Surface Science 600 (2006) 1048–1053
www.elsevier.com/locate/susc

Study of c(2 · 2)-MnAu(0 0 1) layers on Mn(0 0 1) by meansof scanning tunneling microscopy/spectroscopy
T.K. Yamada a,*, A.L. Vázquez de Parga b, M.M.J. Bischoﬀ c,
T. Mizoguchi a, H. van Kempen cba
Department of Physics, Faculty of Science, Gakushuin University, 1-5-1 Mejiro, 171-8588 Toshima, Tokyo, Japan
Departamento de Fisica de la Materia Condensada e Instituto de Ciencia de Materiales ‘‘Nicolas Cabrera’’, Universidad Autónoma de Madrid,
Cantoblanco 28049, Madrid, Spainc
Institute for Molecules and Materials, Radboud University Nijmegen, Toernooiveld 1, 6525 ED Nijmegen, The Netherlands

Received 15 October 2005; accepted for publication 19 December 2005
Available online 19 January 2006

Abstract
Intermixing, growth, geometric and electronic structures of gold ﬁlms grown on antiferromagnetic stacking body-centered-tetragonal
manganese (0 0 1) ﬁlms were studied by means of scanning tunneling microscopy/spectroscopy at room temperature in ultra-high vacuum. We found stable ordered c(2 · 2)-MnAu(0 0 1) alloy layers after depositing Au on pure Mn layers. Since at the fourth layer(5 · 23)-like Au reconstruction appears instead of the c(2 · 2) structure and local density of states peaks obtained on the c(2 · 2)-MnAu
surface disappear, pure Au layers likely grow from the fourth layer.
 2006 Elsevier B.V. All rights reserved.
Keywords: Scanning tunneling microscopy; Scanning tunneling spectroscopies; Surface structure, morphology, roughness, and topography; Metallic ﬁlms;
Gold; Manganese; Alloys; Metal–metal interfaces

1. Introduction
To prevent oxidization of magnetic ﬁlms a gold coating
has been widely used. However, it is diﬃcult to grow a puregold atomic layer on magnetic ﬁlms. Au on Fe(0 0 1) was agood candidate because Fe and Au are immiscible in bulk,
Fe has a much higher surface energy than Au, and the lattice mismatch between Fe(0 0 1) and Au(0 0 1) is only 0.5%.
Despite of these favourable circumstances it was found thatan FeAu surface alloy layer forms with a c(2 · 2) structure[1].
There have been many studies of magnetic ﬁlms grown
on noble metals [2], but much less studies of noble ﬁlms
on magnetic surfaces. Due to the large diﬀerence in surface
energy, the substrate noble metal prefers to segregate on*

Corresponding author. Tel.: +81 3 3986 0221; fax: +81 3 5992 9319.
E-mail addresses: 20030303@gakushuin.ac.jp (T.K. Yamada),
H.vanKempen@science.ru.nl (H. van Kempen).
0039-6028/$ - see front matter  2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.susc.2005.12.027

top of the magnetic ﬁlms during the growth. Manganese
has lower surface energy than noble metals. (Surface energies are 1.333 J/m2 for Au, 1.298 J/m2 for Mn, and 2.123 J/
m2 for Fe [3].) Several studies of Mn ﬁlms grown on noble
metal substrates: Au(0 0 1) [4], Ag(0 0 1) [5], and Cu(0 0 1)
[6–18] have been reported. One monolayer Mn grown on
Cu(0 0 1) forms MnCu alloy layers with a c(2 · 2) orderedstructure. Mn atoms in the c(2 · 2) structure have a highlocal magnetic moment, a ferromagnetic in-plane order,
and an antiferromagnetic coupling between layers [13]. Ac(2 · 2) alloy layer is also observed for Mn/Au(0 0 1) and
Mn/Ag(0 0 1). However, for the Au(0 0 1) substrate, a pureAu layer or a pure Mn layer is also formed depending on
diﬀerent annealing processes [4], i.e., it is diﬃcult to growan well ordered c(2 · 2) alloy structure only.
In this study we show the ﬁrst study of growth, intermixing, and geometric structure of Au ﬁlms grown onan ordered antiferromagnetic stacking body-centeredtetragonal Mn(0 0 1) ﬁlm by means of scanning tunnelingT.K. Yamada et al. / Surface Science 600 (2006) 1048–1053

