Structural transition in Fe ultrathin epitaxial films grown on Ni(111)

Abstract

A structural study of Fe ultrathin epitaxial films, grown at room temperature on Ni͑111͒, has been performed in the 1.5-18 ML coverage range by angle-scanned photoelectron diffraction. Both backscattering and forward-scattering energy regimes have been employed, in order to enhance the structural sensitivity at lower and higher film thicknesses, respectively. Modeling of the experimental data has been performed with multiple scattering calculations. We found indications that Fe atoms in the first layer occupy fcc hollow sites and stack with a pseudomorphic fcc structure up to 2 ML. Concerning the growth mode at these early stages, data suggest that a good substrate wetting and a sharp Fe/Ni interface take place. Between 3 and 6 ML, transition to a bcc͑110͒ phase develops. By quantitative R-factor analysis, we found that Nishiyama-Wassermann ͑NW͒ in-plane orientation of the bcc͑110͒ cell (͗001͘ bcc ʈ ͗11 0͘ fcc ) is favored over the Kurdjumov-Sachs (͗11 1͘ bcc ʈ ͗11 0͘ fcc ) orientation. The best-fit vertical interlayer distance between bcc͑110͒ planes is d NW ϭ2.11 Å ͑ϩ3.9% expansion͒ at 6 ML and relaxes to d NW ϭ2.05 Å ͑ϩ1.0%͒ at 18 ML, in agreement with the angular shift observed for the forward-focusing features. In the same coverage range, the angle between bcc͑110͒ surface basis vectors changes from 67.7°to 69.0°, corresponding to Ϫ1.7% and Ϫ1.0% contractions of the surface cell area, respectively.

Magnetoelectric coupling at metal surfaces

“…The absolute value of the electric-field-induced change of the energy barrier between the two states can be estimated from a comparison of the observed stability at low fields (at least in the order of minutes, τ ≈ 60 s ⇒ ∆E ≈ 6.5 meV) with the switching during the pulse, i.e., τ ≈ 60 µs ⇒ ∆E ≈ 12.2 meV. [57,58], while in other experiments, the fcc sites seemed to be favorable [59]. This fact already indicates a possible coexistence of fcc and hcp phases in the first layers of Fe.…”

“…This fact already indicates a possible coexistence of fcc and hcp phases in the first layers of Fe. With increasing thickness, a transition from fcc to bcc was reported at thicknesses of 3 to 6 ML [60,58]. A considerable difference compared to the system Fe/Cu(111) is given by the ferromagnetism of the Ni(111) substrate.…”

Graphene-protected iron layer on Ni(111)

“…However, Fe films grown on different Cu surfaces have been found to show different growth modes and different ranges of thicknesses in which the ferromagnetic fcc phase with out-of-plane anisotropy persists. Ni is another suitable material due to the small lattice mismatch of +2%, referred to the room temperature lattice parameter of fcc Fe, extrapolated from the high-temperature phase [8,9,10,11,12]. It was shown [8,10] that Fe layer on top Ni(111)/W(110) induces perpendicular to the film magnetic anisotropy for 0.5 − 3 ML thick iron films.…”

Electric-field-induced switching from fcc to hcp stacking of a single layer of Fe/Ni(111)

“…Considering the interatomic distances between nearest neighbors d nn and the resulting lattice mismatch, a Ni(111) surface where d nn = 248 pm should be an ideal candidate to promote the coexistence of Fe fcc (d nn = 253 pm) and bcc (d nn = 247 pm) films. The first layer of Fe/Ni(111) was considered to nucleate in the hexagonal close-packed (hcp) sites in theoretical calculations and photoelectron diffraction experiments 7,8 , while in other experiments, the fcc sites seemed to be favorable 9 . This fact already hints at a possible coexistence of fcc and hcp phases in the first layers of Fe/Ni(111), which is known for Fe/Ir(111) 10,11 .…”

“…This fact already hints at a possible coexistence of fcc and hcp phases in the first layers of Fe/Ni(111), which is known for Fe/Ir(111) 10,11 . With increasing thickness, a transition from fcc to bcc was reported at thicknesses of 3 to 6 ML 8,12 . Altogether this makes the system of Fe/Ni(111) a complex but promising system in the view of possible magnetoelectric coupling (MEC).…”