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2016 Jpn. J. Appl. Phys. 55 08NB14(http://iopscience.iop.org/1347-4065/55/8S1/08NB14)
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REGULAR PAPER

Japanese Journal of Applied Physics 55, 08NB14 (2016)
http://doi.org/10.7567/JJAP.55.08NB14

Temperature control of the growth of iron oxide nanoislands on Fe(001)
Toyo Kazu Yamada1,2*, Yuki Sakaguchi1, Lukas Gerhard3, and Wulf Wulfhekel3
1

Graduate School of Advanced Integration Science, Chiba University, Chiba 263-8522, Japan
Molecular Chirality Research Center, Chiba University, Chiba 263-8522, Japan
3
Institute of Nanotechnology, Karlsruhe Institute of Technology, 76021 Karlsruhe, Germany
2*E-mail: toyoyamada@faculty.chiba-u.jp
Received January 14, 2016; revised March 20, 2016; accepted April 11, 2016; published online July 13, 2016
The control of atomically ﬂat interfaces between iron (Fe) and insulating oxide ﬁlms, such as the Fe/MgO(001) interface, is crucial for tunnelmagnetoresistance (TMR) devices. However, the realization of an ideal atomically ﬂat and clean interface is rather difﬁcult since iron easily binds
to impurities such as oxygen. Atomic step defects and iron oxide at the interface could reduce TMR. In this study, the oxidization of an atomicallyﬂat and clean Fe(001)-whisker single crystal at different substrate and annealing temperatures was investigated with an ultrahigh-vacuum
scanning tunneling microscope (STM). Annealing up to a temperature of 850 K was required to obtain ordered and atomically ﬂat Fe(001)-p(1'1)O
terraces after the oxidization with the coexistence of Fe–O nanoislands (>1 nm in height, >50 nm in size). We found that the growth of suchnanoislands, which enhances interface roughness, strongly depends on the substrate temperature (TS) during the oxidization. A TS lower than
300 K reduces the coverage by the nanoislands to less than 10%. © 2016 The Japan Society of Applied Physics(a) Fe(001)-whisker

Introduction

(b)

1–4)

5 mm

[010]
[100]

(d)

(c)

1.2

z [nm]The bcc-Fe(001) surface has highly spin-polarized states.
With the combination of Fe(001) and a MgO(001) spacer,large tunnel-magnetoresistances (TMR) occurs. Thus, nowadays, the bcc-Fe=MgO(001) system is widely used as a TMR
device for read heads, near-future magneto-random accessmemory (MRAM)5,6) as well as electric-ﬁeld-driven spintronic devices.7) Experimental research on Fe=MgO andrelated tunnel junctions has mainly focused on the performance of full devices depending on the precise preparation
technique and heuristic design rules have been developed.
Despite all of these eﬀorts, the giant TMR values of morethan 1000% that were predicted independently by Butler
et al. and Mathon and Umerski in 20018,9) have never beenachieved experimentally. Recently, a novel exchange bias
eﬀect has been reported for the Fe=MgO interface, where
antiferromagnetic Fe–O nanoclusters could pin the surrounding ferromagnetic Fe, reducing the TMR.10) Several studies
have indicated a partial oxidation of the topmost Fe
layer11,12) and it has been theoretically predicted that suchoxidation will markedly reduce the spin ﬁltering eﬀect andthus the TMR ratio of the junction. However, experiments
on an oxidized Fe surface have shown a much smallerchange in TMR [120% at 300 K for Fe=p(1×1)-O=MgO=
Fe(001)].13–15) This indicates that the reason for thediscrepancy between experiment and theory is still to beunderstood.
The growth of iron oxide on Fe ﬁlms has been studied byscanning tunneling microscopy (STM) in ultrahigh-vacuum(UHV).16,17) When Fe ﬁlms (20–50 nm thick) on a MgO(001)substrate were oxidized (10−4 Pa) at 300 K with subsequentannealing at 925 K, an Fe(001)-p(1×1)O structure wasformed on the surface,15) while, when Fe was deposited on
a MgO(001) substrate at 523 K under dosing O2 (10−4 Pa),
the Fe ﬁlms transformed to Fe3O4(001) ﬁlms,17) i.e., severaliron-oxide phases became energetically close.
In this study, a clean and atomically ﬂat bcc-Fe(001)whisker grown via chemical vapor deposition (CVD) wasused as a substrate.1–4,18–20) We investigated diﬀerent ironoxide phases that form depending on the substrate temperature (TS) during oxidization as well as diﬀerent annealing(dI/dV)/T [arb. units]

1.

