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Cite This: Cryst. Growth Des. XXXX, XXX, XXX−XXX

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Growth Kinetics and Magnetic Property of Single-Crystal Fe
Nanowires Grown via Vapor−Solid Mechanism Using ChemicallySynthesized FeO Nanoparticle Catalysts
Takashi Yanase,*,† Uika Ogihara,‡ Yoshihiro Awashima,† Takeshi Yanagida,§ Kazuki Nagashima,§
Taro Nagahama,† and Toshihiro Shimada††Division of Applied Chemistry, Faculty of Engineering, Hokkaido University, Kita 13 Nishi 8 Kita-ku, Sapporo 060-8628, Japan
Graduate School of Chemical Sciences and Engineering, Hokkaido University, Kita 13 Nishi 8 Kita-ku, Sapporo 060-8682, Japan§
Laboratory of Integrated Nanostructure Materials, Institute of Materials Chemistry and Engineering, Kyushu University, 6-1
Kasuga-koen, Kasuga, Fukuoka 816-8580, Japan

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‡

S Supporting Information*

ABSTRACT: Single-crystal Fe nanowires (NWs) weresynthesized on SiO2/Si substrates by chemical vapor
deposition (CVD) using a two-ﬂow system in the presenceof FeO nanoparticles as catalysts. Transmission electron
microscopy observations and an electron diﬀraction analysis
revealed that the NW was defect-free α-Fe (bcc structure)oriented in the ⟨100⟩ direction. A plausible mechanism of the
Fe NW growth is suggested based on the observations of themorphology and characteristic diameter of the Fe NW. We
found that the NWs became polycrystalline when theirdiameter was approximately 100 nm. The excellent crystal
quality of the single-crystal NW was conﬁrmed by electron diﬀraction and the residual resistance ratio. Magnetic uniformity anda strong shape-anisotropic behavior of magnetization were observed by measuring the hysteresis loop and the angle dependenceof the magneto-optical Kerr eﬀect, respectively.

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nanolithography.14,15 However, the polycrystalline nature,rough surfaces of the NW walls, internal stress, and inevitablecontamination degrade the magnetic properties of the NWs.
The costly process is another disadvantage of the template andnanolithography methods. Therefore, it is crucial to establish amethod for the growth of single-crystal FMNWs using afeasible and scalable CVD approach.
Our research group has revisited the Fe whisker growth thatwas studied in the 1960s−1970s,21−23 then synthesized thesingle-crystal Fe NWs on an Al2O3 substrate with Au
nanoparticles as the catalyst24 and on a YSZ substrate without
a catalyst.25 If one can avoid using expensive single-crystal
substrates, such as Al2O3 and YSZ, the CVD becomes more
realistic as a practical method to synthesize single-crystal Fe
NWs. In this study, we synthesized single-crystal Fe NWs on
SiO2/Si substrates using FeO nanoparticles as a catalyst bythermal CVD based on the simple reduction of FeCl2 with H2.

INTRODUCTION
Iron is recognized as one of the most critical elements fromancient times to now due to its abundance, toughness, wellestablished processing, and magnetism. Therefore, it is widelyused in human society, for example, construction materials,
automobiles, and magnets. Recent nanotechnology has opened
the opportunities for the well-known materials to be improved
and tune their properties, for example, a large variety of singlecrystal nanowires (NWs) has been studied due to theirsuperior properties derived from the 1D geometry. In
particular, semiconducting NWs, such as Si,1−3 GaN,4
GaAs,5 ZnO,6,7 and InAs,8 have been intensively investigatedfor practical applications as ﬁeld-eﬀect transistors,9,10 supercapacitors,3 and sensors11,12 because the semiconducting NWs
can be rationally conﬁgured or structured by chemical vapor
deposition (CVD) with foreign nanoparticles via a vapor−liquid−solid (VLS) mechanism. Ferromagnetic NWs(FMNWs) have also attracted considerable attention due tonot only the interest in magnetic anisotropy13−19 but also
potential applications in spin devices.20 Since the growth of
FMNWs by CVD is still limited due to a vapor−solid (VS)mechanism that remains unclear regarding the microscopic
processes of the NW formation, most FM NWs werepreferably fabricated by electrodeposition using an anodic
alumina oxide template 18 or etching of a ﬁlm using© XXXX American Chemical Society■

