Surface Science 558 (2004) 201–210
www.elsevier.com/locate/susc

Study of Fe/Mn/Feð0 0 1Þ multilayers by means ofscanning tunneling microscopy/spectroscopyzquez de Parga b,*,
T.K. Yamada a, M.M.J. Bischoﬀ a, A.L. Va
T. Mizoguchi c, H. van Kempen a,*ab

NSRIM, University of Nijmegen, NL-6525 ED Nijmegen, The Netherlands
Dpto. de Fısica de la Materia Condensada, Universidad Autonoma de Madrid, Cantoblanco, 28049 Madrid, Spainc
Faculty of Science, Gakushuin University, 171-8588 Mejiro, Tokyo, Japan
Received 17 September 2003; accepted for publication 7 April 2004
Available online 22 April 2004

Abstract
To elucidate the magnetic properties of a magnetic multilayer ﬁlm, an understanding of the local geometry, electronic structure, and possible intermixing at the interfaces is of utmost importance. By gradually increasing the Fe
coverage on the Mnð0 0 1Þ ﬁlm, we obtained this information on the interface region by means of Auger spectroscopy
and scanning tunneling microscopy/spectroscopy. Intermixed atoms and various geometries and density of states arefound to exist in this system.
 2004 Elsevier B.V. All rights reserved.
Keywords: Scanning tunneling microscopy; Scanning tunneling spectroscopies; Epitaxy; Growth; Iron; Manganese; Metal–metal
magnetic thin ﬁlm structures

1. Introduction
Due to the enormous development of themolecular beam epitaxial growth technique during
the last decades, atomically ﬂat ﬁlms can be relatively easily prepared. Specially, the growth of ultra-thin magnetic multilayer ﬁlms has lead toconsiderable practical applications such as spintronic devices: the giant magnetoresistance head,*
Corresponding authors. Tel.: +34-9149-74746; fax: +349149-73961 (A.L. V
azquez de Parga), Tel.: +31-24-365-2121;
fax: +31-24-365-2190 (H. van Kempen).
E-mail addresses: al.vazquezdeparga@uam.es (A.L.
V
azquez de Parga), hvk@sci.kun.nl (H. van Kempen).
URL: http://www-evsf2.sci.kun.nl/.

the tunnel magnetoresistance head, and the magnetoresistive random access memory [1,2].
The magnetic properties of ultrathin magnetic
multilayer ﬁlms are strongly aﬀected by the local
geometry, the electronic structure and possible
intermixing [3,4]. Techniques like scanning electron microscopy with polarization analyzer, Kerr
microscopy, spin-polarized scanning tunnelingmicroscopy/spectroscopy make possible to study
the correlation between magnetism, electronic
structure, morphology, and composition [5–9]. For
example, Pierce et al. studied the relation between
the growth and the magnetic properties of magnetic multilayer ﬁlms (Fe/[Ag,Au,Cu,Cr,Mn,V,Al]/Fe) by scanning electron microscopy with polarization analysis in a combination with reﬂective

0039-6028/$ - see front matter  2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.susc.2004.04.004

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T.K. Yamada et al. / Surface Science 558 (2004) 201–210

high energy electron diﬀraction and scanningtunneling microscopy [5–7]. Still, there is a lack of
information concerning the degree of intermixing,
the crystallographic structure and the electronic
structure at the interface of such multilayers at
atomic level.
In the present paper we focus on the study ofthe growth of a magnetic multilayer by means ofscanning tunneling microscopy (STM), spectroscopy (STS) and Auger spectroscopy. We studied
the growth of Fe on an artiﬁcial Mnð0 0 1Þ surface.
The Mnð0 0 1Þ surface was produced by depositing
seven layers of pure Mn on an Feð0 0 1Þ whisker.
The Mn ﬁlm grown in this way presents a bodycentered tetragonal (bct) structure with the inplane lattice constant dictated by the Fe whisker
and an out-of-lattice constant of 0.165 nm [10].
Body-centered cubic (bcc) Feð0 0 1Þ and bct
Mnð0 0 1Þ have surface states at +0.2 and +0.8 V,
respectively, which can be used for chemical
identiﬁcation [9,11,12].

