diffraction

Abstract The early stages of the interface formation of Fe deposited on Si(111)-(7 × 7) have been investigated as a function of Fe coverage and annealing temperature using the photoelectron diffraction technique. The intensity variation of the Si 2p and Fe 3p core levels was monitored in a full emission cone of a 60° polar angle and a 360° azimuthal angle. Annealing of 1 ML Fe to 150°C produces a well-ordered interface. From a detailed analysis of the intensity modulations of both core levels as a function of emission angles, we deduce that Fe atoms occupy substitutional positions underneath a top silicon layer.

Reversible structural phase transitions in semiconductor and metal/semiconductor surfaces

We review our current understanding of the reversible phase transitions found in clean semiconductor and metal/semiconductor surfaces. The most important phase transitions are considered in detail, in particular those appearing in Si(001), Ge(001), Si(111), Ge(111), 3C-SiC(001), Pb and Sn on Si(111) and Ge(111), Au/Si(111) and Ag/Si(111). Special emphasis is placed on recent experiments and theoretical models, as well as on open or controversial aspects of these interesting surface systems.

Electronic instabilities of the two-dimensional Sn/Ge(111) α-phase

The Sn/Ge(111) interface presents at room temperature an ordered surface reconstruction, formed by a submonolayer array of tin adatoms, which reversibly transforms upon cooling into a 3×3 structure. Recently, it has been claimed that the formation of a commensurate charge density wave is responsible for this transition. We present high-resolution synchrotron radiation photoemission results which support, instead, that the driving force of the transition is rehybridization: fluctuations in the tin adatom type of bonding freeze-in at low temperature to yield the static long-range ordering of the 3×3 ground state which is governed by an ideal 1/2 ratio of the sp3/sp2 hybridization balance.

Oxygen interaction with Si(100) and KSi(100)

The earliest stages of the oxygen interaction with clean and K-covered Si(100) surfaces (8K<0.5 ML) have been investigated using high-resolution core-level photoemission. The sequence of oxidation has been analyzed monitoring the different components of the Si 2p core level. In the case of the clean surface, the contribution coming from the upper dimer atoms is unaffected at the earliest stages of the oxidation. On the contrary, in the presence of K atoms, oxygen reacts rapidly with this type of silicon atoms, and the component is fully depleted after oxygen adsorption. The presence of K atoms induces also the formation of a new oxide component in the Si 2p core level. Its small binding energy shift supports that a fraction of oxygen atoms are bonded at sites modified by the presence of K, forming a K-Si-O complex.

Hydrogen-induced reversible spin-reorientation transition and magnetic stripe domain phase in bilayer Co on Ru(0001)

Imaging the change in the magnetization vector in real time by spin-polarized low-energy electron microscopy, we observed a hydrogen-induced, reversible spin-reorientation transition in a cobalt bilayer on Ru(0001). Initially, hydrogen sorption reduces the size of out-of-plane magnetic domains and leads to the formation of a magnetic stripe domain pattern, which can be understood as a consequence of reducing the out-of-plane magnetic anisotropy. Further hydrogen sorption induces a transition to an in-plane easy axis. Desorbing the hydrogen by heating the film to 400 K recovers the original out-of-plane magnetization. By means of ab initio calculations we determine that the origin of the transition is the local effect of the hybridization of the hydrogen orbital and the orbitals of the Co atoms bonded to the absorbed hydrogen.

Short wavelength, spin-polarized quantum-well states in high quality Cu films on FCC-Co(100)

Abstract The electronic structure of ultrathin Cu films deposited on top of an FCC-Co(1xa00xa00) film has been studied by angle- and spin-resolved photoemission. Deposition at liquid-nitrogen temperature allows the formation of sharp interfaces that produces well-defined quantum-well states in the Cu valence band. Furthermore, short-wavelength quantum-well states originating from the neck of the Cu Fermi surface are only visible after low-temperature deposition. Spin-resolved spectra show that these states, which are predicted to dominate oscillatory magnetic coupling in multilayers, exhibit minority spin character.

Fermi surface of a triangular lattice overlayer: Pb/Ge(111) α-phase

Abstract The α-(√3×√3)R30° phase of the Pb/Ge(111) interface has been extensively studied using angle-resolved photoemission (ARUPS). The contours of the Fermi surface have been measured by two-dimensional ARUPS mapping. The topology of the Fermi surface is characterized by an undulated shape, which resembles the results of theoretical calculations. The experimental Fermi momentum along the ΓK and ΓM direction is 0.30±0.03 and 0.40±0.03 A−1, respectively. These values agree with the wave vector needed for a perfect 3×3 nesting. However, the Fermi surface topology exhibits no large flat areas suitable for electronic nesting, as required by a charge density wave stabilisation.

Probing unoccupied bulk bands via the cross section of quantum well states in thin films

Abstract The photoemission cross section of quantum well states in thin Cu(1xa00xa00) films on Co(1xa00xa00) has been analyzed in normal emission as a function of photon energy. Assuming conservation of the perpendicular wave vector k ⊥ , the cross section maxima can be assigned to direct transitions from the (initial) quantum well state to bulk-like final states. The value of k ⊥ is obtained from the sharp k ⊥ distribution of specific quantum well states, which is centered about discretized k ⊥ values. Our data are compared with free-electron-like models and a low energy electron diffraction calculation of high-energy states in bulk Cu.

Electronic structure and reactivity of the Co/MoS2(0001) interface

Abstract The electronic structure and the surface reactivity of Co deposited on MoS 2 (0 0 0 1) has been investigated using angle-resolved photoemission spectroscopy. After depositing ∼1 ML Co, the surface becomes metallic. The interface is non-reactive at room temperature. Besides the observation of an electronic band due to Co d electrons, the only significant modification of the surface electronic structure after Co deposition, is an overall shift of ∼0.7 eV due to the formation of a Schottky barrier. The interface was annealed to increasingly higher temperatures. It remains unreactive up to 800 K. Above this temperature, the Co-induced intensity diminishes, and further changes are detected in the valence band, signaling partial reaction with the substrate.