Jpn. J. Appl. Phys. Vol. 42 (2003) pp. 4688–4691
Part 1, No. 7B, July 2003
#2003 The Japan Society of Applied Physics

Origin of Magnetic Contrast in Spin-Polarized Scanning Tunneling Spectroscopy:
Experiments on Ultra-Thin Mn Films
Toyo K. Y AMADA1;2 , Maarten M. J. B ISCHOFF1 , George M. M. H EIJNEN1 , Tadashi M IZOGUCHI2
and Herman V AN KEMPEN1; 
1
2

NSRIM, University of Nijmegen, NL-6525 ED Nijmegen, The Netherlands
Faculty of Science, Gakushuin University, Mejiro, Tokyo 171-8588, Japan(Received October 30, 2002; accepted for publication February 3, 2003)

Normalized diﬀerential tunneling conductivities obtained with Fe-coated W tips show a spin-polarized peak around þ0:8 V on
ultrathin bct Mn ﬁlms grown on Fe(001)-whiskers. This spin-polarized peak results in a clear magnetic contrast inspectroscopic images. Our normalization removes the inﬂuence of the tunneling probability and makes the spectroscopic
curves most reliable for a derivation of the spin-resolved sample density of states (DOS) at positive voltages. From thisanalysis we conclude that the magnetic contrast in our spectroscopic maps is caused by a highly polarized DOS. Furthermore,
a tip polarization of about 15% is found. [DOI: 10.1143/JJAP.42.4688]
KEYWORDS: iron, manganese, spin-polarized scanning tunneling microscopy, spin-polarized scanning tunneling spectroscopies,
magnetic films, surface magnetism

1.

Introduction

In the near future a lot of developments in the magnetic
storage technology will be expected due to the present
activation of magnetic and electronic studies in the nanometer range. Spin-polarized scanning tunneling microscopy(SP-STM) and spectroscopy (SP-STS) are the most powerfultools for studying surface magnetism with spatial resolution
on the atomic scale.1,2) Much progress has been made in SPSTM by Wiesendanger and coworkers.1–6) One of the mostimportant issues is to understand the origin of the magnetic
contrast in STS images with magnetic tips. Although it was
already reported that this magnetic contrast is related to thespin-polarized peak in spectroscopy,5) this relation is notstraightforward due to the inﬂuence of the tunnelingprobability. Kleiber et al. obtained the highest contrast in
the dI=dV maps obtained on Cr(001) close to the surface
state energy of 0:02 V but commented on a contrast
inversion inherent to the closed-loop constant current mode
which was used.4) In a more recent publication, it wasreported that the highest contrast is obtained around0:25 V.5) These eﬀects are caused by the tip-sample
separation dependence which is included in dI=dV. Recently, Okuno et al. used ðdI=dVÞ=ðI=VÞ curves obtained on
Co(0001) to extract the spin-polarization of the surface
state.7) However, it was shown by Ukraintsev that, althoughmuch better than dI=dV, this normalization cannot completely remove the tip-sample separation dependence.8)
Ukraintsev showed that, within a one-dimensional WKB
approximation of tunneling, a normalization of dI=dV by its
ﬁtted tunneling probability function leads to the best
recovery of the sample density of states (DOS) on thepositive voltage side.
In this paper we show that a magnetic contrast can beobtained with Fe-coated W tips on ultrathin Mn ﬁlms grownon Fe(001) whiskers, whereas peaks are not observed in the
dI=dV curves. In addition we show that the normalization of
dI=dV curves by tunneling probability functions does reveala spin-polarized peak. We demonstrate experimentally thatthe spin-resolved surface DOS can be found using thisnormalization. With the help of band structure calculations
we trace back the origin of the spin-polarized peak.
2.

