D. Tian

Structural properties of epitaxial films of Fe on Cu and Cu-based surface and bulk alloys

Thin films of Fe were grown epitaxially at room temperature on Cu{001}, on the ordered surface alloys Cu{001}c(2×2)-Au and Cu{001}c(2×2)-Pd, and on the bulk alloy Cu3Au{001}. The maximum thicknesses attained in well-crystallized Fe films were 18 layers on the first three surfaces but only 3 layers on Cu3Au {00}. Low-energy electron diffraction (LEED) intensity data from 1-, 2- and 3-layer films on Cu{001&} could not be fitted by models with complete layers of Fe, suggesting that the growth of these ultrathin films was not layer-by-layer. The 12-layer films were found to have a tetragonally distorted face-centered-cubic (fcc) structure with a= 3.61 A, c= 3.54 A and 4%-expanded first interlayer spacing. Analysis of the elastic strain gave an equilibrium lattice constant of 3.59 A for fcc Fe at room temperature. Comparison with lattice constants from total-energy band calculations shows that the Fe cannot be in the nonmagnetic phase, but could be in the ferromagnetic phase, or possibly in an antiferromagnetic phase with the same lattice constant. It is suggested that the first interlayer spacing is enlarged owing to the larger magnetic moment of the first layer.

Epitaxy of Mn On Pd {001}

Abstract A new metastable body-centered tetragonal phase of Mn has been grown epitaxially at room temperature on Pd {001} up to thicknesses of 21 layers. Low-energy electron diffraction (LEED) intensity analysis has determined two important structural parameters: the bulk interlayer spacing and a 5%-larger first interlayer spacing. Assumption that the film is a cubic phase in plane strain leads to the conclusion that the phase is probably face-centered cubic with an equilibrium lattice parameter of 3.80A, which is much larger than a theoretical non-magnetic equilibrium value and indicates that the phase probably is magnetic.

Epitaxial growth of γ-Fe on Ni{001}

Abstract The epitaxial growth of Fe on Ni{001} is studied by means of low-energy electron diffraction (LEED) and Auger-electron spectroscopy (AES). Films thinner than about 4 atomic layers produce 1 × 1 LEED patterns and are found to be epitaxial, but with markedly expanded interlayer spacings. Films thicker than about 6 layers (up to 25 layers) produce 1 × 1 patterns with weak satellite beams that are attributed to the presence of bcc Fe{110} islands. A LEED analysis of the 1 × 1 component of a 10-layer Fe film reveals a body-centered tetragonal structure with bulk lattice parameters a = 2.489 A (dictated by the Ni{001} substrate) and c = 3.88 A (i.e., interlayer spacing d bulk = 1.94 A, expanded 10% over the Ni spacing 1.76 A). The first interlayer spacing d 12 is expanded 5% with respect to the bulk. Strain analysis estimates that the equilibrium room-temperature lattice constant of the Fe phase grown in these experiments is 3.65 ± 0.04 A, consistent with the results reported elsewhere for Fe films grown on Cu{001}. This information, combined with the results of total-energy band-structure calculations published elsewhere, shows that the γ-Fe phase grown in the present experiments is expanded with respect to the non-magnetic state, and has a strain that is slightly smaller in the ferromagnetic than in the antiferromagnetic state.

A low-energy electron diffraction study of the Pd{0 0 1}c(2 × 2)-Mn adsorbed system

Abstract An intensity analysis is made of the c(2 × 2) LEED pattern of one- to two-layer Mn films on Pd{0 0 1} to see if a theoretically-predicted antiferromagnetic structure contributes to the LEED intensities. We show that a pure magnetic structure, even if buckled, cannot fit the data. However, a mixed, strongly-buckled MnPd surface layer, with the Mn sublayer above the Pd sublayer, gives a moderately good fit. Annealing of the system produces a bulk Pd3Mn alloy with the Cu3Au structure and a buckled relaxed first atomic layer, but with the Mn sublayer now below the Pd sublayer.

