Fe monolayers on InAs(0 0 1): An in situ study of surface, interface and volume magnetic anisotropy

The magnetic anisotropy of epitaxial Fe films with thicknesses in the range of 2-142 monolayers (ML) grown on {4×2} reconstructed InAs(0 0 1) was investigated by in situ ferromagnetic resonance. The easy magnetization direction was found to be parallel to the -direction for Fe films below 4 ML, while it rotates by 45^° toward the -direction. It is observed that both surface-interface and volume contribution to the perpendicular anisotropy favor an easy axis perpendicular to the film plane. The cubic surface-interface anisotropy is relatively large with easy axes along -directions in contrast to the volume contribution which favors easy axes along the -directions. The volume contribution is found to be larger than the Fe bulk cubic anisotropy. A thickness independent uniaxial anisotropy has been found in films with a thickness of 2 up to 142 ML.     

Perpendicular magnetic anisotropy of V/Co(0 0 1)

Based on the full-potential linearized augmented plane wave (FLAPW) calculations, various magnetic properties of ultra thin face centered cubic (fcc) Co(0 0 1) film and V adsorbed systems on Co(0 0 1) surface are explored. It was found that the V film grown on fcc Co(0 0 1) surface has large induced magnetic moment and the direction of magnetization is antiparallel to that of Co atom in the submonolayer coverage. Very interestingly, we found that the surface alloy and 0.5 ML adsorbed V/Co(0 0 1) systems have perpendicular magnetocrystalline anisotropy and the magnitude of anisotropy energy in 0.5 ML V on fcc Co(0 0 1) surface is greatly larger than that of surface alloy, while we observed in-plane magnetization in pure fcc Co(0 0 1) film. It was found that the spin-orbit interaction through spin-flip process cannot be ignored, therefore the simple relation with orbital anisotropy is not applicable in the interpretation of magnetocrystalline anisotropy.     

Growth mode, magnetic and magneto-optical properties of pulsed-laser-deposited Au/Co/Au(1 1 1) trilayers

The growth mode, magnetic and magneto-optical properties of epitaxial Au/Co/Au(1 1 1) ultrathin trilayers grown by pulsed-laser deposition (PLD) under ultra-high vacuum are presented. Sapphire wafers buffered with a single-crystalline Mo(1 1 0) buffer layer were used as substrates. Owing to PLD-induced interfacial intermixing at the lower Co/Au(1 1 1) interface, a close-to layer-by-layer growth mode is promoted. Surprisingly, despite this intermixing, ferromagnetic behavior is found at room temperature for coverings starting at 1 atomic layer (AL). The films display perpendicular magnetization with anisotropy constants reduced by 50% compared to TD-grown or electrodeposited films, and with a coercivity more than one order of magnitude lower . The magneto-optical (MO) response in the low Co thickness range is dominated by Au/Co interface contributions. For thicknesses starting at 3 AL Co, the MO response has a linear dependence with the Co thickness, indicative of a continuous-film-like MO behavior.     

First principles studies of Fe/Co superlattices and multilayers with bcc (0 0 1) and (1 1 0) orientations

The layer resolved magnetic moments and magnetic anisotropy energy of Fe/Co superlattices and multilayers with bcc (0 0 1) and (1 1 0) orientations obtained from first principles simulations are reported here. The magnetic moment of Fe atoms are found to depend on the geometry, coordination number and proximity to Co atoms, whereas that of Co remains almost constant in the superlattices and multilayers. Mixing of atoms at the interface resulted in enhanced Fe magnetic moment while that of Co is unaffected. The magnetic anisotropy energy in superlattices and multilayers are found to be larger than the corresponding values of bulk counterparts. Calculated easy axis of magnetization is in the plane for all superlattice compositions considered in the study, while that in multilayers, changes with crystalline orientation and thickness of Co layers.     

Spin wave excitations in Fe films grown on GaAs(1 1 3)A substrates

The spin wave excitation and its size effect has been studied in Al-capped Fe films grown on low-symmetry GaAs(1 1 3)A substrates. The temperature dependence of saturation magnetization follows an effective Bloch's law as long as magnetization remains larger than about 70% of its saturation value. A significant increase of the spin wave parameter B is found in Al-capped ultrathin Fe films grown on GaAs(1 1 3)A compared to bulk Fe, Fe films on GaAs(0 0 1) and other systems. This is explained as a result of the reduction in uniaxial magnetic anisotropy observed in this orientation for the same thickness range. However, this observed uniaxial magnetic anisotropy is found to be a likely reason for stabilizing the ferromagnetism.     