2. Experimental
STM and STS measurements were performed inan ultra-high vacuum (UHV) chamber (5 · 1011mbar) with a commercial STM (Omicron UHV
STM-1) working at room temperature (RT). In
this study we use W tips that were prepared asfollows. A W polycrystalline wire (purity 99.99%)
with a diameter of 0.5 mm was chemically etchedwith 5 M KOH aq., then washed with distilledwater and introduced in UHV. Once in UHV thetips were sputtered with Arþ (1 kV) and heated byelectron-bombardment (42 W).
The UHV chamber is equipped with a cylindrical mirror analyzer (PERKIN-ELMER PHImodel 10–155) that allows us to do Auger electron
spectroscopy measurements. The detection limit of
our Auger spectroscopy was estimated to be below
1% for oxygen impurities on the Feð0 0 1Þ surface,which was conﬁrmed by atomically and chemicallyresolved STM images.
An Feð0 0 1Þ whisker was used as a substrate[13] and was cleaned by cycles of Arþ sputtering(750 eV) and annealing (up to 730 C). After thiscleaning process, around 1% oxygen contaminants

were measured by Auger spectroscopy and also in
atomically and chemically resolved STM images[14]. In order to produce a clean Mnð0 0 1Þ surface
about 7 mono-layers (ML) of Mn (purity 99.999%)
were grown on the Feð0 0 1Þ whisker which was
kept at 100 C. The evaporation rate was 0.06 nm/
min. During the deposition the pressure remained
below 4 · 1010 mbar. After the deposition, morethan two Mn layers were exposed on the surface.
Using atomically and chemically resolved STM
images we determined that, for this substratetemperature (100 C), the intermixing between Fe
and Mn is limited to the ﬁrst three Mn layers.
Also, the level of contaminants was below 1% on
the Mn surface. Mn layers grown on Feð0 0 1Þ havea bct structure with the same in-plane lattice constant as Feð0 0 1Þ and an interlayer distance of0.165 nm [10]. Theoretical calculations show thatMnð0 0 1Þ with a bct structure has surface states
above the Fermi level that can be detected in thenormalized dI=dV curves as a broad peak centeredat +0.8 V [9]. Finally, Fe layers were grown on theMnð0 0 1Þ surface at RT with a rate of 0.15 nm/
min. Also, in this case, during the deposition thepressure remained below 3 · 1010 mbar.

3. Results and discussion
3.1. Auger spectroscopy
Since Fe has a much higher surface energy thanMn (1.440 J/m2 for Mn and 2.939 J/m2 for Fe) [15],Mn is expected to segregate on top of the Fe ﬁlm
during the deposition. Therefore, to prevent
intermixing between Mn and Fe, the Fe layers
were deposited at RT. In order to test the degree ofintermixing between Mn and Fe an Auger spectroscopy study was performed. During the deposition of the Fe ﬁlm we moved the sample from theevaporation position to the Auger measurement
position without switching oﬀ the Knudsen cell.
After the Auger spectra were measured we moved
the sample back to the growth position to deposit
another fraction of one Fe layer.
It is well known that the mean free path forAuger electrons with energies around 40 eV is very
short (3–5 atomic layers) [16]. Therefore the ratio

T.K. Yamada et al. / Surface Science 558 (2004) 201–210

(a)

(b)

203

(c)

Fig. 1. Auger spectra measured on the Fe layers on a Mnð0 0 1Þﬁlm in diﬀerential mode as a function of Fe coverage. From left
to right the thickness of Fe is increased. The progressive
appearance of the Fe(47 eV) peak and the progressive disappearance of the Mn(40 eV) peak can be observed. Dashed lines
are a guide for the eyes.

