Surface Science 516 (2002) 179–190
www.elsevier.com/locate/susc

STM and STS study of ultrathin Mn layers on Fe(0 0 1)
T.K. Yamada a,b, M.M.J. Bischoﬀ a, T. Mizoguchi b, H. van Kempen a,*a

NSRIM, University of Nijmegen, Toernooiveld 1, 6525 ED Nijmegen, The NetherlandsbFaculty of Science, Gakushuin University, 171-8588 Mejiro, Tokyo, Japan
Received 4 May 2002; accepted for publication 21 June 2002

Abstract
The growth of ultrathin Mn ﬁlms on an Fe(0 0 1) whisker at 370 K is studied by scanning tunneling microscopy(STM) and scanning tunneling spectroscopy at room temperature in ultrahigh vacuum. Atomically and chemicallyresolved STM images show that the Mn ﬁlm grows with the same in-plane lattice constant as Fe(0 0 1) and that Fe
atoms intermix with the ﬁrst (14%), the second (4%), and the third Mn layer (2%), while a negligible amount of Fe
atoms is found above the third layer. The growth mode changes from layer-by-layer to layer-plus-island at a coverage of
3 ML Mn. dI=dV curves which are normalized by voltage-dependent tunneling probability functions show clear peaks
on each Mn layer. These peaks are tentatively ascribed to surface states. On the ﬁrst Mn layer, peaks are found at þ0.35
eV on pure Mn areas and at þ0.28 eV on mixed MnFe areas. The second and the third Mn layer show peaks at þ0.20
and þ0.8 eV, respectively. Mn ﬁlms thicker than three layers reveal besides a strong peak at þ0.8 eV a weaker peak at)0.6 eV. Our apparent step height measurements show that the Mn ﬁlm relaxes at the third layer: the interlayer spacingis 0.16 nm for the ﬁrst two layers, and it increases to 0.18 nm at the third layer. Starting from the fourth layer theinterlayer spacings are geometrically equivalent (0.165 nm).
Ó 2002 Elsevier Science B.V. All rights reserved.
Keywords: Epitaxy; Iron; Magnetic ﬁlms; Manganese; Scanning tunneling microscopy; Scanning tunneling spectroscopies; Surface
structure, morphology, roughness, and topography; Whiskers

1. Introduction
The development of epitaxial growth techniques
has led to considerable practical applications suchas metallic-multilayers devices. Especially, ultrathin magnetic ﬁlms evaporated on magnetic substrates show enormous diﬀerent magnetic andelectronic properties compared to the bulk caused*

Corresponding author. Tel.: +31-24-3653499; fax: +31-243652190.
E-mail address: hvk@sci.kun.nl (H. van Kempen).

by their lower dimensions ([1] and references
therein). Regarding the novel magnetic and electronic properties at the surface and the interface,particularly, the system of Mn ﬁlms on Fe(0 0 1) isremarkable since recent calculations [2–13] andexperiments [14–27] show conﬂicting results indicating the complicated relation between growth
condition, electronic structure, and magnetic coupling.
With reﬂective high energy electron diﬀraction, quantitative low energy electron diﬀraction(LEED), and extended X-ray absorption spectroscopy (EXAFS), it has been shown that Mn0039-6028/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 0 3 9 - 6 0 2 8 ( 0 2 ) 0 2 0 3 2 - 0

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layers grow layer-by-layer on Fe(0 0 1) up to morethan 10 monolayers (MLs). The in-plane lattice
constant is 0.287 nm and interlayer spacings of
0.130 [24] or 0.148 nm [25] for 1 ML, 0.152 nm [25]for 2 ML, and 0.161 nm [24] or 0.164 nm [26] for
14–25 ML are reported, i.e., the structure is bodycentered-tetragonal (bct). These results have beenreported for Mn growth on an Fe(0 0 1)-singlecrystal at room temperature (RT) [24], on Fe(0 0 1)ﬁlms/MgO(0 0 1) at RT [25], and on an Fe(0 0 1)
whisker at 320 K [26]. Auger electron spectroscopy
and grazing ion-surface scattering measurements
showed that a layer-by-layer Mn ﬁlm changes to alayer-plus-island Mn ﬁlm when a 13 ML thick Mnﬁlm, which is deposited at RT on an Fe(0 0 1)single-crystal, is annealed at 420 K [15].
Nonas et al. showed with a Korringa, Kohn,
and Rostoker Green’s function method calculationthat for Mn atoms on Fe(0 0 1) it is energeticallyfavorable to place exchange with the Fe surface
atoms [3,4]. Bischoﬀ et al. showed with chemicallyresolved scanning tunneling microscopy (STM)
and scanning tunneling spectroscopy (STS) theformation of a surface alloy for submonolayer Mnﬁlms grown on Fe(0 0 1) at substrate temperatures
above 370 K [28]. Grazing ion-scattering experiments showed that Fe atoms intermix with Mn
adlayers up to a coverage of 3 ML at a substratetemperature of 420 K [16].
In this paper, a study on the atomic morphology by STM and the electronic structure by STS isreported for Mn ﬁlms grown on an Fe(0 0 1)
whisker at 370 K. Growth at this temperature isbelieved to lead to reasonable ﬂat ﬁlms [17] andlow intermixing [28]. With special tip conditions ahigh chemical contrast can be achieved which isutilized in determining the intermixing behavior.