microscopy (STM)/spectroscopy (STS) at room temperature (RT) in ultra-high vacuum (UHV). STM/STS measurements show that, even at room temperature, stablec(2 · 2)-MnAu(0 0 1) alloy layers are formed. Since on the
fourth layer STM images show (5 · 23)-like Au reconstruction and STS data show an absence of local density ofstates peaks observed on the c(2 · 2)-MnAu(0 0 1) alloy
surface, we conclude that pure Au layers grow from thefourth layer.
2. Experimental details
STM and STS measurements were performed in UHV(5 · 1011 mbar) at RT with a commercial STM.
In this study, we used W tips prepared as follows: in air,a W polycrystalline wire (purity 99.99%) with a diameter of
0.5 mm was chemically etched with 5 M KOH aq. and subsequently, it was moved into UHV (1 · 1010 mbar). Then,the tip was sputtered by Ar+ and heated by electronbombardment.
An Fe(0 0 1) whisker was used as a substrate. This
Fe(0 0 1) whisker was cleaned by sputtering and annealing
cycles. After this cleaning process, less than 1% oxygen
contaminants were measured by Auger spectroscopy andatomically and chemically resolved STM images [19–21].
About 7 ML of Mn (purity 99.999%) was grown on
the Fe(0 0 1) whisker at 370 K at a pressure below
4 · 1010 mbar. There is no intermixing of Fe into Mn layers above the third Mn layer [21]. Mn layers grown on
Fe(0 0 1) have a body-centered tetragonal (bct) structurewith the same in-plane lattice constant as Fe(0 0 1):
0.287 nm and an interlayer distance of 0.165 nm [21]. Au
ﬁlms were deposited on the bct Mn(0 0 1) layers at RT witha rate of 0.25 ML/min at a pressure below 3 · 1010 mbar.
The Au thickness is determined by calibrating the evaporation rate with the atomically and chemically resolved STM
images and the values given by the quartz balance.
Spectroscopy measurements were performed in the following way: I(V) curves were obtained at every pixel of aconstant current topographic image by opening the STM
feedback loop at a given current and voltage (set point).

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dI/dV curves were obtained by numerical diﬀerentiationof the I(V) curves.
3. Results and discussion
Fig. 1(a) shows an atomically resolved STM image obtained on the surface after deposition of 7 ML of Mn at
370 K on the Fe(0 0 1)-whisker. On the Mn(0 0 1) surface,usually, we observed only p(1 · 1)-Mn(0 0 1) atoms andoxygen contaminants at the fourfold hollow sites (<1%concentration). However, the surface of a 1 ML Au layergrown on the Mn(0 0 1) ﬁlm shows a diﬀerent structure(Fig. 1(b)). By comparing Fig. 1(a) and (b) we conclude
that the ﬁrst Au layer forms a c(2 · 2) structure. Our Auger
spectra show a Mn 40 eV peak and a Au 69 eV peak. No Fe
47 eV peak was observed, from which we estimate that the
Fe concentration is below 4%. (The half width of the Auger
peak is 1.5 eV.)Thus, the c(2 · 2) structure consists of only Au and Mn.
Since Au and Mn are miscible and similar c(2 · 2) alloy
layers are also found on Mn ﬁlms grown on noble
metal(0 0 1) [4–18], the observed c(2 · 2) structure isconsidered to be a Mn50Au50 alloy. Fig. 1(c) shows amodel of the p(1 · 1)-Mn(0 0 1) structure and the c(2 · 2)MnAu(0 0 1) alloy structure. Since Au has a larger atomic
size, Au atoms might buckle from the surface and appear
brighter in Fig. 1(b). However, this speculation might bewrong since a study of MnCu-c(2 · 2)-surface-alloy
showed that Mn appears brighter in STM images due toa strong electronic eﬀect [18].
Fig. 2 shows STM images obtained on the surface of a
0.4 ML (a), a 0.7 ML (b), and a 2.2 ML (c) Au ﬁlm grown
on a 7 ML Mn(0 0 1) ﬁlm. Due to a layer-plus-island
growth mode, odd and even Mn layers are exposed asshown in Fig. 2(a). The Au layer grows in a layer-by-layer
mode (Fig. 2(b)). The roughness of the surface increases
with the coverage, but the ﬁlms grow still epitaxially(Fig. 2(c)). In all the layers exposed on the surface, there
are areas that are slightly brighter than the rest of the terrace. The distribution of these bright areas changes withthe thickness: 19% for the ﬁrst layer, 45% for the second