between Fe(47 eV) and Mn(40 eV) peaks gives us
information about the presence of Mn in the top
layers of the Fe ﬁlm. Fig. 1 shows the Auger
spectra in a diﬀerential mode within the energy
range between 35 and 50 eV for diﬀerent Fe coverages. By increasing the Fe coverage the peak at
47 eV that corresponds to Fe is observed to appear
gradually as the Fe thickness is increased andsimultaneously the Mn peak amplitude at 40 eV isgradually lowered and ﬁnally disappears. 1 Fromthese observations we can conclude that there is nosegregation of Mn to the surface of the Fe ﬁlm.
3.2. Growth of Fe on Mn(0 0 1)
Fig. 2(a) shows an STM image of the surface of
5 ML of Mn grown on the Fe(0 0 1) whisker. Four
diﬀerent Mn layers are exposed on the surface.
Hidden Fe steps (marked with black arrows in Fig.
2(a)) and three dimensional (3D) islands produced
during the Mn deposition are also observed. Details about the growth of Mn on Feð0 0 1Þ and thestructure of the ﬁlms are given in Ref. [10]. Fig.
2(b)–(f) show topographic images obtained for
diﬀerent thickness of Fe grown on Mnð0 0 1Þ.

1

For these measurements we can only give an estimation for
the Fe coverage. We estimate that the Fe thickness is below
5 ML.

Fig. 2. STM images obtained for clean Mnð0 0 1Þ and for different thicknesses of Fe ﬁlms grown on the Mnð0 0 1Þ surface at
RT: (a) Mnð0 0 1Þ surface, 200 · 200 nm2 , VS ¼ 0:90 V,
I ¼ 0:05 nA; (b) 0.05 ML Fe, 200 · 200 nm2 , VS ¼ 0:54 V, I ¼
0:44 nA; (c) 0.5 ML Fe, 200 · 200 nm2 , VS ¼ 1:00 V, I ¼ 0:03
nA; (d) 1.5 ML Fe, 200 · 200 nm2 , VS ¼ þ0:17 V, I ¼ 0:19 nA;(e) 3 ML of Fe, 55 · 55 nm2 , VS ¼ 1:00 V, I ¼ 0:04 nA; (f) 7
ML of Fe, 100 · 100 nm2 , VS ¼ 1:00 V, I ¼ 0:03 nA. (a) shows
a topographic image in combination with a diﬀerentiation along
the scanning direction. Black arrows in (a) indicate the positionof the Fe hidden steps. The numbers in (e) indicate the local
thickness of the Fe ﬁlm. Dashed curves in (f) are guide for theeyes to mark the substrate Mn morphology.

When 0.05 ML Fe is deposited on the Mn surface,small islands appear on the surface as shown in
Fig. 2(b). These islands become larger as the Fe
coverage increases (Fig. 2(c)). Fig. 2(d) was
obtained after the deposition of 1.5 ML of Fe.
Although the Fe ﬁlm follows the original morphology of the Mnð0 0 1Þ surface, the whole surface

T.K. Yamada et al. / Surface Science 558 (2004) 201–210

1.0

(a)

0.5

0.0
4
6
8 10
Fe layer number

0.8

(b)

0.6

σ

Distribution of
exposed layer

204

0.4
0 2 4 6 8
Fe coverage [ML]Fig. 3. (a) Distribution of exposed Fe layers at a coverage of
7 ML Fe. The half width at half maximum (HWHM), r, isobtained from a Gaussian ﬁt. (b) HWHM is plotted as afunction of the Fe coverage. The line is a guide for the eye.

is now covered by 1 ML of Fe and small Fe islands
appear on the second layer. For a coverage of 3
ML, up to three diﬀerent layers are exposed on thesurface as can be seen in Fig. 2(e), where the second, the third, and the fourth Fe layers are observed. Fig. 2(f) was obtained after the depositionof 7 ML of Fe. It is still possible to recognize theMnð0 0 1Þ surface morphology. On this ﬁlm up to
ﬁve diﬀerent layers of Fe are exposed and there are
no 3D Fe islands present on the surface.
In order to elucidate if there is a change in thegrowth mode with the thickness we measured the
distribution of exposed Fe layers for various Fe
coverages [17]. We ﬁtted the distribution of exposed Fe layer using a Gaussian curve (Fig. 3(a)).
The half width at half maximum (r) is plotted asfunction of the nominal coverage (Fig. 3(b)). Thereis a linear increase of r as a function of Fe coverage but there is no drastic variation. From thisanalysis we can conclude that the growth of Fe on
Mnð0 0 1Þ is almost layer-by-layer.
3.3. Atomically and chemically resolved STM
imagesFig. 4 shows atomically and chemically resolved
STM images. Fig. 4(a) was measured on theFeð0 0 1Þ surface. In this image, besides a faintsquare lattice randomly distributed depressions
can be seen at the atomic lattice sites. In a theoretical and experimental work, Hofer et al. studied
the atomic corrugation obtained by STM on theFeð0 0 1Þ surface [18]. They used the oxygen contaminants as a marker to identify the atomic

