Atomic surface structure of Fe3O4(001) in different preparation stages studied by scanning tunneling microscopy

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surface science

Surface Science 285 (1993) 1-14 North-Holland

Atomic surface structure of Fe,O,( 001) in different preparation studied by scanning tunneling microscopy

stages

G. Tarrach, D. Biirgler, T. Schaub, R. W~esendanger and H-J. G~ntherodt Institut fik Physik, Unii~~rs~t~t 3asei, ~i~~eibe~strass~ 82, 4056 Rasel, Switzerland

Received 23 September 1992; accepted for publication 9 December 1992

We report a scanning tunneling microscopy study on the structure of the Fe,O,(OOl) surface on the atomic scale. The single-crystalline magnetite sample was characterized in ambient air by X-ray diffractometry and resistance versus temperature curves. Under ultrahigh vacuum condition, the sample was prepared by annealing as well as ion-sputtering. The surface was monitored by Auger-electron and photoelectron spectroscopy in order to determine the chemical composition. The atomic structure was investigated in direct space by scanning tunneling microscopy and these results were compared to the reciprocal space information gained by low-energy electron diffraction. We found different atomic surface reconstructions in all three preparation stages examined.

1. Introduction

The mineral magnetite (Fe,O,) is a ferrimagnet which played an important role in the discovery of magnetism. It is used in wide-spread technological applications due to its magnetic properties and its inertness under ambient conditions. It serves as core material for electromagnetic coils, as catalyst for various chemical reactions and as colloid material in ferro~uid suspensions as well as for recording purposes. Furthermore, it is of great interest in geology and archaeology because of its frequent occurrence in the earth’s crust and its impact on the Iocal magnetic field. The scanning tunneling microscope (STM) and related techniques nowadays allow the investigation and manipulation of surfaces on the nanometer scale. These capabilities and the great demand for high density magnetic recording media focused interest again on magnetite and its related iron-oxides maghemite (y-Fe,O,), hematite (a-Fe,O,) and wustite (FeO). It is well known that the different iron-oxides can be transformed into each other. The transformation process is sensitive to the environment in which it takes place. The metastable y-Fe,O,

phase can be obtained from Fe,O, by moderate annealing in air, whereas higher temperatures lead to the thermodynamically most stable (YFe,O,. It was also found that air-exposed ironoxide samples are covered with a layer of cu-Fe,O, [1,21. The transformation upon annealing is reversed in ultrahigh vacuum (UHV). Fe,O, is transformed into Fe,O, by heating, which can be further reduced to Fe0 at temperatures above 873 K f3f. This variety of chemical processes shows the importance of being able to distinguish various iron-oxides. Wandelt 14) showed that photoelectron spectroscopy of the Fe2p,,, and 0 1s core levels offers this capability for single-crystalline samples. Bulk Fe,O, has a cubic inverse spine1 sJructure (fig. 1) with a lattice constant of 8.3967 A [5]. The oxygen anions form a fee lattice and the iron cations occupy tetrahedrally (A-sites) and octahedrally (B-sites) coordinated interstices. The valence structure is ~Fe3+](Fe3+Fez+X02-)~, where half of the Fe3+ are located on A-sites and the other half together with the Fe*+ ions on B-sites. The spins of the A- and B-sublattices couple anti-ferromagnetically, leading to a net magnetization due to the different total magnetic mo-

OO39-6028/93/$06.00 0 1993 - Elsevier Science Publishers B.V. All rights reserved

ments of the two sublattices. The magnetization persists up to the Curie temperature of 858 K. Electron hopping between neighbouring Fe’+ and Fc’+ on B-sites is the reason for the good electrical conductivity at room temperature. Upon cooling, magnetite undergoes a Verwey transition at a temperature in the range of 115-124 K, where the electron hopping is frozen and the crystal becomes insulating. The full symmetry of the superlattice of the Fe2+ and Fe’+(B) ions resulting from the crystallization of the electrons into a Wigner crystal has not yet been determined. Band structure calculations [6] and inverse photoemission experiments [7] showed that the model of Fe,O, as an ionic crystal is not sufficient for a complete understanding of the electronic properties, since it neglects the strong orbital mixing between Fe 3d and 0 2p states. However, it explains the calculated gap in the majority spin band at the Fermi energy E,. As this gap is absent in the minority band, a negative electron spin polarization is measured near the pho-