-2

+2
0
E – EF [eV]

0.0
0

~0.14

x [nm]

300

Fig. 1. (Color online) (a) Picture of the Fe whisker set in the Mo sample
holder. (b) STM topographic image of the Fe(001) whisker surface after
sputtering and annealing obtained at 300 K: VS = −1 V, I = 100 pA, 300 ×
300 nm2. The inset shows an atomically resolved image. (c) dI=dV curve
normalized by its ﬁtted tunneling probability function obtained on Fe(001).
(d) Line proﬁle along the green arrow in (b).

temperatures after the oxidization. All experiments were
performed with a home-built UHV-STM setup at 300 K.
2.

Experimental methods

The home-built UHV-STM setup consists of preparation andanalysis UHV chambers. In the preparation UHV chamber,the cleaning, oxidization, and annealing of the whisker were
performed, and subsequently, without breaking UHV, thewhisker was transferred to the analysis chamber and put into
the STM setup. The temperature of the sample was examinedusing a pyrometer for T > 550 K and a thermocouple forT < 550 K.
We grew Fe(001) whiskers by a CVD process. One of thewhiskers (1 × 1 × 10 mm3) was placed on a Mo sample
holder [see Fig. 1(a)]. In the preparation UHV chamber, theFe(001) whisker was ﬁrst sputtered with Ar+ ions at 870 K
for 50 h, the sputtering was stopped, and, 5 min later, we
stopped heating. Fe(001) was moved to the analysis UHV

08NB14-1© 2016 The Japan Society of Applied Physics

Jpn. J. Appl. Phys. 55, 08NB14 (2016)

T. K. Yamada et al.

3.

Results and discussion

This study focuses on the oxidization of the atomically ﬂatand clean bcc-Fe(001) whisker (cf. Fig. 1) at diﬀerent TS
during the oxidization and diﬀerent annealing temperatures(TA) after the oxidization.
Firstly, the Fe(001) whisker surface was oxidized at 300 K
by dosing 2 L (1 L = 1.33 × 10−6 mbar s) of oxygen in the
preparation chamber. Figures 2(a1) and 2(a2) show thesurface, on which atomic steps and terraces of the Fe(001)whisker can still be observed. However, the entire surface
seems to be roughened by oxygen exposure. A line proﬁlealong the arrow in Fig. 2(a2) shows a peak-to-peakcorrugation of less than 100 pm on the terrace [cf. Fig. 2(b)].
A dI=dV curve obtained on this surface shows no peak, i.e.,
the Fe(001) d-state peak [Fig. 1(c)] at +0.2 eV was quenched[cf. Fig. 2(c)] after dosing 2 L of oxygen.
In the following, we examined how the oxidized surfacechanged with further annealing. Figures 2(d1) and 2(d2)show STM topographic images obtained after annealing toTA = 540 K. As depicted in Fig. 2(d1), the original Fe(001)terraces are still observed. In Fig. 2(d2), in the same Fe(001)terrace, several layers are exposed. A line proﬁle along
the arrow in Fig. 2(d2) shows that the peak-to-peak corrugation increases to 250 pm [see Fig. 2(e)]. The dI=dV curve
obtained on this surface [see Fig. 2(f)] is comparable to thatin Fig. 2(c).
We further annealed the surface to TA = 740 K. Then, thesurface roughness increased and the original Fe(001) terraces

TA = 540K

TA = 300K

TA = 740K

1㽢1μm

(d1)

1㽢1μm

(g1)

1㽢1μm

(a2) 200㽢200nm(d2)

200㽢200nm(g2)

200㽢200nm

(a1)

A

6

(c)

0
-1

(j)

x [nm] 200

0+1
E – EF [eV]00x [nm] 200
30
(f)

0
-1

0
+1
E – EF [eV]3z [nm]

(e)

B

(h)

0
0
dI/dV [nA/V]

0z [pm]

0

500

(b)

dI/dV [nA/V]z [pm]