Fig. 4. Atomically and chemically resolved STM images obtained on a Feð0 0 1Þ surface, a Mnð0 0 1Þ surface and on 1.5 ML
Fe on Mnð0 0 1Þ. (a) Feð0 0 1Þ clean surface, 19 · 8 nm2 . Few
black holes at atomic positions are clearly seen. The diagonallines that run from the lower left corner to the upper right one
are electrical noise. (b) Image measured on a Mnð0 0 1Þ surface
covered with 0.2 ML Fe: 5 · 5 nm2 , VS ¼ 0:1 V, I ¼ 0:5 nA. (c)was obtained on a 1.5 ML Fe ﬁlm: 20 · 6.7 nm2 , VS ¼ 2 mV,
I ¼ 9:8 nA. The gray scale has been adjusted in every terrace inorder to enhance the contrast between the Mn atoms and the Featoms. (d) Proﬁle measured along the line shown in panel (c),the position of the Mn atoms are marked by arrows. The closepacked directions are shown in (b). The images are slightly
distorted by thermal drift (0.3 nm/min). The in-plane lattice
constants along the ½1 0 0 and the ½1 0 0 directions in the Mn
and the Fe layers are the same as the lattice constant of
Feð0 0 1Þ.

positions and to study the changes in the atomic
corrugation of the Fe atoms as a function of thetip sample distance. For all the set points used intheir experiments and calculations, the oxygen
atoms always appears like depressions. The concentration of these depressions is about 1% and isconstant for all the ﬁlms that we have studied. As
mentioned before, around 1% oxygen contaminants were measured by Auger spectroscopy on
the clean Fe whisker. Therefore, we tentatively
identify these depressions as oxygen atoms adsorbed on the surface. Later on, measuring IðV Þ

T.K. Yamada et al. / Surface Science 558 (2004) 201–210

curves on these defects we will conﬁrm thisassumption.
Fig. 4(b) was measured on the Mnð0 0 1Þ surface
after the deposition of 0.2 ML of Fe. In this imagea square lattice is clearly seen and two diﬀerent
types of lattice positions can be distinguished.
Most of the atomic positions appear bright andfew of them appear dark. Contrary to the previous
case, the number of these dark atomic positions
increases with the amount of Fe deposited, while
we had checked by Auger that the oxygen signal
stays constant during the Fe deposition. Fig. 4(c)was obtained after depositing 1.5 ML of Fe atroom temperature. With this amount of Fe, theMn surface is covered with a complete layer of Fe
and in some areas the second layer can be seen as
is shown in Fig. 4(c). In contrast with the image
shown in Fig. 4(b), at this coverage most of theatomic positions are dark and a small percentage,that depends on the Fe thickness, is bright. The
concentration of these bright atomic positions is
12 ± 2% for the ﬁrst Fe layer and 7 ± 1% for thesecond Fe layer as measured from the atomicallyand chemically resolved images as the one shownin Fig. 4(c). In Fig. 4(d) a line proﬁle taken in Fig.
4(c), four bright atomic positions are seen. The
corrugation of this bright sites is 50 pm and thefull width at half maximum (FWHM) is 0.17 nm.
We can compare these values with the values obtained in two diﬀerent experimental systems like
Pb/Cuð1 1 1Þ [19] where the Pb atoms are embedded in the Cuð1 1 1Þ surface and the apparent
corrugation is 70 pm and the FWHM is 0.20 nm
and Fe/Ptð1 1 1Þ where the Fe atoms are adsorbedon the surface and present an apparent height of0.12 nm and a FWHM of 0.7 nm [20]. We canconclude that the bright spots correspond to foreign atoms embedded in the surface and not to
atoms adsorbed on the surface. If we consider also
the decrease in the number of these bright atoms
with the increase in the Fe deposited and thecontrast between Mn and Fe found in our previous
experiments on the growth of Mn on the Feð0 0 1Þsurface [10], we tentatively identify them as Mn
atoms.
In order to identify better the nature of thesefeatures we measured IðV Þ curves on every pixel ofthe topographic images. In Fig. 5(a) we show a