EXPERIMENTAL SECTION

Figure 1 shows a schematic illustration of the home-built CVD
equipment with a two-ﬂow system for the growth of the Fe NWs in
Received: August 29, 2019
Revised: October 15, 2019
Published: October 21, 2019
A

DOI: 10.1021/acs.cgd.9b01148
Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

ArticleFigure 1. Schematic illustration of the two-ﬂow CVD equipment used for the growth of the Fe NWs.

Figure 2. (a) XRD pattern and (b) TEM image of FeO nanoparticles. FeO nanoparticles were dispersed in hexane and dip-coated on the substrates
under sonication.

Figure 3. (a) SEM image of Fe NWs grown on SiO2/Si using FeO nanoparticles as catalysts. (b) TEM image of a single Fe NW. The inset shows
the detailed morphology of the tip part. The scale bar in the inset of (b) is 100 nm. (c) Electron diﬀraction pattern obtained from the Fe NW
indicates the Fe NWs defect-free single crystal.
controllers that are separately equipped with both the FeCl2 line
and the H2 line.
SiO2(200 nm)/Si wafers cut into 1 × 1 cm2 pieces were used as thesubstrates. The SiO2/Si substrates were cleaned by the Radio
Corporation of America (RCA) cleaning method to remove any
metal and organic surface contaminants.26 After the RCA cleaning,
FeO nanoparticles dispersed in a hexane solution were dip-coatedunder sonication. The FeO nanoparticles were synthesized using aheat-up method according to ref 27. The nanoparticles were identiﬁed
as FeO by X-ray diﬀraction (XRD) as shown in Figure 2(a). A TEM
image shown in Figure 2(b) shows the narrow size distribution of thenanoparticles (8−15 nm). The FeO nanoparticles must be handled
under an inert atmosphere to avoid oxidation because FeO is sensitive
to air.27
The CVD procedure to synthesize the Fe NWs was as follows.
Immediately after the SiO2/Si substrates with FeO nanoparticles wereplaced in the outer quartz tube, the inside of the whole CVD system
was evacuated. When the inside pressure reached lower than 10 Pa,
the quartz tube was ﬁlled with Ar gas through the FeCl2 line. FeCl2·

this study. The CVD equipment was partly modiﬁed to improve its
controllability and reproducibility compared to previous reports.24,25
Our CVD system includes a furnace with three temperaturecontrollable zones, an outer quartz tube with a diameter of 28 mm,
and an inner quartz tube with a diameter of 6 mm. Each zone of thefurnace was labeled zone A, zone B, and zone C from upstream.
FeCl2·4H2O powder (0.2 g, Purity 99%, Kanto Chemical Co., Inc.)
was placed in a pocket of the inner quartz tube as shown by the arrow
in Figure 1. The paths through the inner quartz tube and the outerquartz tube were called the FeCl2 line and H2 line, respectively. The
introduction of the inner quartz tube hinders the reduction of the
FeCl2 powder by the H2 gas during the CVD, while most of thereported studies of the CVD-grown metal NWs ignored the direct
reduction of the source materials due to the single-ﬂow system.16,21,22
In our CVD system, FeCl2 vapor can be continuously and stably
supplied to the substrates for the growth of the Fe NWs. Furthermore,
the ratio of the partial vapor pressures of FeCl2 and H2 can be
optimized to guarantee the reproducibility by the mass ﬂow
B

DOI: 10.1021/acs.cgd.9b01148
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Crystal Growth & Design

