Reversible metamorphosis from Fe3O4 to FeO of epitaxial iron oxide films grown on the Fe-p(1 × 1)O surface

The reduction and oxidation of epitaxial Fe3O4 films grown by reactive deposition on a Fe-p(1 × 1)O surface have been investigated by means of Auger electron spectroscopy (AES), low energy electron diffraction (LEED) and scanning tunneling microcopy (STM). The as-grown iron oxide samples display a square LEED pattern with a lattice constant compatible with a p(1 × 1) bulk terminated Fe3O4(001) surface. STM topographic images of Fe3O4 are characterized by atomically flat terraces separated by highly oriented steps running along the (010) and (100) crystallographic directions of the substrate. Upon annealing at 800 K in an ultra-high vacuum, AES reveals that magnetite transforms to FeO. The sample exposes the (001) surface of the rock salt structure, with a lattice parameter close to that of bulk wüstite. The Fe3O4 phase can be recovered by oxidation at 10−6 mbar of molecular oxygen.


Introduction
Iron oxides have been investigated for decades in several scientic disciplines, spanning physical chemistry 1-3 to medicine. 4 Owing to the different oxidation states assumed by Fe cations, Fe 2+ or Fe 3+ , iron oxides can form various phases, with different stoichiometries and physical properties. Wüstite FeO is an antiferromagnetic insulator crystallizing in a rock salt lattice, where only Fe 2+ cations are present. Magnetite Fe 3 O 4 is a ferrimagnet containing a mixture of Fe 2+ and Fe 3+ ions, which assumes an inverse spinel structure. Above the Verwey transition temperature (T v $ 120 K), Fe 3 O 4 is a half metal, while below T v it is insulating. Haematite (a-Fe 2 O 3 ) adopts a corundum structure containing Fe 3+ ions in octahedral sites. The stoichiometric a-Fe 2 O 3 , below 955 K, is an antiferromagnetic insulator.
The surface science paradigm, i.e. the preparation of well-dened model systems under highly controlled conditions, is the most appropriate to investigate the detailed atomic structure and chemical composition of oxide surfaces. [5][6][7][8] Thin and ultra-thin Fe oxide lms supported on metallic substrates like Pt, 9,10 Ag, 11,12 Fe, 13,14 , Ni, 15,16 have been investigated by using this approach. Aer these investigations, it has been recognized that also new phases, with stoichiometries and physical properties deviating from those occurring in bulk samples, can be stabilized in epitaxial lms with a thickness of few monolayers. 17 The interconversion from one phase to another, depending on parameters like temperature and oxygen partial pressure, is particularly important, both from fundamental and applied points of view. In this frame, several recent papers describe the redox reactions occurring on iron oxide samples: Freindl 19 Jiang et al. investigated by means of ambient pressure scanning tunneling microscopy the effects induced by oxygen and carbon monoxide exposure on FeO(111) nano-islands grown on Au(111). 20 The redox reactions occurring on Fe y O x layers change drastically their physical and chemical properties, affecting the performances of the devices in which they are integrated. In heterogeneous catalysis, during the reaction the oxidation state of Fe oxide can be modied and inuence the catalyst performances. For example, the oxidation of FeO lms grown on Pt(111) induces the formation of a O-Fe-O trilayer, which is a key factor to activate the FeO/Pt(111) catalyst for low temperature CO oxidation. 21,22 In exchange bias systems, where Fe is interfaced with an antiferromagnetic oxide, oen the formation of Fe oxides is observed. [23][24][25] The thermal treatments performed on the layered structure, such as the heating before the eld cooling process, can induce redox reactions [26][27][28] and affect the device properties. In magnetic tunnel junctions, oen there is a thin Fe y O x layer at the interface between the Fe electrodes and the MgO barrier, which can affect the device transport characteristics. 29 In this paper we analyze the effect of annealing and oxygen exposure on an epitaxial Fe 3 O 4 lm grown on a Fe-p(1 Â 1)O surface. The Fe-p(1 Â 1)O sample is characterized by a single layer of oxygen atoms accommodated in the hollow sites of the Fe(001) surface and can be considered as a single layer of iron monoxide compressed by the underlying metal. [30][31][32][33] The Fe-p(1 Â 1)O

