R. F. Willis

Finite‐size scaling behavior of ferromagnetic thin films

We have used molecular‐beam epitaxy to grow high‐quality pseudomorphic Ni and Co1Ni9 films on Cu(001). From temperature‐dependent surface magneto‐optic Kerr effect measurements of these films, we have determined the finite‐size scaling behavior of the Curie temperature of ultrathin films for a thickness range of n=2.5–16 monolayers (ML). The film thickness dependent Curie temperature for each of these ferromagnetic thin‐film systems, TC(n), is described by a finite‐size scaling formula: [TC(∞) − TC(n)]/TC(n) = [(n − n’)/n0]−1/ν, where TC(∞) is the bulk Curie temperature, n0=2.5±0.5 ML for Co films and 3.5±0.4 ML for Ni and Co1Ni9 films is the microscopic length scale, and ν=0.76±0.08 is the bulk correlation length exponent. An interesting result is that TC(n) extrapolates to zero in the single mononolayer limit, n’=1.

Plasmon-phonon strongly coupled mode in epitaxial graphene

We report the dispersion measurements, using angle-resolved reflection electron-energy-loss spectroscopy, on two-dimensional (2D) plasmons in single and multilayer graphene which couple strongly to surface optical phonon (FK-phonon) modes of silicon carbide substrate. The coupled modes show discrete dispersion behaviors in the single and bilayer graphene. With increasing graphene layers on SiC(0001), a transition from plasmonlike dispersion to phononlike dispersion is observed. For plasmonlike modes, the dispersion is strongly damped by electron-hole pair excitations at entering single-particle continuum, while phononlike mode is undamped. In the region free of coupling, the graphene 2D plasmon exhibits acoustic behavior with linear dispersion with slope and damping determined by the Fermi-surface topology.

MAGNETIC INSTABILITY OF ULTRATHIN FCC FEXNI1-X FILMS

The {open_quotes}invar effect{close_quotes} in Fe{sub x}Ni {sub 1{minus}x} alloys occurs when the Fe content approaches 65{percent}. At this point, the magnetization falls to zero, and a martensitic structural transformation from a fcc to a bcc lattice occurs. This paper addresses the question: {open_quotes}What happens if the structural transformation is suppressed in an ultrathin alloy film?{close_quotes} We present results to this effect, showing the variation of the magnetization with changing composition in ultrathin films grown on Cu(100). We find a new low-spin, ferromagnetic phase of matter, which is a sensitive function of the atomic volume. {copyright} {ital 1997} {ital The American Physical Society}

On the atomic structure and electronic properties of decapped GaAs(001) (2×4) surfaces

We have performed scanning tunneling microscopy and spectroscopy on the GaAs(001)(2 × 4) surface. The measurements were performed on arsenic capped samples grown ex-situ by MBE. The n-doped epilayers were grown on flat GaAs(001) n-doped wafers and substrates cut 2° towards (010). The epilayers were terminated with 100 A of intrinsic material. Using STM we establish that capping does not compromise the surface quality of the (2 × 4) reconstruction. We find that provided the ex-situ MBE grown surfaces are of sufficient quality, the arsenic cap can be stripped off to yield surfaces equal to in-situ samples. Results from vicinal samples indicate that As from the cap adheres more strongly to step edges, which can be removed by subsequent annealing. The surfaces of all the samples studied were a mixture of (2 × 4) and c(2 × 8) domains. Tunneling I–V curves are interpreted as primarily due to tunneling out of the doubly occupied lone-pair states located on each surface As atom. Compared with GaAs(110), the spectra are much less sensitive to the distribution of empty states above EF. This behavior was independent of spatial location on the surface. The insensitivity to Ga derived empty states persists even when spectra are obtained above exposed Ga sites in the missing dimer rows where they may be expected to contribute. The measurements serve to locate the Fermi level on these samples pinned 0.6 ± 0.2 eV above the valence band maximum.