Article4H2O was preannealed at 200 °C for 45 min to remove the crystal
water that may cause oxidation of the Fe NWs. The Ar ﬂow was thenstopped, and 4% H2 (diluted by Ar gas) started to ﬂow at the rate of
20 sccm through the H2 line. The sequence of the gas introduction isimportant to avoid direct reduction of the FeCl2 powder placed in the
pocket of the inner quartz tube. When the temperatures of zone A,
zone B, and zone C reached 530, 800, and 800 °C, respectively, theﬂow rate in the H2 line was increased to 200 sccm and the Ar gas
began to ﬂow again through the FeCl2 line at the rate of 20 sccm tosupply the FeCl2 vapor to the substrates. The duration of the gas ﬂow
through the FeCl2 line was deﬁned as the reaction time. After the
CVD was carried out for 60 min, Ar ﬂow in the FeCl2 line wassuddenly cut oﬀ to stop any undesirable reaction while lowering thetemperature. The furnace was naturally cooled to ambient temperature while maintaining the ﬂow of the 4% H2 gas in order for air not
to enter the quartz tube.
The morphology of the Fe NWs was characterized by scanningelectron microscopy (SEM; JEOL JSM-6500F and JSM-6390LVS)
and transmission electron microscopy (TEM; JEOL JEM-2010). The
crystallinity and growth axis of the Fe NWs were determined byelectron diﬀraction. Energy dispersive X-ray spectrometers (EDS)
equipped with SEM and TEM were used for the elemental analysis.
The temperature dependences of the resistivity and magnetoresistance at 9 K were measured using a 4-terminal sensing method.
The magnetization of a single Fe NW was measured by a micro-Kerr
magnetometer (NEOARK BH-L920WF6-FN).

pattern. The d value (0.144 nm) calculated from a diﬀraction
spot of (002) is approximately equal to that of the bulk α-Fe
(0.1433 nm). No diﬀraction spot derived from FeO illustrates
that the FeO nanoparticle was totally reduced to Fe. The
truncated tip rim seems to be general and probably plays acrucial role in the NW growth because the same geometricstructure was also observed for the diﬀerent NWs as shown in
Figure S3. Based on the combined results of the TEM image
and the diﬀraction pattern, the crystallographic orientation of
the truncated tip rim is assignable to the (011) planes as
indicated in the inset of Figure 3(b) by the red arrows. The
details of the growth mechanism are discussed in the next
section.
The VS mechanism can be divided into two growth modes;
i.e., (i) tip growth and (ii) base growth. These two growthmodes can be distinguished by observing the variation in the
diameter of an NW, called the characteristic diameter (orradius), when the growth temperature is changed during the
CVD. It is well-known that the characteristic diameter is amonotonically increasing function of only the reaction
temperature as long as the ﬂux of the reactants is maintained
when the NWs grow by the VS mechanism.29,31 In order toreveal whether tip growth or base growth is true, we carried out
a two-step growth as follows. After the Fe NWs were grown at
800 °C for 30 min, the temperature was increased to 850 °C at
a ramping rate of 5 °C/min, while the other conditions weremaintained. Subsequently, the growth of the NWs wascontinued for another 50 min, then the supply of FeCl2
vapor was shut oﬀ, and the furnace was cooled to room
temperature. The temperature program during the CVD is
schematically shown in Figure 4(a). Figure S4 shows an SEM
image of the NW grown by the two-step growth method. The
diameter varied at the center of the NW. To observe theindividual variation in the diameter and crystallographic■