Strain state in bcc Fe films grown on Si(1 1 1)

The strain state of Fe films grown on Si(1 1 1) has been investigated by X-ray diffraction (XRD) in the thickness range between 11 and 304 monolayers. Fe grows tetragonally distorted with the orientation relationship Fe(1 1 1) // Si(1 1 1) . At low coverage, the films grow pseudomorphic. Above 15 monolayers the films are characterized by the coexistence of a pseudomorphic phase with another one which relaxes with the Fe thickness. This relaxation proceeds rapidly in the earlier stages then slowly with the film thickness. The XRD characterization allows one to obtain quantitative information on the in-plane and out-of-plane strains.     

Ultra-thin films and surface alloying of Pd on Cu(1 1 1) investigated by medium energy ion scattering

The structure of Pd films on Cu(1 1 1) and the alloying between the films and the substrate have been investigated by medium energy ion scattering (MEIS) using 100 keV H⁺ ions. Data are presented for the and alignments (nominal one- and three-layer alignments, respectively). It is found that beyond 1 ML the Pd grows in a twinned fcc structure, the incommensurate nature of which increases the visibility of the Cu(1 1 1) substrate to MEIS. Deposition of 0.2 ML of Pd produces a structure in which Pd mostly occupies the top two layers which have interlayer distances d₁₂ = 208 ± 4 pm and d₂₃ = 211 ± 4 pm. Some twinning is also present in this structure. Upon annealing 1.6 ML of Pd to 600 K for 1 min, the copper and palladium interdiffuse leaving around 0.4 ML of visible palladium. Energy plots show that there are several layers with an altered structure present over at least part of the surface. This may be due to large scale interdiffusion or alloy island formation. Incremental annealing to successively higher temperatures shows that the structural transformation begins around 500 K.     

Surface phase diagram and temperature induced phase transitions of Sn/Cu(1 0 0)

Room temperature deposition of Sn on Cu(1 0 0) gives rise to a rich variety of surface reconstructions in the submonolayer coverage range. In this work, we report a detailed investigation on the phases appearing and their temperature stability range by using low-energy electron diffraction and surface X-ray diffraction. Previously reported reconstructions in the submonolayer range are p(2 × 2) (for 0.2 ML), p(2 × 6) (for 0.33 ML), ()R45° (for 0.5 ML), and c(4 × 4) (for 0.65 ML). We find a new phase with a structure for a coverage of 0.45 ML. Furthermore, we analyze the temperature stability of all phases. We find that two phases exhibit a temperature induced reversible phase transition: the ()R45° phase becomes ()R45° phase above 360 K, and the new phase becomes p(2 × 2) also above 360 K. The origin of these two-phase transitions is discussed.     

Structural and magnetic properties of evaporated Fe thin films on Si(1 1 1), Si(1 0 0) and glass substrates

We present experimental results on the structural and magnetic properties of series of Fe thin films evaporated onto Si(1 1 1), Si(1 0 0) and glass substrates. The Fe thickness, t, ranges from 6 to110 nm. X-ray diffraction (XRD) and atomic force microscopy (AFM) have been used to study the structure and surface morphology of these films. The magnetic properties were investigated by means of the Brillouin light scattering (BLS) and magnetic force microscopy (MFM) techniques. The Fe films grow with (1 1 0) texture; as t increases, this (1 1 0) texture becomes weaker for Fe/Si, while for Fe/glass, the texture changes from (1 1 0) to (2 1 1). Grains are larger in Fe/Si than in Fe/glass. The effective magnetization, 4πM_eff, inferred from BLS was found to be lower than the 4πM_S bulk value. Stress induced anisotropy might be in part responsible for this difference. MFM images reveal stripe domain structure for the 110 nm thick Fe/Si(1 0 0) only.     

Mössbauer spectroscopy and magnetic properties in thin films of FexNi100−x electroplated on silicon (1 0 0)

Fe_xNi_100−x thin films were produced by galvanostatic electrodeposition on Si (1 0 0), nominal thickness 2800 nm, and x ranging 7-20. The crystalline structure of the sample was determined by X-ray diffraction (XRD). The magnetic properties were investigated by vibration sample magnetometry (VSM) and room temperature ⁵⁷Fe Mössbauer spectroscopy. Conversion Electron Mössbauer spectroscopy (CEMS) in both film surfaces for the thick self-supported films showed that the magnetic moment direction is in the plane and conventional transmission (MS) that the directions are out of the plane films. The results were interpreted assuming a three-layer model where the external layer has in-plane magnetization and the internal one, out of plane magnetization.     

