PHYSICAL REVIEW B 68, 045422 共2003兲

Local electronic structure of Fe„001… surfaces studied by scanning tunneling spectroscopy
M. M. J. Bischoff, T. K. Yamada, C. M. Fang,* R. A. de Groot, and H. van Kempen†
NSRIM, University of Nijmegen, Toernooiveld 1, NL-6525 ED Nijmegen, The Netherlands
共Received 23 September 2002; revised manuscript received 15 January 2003; published 30 July 2003兲
Scanning tunneling spectroscopy is used to study the local electronic structure of Fe共001兲 whiskers. The
influence of a voltage dependent background on the apparent peak energies in the dI/dV curves is discussed.
A relation between this background and the apparent barrier height is established. The previously reported
spin-polarized d z 2 -like surface state is shown to shift by an amount of 0.04 eV to higher energies at isolated
oxygen impurities on the Fe共001兲 surface. The surface state disappears on both 关100兴 and 关110兴 directed steps.
Finally, we investigated size effects by studying the behavior of the surface state on nanometer-sized Fe
islands. Besides the surface-state quenching at the step edges, a peak shift to higher energies is observed for
decreasing island sizes.
DOI: 10.1103/PhysRevB.68.045422

PACS number共s兲: 73.20.At, 68.37.Ef

I. INTRODUCTION

direction within a p(2⫻2) unit cell兲.8 It was found that thesurface state disappears at a step and shifts slightly to higher
energies at oxygen impurities. Although the use of a spherical potential leads to lower surface-state energies,7 qualitative results such as shifts and disappearances of surface
states are still believed to be correctly described.
In the present paper, STS measurements on ideal and nonideal Fe共001兲 surfaces are reported. First, it is shown that thepeak energies found in both dI/dV and (dI/dV)/(I/V)
strongly depend on the background in these spectroscopic
curves. This voltage-dependent background is shown to berelated to the apparent barrier height and, consequently, tothe tip configuration. Second, the influence on the local electronic structure of defects such as single oxygen impurities
and monoatomic steps is examined. Finally, size effects arestudied by measuring dI/dV curves on nanometer-sized islands.

The localized d z 2 -like surface state of magnetic Fe共001兲
has been detected in scanning tunneling spectroscopy 共STS兲
by several groups since 1995.1–3 However, the real power of
STS 共i.e., the local sensitivity down to an almost atomic
scale兲 has not been fully utilized, and data on STS measurements on nonideal Fe共001兲 surfaces are still scarce. Nevertheless, this information is of utmost importance for the understanding of electronic and magnetic processes at surfaces
and interfaces, e.g., tunnel magnetoresistance, giant magnetoresistance, and interlayer exchange coupling.
From the few STS experiments on nonideal Fe共001兲 surfaces the following results can be summarized. Biedermann
et al. showed that on an Fe96.5Si3.5(001) surface alloy, onedimensional localization of the Fe共001兲 surface state on antiphase domain boundaries leads to an upward peak shift
from ⫹0.17 eV for pure Fe共001兲 to ⫹0.6 eV. 2 Remarkably,
Oka et al. reported on a c(2⫻2) phase on 10-nm-thick
Fe共001兲 films grown on MgO at 300 °C, which shows a peakin the dI/dV curves around ⫹0.4 V. 4 X-ray photoelectron
spectroscopy measurements did not find any contaminants,
and therefore it was speculated by Oka et al. that the peakshift is a result of a rearrangement of the Fe atoms. Finally,
Kawagoe et al. reported on a strong reduction of the emptystate local density of states 共LDOS兲 up to 1 eV above theFermi level, which was attributed to scattering at the stepedge.3 The same authors also report an increase of the LDOS
at the step edge around ⫺0.5 eV.
Inspired by these STS experiments, many groups havecalculated the band structure for the Fe共001兲 surface. A peakis always found in the minority spin channel of the surface
共or vacuum兲 LDOS, although the exact energy position varies in different works, i.e., 0.2 eV,1 0.3 eV,2 0.17 eV,5 0.19
eV,6,7 and ⫺0.1 eV, 8 mainly depending on whether a fullpotential or a spherical potential approach is utilized.7 Also,
the choice of the exchange-correlation potential and the useof either experimental or theoretical 共i.e., ab initio兲 lattice
constants seems to be crucial.
Until now, the calculations by our group are the only ones
which considered nonideal surface structures such as oxygen
adatoms, Fe adatoms, voids 关all in a p(2⫻2) structure兴, andsteps 共i.e., a periodic row of adatoms running in the 关 100兴
0163-1829/2003/68共4兲/045422共7兲/$20.00

