PHYSICAL REVIEW B 67, 165403 共2003兲

Analysis of the short-range order of the AuÕFe„001… surface alloy
M. M. J. Bischoff, T. K. Yamada, and H. van Kempen*
NSRIM, University of Nijmegen, Toernooiveld 1, NL-6525 ED Nijmegen, The Netherlands
共Received 17 September 2002; revised manuscript received 15 January 2003; published 2 April 2003兲
Scanning tunneling microscopy measurements show that the growth of about half a monolayer Au on
Fe共001兲 at a temperature of 170 °C leads after a postanneal at 700 °C to an island free and homogeneously
alloyed surface layer. The surface alloying is in contrast to the bulk where Au and Fe are immiscible. Strong
chemical contrast in the images allows us to analyze the short-range order of this surface alloy. Two independent methods are considered: the first method extracts pair occupation probabilities directly from digitizedimages, the second method makes use of the autocorrelation of filtered images. Both methods lead to the sameconclusion: only for the nearest-neighbor sites a strong preference for unlike species is found; for further sites
the probabilities follow a random distribution. Scanning tunneling spectroscopy measurements on the alloyed
AuFe surface are used to corroborate the interpretation of the chemical contrast. Furthermore, the spectroscopy
results show that the Fe共001兲 surface state shifts to higher energies on areas where pure Fe patches are confinedby Au atoms.
DOI: 10.1103/PhysRevB.67.165403

PACS number共s兲: 68.35.Bs, 68.37.Ef, 73.20.At

I. INTRODUCTIONof the chemical contrast. Furthermore, it will be examined
how the Fe共001兲 surface state is influenced by the alloying.

In a recent publication we showed that the roughening of
the Au共001兲/Fe共001兲 interface is a result of the creation andannihilation of a surface-confined alloy during growth of thissystem.1 Scanning tunneling microscopy 共STM兲 images withchemical contrast revealed that for growth temperatures
above 100 °C and coverages below 0.5 ML Au atoms become incorporated in the Fe共001兲 surface despite the immiscibility of Au in bulk Fe.2 Growth of about 0.5 ML Au
around 200 °C was found to lead to a homogeneously alloyed surface layer. However, if more Au is added, thesurface-confined nature of the alloy3 leads to a demixing
with a roughened interface as a natural consequence. Understanding of the processes driving the surface alloying/
dealloying and consequently the interface roughening are important since the quality of the interface, i.e., sharp/rough,mixed/pure, determines to a large degree the transport andmagnetic properties of multilayer devices. In this perspective, the Au/Fe system can be considered as a perfect model
system. However, to come to a better understanding of these
phenomena a thorough characterization of the Au-Fe surface
alloy is necessary.
In the present paper we make use of STM images whichshow chemical contrast to analyze the short-range order ofthe Au/Fe surface alloy. For this purpose, two methods ofanalysis are considered. The first method obtains the pairoccupation probabilities directly from the STM images. Thiskind of analysis was, e.g., presented by Davies et al. for the
Cr/Fe共001兲 surface alloy.4 The second method makes use of
the autocorrelation of the STM images and was described indetail by Hebenstreit et al.5 and Schmid6.
The surface state of the Fe共001兲 surface which appears as
a strong peak in the spectroscopic dI/dV curve7–10 can beused as a chemical fingerprint of local Fe areas as was successfully shown by Davies et al. in the case of Cr on
Fe共001兲.11 Also, in the present study scanning tunnelingspectroscopy 共STS兲 is used to corroborate the interpretation
0163-1829/2003/67共16兲/165403共7兲/$20.00

