PHYSICAL REVIEW B 72, 014410 共2005兲

Spin-polarized scanning tunneling microscopy/spectroscopy study of MnAu(001) thin films
T. K. Yamada,1,2 R. Robles,3 E. Martinez,4 M. M. J. Bischoff,2 A. Vega,4 A. L. Vázquez de Parga,5 T. Mizoguchi,1 andH. van Kempen2
1Faculty of Science, Gakushuin University, 171-8588 Mejiro, Tokyo, Japan
2

Institute for Molecules and Materials, Radboud University Nijmegen, Toernooiveld 1, 6525 ED Nijmegen, The Netherlands
3Department of Physics, Uppsala University, SE-75121, Uppsala, Sweden
4Department de Física Teórica, Atómica y Optica, Universidad de Valladolid, 47011 Valladolid, Spain
5Department de Física de la Materia Condensada and Instituto de Ciencia de Materiales “N. Cabrera,”
Universidad Autónoma de Madrid, Cantoblanco 28049, Madrid, Spain
共Received 1 December 2004; revised manuscript received 11 March 2005; published 5 July 2005兲
In this work we explore by means of spin-polarized scanning tunneling microcopy/spectroscopy 共SP-STM/
STS兲 and ab initio calculations the magnetic and electronic properties of thin MnAu alloyed films. The MnAu
films are produced by deposition of small amounts of Au at RT on a Mn共001兲 surface. On the scanningtunneling microscopy images the surface of the AuMn alloy presents a c共2 ⫻ 2兲 reconstruction. From the
SP-STM/STS measurements we find out that the MnAu alloy layers are coupled antiferromagnetically with thelayers above and below. Comparing the result from the ab initio calculations with the spin-polarized measurements we found that the origin of the magnetic contrast is due to a spin-polarized peak at 0.15 eV above theFermi level spatially placed on the Mn sites. In the calculations we also found that the magnetic moments of
the Mn atoms are higher in the alloy layers than in the pure Mn films.
DOI: 10.1103/PhysRevB.72.014410

PACS number共s兲: 75.70.Ak, 68.37.Ef, 71.15.Mb, 73.20.At

I. INTRODUCTION

There are a limited number of studies concerning themagnetic structure of alloys composed of noble metals andantiferromagnetic materials. There are experimental and theoretical studies of the magnetism of the CuMn thin films andalso concerning the c共2 ⫻ 2兲-CuMn共001兲 surface. The calculations for the CuMn共001兲 thin films identified the magnetism of Mn as the driving force for the stability of the alloy1
and the authors tentatively suggested that the magnetic ordering of the alloy layers is determined by an indirect inplane RKKY-type interaction due to the hybridization of theMn d with the Cu sp electrons. In a different work, Rader etal.,2 using inverse-photoemission in combination with theoretical calculations, found a high local magnetic moment for
the Mn atoms in the surface alloy. A more recent calculation3
for CuMn共001兲 thin films shows a ferromagnetic in-planeorder and antiferromagnetic coupling between layers; a highmagnetic moment for the Mn atoms in the alloy was also
reported.
For the MnAu alloy, to our knowledge, there is nothing
published on the magnetic structure of the MnAu thin films.
However, for the bulk there are several studies concerning
the magnetic structure of this alloy. MnAu4 is ferromagnetic
with a Curie temperature of 373 K.4 MnAu3, Mn2Au5, and
MnAu are antiferromagnetic with Néel temperatures of
140 K, 354 K, and 513 K, respectively.5–7 Mn2Au presents ahelical magnetic structure with a Néel temperature of
365 K.4
In this work we focused on the study of the c共2 ⫻ 2兲
-MnAu alloy layer grown on a Mn共001兲 surface by means ofspin-polarized scanning tunneling microscopy/spectroscopy
共SP-STM/STS兲. We cover clean W tips with a ferromagnetic
material 共iron兲 in order to make them sensitive to the in1098-0121/2005/72共1兲/014410共7兲/$23.00

plane magnetization. We found that the layers of MnAu alloy
are coupled antiferromagnetically mutually. With the help of
ab initio calculations we traced back the origin of the magnetic contrast found in the STS experiments. We also foundthat the magnetic moments of the Mn atoms are higher on
the alloy layers than on the pure Mn films.
II. EXPERIMENTAL