II. EXPERIMENTAL DETAILSThe experiments were performed in a homebuilt 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, electronbeam heating, epitaxial growth, and Auger electron spectroscopy 共AES兲. The analysis chamber which contains the scanning tunneling microscope 共STM兲 has a base pressure of lessthan 5⫻10⫺11 mbar.
A single-crystal Fe共001兲 whisker with a size of about 7⫻1⫻1 mm3 was used as the sample. In the early stage of thecleaning 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(750-eV Ar⫹ ) was performed while ramping the annealing
temperature between 150 °C and 750 °C. After a few days ofthese 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 coverage

68 045422-1©2003 The American Physical Society

PHYSICAL REVIEW B 68, 045422 共2003兲

BISCHOFF, YAMADA, FANG, DE GROOT, AND VAN KEMPENTABLE I. Peak energies observed in dI/dV and (dI/dV)/(I/V)
curves and the most likely apparent barrier height during the corresponding STS measurement. The set point during the STS measurements was V s ⫽⫺1 V and I⫽0.5 nA.

FIG. 1. Averaged dI/dV 关black兴 and (dI/dV)/(I/V) 关gray兴
curves measured on clean Fe共001兲 with different tip conditions. In
共a兲 the peak in dI/dV is at ⫹0.22 V 共obtained from a Gaussian fit兲,and an increasing background is visible at the high-energy side of
the peak. In 共b兲 the peak in dI/dV is at ⫹0.16 V; the background isreduced making this peak much more pronounced than the peak of
共a兲. The peak energies are always lower in the normalized curves,
i.e., ⫹0.14 V for 共a兲 and ⫹0.12 V for 共b兲.

of 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.
For the study of the nanometer-sized Fe islands, Fe was
evaporated at a substrate temperature of about 40 °C 共cooling
down after annealing兲 from a Knudsen cell with a rate of 0.2
monolayer 共ML兲 per minute as checked by a quartz microbalance; 1 ML corresponds to a complete coverage of the
Fe共001兲 surface with Fe. The pressure during growth remained in the low range of 1⫻10⫺10 mbar due to a liquidnitrogen-cooled shroud between the furnace and the UHVchamber.
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. STS measurements were performed by recording I(V) curves under open feedback conditions. Typically, such a curve consists of 100 points measured each witha 200 ␮ s delay time and a 320 ␮ s acquisition time. To beable to compare topographic features with spectroscopic
ones, I(V) curves were measured simultaneously with conventional constant current images but at a reduced resolution
共typically 75⫻75 pixels兲 compared to high-resolution STM
measurements 共typically 512⫻512 pixels兲 to avoid strong
distortions due to drift. The I(V) curves were numericallydifferentiated using a five-point window to obtain dI/dV.
III. RESULTS AND DISCUSSION
A. Influence of tip on STS results

The results of two typical STS measurements on a clean
Fe共001兲 area are presented in Fig. 1. The black and graycurves represent dI/dV and (dI/dV)/(I/V), respectively.
The dI/dV curves in 共a兲 and 共b兲 show different apparent
peak energies at ⫹0.22 V and ⫹0.16 V, respectively. The
normalized (dI/dV)/(I/V) curves show peaks at ⫹0.16 V

