Arrays of ferromagnetic iron and cobalt nanocluster wires

Article abstract of DOI:10.1021/jp0122575

We report a fabrication of arrays of ferromagnetic iron and cobalt nanocluster wires (NCWs), ranging from 8 to 10 nm in diameter and Up to a few millimeters in length. The iron and the cobalt nanoclusters served as building blocks of the corresponding ferromagnetic NCWs. The iron and the cobalt nanoclusters were produced by thermally decomposing the corresponding metal carbonyl vapors with a resistive heater placed in the middle of a pair of permanent disk magnets. The NCWs were produced through pile-up of metallic nanoclusters along lines of magnetic flux, perpendicularly to the substrates attached to a pair of permanent disk magnet surfaces. We observed coercivities as large as 248 and 964 oersteds and remanences as high as 61 and 71% for the ferromagnetic iron and cobalt NCWs, respectively.

Full text of DOI:10.1021/jp0122575

© Copyright 2002 by the American Chemical Society
VOLUME 106, NUMBER 9, MARCH 7, 2002
LETTERS
Arrays of Ferromagnetic Iron and Cobalt Nanocluster Wires
G. H. Lee,*,† S. H. Huh,† J. W. Park,† H.-C. Ri,‡ and J. W. Jeong†
Department of Chemistry, College of Natural Sciences, Kyungpook National UniVersity,
Taegu 702-701, South Korea, and Material Science Laboratory, Korea Basic Science Institute,
52 Yeoeun-Dong, Yusung-Ku, Taejeon 305-333, South Korea
ReceiVed: June 13, 2001; In Final Form: August 14, 2001
We report a fabrication of arrays of ferromagnetic iron and cobalt nanocluster wires (NCWs), ranging from
8 to 10 nm in diameter and up to a few millimeters in length. The iron and the cobalt nanoclusters served as
building blocks of the corresponding ferromagnetic NCWs. The iron and the cobalt nanoclusters were produced
by thermally decomposing the corresponding metal carbonyl vapors with a resistive heater placed in the
middle of a pair of permanent disk magnets. The NCWs were produced through pile-up of metallic nanoclusters
along lines of magnetic flux, perpendicularly to the substrates attached to a pair of permanent disk magnet
surfaces. We observed coercivities as large as 248 and 964 oersteds and remanences as high as 61 and 71%
for the ferromagnetic iron and cobalt NCWs, respectively.
There is no doubt that the nanowires (NWs) have drawn a
special attention from all of us because of their properties quite
different from the bulk, resulting from nanometer-scale one-
dimensional structure and their potential applicability to engineer
a variety of state-of-the-art nanodevices.1 For instance, the most
spectacular of these may be arrays of ferromagnetic NWs which
may be useful for a perpendicular magnetic recording.2-4 In
this work, we report a very easy fabrication of arrays of
ferromagnetic nanocluster wires (NCWs), including character-
ization of their structures, physical dimensions, and magnetic
properties. Here, the NCW is named because the NW consists
of metallic nanoclusters.
The experimental setup used in the present experiment is quite
simple, as represented in Figure 1. Here, the stainless steel
reaction chamber is very similar to the one used to generate a
variety of metallic nanoclusters by thermally decomposing metal
Figure 1. A schematic experimental setup.
* Corresponding author. Tel: 82-53-950-5340. Fax: 82-53-950-6330.
carbonyl vapors with a resistive heater in the previous work.5
E-mail: ghlee@bh.knu.ac.kr.
The reaction chamber was evacuated with a 50 L/s mechanical
† Kyungpook National University.
‡ Korea Basic Science Institute.
pump down to 10-3 Torr. Even at this vacuum level, metallic
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2124 J. Phys. Chem. B, Vol. 106, No. 9, 2002
Letters
oxide production was minimal. The NCW production area is
indicated as “A” in Figure 1 and the detailed configuration of
“A” is represented to the right-hand of the reaction chamber.
The “A”, just hung by two electrical copper wires connected to
the resistive heater and through which the alternating current
(AC) flows, consists of a pair of permanent disk magnets, a
tube spacer, a resistive heater, and the substrates. The electrical
copper wires were electrically shielded with Teflon tubes. A
pair of permanent disk magnets, separated from each other by
a tube spacer provided magnetic field strength of 3000-4000
G at the center. To protect the permanent disk magnets from
the ferromagnetic nanoclusters, etc., produced during experi-
ment, they were wrapped with aluminum foils and the aluminum
foils were removed after experiment. An alumina tube spacer
with an inner diameter of 14 mm and a tube length of 15 mm
was used but both dimension and material of the tube spacer