Experimental details
The experiments have been performed in an ultra-high vacuum (UHV) chamber, at a base pressure of 10 À10 mbar. The Fe(001) substrate was obtained by growing in situ a thick Fe lm, about 500 nm, on a MgO(001) substrate. The Fe-p(1 Â 1)O surface was obtained by following a well-established procedure, exposing the Fe(001) sample to 30 L of molecular oxygen (O 2 ) and annealing in UHV to 800 K for 5 minutes. 5 Fe 3 O 4 lms were grown by evaporating Fe in a 10 À6 mbar O 2 atmosphere, with the sample kept at 500 K during the deposition, as measured by a thermocouple positioned close to the sample. The Fe deposition rate was about 1 nm min À1 , as evaluated by a quartz microbalance. The lm thickness was estimated to be about 50 nm. The heating rate during the annealing was on average 1 K s À1 , while during the cooling of the sample was about À2 K s À1 . In the experiments where the reduction of Fe 3 O 4 was induced, the lms were annealed at 800 K in UHV with the same annealing and cooling rates used during the Fe 3 O 4 growth. The scanning tunneling microscopy (STM) measurements were performed in situ by using an Omicron variable temperature STM. The images were acquired at room temperature in constant-current mode with tips obtained by electrochemical etching of W wires.
The Auger electron spectroscopy (AES) and low-energy electron diffraction (LEED) measurements performed by means of an Omicron SPECTALEED with a retarding eld analyzer (total acceptance angle 102 ). The primary energy of the electron beam for AES measurements was 3 keV.  Fig. 1(a) shows the diffraction pattern characteristic of the Fe-p(1 Â 1)O surface, corresponding to a real space square lattice with a lattice parameter equal to a p(1Â1) ¼ 2.86Å. Aer the Fe deposition in O 2 atmosphere, the LEED pattern in Fig. 1(b) indicates the presence of a p(2 Â 2) reconstruction with respect to the Fe-p(1 Â 1)O lattice. This pattern is attributed to the formation of a bulk-terminated Fe 3 O 4 (001) surface, which is characterized by a lattice parameter of about 0.59 nm. The stabilization of Fe 3 O 4 (001) is supported also by the AES and STM data presented in the following. The subsequent UHV annealing at 800 K lis the p(2 Â 2) phase and a square lattice similar to that of the Fe-p(1 Â 1)O substrate appears in Fig. 1(c), although with different spot intensities and unit mesh dimensions. A quantitative evaluation of the ratio between the real space lattice parameter of the annealed sample (a ann ) and that of Fe-p ( 37,38 or aer the deposition of atoms belonging to different species. 39 In our case, the samples have been grown at 500 K in oxygen atmosphere, but the LEED and STM measurements have been performed at room temperature, therefore our results cannot be directly related to those of ref. 37    ðO=Fe 650 Þ ann , which quanties the relative stoichiometries of the as-grown and annealed lms, provides a value of 1.33, in excellent agreement with that expected for a reduction from Fe 3 O 4 to FeO. Finally, the O/Fe 650 ratio increases again to a value of 3.63 aer O 2 exposure, pointing towards a recovery of the original magnetite lm. The Auger peak at low kinetic energy reported in Fig. 2(b) corresponds to the MVV transition, which is particularly sensitive to the oxidation state of Fe atoms. 40,41 The bottom spectrum of Fig. 2(b) acquired on the Fe-p(1 Â 1)O is dominated by the peak of metallic Fe, with a minimum located at 46 eV. Additionally, another feature is visible at lower kinetic energy, partially superimposed onto the metal peak, which can be assigned to the presence of a single layer of iron oxide on the surface. 5 The spectrum taken on the as-grown  31 The step density along the (110) direction is about 0.7 Â 10 À2 nm À1 . The line prole drawn in Fig. 3(a) crosses a bi-layer step [a monoatomic step in Fe(001) is 0.14 nm high], as visible in the topographic curve reported in Fig. 3(b). Aer the Fe 3 O 4 growth, the surface morphology is characterized by a higher density of steps, about 2.5 Â 10 À2 nm À1 along the (110) direction, which are highly  oriented along either the (001) or (010) crystallographic direction of the substrate. Panel (d) displays the line prole acquired across Fe 3 O 4 terraces. Considering that the interlayer spacing between layers with the same termination is 0.21 nm, the measured topography corresponds to bunches of two and three steps of magnetite. The mesoscopic morphology of FeO reported in Fig. 3(e) is very similar to that of Fe 3 O 4 in terms of steps density, orientation and height. The line prole drawn in Fig. 3(f) corresponds approximatively to a two-layer high step along the [001] direction of the rock-salt FeO [a monoatomic step in FeO(001) is 0.215 nm high].