500

dI/dV [nA/V]

chamber without breaking the UHV, and the Fe(001) surface
was studied by STM. As shown in Fig. 1(b), atomically ﬂat
terraces were observed. The atomically resolved image in theinset in Fig. 1(b) shows a bcc Fe(001)-p(1×1) symmetry. A
normalized (dI=dV)=T curve obtained on the Fe(001) surface
displays an LDOS peak at +0.2 eV above the Fermi energy inagreement with previous results.1,3) A line proﬁle along thearrow in Fig. 1(b) conﬁrmed monolayer steps of ∼0.14 nm,
which corresponds to the literature value of a bcc-Fe(001)interlayer distance of 0.143 nm [see Fig. 1(d)].
We used tungsten (W) tips as STM probes, which were
etched from commercial polycrystalline W wires with a KOH
solution. The apex of the tips was examined using a scanningelectron microscope (Technex. Tiny-SEM, ∼10−3 Pa). Sharp
tips were introduced into the STM setup and, in the
preparation chamber, the tip apex was heated by electron
bombardment (10–30 W: 500 V × 20–60 mA, 10 s)21) toremove oxide from the apex. The cleaned tips were placedin the STM without breaking UHV.
All STM topographic images were obtained in the constantcurrent mode (tunneling current feedback loop ON). We
performed scanning tunneling spectroscopy (STS) measurements, in which the diﬀerential conductance dI=dV is ﬁrstorder proportional to the sample local density of states(LDOS). During the spectroscopy, the tip–sample separation
was ﬁxed by switching oﬀ the feedback loop. The tunnelingcurrent was measured as a function of the sample bias
voltage, typically from −1 to +1 V (I–V curve). The obtained
I–V curves were numerically diﬀerentiated, and diﬀerential
conductivity (dI=dV) curves were obtained and normalized
by the tunneling matrix element in order to extract the sample
LDOS even for large voltages.22)

4 (i)

0
-1

x [nm] 200
A
B

0
+1
E – EF [eV]~1.2nmFig. 2. (Color online) STM topographic images obtained on the oxidizedFe(001) whisker single-crystal surface at 300 K in UHV. (a1, a2) STM
images obtained after oxidization of the clean Fe(001) surface by dosing 2 L
of O2 at 300 K (VS = +2.5 V, I = 50 pA). (b) Line proﬁle along the arrow in(a2). (c) dI=dV curve obtained on the terrace in (a2) (VS = −1 V, I = 500 pA).
(d1, d2) STM images obtained after UHV annealing to TA = 540 K (ﬁlament
power: 2 W, 5 min) (VS = −2.5 V, I = 200 pA). (e) Line proﬁle along thearrow in (d2). (f) dI=dV curve obtained on the terrace in (d2) (VS = −2 V,
I = 150 pA). (g1, g2) STM images obtained after UHV annealing to TA =740 K (ﬁlament power: 3.5 W, 5 min) (VS = −2.5 V, I = 50 pA). (h) Line
proﬁle along the arrow in (g2). (i) dI=dV curves obtained on the terrace (A)and the island (B) in (g2) (VS = −1 V, I = 500 pA). ( j) Nanostructures in theboxed area in (g2) (31 × 16 nm2, VS = 2.1 V, I = 50 pA).

completely disappeared. Instead, new terraces and islands
appeared [cf. Figs. 2(g1) and 2(g2)].
A line proﬁle along the arrow in Fig. 2(g2) shows that theislands have a typical height of ∼1 nm with a width of∼50 nm [Fig. 2(h)], while the ﬂat area has a roughness of
50–100 pm. The dI=dV curves obtained on the terrace [A inFig. 2(g2)] and islands [B in Fig. 2(g2)] are diﬀerent[Fig. 2(i)], i.e., they have diﬀerent LDOSs, indicating thatthe islands and terrace are diﬀerent Fe–O phases. On theterrace, we succeeded in obtaining a high-resolution image[Fig. 2( j)] whose area corresponds to the box in Fig. 2(g2).
We observed two patterns: (1) one-dimensional (1D) lines
along [100] with a periodicity of ∼0.6 nm and (2) 1D lines
along [010] with a periodicity of ∼1.2 nm, which areseparated every ∼3 nm. Since similar patterns have been
observed on Fe3O4(001) ﬁlms grown on 4-nm-thick Fe buﬀer

08NB14-2© 2016 The Japan Society of Applied Physics

Jpn. J. Appl. Phys. 55, 08NB14 (2016)

TA = 850K(a)10z [nm]