subsequently rinsed with ethanol in air. In UHV,the tip was heated up to 1000 K and self-sputteredwith Ar or Ne in order to clean the tip and to
decrease its radius below 10 nm as veriﬁed by ﬁeldemission spectroscopy [29]. To obtain a clean andﬂat substrate surface an Fe(0 0 1) whisker was usedin this study [30]. Using Ar sputtering (750 eV)
cycles at 1020 K, the Fe(0 0 1) whisker, with dimensions 7  0:5  0:5 mm3 , was cleaned until theconcentration of surface contaminants became
lower than 1% as measured by Auger spectroscopy
and atomically resolved STM images. Mn (purity
99.999%) was sublimated from a Knudsen cell at adeposition rate of 0.3 ML/min as estimated by aquartz crystal oscillator. Here, one ML is deﬁned
as one Mn atom per Fe atom on the (0 0 1) surface.
In the present paper, the local Mn coverage is
obtained from a Gaussian ﬁt to the distribution of
exposed layers (see Fig. 2a). During the growth,
the pressure was always around 4  1010 mbar.
The substrate was radiatively heated to 370 K. It
must be noted that the inaccuracy of the temperature measurements is estimated to be 20 K at
370 K since the thermocouple was not in directcontact with the sample. Using the Fe single stepheight of bcc-Fe(0 0 1) which is 0.143 nm, the apparent step heights of our STM measurements
were calibrated. STS measurements were performed by recording an IðV Þ curve at every pixelof a topographic scan which is obtained at the
typical setpoint of VS ¼ )0.5 V, I ¼ 0:5 nA. dI=dV
curves were obtained by numerical diﬀerentiationof the IðV Þ curves.

3. Results and discussion
3.1. The growth of Mn layers on Fe(0 0 1)

2. Experimental
STM and STS measurements were performed inultrahigh vacuum (UHV) (5  1011 mbar) at
RT with a commercial STM (Omicron UHV
STM-1). For getting a sharp STM tip, a W polycrystalline wire with a diameter of 0.5 mm (purity
99.99%) was etched with a 5 M KOH solution andMonoatomic steps can be observed with an interval of about 200 nm at the clean Fe(0 0 1)
whisker substrate (Fig. 1a). When 1.5 ML Mn isdeposited at a substrate temperature of 370 K, Mn
grows layer-by-layer. This follows from Fig. 1b inwhich it can be seen that the second layer islands
have started to grow after the ﬁrst layer has been
completed. At a coverage of 3 ML, the second, theT.K. Yamada et al. / Surface Science 516 (2002) 179–190

181

Fig. 1. STM topographic images of Mn layers grown on the Fe(0 0 1) whisker at 370 K. (a) The bare whisker, 310  80 nm2 ,VS ¼ )1.00 V, I ¼ 0:06 nA; (b) 1.5 ML, 165  157 nm2 , VS ¼ )0.38 V, I ¼ 0:05 nA; (c) 3 ML, 100  100 nm2 , VS ¼ )0.34 V, I ¼ 2:00
nA. The second, third, and fourth layer are visible. (d) 5 ML, 200  200 nm2 , VS ¼ )1.00 V, I ¼ 0:05 nA; (e) 6 ML, 300  300 nm2 ,VS ¼ )1.00 V, I ¼ 0:05 nA. (e) shows the data in the diﬀerential mode in which the grey scale is proportional to the gradient of theheight variation along each scan line.