II. EXPERIMENTAL DETAILSThe experiments were performed in a home-built ultrahigh vacuum 共UHV兲 system consisting of a fast-entry loadlock, a preparation chamber and an analysis chamber. The
preparation chamber has a base pressure of about 1⫻10⫺10mbar and contains equipment for sample cleaning, electron
beam heating, epitaxial growth, and Auger electron spectroscopy 共AES兲. The analysis chamber which contains the STM
has a base pressure of less than 5⫻10⫺11 mbar.
A single-crystal Fe共001兲 whisker with a size of about7⫻1⫻1 mm3 was used as the sample. In the early stage ofthe cleaning process, AES showed that oxygen and carbon
impurities preferentially segregate to the surface at temperatures between 200 °C and 300 °C. The AES signals of these
impurities were found to decrease again at higher annealing
temperatures. Therefore, in order to get rid of these bulk
impurities most efficiently the ion bombardment 共750 eVAr⫹ ) was performed while ramping the annealing temperature between 150 °C and 750 °C. The sample temperature of
750 °C was verified with an infrared pyrometer. After a few
days of these cycles, only oxygen could be detected by AES.
The ratio of the oxygen and iron peaks was about 2% at thisstage. From the concentration of depressions in the STM
images, this ratio can be translated into an oxygen coverageof about 1%. After this bulk-depletion stage, the whisker was
sputtered for about 45 min at 750 °C before the actual measurement. The radiative cooling time to room temperature
was sufficient to repair any sputter damage as verified by the
STM measurements which generally show terraces wider
than 200 nm. This treatment also kept the surface impurity
concentration below 1%.
Gold (99.99% pure兲 was evaporated from a Knudsen cellwhich is shielded by a liquid nitrogen cooled shroud. This
keeps the pressure in the low 1⫻10⫺10 mbar range during

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M. M. J. BISCHOFF, T. K. YAMADA, AND H. van KEMPEN

FIG. 1. Chemically resolved
STM image of the Fe共001兲
surface which was covered
with 0.5 ML Au and postannealed at 700 °C (30 nm⫻30 nm, ⫺0.28 V, 0.10 nA兲.
60 pm depressed lattice sites canbe observed in this image whichshow an apparent short-range order: The areas marked ‘‘A’’,‘‘B’’, and ‘‘C’’ show a localc(2⫻2) ordered area, a line
along the close-packed 关100兴 direction, and a line along the关110兴 direction, respectively. ‘‘
D’’ shows one of the deeper andlarger depressions, which are attributed to impurities.

growth. The evaporation rate was typically 0.2 monolayer/
min as determined by a quartz crystal microbalance and histograms of STM images obtained after submonolayer growthof Au at temperatures below 100 °C. Here one monolayer
共ML兲 is defined as a complete overlayer with Fe共001兲 bulk
lattice constant (1.214⫻1019atoms m⫺2 ). During growth, thesubstrate can be resistively heated to a maximum temperature of about 250 °C. The sample temperature during growth
was measured by a thermocouple mounted close to 共but notin direct contact with兲 the sample making the inaccuracy inthese temperature measurements large (⫾30 °C).
The STM experiments were performed with an Omicron
UHV STM-1 operating at room temperature. Electrochemically etched W tips were cleaned and sharpened in situ by Ar
self-sputtering. During the STM measurements, the tips were
often ‘‘reconditioned’’ by applying voltage pulses of ⫾10 V
at about 0.1 nA. To be able to compare topographic featureswith spectroscopic ones, STS measurements were performed
simultaneously with conventional constant current images: ateach pixel the feedback loop was temporarily switched off torecord an I(V) curve. These images were measured at a reduced resolution of typically 75⫻75 pixels to avoid strong
distortions due to drift. The images presented in this paper
were not corrected for drift. The I(V) curves were numerically differentiated using a five-point window to obtain
dI/dV.

III. RESULTS AND DISCUSSION

In Ref. 1 we showed that for submonolayer growth of Au
共coverages below 0.5 ML兲 bright features appeared in the
Fe共001兲 terrace for growth temperatures above 100 °C. Since
the concentration of these features increased with growthtemperature and Au coverage, we identified them as Au atoms which have place exchanged with substrate Fe atoms.
For growth of about 0.5 ML Au at about 200 °C a homogeneously alloyed overlayer was found. Nevertheless, this alloyed overlayer apparently did not show any long-range order. To study this overlayer in detail, 0.5 ML of Au is grownon Fe共001兲 at a substrate temperature of 170 °C. In contrast
to the experiments in Ref. 1, after growth we postannealed
the sample for 1 h at a temperature of 700 °C. The motivation for this postannealing step was to see whether this treatment could result in an ordered surface alloy. Furthermore,postannealing reduces the island density which facilitates theanalysis. The sample was radiatively cooled down to room
temperature after the annealing step. Figure 1 shows an
atomically and chemically resolved STM image of the surface after this preparation. It must be noted that this strong
chemical contrast is only achieved with special tip conditions. Possibly, in this case an adsorbate at the tip apex interacts differently with the various species of surface
atoms.12 This contrast does not show any strong influence of

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ANALYSIS OF THE SHORT-RANGE ORDER OF THE . . .