Experimental

STM and STS measurements were performed in ultrahigh vacuum (5  1011 mbar) at room temperature (RT)with an Omicron UHV STM-1. Mn layers with a bodycentered tetragonal (bct) structure were grown on anFe(001)-whisker at 370 K. Details are given in refs. 9 and10. In the present study, we used two kinds of tips: W tips
(non-magnetic tips)9) and Fe-coated W tips (magnetictips).11) A magnetic contrast was reproducibly obtainedwith W tips covered with 7–10 nm Fe at RT. Radii of ourtips were more than 300 nm, which was measured by ﬁeld
emission spectroscopy. The magnetic contrast depends onthe factor cos , where  is an angle between the magnetization directions of the Fe-coated tip and the Mn layers.
The tip magnetization direction is randomly orientated withrespect to the magnetization direction of the Mn layers and is
diﬀerent from tip to tip. However, highest contrasts areobserved when these magnetizations are parallel or antiparallel. Therefore, we believe that the largest experimentallyobserved magnetic contrasts (about 20 diﬀerent Fe-coatedtips were used) such as reported in this paper correspond tonearly (anti)parallel alignments. The tip was carefully
brought close to the 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 (‘‘setpoint’’). dI=dV curves wereobtained by numerical diﬀerentiation of the IðVÞ curves.
Band structure calculations were performed using theVienna Ab initio Simulation Program (VASP).13–15) Aneight layer slab with the experimental values for the in-planeCorresponding author. E-mail address: hvk@sci.kun.nl
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T. K. YAMADA et al.

4689

dI/dV [nA/V]

3

0.1
-0.5
0.0
0.5
Sample voltage [V]25(dI/dV)/T

dI/dV [nA/V]

10 0.71
Fig. 1. SP-STM and SP-STS measurements on Fe(001) covered with
about 10 ML Mn at 370 K. These measurements were performed with an
Fe-coated W tip. (a) is the topographic image which was obtained at
VS ¼ 0:5 V, I ¼ 0:5 nA. Scan size is 100  100 nm2 and ﬁve layers are
exposed. Numbers in Fig. 1(a) denote the stacking numbers of the Mn
layers. (b) shows the dI=dV map at þ0:2 V measured at the same area as(a), which was numerically obtained from dI=dV curves measured at a
setpoint of VS ¼ 0:5 V, I ¼ 0:5 nA.

 and out-of-plane (2  1:64 A) lattice constants was(2.87 A)
used. Furthermore, an antiferromagnetically stacking of theMn(001) layers was assumed.
3.

Results and Discussion

Since the growth of Mn layers on Fe(001) changes atcoverages above 3 ML from layer-by-layer to layer-plusislands,9) ﬁve levels can be exposed at a coverage of about
7 ML Mn as shown in Fig. 1(a). Numbers in Fig. 1(a) denotethe stacking numbers of the Mn layers. Previous results
showed that from the fourth layer the Fe intermixing in theMn ﬁlm can be neglected.9) At this surface, dI=dV curves
were measured with Fe-coated W tips at 75  75 pixels at a
setpoint of VS ¼ 0:5 V, I ¼ 0:5 nA. At every pixel an IðVÞcurve was measured which was numerically diﬀerentiated toobtain the dI=dV curve. The dI=dV map at þ0:2 V in Fig.
1(b) shows a strong dark-bright contrast between thediﬀerent levels. Clearly, the contrast oscillates with a periodof two layers. This oscillating contrast starts from the fourth
Mn adlayer (not shown here).12) The contrast changes
between dark (1.50 nA/V) and bright (1.85 nA/V) within alateral length of 1:0  0:5 nm.16) This contrast was neverobserved with clean W tips:9,11) dI=dV curves obtained on
Mn layers with local coverages (L ) larger than 3 monolayer (ML) are all equivalent.16) In a recent publication thiscontrast was explained to be caused by spin-polarizedtunneling into the antiferromagnetically coupled Mn(001)sheets.11) Arguments for this interpretation are that thebright-dark contrast in the dI=dV maps is only observed with
Fe-coated tips and that the onset (L > 3 ML) and period(two layers) are in agreement with previous results obtainedwith other, non-local, techniques.18–20) Furthermore, thestrength and sign of the contrast (i.e., dark-bright might bereversed) depend strongly on the Fe-coated tip used which
demonstrates the degree of freedom of the tip magnetization
direction.
In the present paper we will analyze the origin of thiscontrast in more detail. Figure 2 shows dI=dV curves (solid
curves) measured with an Fe-coated tip (VS ¼ þ0:5 V,
I ¼ 0:5 nA) on Mn ﬁlms with local coverages larger than
3 ML. dI=dV curves were obtained within a large voltage
range from 2 V to þ3 V. dI=dV curves obtained on the odd