All measurements were performed in an ultra-highvacuum 共UHV兲 chamber 共⬃5 ⫻ 10−11 mbar兲 at room temperature 共RT兲. A STM was attached to the chamber which isequipped with molecular beam epitaxy, Auger spectroscopy,
field emission spectroscopy 共FES兲, sample heating and sputtering facilities.8
An Fe共001兲 whisker with dimensions of ⬃0.1⫻ 0.1⫻ 0.5 cm3 was used as a substrate.9 Fe whiskers are an ideal
substrate because they are extremely flat, presenting terraces
up to 1 ␮m wide. The magnetic structure of the Fe whisker
is well known10 and the size of the magnetic domains is big
enough to be sure that we are measuring with the STM in asingle magnetic domain. The Fe共001兲 whisker was cleaned
by Ar+ sputtering and annealing cycles. After this cleaning
process, less than 1% oxygen contaminants were measured
by Auger spectroscopy and atomically and chemically resolved STM images.
About 7 ML of Mn 共purity 99.999%兲 was grown on the
Fe共001兲 whisker at 100 ° C. The evaporation rate was
0.06 nm min−1 and the pressure was always below 4⫻ 10−10 mbar. After deposition, more than two Mn layers
were exposed on the surface due to the layer-plus-island
growth. Using atomically and chemically resolved STM images, we confirmed that there is no intermixed Fe into the
Mn layers above the third Mn layer.11 Less than 1% of con-

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YAMADA et al.

taminants was observed on the Mn surface. Mn layers grownon Fe共001兲 have a structure of body-centered tetragonal 共bct兲
with the same in-plane lattice constant as Fe共001兲 and aninterlayer distance of 0.165 nm.11
In this study, we used clean W tips as nonmagnetic tips
and Fe-coated W tips as magnetic tips. A clean W tip was
prepared as follows: in air, a W polycrystalline wire 共purity
99.99%兲 with a diameter of 0.5 mm was chemically etchedwith 5 M KOH aq. Once in UHV the tip was sputtered using
Ar+ and heated by electron bombardment. After this treatment, a tip radius over 300 nm was measured by FES.12
Fe-coated W tips were made by depositing 10 nm Fe on theclean W tips at RT. The Fe evaporation rate was
0.6 nm min−1 and the pressure remained below 2⫻ 10−10 mbar during deposition. We did not apply any external magnetic field to the tips.
The Au was evaporated on the Mn共001兲 at RT with adeposition rate of 0.15 nm min−1. During the deposition thepressure was always below 3 ⫻ 10−10 mbar.
For the spectroscopy measurements, I共V兲 curves were obtained at every pixel of a constant current topographic image
by opening the STM feedback loop at a given current andvoltage 共set point兲. dI / dV curves were obtained by numericaldifferentiation of the I共V兲 curves.
III. THEORETICAL METHODS

We have performed ab initio all-electron calculations of
the electronic structure of two systems: two Mn– Au 50%
alloyed monolayers on top of a Mn共001兲 film and the pure
Mn共001兲 film. We have not included in the calculation the
Fe共001兲 substrate because the STM experiments are mainly
sensitive to the surface properties and the Fe– Mn hybridization effect is negligible at the surface sites if the Mn film isthick enough. The Mn films are taken large enough to simulate the semi-infinite substrate while using the supercell technique in the reciprocal space. The interlayer distance of thealloy layers is the same as that of the pure Mn substrate, as
experimentally observed. For both systems this interlayer
distance is 0.165 nm, while the in-plane distance is given by
the Fe共001兲 substrate with a lattice parameter of 0.287 nm.
The calculations are performed using a scalar-relativistic
version of the k-space TB-LMTO method13,14 within theatomic-sphere approximation. This all-electron method isbased on the density functional theory15 and we have used
the generalized gradient approximation for the exchange andcorrelation potential in the form of Perdew-Wang.16
In order to construct the supercell we consider enoughlayers of empty spheres to assure that there is no interaction
between the surfaces of adjacent supercells17 共5 monolayersof empty spheres were enough兲. The calculations are performed using an increasing number of k points in the irreducible Brillouin zone 共integrations are done with the tetrahedron technique兲 until the total energy does not change
within our convergence error 共10−5 eV兲. A total number of
98 k points fulfills this requirement.
IV. RESULTS AND DISCUSSION

