APPLIED PHYSICS LETTERSVOLUME 82, NUMBER 93 MARCH 2003

Use of voltage pulses to detect spin-polarized tunneling
T. K. Yamada
NSRIM, University of Nijmegen, NL-6525 ED Nijmegen, The Netherlands and Faculty of Science,
Gakushuin University, 171-8588 Mejiro, Tokyo, JapanM. M. J. Bischoff
NSRIM, University of Nijmegen, NL-6525 ED Nijmegen, The Netherlands

T. Mizoguchi
Faculty of Science, Gakushuin University, 171-8588 Mejiro, Tokyo, Japan

H. van Kempena)
NSRIM, University of Nijmegen, NL-6525 ED Nijemegen, The Netherlands

共Received 2 December 2002; accepted 3 January 2003兲
The present letter describes a method to make a spin-polarized scanning tunneling microscopy tip
by applying voltage pulses between a W tip and a magnetic sample. This spin-polarized tip has thesimilar characteristics as an Fe-coated W tip, which was confirmed by observations ofantiferromagnetically coupled ferromagnetic Mn共001兲 layers 共⬎3 ML兲 grown on an Fe共001兲
whisker at 370 K. Furthermore, we demonstrate that these voltage pulses can vary the tip
magnetization direction. © 2003 American Institute of Physics. 关DOI: 10.1063/1.1556958兴

Spin-polarized scanning tunneling microscopy 共SP–STM兲 and spectroscopy 共SP–STS兲 are the most powerfultools for studying surface magnetism at an atomic scale.1–7
To detect SP tunneling, W tips coated with ultrathin magnetic
films,1–3 tips made of soft magnetic materials whose magnetization can be easily switched,4 and optically pumped GaAs
tips5–7 have been used. Although the use of tips covered withultrathin magnetic layers has great advantages compared tothe use of bulk magnetic tips with respect to stray fieldeffects4 and to optically pumped GaAs tips with respect tosimplicity,5–7 it still needs a lot of efforts to prepare thesetips for SP tunneling experiments. In this letter we demonstrate that a W tip can detect SP tunneling after voltage
pulses have been applied on magnetic materials 共‘‘voltagepulsed tip’’兲. We also show that these voltage pulses can vary
the tip magnetization direction.
STM and STS measurements were performed in ultrahigh vacuum (⬃5⫻10⫺11 mbar兲 at room temperature with acommercial STM 共Omicron UHV STM-1兲. In this study wecompared three kinds of tips: 共1兲 clean W tips 共nonmagnetictips兲,8 共2兲 Fe-coated W tips 共magnetic tips兲,1–3 and 共3兲
voltage-pulsed W tips. For applying the voltage pulses, weused the Omicron STM software and control unit. During
constant current STM topographic measurements 共typically,V S ⫽1 V, I⫽0.1 nA兲, we changed the voltage by means of thecontrol software. After requesting 10 V the actual bias voltage increased from 1 to 10 V within 60 ␮s. We monitored
that the feedback system is too slow to maintain a constantcurrent during this process. We found that it takes about 15
ms to recover the constant current (I⫽0.1 nA兲 level. Therefore, during this period, the current density was drasticallyincreased, which likely resulted in a tip-sample contact 共‘‘tipcrash’’兲 or even in a local melting of the tip-sample contact
area.9 Always, mountains with a height of 10–30 nm werea兲

Electronic mail: hvk@sci.kun.nlobserved on the sample surface. Then, we changed the voltage range back to 1 V and moved to a different area. Several
processes can occur simultaneously during a voltage pulse,9but it is unclear which process dominates. However, clean W
tips showed magnetic contrast after application of voltagepulses 关Fig. 1兴. As samples, we used Mn films 共⬎3 ML兲which were grown on an Fe共001兲 whisker. Previous resultswith different techniques showed that these Mn films consist

FIG. 1. 共Color兲 SP–STS measurement with the voltage-pulsed tip on
Fe共001兲 covered with 5 ML Mn at 370 K. 共a兲 is a topographic image obtained at a setpoint of V S ⫽⫺0.5 V, I⫽0.5 nA. Scan size is 50⫻50 nm2 .
Seven different levels are exposed on the surface due to the threedimensional growth mode. 共b兲 shows the dI/dV curves representative ofeach level. These curves are averages of typically 10 single curves. Only thelowest level dI/dV curve is a single curve. 共c兲 shows the dI/dV map at⫹0.1 V measured at the same area as 共a兲. Numbers in 共a兲 and 共c兲 denote thestacking numbers of Mn layers which are deduced from the STS results. 共d兲shows a line profile across the two steps measured at the black line in 共a兲. h i jindicates the step height between level i and level j.