205

STM topography measured on a Mnð0 0 1Þ surface
covered with 0.2 ML of Fe. As we have shown
before (Fig. 2(b) and (c)) the deposition of submonolayer amounts of Fe leads to the formationof monoatomic high islands. The Fe islands present on the surface are labelled as 1 ML Fe. In Fig.
5(c) we show the value of the IðV Þ curve at +0.3 V.
From this image it is clear that after the depositionof 0.2 ML of Fe the Mn substrate and thedeposited Fe island are far from been homogeneous in composition.
The dI=dV curves measured on this area areplotted in panels (e) and (f) of Fig. 5. In panel (e)of Fig. 5 we show the dI=dV curves measured on
the bright areas of the Fe island (solid black curve)
and on the grey areas on the Mn terrace (solid grey
curve). With the help of our previous experiments
and theoretical calculations on the Mn ﬁlms grownon Feð0 0 1Þ whiskers we can identify one of thedI=dV curves as originated from the Mnð0 0 1Þelectronic structure [9]. It has been shown that thedI=dV curves measured on the Mn(0 0 1) surface
always present a dip at +0.1 V (grey arrow in Fig.
5(e) and (f)). The dI=dV curves measured on the
Fe areas present a shoulder at +0.3 V. Therefore,
the dark areas in panels (b)–(d) of Fig. 5 correspond to the Mn atoms present on the surface(grey solid curve in panels (e) and (f) in Fig. 5) andaccordingly the bright spots corresponds to the Fe
positions (black solid curve in panels (e) and (f) in
Fig. 5). For comparison we included in Fig. 5(f)(dashed black curve) the dI=dV curve taken on oneof the impurities present on the surface, as the ones
shown in Fig. 4(a). This curve does not show anyof the characteristic signatures found on the Mn
and Fe curves [10]. Panels (b)–(d) in Fig. 5 areconstructed with the value of the tunnelling current for three diﬀerent values of the bias voltage. It
is clear that the position and contrast between
bright and dark spots are perfectly preserved in thethree panels. These spectroscopic measurements
proof that the surface is not homogeneous and it iscomposed by Mn and Fe atoms.
Once we have been able to identify the chemical
identity of the features observed on the surface ofthe diﬀerent ﬁlms we proceed to estimate the relative abundance. We found that the ﬁrst layer
consists of 12% of Mn and 88% of Fe. The second

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T.K. Yamada et al. / Surface Science 558 (2004) 201–210

Fig. 5. (a) STM image of 0.2 ML of Fe deposited on Mnð0 0 1Þ at RT, 20 · 22 nm2 taken with VS ¼ 0:5 V and I ¼ 0:5 nA. (b)–(d)Images constructed with the value of the IðV Þ curves at +0.2, +0.3 and +0.4 V respectively measured in every pixel of the image shownin panel (a). (e) dI=dV curves obtained from the IðV Þ curves measured in the Fe island (black solid line) and in the Mn substrate (solid
gray line). (f) dI=dV curves obtained from the IðV Þ curves measured in the spots marked in the STM image in panel (a).