TABLE I. Pair occupation probabilities for equivalent surface near-nearest neighbor sites 共up to fifthnearest neighbor兲. Three STM images are analyzed and are labeled ‘‘1’’, ‘‘2’’, and ‘‘3’’ for convenience.
However, only the image labeled ‘‘1’’ is shown in this paper 共i.e., Fig. 1兲. C Au and C Fe are the Au and Fe
concentrations, respectively. P AuAu(n) and P FeFe(n) are the probabilities to find a Au-Au or Fe-Fe pair at nthsurface nearest-neighbor sites.
Image

C Au

P AuAu(1)P AuAu(2)P AuAu(3)P AuAu(4)P AuAu(5)

1
2
3

0.265
0.270
0.279

0.162
0.151
0.160

0.323
0.286
0.292

0.268
0.270
0.278

0.256
0.271
0.280

0.262
0.271
0.289

C Fe

P FeFe(1)P FeFe(2)P FeFe(3)P FeFe(4)P FeFe(5)

0.735
0.730
0.721

0.695
0.688
0.680

0.756
0.739
0.733

0.732
0.730
0.721

0.730
0.730
0.724

0.730
0.731
0.728

1
2
3

the particular set points of tunneling current or bias voltage.
No islands are visible after the high-temperature postannealing in contrast to the growth at about 200 °C. 1 The terraces
show 60 pm deep depressions at the lattice sites of a weakly
corrugated (⬍10 pm)p(1⫻1) lattice. A long-range orderingof the depressions is missing although locally smallc(2⫻2) ordered patches 共cf. the area marked ‘‘A’’兲, lines
along the close-packed 关100兴 directions 共cf. marked ‘‘B’’兲
and lines along second nearest-neighbor 关110兴 directions 共cf.
marked ‘‘C’’兲 are often observed. The larger and deeper
共about 100 pm deep兲 depressions 共cf. marked ‘‘D’’兲 are attributed to oxygen impurities. Their concentration is about
1%. Furthermore, tip conditions not allowing chemical contrast image them as well.
STM images like Fig. 1 are digitized by manually transforming the square lattice into an m⫻n 共with m,n typicallyaround 50兲 matrix of which each element 共corresponding to alattice site兲 can possess only three height values: ‘‘0’’ if thesite is occupied by an impurity, ‘‘1’’ if occupied by a brightatom, and ‘‘2’’ if occupied by a depressed atom. From thesematrices the surface coverage of the two species (C Au andC Fe) and the pair occupation probabilities for equivalent( P AuAu(n) and P FeFe(n) ) and inequivalent surface nearneighbor sites ( P AuFe(n) and P FeAu(n) ) can be determined.
Near-neighbor sites up to the fifth nearest neighbor (n⫽5)
are considered. Sites having impurities at near-neighbor sites
closer than five times the nearest-neighbor distance are discarded from the analysis. The results of this analysis areshown in Table I for three STM images. First, it follows thatthe concentration of the depressed sites in Fig. 1 is
0.265 ML. From this it can be concluded that the depressed
lattice sites are the Au atoms, since only 0.50 ML Au wasdeposited. It also means that about half of the deposited Au
disappears into the bulk after the high-temperature postannealing step. Table I shows that only the pair occupation
probabilities for equivalent nearest- and next-nearestneighbor sites deviate from a random distribution for which

FIG. 2. Topographic STM image showing chemical contrast
across a step edge (11 nm⫻11 nm, ⫺0.28 V, 0.10 nA兲. The Au
and Fe atoms at the step edge are marked. A stepped linear grayscale has been used: the black-white range corresponds to about
0.15 nm on both terraces. The black 共white兲 perimeter along the stepedge is a gray scale artifact.