dI/dV
Peak energy 共eV兲(dI/dV)/(I/V)
Peak energy 共eV兲

␾ 共eV兲

Shoulder
0.19
0.18
0.17
0.16

0.17
0.15
0.14
0.14
0.12

3.3
4.2
4.4
5.1
5.1

共a兲 and ⫹0.12 V 共b兲, respectively. It can also be clearly seenthat the peak energies in corresponding dI/dV and(dI/dV)/(I/V) curves are not the same. The curves in Fig.
1共a兲 have a strong exponential background on the highenergy side of the peak, while for the curves in Fig. 1共b兲 thisbackground is completely absent. Due to this background,the peaks are found at a slightly higher energy for the curves
shown in Fig. 1共a兲 compared to those shown in Fig. 1共b兲.
Since both measurements are performed on clean Fe共001兲
areas (⬎2⫻2 nm2 ), the differences in the curves can only
be a result of the different electronic structures of the tip.
Table I shows the apparent peak energies in the dI/dV
curves and the corresponding peak energies in(dI/dV)/(I/V) obtained with different tip configurations.
The tip configuration changed spontaneously or was deliberately changed by applying voltage pulses. Obviously, thepeak energy in (dI/dV)/(I/V) is always lower than in
dI/dV.
The different apparent peak energies in the dI/dV curves
can be traced back to the voltage dependence of the exponential background. Normalization of dI/dV by I/V does notcancel this background, as can be seen from the curves inFig. 1. It has been shown by Ukraintsev that neither dI/dVnor (dI/dV)/(I/V) perfectly represents the LDOS.9 Rather,
Ukraintsev showed that dI/dV normalized by its fit to thetunneling probability function gives the best representationof the LDOS, at least within a one-dimensional WKB description of the tunneling process. However, for accurate fitting, dI/dV curves need to be measured over a large voltage
range to describe the background accurately, which is notalways straightforward experimentally 共i.e., due to the nonlinear dependence of the tunneling current on the sample
voltage, conventional current amplifiers will become overloaded at high bias voltages兲. Nevertheless, Fig. 2 shows anaveraged dI/dV measurement obtained on a clean Fe共001兲
area, and the result of fitting the exponential tunneling probability function9 multiplied by an offsetted Gaussian line
shape to this dI/dV curve. Thus, a surface-state energy of⫹0.17 eV is found. It was verified that dI/dV curves with abroad range of different backgrounds lead to this surfacestate energy when normalized in the proposed manner. Consequently, ⫹0.17 eV is considered to be the most accurate
value of the surface-state energy.
In order to obtain a better characterization of the tips leading to the different backgrounds, apparent barrier height045422-2

PHYSICAL REVIEW B 68, 045422 共2003兲

LOCAL ELECTRONIC STRUCTURE OF Fe共001兲 . . .

FIG. 2. Averaged dI/dV curve 共about 100 single curves; bold兲
obtained on a clean Fe共001兲 area. Set point was V s ⫽⫺0.5 V andI⫽0.5 nA. The dotted curve is the best-fit to this dI/dV curve of
the exponential tunneling probability function multiplied by an offsetted Gaussian as proposed by Ukraintsev 共see text兲. This leads tothe most accurate determination of the peak energy, ⫹0.17 eV.

measurements were performed. At a fixed bias voltage of⫺0.1 V, the tunneling current was increased in a few steps
to a maximum of ⬃25 nA, after which it was decreased
again. The corresponding tip displacements z were measured
as a function of the tunneling resistance R. Figure 3 shows
the results of these measurements. The vertical scale denotes
relative tip displacements. In a simple one-dimensional tunneling model 共see, e.g., Refs. 9,10兲, it can be shown that on
a logarithmic scale there is a linear relationship between the
tip displacement and the tunneling resistance (z⫽ ␤ R). From␤ the apparent barrier height ␾ can be calculated ( ␾⫽0.051/␤ 2 eV nm2 ), which can be interpreted as the average of the tip and sample work functions. The slopes of thecurves in Fig. 3 are determined from linear fits 共which arealso shown in this figure兲. High tunneling currents often leadto changes of the tip configuration, which are characterizedby tip displacements of less than 0.1 nm 共see arrow in Fig. 3
marking the tip change in measurement B). A different slope
is observed during tip retraction if a tip change occurs. In
these cases different functions were fitted to the tip-approachand tip-retraction data.
The apparent barrier heights obtained from the fits to thez(R) measurements are also shown in Fig. 3. Before andafter a particular barrier height measurement, an STS measurement was performed as well. This allows for a directcomparison between the surface-state peak energies observed
in dI/dV and (dI/dV)/(I/V) and the apparent barrier height
present during the STS measurement. The results of Table I
show that low 共high兲 apparent barrier heights result in a high
共low兲 exponential background, and therefore to a high 共low兲
peak energy. The inset in the top-left side of Fig. 3 shows
clearly the impact of this effect. The Gaussian-shaped curve
共black兲 was measured before tip displacement A. From thisz(R) a barrier height of 5.1 eV is deduced. The gray curve
which only shows a shoulder on a strong rising background
共the surface state is not observed as a peak in this case兲 was
measured after tip displacements A, B, and C. From tip displacement C, an apparent barrier height of 3.3 eV is deduced.
The absolute tip-sample distance can be estimated from
Fig. 3 by extrapolation of the tunneling resistance to thecontact point 关i.e., ⬃h/(2e 2 )⫽12.9 k⍀] where a tip-sample