third, and the fourth layer are observed as trenches, terrace, and islands, respectively (Fig. 1c).
Before the second layer has been completely covered by Mn, the fourth layer has started to growwhich means that the growth mode is not layer-bylayer anymore. At coverages above 3 ML, theappearance of the surface changes drastically andmany levels are exposed on the surface (Fig. 1d).
Whereas, up to a coverage of 6.5 ML Mn, hidden
Fe steps are clearly observed on the Mn(0 0 1)

surface as about 0.02 nm high steps, the Fe stepheight of 0.143 nm is never observed, which meansthat the overlaying Mn ﬁlm tends to make thesurface ﬂat. Three dimensional (3D) islands (5nm high) are observed at the position of hidden Fe
steps and defects marked by the white arrow in
Fig. 1e. This ﬁgure is displayed in the diﬀerential
mode and shows that the islands have pyramidlike shapes which corresponds to the results reported by Pfandzelter et al. [15].

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Fig. 2. (a) Distribution of exposed Mn layers at a coverage of
6.5 ML Mn. Average Mn coverage and its deviation, r, areobtained from a Gaussian ﬁt and (b) the dependence of the 3D
islands density on Mn coverage. The 3D islands appear at acoverage of 3 ML and their density increases with the coverage.
The solid line is a guide to the eye.

Pierce et al. found that the distribution of the
exposed layers can be approximated by a Gaussian. The center of the Gaussian gives the averagecoverage, and the deviation r the root-meansquare height variation or roughness [14,31]. Thus,r can be obtained from our STM images (Fig. 2a).
r is 0:40  0:05 and 0:7  0:1 ML for coverages
below and above 3 ML Mn, respectively. If thelayer-by-layer growth continued for coverages
above 3 ML, we do not expect an increase of r.
However, r drastically increases by a factor of two
at coverages above 3 ML in our experiments,
which is a clear indication that the growth modechanges. Furthermore, starting from around acoverage of 3 ML Mn, the number of 3D islands
strongly increases as can be observed in Fig. 2b.
The density of the 3D islands at a coverage ofaround 5 ML Mn (1010 cm2 ) corresponds withthe results reported by Pfandzelter et al. [15].
Based upon both the increase of r and the appearance of 3D islands, the critical coverage where
the growth mode changes from layer-by-layer tolayer-plus-island is estimated to occur around 3
ML.
The layer-plus-island growth can be understoodby equilibrium thermodynamics [15,32,33] whichstate for the Mn/Fe system that if Dc ¼ cMn þ
cin þ cs  cFe is negative (positive), the layers growlayer-by-layer (layer-plus-island). cMn and cFe aresurface energies of Mn and Fe, respectively. cin is
the interface energy and cs is the strain energy. The
Fe surface energy (2.6 J/m2 ) is higher than theMn surface energy (1.6 J/m2 ) [34] and the interfacial energy is assumed to be much smaller than
the surface energy [15]. The strain energy is causedby the lattice mismatch. The lattice mismatch between Fe and Mn is diﬃcult to determine since astable bulk bcc or fcc structure does not exist at
RT and consequently lattice parameters for thebcc and fcc structures must be extrapolated fromeither high temperature phases or alloyed phases[24]. Kim et al. showed that the observed bct
structure of Mn ﬁlms might be a distortion of either a fcc or a bcc phase. Based upon their analysis
the in-plane lattice mismatches are estimated )8%and þ4% for fcc and bcc, respectively [24]. Due tothis high lattice mismatch the strain energy plays a
key role for the change of the growth mode. In ourcase, due to an increase of the strain energy, thetotal energy Dc is likely to change from negative topositive at a coverage of 3 ML and consequently
layer-plus-island growth sets in.
Our STM topographic results diﬀer from theresults reported by Tulchinsky et al. which showedthat for comparable growth rate and temperature
the layer-by-layer growth continues up to 10 ML
without any 3D islands [14,17]. The Fe whisker
used in this study is not of the high ﬂatness (seeFig. 1a) as the ones used by Tulchinsky et al. [17],i.e., our Fe whisker has a higher Fe step density.
Furthermore, various defects can be observed on
the Fe(0 0 1) surface [35], e.g., ‘‘Ar bubble-like’’
defects [36,37], holes (5 nm deep) caused by the
sputtering, and screw dislocations. Since this nonideal surface morphology is expected to lead to anincrease of the interface energy, an inﬂuence on thegrowth process is likely. Therefore, the onset seems
to depend strongly on the quality of the Fe(0 0 1)
whisker although it may depend on the growthrate and temperature as well.