Results and discussion
It should be emphasized that, on the FeO samples, it was possible to obtain stable STM images only at high tip-sample voltage (above 2 V). Conversely, on Fe 3 O 4 it was possible to measure also with small sample-tip bias, in agreement with the insulating and conductive nature of FeO(001) and Fe 3 O 4 (001) surfaces, respectively.
On the Fe 3 O 4 , thanks to the good electrical conductivity of the surface, it was also possible to obtain atomically resolved images, shown in Fig. 4(a). The atomic corrugation is dominated by periodic rows with an apparent height of about 0.3Å and separated by about 6Å, as measured from the topographic line of Fig. 4(b). The rows belonging to different terraces are mutually rotated by 90 . These observations are similar to those reported for the reconstructed Fe 3 O 4 (001) surface of bulk sample and ultrathin lms, 1 despite we do not observe theð ffiffiffi 2 p Â ffiffiffi 2 p ÞR 45 superstructure in the LEED measurements, as already mentioned above. In Fig. 5 a highly resolved image is displayed, in which elongated features are present. In order to rationalize the structural data acquired by LEED and STM, the right panel of Fig. 5 reports a schematic model of the unreconstructed Fe 3 O 4 (001), which is characterized by a square unit cell compatible with the diffraction pattern measured by LEED. We suggest that, in our STM measurements, dimers of oxygen atoms separated by the octahedral Fe row are imaged as bright ovals, while the octahedral Fe atoms are imaged as depressions. We notice that our STM images of Fe 3 O 4 differ noticeably from those reported in most of the literature, where the Fe cations are imaged as protrusions, despite there are reports in which anions are imaged as bright spots. 42 In order to discuss the Fe 3 O 4 -FeO interconversion we recall that both phases are characterized by an oxygen face-centeredcubic lattice and differ only for the cation disposition. The Fe cations completely occupy the octahedral sites in FeO, while in magnetite there are Fe 3+ in tetrahedral sites and a 50 : 50 mixture of Fe 3+ and Fe 2+ in octahedral sites. During the reduction of Fe 3 O 4 to FeO oxygen atoms should be removed by the annealing and/or additional cations added to the iron oxide lattice. Generally, the UHV annealing of Fe 3 O 4 bulk or Fe 3 O 4 lms grown on MgO modies the surface structure but does not induce a complete reduction of magnetite to FeO, 35 in agreement with the fact that even at a temperature of 1000 K and pressure of 10 À10 mbar the free energies of Fe 3 O 4 and FeO are À96.1 kJ mol À1 and À71.3 kJ mol À1 , respectively. 43

Conclusions
Fe 3 O 4 (001) lms have been grown by reactive molecular beam epitaxy on a Fe-p(1 Â 1)O surface. The Fe 3 O 4 (001) LEED pattern forms a p(2 Â 2) superstructure with respect to the substrate, revealing a surface structure compatible with a bulk terminated Fe 3 O 4 (001) sample. Highly resolved STM images have been obtained on magnetite, tentatively assigned to an imaging mode in which oxygen atoms are measured as protrusions. The Fe 3 O 4 (001) lms can be converted in rock-salt FeO(001) by annealing the sample at 800 K in UHV. The FeO surface is characterized by a p(1 Â 1) diffraction pattern with respect to the substrate, with a lattice constant similar to that of bulk Wüstite. The Fe 3 O 4 (001) and FeO(001) phases are clearly discernible also by AES, both in the high and low kinetic energy transitions. Finally, it has been shown that the Fe 3 O 4 (001) phase can be restored by the oxidation of FeO(001) at high temperature.

Conflicts of interest
There are no conicts to declare.