0

0
-2

-1

0

1

2

3

Sample voltage [V]Fig. 2. dI=dV curves normalized by the voltage-dependent tunnelingprobability functions (T) were obtained with an Fe-coated W tip on Mn
ﬁlms thicker than three layers. Solid and dotted curves are the dI=dV andthe ðdI=dVÞ=T curves, respectively. dI=dV curves averaged over 30 single
curves were obtained at a setpoint of VS ¼ þ0:5 V, I ¼ 0:5 nA. T (dashedcurves) were obtained by a ﬁt to the dI=dV curves. Black and grey curves
are representative of the odd and the even layers, respectively. An inset in
the top-left of this ﬁgure shows dI=dV curves of odd and even layers
within a voltage range from 0:5 V to þ0:5 V. No diﬀerence is observed
around þ0:2 V.

(black) and the even (grey) layers show as a function of bias
voltage a steady increase above the Fermi level and an
exponential increase above þ2 V. These dI=dV curves do
not show any peak. Spectroscopic measurements in Figs. 1
and 2 were performed at voltage-setpoints of VS ¼ 0:5 V
and VS ¼ þ0:5 V (I ¼ 0:5 nA), respectively. Diﬀerent setpoints cause diﬀerent tip-sample separations, which has astrong inﬂuence on the dI=dV signals. This is the reason why
the dI=dV curves of Fig. 2 are the same at þ0:2 V, whereas
the dI=dV map of Fig. 1(b) shows a clear contrast at þ0:2 V.
To remove the tip-sample separation dependence, dI=dV
curves were normalized by tunneling probability functions(T). Theoretically, this normalization was shown to lead tothe best sample DOS recovery.8) Also, our recent publications show that this method can successfully recover the
sample DOS from experimental dI=dV curves.9,10) The tipsample separation is included in the tunneling probability
function:"1=2 #
2m 
T ¼ K exp 2S 2 ð  eV=2Þ;ð1Þh
where V > 0. K is a proportionality coeﬃcient related to the is the average of sample
eﬀective tip-sample contact area. 
and tip barrier height, S the tip-sample separation, e theelectron charge, and m the electron mass. We ﬁtted T
multiplied by an oﬀsetted Gaussian line shape, representing
the surface state peak in the DOS, to the dI=dV curves.
Tunneling probability functions for odd (Todd ) and even
(Teven ) layers are shown as dashed curves in Fig. 2. The
normalized dI=dV curves (ðdI=dVÞ=T: dotted curves inFig. 2) show a strong peak around þ0:8 V. The peak
amplitude oscillates with a period of two layers. Since theMn layers couple antiferromagnetically and only magnetic
tips show these characteristics, we can conclude that this

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Jpn. J. Appl. Phys. Vol. 42 (2003) Pt. 1, No. 7B

T. K. YAMADA et al.

oscillating amplitude of the peak is caused by spin-polarizedtunneling.
An interesting point is the origin of this spin-dependent
peak at þ0:8 V. To elucidate this we will use theapproximation made in ref. 8 to recover the spin-resolved
DOS from our normalized dI=dV curves. For positive
sample bias voltages, the dI=dV curves for the odd and theeven layers can be described as follows:maj#ðdI=dVÞeven ¼ ðD"Fe  Dmin
Mn þ DFe  DMn Þ  Teven#minðdI=dVÞodd ¼ ðD"Fe  Dmaj
Mn þ DFe  DMn Þ  Todd :ð2ÞD"Fe (D#Fe ) indicates the DOS at the Fermi level (EF ) for the
majority (minority) bands of the ferromagnetic Fe-coatedtip. Here, since Fe is ferromagnetic, majority-spin andminority-spin are deﬁned as spin-up and spin-down. Dmaj
Mn(Dmin
Mn ) indicates the DOS at (EF þ eV) for the majority(minority) bands of the sublattices of antiferromagnetic bct
Mn ﬁlms. Since spin-up and spin-down are not deﬁned for
antiferromagnetic slabs, we prefer to use majority andminority DOS for the Mn layers, instead. In eq. (2) we
assume that spin-up(-down) electrons of the Fe-coated tip
tunnel into the minority-(majority-)spin bands of the odd Mnlayers and the majority-(minority-)spin bands of the even
Mn layers without spin-ﬂipping. However, remember thatthe tip magnetization direction is random and might be either
parallel to the even or odd Mn layers. Using the even and theodd ðdI=dVÞ=T curves in Fig. 2, the averaged ðdI=dVÞ=T
curve and the asymmetry in ðdI=dVÞ=T are obtained (Fig.
3(a)). The asymmetry (AðdI=dVÞ=T ) is deﬁned as follows:AðdI=dVÞ=T ¼¼