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183

Fig. 3. Atomically and chemically resolved STM images of (a) the Fe(0 0 1) substrate (3:5  3:5 nm2 , VS ¼ )2 mV, I ¼ 2:3 nA), (b) theﬁrst (6  9 nm2 , VS ¼ )2 mV, I ¼ 2:6 nA), and (c) the ﬁfth Mn layer (3:5  3:5 nm2 , VS ¼ )7 mV, I ¼ 2:6 nA), respectively. The black
dots in (a) indicate the positions of the atoms. The close-packed directions are shown in (a). Numbers in (b) denote three kinds ofdepressions on the surface (see text). The images are slightly distorted by thermal drift (0.3 nm/min). The in-plane lattice constantalong the [1 0 0] and [0 1 0] directions in the Mn layer is the same as the lattice constant of Fe(0 0 1).

3.2. Atomically and chemically resolved STM
topographic images
Fig. 3 shows atomically resolved STM topographic images obtained on the Fe(0 0 1) substrate(a), the ﬁrst (b), and the ﬁfth (c) Mn adlayer, respectively. Atomically resolved images could beobserved on all Mn adlayers studied (i.e., up to theseventh Mn layer). The STM images include aslight distortion due to thermal drift (0.3 nm/min) resulting in a deviation from 90° of the angle
between the close-packed lattice directions (which
are indicated in Fig. 3a) [28]. From the atomicallyresolved STM images it can be concluded that for
all the Mn adlayers the in-plane lattice constantequals the Fe(0 0 1) lattice constant within the accuracy of STM, which corresponds with the resultsof quantitative LEED [24]. Furthermore, since
there is no indication for an in-plane relaxationsuch as dislocation networks, the Mn adlayer must
follow the Fe substrate lattice.

With a normal tip condition 1 the corrugation of
the lattice atoms is about 1 pm which is of the orderof the noise level of our STM. Consequently, theatoms can hardly be observed in the topographic
image. Fortunately, occasionally special tip conditions could be obtained during STM measurements, e.g., the corrugation of the Fe atoms
increases to more than 10 pm as shown in Fig. 3a.
With this tip condition, the corrugation of the Mn
lattice atoms increases to 35 pm and atomicallyresolved images could be obtained with highchemical contrast (Fig. 3b).
The chemically resolved image obtained on theﬁrst Mn layer shows three kinds of depressions. The
depressions marked ‘‘1’’ (60 pm deep) andmarked ‘‘2’’ (60 pm deep) are observed at lattice1
A normal tip condition is deﬁned as a tip which does notshow chemical contrast. Normal refers to the higher chance of
having this kind of tip compared to a tip leading to chemical
contrast.

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sites and hollow sites, respectively. The concentration of both depressions is the same, i.e., 7  1%. In
Fig. 3b, depressions marked ‘‘3’’ are observed aswell. Their concentration is much lower (1%) andtheir contrast is much weaker (25 pm deep).
To identify these three kinds of depressions, thepossibility of oxidation must be considered. Mn
atoms are easily oxidized to MnO [27]. Andrieu et al.
proposed that the main cause of the oxidation of theMn layer is CO dissociation which is already eﬀective at RT on Mn ([27] and references therein). In
their experimental setup, the oxidation speed of
the Mn layer was 0.03 ML/h from which a CO
partial pressure of 2:5  1011 Torr was estimated.
Using mass spectrometry, the CO partial pressurein our STM chamber is estimated to be about 10
times better, which implies that the oxidation speed
in our system is below 0.003 ML/h. Our atomicallyresolved STM images were measured about 5 h after
the Mn layers had been grown, which implies thatthe total amount of oxygen contamination is below
1.5%. Furthermore, with Auger spectroscopy, theconcentration of impurities (mainly oxygen) on ourFe(0 0 1) whisker is estimated to be 1% which isabout the detection limit of our Auger setup.
Since the concentration of the atoms marked‘‘1’’ and ‘‘2’’ is much higher than the impurity
concentration, these depressions are not likely to
be contaminants but rather the eﬀect of intermixed
Fe atoms. The two types of depressions are tentatively explained by diﬀerent local environmentsof the Fe atoms: i.e., having 4 Fe nearest neighbors
or 3 Fe and 1 Mn nearest neighbors in the layer
below. Calculations and experiments with othertechniques are needed to verify this assumption.
On the other hand, the similar concentration andappearance in STM images obtained with normal
tip conditions on Fe(0 0 1) with and without Mn
deposition make us to identify the depressions ‘‘3’’as oxygen impurities.
The eﬀect of intermixed Fe is observed until
the fourth Mn layer but decreases with layer
thickness (see Fig. 4). On the fourth layer theconcentration of these features equals the impurity
concentration. Therefore, pure Mn layers areformed above the fourth layer. This is in agreement with the results of grazing ion-scattering
measurements [16].