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1437© 2003 American Institute of Physics
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Appl. Phys. Lett., Vol. 82, No. 9, 3 March 2003

of ferromagnetic 共001兲 planes which are antiferromagnetically aligned.3,10–12 The growth and local electronic structureof the Mn layers on Fe共001兲 were described in Refs. 8 and
13. STS measurements were performed at every pixel of aconstant current topographic image by opening the STM
feedback loop at a given current and voltage 共‘‘setpoint’’兲.
dI/dV curves were obtained by numerical differentiation of
the I(V) curves. An external magnetic field was not applied.
Figure 1 was obtained with the voltage-pulsed tip. Figure 1共a兲 shows a topographic image of the Fe共001兲 surface
after coverage of 5 ML Mn. Seven levels are exposed. Averages of typically 10 dI/dV curves representative of the various layers in Fig. 1共a兲 are shown in Fig. 1共b兲. Since thedI/dV curve obtained on the lowest level of Fig. 1共a兲 issimilar to the curve obtained on the second layer,8 this levelcan be identified as the second Mn layer. The dI/dV curves
obtained on Mn films thicker than four layers show a dip
around ⫹0.1 V and a steady increase at positive voltages.
The dI/dV curves obtained on the fourth layer are the sameas those obtained on the sixth and eighth layer, while thedI/dV curves obtained on the fifth layer are the same as
those obtained on the seventh layer. Figure 1共b兲 also showsthat at positive voltages the dI/dV signal is larger on theeven layers 共i.e., 4th, 6th, and 8th兲 compared to the oddlayers 共i.e., 5th and 7th兲. In the dI/dV map at ⫹0.1 V, thecontrast is observed to oscillate with a period of two layers
starting from the fourth layer 关Fig. 1共c兲兴. The cross section at
the black line in Fig. 1共a兲 clearly shows the different stepheights in Fig. 1共d兲.14 The present results obtained with thevoltage-pulsed tip are equivalent to those obtained with Fecoated tips.3 Furthermore, clean W tips do not show any
contrast in dI/dV maps.3,8 Therefore, it is concluded thatvoltage-pulsed tips can detect SP tunneling, i.e., voltage
pulses can make a nonmagnetic W tip sensitive to the magnetic structures of the sample surface.
The asymmetry obtained from the even and the odd
layer dI/dV curves reveals the sample polarization multiplied by the tip polarization,2 whereas the tip-sample separation dependence is still included.3 The asymmetry is definedas A dI/dV ⫽ 关 (dI/dV) even⫺(dI/dV) odd兴 / 关 (dI/dV) even⫹(dI/dV) odd兴 , where (dI/dV) even and (dI/dV) odd are dI/dV
curves obtained on the even and the odd layers. Figure 2shows the asymmetries (A dI/dV ) at ⫹0.8 V, which were obtained at the setpoint of V S ⫽⫺0.5 V, I⫽0.5 nA, as a functionof the dI/dV value at ⫹0.8 V. The data were obtained withdifferent Fe-coated tips, different clean W tips, and different
voltage-pulsed tips.15 The dI/dV maps obtained with W tips
never show magnetic contrast, i.e., A dI/dV ⫽0 共triangles inFig. 2兲. The dI/dV maps obtained with Fe-coated tips always
show a magnetic contrast of 5%–10% 共circles in Fig. 2兲.
About 50% of the dI/dV maps obtained with voltage-pulsed
tips show a magnetic contrast, i.e., in Fig. 2, 13 data points
共some fall on top of each other兲 reveal a magnetic contrast
between 5% and 15% while 13 other data points do notreveal a contrast 共⬍2%兲. The dI/dV value at ⫹0.8 V is usedas the horizontal axis of Fig. 2 since this value indicates theamplitude of the exponential background in dI/dV. 13,16,17
dI/dV values at ⫹0.8 V between 1.5 and 2.5 nA/V for Fecoated tips and between 2.8 and 4.5 nA/V for W tips areobserved, which indicate that W tips have a higher exponen-Yamada et al.

