Symmetry-Induced Structuring of Ultrathin FeO and Fe3O4 Films on Pt(111) and Ru(0001)

Abstract

Iron oxide films epitaxially grown on close-packed metal single crystal substrates exhibit nearly-perfect structural order, high catalytic activity (FeO) and room-temperature magnetism (Fe3O4). However, the morphology of the films, especially in the ultrathin regime, can be significantly influenced by the crystalline structure of the used support. This work reports an ultra-high vacuum (UHV) low energy electron/synchrotron light-based X-ray photoemission electron microscopy (LEEM/XPEEM) and electron diffraction (µLEED) study of the growth of FeO and Fe3O4 on two closed-packed metal single crystal surfaces: Pt(111) and Ru(0001). The results reveal the influence of the mutual orientation of adjacent substrate terraces on the morphology of iron oxide films epitaxially grown on top of them. On fcc Pt(111), which has the same mutual orientation of adjacent monoatomic terraces, FeO(111) grows with the same in-plane orientation on all substrate terraces. For Fe3O4(111), one or two orientations are observed depending on the growth conditions. On hcp Ru(0001), the adjacent terraces of which are ‘rotated’ by 180° with respect to each other, the in-plane orientation of initial FeO(111) and Fe3O4(111) crystallites is determined by the orientation of the substrate terrace on which they nucleated. The adaptation of three-fold symmetric iron oxides to three-fold symmetric substrate terraces leads to natural structuring of iron oxide films, i.e., the formation of patch-like magnetite layers on Pt(111) and stripe-like FeO and Fe3O4 structures on Ru(0001).

Ostwald Ripening in an Oxide‐on‐Metal System

“…The origin of the Ostwald ripening process and the FeO‐to‐Fe 2 O transformation is believed to be related to energy minimization, that is, preferred reduction of the number of FeO/Ru edge sites, as well as the structural characteristics of the FeO/Ru(0001) system. First of all, the oxide on this particular substrate preferably grows in the bilayer (O–Fe–O–Fe) form, and not as a monolayer (O–Fe) like on most single‐crystal supports (e.g., Pt(111) [ 17 ] ). The monolayer can be stabilized, however, only within a limited range of oxygen pressures.…”

A Platform for Addressing Individual Magnetite Islands Grown Epitaxially on Ru(0001) and Manipulating Their Magnetic Domains

“…Depending on the particular details of the growth temperature and postprocessing of the samples (like further annealing steps), the magnetite islands either extend down to the Ru substrate or sit on top of the FeO wetting layer. Our goal here is to establish that the islands correspond to magnetite, so we refer the reader to the published works on this subject. , The density of islands on the Ru films is comparable to that of magnetite islands grown in single-crystal Ru substrates. It is likely that the presence of the wetting layer decouples the nucleation of the magnetite islands from the local density of the steps to some extent.…”

“…Upon Fe deposition under an oxygen atmosphere, the Ru film is first covered by a FeO(111) wetting layer whose thickness is two atomic layers for the conditions we used, − followed by the growth of 3-dimensional magnetite islands with thicknesses ranging from a few nanometers to a hundred nanometers and a lateral extension of several micrometers . Depending on the particular details of the growth temperature and postprocessing of the samples (like further annealing steps), the magnetite islands either extend down to the Ru substrate or sit on top of the FeO wetting layer.…”

“…Our goal here is to establish that the islands correspond to magnetite, so we refer the reader to the published works on this subject. 21,26 The density of islands on the Ru films is comparable to that of magnetite islands grown in single-crystal Ru substrates. It is likely that the presence of the wetting layer decouples the nucleation of the magnetite islands from the local density of the steps to some extent.…”

Magnetic-Field-Assisted Molecular Beam Epitaxy: Engineering of Fe3O4 Ultrathin Films on MgO(111)

“…Complex structural, electronic, and magnetic properties become even more intricate in epitaxial magnetite films. Epitaxial magnetite films can be grown on different substrates, both metallic, e.g., Pt [ 17 ], Ru [ 18 ], and Fe [ 19 ], or oxidic, such as Al 2 O 3 [ 20 , 21 , 22 , 23 ], SrTiO 3 [ 24 ], MgAl 2 O 4 [ 12 ], and MgO [ 25 , 26 , 27 , 28 , 29 , 30 ]. Among the latter, MgO, used in the present work, has been the most exploited, and the hetero-epitaxial Fe 3 O 4 /MgO system is important, albeit not the only case of thin film modification of the bulk magnetite characteristic.…”