tothreshold in photoemission experiments [Xl. The observed high value of spin polarization was the trigger for our spin-polarized tunneling cxpcriments on magnetite by means of a STM [0,10]. The comparison of the atomic corrugation mcasured with non-magnetic tungsten tips and with ferromagnetic iron tips lead to the conclusion that the differences in the images have to he attributed to the magnetic structure of the sample surface on an atomic scale. This report presents further results on the same crystal. Here. only the atomic structure has been invcstigatcd and the magnetic effects will not be discussed.

2. Instrumental The experiments were performed in a fourchamber UHV system. Each chamber is individually pumped by an ion pump and can be operated in the low 10 _ ” mbar range. One chamber offers facilities for thermal annealing by resistive heat-

L/

0

O'- ions Fe”+: tetrahedral

Fig. 1. Conventional

+ interstices

loo11

(A-sites)

cubic unit cell of the inverse spine1 structure

of Fe,O,.

For clarity,

the atoms are shown only in the front half.

3

G. Turrach et al. / STM study of Fe,O,(OO1)

ing and for ion-sputtering. A standard commercial surface analysis chamber is equipped with a hemispherical electron analyser for Auger-electron spectroscopy CAES) and X-ray as well as ultraviolet photoelectron spectroscopy (XPS and UPS). Furthermore, it offers the capability of scanning electron microscopy (SEMI, ion sputtering and resistive heating. Low-energy electron diffraction (LEED) can be performed in a third chamber, while the last contains a STM. There, as well as in the analysis chamber, a pressure below the lower operation limit of the manometer (1 x 10-l’ mbar) can be maintained for several hours by supplemental cryogenic pumps. Samples are mounted onto standard specimen holders which can be picked up with wobble-stick manipulators. They are moved to any experimental location within the UHV system by use of two linear transport systems. The magnetite crystal was mounted on a Taholder with an integrated W-wire for resistive heating. For annealing it was placed on the heating stage in the analysis chamber, where the pressure could be kept below 5 X lo- I0 mbar at the m~mum annealing temperature. The filament current through the W-wire was used as control parameter for annealing. The temperatures of the sample surface and of the specimen holder were measured with a dual-wavelength infrared pyrometer. The latter temperature, more accurately measured because of the smaller working distance, was used to reproduce the annealing conditions. All temperature values had to be corrected according to the disturbing effect of the Kodial viewport and cannot be given more precisely than rt30 K. Sputtering was done in the analysis chamber with a scanning ion gun operated with Ar at a beam energy of 1.5 keV. The intensity was 2 PA/cm2 for 25 min at normal incidence. We used Mg Ka radiation (1253.6 eV> of a twin-anode X-ray source for the XPS measurements. The analyser was operated at a constant pass energy (CAE-mode) of 20 eV resulting in an instrumental resolution below 0.2 eV. The photoemission for UPS was induced by the He I (21.2 eV1 and He II (40.8 eV> lines of a windowless gas discharge lamp. There, the pass energy of the

analyser was chosen as 1.0 eV in order to give an instrumental resolution of 0.01 eV, which is less than the linewidth of the lamp. The AES measurements were performed with a scanning electron gun using a beam energy of 3 keV. In conjunction with a scintillator/ photomultiplier we could select the area of interest by taking SEM images. The rear-view LEED can be operated at a pressure of 1 x lo-” mbar. The spot size of the electron beam is below 1 mm. Since we have no facilities for intensity measurements or spot profiling, we only use LEED for qualitative interpretation of the spot patterns and for the determination of the lattice constants. Detailed structural information on the atomic scale is gained with the home-built STM in combination with commercial electronics. The STM is mounted on a stack of steel plates with Viton isolation elements in between. The whole setup is suspended by a double-stage spring system with eddy-current damping in order to isolate the microscope from external vibrations. The details of the instrument together with a description of the coarse approach of the sample to the tip are given in ref. fll]. All STM images presented in this letter were obtained at room temperature with electrochemitally etched tungsten tips. The electrical resistance as a function of temperature and also the X-ray diffractometry (XRD) of the crystal were performed ex situ in ambient air. The radiation in the 2B-diffractometer was the CuKc~i-line (8047.8 eV) with a significant CuKa,-satellite (8028.1 eV). XRD and all in situ analysis were performed at room temperature.