FIG. 3. Histogram of the apparent heights of the pixels corresponding to the atom positions of Fig. 1 共gray curve兲. This histogram is fitted to a sum of two Gaussians 共black curve兲. The results
for the fitting parameters m 1 –m 6 are shown as well. The Au coverage can be determined from the ratio between the Gaussian peak
amplitudes: C Au⫽1⫺m 4 /(1⫹m 4 ).

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M. M. J. BISCHOFF, T. K. YAMADA, AND H. van KEMPEN

FIG. 4. Autocorrelation analysis of STM image with chemical
contrast 共Fig. 1兲. The STM image is modified as described in the
text. 共a兲 shows the autocorrelation of the atomic image 共i.e., allatoms have the same height兲. 共b兲 shows the autocorrelation of thechemical image 共the Au and the Fe atoms have a different height兲.
The white square in the center of the image is a gray scale artifact关 AC(0) is one, while at the other lattice sites it is much lower兴.
Equivalent points in these images are marked. White circles correspond to AC(1), black circles to AC(2), dotted circles to AC(4).
See text for discussion.

P AuAu(n) ( P FeFe(n) ) would be simply C Au (C Fe). More specifically, P AuAu(1) is only 58⫾2% of the value expected fora random distribution, while P FeFe(1) is reduced to 94⫾1%. From these values it can be concluded that nearest
neighbors of different species are preferred. The values forthe second nearest neighbors are less different from a random distribution although there is a slight preference for
equivalent species: P AuAu(2) and P FeFe(2) are 111⫾8% and102⫾1% of the values expected for a random distribution.
The probabilities for n⬎2 are equivalent to the random distribution within the accuracy of the analysis.
Figure 2 shows an atomically and chemically resolved
STM image across a monoatomic step. It can be seen that thesteps have a preference for Au atoms. About 80% of the stepedge sites are occupied by Au atoms which is three times
higher than the value expected for a random distribution.
To check the reliability of the results obtained above, themethod based on the autocorrelation as described byHebenstreit5 and Schmid6 is also used to analyze the shortrange order of the Au-Fe surface alloy. For this purpose thechemically resolved STM image of Fig. 1 is first bandpass
filtered to enhance the atomic lattice. A local maxima search
is implemented to find the pixel positions of the atoms. The
histogram of the apparent heights of these pixels is shown in
Fig. 3 and shows two well-separated peaks. This histogram isfitted to a sum of two Gaussians,f 共 x 兲 ⫽ 共 m 1 /m 2 兲 exp兵 ⫺ 关共 z⫺m 3 兲 /m 2 兴 2 其⫹ 共 m 1 m 4 /m 5 兲 exp兵 ⫺ 关共 z⫺m 6 兲 /m 5 兴 2 其 .

共1兲FIG. 5. Pair occupation probabilities for Au-Au pairs 共image 1of Table I兲 specified for each lattice direction. The values at the graysites are slightly enhanced in agreement with the autocorrelation
results of Fig. 4共b兲. The gray sites make up an apparent surface
alloy unit cell with an Au concentration of 1/3.

The values of m 1 –m 6 which are the results of the fitting
procedure are shown in Fig. 3 as well. The Au concentrationon the surface can be extracted from m 4 , 5,6
C Au⫽1⫺m 4 / 共 1⫹m 4 兲 .

Thus, for the image of Fig. 1 we find an Au concentration of
0.269 which is in good agreement with the result obtained by
the first 共direct兲 method 共see image ‘‘1’’ in Table I兲. The
apparent height which separates the two histogram peaks
共roughly ⫺0.01 nm) is used as a discriminator: pixels withheight smaller than this value are put at ⫺ 冑C Au /C Fe, while
pixels with a larger height are put at ⫹ 冑C Fe /C Au. 5,6 Then,the single-pixel atom positions are broadened to 3⫻3 pixel
clusters. The values of the remaining background pixels are
put at a value that assures that the average of the total image
is zero. The autocorrelation of this ‘‘chemical’’ image is calculated.
By setting the pixel clusters that describe the atom positions at equal values, the chemical information is lost. By
definition, the autocorrelation of this ‘‘atomic’’ image must
be one at lattice distances. However, deviations from a perfect square lattice 共due to distortions兲 result in autocorrelations less than one. These deviations influence the autocorrelation of the chemical image equivalently. Therefore, tocompensate for these errors the autocorrelation values obtained from the chemical image are normalized by the auto-

TABLE II. Autocorrelations (AC) for the five nearest-neighbor distances obtained directly from thechemically resolved STM image 共Fig. 1兲 and obtained from the probabilities of Table I 共averages of three
images兲 and Eqs. 共3兲 and 共4兲.