FIG. 3. Tip displacement z as a function of tunneling resistanceR. Three datasets are shown (A, B, and C). Bold, dashed, anddotted lines are linear fits to datasets A, B, and C, respectively. The
slope ␤ of the linear fits is related to the apparent barrier height ⌽共also shown in the figure兲. Except for the displacement data A, thetip configuration changed at low resistances, which changed theslope 共i.e., the barrier height兲 for the retraction (⌽ retr.兲 comparedto the approach (⌽ appr.兲. The arrow marks a tip change in the B
dataset, which also leads to a 0.1-nm tip retraction at constant resistance. The upper inset shows the dI/dV curve measured before
tip displacement data A 共Gaussian line shape; black curve兲 and after
data C 共large background, and instead of a peak only a weak shoulder is observed; gray curve兲.

distance of around 0.2–0.3 nm is assumed. Therefore, at a
typical STS set point of 1 G⍀, the tip-sample distance canbe roughly estimated to be 0.8 nm for the dI/dV measurement showing a strong peak and 1 nm for the dI/dV measurement showing a shoulder.
The different backgrounds in the dI/dV curves are therefore a result of the variations in the apparent barrier heights
in a twofold way. First, lower 共higher兲 barrier heights decrease 共increase兲 the onset voltage of the exponentially increasing tunnel current. Second, the larger 共smaller兲 tipsample distance d at the set point of the spectroscopy
increases 共decreases兲 the voltage dependence of the tunneling probability function T, i.e., T(V)/T(0)⬃exp关⫺2„␬ (V)⫺ ␬ (0)…d 兴 , with ␬ the voltage-dependent inverse decay
length 共see, e.g., Refs. 9,11兲.
Imaging of monoatomic steps with tips leading to the two
extreme backgrounds observed in dI/dV both show a stepsmearing of 0.7 nm at the same STS set point (V s ⫽⫺0.5 V, I⫽0.5 nA). The lateral resolution in STM has been
estimated to be 关 (0.2 nm)(r⫹d) 兴 1/2, where r is the tip radiusand d is the width of the vacuum gap between tip andsample.12 This formula shows that with a single-atom tip (r⬃0.3 nm) and tip-sample distances of 0.8 nm and 1.0 nm
resolutions of 0.47 nm and 0.51 nm are predicted. These
predictions show that the 0.2-nm difference in tip-sample
distance hardly affects the resolution. This and the observation that tip changes leading to apparent barrier height variations from 1.4 eV to 5.1 eV only involve tip displacements
smaller than 0.1 nm seem to indicate that only the apex atom
is involved in these tip changes. Furthermore, oxygen impu-

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BISCHOFF, YAMADA, FANG, DE GROOT, AND VAN KEMPEN

rities seem to be imaged slightly sharper with the high background tip compared to the low background tip. Therefore,
these effects might be attributed to the difference in tunneling behavior between a metallic tip apex atom 共such as W,
Fe, and Au兲 and a nonmetallic one 共such as O兲. Electronegative atoms such as oxygen are known to increase work
functions,13 which corroborates this interpretation. The expected lower conductivity of a nonmetallic tip 共compared toa metallic tip atom兲 may lead to a shorter tip-sample distanceat a fixed current set point, and therefore diminishes the effect of the voltage-dependent background. The oxygen contamination on the Fe共001兲 surface makes it likely that 共atextremely low tunneling resistances兲 oxygen atoms are
picked up/dropped by the tip. It also should be noted that the
tip configuration, and consequently, the shape of the dI/dV
curves, can be easily changed by applying voltage pulses of⫾10 V at set point currents of about 0.2 nA. Accordingly,
tips are typically optimized for STS measurements to a configuration which shows the Fe共001兲 surface state as a
Gaussian-like peak in the dI/dV curves.
B. Influence of oxygen impurities