3. separation

and results

The sample is a natural single crystal of magnetite originating from Zillertal (Austria). It was cut with a diamond saw to a rectangular plate of 2 mm thickness and has a size of 10 mm. The resulting Fe,O,(OOl) surface was first abraded and then polished using diamond paste with decreasing grain sizes of 3, 1 and 0.25 pm. After rinsing in ethanol, the sample was screwed onto the well-degassed Ta-holder.

The initial ex situ measurements include the acquisition of the electrical resistance versus temperature curves and XRD in ambient air. The resistance curves showed the Verwey transition at 98 K. From the positions of the (004) and (008) peaks in XRDDwe calculated a lattice constant of 8.406 If: 0.010 A in the direction perpendicular to the investigated surface. The diffractograms of a powdered part of the sample are in good agreement with the data base spectrum for magnetite as well as with the theoretical line positions. After these measurements the crystal was introduced into UHV and spent nine months there with preparation and analysis cycles. Back in air, a second set of measurements was performed ex situ in order to detect changes in the structure of the bulk. The [OOl] lattice constant was determined once more with the (004) and (008) peaks resulting in a value of 8.404 _t 0.010 A. This is

equal to the initial value within the limit of the experimental error. In the following we give a detailed description of the history of this magnetite crystal in IJHV, where we can discern three different states of the magnetite surface: annealed to 810 K (stage A), annealed to 880 K (stage B) and Ar-sputtered with subsequent annealing to 880 K (stage 0. 3.1. Annealed to 810 K The first preparation steps in UHV were successive annealing cycles with increasing maximum temperatures up to 810 f 30 K. The chemical composition of the surface was monitored by AES. As soon as only the Fe and 0 peaks were left in the spectrum range of 50 to 800 eV, LEED and a whole series of STM measurements were performed. The images show stepped surface ar-

Fig. 2. Atomic surface structure on two adjacent terraces separated by a 2.3 A high step edge. The image contains an area of 70 A x 76 A and was acquired at stage A (1.0 nA tunneling current at t3.0 V sample bias). The lateral separation of the rows is h A.

G. Tarrach et al. / STM study of

eas with a step height of 2.3 + 0.3 A. Each terrace contains parallel rows separated by 6 A. The orientation of these rows changes from [liO] to [1101 direction on 2.3 A higher or lower terraces, as can be seen in fig. 2. More results of this surface state have already been published in refs. [9,10,12]. The sample had to be taken out of UHV for a short time once during this phase in order to adjust the mounting of the crystal on the holder. After the interruption, the magnetite sample was prepared the same way as described above and further STM measurements were taken. These images revealed the same surface topography as before. We call this preparation phase stage A for further reference. 3.2. Annealed to 880 K After having exposed the crystal to ambient conditions for a longer time, the preparation process of stage A could not reproduce a clean surface anymore. The AES spectrum taken at that time indicated contaminations of C, K and S. Therefore, we increased the annealing temperature to 880 + 30 K and these peaks vanished. Additional XPS measurements confirmed the re-

Fig. 3. LEED

pattern of the Fe,O,(OOl) pattern (arrows) correspond

Fe,O,(OOl)

5

sult. This state of the clean surface will be referred to as stage B. The LEED patterns reveal a square lattice, where the (20}-spots dominate in intensity. One of these spots is marked with an arrow in fig. 3a. Their first order spots c?rrespond to a lattice periodicity of 6.3 + 0.5 A along the [liO] and [llO] directions. Therefore, the patterns can be interpreted in terms of a ccl X 1) surface symmetry with respect to the conventional cubic unit cell. We performed extensive STM measurements on the clean surface at many different locations on the crystal, inserting intermediate annealing cycles in order to avoid contamination. Stable tunneling was possible in the range of 2.5 to 3.5 V bias voltage for either polarity. Survey images as the example presented in fig. 4a typically show stepped surface areas with orthogonal $ep edge structures. A step height of 2.3 _t 0.3 A can be evaluated from the histogram of the height values in fig. 4b. The appearance of the atomic-scale corrugation is different for the two polarities of the bias voltage. A pattern of elongated worm-like shapes is observed at positive bias of the sample with respect to the tip. The periodicity along the elon-

surface in stage B at 61 eV (a) and in stage C at 66 eV (b). The brightest spots in each to the same lattice periodicity along the [liOl direction of the cubic unit cell.