RESULTS AND DISCUSSION
Figure 3(a) shows an SEM image of Fe NWs grown at random
angles on the SiO2/Si substrate. The diameter and length of
the Fe NWs ranged from 100 to 500 nm and from 5 to 35 μm,
respectively. It was conﬁrmed that nothing had grown on the
SiO2/Si substrate without the FeO nanoparticles, whichindicates that the FeO nanoparticles are essential to forming
the Fe NWs. An EDS analysis shown in Figure S1 revealed that
the NWs were composed of Fe, while Si and O were detected
due to the substrate. A single NW was transferred to a TEM
grid using a micromanipulator and observed by the TEM tocharacterize the detailed morphology as shown in Figure 3(b).
The growth mechanism of the NWs is generally classiﬁed into
two types; i.e., (i) VLS mechanism and (ii) VS mechanism. It
is straightforward to distinguish the diﬀerence between these
two mechanisms by observing the tip of an NW. While around-shaped droplet as a catalyst is always found on the tip by
the VLS mechanism,1 the ﬂat surface of the tip is standard
geometry for the NWs grown by the VS mechanism as
previously reported for the Sn,28 ZnO,6,29 and W18O4930 NWs.
The very ﬂat surface as seen in the inset of Figure 3(b) showsthat the Fe NWs grew by the VS mechanism. The EDS analysis
shows that the NW was composed of Fe and there was no
impurity, such as Cl and O, which illustrates that the FeO(catalyst) and FeCl2 were completely reduced to Fe by the H2(see Figure S2). The Cu peaks were always found andoriginated from the TEM grid. An electron diﬀraction analysis
shown in Figure 3(c) revealed that the NW was the defect-freesingle-crystal α-Fe (bcc structure) and the growth directionwas ⟨100⟩. The growth axis of the NW is consistent with aprevious report in the 1970s21,22 that studied Fe whiskers.
While Fe whiskers were preferably grown along the ⟨110⟩ and⟨112⟩ directions when the pure iron boat was used as a
substrate, iron oxide particles promoted the growth of the⟨100⟩ oriented Fe whiskers. Since we used FeO nanoparticles
as the catalyst for the growth of the Fe NWs, the Fe NWs grew
along the ⟨100⟩ axis. The orientation of the sidewalls wasdetermined as {100} from the TEM image and the diﬀraction

Figure 4. (a) Temperature program for the two-step growth method.
(b−d) TEM images show the base (b), center (c), and tip (d) of theNW grown by the two-step growth method. The electron diﬀraction
patterns in the insets illustrate no variation in the crystallographic
orientation of a single NW.
C

DOI: 10.1021/acs.cgd.9b01148
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Crystal Growth & Design

ArticleFigure 5. (a) Orientation of the Fe NW determined by TEM observation shown in Figure 3. (b) Preferable adsorption of dissociative hydrogenatoms on {110} surface due to the highest Fe−H binding energy among the low indexed planes. (c) Reduction of FeCl2 on the surface of {110}and 2D island growth. (d) Mass transport from the tip rim to tip terrace and nucleation for the Fe NW growth.

Figure 6. TEM image (left) and electron diﬀraction pattern (right) of an NW with a diameter of 100 nm. The center of the NW was magniﬁed asshown in the inset (left) to see the diameter of the NW. The electron diﬀraction pattern includes two distinctive spots enclosed by the blue and red
rectangles, which correspond to the bcc structure rotated ±20° and ±25° from the original bcc with a growth direction as an axis, respectively. A
crystallographic relationship for th e±20° rotated structure is illustrated in the insets.

on the Fe surface both theoretically32,33 and experimentally,34
step (i) is reasonable. Step (ii) is supported by the fact that thereactivity of a dissociatively adsorbed hydrogen is much greaterthan that of a hydrogen molecule.35 This fact is consistent withthe result that no Fe NW grew on the SiO2/Si substrate
without FeO nanoparticles. The desorption of HCl in step (iii)
is consistent with no detection of Cl atoms by an EDS analysis.
Figure 5(a) illustrates the surrounding planes of a Fe NW,
which were determined by observing Figure 3(b,c). To explain
the growth of the Fe NWs, we suggest a plausible mechanism
as shown in Figure 5(b−d). The hydrogen concentration
becomes the highest at the tip rim due to the Berg eﬀect.30,36
Besides, the {110} surface is more active than the {100}surface to dissociatively adsorb hydrogen (Figure 5(b))
because the Fe−H binding energy of {110} is the highest
among the low indexed planes.32 Therefore, the reduction of
FeCl2 and 2D island growth of Fe preferentially takes place onthe {110} surfaces (the truncated tip rim) as shown in Figure5(c). The 2D island growth at the tip rim results in decreasing
the surface area of the {110} planes and increasing that of the{100} planes. This 2D island growth causes the increase in the
total surface free energy of the Fe NW because the surface freeenergy of the {110} plane is lower than that of the {100} plane.
When the surface area of the {110} plane reaches a certain
value, Fe atoms were transported to the main facet (001) todecrease the surface free energy, and nucleation occurs at the
tip terrace (Figure 5(d)). Since the tip terrace was surroundedby four {110} planes, the collision rate of Fe atoms at the tip
terrace was approximately four times larger than that at the
sidewalls. Therefore, the nucleation took place more often at
the tip terrace than sidewalls, which results in the uniaxial
growth of a Fe NW. A similar NW growth process has been
observed in the W18O49 NW grown by physical vapor