Study of the growth of Fe on Ru(0001) by low-energy electron diffraction

Abstract Qualitative and quantitative LEED (low-energy electron diffraction) and AES (Auger electron spectroscopy) studies of the growth of Fe films on Ru(0001) show that the growth mode is the Stranski-Krastanov mode: monolayer growth followed by island growth. The first monolayer of Fe is pseudomorphic with the hexagonal Ru(0001) net at an interlayer spacing of 2.05 A: the Fe atoms are in the sites dictated by continuation of the substrates hexagonal close-packed (hcp) stacking. Thicker films consists of 3-dimensional bcc- Fe{ 100} domains in two equivalent Kurdjumov-Sachs orientations, rotationally related to one another by the 6-fold symmetry of the substrate net. These results do not support recent reports of the growth of hcp Fe on Ru(0001). Possible connections between the present structure results and the magnetic properties of Fe/Ru ultra-thin films reported by others are discussed.

Study of ultrathin Fe films on Pd{111)}, Ag{111} and Al{111}

Abstract The growth of epitaxial Fe films of 1 to 10 layer equivalents (LE) on Pd{111}, Al{111} and Ag{111} has been studied with quantitative low-energy electron diffraction and Auger electron spectroscopy. On Pd{111}, both at room temperature and at 200° C, the growth starts with pseudomorphic layers which may involve intermixing of Fe and Pd. At both temperatures, when the coverage reaches 6 LE, the Fe films develop into large bcc {110} domains related to the substrate by the Kurdjumov-Sachs orientation. On Ag{111} Fe grows initially in a very similar manner to Fe on Ag{001}, i.e., by way of small islands of bcc Fe. These islands then grow into bcc Fe{110} domains in the Nishiyama-Wassennan orientation. On Al{111} Fe behaves differently, despite the fact that the lattice mismatch for Fe on A1{111} is nearly identical to that for Fe on Ag{111}: for coverages of less than 1 LE the surface region becomes completely disordered and no LEED pattern is visible.

Epitaxial ordered-alloy formation through surface reactions: Al on Ni{0 0 1}

Abstract We have studied the products of vacuum deposition of Al on Ni{0 0 1} to compare with Au on Cu{0 0 1}, which is known to develop a surface alloy. Slow vacuum deposition of Al onto unheated Ni{0 0 1} produces partially-ordered one- and two-layer-thin epitaxial Al films. These films have larger interlayer spacing (by about 15%) than the Ni substrate and a substantial disordered component. Anneals of the films, or slow deposition of Al onto a hot Ni{0 0 1} substrate, produce well-crystallized epitaxial films of Ni 3 Al{0 0 1}. The alloy is not confined to just the surface layer, hence it is not a surface alloy. The thickness of the alloy films can be controlled via the deposition rate of Al and/or the temperature of the Ni substrate. Deposition of Ni on Al{0 0 1}; does not result in ordered alloy formation.

Incommensurate epitaxial growth of Cu on Pd{111}

A low-energy electron diffraction (LEED) study of the growth of Cu on Pd{111} shows that the Cu films are epitaxial but are incommensurate with the substrate and have the equilibrium fcc Cu structure, in contrast to the growth on Pd{001}, where the Cu films are both epitaxial and pseudomorphic. The different behavior of the growth of the same material on two different surfaces of the same substrate is probably due to different magnitudes of the ratio of adatom-adatom interactions to the adatom-substrate interactions on the two surfaces.

Electronic properties of body-centred-tetragonal copper

Angle-resolved photoemission with synchrotron light is used to determine the band structure of five- and six-layer films of Cu grown epitaxially on Pd(001) and on Pt(001). The films have body-centred-tetragonal structure resulting from the large plane strains of 8 to 9% imposed by the substrates. The band structure is the same, within experimental error, for films grown either on Pd(001) or Pt(001), and is similar to that of stable FCC Cu.