800㽢550nm(d)(b)
1.0㼼0.2

0(c)

T. K. Yamada et al.

0x [nm](e)

800
Islands

Islands

ρ [%]

(f)

100
80
60
40
20
0
200

300

400 500
TS [K]

600

700

Fig. 3. (Color online) (a) STM topographic image of Fe(001) after dosing
2 L of O2 at 300 K with subsequent annealing to TA = 850 K (800 × 550 nm2,
VS = +1.5 V, I = 200 pA). (b) Line proﬁle along the arrow in (a). (c–e) STM
topographic images of Fe(001) surfaces oxidized (2 L of O2) at diﬀerent TSwith subsequent annealing to TA = 850 K: (c) TS = 300 K, VS = +0.5 V,
I = 50 pA, and 500 × 500 nm2. (d) TS = 330 K, VS = +1.5 V, I = 100 pA, and500 × 500 nm2. (e) TS = 590 K, VS = −1.5 V, I = 100 pA, and 400 ×
400 nm2. (f) Occupation of the surface by nanoislands as a function of the TS
during the oxidization.

layers UHV-annealed to 775 K,23–25) the surface after 740 K
annealing could be covered by a Fe3O4 ﬁlm.
Annealing to TA = 850 K leads to a rather marked changeon the surface as shown in Fig. 3. On the surface, two
diﬀerent Fe–O phases coexist, i.e., atomically ﬂat terraces
and islands. The islands have a height of ∼1 nm and a widthof ∼50 nm [see the line proﬁle in Fig. 3(b)]. The atomicallyﬂat terraces have a typical width of ∼100 nm, i.e., theannealing temperature of 850 K is necessary to obtain an
atomically ﬂat surface.
We consider that the islands could be Fe–O nanoparticles
that formed at the Fe=oxide interface as previously
reported10) and disturbed the atomically ﬂat interface=surface.
Therefore, we studied the island growth depending on the TS
during the oxidization. Results are shown in Figs. 3(c)–3(f);subsequent annealing to TA = 850 K was always performed
before STM measurements of the surface. When Fe(001) was
oxidized at TS = 300 K, ρ ∼ 10% of the surface was coveredby islands [Fig. 3(c)]; however, at TS = 340 K, ρ ∼ 20% of
the surface was covered by islands [Fig. 3(d)], and further, at
TS = 570 K, islands covered ρ = 70–80% of the surface[Fig. 3(e)]. Figure 3(f) shows how much the surface wascovered by islands [ρ(%)] as a function of the TS. These
results show that island growth strongly depends on the TS
during oxidization. To prevent nanoisland growth andmaintain an atomically ﬂat interface, it is better to use a TS
lower than 300 K during the oxidization. We tested to expose
the surface to up to 6 L of oxygen, but the island growth did

