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Observation of Spin-Polarized Surface States on Ultrathin bct Mn(001) Filmsby Spin-Polarized Scanning Tunneling Spectroscopy
T. K. Yamada,1,2 M. M. J. Bischoff,1 G. M. M. Heijnen,1 T. Mizoguchi,2 and H. van Kempen1,*
1
2

NSRIM, University of Nijmegen, NL-6525 ED Nijmegen, The Netherlands
Faculty of Science, Gakushuin University, 171-8588 Mejiro, Tokyo, Japan(Received 1 August 2002; published 6 February 2003)

We report the observation of a magnetic contrast of up to 20% in the scanning tunneling spectroscopy
dI=dV maps obtained with Fe-coated tips on Mn(001) layers grown on an Fe(001) whisker at 370 K.
These nanometer resolution microscopy results show that the layers couple antiferromagnetically. By
normalizing the dI=dV curves by tunneling probability functions, we found a spin-dependent peak on
the body-centered-tetragonal (bct) Mn(001) surface at 0:8 V, whose high spin polarization gives riseto the dI=dV map contrast. Band structure calculations allow one to identify the 0:8 V peak as due to
two spin-polarized dz2 surface states.
DOI: 10.1103/PhysRevLett.90.056803

PACS numbers: 73.20.–r, 68.37.– d, 75.70.–i

The (001) surfaces of the bcc 3d transition metals Cr
and Fe are characterized by highly spin-polarized surface
states [1–3]. In the periodic table, Mn is not only between
Cr and Fe, it is also exactly in the middle of the 3d seriesfavoring higher magnetic moments [4]. Although Mndoes not exist in a bulk bcc phase at room temperature(RT), previous results showed that body-centeredtetragonal (bct) (001) surfaces can be stabilized by thinfilm growth at Fe(001) surfaces [5–7]. Recent scanningelectron microscopy with polarization analysis results
suggested that for films thicker than three layers theferromagnetic Mn(001) sheets couple antiferromagnetically, whereas the local surface magnetism is unknown[8]. A natural question that arises is whether the magnetization of these artificial bct Mn(001) layers can be
related to the existence of highly spin-polarized d-likesurface states. Furthermore, the study with local magnetic microscopy with nanometer resolution of a system
consisting of an antiferromagnet on top of a ferromagnet
is of utmost importance for fundamental understandingof nanomagnetism and possible applications such as spintronical devices. Spin-polarized scanning tunneling microscopy (SP-STM) and spectroscopy (SP-STS) are themost powerful techniques to tackle these issues [9].
In this Letter, we report on the observation of a magnetic contrast in SP-STM and SP-STS measurements on
bct Mn(001) layers giving evidence that the layers couple
antiferromagnetically. After a careful background correction a peak is found at 0:8 V in the spectroscopic dI=dV
curves. Band structure calculations allow us to interpret
this peak in terms of spin-polarized dz2 surface states.
Problems that arise in determining the sample polarization are discussed.
STM and STS measurements were performed in ultrahigh vacuum (UHV) (51011 mbar) at RT withan Omicron UHV STM-1. Mn layers were grown on anFe(001) whisker at 370 K. Details are given in Refs. [7,10].
In the present study, we used two kinds of tips: (1) W tips
self-sputtered in UHV by Ar were used as clean W tips

[7]. (2) Fe-coated W tips were used as SP tips. A magnetic
contrast was reproducibly obtained with relatively blunt
W tips covered with 7–10 nm Fe at RT [11]. It is believedthat for these tips the magnetization is parallel to thesample surface [9]. The tip was carefully approached tothe sample to exclude any mass transport between tip
and sample. Furthermore, no voltage pulses were applied
during these SP-STS measurements [12]. An external
magnetic field was not applied. STS measurements were
performed at every pixel of a constant current topographic image by opening the STM feedback loop at agiven current and voltage (‘‘set point’’). dI=dV curves
were obtained by numerical differentiation of the IV
curves.
Since the growth of Mn layers on Fe(001) changes atcoverages above 3 ML (monolayer) from layer-by-layer tolayer-plus-islands [7], more than four levels can be exposed at a coverage of 7.2 ML Mn as shown in Figs. 1(a)and 1(c). Previous results showed that from the fourthlayer the Fe intermixing in the Mn film can be neglected[7]. At this surface, dI=dV curves were measured at 75 
75 pixels. A map of the dI=dV signal at 0:2 V is shownin Figs. 1(b) and 1(d). Numbers in Fig. 1 denote thestacking numbers of the Mn layers, which were estimatedby a quartz crystal oscillator calibration and Gaussianfits to the fraction of exposed layers on the surface [7].
Using clean W tips, the dI=dV map shows no contrast[Fig. 1(b)]. All step heights in Fig. 1(a) are 0.16 nm [14].
Furthermore, since the in-plane lattice constant equals
that of Fe(001), i.e., 0.287 nm [7], it can be concluded that
the Mn films grow in a bct phase. Figures 1(c) and 1(d)show STM and STS measurements, respectively, performed with an Fe-coated W tip. Clearly, the contrast in
the dI=dV map oscillates with a period of two layers.
From higher resolution images [Figs. 1(e) –1(g)], it can beconcluded that the magnetic contrast changes within alateral length of 1:0  0:5 nm. Although the even layers(> 3 ML), i.e., layers ‘‘8’’ and ‘‘10’’, are darker than the
odd layers (‘‘7’’ and ‘‘9’’) in the dI=dV map, this is not