Direct calculation
From probabilities

共2兲

AC(1)

AC(2)

AC(3)

AC(4)

AC(5)

⫺0.21⫾0.01⫺0.16⫾0.01⫹0.04⫾0.02⫹0.05⫾0.02

0
0

0
0

0
0

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ANALYSIS OF THE SHORT-RANGE ORDER OF THE . . .

FIG. 6. 共a兲 and 共b兲 are consecutively obtained STM imagesof the same area of the Au-Fe
surface alloy (15 nm⫻15 nm).
The set points for these images
are ⫺1.0 V, 0.04 nA 共a兲 and⫺0.28 V, 0.01 nA 共b兲, respectively. A change of the tip apex
configuration leads to a different
contrast: the protrusions in 共a兲correspond to the dark latticesites in 共b兲. A spectroscopy measurement is performed at the
same
area
(V s ⫽⫺0.5 V,
I⫽0.20 nA). 共c兲 shows thedI/dV map at V s ⫽⫹0.20 V. 共d兲shows dI/dV curves obtained byaveraging the single dI/dV
curves obtained in the encircled
areas in Figs. 3共a兲–3共c兲.

correlation values from the atomic image. Both autocorrelation images are shown in Fig. 4.
The interpretation of this method is as follows. An autocorrelation value of zero corresponds to a random distribution. An autocorrelation of ⫹1 means that atoms of the same
species are preferred, while a negative value means that atoms of different species are preferred. For the latter case, theminimum value is given by 1⫺1/C Fe , i.e., ⫺0.37 (⫺1 isonly obtained for a perfectly ordered c(2⫻2) structure兲.
Figure 4共b兲 shows that a large negative value for the autocorrelation 共AC兲 is found at the nearest-neighbor distances.
Averaging over the four equivalent sites gives AC(1)⫽⫺0.21⫾0.01. The autocorrelation of the averaged second
nearest-neighbor sites is slightly positive: AC(2)⫽⫹0.04⫾0.02. The autocorrelations for larger distances average tozero. However, a more careful look at Fig. 4共b兲 reveals that
there is a weak asymmetry in AC(2) and AC(4): slightly
larger values are found at two diagonally linked sites for bothAC(2) and AC(4).
Now the results of both methods can be compared byusing the relation between the autocorrelations and the pairoccupation probabilities,5,6
W diff共 n 兲 ⫽2C FeC Au关 1⫺AC 共 n 兲兴 .

共3兲

W diff(n) is the probability to find a pair of different atoms at
nth nearest-neighbor sites and can be expressed in the probabilities given in Table I,

W diff共 n 兲 ⫽C AuP AuFe(n) ⫹C FeP FeAu(n) .

共4兲

P AuFe(n) and P FeAu(n) are simply given by 1⫺ P AuAu(n) and1⫺ P FeFe(n) , respectively. Using the values of Table I, autocorrelation values are calculated from Eqs. 共3兲 and 共4兲. The
results are shown in Table II together with the autocorrelations obtained from Fig. 4. It is clear that both methods leadto similar results.
To confirm whether the weak asymmetry in the AC(2)and AC(4) values is a real effect or an artifact of the analysis, the pair occupation probabilities for image 1 of Table I
are specified for the different directions. Figure 5 shows thatthe asymmetries are also found in the directly obtained pairoccupation probabilities. This seems to point at a unit cell forthe surface alloy which contains 6 atoms of which 2 are Au.
Remarkably, the Au concentration in this cell is 1/3 which iscomparable to the Au concentration in the surface. Therefore,
this cell might be the result of a balance between the Au
surface concentration and repulsive Au-Au interactions. The
asymmetry might be imposed by the monoatomic steps on
the Fe共001兲 surface. Indeed, comparison with Fig. 2 showsthat the short direction of the unit cell of the surface alloy isparallel to the step direction. The preference for Au atoms at
the step edge sites might thus induce the observed asymmetry.
In contrast to the images shown in Ref. 1, in which only
the Au atoms are visible, Figs. 1 and 2 show both species of