Evolution of the GaAs(001) surface structure during the transition from the As-rich (2×4) to the Ga-rich (4×2) reconstruction

Abstract Evolution of the GaAs(001) surface during the transition from the As-rich (2×4) to the Ga-rich (4×2) reconstruction has been studied by scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED). It has been found that the (2×4)–(4×2) transition proceeds via the formation of intermediate phases exhibiting (3×6) and (4×6) LEED patterns. STM images indicate that these phases are multi-domain. In particular, the (3×6) phase is locally composed of domains of the “(2×6)” and disordered phases, while the (4×6) phase in addition contains (4×2) domains. To explain STM images taken in a dual polarity bias mode, structural models for the “(2×6)” and disordered phases are proposed. These structures are electrostatically compensated via disorder of As (“2×6” phase) and Ga (disordered phase) dimers. A comprehensive picture of the (2×4)–(4×2) phase transition based on the analysis of the STM data is presented.

Interaction of Sn atoms with the intrinsic dangling-bond states of Si(111)-(7 × 7)

Abstract At the initial stages of room-temperature deposition we have observed the preferential adsorption of the individual Sn atoms on the Si center-adatom (T 1 ) sites on Si(111)-(7 × 7) using scanning tunneling microscopy. A spectroscopic analysis of changes in the energy spectrum of surface states around the Fermi level ( E f ) suggests that covalent bonding occurs between individual Sn and Si adatoms. This results in a surface metal-insulator transition due to the removal of Si adatom surface states at E f while preserving the (7 × 7) sub-lattice. An energy-level scheme is proposed to explain this behavior.

The effect of microstructure on the magnetic behavior of epitaxial cobalt layers

We have studied the magnetic behavior of thin cobalt films epitaxed on a Cu (001) substrate as a function of their growth temperature. At 150 K, the film texture is rough and the surface coverage is incomplete for film thicknesses <1.2 monolayers. Smoother films are obtained as the substrate temperature is increased, but at the expense of increased copper interdiffusion. Growth of films at 450 K produces smooth continuous epitaxial layers but coated with a layer of interdiffused copper which serves to lower the surface free energy. Copper interdiffusion can be controlled at intermediate temperatures between 300 and 450 K. We report on the effect of annealing cycles on the microstructure of these films in relation to their magnetic properties, as revealed by surface magneto‐optic Kerr effect hysteresis loop behavior.

Micromagnetic properties of ultrathin cobalt films

The properties of the magnetic domain structures of ultrathin fcc cobalt films epitaxially grown on Cu (001) have been examined using an ultrahigh vacuum surface magneto‐optic Kerr effect instrument. The evolution of magnetic behavior is observed for film thicknesses ranging from 1.4 to 7.5 monolayers. The coercivity is sensitive to film growth temperature and thermal cycling history. The coercivity decreases with diminishing film thickness and falls to very low values for the thinnest layers. The results are discussed in terms of Neel domain‐wall micromagnetics for ultrathin films.

Magnetic x‐ray linear dichroism in the photoelectron spectroscopy of ultrathin magnetic alloy films

The magnetic structure of nanoscale alloy films has been probed using the magnetic x‐ray linear dichroism in photoelectron spectroscopy. FeNi and CoFe epitaxial films were grown on Cu(001), in situ and using molecular beam epitaxy techniques. The magnetic x‐ray linear dichroism measurements were made at the Spectromicroscopy Facility of the Third Generation Advanced Light Source. Because soft x‐rays were used to generate photoemission from the 3p core levels, both elemental selectivity and magnetic sensitivity were achieved simultaneously.

Cs adsorption on n- and p-type GaAs(001)(2 × 4) surfaces

Abstract A study of room temperature Cs adsorption on both n- and p-type GaAs(001)(2 × 4) surfaces is presented. We find that initially the work function decreases linearly with Cs exposure saturating 3 eV below the clean surface value. Unlike GaAs(110) which forms an ordered overlayer, Cs deposition on (001) is amorphous. Adsorption steadily degrades the initial c(2 × 8) LEED pattern, and no additional superstructure is observed. Preliminary STM images obtained at low coverage indicate that at room temperature the Cs does not adsorb at any particular atomic site. Cs atoms adsorb both on top of the arsenic dimers and above the Ga in the missing dimer row. Tunneling spectroscopic measurements obtained at a range of coverages show that the surface remains semiconducting up to saturation coverage.