The Au– Mn alloy was produced by evaporation of small
amounts of Au on the surface of 7 ML of Mn grown on the

FIG. 1. 共a兲 The 100⫻ 100 nm2 STM topography of 9.5 ML ofMn grown on an Fe共001兲 whisker. The numbers denote the local
Mn thickness. 共b兲 The dI / dV map at +0.2 V showing the magnetic
contrast between different Mn layers.

surface of an Fe共001兲 whisker. We will first discuss the electronic and magnetic characteristics of the Mn共001兲 films.
Scanning electron microscopy with polarization analysis results suggested that for films thicker than three layers theferromagnetic Mn共001兲 sheets couple antiferromagneticallyamong each other.18 In a recent publication Yamada et al.19
showed that above the third layer each ferromagnetic Mnlayer, whose magnetization direction is in-plane, couples antiferromagnetically with the Mn layer above and below. The
antiferromagnetic coupling between layers produces the existence of different magnetic domains on the surface every
time the thickness of the Mn film changes, i.e., every time asurface step is crossed. The body-centered-tetragonal 共bct兲
Mn共001兲 surface has two dz2 surface states that are responsible for the magnetic contrast observed in the SP-STM.19
Therefore we used this well-known and well-characterized
surface to check the sensitivity of our Fe-coated W tips to thein-plane surface magnetization.
Figure 1共a兲 shows an example of the surface topography
after the deposition of 9.5 ML of Mn. In the image it ispossible to identify five different Mn layers, from 8 to 12, aclear indication of a quasi-layer-by-layer growth mode.11 In
STM the tunneling probability depends on the local densityof states 共LDOS兲 of tip and sample in an energy window
determined by the bias voltage applied between them. If the
STM tip used in the experiment is covered with ferromagnetic material, then the tunneling probability also depends on
the relative orientation between the magnetizations of sample
and tip.20
Taking I共V兲 curves in every pixel of the topographic image it is possible to construct an image of the value of thedifferential tunneling conductance 共dI / dV兲 at certain bias
voltage. Figure 1共b兲 shows the dI / dV map at +0.2 V constructed in this way. From previous studies19,11 we know that
the LDOS of the Mn layers does not change from layer tolayer for films thicker than 4 ML. Depending on the Mn
local thickness the magnetization of the sample is parallel
共antiparallel兲 to the one of the tip, and therefore the tunnelingprobability is higher 共lower兲, giving the contrast observed inFig. 1共b兲. Using these Mn layers we can check the sensitivityof our Fe covered tips to the in-plane sample magnetization.
Figure 2 shows STM images obtained on the surface ofthe sample after the deposition at RT of 0.4 ML 共a兲 and 0.9014410-2

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SPIN-POLARIZED SCANNING TUNNELING…

FIG. 2. 共a兲 The 100⫻ 100 nm2 STM topographic image obtained on a Mn surface covered by 0.4 ML of Au. The white brokenlines mark the position of the original Mn steps. 共b兲 The 100⫻ 100 nm2 STM topographic image obtained on a Mn surface covered by 0.9 ML of Au. 共c兲 6.6⫻ 6.6 nm2 atomic resolved image
obtained on a 2.2 ML Au film deposited at RT showing thec共2 ⫻ 2兲 surface reconstruction and several defects.