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M. M. J. BISCHOFF, T. K. YAMADA, AND H. van KEMPEN

FIG. 7. STS measurement
performed at Fe共001兲 covered
with 0.35 ML Au at 200 °C. The
size of 共a兲 and 共b兲 are 13 nm⫻13 nm. 共a兲 is the topographic
image obtained at a set point of⫺0.01 V, 0.25 nA and 共b兲 is thedI/dV map at V s ⫽⫹0.20 V.
The spectroscopic measurement
共set point: ⫺1.0 V, 0.5 nA兲 was
performed after the topographic
image. Squares in 共a兲 and 共b兲 indicate equivalent areas. 共c兲shows four dI/dV curves thatare representative of the four different contrasts in the dI/dV
map. These curves are averagesof the curves obtained within thelabeled squares and circles
共1– 4兲.

the Au-Fe alloyed overlayer. Occasionally, changes in the tip
configuration, probably caused by the exchange of the tip
apex atom, lead to large changes in contrast. As an example,
Figs. 6共a兲 and 6共b兲 show two consecutively obtained STM
images of the same area. It is obvious that the 15 pm highprotrusions in Fig. 6共a兲 correspond to the dark lattice sites inFig. 6共b兲. Some adsorbates at the tip 关e.g., in Fig. 6共a兲兴 leadto a strong interaction with the embedded Au atoms causing
an increase in the tunnel current, but not with the Fe atoms
that are not resolved; other adsorbates 关e.g., in Fig. 6共b兲兴
cause an interaction that decreases the current at the Au atoms, which probably allows the tip to come close enough tothe surface to resolve the Fe lattice.
Between the measurements of Figs. 6共a兲 and 6共b兲 an STS
measurement was performed at the same area 共set points:
V s ⫽⫺0.5 V and I⫽0.20 nA). Figure 6共c兲 shows the dI/dV
map at the bias voltage of ⫹0.20 V. This voltage is chosen
since Fe共001兲 is known to have a localized d z 2 -like surface
state around this energy.7–10 The dark areas in Fig. 6共a兲 arebright in the dI/dV map which gives additional evidence thatthe dark areas in Fig. 6共a兲 are the Fe atoms and the brightprotrusions are the Au atoms. Also, since the dark atoms in
Fig. 6共b兲 are positioned at the positions of the bright protrusions in Fig. 6共a兲, these dark atoms must be the Au atoms.
Two averaged dI/dV curves are shown in Fig. 6共d兲. The
areas in which the pixels have been averaged to obtain thesecurves are marked in Figs. 6共a兲 to 6共c兲. Unfortunately, the tip
leading to the strong atomical and chemical contrast does notallow the Fe共001兲 surface state to be resolved in the dI/dV
curve as a peak due to the rapidly rising background withincreasing voltage. Instead, only a shoulder is observed. The
influence of the tip configuration on the background waspreviously discussed for STS experiments on Mn/Fe共001兲.10
The reversed contrasts of Figs. 6共a兲 and 6共b兲 show that a
relation between the chemical contrast and the Fe共001兲 surface state is not straightforward. Both images are recorded at
negative bias voltages, where the influence of the surface
state on the electronic structure is negligible. Furthermore,these types of contrasts change frequently 共when the tip con-dition is changed兲 and show no particular influence on thetunneling setpoints. In other words, no evidence was foundthat the chemical contrast is enhanced around the surfacestate energy. Therefore, instead of explaining the chemical
contrast in terms of the Fe共001兲 surface state, the best we cando at present is to identify both types of atoms by making
use of the Fe共001兲 surface state.
Figure 7 shows STS results obtained with a tip showing
the Au atoms as bright protrusions 共set points: ⫺0.01 V,
0.25 nA兲. For these measurements the postannealing step
was not performed: 0.35 ML Au was grown on Fe共001兲 at a
substrate temperature of 200 °C. For this preparation, theisland coverage corresponds to the amount of Au. In thiscase, the dI/dV curves that are obtained on the same area asin Fig. 7共a兲 共but after the topographic image was obtained
and with different set points: ⫺1.0 V, 0.5 nA兲 show alargely suppressed background at the high-voltage side 关e.g.,
Fig. 7共c兲兴. The dI/dV map at V s ⫽⫹0.20 V 关Fig. 7共b兲兴
shows bright and dark areas on both the terrace and the island. The island appears darker in the dI/dV map than theterrace that can be explained by the higher Au concentrationin the island compared to the terrace for this particular preparation. However, it is not trivial to correlate the contrasts inthe dI/dV map to the atomic scale features in Fig. 7共a兲. The
reason is that the larger tunneling resistance during the spectroscopy measurements leads to a larger tip-sample distancewhich deteriorates the resolution 共typically 0.7 nm, seee.g.13–15兲. Nevertheless, some tendencies can be recognized.
To facilitate this purpose, equal areas are marked withsquares and circles in Figs. 7共a兲 and 7共b兲. The brightest
dI/dV map contrasts on the terrace are found at pure Fe共001兲
patches of at least 1 nm⫻1 nm. Although less clear, thedarkest dI/dV map contrasts on the terrace seem to be localized on areas which contain many Au atoms. The same conclusion can be drawn for the island: The brightest dI/dV
map contrasts are found in areas which contain many dark
spots 共i.e. Fe兲, while the darkest dI/dV map contrasts on theisland are found on close-packed areas of bright protrusions
共Au兲.

