Chapter 103

Spin-Polarized Scanning TunnelingMicroscopy
Toyo Kazu Yamada

Keywords Spin-polarized tunneling current

103.1

 Spin polarization  MagnetismPrinciple

Spin-polarized scanning tunneling microscopy (SP-STM) is a powerful tool tovisualize spin-polarization vectors of sample surface atoms. Variety of unique newmagnetic properties have been discovered and studied, such as non-collinear
magnetic exchange coupling between monolayer ﬁlms, magnetoresistance of single
atoms, single molecules, and nanomagnets, as well as skyrmion or magnon inmagnetic ﬁlms [1–5].
SP-STM measurements can be performed with an ultrahigh-vacuum (UHV)STM setup combined with a scanning tunneling spectroscopy (STS) technique,while SP-STM requires a spin-polarized tip. Figure 103.1 shows the principle, inwhich local density of states (LDOS) of a tip and a sample is spin-polarized near theFermi energy (EF). Now only 3 spin-up electrons exist for the tip [spin polarization(P) = (3 − 0)/(3 + 0) = 100%], while 3 spin-up and 1 spin-down electrons for thesample [P = (3 − 1)/(3 + 1) = 50%]. In (a), tip and sample spin-polarization vectors are parallel. Then, tunneling probability can be described as 3  3 + 0  1 = 9,
where no spin-flip during tunneling is assumed. Next, the sample spin-polarization
vector is reversed (antiparallel conﬁguration, see Fig. 103.1b). In this case, tunneling probability is 3  1+ 0  3 = 3. SP-STM detects this difference in current
due to different spin-polarization vector conﬁgurations between the tip and thesample. This mechanism is comparable to tunnel magnetoresistance.

T.K. Yamada (&)
Graduate School of Advanced Integration Science, Chiba University, Chiba, Japane-mail: toyoyamada@faculty.chiba-u.jp© Springer Nature Singapore Pte Ltd. 2018
The Surface Science Society of Japan (ed.), Compendium of Surface
and Interface Analysis, https://doi.org/10.1007/978-981-10-6156-1_103

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T.K. Yamada

Fig. 103.1 Spin-polarized electron tunneling process between the spin-polarized tip and thesample in the case of a parallel and b antiparallel spin-polarization (magnetization) conﬁgurations.
[1–5]

103.2Features••••Spin-polarization vectors of sample surfaces can be detected.
Magnetoresistance can be detected at the atomic scale.
Spin-polarized STM tips are required.
Combined with IETS or magnetic coils, magnon or hysteresis loop can bemeasured.
• SP-STM has been performed in UHV at *10 mK–300 K.

103.3

Instrumentation

SP-STM system is based on a UHV-STM setup. All experiments have been performed in UHV to prevent contamination adsorption on the tip and the samplesurfaces. Single impurity atoms break local symmetry, and therefore, spin polarization can be quenched. To detect spin-polarized current, it is necessary to decrease
current noise as much as possible (typical current noise: Inoise < 1 pA).
Figure 103.2 shows an example of SP-STM measurement system, where a layerwise antiferromagnetically coupled Mn(001) ﬁlm is scanned by a spin-polarized tip.
The most important (and difﬁcult) point to make success SP-STM measurements
is how to obtain a stable spin-polarization vector at the tip apex. Recent trends
are uses of Fe-coated W tips, Mn-coated W tips, Cr-coated W tips, and bulk-Cr tips
[1–6]. The Fe-coated tips have been used since they have the highest spinpolarization of *40%, while recently most of the SP-STM researchers use theantiferromagnetic tips because of no stray ﬁelds (P * 10%) [2].

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STM spin-polarized tipbct-Mn(001) film

Spinpolarizedtunnelingcurrent

10ML
9ML

z

8ML

xyFig. 103.2 Experimental setup of spin-polarized STM measurements. Spheres and black andwhite arrows denote atoms and spin-polarization vectors, respectively. Blue and red arrows denotespin-up and spin-down electrons, respectively. SP-STM measurements can be performed in UHV
at 10 mK–300 K. [2–5]

Table 103.1 Spin-polarization vectors of spin-polarized tips
SP tipsThickness

Direction

Spin polarization

Refs.

Fe-coated W
Mn-coated W
Cr-coated W
Cr-coated W
Bulk-Cr

2–10 nm
3–6 nm>10 nm
2–10 nm–In-plane
In-plane
In-planeOut-of-planeOut-of-plane

*40%
*9%
*10%
*10%
–

[1–5]
[2]
[2]
[2]
[6]

These tips were prepared in a UHV preparation chamber before setting in STM.
First, a W tip chemically etched in air was introduced into the preparation chamber
through a load lock chamber and cleaned (Ar+ sputter with a subsequent anneal to2200 K). Next, magnetic ﬁlms were deposited at 300 K on the cleaned W tip. Proper
annealing of 500–600 K could increase crystalline quality of the ﬁlm [2].
On the other hand, the bulk tip requires a different manner. A chemically etchedbulk-Cr tip was cleaned inside the UHV-STM setup by applying voltage pulses(  10 V) between the tip and the sample to remove oxide layers at the apex [6].
Depending on materials and thickness, the tip spin-polarization directions can becontrolled by different magnetic anisotropies. Table 103.1 shows experimentallyobtained tip spin-polarization vectors of SP-STM tips. Using these clean
spin-polarized tips combined with STS, the spectroscopy dI/dV map shows spincontrast.