6

/ -5

0 2

ii)

5

I

ID

Fig, 4. (af STM image of stage B, which shows the step structure with 2trtmie rcsoluticm on an area of 350 A X 3% A tO.3 nA tunneling current at +3.0 V sample bias). Cbt A step height of 2.3 A can be determined in this histogram of image fzr).

Fig. t. Atomic surface structure on two adjacent terraces separated by a 2.3 A high step edge. The image contains an area of and was acquired at stage B (0.3 nA tunneling current at + 3.0 V sample bias). The ends of the line mark a phase shift of the atomic features at two different locations on the same terrace.

(198 A?

Fig. 6. Rotation

of the atomic

structure at a step edge by 90”. The image was taken in stage B with negative tunneling current at -3.5 V). The area covers 198 A X 102 A.

sample

bias (0.3 nA

gation in [110] direction is 12.0 & 0.5 A and the corrugation amplitude is 1.0 _+0.3 A. The corresponding values perpendicular to the worm-like structures (in [liOl direction) are 6.0 k 0.3 and 0.4 +_0.2 A. Vacancy positions of the worm-like entities reveal parallel rows running alon [liO]. They are separated by a distance of 6.0 A. The orientation of the pattern rotates by 90”0comparing terraces vertically kept apart by 2.3 A (fig. 5). The rows can be recognized on both terraces of fig. 5. Fig. 6 shows a surface area with two terraces separated by a step of 2.3 A height taken at a negative bias voltage. Rows of spherical components along [liOl are recognized with every second row appearing enhanced. The spheresOof the enhanced rows are strictly ordered on a 6 A x 12 A lattice, whereas those of the rows in between are often shifted along the direction of the rows. This direction rotates at step edges in the same way as described above for the positive polarity. The corrugation of, enhanced spheres are0 measured as 0.4 + 0.1 A along and 0.6 + 0.2 A perpendicular to the rows. A direct comparison between images simultaneously acquired with opposite bias voltages enables us to determine the positions of spheres and worm-like structures relative to each other.

725

720

715

710 705 Binding

We find that the centers of the enhanced sphorrs correspond to the centers of the worm-like clements and the rows of spheres are orientated perpendicular to the elongation of the former. Furthermore, it can be seen in fig. 5 that the worm-like structures are placed between the lower-lying rows. Comparing the translational symmetry of the STM images to the LEED patterns we ascertain that the period along the [llO] direction differs by a factor of two. Thus, the atoms are located on a (fi/2 X fi/2)R45” lattice while the local elcctronic structure has (fi X fi/2)R45’ symmetry, both with respect to the [loo] and [OIO] vectors of the conventional bulk unit cell (fig. 1). 3.3. Sputtered We monitored the effects of sputtering and annealing on the chemical composition and the surface structure with AES and LEED. Two sputter cycles succeeded by annealing to a temperature of 880 K lead to a new and stable surface structure, which we call stage C. This stage was investigated using all available techniques, namely AES, XPS, UPS, LEED and STM. The AES measurements showed no other peaks than the Fe and 0 series. The shape of the

700

695

690

685

660

675

Energy / eV

Fig. 7. Comparison of the shapes and positions of the Fe 2p lines in stage B (lower curve) and stage C (upper curve). The Fc 2~3, z line is identified at 710.1 eV in both spectra, where the 0 Is peaks (inset) are positioned at 529.9 eV. The two bumps on the right are due to satellites of the Mg Km-radiation.