Fig. 1. (a) shows an atomically resolved STM image obtained on the surface of a 7 ML Mn ﬁlm grown at 370 K on the Fe(0 0 1)-whisker (1.7 · 1.7 nm2). Ap(1 · 1)-Mn(0 0 1) structure is observed. (b) shows an atomically resolved STM image obtained on the surface of a 1 ML Au ﬁlm grown at RT on theMn(0 0 1) ﬁlm (1.7 · 1.7 nm2). A c(2 · 2)-MnAu alloy structure is observed. (c) A model of the atomic conﬁgurations. Dots and white-circles denote Mn
and Au atoms, respectively. Lines denote the atomic lattice.

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T.K. Yamada et al. / Surface Science 600 (2006) 1048–1053

Fig. 2. STM topographic images obtained on the Mn surface covered byAu ﬁlms with coverages of (a) 0.4 ML (VS = 1 V, I = 0.1 nA), (b)0.7 ML (VS = 0.5 V, I = 0.1 nA), and (c) 2.2 ML (VS = +0.8 V,
I = 0.1 nA) (100 · 50 nm2). Arrows and the dotted circle denote brightareas, which are 0.05 ± 0.01 nm higher than the surrounding terrace.
Numbers denote the stacking numbers of the layers.

layer, and 43% for the third layer. On the ﬁrst layer, theseare observed around steps as marked by arrows in Fig. 2(a)and (b) and dotted circles in Fig. 3(a). On the second layer,these are observed in the middle of the terraces, e.g., asshown in the dotted-circle in Fig. 2(c). The height of thesebright areas/lines is about 0.05 ± 0.01 nm with respect tothe surrounding terrace. The origin of these bright areas/lines is discussed later.
Fig. 3(a) shows an atomically resolved STM image obtained on the Mn surface after deposition of 0.6 ML of
Au at RT. The Mn terrace and the ﬁrst Au layer show ac(2 · 2) structure. The observed structure in Fig. 3 is considered to be a c(2 · 2)-MnAu surface alloy, i.e., now the
top Mn layer (‘‘0’’) and the ﬁrst Au layer (‘‘1’’) mix completely and form the c(2 · 2) structure.
Fig. 3(b) shows another atomically resolved STM image
obtained on the surface of Mn ﬁlms covered by a 1 ML Au
ﬁlm. Most terraces are the ﬁrst layer (‘‘1’’) and the second
layer is observed as islands (‘‘2’’). The original Mn surface

Fig. 3. (a) shows an atomically resolved STM image obtained on the Mnsurface covered by a 0.6 ML Au ﬁlm at RT (20 · 20 nm2, VS = 14 mV,
I = 3.3 nA). The surface shows the c(2 · 2) structure of the MnAu surface
alloy. Near the step edge, the ﬁrst layer appears brighter (dashed circles).
The white lines outside the islands marked by black arrows are an artifactof the jumped grey-scale. Numbers denote the stacking numbers of thelayers. (b) shows an atomically resolved STM image obtained on the Mnsurface covered by a 1 ML Au ﬁlm at RT (16 · 13 nm2, VS = 3 mV,
I = 8.5 nA). In (b) arrows: A–D are guides for the eye. Numbers denotethe stacking numbers of the layers. The dashed line denotes a domainboundary. To make the atomic features more prominent, the contrast in (aand b) has been adjusted in every terrace.