majD"Fe  D#Fe Dmin
Mn  DMn
majD"Fe þ D#Fe Dmin
Mn þ DMn¼ PT ðEF Þ  PS ðEF þ eVÞ;

ð3Þ
where

V > 0:

ðdI=dVÞeven =Teven and ðdI=dVÞodd =Todd are ðdI=dVÞ=T curves
obtained on the even and the odd layers, respectively, PT the
tip polarization at EF , PS the sample polarization at(EF þ eV). AðdI=dVÞ=T shows a maximum value of about
10% around the peak energy and reaches 0% above þ2 V
(Fig. 3(a)). Using eqs. (2) and (3) the spin-resolved DOS ofMn(001) can be experimentally obtained as follows:1AðdI=dVÞ=T
Dmaj
¼
1

½ðdI=dVÞ=Tave
Mn
c
PT


ð4Þ
1
AðdI=dVÞ=T
1
þ
Dmin
¼
;
½ðdI=dVÞ=T
ave
Mn
c
PT
where ½ðdI=dVÞ=Tave is the average of ðdI=dVÞodd =Todd andðdI=dVÞeven =Teven . c is the sum of D"Fe and D#Fe .
By following eq. (4), the majority (grey) and the minority(black) DOS of Mn(001) can be obtained experimentally as
shown in Fig. 3(b). A broad peak is observed around þ0:8 V
only in the minority DOS. Figure 3(c) shows the majority
and the minority local DOS at the surface of Mn(001)obtained by band structure calculations.11) There are peaksin the minority DOS, while the majority DOS is low and ﬂatwithout any structure above the Fermi energy. In Fig. 3(c),‘‘SS’’, ‘‘SR’’, and ‘‘B’’ refer to surface states, surface
resonance, and bulk states, respectively. Fitting of theexperimentally observed majority DOS to the calculated
DOS leads to a tip polarization of 15%. Qualitatively, Fig.
3(b) is in agreement with Fig. 3(c). This gives evidence thatthe spin-dependent peak at þ0:8 V in the ðdI=dVÞ=T curves
is caused by this highly polarized DOS. Although, from Fig.
3(b), a tip polarization of 15% is found, the polarization of
an Fe-coated tip was assumed to be as high as 44% in ref. 6.
When we use such a high tip polarization, our experimental
results produce a pronounced peak in the majority DOS
which is qualitatively inconsistent with our theoretical
calculation. Also, this high tip polarization (40%) likely
includes uncertainties; (1) This high tip polarization was
concluded from tunneling measurements of Fe/Al2 O3 /super-conductor junctions,21) but the barrier materials
greatly inﬂuence the measured spin polarization.22) (2)The
spin polarization of Cr(001) was deduced to be 17% byassuming about 40% polarization for the Fe-coated tip4)
whereas Cr(001) has highly spin-polarized d surface
states.23)
4.

Fig. 3. (a) shows the averaged dI=dV curve of the odd and the evenðdI=dVÞ=T curves in Fig. 2 (white circles) and the asymmetry of theðdI=dVÞ=T curves obtained by eq. (3) (black dots). (b) shows the majority(grey) and minority (black) DOS of Mn(001) obtained by experimentallyobtained ðdI=dVÞ=T curves (Fig. 2). (c) shows the majority (grey) andminority (black) DOS of Mn(001) obtained by band structure calculations.