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FIG. 2. (a),(b) dI=dV curves normalized by the voltagedependent tunneling probability functions, which were obtained on Mn films thicker than three layers with clean W
tips and Fe-coated tips, respectively. Solid and dotted curves
are the dI=dV and the dI=dV=T curves, respectively. Tunneling probability functions (dashed curves) were obtained by a fit
to the dI=dV curves. In (b), black and grey curves are representative of the odd and the even layers, respectively.
FIG. 1. (a),(b) STM and STS measurements, respectively,
obtained with a clean W tip on Fe(001) covered with 7.2 ML
Mn at 370 K. (a) A topographic image obtained at a set pointof VS  0:5 V, I  0:5 nA. Scan size is 150 100 nm2 .
(b) The dI=dV map at 0:2 V measured at the same area as(a). (c),(e) SP-STM and (d),(f) SP-STS measurements obtainedwith Fe-coated tips. (c),(e) Topographic images obtained at a
set point of VS  0:5 V, I  0:5 nA. Scan size is 100 
67 nm2 for (c) and 14 20 nm2 for (e). (e) A hidden Fe step[7], where step height is only 0.02 nm and magnetizations arereversed. (d),(f) The dI=dV map at 0:2 V measured at thesame area as (c) and (e), respectively. For (d) black-white scale
ranges from 0.75 nA/V to 1:00 nA=V. In (f), one pixel is0.25 nm. (g) The line profile along the white line in (f).

always the case, i.e., with some Fe-coated tips the odd
layers appear darker than the even layers. With the Fecoated tip also inequivalent step heights are measured forsteps from even to odd (0.15 nm) and from odd to even(0.17 nm) layers in Fig. 1(c) [14]. We explain the alternating contrast in the dI=dV maps obtained with Fe-coatedtips by the layered antiferromagnetic magnetization of
the Mn(001) layers. This explanation is corroborated by
the following observations: (i) The alternating contrast
in the dI=dV maps has the same onset (> 3 ML) andperiod (two layers) as previous results obtained with otherless local techniques [6,8,15]. (ii) The contrast is observed with the Fe-coated tips only; it is not observedwith clean W tips. (iii) Reversed contrasts are also observed using different Fe-coated tips. This indicates therandom character of the tip magnetization direction.
The dI=dV curves obtained on the Mn(001) surface donot show peaks (Fig. 2). dI=dV curves normalized by I=V
do not show peaks either. Conversely, normalization of
the dI=dV curves by the tunneling probability function(T) is more successful. Theoretically, this normalization
was shown to lead to the best sample density of states(DOS) recovery [16]. Also, our recent publications show
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that this method can successfully recover the sample
DOS from experimental dI=dV curves [7,10]. In thepresent work we fitted T multiplied by an offsetted
Gaussian line shape (which represents the surface statepeak in the DOS) to the dI=dV curves. This fitting
procedure was leading only to good fitting results when
we used as input work function ( ) values 4:0  0:25 eV.
For ’s varying between those limits the peak amplitudechanges 15% and the width 5%, but the peak energy
only 10 meV. These changes produce errors in asymmetries 2% as discussed below [see Fig. 4(d) below].
Figures 2(a) and 2(b) show dI=dV=T curves averaged
over 30 single curves measured on Mn films thicker than
three layers with a clean W tip and an Fe-coated tip,
respectively. dI=dV curves were obtained within a large
voltage range from 2 V to 3 V. Normalized dI=dV
curves measured with the clean W tip show two peaks: astrong peak around 0:8 V and a weak peak around0:5 V. In Fig. 2(b), normalized dI=dV curves measured
on even layers (grey curve) and odd layers (black curve)
show two peaks at the same energies as those obtainedwith the clean W tips. However, now the amplitude ofboth peaks oscillates with a period of two layers. The
curves measured on even and odd layers cross around the
Fermi level. In the approximation of Ref. [16], dI=dV=Tat positive (negative) voltages is proportional to thesample surface (tip) DOS multiplied by the tip (sample
surface) DOS at the Fermi level [17].
To determine the nature of these peaks, band structure
calculations were performed using the Vienna Ab initio
Simulation Program (VASP) [18,19]. An eight layer slab
with the experimental values for the in-plane (2.87 Å) and lattice constants were used.
out-of-plane (2  1:64 A)
Furthermore, an antiferromagnetically stacking of theMn(001) layers was assumed. Peaks are found above theFermi level in the ‘‘minority’’ band of the surface DOS.
To confirm that these peaks are surface states which
protrude far enough into the vacuum to be detected by
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the STM tip, bands with the largest dz2 character at  areselected [1]. These are indicated by thick lines in Fig. 3(a).
Three bands are possible candidates for the experimentally observed empty state dI=dV=T peak: band at
0.27 eV, at 0.50 eV, and at 0.87 eV above the Fermi
level. A plot of the surface charge density distribution ofthese three bands shows that the last one has a stronger
decay into the vacuum (not shown in Fig. 3). The first two
show surface-state-like behavior and their charge densities are plotted in Figs. 3(b) and 3(c). Opposed to theminority band, no surface states were found above theFermi level for the ‘‘majority’’ band. The width ofthe 0:8 V peak in dI=dV=T is about 1 V (Fig. 2).
This width is larger than the peak width obtained on
Fe(001) (0.13 eV) [1] and Cr(001) (0.2 eV at RT and
0.015 eV at 4.2 K) [2,3]. One reason is that the 0:8 V
peak in dI=dV=T is caused by two surface states: and. The difference between the calculated peak energies
and the experimental peak energy is attributed to thelimitations of our VASP calculations. Nevertheless, based
upon the symmetry of the calculated and states, thesestates must be the origin of the experimentally obtained0:8 V peak. Below the Fermi level a weak dz2 state is