T.K. Yamada et al. / Surface Science 600 (2006) 1048–1053

is observed in depressions (‘‘0’’). All layers completely mix
with Mn and form the c(2 · 2)-MnAu(0 0 1) structure. It isfound that there is a default line (a black dashed line in
Fig. 3(b)). The lower and the upper sides of the default line
are named ‘‘Area I’’ and ‘‘Area II’’, respectively. The arrows marked ‘‘A, B’’ (‘‘C, D’’) follow the [1 1 0] ð½1 1 0Þ
direction. Following the arrows ‘‘A’’ shows that the protrusions are on the arrows. On the other hand, following thearrows ‘‘B’’ from the area I to the area II shows that thearrows have to be slipped half a lattice constant alongthe ½1 
1 0 direction to keep on the protrusions. The sameatomic conﬁguration is also observed along the ½1 
1 0 direction. Although following the arrows ‘‘C, D’’ in the area I
shows that the arrows are on the protrusions for the layers‘‘0, 1, and 2’’, following the arrows ‘‘D’’ from the area I to
the area II shows that the arrows have to be slipped half alattice constant along the [1 1 0] direction to keep on the
protrusions. Thus, from the area I to the area II, protrusions shift half a lattice constant along both [1 1 0] and½1 1 0 directions. These domain walls are a straightforward
result of the fact that two diﬀerent c(2 · 2) lattices can beaccommodated on the p(1 · 1) lattice, shifted with ap(1 · 1) period.
Fig. 4 was obtained on the Mn surface covered by a
2.2 ML Au ﬁlm. The second (‘‘2’’) and the third (‘‘3’’) layers are observed as a terrace and islands, respectively. The
second and the third layers show the c(2 · 2) structure, butislands that consist of less than 80 protrusions show ap(1 · 1) structure. Probably, this structure is formed by

Fig. 4. This ﬁgure shows an atomically resolved STM image in constantcurrent mode obtained on the Mn surface covered by a 2.2 ML Au ﬁlm at
RT (14 · 14 nm2, VS = 12 mV, I = 3 nA). Numbers denote diﬀerent
heights on the image. Black lines are guides for the eye. Dashed lines
denote domain boundaries. To make the atomic features more prominent,
the derivative of the constant current image is shown.

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one single element, i.e., Au or Mn. If these are pure Au islands, these Au atoms have in-plane lattice constants similar to the substrate bct-Mn(0 0 1), which has the same inplane lattice constant as the bcc-Fe(0 0 1), i.e., 0.287 nm.
The height of the p(1 · 1) islands is the same as the heightof the c(2 · 2) islands of the third layer. On the surface ofthe layer ‘‘2’’, the presence of domain boundaries is veryclear (dashed lines in Fig. 4).
Although the c(2 · 2) structure was observed up to thethird layer, no clear ordered structure was observed on
the fourth layer. Fig. 5(a) was obtained on the Mn surface
covered by a 4.0 ML Au ﬁlm at RT. The STM images show
not only ﬂat terraces, but also many bright areas (47%) on
the fourth layer (Fig. 5(a)). Although this is comparable to
the results obtained on the ﬁrst three layers, the diﬀerenceis that now the bright areas make periodic rows (Fig. 5(b)).
The width of each row is about 1.5 nm (Fig. 5(c)). The corrugation of the rows is about 0.05 nm. These parallel
arrangement and the width are similar to the (5 · 23)
reconstruction of Au(0 0 1) [22]. The reconstruction is alsopresent on Au layers deposited on Fe(0 0 1) [1]. The presence of these structures indicates that a pure Au layer starts
from the fourth layer. It should be mentioned that thebright rows follow the [1 0 0] or the [0 1 0] direction.
The step heights (=interlayer distances) were measured
from the STM images. First, we diminished the distortionin the STM image with a plane-ﬁtting. Then, we made ahistogram from this STM image. Each atomic layer appears as a sharp peak. By measuring the distance between
the peaks, we obtained the step heights with an error of

