Effects of unreconstructed and reconstructed polar surface terminations on growth, structure, and magnetic properties of hematite films

The effects of polar surface stabilization mechanisms on the film growth, phase composition, surface and interface structure, and magnetic properties are explored for polar oxide interfaces formed by the epitaxial growth of hematite films on magnesia and alumina single crystals. Growth of α-Fe${}_2$O${}_3$(0001) on the ($\sqrt{3}$×$\sqrt{3}$)R30° and (2 × 2) reconstructed MgO(111) surfaces results in formation of a self-organized Fe${}_3$O${}_4$(111) interfacial nano buffer that persists after growth. The interfacial magnetite-like phase is absent from the hematite films formed on hydrogen-stabilized unreconstructed MgO(111)-(1 × 1) and on Al${}_2$O${}_3$(0001)-(1 × 1) surfaces under equivalent conditions. This study suggests that in addition to the customary strain, spin, and band-gap engineering, control of surface polarity stabilization could also be important for electronic and magnetic device engineering.

Figure 1

Atomic models in side and top view of (a) α-Fe${}_2$O${}_3$(0001)—corundum (also α-Al${}_2$O${}_3$), (b) Fe${}_3$O${}_4$(111)—spinel, and (c) MgO(111)—rocksalt slabs with bulk-terminated polar surfaces. The side view illustrates alternate stacking of planes of oxygen anions (O${}^{2-}$ in green/gray) and magnesium (Mg${}^{2+}$, orange/light gray) or iron (Fe${}^{3+}$ and Fe${}^{2+}$, purple/medium gray) cations needed for polar oxide surface and/or interface creation. In all (111) cubic planes and in the α-Al${}_2$O${}_3$ (0001) hexagonal planes, the O${}^{2-}$ ions are in a close-packed hexagonal lattice, with in-plane distortions from the perfect lattice sites for hematite (0001) planes. Iron (aluminum) is stacked in bilayers of Fe${}^{3+}$ (Al${}^{3+}$) in the α-Fe${}_2$O${}_3$ (α-Al${}_2$O${}_3$) polar (0001) direction and in alternating monolayers of Fe${}^{3+}$ and trilayers of Fe${}^{2+}$Fe${}^{3+}$Fe${}^{2+}$ in the Fe${}_3$O${}_4$ polar (111) direction. Top-view drawings of the unreconstructed surface unit cells for all three structures illustrates that the α-Fe${}_2$O${}_3$(0001)-(1 × 1) cell is nearly commensurate with MgO(111)-($\sqrt{3}$ × $\sqrt{3}$)R30° and Fe${}_3$O${}_4$(111)-(1 × 1) with MgO(111)-(2 × 2).

Figure 2

Thickness-normalized VSM magnetization curves for films grown on magnesia and alumina polar surfaces in continuous fashion. Saturation moment and coercivity data are presented in Table  with a drastically higher magnetic moment for the film grown on reconstructed magnesia (MR).

Figure 3

XRD intensity profiles of iron oxide films grown in continuous mode on unreconstructed (MU) and reconstructed (MR) MgO(111) surfaces displaying hematite reflections in α-Fe${}_2$O${}_3$ (0001) epitaxial orientation. Lines indicate peak positions for magnesia, hematite, magnetite and maghemite.

Figure 4

Fe L (a) and O K (b) XAS and Fe L XMCD (a) spectra of hematite films grown by continuous OPA-MBE deposition on hydrogen-stabilized MgO(111) and Al${}_2$O${}_3$(0001) unreconstructed surfaces (MUc, AUc) and on reconstruction-stabilized MgO(111) polar surfaces (MRc).

Figure 5

In situ Fe 2$p$ XPS spectra from hematite films grown by continuous OPA-MBE on unreconstructed (MU) and reconstructed (MR) MgO(111) and unreconstructed (AU) Al${}_2$O${}_3$ (0001) polar surfaces. Dashed lines denote positions of Fe${}^{3+}$ peaks in close agreement with bulk Fe${}_2$O${}_3$ values of 711 eV for Fe $2p_{3/2}$ and 719 eV for the satellite.

Figure 6

In-situ RHEED patterns from magnesia and alumina substrates after surface preparation for growth (top row) and from respective hematite film surfaces after completion of the continuous OPA-MBE growth (bottom row): (a) MU film on unreconstructed MgO(111) substrate; (b) MR film on reconstructed MgO(111)-($\sqrt{3}\times \sqrt{3}$)R30° with minority (2 × 2) domains; and (c) AU film on unreconstructed Al2O3(0001) surface. (d) RHEED in-plane intensity profiles of above films are consistent with α-Fe${}_2$O${}_3$(0001)-(1 × 1) termination structure. The rods and their intensity profiles are indexed; arrows in patterns denote location of magnesia (green/gray), alumina (blue/medium gray), and hematite (orange/light gray) reflections, lines of same color indicate the substrate 1 × 1 rods.

Figure 7

Bright-field TEM image of MUc film, imaged in cross section under weakly diffracting conditions, allows measurement of film thickness and ascertains absence of dense Fe nanoparticles.

Figure 8

Selected area diffraction patterns from films grown on (a) unreconstructed (MUc) and (b) reconstructed (MRc) magnesia surfaces recorded with the incident beam parallel to MgO [11-2] direction. The hematite (orange/light gray) and magnesia (green/gray) unit cells are denoted on the experimental patterns, showing that additional reflections, due to magnetite (lilac/medium gray), are present only in the MRc sample.

Figure 9

HRTEM images of interface regions of MUc (a) and MRc (b) samples and indexed digital diffractograms (c) from MgO substrate (green/gray), Fe${}_3$O${}_4$ interfacial band (lilac/medium gray), and α-Fe${}_2$O${}_3$ film (orange/light gray) image regions. The self-assembled interfacial nano buffer with magnetite structure is seen in the films grown on reconstructed MgO(111). Cubic γ-Fe${}_2$O${}_3$ is not present, as demonstrated by absence of (1-10) and (201) reflections (red/dark gray).

Figure 10

Position-resolved EELS spectra of the Fe-L transition from (a) hematite regions of film MRc; and (b) interfacial magnetite-like band of the same film grown on reconstruction-stabilized MgO(111)-($\sqrt{3}$ × $\sqrt{3}$)R30° polar surfaces. The grey area was subtracted prior to L${}_3$/L${}_2$ calculation. The L${}_3$/L${}_2$ ratio and the FWHM values of the L${}_3$ peak indicate a lower Fe oxidation state at the interface. Background-subtracted spectra.