ML 共b兲 of Au on a 7 ML Mn共001兲 film. A close inspection of
the surface 关Fig. 2共c兲兴 reveals a c共2 ⫻ 2兲 structure that covers
all the sample surface. This surface reconstruction appears

after the deposition of 0.5 ML of Au on the surface and exists
on the surface upon deposition of up to 3 ML of Au. Figure
2共a兲 is taken after deposition of 0.4 ML of Au. The big islands present on the image 共marked with white broken lines兲
correspond to Mn islands already present before the deposition of the Au. The areas marked with “0” correspond withthe level of the original Mn surface. Figure 2共b兲 is taken after
the deposition of 0.9 ML of Au. The growth is almost layer
by layer; the second layer 共marked as 2 in the image兲 isstarting to develop before the first layer 共marked as 1 in theimage兲 is completed. For thicker Au films the c共2 ⫻ 2兲 reconstruction disappears and it is replaced by a p共1 ⫻ 1兲 due tothe growth of pure Au layers.
Several studies concerning the bulk magnetic structure ofdifferent AuMn alloys4–7 prove the existence of bulk alloys
of Au and Mn. On the other hand, experimental studies on
Mn/ Au共001兲,21 Mn/ Ag共001兲,22 and Mn/ Cu共001兲 共Refs. 23
and 24兲 show that in all cases the Mn forms a c共2 ⫻ 2兲 surface alloy with the noble metals. In the case of one monolayer thick epitaxial c共2 ⫻ 2兲 MnCu surface alloy film on
Cu共100兲, the Mn atoms show an unusual buckling relaxation1
that is the origin of the structure that we observe in the STM
measurements. The details of the geometric structure of thefilms are described elsewhere.25
As mentioned before, in SP-STM experiments the information concerning the relative orientation of the sample
magnetic domains appears as small changes in the intensityof the differential tunneling conductance 共dI / dV兲 measured
in the different magnetic domains. Therefore, in order to beable to use this technique to study the magnetic properties ofthe MnAu alloy films, it is important to carefully characterize the dI / dV curves measured with a clean W tip. In fact,
we have to be sure that the LDOS of the sample does notchange from layer to layer in the alloy or from one area tothe other on the surface. For this reason we first performed
spectroscopic measurements using a clean W tip.
Figure 3共a兲 shows a representative STM topography ofthe surface after deposition of 0.6 ML of Au on the Mn共001兲
surface at RT. As mentioned before the surface presents ac共2 ⫻ 2兲 reconstruction in all the levels exposed. Figure 3共b兲shows the dI / dV curves 共solid lines兲 measured on the substrate 共black line兲 and the first layer 共gray line兲 of the image
shown in panel 共a兲 with a clean W tip. It is clear that the
curves measured in every layer exposed on the surface
present the same shape and amplitude in the voltage range
explored in our experiments.
The energy position of the features in the sample LDOS isdifficult to estimate from the dI / dV curves, due to thevoltage-dependent exponential background. In addition thefeatures appear as shoulders in the dI / dV curves, making it
more difficult to determine their exact energy position. In
order to circumvent these problems we normalized the dI / dV
curves to reduce the influence of the exponential background.
For metallic samples it has been proposed that the best
way to recover the sample density of states from the dI / dV
curves is normalizing them by their fit to a tunneling probability function 共“T”兲 共Ref. 26兲 that is given by the following
expression:

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YAMADA et al.

FIG. 3. 共a兲 The 40⫻ 40 nm2 STM image taken after the deposition of 0.6 ML of Au on the Mn共001兲 surface at RT. “0” denotes
the substrate and “1” denotes the islands. 共b兲 The dI / dV curves
measured simultaneously with the image shown in panel 共a兲 共solidlines兲. The exponential background fit for both curves is shown withdashed lines. The black curves correspond to the data measured on
the substrate 共0兲 and the gray curves correspond to the ones measured on the islands 共1兲. 共c兲 The normalized tunneling conductanceusing the exponential fitting shown in panel 共b兲. Three clear contributions to the tunneling conductance can be distinguished in the
spectra.

冋 冋 冉 冊册 册冋 冋 冉 冊册 册

T = at exp − 2S

2m ¯ V␾−ប22+ as exp − 2S

2m ¯ V
␾+
ប2
2

2

2

.