Fig. 5. (a) shows an STM image obtained on the Mn surface covered by a
4.0 ML Au ﬁlm at RT (100 · 50 nm2, VS = 0.5 V, I = 0.5 nA). Numbersin (a) denote the stacking numbers of the layers. (b) shows an enlarged
image from (a) (14 · 11 nm2). Periodic rows are observed, which follow
the [1 0 0] or the [0 1 0] direction. (c) shows a line proﬁle along the blackarrow in (a). The periodic rows have a corrugation of 0.05 nm. The
width of a bright row is about 1.5 nm.

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T.K. Yamada et al. / Surface Science 600 (2006) 1048–1053±0.007 nm. The error was obtained from the half width at
half maximum of the peak.
The obtained apparent step heights are inﬂuenced by theelectronic structure of the sample surface (the sample local
density of states (LDOS) as well as the barrier heights). To
check the inﬂuence of the sample LDOS, the apparent stepheights were measured as a function of the sample bias
voltage. Barrier height measurements were not performed.
(All measurements shown in Fig. 6 were obtained between
the c(2 · 2)-alloy layers. It is assumed that all c(2 · 2)-alloy
layers have the same work function.) Fig. 6 shows theapparent step heights as a function of the sample bias voltage. hij denotes the step height between the layer i and j.
Since the STM images obtained at the voltage set point
above the Fermi energy include inﬂuences of the sample
LDOS, the step heights obtained at high negative voltages(<1 V) are believed to show the real geometric step heights.
The interlayer distances obtained are 0.168 ± 0.007 nm for
h01, 0.170 ± 0.007 nm for h12, and 0.170 ± 0.007 nm forh23. This is a little larger than the interlayer distance ofthe bct-Mn(0 0 1) (0.165 nm).
From all these STM images we propose a possible modelfor each Au coverage (Fig. 7). Since the Au and Mn are
miscible in the bulk, dealloying as in the case of Au on
Fe is unlikely [1]. So, after deposition of 3 ML of Au thesurface layer is still a c(2 · 2) MnAu alloy, and we speculate that the layers underneath are also alloyed layers. On
the MnAu-alloy layers bright areas were always observedwith a height of 0.05 ± 0.01 nm. This height was also
checked to be independent of the sample bias voltage (Figs.
2–5 were obtained at several voltages between 1.2 and+0.8 V). Thus, we believe that this height diﬀerence is notdue to an electronic eﬀect, but to a change in geometricstructure. After deposition of 4 ML of Au, six layers of
MnAu-c(2 · 2)-alloy are formed and an additional layer
is formed on the top. If the bright rows of the top layer
are an indication of Au (5 · 23) reconstruction [22], we
speculate that the layers on the MnAu-alloy-layers consistof pure Au. The experimentally obtained interlayer dis-

(a)

(b)

(c)

Fig. 6. Apparent step heights of the ﬁlms of the Au/Mn(0 0 1) layers. hij
denotes the step height between the layer i and j. The step heights wereobtained by measuring the distance between the sharp peaks in thehistogram.