Fig. 4. The concentration of the intermixed Fe atoms in thevarious Mn layers. The dashed line indicates the impurity
concentration.

3.3. Scanning tunneling spectroscopy
Fig. 5a shows an STM topographic image ofFe(0 0 1) covered with 1 ML Mn at a substratetemperature of 370 K. In Fig. 5a, three layers areobserved: the Fe substrate layer (marked ‘‘0’’), theﬁrst (marked ‘‘1a þ 1b’’), and the second Mn adlayer (marked ‘‘2’’). Less than 1% of the substrateis still exposed. IðV Þ curves are measured at
75  75 pixels at the area within the square indicated in Fig. 5a. The dI=dV map at the bias voltage of þ0.8 V is shown in Fig. 5b. Based upon thisdI=dV map, four kinds of dI=dV curves can be
distinguished. Averages of typical 10 neighboring
pixels are shown in Fig. 5c. The averaged dI=dV
curve from the Fe substrate shows a peak at 0.22
eV above the Fermi energy, which is attributed toa minority spin state of dz2 symmetry [38,39]. On
the ﬁrst Mn layer, both depressed (cf. ‘‘1a’’) andbright (‘‘1b’’) areas can be observed in the dI=dV
map (Fig. 5b). The concentration of the depressions in Fig. 5b is 7%, 2 which corresponds to the
Due to the high tunneling resistance (VS ¼ )0.5 V, I ¼ 0:5nA), atomic resolution cannot be obtained in the spectroscopy
measurement. If one spherical depression in Fig. 5b (i.e., one
marked by circle) is related to a single atom, the concentrationof the depressions is roughly estimated to be 7%.
2

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185

Fig. 5. STS measurement on Fe(0 0 1) covered with 1 ML Mn at 370 K: (a) is a topographic image obtained at a setpoint of VS ¼ )17mV, I ¼ 0:5 nA. Scan size is 15  15 nm2 . The Fe(0 0 1) surface, the ﬁrst, and the second Mn layer are observed as the black (marked‘‘0’’), the grey (marked ‘‘1a þ 1b’’), and the white areas (marked ‘‘2’’), respectively. (b) dI=dV map at þ0.8 V measured at the area
within the grey square in (a). Depressed (cf. ‘‘1a’’) and bright (‘‘1b’’) areas are visible on the ﬁrst Mn layer. One spherical depression ismarked by a black circle. Scan size is 10  10 nm2 . (c) dI=dV curves representative of the areas indicated in (b). Curves are typicallyaverages of 10 neighboring pixels. The quadratic functions ﬁtted to the dI=dV curves are shown as dotted curves (‘‘T ’’). (d) dI=dV
curves of (c) normalized by quadratic background ﬁts. Labels in (a) and (b) correspond to the ones in (c) and (d).

concentration of one kind of depressions observedin Fig. 3b. The equal concentration of the depressions implies that the depressed areas in the
dI=dV map of Fig. 5b (i.e., those marked ‘‘1a’’)correspond to one type of Fe atoms which havebeen incorporated in the ﬁrst Mn adlayer. The
averaged dI=dV curve measured at the depressions
shows a peak at 0.43 eV above the Fermi level(Fig. 5c). The averaged dI=dV curve of the brightareas ‘‘1b’’ does not show a peak, but it is clearthat at positive voltages there is an additional
contribution on top of the background (Fig. 5c).
The averaged dI=dV curve obtained on the second
Mn layer shows a strong shoulder around 0.3 eV
above the Fermi level. The diﬀerent dI=dV curves

obtained on the ﬁrst and the second layer must be
related to the diﬀerent coordination of the Mn
atoms in the ﬁrst and the second layer: Mn atoms
in the ﬁrst layer have an Fe layer underneath,
while Mn atoms in the second layer cover a Mn
layer.
The accurate energies of the peaks in the sample
density of states (DOS) are diﬃcult to estimate
from the dI=dV curve since the apparent energies
may shift due to the voltage dependence of the
exponential background in the dI=dV curve [28].
Although normalization of ðdI=dV Þ by ðI=V Þ
has been suggested as a method to remove theinﬂuence of the background, this ðdI=dV Þ=ðI=V Þ
method cannot completely remove it [40].