layer consists of 7% of Mn and 93% of Fe. Theseobservations are consistent with the Auger measurements that show a decrease on the amplitudeof the Mn(40 eV) peak with the Fe coverage. Insummary, from the observations of the atomicallyand chemically resolved STM images, Auger
spectroscopy and STS data we can conclude thatfor Fe ﬁlms grown on Mnð0 0 1Þ at RT, the intermixing between Mn and Fe decreases with theincrease in the Fe thickness and it is observable forthe ﬁrst four layers.
3.4. Geometric structure of the Fe layers
In order to determine the geometric structure of
the Fe layers we measured the in-plane and out-ofplane distances with the STM. In a previous work
[10] we had shown that Mn ﬁlms grown pseudomorphically on Feð0 0 1Þ, having a bct structurewith the same in-plane lattice constant than theFeð0 0 1Þ, i.e. 0.287 nm. An atomically resolved
image of such a surface can be seen in Fig. 4(b). The
Fe ﬁlms deposited on these Mnð0 0 1Þ surfaces also
grown pseudomorphically, presenting the same inplane lattice constant than the bcc Feð0 0 1Þ surface.
We measured the apparent step height for different Fe layers as a function of sample bias
voltage. We did the measurements in this way torule out any inﬂuence due to changes in the electronic structure of the ﬁlms with the thickness. The
heights were obtained from histograms of planeﬁtted STM images taken at diﬀerent bias voltages.
Fig. 6(a) shows one of these STM images measured at V ¼ 1:0 V. In Fig. 6(b) we show thehistogram of heights present on the image of panel(a). By measuring the distance between the peaks
we can determine the height of the steps present on
the image. We took the half width at half maximum as error bar for the step heights values. Thismethod for measuring the steps heights is more

T.K. Yamada et al. / Surface Science 558 (2004) 201–210

207

(a)

(b)

(c)
Fig. 6. (a) STM image of 1.5 ML of Fe deposited on aMnð0 0 1Þ substrate at RT, 50 · 45 nm2 , measured at VS ¼ 1:0
V and I ¼ 0:04 nA. (b) Histogram showing the heights distributions present in the image of panel (a). By measuring the
distance between the peaks in the histogram we obtain the stepheight. The numbers on the STM image do not represent the Fe
thickness.

accurate than making individual line proﬁlesacross the steps.
Fig. 7(a)–(e) correspond to h01 , h12 , h23 , h34 , andh67 , h78 , respectively. hij denotes the interlayer
distance between layers i and j. The step height
does not show a strong dependence on the sample
bias voltage, but changes slightly with the thickness. h01 and h12 scatter around 0.160 ± 0.007 nm.
h23 is slightly higher (0.165 ± 0.007 nm), butalmost the same as h01 and h12 . h34 is around0.155 ± 0.007 nm. Above the fourth layer the interlayer distances are around 0.145 ± 0.007 nm asshown in Fig. 7(e).
The averaged values of hij ’s below the Fermi
level taken from panels (a)–(e) of Fig. 7 are plottedin Fig. 7(f). 2 This ﬁgure shows clearly that there isa change in the step height with the Fe thickness.
The ﬁrst, the second, and the third Fe layer havethe same step height: 0.162 ± 0.007 nm. The fourth
Fe layer has a lower step height: 0.155 ± 0.007 nm.
Then, the Fe ﬁlm above the fourth layer has anequivalent interlayer distance of 0.145 nm ± 0.007
nm. In the studies of bct-Fex Mn1x /Irð0 0 1Þ superlattices [21], a sharp change of the interlayer
distance was observed from 0.165 ± 0.001 nm forFe0:7 Mn0:3 to 0.153 ± 0.001 nm for Fe0:9 Mn0:1 by
X-ray diﬀraction measurements. In that work theauthors linked the reduction in the Mn percentage
in the ﬁlms with the change in the interlayer dis-

(d)

(e)

(f)

Fig. 7. (a)–(e) Apparent step heights of the Fe layers as afunction of the sample bias voltage. hij indicates the step height
between layers i and j. Since the heights are independent of thecurrent, the height at each voltage is obtained from the STM
images measured at current set points between 0.02 and 1 nA. (f)shows the interlayer distances obtained at negative voltages 2.