G. Tarruch et al. / STM study of Fe,O,(OUlj

Fe2p lines shown in the XPS spectra of fig. 7 (upper curve) is compared to the shape of the lines acquired in stage B (Iower curve). The energy scales of the spectra have been adjusted so that the 0 1s transition occurs at 529.9 eV. This position is independent of the oxidation state of the surface as was shown in ref. [3]. Comparing the two measurements, no difference either in the line shape or in the position of the Fe2p,,* line (710.1 eV> can be recognized. In stage C we additionally measured the valence band structure with UPS and compared the spectra with measurements on a Pd(OO1) single crystal in order to determine the Fermi edge (E,). At 4.8 eV below E, we observe the maximum intensity, which is mainly the contribution of the 02p states. Shoulders at 6.5 and 3 eV originate in the Fe4+ fina state multiplets [13,4]. The measured intensity just below E, is due to

9

the 6A,9 final-state configuration of the Fe2+ ions at B-sites. The LEED patterns (e.g. see fig. 3b) indicate a square lattice with intensity variations of the different spots. The Ill}-spots and their higher orders dominate the pattern at all energies. The lattice constant of 8.44 + 0.07 A was determined by direct comparison to the patterns of a Si(OOl)(2 X l)-reconstruction. It measures the periodicity along the [loo] and [OlOl directions. Consequently, the LEED pattern reveals a ~(3 x 3) symmetry relative to the bulk unit cell. Therefore, the comparison to the cfl x I> pattern of stage B implies a transition of the surface structure from stage B to stage C. The morphology of the crystal surface on a larger scale is shown by a STM image in fig. 8. Again we recognize the orthogonal&y of the underlying lattice and the occurrence of mainly 2.3

Fig. 8. Crystallographic topography of the Fe304(OOl) surface in stage C imaged by STM (0.22 nA tunneling current at +3.0 V sample bias). The area measures f2000 A)‘.

A high steps at a higher magnification (fig. 9). The atomic surface structure is dominated by a X.3 + 0.3 A square lattice aligned with the bulk unit cell. It shows a slightly asymmetric appearance of the atomic features towards [l 101. The underlying rows, visible on extended areas of vacancy sites (see upper left of fig. 9a), are 45” rotated with respect to the square lattice. Notice that the step edge at the left of the image runs exactly along an underlying row parallel to [IjO]. The direction of asymmetry of the atomic features runs perpendicular to the rows (along [I lo], see fig. 9b) a!d rotates by 90” on a terrace being a step of 2.3 A higher (compare lower with upper part of fig. 9a). The difference in the apparent height between the top of the bright atomic features and the top of the underlying rows is with

‘,.5 _+0.2 A significantly higher than the 0.h + 0.2 A corrugation of the bright spots. All images presented for stage C were acquired with negative bias voltage, since tunneling was more stable with this polarity and the resolution was poor using positive sample bias. In contrast to stage B, the STM images of stage C have the same ~(1 x 1) translational symmetry as the LEED pattern.

4. Discussion There are several points to be discussed for the interpretation of our results. First, we check whether the bulk material is Fe,O,. Second, we confirm that the surface has the same structure

Fig. 9. STM images of stage C taken with 0.1 nA tunneling current at -3.0 V sample bias. (a) On an area of 140 A X 180 A, the rotation of the atomic surface structure at a step is visible. The step edge in the upper left reveals the lower-lying rows. (b) The image with higher magnification

shows the orientation

of the asymmetric atomic features on an area of (65 A)‘.

