Multiscale investigation of the structure and morphology of the $\text{Co}/{\text{Fe}}_2{\text{O}}_3\left(0001\right)$ interface

We report a detailed structural characterization of Co films grown on $\alpha {\text{-Fe}}_2{\text{O}}_3\left(0001\right)$ for thicknesses up to 25 nm. Epitaxial and single-crystalline 20 nm thick $\alpha {\text{-Fe}}_2{\text{O}}_3\left(0001\right)$ layers deposited on $\alpha {\text{-Al}}_2{\text{O}}_3\left(0001\right)$ and Pt(111) single crystals were used as substrate. The $\text{Co}/\alpha {\text{-Fe}}_2{\text{O}}_3\left(0001\right)$ interface is a prototypical magnetic exchange coupled system for which the magnetocrystalline parameters are crucial to address. We evidence medium range order requiring a multiscale approach to reach a reliable description of the crystalline structure. Surface extended x-ray absorption fine structure (EXAFS) and surface x-ray diffraction (SXRD) have been combined to describe the structure in an extended thickness range. Additional grazing incidence small angle x-ray scattering (GISAXS) measurements show that the growth is three dimensional up to $\sim 3\text{nm}$ and follows a nucleation—growth—coalescence scheme. For all thicknesses cobalt is found to grow with a lattice parameter close to its bulk lattice parameter. In the early stages of growth a disordered 2–3 monolayer thick interface exhibiting oxidized cobalt and metallic iron is evidenced. Long range order sets in for thickness above 4 nm showing the coexistence of fcc, twinned fcc, and hcp stacking within direct and in-surface-plane $30°$ rotated epitaxial relationships.

Figure 1

(Color online) SXRD in surface plane scans recorded on $\text{Co}/\alpha {\text{-Fe}}_2{\text{O}}_3/\text{Pt}\left(111\right)$ samples along the (a) [hh0] and (b) [0k0] reciprocal space directions with respect to the Co thickness: from bottom to top 0, 0.1, 0.2, 0.4, 0.8, 1.6, 3.2, 6.4, and 25.6 nm. The horizontal axes are labeled in $\alpha {\text{-Fe}}_2{\text{O}}_3$ reciprocal lattice units (r.l.u.). The (110), (220), and (300) peaks are Bragg reflections from the hematite layer and the (3.3,0,0) peak is a Pt(111) Bragg peak. The satellite structures around (110) and (220) originates from an interfacial well ordered coincidence lattice as described in Ref. 24.

Figure 2

(Color online) SXRD out of surface plane scans (crystal truncation rods of Co) recorded on $\text{Co}/\alpha {\text{-Fe}}_2{\text{O}}_3/\text{Pt}\left(111\right)$ samples along the (a) ${\left[10\text{L}\right]}_{Co}$ and (b) ${\left[11\text{L}\right]}_{Co}$ reciprocal space directions with respect to the Co thickness.

Figure 3

(Color online) EXAFS spectra recorded at 300K at the Co $K$-edge for 0.4 and 2 nm of Co deposited on ${\text{Fe}}_2{\text{O}}_3/{\text{Al}}_2{\text{O}}_3\left(0001\right)$ in grazing incidence, compared with spectra of bulk Co and bulk CoO

Figure 4

(Color online) (a) XPS Fe 2p for ${\text{Fe}}_2{\text{O}}_3$ film and for 1.2 nm $\text{Co}/{\text{Fe}}_2{\text{O}}_3$, (b) XPS Co 2p for Co bulk, 0.6 and 1.2 nm $\text{Co}/{\text{Fe}}_2{\text{O}}_3/{\text{Al}}_2{\text{O}}_3\left(0001\right)$ taken at a photon energy $h\nu =1253.6\text{eV}$, (c) XANES spectra at the Co $K$-edge for CoO, 0.4, 0.8, 1.4, and 2 nm $\text{Co}/\alpha {\text{-Fe}}_2{\text{O}}_3/\alpha {\text{-Al}}_2{\text{O}}_3\left(0001\right)$ and bulk Co. Dashed lines indicate the position of (a) metallic $\text{Fe}2{\text{p}}_{3/2}$ and ${\text{Fe}}^{3+}$ satellite, (b) ${\text{Co}}^{2+}$ satellite and (c) 7716.5 and 7730.5 eV photon energy

Figure 5

(Color online) FT (from $k=2.51$ to $12.7{\text{Å}}^{-1}$ of $k\chi \left(k\right)$ for 0.4, 0.8, 1.4 nm and 2 nm in grazing incidence $\text{Co}/\alpha {\text{-Fe}}_2{\text{O}}_3/\alpha {\text{-Al}}_2{\text{O}}_3\left(0001\right)$, Co bulk and CoO bulk.

Figure 6

(Color online) Geometry of the GISAXS experiment and angles description of the scattered ${\mathbf{k}}_f$ and incident ${\mathbf{k}}_i$ wave vectors yielding the momentum transfer (i.e., the reciprocal space vector) $\mathbf{Q}={\mathbf{k}}_f-{\mathbf{k}}_i$. The angles ${\alpha }_i$, ${\alpha }_f$ and $2{\theta }_f$ are related to the components of the momentum transfer, either parallel ($Q_x$ and $Q_y$, with $Q_{\parallel }^2=Q_x^2+Q_y^2$) or perpendicular ($Q_z$ or $Q_{\perp }$) to the sample surface. $D$ is the interisland distance and $R$ and $H$ the radius and the height of islands.

Figure 7

(Color online) In-surface-plane GISAXS scans performed on $\text{Co}/\alpha {\text{-Fe}}_2{\text{O}}_3/\text{Pt}\left(111\right)$ films in critical hematite incidence and emergence conditions $\left({\alpha }_i={\alpha }_f\right)$ for increasing Co coverage (from bottom to top). The signal located at $Q_{\parallel }=\pm 0.133{\text{Å}}^{-1}$ belongs to the interface dislocation network located at the $\text{Pt}\left(111\right)/\alpha {\text{-Fe}}_2{\text{O}}_3$ interface. It is best evidenced when the incoming beam runs along the [010] crystalline direction of $\alpha {\text{-Fe}}_2{\text{O}}_3$ and has sixfold symmetry. The signal corresponding to the growing Co islands moves progressively toward $Q_{\parallel }=0$ and is isotropic with respect to the sample azimuth.

Figure 8

(Color online) Experimental (symbols) and calculated (straight lines), in (left) and out (right) of surface plane GISAXS scans taken for $t_{Co}=0.4$, 1.6, 3.2 and 12.8 nm, at the $Q_{\parallel }$ and $Q_{\perp }$ values indicated in the graphs. The parameters of the fit are indicated in Table . Dashed line is calculation assuming ${\sigma }_H=0$.