tance. It is worth to mention that STM is not thebest technique to get precise crystallographic
information because the step height measured canbe aﬀected by changes in the electronic structure
between diﬀerent layers and also by changes in thework function. In our case the former has been
excluded by measuring the apparent step height at
diﬀerent bias voltages but with our experimental
data the later cannot be excluded. Therefore, fromthese step height measurements we can conclude
that for Fe ﬁlms thicker than four layers, the Fe
ﬁlm presents a bcc structure. For the ﬁrst four
layers we can tentatively conclude that the interlayer distance is diﬀerent.
3.5. STS measurements on the Fe layers

2
Since the dI=dV curves are featureless for negative bias
voltages, we expect smaller electronic eﬀects.

Using the spectroscopic capabilities of the STM
we studied the surface electronic structure of the208

T.K. Yamada et al. / Surface Science 558 (2004) 201–210

Fe layers. STS measurements were performed in
the following way. IðV Þ curves were obtained atevery pixel of a constant current topographic image by opening the STM feedback loop. dI=dV
curves were obtained by numerical diﬀerentiationof the IðV Þ curves.
We will ﬁrst focus on the electronic structure of
the thick Fe ﬁlms. As we have shown before Fe
ﬁlms thicker than four layers present a bcc structure with a lattice parameter that corresponds tobulk Fe. We also know that for these thick ﬁlms
there is no intermixing with the atoms from thesubstrate. Fig. 8 was obtained on a 7 ML Fe ﬁlm.
As was explained before, with the increase of the
Fe thickness, the number of exposed layers increases and therefore the surface becomes rougher(Fig. 8(a)). For a 7 ML Fe ﬁlm four Fe layers are
exposed. All these layers reveal the same dI=dV
curve that presents a clear peak at +0.2 V (Fig.

Fig. 8. (a) STM topographic image obtained on the Mn layers
covered by about 7 ML Fe (VS ¼ 0:5 V, I ¼ 0:5 nA (90 · 90nm2 )). About four Fe layers are exposed. At this area spectroscopy measurements were performed. (b) shows an averaged
dI=dV curve. A peak at +0.2 V is visible.

8(b)). The presence of this peak in the STS spectra
measured on the surface of the Feð0 0 1Þ whisker
was ﬁrst described by Stroscio et al. [11]. With thehelp of a theoretical calculation they traced theorigin of this peak back to a nearly unperturbed
d orbital. The dI=dV curves shown in Fig. 8(b)are identical to the ones that we measured on aFeð0 0 1Þ whisker [14].
The situation for the ﬁrst four Fe layer is more
complicated because, as we have shown before inthis paper, there is intermixing between Fe and Mn
that changes from layer to layer. In order to retrieve the surface density of states the dI=dV
curves were normalized. We used the curves measured on the thick Fe layers as a reference during the normalization procedure. Following themethod proposed by Ukraintsev [22], we ﬁtted theexponential background of the dI=dV curves witha function T , and then we used this function tonormalize the dI=dV curves. Fig. 9(a) shows a
representative dI=dV curve for the four ﬁrst layersof Fe on Mnð0 0 1Þ (solid curve). The dI=dV curveshows two shoulders at +0.2 and +1.7 V. For
negative and higher (>+2 V) positive voltages the
dI=dV curve increases exponentially. This energy
range was used to ﬁt the exponential background(T ) (grey dashed curve in Fig. 9(a)). In Fig. 9(b) weshow the normalized conductance (ðdI=dV Þ=T )curve (black curve). In the normalized conductance the two shoulders that appear in the dI=dV
curves show up as two peaks. In order to determine the position in energy of these two peaks weﬁt the normalized conductance with two Gaussian
curves that are shown in Fig. 9(b) (grey curve).
From this ﬁt we found that the energy position of
the two peaks is +0.3 ± 0.1 V and +1.0 ± 0.1 V.
Figs. 8(b) and 9(c) show the tunneling conductance (dI=dV ) obtained on an Fe ﬁlm thicker thanfour layers. In this case the dI=dV curve shows apeak at 0.2 V above the Fermi level. Following thesame procedure described above we just ﬁt theexponential background of the dI=dV curve (greydashed curve in Fig. 9(c)). Fig. 9(d) shows thenormalized tunneling conductance (ðdI=dV Þ=T )with a solid curve and using a Gaussian curve wedetermined the energy position of the peak present
in the spectra (grey solid curve in Fig. 9(d)). We
found that the energy position is 0.2 V above theT.K. Yamada et al. / Surface Science 558 (2004) 201–210