C. Tarrach et al. / STM study of Fe,30,(OOl)

and stoichiometry as the bulk. And third, we present a model for the atomic surface structures observed by STM that is consistent with XRD, LEED and the spectroscopic measurements. 4.1. Bulk From XRD diffractograms we can exclude the existence of significant amounts of Fe, Fe0 and a-Fe,O, in the bulk. Discriminating Y-Fe20, is not as easy as the other phases since magnetite and maghemite have nearly the same structures and lattice constants. However, taking into account both XRD peaks of the single crystal spectrum and the powder spectrum, it is the database entry for magnetite that fits the experiment better. This conclusion is also supported by the resistance curves, which indicate a Verwey transition The transition temperature of 98 K is lower than for stoichiometric Fe,O, (115-124 K). This fact may be attributed to an iron deficiency of about 0.5% [14] or to bulk impurities, which we might expect in a natural crystal. Comparing initial and final XRD measurements, we can exclude a bulk transformation induced by the preparation and analysis in UHV. Also the [OOl] lattice constant, which is a sensitive indicator for the transition between magnetite and maghemite, did not change within the limits of the experimental accuracy, 4.2. Surface By permanently monitoring the surface composition by AES: we know that the relative ratio of iron to oxygen did not vary significantly during every stage of the experiment. Unfortunately, we cannot determine an absolute value for the concentration ratio of Fe to 0. The quantitative peak analysis strongly relies on the correct consideration of the background and the transmission characteristics of the experimental setup, which we do not know accurately. These problems are even more serious in the XPS spectra. We restrict our analysis therefore to the interpretation of the peak shapes and positions of the Fe 2p and 0 1s lines in stages B and C. The maximum of the Fe2~,,~ peak lies between the values of Fe,O,

11

and Fe0 as determined in refs. 11-31. The peak shape shows the Fe2+ final state satellite in the tail to higher binding energies. The classification of this satellite is also supported by the UPS measurement, where due to the final-state configuration of Fe2+ a significant intensity near E, was detected. In XPS, the 0 1s peak is very sharp and contains only a slightly asymmetric tail, which is also reported by the mentioned authors. The extent of this tail is not so large as to expect much chemisorbed oxygen on the surface. Considering the spectroscopic results we conclude that the surface consists of Fe,O, just as the bulk, with the reservation that from XPS/UPS data only a Fe0 surface cannot be excluded. 4.3. Atomic structure The discussion of the atomic surface structure is based upon STM measurements. We first note that the [loo] and [OlO] lattice constants determined in the images are at all stages consistent with the values observed by LEED and match well with the [OOl] value from XRD, as required by the cubic structure. The characteristic features of the atomic corrugation in the STM images are shown in fig. 10. The extent of four conventional unit cells (fig. 1) is indicated by thin lines. The rows (thick lines) are common to all three stages. They are apparent in stage A and hidden by an atom layer above them in stages B and C. The worm-like structures appearing at positive bias voltages in stage B are represented by open ovals, whereas filled large (small) circles indicate the position of the enhanced (dark) spheres in the images taken at negative bias voltages. Inspecting the bulk structure of magnetite, we conclude that the rows can be identified with the layer built up from oxygen and iron B-sites, since only this layer has two-fold symmetry (figs. 1 and lla). We also note that these layers are 2.1 A apart and that the symmetry line rotates by 90” in correspondence with the observed behaviour at step edges. Since the rock salt structure of Fe0 does not contain any family of lattice planes with the observed symmetry properties, this iron-oxide phase can be excluded for the surface of our sample.

(a) stage

Fig. 10. Scheme

A

(b) stage

B

(c) stage

C

of the atomic corrugation in the STM images of stages A (left), B (middle) and C (right). The structure an extent of four conventional unit cells, as indicated by the thin lines.

From band structure calculations [6] and experimental work El33 it is known, that the 02p states lie well below E, and are therefore not accessible for tunneling experiments. This means that the contrast in STM images mainly originates in the Fe states. Consequently, the rows common to all STM images can be interpreted as the Fe B-sites, where the individual ions are not resolved along the rows due to the small nearest neighbour distance of 3 A. For a more thorough explanation of the apparent shape of the atomic features, and how their asymmetry relates to the

(a) stage

A

is shown in

electronic structure, Iocal tunneIing spectroscopy (CITS, see ref. [IS]> would be required. Unfortunateiy, we were not successful in taking these measurements with simuItaneous atomic rcsolution. For the description of the top-most layer in stages B and C we take a closer look at the structure and coordination of Fe30, in terms of an ionic crystal. We are aware of neglecting the covalent orbital mixing and ignore the Wigner crystallization [16] for this discussion. In the bulk, the tetrahedrally coordinated Fe atoms transfer 3

(b) stage

B

(c) stage

C

Fig. 11. Atomic model of the Fe,04 (001) surface at the three different preparation stages A, B, and C. The layer with 0 (open circles and squares) and Fe(B) (filled smalt circles) from stage A is covered with a different number of Fe(A) in stages 13 and C (large filled circles).

