Structure and stability of ultrathin Fe films on W(110)

B. Santos, M. Rybicki, I. Zasada, E. Starodub, K. F. McCarty, J. I. Cerda, J. M. Puerta, and J. de la Figuera

Phys. Rev. B 93, 195423 – Published 17 May 2016

Abstract

The growth of one and two atomic layers of iron on a W(110) substrate was followed by low-energy electron microscopy. The near-surface structural properties of the perfectly flat pseudomorphic films were studied by quantitative low-energy electron diffraction analysis from areas of uniform thickness as well as by the density functional theory. A strong relaxation of the outermost atomic layers was found in Fe mono- and bilayers on W(110). By calculating the phonon dispersion relations and phonon density of states, the stability of the pseudomorphic iron bilayer on a tungsten substrate has been addressed. To complete the physical picture, an iron trilayer has also been analyzed in order to identify the source of instability for its pseudomorphic phase. Our results show that the surface instability originates from the softening of the in-plane surface modes along the $[1 \bar{1} 0]$ direction, although the soft modes were not observed. The enhanced magnetic moments calculated within the density functional theory are in good agreement with experimental findings reported for these systems.

Received 19 March 2014
Revised 11 February 2016

DOI:https://doi.org/10.1103/PhysRevB.93.195423

Physics Subject Headings (PhySH)

Phonons Structural properties

Transition metals Ultrathin films

Current-voltage low-energy electron diffraction Low-energy electron microscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

B. Santos

Instituto de Química-Física Rocasolano, CSIC, Madrid 28006, Spain and European Quality Assurance Spain, S.L. Madrid 28023, Spain

M. Rybicki and I. Zasada^*

Solid State Physics Department, University of Lodz, ul. Pomorska 149/153, Lodz, Poland

E. Starodub and K. F. McCarty

Sandia National Laboratories, Livermore, California 94550, USA

J. I. Cerda and J. M. Puerta

Instituto de Ciencia de Materiales de Madrid, CSIC, Madrid 28049, Spain

J. de la Figuera

Instituto de Química-Física Rocasolano, CSIC, Madrid 28006, Spain

^*izasada@wfis.uni.lodz.pl

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Vol. 93, Iss. 19 — 15 May 2016

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Images

Figure 1
LEEM images acquired during the deposition of 2 ML of Fe on W(110). The growth takes place layer by layer in the step-flow mode. (a) Image of the bare W(110) surface prior to the Fe deposition, (b)–(d) completion of the first layer, and (e), (f) images acquired during the growth of the second layer of Fe. The temperature of the substrate is 550 K, the field of view is $7 μ m$ , and the electron beam energy is 5.5 eV.
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Figure 2
(a) LEEM image acquired after the deposition of a 1 ML Fe/W(110). The field of view was $7 μ m$ and the electron beam energy was 5.5 eV. LEED pattern (150 eV) obtained from a 1 ML Fe area on a single W(110) terrace. The $(1 \times 1)$ pattern shows that the film is pseudomorphic. (b) LEED- $I (V)$ data (solid lines) and best fit $I (V)$ curves (dashed lines) for different beams.
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Figure 3
(a) LEEM image acquired after the deposition of a 2 ML Fe/W(110). The field of view was $7 μ m$ and the electron beam energy was 5.5 eV. LEED pattern (150 eV) obtained from a 2 ML Fe area on a single W(110) terrace. The $(1 \times 1)$ pattern shows that the 2 ML film is pseudomorphic. (b) LEED- $I (V)$ data (solid lines) and best fit $I (V)$ curves (dashed lines) for different beams.
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Figure 4
The calculated phonon dispersion curves for the 2Fe/7W/2Fe 11-layer slab, (a) for the surface Fe layer and (b) for the subsurface Fe layer. Squares (green) and circles (red) denote the highest intensity vibration modes with polarization along the in-plane [001] and $[1 \bar{1} 0]$ directions, respectively. The Rayleigh wave mode, with [110] out-of-plane polarization is marked by triangles (blue).
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Figure 5
In-plane phonon DOSs for tungsten and iron bulk [panel (a)]. In-plane (solid lines) phonon DOSs for Fe layers in the 1Fe/7W/1Fe system [panel (b)] and the 2Fe/7W/2Fe system [panels (c) and (d)]. Dotted lines in each panel show the modes with polarization along the in-plane [001] and $[1 \bar{1} 0]$ directions. Arrows indicate the position of main features in the spectra. The iron calculated and experimental phonon DOSs (filled contour) are reproduced from Ref. [50], while the tungsten phonon DOSs are calculated for middle W layers in slabs with 7 and 9 tungsten MLs.
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Figure 6
In-plane (solid lines) phonon DOSs for Fe surface [panel (c)], subsurface [panel (b)], and interface [panel (a)] Fe layers in the 3Fe/9W/3Fe system. Dotted lines in each panel show the modes with polarization along the in-plane [001] and $[1 \bar{1} 0]$ directions. Arrows indicate the position of main features in the spectra.
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