(a)

(b)

(c)

(d)

Fig. 7. Models of the Mn ﬁlms covered by Au ﬁlms of (a) 0 ML, (b)0.6 ML, (c) 2.2 ML and (d) 4.0 ML. (b)–(d) correspond to Figs. 3–5,
respectively.

tances are 0.165 nm for Mn(0 0 1), 0.170 nm for the
MnAu-alloy, and 0.220 nm for the bright rows on the Au
layers.
At the bright areas on the MnAu-alloy layers, atomically resolved images showed the c(2 · 2) structure, i.e.,
no diﬀerence in the in-plane lattice constant, and only theinterlayer distance expands 0.05 ± 0.01 nm. These brightareas were observed near steps. The atoms at steps havelower coordination and therefore they can relax the strain
easily, so the bright areas are probably caused by the relaxation of strain. In bulk, it is known that MnAu alloy has abct (CsCl-type) structure with an in-plane (out-of-plane)
lattice constant of 0.318 nm (0.328 nm) [23]. Since the inplane lattice constant of the observed MnAu(0 0 1)-alloy
is 0.405 nm, the large diﬀerence in the in-plane lattice constant might be a cause of the strain.
By taking spectroscopy curves LDOS on the sample surface were studied. In our previous study LDOS peaks ofthe c(2 · 2)-MnAu(0 0 1) were found at +0.1 V, +0.9 V,
and +1.9 V above the Fermi energy in dI/dV curves, whichwere normalized by the tunneling probability function [24].
The atomically resolved images show that pure Au layers
likely grow from the fourth layer. To conﬁrm this statement LDOS of the fourth and the ﬁfth layers were studied.
Fig. 8 was obtained on the Mn surface coated by a 4 ML
Au ﬁlm. The STM image shows the fourth and the ﬁfth layers (Fig. 8(a)). The dI/dV curves obtained on the fourth(‘‘4’’) and the ﬁfth (‘‘5’’) layers are the same, but thedI/dV curves do not show any peaks or shoulders above
the Fermi energy (Fig. 8(b)). Also, dI/dV curves obtained
from the reconstructed areas (‘‘R’’ in Fig. 8(a)) show nopeaks. The normalized (dI/dV)/T curves represent the sample LDOS [21,24]. (dI/dV)/T curves in Fig. 8(b) show thatthe fourth and the ﬁfth layers do not have any LDOS
peaks, i.e., the fourth and the ﬁfth layers have a diﬀerent
crystalline structure or composition compared to thec(2 · 2) MnAu layer. The diﬀerence in the dI/dV curves obtained on the fourth layer and the reconstructed areas isprobably caused by a diﬀerent work function on the recon-

T.K. Yamada et al. / Surface Science 600 (2006) 1048–1053

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edges the ﬁnancial support by the Ministerio de Educaciony Ciencia (MEC) through Project No. FIS2004-01026.
T.K.Y. acknowledges Grant-in-Aid for JSPS Fellows.
H.v.K. acknowledges a DVF from the International Research Center for Experimental Physics (IRCEP) at the
Queen’s University, Belfast (Ireland) and Instituto Universitario de Ciencia de Materials ‘‘Nicolas Cabrera’’ at the
Universidad Autónoma de Madrid (Spain).
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Fig. 8. (a) shows an STM image obtained with a W tip on the Mn surface
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were obtained from ‘‘4’’, ‘‘5’’, and ‘‘R’’ in (a), respectively.

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4. Conclusion
Intermixing, growth, geometric and electronic structuresof gold ﬁlms grown on antiferromagnetic stacking bodycentered-tetragonal manganese (0 0 1) ﬁlms were studied
by means of scanning tunneling microscopy/spectroscopy
at room temperature in ultra-high vacuum. We found stable ordered c(2 · 2)-MnAu(0 0 1) alloy layers after depositing Au on pure Mn ﬁlms. The alloy layers have an in-plane(an out-of-plane) lattice constant of 0.405 nm (0.340 nm).
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Acknowledgments
This work was supported by the Stichting voor Fundamenteel Onderzoek der Materie (FOM), which is fundedby the Nederlandse Organisatie voor Wetenschappelijk
Onderzoek (NWO), and the European Growth Project
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