electrons (e-1 to each of the four neighbouring 0, leaving Fe3+ ions at A-sites. The octahedral Fe atoms transfer & e- to each of the six surrounding 0, resulting on the average in Fe’12+ ions at B-sites. Cutting the bulk to expose a (0011 surface consisting of 0 and Fe B-sites (filled circles in fig. lla), for each of the octahedral Fe sites an 0 atom is absent above it and thus seare in excess. On the other hand, &e- are missing for one half of the 0 atoms (open circles in fig. lla) and Se- for the other half (squares in fig. lla) due to the removal of neighbouring Band A-site Fe atoms. Although the first half of 0 (open circles) can be satisfied by the excess charge on Fe ions, three electrons per unit cell are still required by the other half (open squares). The charge for the 02- ions can be supplied by covering the surface partially with Fe ions. There are two preferred positions per unit cell, namely the A-sites of the buIk crystal. In our mode1 the surface of stage A is frustrated, since no Fe atoms satisfy the charge needed. The existence of this surface structure might be explained by orbital mixing or by a lack of Fe. The transition into stage B is understood by the higher annealing temperature, which is in the range where Fe,O, is reduced to Fe0 [31. Although we can exclude this transformation, the surface tends to enlarge the amount of iron relative to oxygen. This leads to the observed structure, where an Fe atom is located at every A-site on top of the Fe-O layer. The only possibility for the transfer of three electrons per unit cell from the two Fe atoms to the oxygen is to have the same number of Fe”’ and Fe’+, since the oxidation stage Fe’+ does not exist. Having the same number of two electronically inequivalent types of Fe ion, the reconstruction of stage B can be understood as the worm-like features and enhanced spherical entities being the Fe”+ and the dark spheres the FeO* ions. The position of the Fe”* ions along the rows is not very well defined in the STM images, which can be attributed to their weak binding. It is obvious from fig. lob that in principle two anti-phase domains exist with equivalent reconstructions. They can be transformed into each other by exchanging the rows of enhanced and dark spheres. Therefore

we would expect the occurrence of anti-phase domains in the images of stage B. On flat areas with a high number of Fe-vacancies, they are indeed observed. An example is indicated by the line mark in fig. 5, where one end of the line crosses the worm-like features while the other end lies between them. The sputtered surface of stage C is only covered with half the number of Fe atoms compared to stage B. In the context of electron counting, all of the top-layer Fe atoms have to transfer three electrons. The reason for the transition from stage B to C is not clear. We may not draw conclusions from the comparison of the atomic structure before and after the treatment, since sputtering is a destructive preparation method and usually changes the surface topography in an uncontrolled way. It was shown by Aeschlimann et al. [S] that Ar sputtering changes the magnetic properties of a sample as well. The transformation of Fe,O, into y-Fe,O, was presumed in order to explain the observed changes. Our results give clear evidence to exclude this transformation, at least for the combined Ar sputtering with successive annealing to a temperature about 200 K higher than in ref. [81.

5. Conclusions We have applied different analysis techniques to investigate the detailed structure of the Fe,O,(OOl) surface. We emphasize the use of the STM in such studies as a method complemental to the other techniques used. While it is difficult to distinguish the individual iron-oxides by spectroscopic techniques and on the other hand, while STM does not provide element-specific information, the combination of these allowed us to identify the iron-oxide phase of our sample surface and its atomic structure. Furthermore, we have confirmed the strong dependence of the Fe,O, surface properties on the preparation parameters. This work shows that such dependences are observable on single crystalline samples and detectable on the atomic scale. We believe that studies combining integral techniques and local methods, such as STM, will

14

G. Tarrach et al. / STM study of Fe.,O,(OOl)

provide more insight into the dependence electronic and magnetic surface properties the atomic structure.

of the upon

Acknowledgements We would like to thank I.V. Shvets for his contributions at the initial stage of this work, H.P. Lang for his help with the X-ray diffractograms and Professor S. Graser for providing the single crystals of magnetite. Financial support from the Swiss National Science Foundation and the Kommission zur Forderung der wissenschaftlichen Forschung is gratefully acknowledged.

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