FIG. 2. The asymmetry in dI/dV curves (A dI/dV ) at ⫹0.8 V obtained on Mn
films thicker than three layers are shown as a function of the dI/dV value at⫹0.8 V. All data shown are obtained from dI/dV curves obtained at a
setpoint of V S ⫽⫺0.5 V, I⫽0.5 nA. Circles, triangles, and diamonds wereobtained by different Fe-coated W, different clean W, and different voltagepulsed tips, respectively.

tial background. One can conclude that this is caused by thelower barrier height of the tip apex of W tips compared toFe-coated tips. On the other hand, dI/dV values at ⫹0.8 V
obtained with voltage-pulsed tips scatter between 1.5 and 5.0
nA/V, i.e., voltage pulses can vary the exponential background. This suggests that when a peak cannot be observed
in dI/dV curves due to a high exponential background, thistechnique can help to resolve the peak by lowering the exponential background as was experimentally shown in Refs.
13 and 17.
We found that voltage pulses can change the magnetization direction of the SP tip. Figure 3 shows the dependenceof the alternating contrast in the dI/dV maps on the tip condition. Figure 3共a兲 is a topographic image. Numbers in Fig.

FIG. 3. SP–STS measurements on Fe共001兲 covered with 9.5 ML Mn at 370
K showing a reversal of the magnetic contrast. 共a兲 is a topographic image
obtained at a setpoint of V S ⫽⫺0.5 V, I⫽0.5 nA. Scan size is 100⫻80 nm2 .
Five different levels are exposed on the surface. Numbers denote the stacking number of the Mn layers. 共b兲–共d兲 are dI/dV maps at ⫹0.2 V measured
at the same area as 共a兲. Between 共b兲, 共c兲, and 共d兲 the tip was moved ⫾500
nm away from this area and a voltage pulse was applied. A line profile along
the line in 共a兲–共d兲 is shown below each image.
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Yamada et al.

Appl. Phys. Lett., Vol. 82, No. 9, 3 March 2003

3共a兲 denote the stacking numbers of the Mn layers, which are
estimated from a quartz crystal oscillator calibration.8 Figures 3共b兲–3共d兲 are dI/dV maps at ⫹0.2 V, which were measured at the same setpoint (V S ⫽⫺0.5 V, I⫽0.5 nA兲 and on
the same area as Fig. 3共a兲. A clear contrast 共⫹5%兲 can beobserved in Fig. 3共b兲. After the measurement of Fig. 3共b兲, thetip was moved 共⬎500 nm兲 to a different area and a voltage
pulse was applied. The tip was moved back to the same areaas Fig. 3共a兲, and Fig. 3共c兲 was measured. The contrast in Fig.
3共c兲 is almost negligible 共⬍⫾1%兲. Again a voltage pulse wasapplied in the same way; afterwards Fig. 3共d兲 was measured.
Figure 3共d兲 shows a clear contrast again, but the contrast is
inversed 共⫺6%兲 compared to that in Fig. 3共b兲. This contrast
clearly depends on the tip condition. The variation in thecontrast can be explained by different angles between the tip
and the sample magnetizations,2 i.e., the contrast becomesstrong when the tip magnetization is parallel or antiparallelto the sample magnetization 关Figs. 3共b兲 and 3共d兲兴 and becomes negligible when the tip magnetization is orthogonal to
the sample magnetization 关Fig. 3共c兲兴. These results show thatthe tip magnetization is not fixed to one direction by, e.g., thestray field of the Fe whisker. The tip magnetization apparently is frozen in immediately after the mass transport fromthe sample to the tip during pulsing because spontaneouscontrast reversals during scanning do not occur.
Magnetic contrasts in SP–STS were found with Fecoated tips.1–3 However, the present study showed that W
tips can show the same characteristics as Fe-coated tips after
applying voltage pulses. These pulses apparently lead tomass transport from the sample to the tip. This results in thetip apex becoming covered with Fe or Mn, thus, giving it
magnetic properties. Also, the voltage pulses probably makethe tip blunter which is in favor of detection of SP tunnelingfor systems with an in-plane magnetization.18 Therefore, thisvoltage pulsing technique is proposed as an alternative way
to prepare SP–STM tips. In our study, we could obtain SP
voltage-pulsed tips with a probability of 50% after applying
voltage pulses. We also found that this technique can vary
the tip magnetization direction and, furthermore, that voltage

1439

pulses can vary the background in the dI/dV curves and canchange a double tunneling tip to a single tunneling tip 共notshown兲 since voltage pulses can change the chemical composition, shape, and radius of the tip apex.
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, and the European Growth Project MAGNETUDE. The authors are especially grateful to D. T. Pierce
for supplying them with the Fe whiskers.
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Using the single step height of bcc–Fe共001兲 which is 0.143 nm, the apparent step heights of our STM measurements were calibrated.
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M. M. J. Bischoff, T. K. Yamada, C. M. Fang, R. A. de Groot, and H. vanKempen 共unpublished兲.
18
Using field emission spectroscopy we checked that a sharp tip with a tip
radius of 20 nm became a blunter tip with a tip radius of 250 nm after
applying many voltage pulses.

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