orientation at the base, center, and tip of the NW, TEM images
and electron diﬀraction patterns were obtained as shown in
Figure 4(b−d). The characteristic diameter varied from thebase (400 nm) to tip (700 nm). The temperature dependenceof the characteristic diameter was rationalized by the EhrlichSchwoebel barrier that prevents the step down of an adatom
from terrace to terrace.29,31 Electron diﬀraction patterns
clariﬁed that the Fe NWs grew along the ⟨100⟩ direction,and there is no variation in the crystallographic orientation at
the base, center, and tip as shown in the insets. Whenconsidering that the characteristic diameter is a monotonicallyincreasing function, Figure 4(b−d) indicate that the growthmode of the Fe NW was tip growth, not base growth. It should
be noted that it was diﬃcult to make a plot of the temperature
vs diameter in our experiment because the temperature rangeof the NWs growth was narrow. Almost nothing formed when
the reaction temperature was 700 °C, and only chunks of Fe
were generated when it was 1000 °C as shown in Figure S5.
Although the growth mechanism was experimentally
determined, it is crystallographically curious that the Fe NWs
were surrounded by (100) planes because the equilibrium
shape (Wulﬀ shape) of the bcc structure is surrounded by the(110) plane that is the most stable. Thus, another anisotropic
eﬀect for the Fe NW growth that is based on kinetics should betaken into account. We consider the reduction reaction thatcan be written as H2 + FeCl2 → Fe + 2HCl to discuss themechanism. It can be assumed that the reaction includes threesteps; i.e., (i) dissociative adsorption of a hydrogen moleculeon the Fe surface, (ii) two adsorbed hydrogen atoms react with
the FeCl2 molecule adsorbed on the Fe surface, forming a(reduced) Fe atom and two HCl molecules, and (iii) the HClmolecules desorb from the Fe surface. Since it is wellrecognized that a hydrogen molecule dissociatively adsorbs
D

DOI: 10.1021/acs.cgd.9b01148
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Crystal Growth & Design

ArticleFigure 7. (a) SEM image of the sample geometry for 4-terminal sensing measurement. Four gold electrodes were prepared by the typical electron
beam lithography. A single Fe NW was transferred onto the electrodes using a micromanipulator. W metal was deposited at the junction between
the gold electrodes and the Fe NW to make good electrical contacts for the measurement. Ditches between the gold electrodes are necessary inorder not to form a short circuit due to the W deposition. (b) Temperature dependency of resistance for the Fe NW. (c) Magnetoresistance of the
Fe NW when the magnetic ﬁeld was applied (i) perpendicular, (ii) 30 degrees, and (iii) parallel to the NW.