ðdI=dVÞeven =Teven  ðdI=dVÞodd =Todd
ðdI=dVÞeven =Teven þ ðdI=dVÞodd =Todd

Conclusion

A magnetic contrast was obtained on ultrathin bct Mnﬁlms grown on Fe(001) whiskers by SP-STM with Fe-coated
W tips. Normalizing the dI=dV curves by the tunnelingprobability function removed the tip-sample separation
dependence and revealed a spin-polarized peak at þ0:8 V.
We demonstrated that the spin-resolved sample DOS which
we extracted from the ðdI=dVÞ=T curves is in agreementwith our band structure calculations. Consequently, theJpn. J. Appl. Phys. Vol. 42 (2003) Pt. 1, No. 7B

magnetic contrast in the SP-STS can be explained by thishighly SP-DOS around þ0:8 eV. A tip polarization ofaround 15% can be derived by a comparison between thecalculated DOS and experimentally obtained DOS.
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), and the European Growth Project MAGNETUDE. We are especially grateful to D. T. Pierce forsupplying us with the Fe whiskers and to A. L. Vazquez
de Parga and A. I. Lichtenstein for helpful discussions.
1) S. Heinze, M. Bode, A. Kubetzka, O. Pietzsch, X. Nie, S. Blügel and
R. Wiesendanger: Science 288 (2000) 1805.
2) O. Pietzsch, A. Kubetzka, M. Bode and R. Wiesendanger: Science 292(2001) 2053.
3) R. Wiesendanger, H.-J. Güntherodt, G. Güntherodt, R. J. Gambino and
R. Ruf: Phys. Rev. Lett. 65 (1990) 247.
4) M. Kleiber, M. Bode, R. Ravlić and R. Wiesendanger: Phys. Rev. Lett.
85 (2000) 4606.
5) M. Kleiber, M. Bode, R. Ravlić, N. Tezuka and R. Wiesendanger: J.
Magn. & Magn. Mater. 240 (2002) 64.
6) R. Wiesendanger, M. Bode and M. Getalaﬀ: Appl. Phys. Lett. 75(1999) 124.
7) S. N. Okuno, T. Kishi and K. Tanaka: Phys. Rev. Lett. 88 (2002)

T. K. YAMADA et al.

4691

066803.
8) V. A. Ukraintsev: Phys. Rev. B 53 (1996) 11176.
9) T. K. Yamada, M. M. J. Bischoﬀ, T. Mizoguchi and H. van Kempen:
Surf. Sci. 516 (2002) 179.
10) M. M. J. Bischoﬀ, T. Yamada, A. J. Quinn and H. van Kempen: Surf.
Sci. 501 (2002) 155.
11) T. K. Yamada, M. M. J. Bischoﬀ, G. M. M. Heijnen, T. Mizoguchi and
H. van Kempen: Phys. Rev. Lett. 90 (2003) 056803.
12) T. K. Yamada, M. M. J. Bischoﬀ, T. Mizoguchi and H. van Kempen:
Appl. Phys. Lett. 82 (2003) 1437.
13) G. Kresse and J. Furthmüller: Phys. Rev. B 54 (1996) 11169.
14) G. Kresse and J. Hafner: Phys. Rev. B 47 (1993) 558.
15) G. Kresse and J. Hafner: Phys. Rev. B 49 (1994) 14251.
16) The resolution was conﬁrmed in higher resolved dI=dV maps
(0.25 nm/pixel) at hidden Fe steps:9) here the smoothly overgrown
 in the topography and a change in
Mn ﬁlm leads to steps of 0.2 A
magnetic contrast between the even and the odd Mn layers in the
dI=dV map.
17) The ﬁrst three Mn adlayers on Fe(001) have a diﬀerent electronic
structure (see refs. 9 and 10).
18) D. A. Tulchinsky, J. Unguris and R. J. Celotta: J. Magn. & Magn.
Mater. 212 (2000) 91.
19) S. Andrieu, M. Finazzi, Ph. Bauer, H. Fischer, P. Lefevre, A. Traverse,
K. Hricovini, G. Krill and M. Piecuch: Phys. Rev. B 57 (1998) 1985.
20) T. G. Walker and H. Hopster: Phys. Rev. B 48 (1993) 3563.
21) P. M. Tedrow and R. Meservey: Phys. Rev. B 7 (1973) 318.
22) Private communication with R. A. de Groot.
23) J. A. Stroscio, D. T. Pierce, A. Davies, R. J. Celotta and M. Weinert:
Phys. Rev. Lett. 75 (1995) 2960.