103.4
103.4.1

Applications
Magnetic Imaging

Experimentally obtained SP-STM results of Mn(001) ﬁlms grown on Fe(001) areshown in Fig. 103.3 [2–5]. Seventh to tenth monolayers (ML) are exposed on the640

T.K. Yamada

Topographic image

Spin-polarization map

8ML
9MLyzx

10MLFig. 103.3 Spin-polarized STM results on bct-Mn(001) ﬁlms with an Fe-coated W tip in UHV at
300 K. Topographic image (100  100 nm2) and spin-polarization dI/dV map at +0.2 V wereobtained simultaneously. [2–5]

surface as shown in the STM topographic image (100  100 nm2). Since Mn(001)ﬁlms higher than fourth layer have the same LDOS, non-spin-polarized W tips
show no contrast; however, spin-polarized Fe-coated W tips with an in-planesensitivity (see Table 103.1) show spin contrast in the dI/dV map at +0.2 eV(Fig. 103.3). Alternating spin contrast between the monolayers means that Mnatoms in the monolayer couple ferromagnetically, while they couple antiferromagnetically between the layers. Thus, we could obtain spin structures of nanomaterial by means of SP-STM.

103.4.2

Single-Molecule Junction

Using the STM tip, we can manipulate single atoms or single molecules. Left panels
as shown in Fig. 103.4 show examples, where letters “N” and “S” were written with
17 Fe atoms, and “smile” was drawn with 11 CO single molecules. Using this STM
manipulation technique, we can measure conductance through the target (seeFig. 103.4). Here, we measure I–z curve instead of dI/dV. By measuring the tunneling current (I), the tip is approached to the target molecule. When the tip contacts
with the molecule, the measuring current becomes constant, forming asingle-molecule junction. Thus, we can measure conductance through the molecule.
Using the spin-polarized tips, we can measure magnetoresistance (MR) througha single molecule (see right panels as shown in Fig. 103.4). Single phthalocyanine
molecules were deposited on the Mn(001) ﬁlms as shown in Fig. 103.3. First, theFe-coated W tip was gently contacted to the molecule adsorbed on the ninthmonolayer, in which spin polarization is parallel to the tip spin polarization andmeasured conductance “I”. Next, the same tip was contacted to the molecule
adsorbed on the eighth monolayer, in which spin polarization is antiparallel to thetip spin polarization and measured conductance “II”. Thus, MR through a single

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Fig. 103.4 STM manipulations of single atoms and molecules in UHV at 5 K. Magnetoresistancemeasurements through a single molecule by means of SP-STM [3]phthalocyanine molecule was obtained by MR = (I–II)/II = −50% [3], whichsuggests that such a 1-nm-size p-conjugated organic molecule can be useful as a
new nanomaterial for near-future spintronic devices.

References
1. Wiesendanger, R.: Spin mapping at the nanoscale and atomic scale. Rev. Mod. Phys. 81, 1495
(2009)
2. Yamada, T.K., Vazquez de Parga, A.L.: Room temperature spin-polarizations of Mn-based
antiferromagnetic nanoelectrodes. Appl. Phys. Lett. 105, 183109 (2014)
3. Bagrets, A., Schmaus, S., Jaafar, A., Kramczynski, D., Yamada, T.K., Alouani, M., Wulfhekel,W., Evers, F.: Single molecule magnetoresistance with combined antiferromagnetic andferromagnetic electrodes. Nano Lett. 12, 5131–5136 (2012)
4. Yamada, T.K., Martinez, E., Vega, A., Robles, R., Stoeffler, D., Vazqeuz de Parga, A.L.,
Mizoguchi, T., van Kempen, H.: Spin conﬁguration in a frustrated ferromagnetic/
antiferromagnetic thin-ﬁlm system. Nanotechnology 18, 235702 (2007)
5. Yamada, T.K., Bischoff, M.M.J., Heijnen, G.M.M., Mizoguchi, T., van Kempen, H.:
Observation of spin-polarized surface states on ultrathin bct Mn(001) ﬁlms by spin-polarizedscanning tunneling spectroscopy. Phys. Rev. Lett. 90, 056803 (2003)
6. Rodary, G., Girard, J.-C., Largeau, L., David, C., Mauguin, O., Wang, Z.-Z.: Atomic structureof tip apex for spin-polarized scanning tunneling microscopy. Appl. Phys. Lett. 98, 082505(2011)