Figure 8. (a) MOKE signal when the magnetic ﬁeld was applied perpendicular to the NW. Inset shows the expansion of the range from −300 to
300 Oe to observe the sharp ﬂip of the magnetization. (b) Angle dependence of coercivity of the NW. The blue curve is ﬁtted by H0/cos θ.

deposition.30 A high concentration of FeCl2 at the tip due tothe Berg eﬀect is another reason why nucleation preferentiallyoccurs at the tip terrace than the sidewall. Therefore, the {100}planes at the sidewalls of the Fe NW do not contribute to thetransverse growth, and only the tip is the growing point
resulting in the NW shape.
We noticed that an NW with a diameter of approximately
100 nm was polycrystalline as shown in Figure 6. It is unusual
for the polycrystalline NW to grow in one direction without atemplate. One possible reason to explain the polycrystalline
NW growth is lowering the temperature of the martensitic
transition (fcc-to-bcc phase transition). Although the temperature of the phase transition (fcc → bcc) in bulk is 912 °C, a
theoretical calculation predicts that the temperature of thephase transition becomes lower as the size is smaller.37 In otherwords, NWs became polycrystalline (bcc phase) during coolingafter the NWs grew as a single crystal in the fcc phase. The
orientation of the NW is not completely random because thediﬀraction spots indicated by the blue and red rectangles in
Figure 6 are assignable to ones when it is rotated ±20° and±25° with the growth direction as an axis. The unusual
orientation that is not a common habit plane in bulk38−41 is
probably due to a sizable uniaxial anisotropy (NW shape) thatcauses diﬀerent stress as compared to that of bulk. In order tomake thinner single-crystal Fe NWs, a more suitable precursor
such as an organometallic compound is needed to decrease thegrowth temperature because Fe NWs could grow only ataround 800 °C as long as FeCl2 was used as the precursor.
The temperature dependence of the resistivity (R−T) for asingle-crystal NW with a diameter of 450 nm was measured
using a 4-terminal sensing method. The detailed geometry andpreparation for the measurement are shown in Figure 7(a).
Figure 7(b) shows the R−T curve in the range of 9−293 K.
The residual resistance ratio (RRR) of the Fe NW (11.6) isgreater than that (2−5) of the NWs synthesized byelectrochemical deposition in the literature.42,43 This resultclearly illustrates that the crystallinity of the Fe NWs grown by
CVD is much better than that grown by electrochemical
deposition. Considering only the diﬀusive scattering based on
the Fuchs−Sondheimer model,44 the residual resistance for the
Fe NW with a diameter of 450 nm can be calculated as only
0.105 Ω (RRR = 136). The discrepancy between theexperimental (1.30 Ω) and theoretical (0.105 Ω) values isprobably due to the silicon impurity arising from the substratebecause even ppm-order Si impurity that cannot be detectedby EDS to reduce the RRR values.16,45 However, it should be
noted again that the RRR of our Fe NW is much better thanthat grown by electrochemical deposition. Magnetoresistance(MR) measurements were carried out using the same
geometry of the R−T measurement at 9 K with the NW
oriented (i) perpendicular, (ii) 30 degrees, and (iii) parallel tothe direction of the applied magnetic ﬁeld as shown in Figure
E

DOI: 10.1021/acs.cgd.9b01148
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Crystal Growth & Design