共1兲

The first 共second兲 term of T describes the tunneling fromthe tip 共sample兲 Fermi level to the unoccupied sample 共tip兲
states. at and as are proportionality coefficients related to thetip-surface effective contact area and are proportional to the¯ is thetip and sample DOS at the Fermi level, respectively. ␾
average of tip and sample work function, S is the tip sample
distance, and m is the electron mass. To use this normalization procedure it is necessary to fit the exponential background accurately. Therefore it is necessary to have the I共V兲

curves measured over a wide voltage range as shown in Fig.
3 where the I共V兲 curves have been measured between −2 V
up to +3 V.
In Fig. 3共b兲 we plot the exponential background fit
共dashed lines兲 for the dI / dV curves 共solid lines兲. The dI / dV
curves deviate from the exponential background. This is aclear indication of the contribution to the differential tunneling conductance from well-defined features of sample
LDOS. Using the exponential background fit 共“T”兲, we normalize the dI / dV curves to get a precise determination of theenergy position of the different features present in the dI / dV
curves. As shown in Fig. 3共c兲 the normalized conductancecurves obtained in the substrate and first layer are also identical and present three clear features: a peak at −0.6± 0.1 V,a small feature marked with a black arrow at +0.1± 0.1 V,
and a broad peak at +0.9± 0.1 V. All these peaks are present
in all the curves measured with different clean W tips. The
amplitude of all these features is identical in all the layers
exposed when they are measured with a clean W tip. These
curves are very different from the ones measured in the
Mn共001兲 surface,19 therefore we conclude that these curves
are representative of the sample DOS for the AuMn surface
alloy.
In the dI / dV curves obtained with the Fe covered tips wefound the same spectral features as in the curves measured
with the clean W tips. A careful analysis reveals that in thedI / dV curves measured with the Fe covered tip the intensityof the small peak at +0.1 V 关marked with an arrow in Fig.
3共c兲兴 changes when the tip goes from one terrace to the other.
In the following we will only plot the dI / dV curves in anenergy range of ±0.4 V around the Fermi level to make thesechanges in intensity more visible.
Figure 4共a兲 shows a STM image after a deposition on 0.7
ML of Au on the Mn共001兲 surface. The broken white line
marks the approximate position of the original Mn step.
Therefore the upper right corner 共labelled “odd”兲 and the restof the image 共labelled “even”兲 correspond to different terraces of the Mn film 关see Fig. 1共a兲兴. Figure 4共b兲 shows animage constructed with the value of the differential conductance 共dI / dV兲 at +0.1 V for the individual I共V兲 curves measured in every pixel of the image shown in panel 共a兲. The
magnetic contrast between the terraces and the islands is inverted after crossing the Mn step. For the terrace labelled
odd the islands are dark and the terrace is bright. On thecontrary in the terrace labelled even the islands are bright andthe terrace is dark. In panels 共c兲 and 共d兲 we show the dI / dV
curves measured in the even and odd areas of the image forthe substrate 共solid line兲 and islands 共dashed line兲. This result
indicates that the magnetic structure of the MnAu alloy iscoupled with the magnetic structure of the Mn共001兲 film underneath.
Figure 5共a兲 shows a STM image of the surface of 1.1 MLof Au deposited on Mn共001兲 surface. In this image there are
four different levels exposed due to the quasi-layer-by-layer
growth mode. In this surface there are present three different
thicknesses of the MnAu alloy marked in the image as “1,”“2,” and “3.” In Fig. 5共b兲 we present a narrow energy rangeof the dI / dV curves taken in three different areas mentioned
above. The curves are identical in the energy range measured
and the only difference appears around +0.1 V. The intensity