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T.K. Yamada et al. / Surface Science 516 (2002) 179–190

Ukraintsev showed that normalizing dI=dV with
its ﬁtted tunneling probability function leads to thebest recovery of the sample DOS within a one dimensional WKB approach [40]. The tunnelingprobability function is:"1=2 #
2m 
T ¼ at exp  2S
ðU  V =2Þh2"1=2 #2m 
þ as exp  2S
ðU þ V =2Þ
:
h2



The ﬁrst (second) term of T describes tunnelingfrom the tip (sample) Fermi level to unoccupied
sample (tip) states. at and as are proportionality
coeﬃcients related to the tip–surface eﬀective
contact area and are proportional to the tip andthe sample DOS at the Fermi level, respectively. U
is the average of sample and tip work functions, S
the tip-sample separation, and m the electronmass. However, this recovery procedure is not verypractical since it needs dI=dV data over a larger
voltage range compared to usual dI=dV measurements (e.g., Fig. 5c). When there is not a large
voltage range available, a ﬁt by a quadratic function might work as well as Ref. [28] has shown.
The Fe(0 0 1) dI=dV curve in Fig. 5c has been ﬁtted
on the high positive voltage side to a quadratic
function. Normalizing dI=dV by this ﬁt shifts thepeak from þ0.22 eV in the dI=dV curve to þ0.17
eV in the normalized curve (Fig. 5d) in agreementwith the reported energy of the Fe(0 0 1) surface
state [39].
Using the quadratic function ﬁt, the sample
DOS of the ﬁrst and second layer are recovered(Fig. 5d). By analyzing many normalized dI=dV
curves, this method is believed to ﬁnd the DOS
peaks with an accuracy of 0.02 eV. Althoughband structure calculations are needed to reveal
the nature of these peaks, it seems reasonable tointerpret them as Mn(0 0 1) surface states. On theﬁrst layer, the bright and depressed areas in Fig. 5bshow a peak at 0.35 eV and 0.28 eV above theFermi energy, respectively. The peak energies atthe bright and depressed areas in Fig. 5b correspond to the peak energies observed on the pureMn pð1  1Þ and the alloyed FeMn cð2  2Þ

structures obtained for submonolayer Mn growthat 420 K [28]. Therefore, the bright and depressed
areas in Fig. 5b can be identiﬁed as pure Mn andalloyed FeMn areas. The second layer shows apeak at 0.20 eV above the Fermi energy after
normalization.
At a coverage of 2.8 ML Mn, three levels are
exposed as shown in Fig. 6a. At this area, dI=dV
curves were obtained at 75  75 pixels and averages of typically 10 curves representative of thevarious layers are shown in Fig. 6b. Since thedI=dV curve obtained on the lowest level of Fig. 6a
is similar to the curve obtained on the second layerof Fig. 5, this level can be identiﬁed as the second
Mn layer. The averaged dI=dV curve of the third
Mn layer (Fig. 6b) shows a diﬀerent behavior
compared to the second layer. The dI=dV curve ofthe third layer steadily increases above the Fermi
level without showing a peak or a shoulder.
Whereas the averaged dI=dV curve of the fourthlayer (Fig. 6b) shows a steady increase above theFermi energy as well, the dI=dV values are lower
compared to the third layer. Averaged dI=dV
curves measured on ﬁlms thicker than four layers
are comparable to the fourth layer dI=dV curve(not shown). Fig. 6c shows the dI=dV map at þ0.6
V measured at the same area as Fig. 6a. Fig. 6dshows the line proﬁle along the white line in Fig.
6c. This line proﬁle reveals sharp dips at the steps
since the dI=dV curves measured at the steps
show lower dI=dV values (Fig. 6b), which is related to a quenching of the surface states at steps[28].
For dI=dV curves measured on Mn ﬁlms
thicker than two layers, the quadratic function ﬁt
should also allow for an accurate evaluation of theDOS. However, the dI=dV curves show a linearlike increase up to þ1.5 V. Therefore, it is impossible to ﬁt this background to a quadratic
function. For Mn ﬁlms thicker than two layers,
dI=dV measurements were performed within alarge voltage range from )2 V to þ3 V. 3 The
dI=dV curves measured within this large voltage
3
Although the IðV Þ measurements at large voltages werevery unstable and, frequently, some small drops (3  3  0:3
nm3 ) were observed on the surface after the measurement, somereliable dI=dV curves could still be obtained.