Article7(c). The magnetic ﬁeld was swept from 10 to −10 kOe and aconstant dc current (100 μA) was in the growth direction⟨100⟩. Two major contributions to the MR, ordinary(Lorentz) MR (OMR) and anisotropic (spin−orbit) MR(AMR), must be considered in a ferromagnet. When ΔR/R0 =(R∥ − R⊥)/R0, where R0 is the MR at a zero ﬁeld, R∥ is the MR
when the magnetic ﬁeld is parallel to the current, and R⊥ is the
MR when perpendicular, is deﬁned as the MR ratio, theLorentz force and spin−orbit eﬀect yield ΔR/R0 > 0 and ΔR/
R0 < 0, respectively.46,47 This fact shows that the OMR is amajor factor in our Fe NW. According to ref 45, which
reported the case of a thin ﬁlm, ΔR/R0 becomes negative when
the thickness is below 30 nm because the surface scattering
limits the electron mean free path. Moreover, the AMR of the⟨100⟩ direction is the lowest (less than 0.1%) among the threemajor axes (⟨100⟩, ⟨110⟩, ⟨111⟩). Therefore, the AMR eﬀect
was not observed in our Fe NW due to the limitation of ourmeasurement system (limit of resolution).
Figure 8(a) shows the magneto-optic Kerr eﬀect (MOKE)
signal of a single Fe NW at room temperature when theapplied magnetic ﬁeld was parallel to the growth direction of
the Fe NW. The single Fe NW was transferred by amicromanipulator to a clean SiO2/Si substrate to eliminate
the contamination eﬀect. The angle dependence of themagnetization for the 30, 60, 80, and 90 deg is shown in
Figure S6, and the hysteresis loop ﬁnally disappeared when theapplied magnetic ﬁeld was perpendicular to the growth
direction because the magnetization did not saturate in the
maximum magnetic ﬁeld (2000 Oe). The angle dependence ofthe coercivity was plotted every 5° in the range from 0° to 80°
as shown in Figure 8(b). The blue curve was ﬁtted by H0/cos
θ, where H0 is the coercivity when the magnetic ﬁeld is parallelto the growth direction of the NW and θ is the angle between
the magnetic ﬁeld and the NW (see the inset of Figure 8(b)).
The well-ﬁtting curve conﬁrms that the magnetic response of
the single-crystal Fe NW is almost ideal. H0 was determined tobe 110 Oe from the ﬁtting curve, and this is much higher thanthat of bulk Fe (∼1 Oe). The enhancement of the coercivity is
due to the high aspect ratio of the Fe NW. The anisotropy ﬁeld
was calculated to be 11000 Oe for our Fe NW using the aspectratio (30) and magnetization of the bulk α-Fe (Ms = 1750
e.m.u. cm−3), which is consistent with the result that themagnetization did not saturate at 2000 Oe when the magnetic
ﬁeld is perpendicular to the growth direction of the NW. The
square ratio (Mr/Ms, where Mr and Ms are the residual
magnetization and saturation magnetization, respectively) isroughly estimated from Figure 8(a) as 0.9, which is higher thanthat of the NWs grown by electrochemical deposition. Sharp
ﬂipping of the magnetization and 1/cos θ relation show nomajor domain-wall pinning site such as grain boundary andprecipitate in the Fe NW, which means our Fe NW showedmagnetic uniformity.

nanostructured magnetic materials, such as the core−shellstructure, with high quality and heterojunction.

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ASSOCIATED CONTENT

S Supporting Information*

The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.cgd.9b01148.

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EDS and SEM images, surface morphologies, and angle
dependence (PDF)

AUTHOR INFORMATION

Corresponding Author*E-mail: yanase42@eng.hokudai.ac.jp.
ORCID

Takashi Yanase: 0000-0002-5625-9308
Takeshi Yanagida: 0000-0003-4837-5701
Kazuki Nagashima: 0000-0003-0180-816X
Notes

The authors declare no competing ﬁnancial interest.

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ACKNOWLEDGMENTS
Instrumental analyses were supported by the Nanotechnology
platform at Hokkaido University by METI, Japan. T.Y.
received ﬁnancial support from the Nippon Sheet GlassFoundation for Materials Science and Engineering and JSPS
KAKENHI Grant Number JP19K15386.

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CONCLUSION
We have synthesized Fe NWs on SiO2/Si substrates using FeO
nanoparticles by the two-ﬂow CVD. Electron diﬀraction, R−T
measurement, and MOKE measurement showed the excellent
crystalline quality of the Fe NW grown by the CVD. The
detailed observations of the morphology revealed that the Fe
NW grew via the VS mechanism in the tip growth mode. The
strong anisotropy of the magnetization for the Fe NW
indicated that our CVD approach is suitable to makeF

DOI: 10.1021/acs.cgd.9b01148
Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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DOI: 10.1021/acs.cgd.9b01148
Cryst. Growth Des. XXXX, XXX, XXX−XXX