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FIG. 4. 共Color online兲 共a兲 The 50⫻ 37 nm STM image taken
after deposition of 0.7 ML of Au on the Mn共001兲 surface. Threedifferent levels are seen in the image. The white broken line marks
the approximate position of the original Mn step. The odd and evenareas correspond to different terraces of the Mn film. On the image
the substrate is marked with “0” and the islands with “1.” 共b兲 The
image constructed with the value of the dI / dV at +0.1 V measured
in every pixel of the image shown in 共a兲. 共c兲 The averaged dI / dV
curves measured on the substrate marked as “0” on panel 共a兲 共continuous line兲 and on the islands marked as “1” on panel 共a兲 共broken
line兲 of the area marked as even on panel 共a兲 共lower left corner兲. 共d兲
The averaged dI / dV curves measured on the substrate marked as“0” on panel 共a兲 共continuous line兲 and on the islands marked as “1”on panel 共a兲 共broken line兲 of the area marked as odd on panel 共a兲共upper right corner兲.
2of the differential tunneling conductance oscillates with the
thickness. It is high for areas “1” and “3” 共solid and dottedcurves, respectively兲 and low for area “2” 共dashed curve兲.
This result indicates a nonferromagnetic coupling between
layers in the MnAu ordered alloy.
In order to fully understand these results we have performed ab initio all-electron calculations of the electronic
structure of two systems: two Mn– Au 50% alloyed monolayers on top of a Mn共001兲 film and the pure Mn共001兲 film.
For both the pure Mn共001兲 film and the two Mn– Au 50%
alloyed monolayers on top of the Mn共001兲 film we obtain the

FIG. 5. 共a兲 The 80⫻ 80 nm2 STM image taken after depositionof 1.1 ML of Au on the Mn共001兲 surface. Four different levels areseen in the image. 共b兲 The dI / dV curves measured with Fe covered
W tips on terraces marked 1 共solid curve兲, 2 共dashed curve兲, and 3
共dotted curve兲.

FIG. 6. The spin-resolved DOS for the Mn and Au atoms on the
Mnu Au alloyed layers for the surface layer 共S兲 and the subsurface
layer 共S-1兲.

layered antiferromagnetic structure 共LAF兲 as experimentallyobserved. Although the presence of Au does not modify themagnetic coupling in the system, quantitative differences in
the magnetic moments of the surface Mn atoms exist as aconsequence of the change of the local chemical environment.
In Fig. 6 we plot the calculated spin resolved DOS for the
Mn and Au sites of the first two layers of the MnAu alloy on
top of 7 ML of Mn. The change of sign of the spin polarization at the different 共001兲 planes is characteristic of the LAF
configuration. Au is a noble metal, thus the Mn– Au hybridization is small and for this reason the spin polarization of
the Au atoms is negligible. The spin polarization of the Mn
atoms is clear and also the oscillation in the sign from layer
to layer. This oscillation in the spin polarization on the Mnsites is the origin of the contrast found in the dI / dV curves
measured with the Fe covered tips.
To get a better understanding of the role played by the Mn
and the Au atoms on the magnetic structure of the thin MnAu
alloy film, we calculated the number of electrons per atomic
site for the s, p, and d bands in the pure Mn共001兲 films andin the 2 ML thick MnAu alloy. The result is shown in Table
I for the Mn and Au atoms. For the theoretical calculation of

FIG. 7. 共a兲 The local magnetic moments of the Mn atoms for the
first four layers of a pure Mn共001兲 film. 共b兲 The local magnetic
moments for the Mn 共empty circles兲 and Au atoms 共gray circles兲 forthe 2 ML of MnAu alloy on a Mn共001兲 film.

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TABLE I. This table shows the electron population in the s, p, and d bands for the pure Mn共001兲 layers
and for the 2 ML thick MnAu alloy as well as the value of the magnetic moment in Bohr magnetons.
MnAu c共2 ⫻ 2兲Mn共001兲
s