not change, indicating that 1–2 L of oxygen might be enoughto cover the Fe(001) surface and additional oxygen desorbed
during the annealing.
While the ﬁrst layer of iron oxide forms as a continuous
ﬁlm, the growth mode changes upon the termination of theﬁrst layer. Then, the ﬁlm atoms more strongly bind to eachother than to the substrate, which leads to the growth of 3D
islands. These results are in agreement with those of previous
studies where the growth of a mixed iron layer on top of the
FeO layer,27) as well as that of oxide islands on top of thecompleted FeO layer,15) was found.
We were interested in identifying the iron-oxide phases on
the ﬂat terraces and islands. Figure 4(a) shows the focusedarea, where one big island (area II) and atomically ﬂat
terraces (area I) are observed.
Firstly, we took an atomically resolved STM image on theﬂat terrace [the enlarged image in Fig. 4(a)], which shows thesame symmetry as bcc-Fe(001). By comparison of our resultswith previously reported results,15) we conclude that the ﬂat
terrace surface has a Fe(001)-p(1×1)O structure.
On the nanoislands (area II), ﬂaky and rectangular structures were found. The line proﬁle [cf. Fig. 4(b)] along thearrow in Fig. 4(a) shows that the distance between layers is∼180 pm. Figure 4(c) shows an enlarged STM image obtained from an area with a rectangular structure. Bright spotsthat align along the [010] direction with a periodicity of∼0.6 nm and along the [100] direction with a periodicity of∼0.3 nm are observed, i.e, the spots form a p(2×1) structurewith respect to the substrate bcc-Fe(001) lattice. When thesame area as that in Fig. 4(c) was scanned with the STM
tip, we frequently observed a diﬀerent atomic structure asshown in Fig. 4(d), which has a similar lattice to the bccFe(001) symmetry, i.e., the p(1×1) structure. Since these
diﬀerent observations can be obtained by a slight change at
the tip apex, showing sublattices of diﬀerent chemical species
in metal-oxide ﬁlms,26) the observed p(2×1) and p(1×1)
structures are probably alternating subterraces in iron oxide.
Figure 4(e) shows a sphere model. Black dots denote Fe
atoms in a bcc(001) symmetry with a lattice constant of
287 pm. Grey circles are oxygen atoms adsorbed at hollow
site positions on the Fe(001), corresponding to the p(1×1)
structure. White circles align along the [010] direction with aperiodicity of 574 pm (= 2 × 287 pm), corresponding to thep(2×1) structure.
It is well known in bulk that iron reacts with oxygen at
300–900 K via (1) 3=2Fe + O2 = 1=2Fe3O4, (2) 2Fe + O2 =2FeO, (3) 6FeO + O2 = 2Fe3O4, and (4) 4Fe3O4 + O2 =6Fe2O3. Therefore, possible iron oxide structures are Fe3O4(magnetite): cubic, a = 0.8396 nm, Fe1−xO (wustite): cubic
a = 0.4302 nm (Fe-rich), a = 0.4275 nm (Fe-poor), α-Fe2O3(hematite): hcp a = 0.5034 nm, c = 1.3752 nm, and γ-Fe2O3(maghemite): cubic or tetragonal a = 0.834 nm, c = 2.501
nm. Also, above 840 K, FeO is energetically stable. Since theobtained structures in our study are based on a bcc(001)symmetry, iron oxides with cubic structures, i.e., Fe3O4(001),
Fe1−xO(001), and γ-Fe2O3(001), could be fabricated.
Although Fe3O4 ﬁlms similar to those in Figs. 2(g) and2( j) were observed at TS = 550 K with TA = 645–775 K,24)
studies of iron-oxide ﬁlms grown on a Fe(001) substrate at
300 K with higher annealing temperatures suggested thecoexistence of FeO and γ-Fe2O3 phases.27,28) By comparing

08NB14-3© 2016 The Japan Society of Applied Physics

Jpn. J. Appl. Phys. 55, 08NB14 (2016)160㽢150nm
I(b)

Fe(001)-p(1x1)O

II

400

I

z [pm](a)T. K. Yamada et al.

300

~180

200
100

[010]

II

(e)

(d)

~0.3nm~0.3nm~0.3nm~0.3nm[100]~0.6nm

(f)

(g)

II

II

100

(i)

~0.6

0
0

x[nm]

z[pm]
0

[100]

II
-1

4

0

x[nm] 20

0

+1 +2

E-EF [eV]180㼼30

I

I

(k)

300

[010]

574pm

(j)

(h)
z[pm]

I

60

x [nm]

dI/dV[nA/V]

II0[010] 287pm

(c)

0

[100]

γ-Fe2O3(001)

1MLFeO(001) Fe(001)Fig. 4. (Color online) STM=STS measurements on Fe(001) surfaces oxidized (2 L of O2) with subsequent annealing to TA = 850 K. (a) STM image of thearea including one large nanoisland (area II) and atomic terraces (area I). An enlarged image of area I shows an Fe(001)-p(1×1)O structure. (b) Line proﬁle ofarea II along the arrow in (a). (c, d) Atomically resolved STM images obtained in area II [(c) 8 × 8 nm2, VS = −9 mV, and I = 2 nA; (d) 5 × 5 nm2,
VS = −15 mV, and I = 3 nA]. (e) Sphere model of the obtained structures on a bcc-Fe(001) substrate. Black and grey dots denote Fe and oxygen atoms,
respectively, and white circles denote (2×1). (f) STM current image (75 × 150 nm2) of the area including one large nanoisland (area II) and atomic terraces(area I). Step edges are enhanced. (g) High-resolution STM image on an island (area II). (h) and (i) Line proﬁles along the green and blue arrows in (g),respectively. ( j) dI=dV curves obtained on the FeO terrace (purple line) and on the island (green line). (k) Model of the cross-sectional view of the obtained
surface.