GaN/LiNbO3 (0 0 0 1) interface formation calculated from first-principles

The stable adsorption sites for both Ga and N ions on the ideal and on the reconstructed LiNbO₃ (0 0 0 1) surface are determined by means of first-principle total energy calculations. A single N layer is found to be more strongly bound to the substrate than a single Ga layer. The adsorption of a GaN monolayer on the polar substrate within different orientations is then modeled. On the basis of our results, we propose a microscopic model for the GaN/LiNbO₃ interface. The GaN and LiNbO₃ (0 0 0 1) planes are parallel, but rotated by 30° each other, with in-plane epitaxial relationship [1 0 0]_GaN‖ [1 1  0]_LiNbO3. In this way the (0 0 0 1) plane lattice mismatch between GaN and LiNbO₃ is minimal and equal to 6.9% of the GaN lattice constant. The adsorbed GaN and the underlying LiNbO₃ substrate have parallel c-axes.     

The plasma oxidation of Pd(1 0 0)

The oxidation of Pd(1 0 0) by an oxygen plasma was characterized using X-ray photoelectron spectroscopy (XPS), low energy ion scattering spectroscopy (ISS), temperature programmed desorption (TPD), and low energy electron diffraction (LEED). The oxygen uptake followed a typical parabolic profile with oxygen coverages reaching 32 ML after 1 h in the plasma; a factor of 40 higher than could be achieved by dosing molecular oxidants in ultra high vacuum. Even after adsorbing 32 ML of oxygen, XPS revealed both metallic Pd and PdO in the surface region. The R27^o LEED pattern previously attributed to a surface oxide monolayer, slowly attenuated with oxygen coverage indicating that the PdO formed poorly ordered three dimensional clusters that slowly covered the ordered surface oxide. While XPS revealed the formation of bulk PdO, only small changes in the ISS spectra were observed once the surface oxide layer was completed. The leading edges of the O₂ TPD curves showed only small shifts with increasing oxygen coverage that could be explained in terms of the lower thermodynamic stability of small oxide clusters. The desorption curves, however, could not be adequately described as simple zero order decomposition of PdO. There has been an ongoing debate in the literature about the relative catalytic activities of PdO and oxygen phases on Pd, the results indicate that any differences in the reactivity between bulk PdO and surface oxides are not associated with differences in the density of exposed Pd atoms or the decomposition kinetics of these two phases.     

Surface alloying and anomalous diffusion of Bi on Cu(1 1 1)

We have used Low-Energy Electron Microscopy (LEEM) to study the formation and structure of the surface alloy of Bi on the Cu(1 1 1) surface. As a function of Bi coverage we find a progression from a dilute surface alloy to a structure, which coexists with a Bi overlayer when the Bi coverage exceeds 1/3 ML. At a temperature of 401 K, an unusual formation of alloy domains is observed. The origin of this effect is traced down to the mobility of Bi adatoms, which is found to strongly depend on the Bi coverage of the surface alloy on which they reside.     

Co-induced nano-structures on Si(1 1 1) surface

The interaction of cobalt atoms with silicon (1 1 1) surface has been investigated by means of scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED). Besides the Co silicide islands, we have successfully distinguished two inequivalent Co-induced reconstructions on Si(1 1 1) surface. Our high-resolution STM images provide some structural properties of the two different derived phases. Both of the two phases seem to form islands with single domain. The new findings will help us to understand the early stage of Co silicide formations.     

CO oxidation on Pd(1 1 1), Pt(1 1 1), and Rh(1 1 1) surfaces studied by infrared chemiluminescence spectroscopy

The infrared (IR) chemiluminescence studies of CO₂ formed during steady-state CO oxidation over Pd(1 1 1), Pt(1 1 1), and Rh(1 1 1) surfaces were carried out. Analysis of their emission spectra indicates that the order of the average vibrational temperature () values of CO₂ formed during CO oxidation was as follows: Pd(1 1 1) > Pt(1 1 1) > Rh(1 1 1), and the order is coincident with the potential energy in the transition state expected by the theoretical calculations. Furthermore, it is suggested that the bending vibrational temperature () can also be influenced by the angle of O-C-O (∠_OCO) of the activated complex in the transition state, which has also been proposed by the theoretical calculations.     

Photoelectron spectroscopy study of Pb/Ag(1 1 1) in the submonolayer range

The vacuum deposition of Pb onto Ag(1 1 1) gives rise to two different surface structures depending on coverage and deposition temperature. At room temperature (RT), low energy electron diffraction (LEED) reveals a sharp reconstruction completed at 1/3 Pb monolayer (ML). Beyond, a close-packed Pb(1 1 1) incommensurate overlayer develops. At low temperature (LT, ∼100 K) the incommensurate structure is directly observed whatever the coverage, corresponding to the growth of close-packed two-dimensional Pb(1 1 1) islands. Synchrotron radiation Pb 5d core-level spectra clearly demonstrate that in each surface structure all Pb atoms have essentially a unique, but different, environment. This reflects the surface alloy formation between the two immiscible metals in the reconstruction and a clear signature of the de-alloying process at RT beyond 1/3 ML coverage.