T.K. Yamada et al. / Surface Science 516 (2002) 179–190

187

Fig. 6. STS measurement on Fe(0 0 1) covered with 2.8 ML Mn at 370 K: (a) is a topographic image obtained at a setpoint of
VS ¼ )0.54 V, I ¼ 0:40 nA. Scan size is 50  50 nm2 . Three diﬀerent levels are exposed on the surface. (b) shows the dI=dV curves
representative of each level. These curves are averages of typically 10 single curves. Since the dI=dV curve measured on the lowest levelis similar to the one measured on the second layer, which is shown in Fig. 5c, this layer is identiﬁed as the second layer. (c) shows the
dI=dV map at þ0.6 V measured at the same area as (a). Numbers in (a) and (c) denote the stacking number of the Mn layers. (d) The
line proﬁle along the white line in (c) shows sharp dips at the steps.

range can be more accurately ﬁtted by the tunneling probability function. With this normalizing
technique a peak can be recovered as shown in Fig.
7. The normalized dI=dV curve measured on thethird layer shows a peak around 0.8 eV above theFermi level. For Mn ﬁlms thicker than three layers, besides this þ0.8 eV peak a weak peak at )0.6
eV is observed in ðdI=dV Þ=T curves. The þ0.8 eV
peak amplitude is about 1.3 times higher at thethird layer than at thicker layers, which explains
the dI=dV curves of Fig. 6b. More details about
these states will be published in a forthcoming
publication.
3.4. The step height
Apparent step heights were obtained from histograms of STM images. Diﬀerent levels appear as

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T.K. Yamada et al. / Surface Science 516 (2002) 179–190

Fig. 7. dI=dV curves measured at the third layer (black dots)
and layers above the third layer (grey dots). The same setpointof VS ¼ )0.5 V, I ¼ 0:5 nA was used and curves are averages oftypically 20 single curves. Fits of the tunneling probability
function to the dI=dV curves are also shown (‘‘T’’). The dI=dV
curves normalized by this ‘‘T’’, i.e., ‘‘ðdI=dV Þ=T ’’, reveal astrong peak around þ0.8 eV for the third Mn layer. For thicker
Mn ﬁlms, besides this þ0.8 eV peak, a weak peak at )0.6 eV isobserved in the ðdI=dV Þ=T curves as well.

sharp Gaussian peaks in this distribution with
their spacings being the step heights. Only sharp
Gaussian distribution proﬁles for which stepheights unambiguously can be deﬁned were included in the analysis.
Since variation in the apparent barrier heights
may lead to diﬀerent apparent step heights [41,42],
apparent barrier heights were estimated from ameasurement of the dependence of the tip–sample
separation on the current (voltage was kept constant). However, the same apparent barrier height
was obtained at each Mn layer (no tip change wasobserved during these measurements) and therefore this eﬀect can be excluded.
Fig. 8 shows that the apparent step heightsof the Mn layers strongly depend on the sample
voltage. Since the heights are independent of thecurrent, which was conﬁrmed for a range from 0.1
nA to 2 nA at a constant voltage, the results ofFig. 8 are obtained from STM topographic images
recorded within this current range. hij indicates theFig. 8. Apparent step heights of the Mn layers as a function of
the sample bias voltage. hij indicates the step height between
level i and j. Since the heights are independent of the current,
the height at each voltage is obtained from STM topographic
images measured at current setpoints between 0.1 nA and 2 nA.
The strong variation in h01 around the Fermi level is due to the
Fe surface state. (g) shows the interlayer distances obtained atlarge negative voltages, which are believed to reﬂect the realgeometric step heights.

apparent step height between level i and level j. h01 ,h12 , and h23 show a considerable voltage dependence.
Since the Fe(0 0 1) substrate layer, the ﬁrst,
the second, and the third Mn layer reveal peaks
at diﬀerent energies above the Fermi level inðdI=dV Þ=T (see Figs. 5d and 7), h01 , h12 , and h23 areinﬂuenced by the electronic structure (Figs. 8a–c).