can be changed. A resistive heater placed in the middle of the
alumina tube spacer was used to decompose the metal carbonyl
vapors into the metal atoms and the COs. A Nichrome alloy
wire was used as a resistive heater in the present experiment
but any kind of material can be used as a resistive heater. The
glass substrates with 1 mm thickness were used in the present
experiment but any kind of material can be used. The glass
substrates were attached to the permanent disk magnet surfaces
on which the NCWs grew. The metallic nanoclusters were
produced near to the resistive heater environment through
numerous collisions between the decomposed neutral metal
atoms. To evenly decompose the metal carbonyl vapors and
thus, to evenly produce the NCWs over the glass substrates,
the resistive heater was shaped such as the top view represented
in Figure 1. Here, the resistive heater temperature was kept
at 300-400 °C in order to break only the metal atom-CO
bond. Above this temperature, the CO was also dissociated into
C + O, from which both undesirable metal carbides and oxides
were also produced. The vapor pressures of both Fe(CO)5 and
Co2(CO)8 metal carbonyls used in the present experiment were
15-20 Torr, but can be changed. The Fe(CO)5 metal carbonyl
vapors were directly introduced from the lecture bottle into the
vacuum chamber, whereas the Co2(CO)8 metal carbonyl vapors
were obtained by heating the vacuum chamber up to ∼100 °C
into which the solid Co2(CO)8 metal carbonyls were loaded.
The iron or the cobalt nanoclusters produced near to the resistive
heater environment were equally pulled in two opposite direc-
tions by the magnetic field applied by a pair of permanent disk
magnets and piled up through aggregation perpendicularly to
the glass substrates (see Figure 1). It took only a few minutes
to complete the fabrication of arrays of NCWs. At the above
conditions, arrays of NCWs, ranging 8 to 10 nm in diameter
and up to a few millimeters in length were fabricated.
Figure 2. The SEM micrographs of arrays of (a) - (d) the iron NCWs
and (e) - (h) the cobalt NCWs.
straightness and thus, are tangled together with neighboring
NCWs to form bundles of NCWs. Each magnified picture of
one bundle of NCWs is represented in Figure 2d,h for the
ferromagnetic iron and cobalt NCWs, respectively.
Figure 3a-c represents the HRTEM micrographs of the ferro-
magnetic iron (Figure 3a) and cobalt (Figure 3b,c) NCWs. Here,
the diameter of NCW ranging from 8 to 10 nanometers can be
observed for both ferromagnetic iron and cobalt NCWs. Figure
3c represents two cobalt NCWs connected to each other (the
connected area is indicated as an arrow), which is produced
while the NCWs are tangled together with neighboring NCWs
because the NCWs cannot maintain straightness due to their
long length as mentioned before. Examples of individual
nanoclusters are represented as arrows in Figure 3a,b for the
iron and the cobalt nanoclusters, respectively, which are barely
visible due to a tight aggregation between nanoclusters.
The structures of the ferromagnetic iron and cobalt NCWs
were characterized by using the XRD patterns, as represented
in Figure 4a,b, respectively. Since the ferromagnetic NCWs
consist of metallic nanoclusters as roughly observed in the
HRTEM micrographs (Figure 3a-c), the structures of NCWs
should correspond to those of metallic nanoclusters. The XRD
patterns indicate that the iron nanoclusters consist of a body-
centered-cubic (bcc) structure whereas the cobalt nanoclusters
consist of a fcc structure with some contribution from a
hexagonal close packing (hcp) structure because the broad
feature below the fcc (111) peak fits well to the three hcp peaks.
The cell constants are estimated to be 2.8697 ( 0.0011 and
3.5413 ( 0.0068 Å for the iron and the cobalt nanoclusters,
The NCWs grew through aggregation of metallic nanoclusters
along lines of magnetic flux. Without a magnetic field, however,
only the metallic nanoclusters had been produced.5 Thus, the
size and the structure of individual nanoclusters are conserved
in the NCWs, as confirmed from the high-resolution transmis-
sion electron microscope (HRTEM) micrographs and the X-ray
diffraction (XRD) patterns, respectively, which will be discussed
later.
Figure 2a-h represents the scanning electron microscope
(SEM) micrographs of arrays of the ferromagnetic iron and
cobalt NCWs. It is interesting to note that the NCWs exist as a
bundle of NCWs somewhat like woolen yarn, as can be seen
in Figure 2c,g for the ferromagnetic iron and cobalt NCWs,
respectively, which likely originates from the fact that the NCWs
are so long (∼ a few millimeters) that they cannot maintain