Figure 4 shows the results of an STS measurement on
Fe共001兲 contaminated with oxygen impurities. The oxygen
impurities are visible as 5–10 pm deep depressions in theconstant current image 共a兲 obtained at the set point of STS(V s ⫽⫺0.50 V,I⫽0.55 nA). From this image the impurity
concentration is estimated to be around 0.02 ML. dI/dV
curves were measured at every pixel simultaneously withthis constant current image. Figure 4共b兲 shows two single
dI/dV curves measured at the clean position ‘‘1’’ and thedepressed position ‘‘2,’’ respectively. These positions aremarked in Fig. 4共a兲 and lie along the cross section shown in
Fig. 4共d兲. Although single dI/dV curves obtained at RT show
a low signal-to-noise ratio, the surface-state peak is clearly
resolved. It is also clear that at the impurity the peak has
shifted slightly to a higher energy 共note the vertical line as a
guide to the eye兲. Furthermore, the amplitude of the peak isreduced by about 30%.
Since the peaks in the dI/dV curves obtained with thisparticular tip configuration follow an almost ideal Gaussianline shape, the single dI/dV curves were fitted to the Gaussians. A constant background of 1.0 nA/V is used for the fits
to the data of Fig. 4共a兲. It was not possible to obtain thisbackground as an independent fitting parameter. However, it
was verified that a change of 20% in this background parameter changes the peak energies found by the fitting routine by
only 10 mV. Furthermore, dI/dV is fitted only in a smallinterval around the peak 共between ⫺0.5 V and ⫹0.5 V for
this measurement兲. The best-fit Gaussians are also shown in
Fig. 4共b兲. In spite of the low signal-to-noise ratio in the
single curves, the fitted Gaussians show a reasonable agreement. Figure 4共c兲 shows for each pixel the peak energy
found by the fitting routine. A clear anticorrelation between
this image and the topographic STM image 共a兲 can be recognized: the topographic depressions appear as bright areasin the peak energy image, i.e., higher peak energies compared to clean Fe共001兲 are always found at impurities. The

FIG. 4. STS measurement performed on a typical Fe共001兲 whisker surface; 共a兲 shows the constant current image obtained at the set
points of the spectroscopy measurements (10⫻10 nm2 , ⫺0.50 V,
0.55 nA兲; 共b兲 shows two single dI/dV curves 共note the typicalsignal-to-noise ratio兲 representing two extremes: curve ‘‘1’’ is obtained on the clean Fe共001兲 area along the line in 共a兲, curve ‘‘2’’ on
the dark spot 共oxygen impurity兲 along this line. The vertical line isa guide to the eye for the oxygen-induced peak shift. All the 100⫻100 dI/dV curves were fitted to the Gaussians 关shown in 共b兲 aswell兴. Peak energies of these Gaussians are shown in 共c兲. The grayscale of this image is linear: black corresponds to ⫹0.16 V andwhite to ⫹0.20 V. Clearly, 共c兲 is anticorrelated with 共a兲: impurities
correspond to a higher peak energy. The line profiles along theblack and white lines in 共a兲 and 共c兲 are shown in 共d兲.

cross section of the peak energy image 关Fig. 4共d兲兴 shows thata single oxygen impurity shifts the Fe共001兲 surface state towards a 0.04 eV higher energy. The width of the dip in the z
cross section is equivalent to the width of the peak in thepeak energy cross section, i.e., about 1 nm. Therefore, thiswidth is related to the lower resolution during typical STS
measurements compared to atomically resolved STM imaging 共typically three orders of magnitude difference in tunneling resistance兲. The resolution during STS was tried to increase by changing the set points to lower bias voltages
and/or higher tunneling currents. However, bias voltages
lower than 0.5 V and tunneling currents higher than a few nA
generally lead to overloads in the current amplifier at theends of the voltage range during I(V) measurements 共typically from ⫺1 V to ⫹1 V). Observable effects on the resolution are, however, only expected if the set points arechanged by some orders of magnitude.12
At this point it is interesting to note that the same order ofsurface-state peak shifts on top of impurities have been recently reported for carbon impurities on V共001兲.14 –16 Apparently, electronegative impurity atoms interact with localizedd z 2 -like surface states comparably. Furthermore, the experi-

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LOCAL ELECTRONIC STRUCTURE OF Fe共001兲 . . .

FIG. 5. STS measurements
performed over monoatomic steps
on the Fe共001兲 surface. Stepsalong the 关 11̄0 兴 direction are observed in the constant current image 共a兲 which is imaged at the set
points of the spectroscopy measurements(15⫻15 nm2 ,⫺0.50 V, 0.50 nA兲. The dI/dV
values at V s ⫽⫹0.17 V are shownin 共b兲. The line profiles along thedashed lines in 共a兲 and 共b兲 areshown in 共c兲. 共d–f兲 are the constant current image (10⫻10 nm2 ,⫺0.50 V, 0.50 nA兲, dI/dV(⫹0.17 V), and corresponding
line profiles, respectively, for a
step along the 关 010兴 direction.

