Mesoscopic organization of cobalt thin films on clean and oxygen-saturated Fe(001) surfaces

Abstract

The different morphologies of Co films grown on either the clean Fe(001) surface and the oxygen-saturated Fe(001)-p(1 × 1)O substrate are investigated by means of scanning tunneling microscopy, Auger electron spectroscopy, and density functional theory. The considered Co coverage range extends beyond the thickness at which layer-by-layer growth is destabilized by plastic deformations induced by the relaxation of the strain accumulated in the film. Our findings indicate that the oxygen overlayer of the Fe(001)-p(1 × 1)O surface floats on top of the growing Co film and strongly influences both the Co nucleation process and the film structural evolution. The layer-dependent islands nucleation of Co films grown on clean Fe(001) substrates, recently associated with a thickness-dependent adatom mobility [A. Picone et al., Phys. Rev. Lett. 113, 046102 (2014)], is found to be suppressed by the oxygen overlayer. The latter also significantly delays the layer-by-layer instability with respect to the oxygen-free growth. Furthermore, the body-centered-tetragonal/hexagonal-close-packed transition is not observed in the case of Co/Fe(001)-p(1 × 1)O sample, replaced by the development of highly ordered surface undulations. These form a mesoscopic square pattern with the sides aligned to the Fe 110 directions, while the surface atomic structure retains the square p(1 × 1) symmetry in registry with the substrate. Such undulations are likely generated by a highly ordered array of interfacial misfit dislocations running along the Fe 110 directions.

Electronic structure and magnetism of strained bcc phases across the fcc to bcc transition in ultrathin Fe films

“…The QUAN-TUM ESPRESSO package [37] was used for the calculations, within plane waves and ultrasoft pseudopotentials. The Fe pseudopotential was generated starting from scalar-relativistic all-electron atomic calculations and using nonlinear core corrections, and has been extensively studied in our previous works [38][39][40]. We used a kinetic energy cutoff of 45 Ry for the wave functions and 200 Ry for the effective potential and the charge density.…”

Magnetic anisotropy at the buried CoO/Fe interface

“…10,19,23 This result has been accomplished by exploiting an ultra-thin Co buffer layer with a bct cubic structure. 21,24 In that case, the CoO films were also a) Currently at Institute of Applied Physics, TU Wien, Vienna, Austria b) Electronic mail: alberto.brambilla@polimi.it characterized by a well-ordered mesa mound morphology for CoO thicknesses above a few monolayers (ML), occurring on account of stress relaxation through the formation of a network of misfit dislocations. 25,26 In this Letter, we explore the magnetic properties of such an interface and we report on the occurrence of a uniaxial magnetic anisotropy that gives rise to multijump magnetic hysteresis loops.…”

“…21. Subsequent annealing at 470 K for 5 minutes produces the Co(001)-p(1 × 1)O surface, 24 characterized by a single atomic layer of chemisorbed oxygen, while iron at the interface remains free of oxygen. Reactive deposition of CoO, with a thickness t CoO up to about 25 ML (1 CoO ML = 0.213 nm), was performed with the sample kept at 470 K by evaporating metallic Co in a pure O 2 atmosphere with partial pressure p O2 = 1 · 10 −4 Pa. 21 Magnetic hysteresis loops were acquired at room temperature on samples characterized by t CoO = 25 ML with the Magneto-Optical Kerr Effect (MOKE) apparatus described in Ref.…”

Magnetic properties of the CoO/Fe(001) system with a bottom-up engineered interface

“…Thin CoO films were grown onto the Co(001)-p(1 × 1)O surface of a ultrathin Co film (5 monolayers, ML) grown on a 500 nm-thick Fe(001) film, as described in detail in previous publications [4,[6][7][8][9]18]. The oxygensaturated Co surface is obtained by annealing the Co/Fe sample at 470 K for a few minutes; this also induces the formation of atomically flat terraces on the surface [19].…”

“…Co layers on Fe [6,7], to obtain a CoO/Fe interface free of any Fe oxide. The latter, on account of a misfit dislocation network developing at the interface, was also, interestingly, characterized by a dislocation-driven nanostructuration of CoO [8].…”