(a)

(b)

(c)

(d)

Fig. 9. (a) The dI=dV curve obtained on the ﬁrst four Fe layers(black solid curve) was normalized by a ﬁtted tunneling probability function (T ) (grey dashed curve). The grey solid curve in(a) denotes a ﬁtted dI=dV curve. (b) ðdI=dV Þ=T curve (blacksolid curve) shows a broad peak in the energy range between the
Fermi level and +2.5 V, it can be ﬁtted with two Gaussian
curves (grey solid curve) centered in energy at +0.3 ± 0.1 V and
1.0 ± 0.1 V respectively. (c) The dI=dV curve obtained on the Fe
ﬁlm thicker than four layer (black solid curve) was normalized
by a ﬁtted T (grey dashed curve). (d) ðdI=dV Þ=T curve (blacksolid curve) shows a peak at 0.2 V above the Fermi level. It canbe ﬁtted by a single Gaussian curve (grey solid curve). (a) and(c) were measured at set points of VS ¼ 0:6 V, I ¼ 0:3 nA andVS ¼ 0:5 V, I ¼ 0:5 nA, respectively.

Fermi level, this energy corresponds to the energyof the surface state of bcc Feð0 0 1Þ [11].
According to the above observations, we conclude that (1) the ﬁrst four Fe layers (which have a
bct structure and include intermixed Mn) show
peaks at +0.3 ± 0.1 V and +1.0 ± 0.1 V in thedensity of states (ðdI=dV Þ=T ); (2) the Fe layers (>4ML) (which have a bcc structure and do notcontain intermixed Mn) show a peak at +0.2 V in
the density of states (ðdI=dV Þ=T ).

4. Conclusion
In this study we investigated the geometricstructure, the concentration of the intermixed Mn

209

atoms, and density of states for the Fe/Mn magnetic multilayer ﬁlm with STM, STS, and Auger
spectroscopy. The Fe ﬁlm grows almost layer-bylayer. The in-plane lattice constant is the same asbcc-Feð0 0 1Þ. Intermixing Mn atoms were observed with a concentration of 12 ± 2% for the ﬁrst
and 7 ± 1% for the second Fe layer. The concentration of Mn was found to decrease with an increase of the number of Fe layers. Interlayer
apparent distances of 0.161 ± 0.007 nm for the ﬁrst
and the second layer, 0.163 ± 0.007 nm for thethird layer, 0.155 ± 0.007 nm for the fourth, and
0.145 ± 0.007 nm for thicker Fe layers were found.
From this it follows that Fe ﬁlm thicker than four
layers grow in a bcc structure and we can tentatively conclude that the ﬁrst four Fe layers grow ina bct one. The surface density of states of the Felayers show that: (1) the ﬁrst four Fe layers havepeaks around +0.3 ± 0.1 V and +1.0 ± 0.1 V; (2)the Fe ﬁlm thicker than four layers has one peak at+0.2 V, which energy corresponds to the bccFeð0 0 1Þ surface state energy.
One can conclude that around the fourth layer
both the electronic structure and the interlayer
distance change. We attribute these change to therelaxing of the Fe ﬁlm from a bct to a bcc structure. The onset of this structural transition isprobably related to the intermixing behavior at the
Fe/Mn interface [21].
This indicates that in typical magnetic multilayer ﬁlms some monolayers at the interface havediﬀerent geometric and electronic structures. Thismight cause diﬀerent magnetic structure at theinterface of the magnetic multilayer ﬁlms.

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.
We are especially grateful to D.T. Pierce for supplying us with the Fe whiskers. One of us, ALVP,
thanks the Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO) and Ministeriode Ciencia y Tecnologıa for the ﬁnancial support.

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T.K. Yamada et al. / Surface Science 558 (2004) 201–210

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