mental peak shift is comparable to the peak shift predictedby our calculations for oxygen on Fe共001兲, i.e., ⬃0.05 eV. 8
C. Monoatomic steps

Among defects on surfaces, steps are the most common.
Not only is the coordination of the step Fe atoms different
compared to the surface atoms, there is also a difference incoordination between Fe atoms along 关 100兴 and 关 110兴 directed steps. Consequently, variations in the local electronic
structure are to be expected.
In Fig. 5 STS measurements are shown over monoatomic
steps on the Fe共001兲 surface. Figure 5共a兲 shows the constantcurrent image taken at the set point of spectroscopy (V s ⫽⫺0.5 V, I⫽0.5 nA, 15⫻15 nm2 ): three steps which runparallel to the 关 11̄0 兴 direction are observed. At each pixel ofthis image (75⫻75 pixels兲 an I(V) curve was measured.
Figure 5共b兲 shows the dI/dV map at the surface-state energyof ⫹0.17 V. The steps appear as dark lines in this image,which is a clear indication that the surface state is quenchedat the steps. The line profiles along the dashed lines in Figs.
5共a兲 and 5共b兲 show that the width of the depressed lines inthe dI/dV map (⬃1.0 nm) correspond to the smearing of themonoatomic steps at this particular STS set point 关Fig. 5共c兲兴.
Figure 5共d兲 shows an STS measurement on an area showing
monoatomic steps running parallel to the close-packed 关 010兴
direction (V s ⫽⫺0.5 V, I⫽0.5 nA, and 10⫻10 nm2 ). The
corresponding dI/dV map 关Fig. 5共e兲兴 also shows these steps
as depressed lines. The line profiles along the dashed lines in
共d兲 and 共e兲 are shown in Fig. 5共f兲, which show that for the关 010兴 steps the width of the step broadening and dI/dV reduction also equals ⬃1.0 nm. Obviously, at both 关 010兴 and关 11̄0 兴 steps the Fe共001兲 surface state is quenched. The observation that the width of the depression in dI/dV equals
the width of the step smearing seems to imply that bothwidths have the same origin: the reduced resolution at thehigh tunneling resistance set point used during STS. Notethat the finer structures in the line profiles of Figs. 5共c兲 and5共f兲 are caused by the impurities as discussed in the preced-

ing section. These impurities can be clearly recognized as thedepressed spots in the dI/dV maps 关Figs. 5共b兲 and 5共e兲兴.
To investigate whether at the steps the surface state is
only reduced in amplitude, completely quenched, or shiftedto another energy dI/dV curves obtained at the center of thesteps,17 are shown in Fig. 6. With this tip configuration the
exponentially increasing background at the high-voltage side
is suppressed, which is most advantageous for observing
weak peaks. Clearly, at the step the Fe共001兲 surface state iscompletely quenched. This is in agreement with our
calculations8 and the calculations of Kawagoe et al.3 Remarkably, the same quenching of the electronic structure wasobserved on Cr共001兲,18 while no evidence for such a quenching was found on V共001兲.14,15
D. Size effects on nanometer-sized islands

An interesting question regarding the localized Fe共001兲
surface state is how large an Fe共001兲 area must be to show

FIG. 6. 共a兲 Averaged dI/dV curves measured on the terrace andalong the steps of Fig. 5共a兲. To obtain the black curve, about 100
single curves were averaged, whereas for the gray curve only about
ten curves were averaged. This explains the better signal-to-noise
ratio for the first one. The set point was V s ⫽⫺0.5 V, I⫽0.5 nA.
Note that with this particular tip configuration the exponentially
increasing background at the high-voltage side is suppressed. 共b兲.
Corresponding I(V) curves showing the effect of the surface stateon the averaged curve obtained on the terrace, and the almost
Ohmic 共linear兲 behavior along the steps.

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FIG. 7. STS measurement on Fe共001兲 covered with 0.1-ML Fe
at 40 °C; 共a兲 is the topographic image (10⫻10 nm2 , ⫺0.52 V, 1.0
nA兲. A dI/dV curve was measured at every pixel of this image; 共b兲
shows the dI/dV map around the surface-state energy of V s ⫽⫹0.18 V. The numbers in 共b兲 refer to the dI/dV curves shown in
共c兲. These curves are averages of typically ten single curves. The
dI/dV and (dI/dV)/(I/V) peak energies obtained on thenanometer-sized islands are shown in 共d兲 as a function of 1/L 21⫹1/L 22 . L 1 and L 2 are the island sizes along the short and long
directions, respectively, as indicated in 共a兲. Although the data points
show a large scatter, both data clouds can be fitted with linear
functions 共regression constants are 0.6兲. The vertical lines in 共d兲marked with ‘‘1– 4’’ denote the data points corresponding to theislands marked in 共b兲 and the dI/dV curves in 共c兲.