our STM images in Figs. 4(c) and 4(d) with those of
γ-Fe2O3(001),29) the experimentally obtained p(1×1) andp(2×1) structures could correspond to sublattices in
γ-Fe2O3(001), i.e., p(2×1) octahedral Fe3+ and p(1×1)
oxygen layers.
γ-Fep2O
ﬃﬃﬃ 3 has a lattice constant of 834 pm, which ﬁts
the 2 2  287 pm = 812 pm in a bcc-Fe(001) shown in
Fig. 4(e), i.e., γ-Fe2O3 ﬁlms on bcc-Fe(001) substrate display a small lattice mismatch of only ∼2.6%. The constantisland height of ∼1 nm is similar to the lattice constant of
γ-Fe2O3.
Figure 4(f) shows another island on the same surface asthat in Fig. 4(a). This image was taken in the constant height
mode, and the current map emphasizes step edges. In thisimaging mode, 1D lines are clearly observed. Figure 4(g)shows an enlarged image. On terraces on the island in
Fig. 4(g), 1D lines along the [010] direction, whose
periodicity is ∼0.6 nm [see the line proﬁle in Fig. 4(h) along
the green arrow in Fig. 4(g)], can be seen corresponding tothe (2×1) structure in Fig. 4(c). On top of the (2×1) structure,
the next layer starts to grow along the [100] direction. The
distance between layers is ∼180 pm [see the line proﬁle inFig. 4(i) along the blue arrow in Fig. 4(g)], which is the sameas that in Fig. 4(b), indicating that the stacking structure
seems to be the same for all nanoislands.
Furthermore, we performed STS measurements to determine the LDOS. Figure 4( j) shows dI=dV curves obtainedon the Fe(001)-p(1×1)O ﬂat terrace (area I, purple line) and
γ-Fe2O3(001)-like nanoislands (area II, green line). The FeO
terrace has peaks at −0.5 and +1.0 eV, which are in good
agreement with Ref. 30. However, on the islands, the −0.5
peak seems to shift below −1 eV, while the +1 eV peak issuppressed.
From these studies, we can conclude the following. At
TS ∼ 550 K, mainly Fe3O4 ﬁlms are grown. Annealing to850 K might change the structure from Fe3O4 to Fe2O3, butwith a very low possibility to generate FeO. On the other
hand, when we oxidize Fe(001) at TS = 300 K with subsequent annealing, diﬀerent iron-oxide ﬁlms are grown at
diﬀerent TA: TA ≤ 540 K amorphous ﬁlms, TA = 740 K
Fe3O4(001)-like ﬁlms, and TA = 850 K atomically ﬂat
FeO(001) ﬁlms and γ-Fe2O3 three-dimensional nanoislands.
Iron oxides, such as Fe3O4 and Fe2O3, are ferrimagnets. Ifiron oxide islands grow at iron=oxide ﬁlm interfaces, owing
to an exchange bias eﬀect, the islands could pin thesurrounding ferromagnetic Fe and decrease TMR.

08NB14-4© 2016 The Japan Society of Applied Physics

Jpn. J. Appl. Phys. 55, 08NB14 (2016)

4.

T. K. Yamada et al.

Conclusions

An atomically ﬂat and clean bcc-Fe(001) whisker single
crystal surface was oxidized (2 L) in UHV. The producediron oxide was studied as a function of the TS during theoxidation and the annealing temperature after the oxidation.
The annealing temperature of 850 K is necessary to obtainan ordered atomically ﬂat Fe(001)-p(1×1)O surface with
Fe–O nanoislands. The growth of islands was found to becontrolled by the TS during oxidation. By setting the TS lower
than 300 K during oxidization, the number of islands on thesurface could be reduced to less than 10%. However, atsubstrate temperatures higher than 500 K, most of the surface(70–80%) was covered by Fe–O nanoislands. The island
growth was not aﬀected by the oxygen dose between 1 and6 L. These experimental ﬁndings are important in producing
an atomically ﬂat ordered Fe=oxide interface.

8)
9)
10)
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12)
13)
14)
15)
16)

17)
18)

Acknowledgements

We thank Dr. D. T. Pierce (NIST) for his guidance regarding
the chemical vapor deposition of the Fe whisker. This work
was funded by the ImPACT Program of Council for Science,
Technology and Innovation (Cabinet Oﬃce, Government ofJapan) and JSPS KAKENHI Grant Number 15K13357.

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