FIG. 3. (a) Band structure of an eight-layer Mn(001) slab. The
plot shows both ‘‘majority’’ and ‘‘minority’’ bands, which areequivalent for this even-layered antiferromagnetically orderedslab. The magnetization for each layer is defined by the difference in integrated local DOS of the ‘‘majority’’ and ‘‘minority’’ bands. Thick lines indicate dz2 -like states around  , andthe dashed line the Fermi level. , , and could contribute to
the experimentally observed 0:8 V peak. (b),(c) The isocharge density distributions of the and the states at  ina (110) plane. Dots and dashed lines indicate atom positions andthe boundary between vacuum and bulk, respectively.

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found at 0:37 eV in the majority band: its charge density does not show a strong vacuum contribution.
Nevertheless, since the weak peak around 0:5 V inthe dI=dV=T curve is reproducibly observed with various tips, we think it is a real feature of the Mn(001)electronic structure.
Various methods for extracting the sample polarization from SP-STS were reported [9,20]. However, thesemethods include a certain ambiguity. For example,
when extracting the sample polarization from the SPSTS results one encounters the problem that for a given
set point voltage the tip-sample distance (and so thetunneling probability) is different for the even and odd
Mn layers. This is due to the SP part of the tunnel current
which has different bias voltage dependencies for evenand odd layers. Figures 4(a) and 4(b) show dI=dV curves
measured on the even and the odd layers (> 3 ML) at
negative- (VS  0:5 V) and positive-voltage set point(VS  0:5 V). The dI=dV curves measured at thepositive-voltage set point are clearly different from theones measured at the negative-voltage set point.
Figure 4(c) shows the asymmetry in the dI=dV curvesof Figs. 4(a) and 4(b), which is defined as AdI=dV dI=dVeven  dI=dVodd = dI=dVeven  dI=dVodd ,where dI=dVeven and dI=dVodd are dI=dV curves obtained on the even and the odd layers. The asymmetrystrongly depends on the voltage set point, whereas it does
not depend on the current set point in the range between
0.1 and 1 nA. Strictly speaking, a definition of the asymmetry in terms of dI=dV curves is valid only for low bias
voltages where the effects of an increasing backgroundare negligible [21]. The dI=dV=I=V normalization
which was used by Okuno et al. [20] cannot exclude theexponential background completely [16] and dI=dV=I=V is by definition 1 at 0 V, which might influence the asymmetry. On the contrary, the asymmetry(AdI=dV=T ) obtained from the normalized dI=dV=T
curves does not depend on set points and tips, at least at
positive voltages [Fig. 4(d)]. AdI=dV=T gives at positive
voltages the sample polarization multiplied by the tip
polarization at the Fermi level. The asymmetries inFig. 4(d) are obtained from dI=dV=T curves whichshow a clear spin-dependent peak. The three curves inFig. 4(d) are similar at positive voltages ( 10%). The
influence of the fitting parameters on the asymmetry canbe observed from the error bars in Fig. 4(d). If we assume
that the surface states have a 100% polarization and thatthe tip and sample magnetization are perfectly (anti)parallel, we find a minimum Fermi level tip polarization of
10% which is in agreement with previous results [2].
AdI=dV=T ’s larger than 10% were never observed whichmakes it likely that the magnetization directions are indeed parallel. Using this tip polarization, the black solidline in Fig. 4(d) is obtained from the calculated spindependent sample total states. AdI=dV=T has some advantages compared to AdI=dV and AdI=dV=I=V as used in
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0:8 V on the artificial bct Mn(001) layers. Our band
structure calculations confirmed that this peak is possibly
related to two spin-polarized dz2 surface states. The
asymmetries of the dI=dV=T curves are independentof the voltage set points and tips.
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
MAGNETUDE. We are especially grateful to D. T. Pierce
for supplying us with the Fe whiskers and to A. L. Vazquez
de Parga and A. I. Lichtenstein for helpful discussions.