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ANALYSIS OF THE SHORT-RANGE ORDER OF THE . . .

Four dI/dV curves which are representative of these four
different dI/dV map contrasts are shown in Fig. 7共c兲.
Clearly, the pure Fe共001兲 patches reveal a strong peak at⫹0.20 V 共e.g., curve ‘‘1’’兲. The areas of close-packed Au
atoms on the island 共e.g., ‘‘4’’兲 show the other extreme: apeak around ⫹0.70 V. Curves ‘‘2’’ and ‘‘3’’ show peaks atenergies between ⫹0.20 and ⫹0.70 V. It should be stressedthat the dI/dV curves shown in Fig. 7共c兲 are only presenting
four typical curves measured at this complex surface; curves‘‘1’’ and ‘‘4’’ only represent the extremes that are observed.
Instead of relating the ⫹0.70 V peak to the Au, it seems
more reasonable to relate it to an upward shift of the d z 2
surface state of Fe共001兲. The incorporated Au atoms may
confine the Fe共001兲 surface-state electrons to small patches
thereby increasing their energy. The same kind of behavior
was observed on a Fe96.5Si3.5 surface alloy:8 here onedimensional localization of the Fe共001兲 surface state on antiphase domain boundaries leads to an upward shift to⫹0.6 V. Geometrically, the local structure of these domainboundaries resemble the present local structures.
IV. CONCLUSION

We presented two independent analyses of the short-rangeorder of the surface-confined AuFe alloy. This surface alloy*FAX:⫹31 24 3652190. Email address: hvk@sci.kun.nl
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was prepared by growth of about 0.5 ML Au on an Fe共001兲
whisker at a temperature of about 170 °C. After growth, thesurface was annealed at 700 °C. A strong preference for unlike nearest-neighbor sites was found. The pair occupation
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images with chemical contrast was corroborated by STS results which reveal the well-known Fe共001兲 surface state atareas as small as 1 nm2 . Upward peak shifts of the Fe surface state were found on small Fe areas which are enclosed
by incorporated Au atoms. It is clear that this shift strongly
depends on the local geometry, although, the typical resolution during STS measurements is too low to allow for a more
precise determination.

ACKNOWLEDGMENTS

We would like to thank D. T. Pierce for kindly providing
us with the Fe whiskers. This work was supported by theEuropean Growth project MAGNETUDE and the Stichting
voor Fundamenteel Onderzoek der Materie 共FOM兲, which isfunded by the Nederlandse Organisatie voor Wetenschappelijk Onderzoek 共NWO兲.

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