the surface state at its converged value. In order to clarify
this question, small amounts of Fe were deposited on the
Fe共001兲 surface at relatively low temperatures. Figure 7共a兲shows the results of such an experiment: 0.1-ML Fe wasdeposited at a sample temperature of 40 °C with a rate of 0.2
ML/min. At each pixel of this figure (V s ⫽⫺0.52 V, I
⫽1.0 nA, and 10⫻10 nm2 ), a dI/dV curve was obtained.
Figure 7共b兲 shows the dI/dV map around the surface-stateenergy (⫹0.18 V). In correspondence with the previous results for monoatomic steps, the rims of the islands appear
depressed in this dI/dV map. dI/dV curves obtained on various islands 共marked in the dI/dV map兲 are shown in Fig.
7共c兲. Compared to the clean terrace 共curve 1; peak at⫹0.18 V), the peak in dI/dV has shifted by ⬃0.06 V towards higher energy on the island marked 2 共curve 2; peak at⫹0.24 V). On island 3 the peak has further shifted to⫹0.33 V. Finally, on island 4 only a very small bump on the
background in dI/dV is observed. This island is about 0.10
nm high, and has a full width at half maximum of 1.1 nm
along the long direction, and 0.6 nm along the short direction. The height and widths seem to suggest that this island
consists of only a few atoms. However, it is unlikely that Fe
adatoms are stable at the growth temperature of 40 °C. It istherefore more likely that the smallest islands represent
dimers or trimers.

In order to check for a possible relationship between
surface-state energy and island size, dI/dV and(dI/dV)/(I/V) peak energies have been determined on various islands. As a first-order approximation, these islands arecharacterized by two lengths, L 1 and L 2 . These lengths aredefined as the distance from half maximum island height tohalf maximum island height along the long (L 1 ) and shortlengths of the islands (L 2 ), whose directions roughly follow
the close-packed 关100兴 and 关010兴 directions. For the growth
conditions leading to the islands shown in Fig. 7共a兲, theselengths vary roughly between 0.5 nm and 1.6 nm. In Fig.
7共d兲 the peak energies in both dI/dV and normalized(dI/dV)/(I/V) are plotted as a function of 1/L 21 ⫹1/L 22 . Although the scatter of the data points is large, the clouds canbe fitted to linear functions with regression constants ofabout 0.6. Therefore, a weak trend is obvious: the Fe共001兲
surface state shifts towards higher energies for decreasingisland sizes. The highest dI/dV peak energy found was⫹0.33 V. The (dI/dV)/(I/V) data in Fig. 7共d兲 suggest thateven for smaller islands (1/L 21 ⫹1/L 22 ⬎3 nm⫺2 ) the Fe共001兲
state is still present but has shifted to an even higher energy.
However, the steeply rising background in these measurements is unfavorable to detect peaks in dI/dV
关 (dI/dV)/(I/V) 兴 at voltages higher than ⬃0.33 V(⬃0.28 V). A tip which shows more Gaussian-like peaks
was difficult to obtain during these measurements, whichmight be an indication for a metallic atom on the tip apex
共the small Fe islands make it probably easy to pick up a Fe
atom兲. Nevertheless, on the smallest islands 关e.g., curve 4 inFig. 7共c兲兴, dI/dV is different compared to the dI/dV curve
obtained on the steps 共for which dI/dV above the Fermi
level is lower, as curve 5 and the dI/dV map show兲. In thenormalized (dI/dV)/(I/V) 共not shown兲, still a weak peakcan be observed around ⫹0.24 V, which might be an indication that on the smallest islands, the surface state is notcompletely quenched but rather shifted towards an evenhigher energy where it cannot be resolved from the background.
The upward peak shift with decreasing island size seems
to imply that this peak shift is caused by confinement of thesurface-state electrons. The energy band of the Fe共001兲 surface state shows an upward dispersion around ¯⌫ : the energy
increases with increasing k 储 . 2 The observed peak shift to 0.6
V on one-dimensional Fe lines was explained in terms ofconfinement of the Fe共001兲 surface state.2 If space is confined to length L, the wave vector of the surface state becomes quantized: i.e., k 储 ⫽n( ␲ /L) 共with n an integer兲. The
lowest possible k 储 is ␲ /L, and due to the upward dispersion
the lowest available energy in the surface state band shifts toa slightly higher value. Consequently, larger surface-state energies are expected for smaller Fe islands. Although a proper
physical description of the localization on the islands is more
complicated 关taking into account the real island sizes/
confinement barriers and a full description of the Fe共001兲
band structure兴, in a first-order approximation the peak shifts
are expected to be proportional to 1/L 21 ⫹1/L 22 . Therefore,both the dI/dV and (dI/dV)/(I/V) data clouds are fitted tolinear functions, which are also shown in Fig. 7共d兲. The peak045422-6