FIG. 4. (a),(b) SP-STS measurements representative of
the even (grey curve) and the odd layers (black curve)(>3 ML) at negative- (VS  0:5 V, I  0:5 nA) and positivevoltage set points (VS  0:5 V, I  0:5 nA), respectively.
(c) The asymmetry in the dI=dV curves. Dotted and solid
curves are obtained from (a) and (b), respectively. (d) The
asymmetry in the normalized even and odd layer dI=dV=T
curves with respect to the sample bias voltage. Open circles andtriangles were obtained with the same Fe-coated tip using
negative and positive voltage set points, respectively. Solid
dots were obtained with a different Fe-coated tip (VS 0:5 V). The black solid curve is obtained from the calculated
spin-dependent DOS.

Refs. [9,20]: (1) The exponential background is completely removed. (2) AdI=dV=T above the Fermi energy
is independent of set points. AdI=dV=T obtained at positive
and negative set points are shown as open circles andtriangles in Fig. 4(d), respectively. (3) Reproducible
AdI=dV=T values are observed with different Fe-coatedtips at the positive-voltage side [Fig. 4(d)]. Scattering ofAdI=dV=T at the negative-voltage side may be a contribution of the tip DOS [16].
We reported the observation of a magnetic contrast in
the dI=dV maps obtained with Fe-coated tips on Mn
layers grown on an Fe(001) whisker at 370 K. This contrast was not observed with clean W tips. Therefore, ourresults give strong evidence that the Mn(001) sheets
couple antiferromagnetically even on a nanometer scale.
By normalizing the dI=dV curves by the tunnelingprobability function, we found a spin-polarized peak at

056803-4*Corresponding author.
Email address: hvk@sci.kun.nl[1] J. A. Stroscio et al., Phys. Rev. Lett. 75, 2960 (1995).
[2] M. Kleiber et al., Phys. Rev. Lett. 85, 4606 (2000).
[3] O. Yu. Kolesnychenko et al., Nature (London) 415, 507(2002).
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[6] S. Andrieu et al., Phys. Rev. B 57, 1985 (1998).
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[8] D. A. Tulchinsky et al., J. Magn. Magn. Mater. 212, 91(2000).
[9] R. Wiesendanger and M. Bode, Solid State Commun. 119,341 (2001).
[10] M. M. J. Bischoff et al., Surf. Sci. 501, 155 (2002).
[11] A tip with a radius > 300 nm was prepared by annealingat 1500 K. The radius of the tip was estimated by fieldemission spectroscopy.
[12] In order to reshape or clean the tip voltage pulses( 10 V) were applied between the tip and the sample
in our previous results [10]. These pulses likely lead tomass transport from the sample to the tip [13].
[13] T. K. Yamada et al., Appl. Phys. Lett. (to be published).
[14] Using the single step height of bcc-Fe(001) which is
0.143 nm, the apparent step heights of our STM
measurements were calibrated. Step heights were measured at large negative bias voltages to exclude electronic
effects [7].
[15] T. G. Walker and H. Hopster, Phys. Rev. B 48, 3563
(1993).
[16] V. A. Ukraintsev, Phys. Rev. B 53, 11 176 (1996).
[17] The different amplitudes of the dI=dV=T curve in
Fig. 2(a) and the average of the even and odd
dI=dV=T curves in Fig. 2(b) are related to the different
Fermi level DOS of the Fe-coated tip and the W tip.
[18] G. Kresse and J. Hafner, Phys. Rev. B 47, 558 (1993); 49,
14251 (1994).
[19] G. Kresse and J. Furthmüller, Phys. Rev. B 54, 11 169(1996).
[20] S. N. Okuno et al., Phys. Rev. Lett. 88, 066803 (2002).
[21] S. Heinze, Ph.D. thesis, University of Hamburg, 2000.

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