T.K. Yamada et al. / Surface Science 516 (2002) 179–190

Since ðdI=dV Þ=T curves measured at the substrate,
the ﬁrst, the second, and the third layer are notcompletely the same at and below the Fermi level,
electronic eﬀects cannot be fully excluded from thestep height measurements at large negative voltages, for which the tunneling current is dominatedby sample states at the Fermi level [40]. Nevertheless, the inﬂuence of the electronic structure on
the step heights is expected to be much lower atlarge negative voltages than at positive voltages
since no strong peaks are observed at negative
voltages in ðdI=dV Þ=T . Therefore, it can be assumed that h01 and h12 ¼ 0:16 nm and h23 ¼
0.18 nm give the geometric step heights. It canbe concluded that: (i) The ﬁrst two layers show
signiﬁcantly higher step heights than the Fe single
step height (0.143 nm). (ii) The step height increases about 0.02 nm at the third layer comparedto the ﬁrst and the second layer, i.e., Mn layers on
Fe(0 0 1) strongly relax at the third layer, which isin agreement with EXAFS measurements [25].
Fig. 8d shows the apparent step height between
the third and the fourth layer. Whereas the scatterin h34 is of the same order as that in h45 and h56(Figs. 8e and 8f), h34 might contain some inﬂuenceof the electronic structure as well. The third layer
reveals a higher amplitude of the peak at þ0.8 eVcompared to the fourth layer. Furthermore, theweak )0.6 eV peak present on the fourth layer isabsent on the third layer (Fig. 7). In order tomaintain the constant current, the tip has to movecloser to the third layer at negative voltages andcloser to the fourth layer at positive voltages,
which qualitatively explains the variation in h34 .
Since at both negative and positive voltages h34
includes electronic eﬀects, the real geometrical stepheight is diﬃcult to disentangle.
Step heights between successive Mn layers (h45 ,h56 , etc) show the same values (0.165) as shownin Figs. 8e and 8f. These step heights (h45 and h56 )are almost independent of bias voltage, which canbe explained by the equal dI=dV and ðdI=dV Þ=T
curves measured at the fourth and higher layers.
Therefore, it can be concluded that the interlayer
distance for Mn ﬁlms thicker than 3 ML is geometrically the same in agreement with previous
results [14]. The interlayer distances between thevarious layers, which are obtained at large nega-189

tive voltages and are believed to be the most reliable values, are shown in Fig. 8g.

4. Conclusion
Surface topographic and electronic informationof Mn ﬁlms grown on an Fe(0 0 1) whisker at 370
K were obtained by STM and STS at RT in UHV.
Atomically and chemically resolved STM images
show that the Mn ﬁlm grows with the same inplane lattice constant as Fe(0 0 1) and that Fe
atoms intermix with the ﬁrst, the second, and thethird Mn layer. The concentration of intermixed
Fe decreases with ﬁlm thickness. Furthermore,
STM shows that the growth mode changes fromlayer-by-layer to layer-plus-island at a coverage of
3 ML Mn.
dI=dV curves normalized by voltage-dependent
tunneling probability functions show peaks oneach Mn layer, which can be interpreted as features in the sample DOS. The ﬁrst Mn layer shows
peaks at þ0.35 eV on pure Mn areas and at þ0.28
eV at mixed MnFe areas. The second and thethird layers show peaks at þ0.20 eV and þ0.8 eV,
respectively. Thick Mn ﬁlms (>3 ML) reveal
a weaker peak at )0.6 eV besides a strong peak at
þ0.8 eV. The þ0.8 eV peak amplitude in ðdI=dV Þ=
T curves obtained on Mn ﬁlms thicker than threelayers is about 1.3 times lower than that on thethird Mn layer.
Based upon our apparent step height measurements, we conclude that the ﬁrst two Mn overlayers show signiﬁcant higher step heights than the
Fe single step height and that the Mn ﬁlm relaxes
by about 0.02 nm at the third layer. From thefourth layer the interlayer spacings are geometrically the same (about 0.165 nm).

Acknowledgements
This work was supported by the Stichting voor
Fundamenteel Onderzoek der Materie (FOM),
which is funded by the Nederlandse Organisatie
voor Wetenschappelijk Onderzoek (NWO), andthe European Growth Project MAGNETUDE.
T.K.Y. likes to thank Gakushuin University for a190

T.K. Yamada et al. / Surface Science 516 (2002) 179–190

travel grant. We are especially grateful to B.
Heinrich for supplying us with the Fe whiskers.

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