PHYSICAL REVIEW B 68, 045422 共2003兲

LOCAL ELECTRONIC STRUCTURE OF Fe共001兲 . . .

energies for the clean terrace 共i.e., 1/L 21 ⫹1/L 22 ⫽0) are assumed to be most accurate, and consequently, only one free
fitting parameter is used. Due to both the large inaccuracy inthe data points and the simplicity of the theoretical model,
the fitted lines are merely considered as rough estimates.
Nevertheless, the description may be quite reasonable as thefollowing calculation shows. If a shift of 0.43 V is assumed
for localization along a one-dimensional line,19 a phenomenological formula for the peak shift can be derived:⌬E⫽0.43V

冋冉 冊 冉 冊 册

0.3 2
0.3 2
⫹
.
L1
L2

Here 0.3 nm is the width of the one-dimensional Fe lines
in Ref. 2, and L 1 and L 2 are the lengths of the long and shortaxes of the island. If L 1 and L 2 are both taken to be 1.2 nm,
1/L 21 ⫹1/L 22 ⫽1.4 nm⫺2 , which is frequently experimentallyobserved, see Fig. 7共d兲, a peak shift of 0.05 V is predicted by
the phenomenological formula which is in good agreementwith the shifts observed. These considerations show at leastthat the order of magnitude of the peak shifts found on differently confined features 共lines, and small and large islands兲
are compatible.

atomic distances can change the appearance of the Fe共001兲
surface state from a Gaussian-like line shape to a weakshoulder on a strongly increasing background and vice versa.
The shape of the dI/dV curve was related to the apparent
barrier height. Very low apparent barrier heights correspondto strongly increasing backgrounds; high apparent barrierheights correspond to Gaussian-like dI/dV curves. By applying voltage pulses, Gaussian-like line shapes could be obtained. With these type of curves the effect of single oxygen
impurity atoms was studied. It was shown that an isolated
oxygen impurity shifts the Fe共001兲 surface state towards ahigher energy of 0.04 eV. This result is comparable to earlier
results obtained for single carbon impurities on V共001兲. Thisis also in agreement with our recently published calculations
for oxygen on Fe共001兲. The surface state is completely
quenched on both 关 100兴 and 关 110兴 directed monoatomic
steps. The quenching of the Fe共001兲 surface state at monoatomic steps was previously predicted by our calculations.
Finally, it was shown that the Fe共001兲 surface state shifts tohigher energies on islands which are smaller than about 1⫻1 nm2 . These shifts were explained in terms of confinement of the surface-state electrons and are compatible withthe earlier reported confinement effects of this surface state.

IV. CONCLUSION

In summary, an STS study was presented on the local
electronic structure of the Fe共001兲 surface. First, the influence of the tip on the STS measurements was examined. It
was found that the voltage dependence of the exponential
background in dI/dV and (dI/dV)/(I/V) curves strongly depends on the tip configuration. It was observed that small tipchanges leading to tip displacements of less than typical*Present address: Laboratory of Solid State and Materials Chemistry, Eindhoven University of Technology, P.O. Box 513, NL-5600
MB Eindhoven, The Netherlands.
†
Corresponding author. FAX: ⫹31 24 3652190. Email address:
hvk@sci.kun.nl
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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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17
In practice, dI/dV curves are selected from the dI/dV maps at⫹0.17 V: curves measured in the depressed step zone are averaged.
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19
Biedermann et al. reported a shift of 0.3 eV at the onedimensional lines based upon an Fe共001兲 surface-state energy of⫹0.3 eV 共Ref. 2兲. However, in the present work and also inmany other works 共see, e.g., Ref. 1兲 the surface state of clean
Fe共001兲 is found at ⫹0.17 eV, which makes a shift of 0.43 eV
more likely.

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