p

d

␮

Mn共S兲

0.62

0.49

5.46

−3.81

Mn共S-1兲

0.67

0.84

5.68

1.64

Mn共S-2兲

0.67

0.81

5.49

−2.97

the alloy layers, it was assumed that the S-2 layer is the firstone composed only of Mn. In this layer there are two inequivalent Mn atoms. For one of them all the second nearest
neighbors are Mn and for the other some are Au. Comparing
the occupancy of the d bands for the Mn atoms in the pure
Mn共001兲 layers and in the 2 ML thin MnAu alloy we foundthat this is almost identical for every layer. This is a clear
indication of the low hybridization between the Mn and the
Au atoms.
In Fig. 7 we plot the magnetic moment distribution obtained in both systems. Mn atoms at the Mn– Au surface
alloy have larger magnetic moments than the surface atomsof the pure Mn共001兲 film. Moreover, the local magnetic moments of the Mn atoms of the first three layers are different
in both systems. Due to the small Mn– Au hybridization, theMn atoms close to the Au atoms in the Mn– Au alloy behave
as if they were less coordinated than the Mn atoms of thepure Mn共001兲 system. This situation is more extreme for thesurface Mn atoms in the MnAu alloy where they are closer tothe magnetic saturation limit. As mentioned earlier, the spinpolarization of the Au atoms is negligible as expected. The

FIG. 8. 共a兲 The majority bands in the Mn site at the surface of
the alloy. 共b兲 The isocharge density distribution in an energy window of 0.25 eV above the Fermi level layers. The dashed line indicates the boundary between vacuum and the bulk.

Mn共S兲
Au共S兲
Mn共S-1兲
Au共S-1兲
Mn共S-2兲
Mn共S-2兲

s

p

d

␮

0.58
0.87
0.67
0.92
0.65
0.65

0.51
0.50
0.85
0.90
0.82
0.81

5.45
9.12
5.67
9.04
5.58
5.58

4.08
0.00
−3.17
0.08
2.68
2.59

magnetic moments found in our calculation for the Mn and
Au atoms in the MnAu alloy are similar to the values foundfor MnCu alloy.2 It has been claimed that the large magnetic
moment of the Mn atoms is the basic source for the formation and stability of the surface alloys.1
In Fig. 8 we plot the projection of the majority bands 共a兲
in the Mn site at the surface layer of the alloyed system. The
thickness of the bands is proportional to the percentage ofthe atom in this band. So, if the band is very thick, it meansthat it is due mainly to this atom. A broad band can be observed at 0.15 eV above the Fermi level close to the X pointof the Brilluoin zone. Thus we can identify the peak at
0.1 eV in our dI / dV curves as coming from the Mn atoms at
the surface. In panel 共b兲 of Fig. 8 we plot the isocharge
density distribution in an energy range of 0.25 eV above theFermi level. This is the energy range where differences in theintensity have been found in the dI / dV curves measured inthis system. In this plot only the atoms of the surface 共S兲 andthe second layer below the surface 共S-2兲 are visible. Due tothe geometric structure in this plot the atoms of the subsurface layer 共S-1兲 do not lay in the same plane. As mentioned
before, for the alloy layers the S-2 layer is the first layercomposed only by Mn.
In conclusion, we have studied the magnetic and electronic structure of the MnAu alloy grown on a Mn共001兲 surface by means of spin-polarized scanning tunneling experiments and ab inito all-electron calculations. The MnAu films
are produced by deposition of small amounts of Au at RT on
a Mn共001兲 surface. On the scanning tunneling microscopy
images, the surface of the MnAu alloy presents a c共2 ⫻ 2兲reconstruction. By means of SP-STS measurements wefound an in-plane magnetization with a ferromagnetic coupling between the Mn atoms in the same layer and a layered
antiferromagnetic structure. The calculated density of statesclose to the Fermi level is dominated by a spin-dependent
peak at +0.15 eV that is located mainly at the Mn atomic
sites. This peak is the origin of the contrast in the SP-STS
images. The hybridization between Mn and Au atoms is negligible and a very high magnetic moment of the Mn atoms on
the MnAu alloy is found, especially at the surface.
ACKNOWLEDGMENTS

ALVP acknowledges the financial support by the Spanish
Ministry of Education 共MEC兲 through Project No. FIS2004-

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01026. RR and AV aknowledge financial support by the
Spanish Ministry of Science and Technology through Project
No. MAT2002-03142. TKY acknowledges the Grant-in-aid
for JSPS fellows. The authors thank Dr. D.T. Pierce for sup-plying the Fe whiskers.This work was supported by the
Stichting voor Fundamenteel Onderzoek der Materie 共FOM兲,
which is funded by the Nederlandse Organisatie voor Wetenschappelijk Onderzoek 共NWO兲.

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