Letters
J. Phys. Chem. B, Vol. 106, No. 9, 2002 2125
Figure 4. The XRD patterns of (a) the iron NCWs and (b) the cobalt
NCWs. The assignments are the Miller indices (hkl).
Figure 3. The HRTEM micrographs of (a) an iron NCW, (b) a cobalt
NCW, and (c) two cobalt NCWs connected to each other. Examples
of individual nanoclusters are represented as arrows in both original
(10 nm scale) and further magnified (shown in a box, 3.8 nm scale)
micrographs for the same nanoclusters, which are barely visible due
to a tight aggregation between nanoclusters. The arrow in (c) indicates
a connected area between two cobalt NCWs.
Figure 5. The hysteresis loops of (a) the iron NCWs and (b) the cobalt
NCWs with the magnetic field applied parallel (|) and perpendicular
( ) to the NCWs at 10 and 300 K. M/Ms indicates the magnetization
(M) normalized by the saturated magnetization (Ms).
respectively, which are in good agreement with those of the
corresponding bulk metals, respectively.6,7 The cluster diameters
of the iron and the cobalt nanoclusters estimated by using the
full width at half-maximums (fwhms) of 1.483 ( 0.010 and
2.027 ( 0.137°, for the iron and the cobalt nanoclusters,
respectively, and the peak centers of 44.657 ( 0.019 and 44.303
( 0.090° for the iron and the cobalt nanoclusters, respectively,
and by using the Scherrer’s formula8 are 5.82 ( 0.34 and 4.26
( 0.29 nm, for the iron and the cobalt nanoclusters, respectively,
which are consistent with those roughly estimated from the
HRTEM micrographs, respectively. Here, both fwhms and peak
centers were obtained by fitting the (110) peak of the iron
nanoclusters and the (111) peak of the cobalt nanoclusters to
the Lorentzian function. It is likely that both metallic atoms
and metallic molecules were also drawn by the magnetic field
along lines of magnetic flux and deposited together with the
metallic nanoclusters to become the part of NCWs. The NCWs,
however, mostly consist of metallic nanoclusters because
amorphous peaks which originate from both metallic atoms and
metallic molecules are negligible in the XRD patterns (see
Figure 4a,b).
System (MPMS). Figure 5a,b represents the hysteresis loops
of the ferromagnetic iron and cobalt NCWs with magnetic fields
applied parallel (H|) and perpendicular (H ) to the NCWs,
respectively. As expected, the remanences of the H| (i.e., the
“easy axis”) are much higher than those of the H for both
ferromagnetic iron and cobalt NCWs due to the demagnetizing
field in the H .9 Here, the magnetic properties of the H| are
important for the perpendicular magnetic recording. The coer-
civities as large as 248 and 964 oersteds and the remanences
up to 61 and 71% for the H| have been observed at 300 K for
the ferromagnetic iron and cobalt NCWs, respectively. We
noticed that both coercivities and remanences for the H| could
be improved by improving the collection technique of the NCWs
because the number of the NCWs which possessed the original
straight direction could be maximized.10 Thus, the actual
coercivities and remanences for the H| will be higher than the
measured values given above. At 10 K, the coercivities as large
as 439 and 1576 oersteds and the remanences up to 69 and 72%
for the H| have been observed for the ferromagnetic iron and
To characterize the magnetic properties, we measured the
hysteresis loops by using the Magnetic Property Measurement

2126 J. Phys. Chem. B, Vol. 106, No. 9, 2002
Letters
cobalt NCWs, respectively, and are all higher than those mea-
sured at 300 K, simply because of lower temperature. All of
these values are much higher than the corresponding bulk values.
The possible application area of arrays of ferromagnetic
NCWs may be a high density perpendicular magnetic record-
ing.3,4 The high remanences and the large coercivities, resulting
from the single domain nature of the small iron and cobalt
nanoclusters which are the components of the corresponding
NCWs, together with a dense population of NCWs in the arrays,
may make these arrays extremely useful for a high density
perpendicular magnetic recording. In addition, fabrication of
arrays of NCWs is quite simple as described previously and
more importantly is suitable for a large scale production and
costs low, and dimension of arrays of and length of NCWs can
be easily modified by using a pair of permanent disk magnets
with a different disk area and a different metal carbonyl vapor
pressure, respectively, as described before, which are certainly
the great advantages over the other fabrication methods used
to produce arrays of NWs.1,2 There is, however, no doubt that
a further technical improvement should be made to the present
arrays in order to make these arrays useful for a high density
perpendicular magnetic recording.
References and Notes
(1) See, for example, Komarneni, S.; Parker, J. C.; Hahn, H. Nanophase
and Nanocomposite Materials III; Material Research Society: Warrendale,
2000; Vol. 581, pp 213-250.
(2) Whitney, T. M.; Jiang, J. S.; Searson, P. C.; Chien, C. L. Science
1993, 261, 1316.
(3) Speliotis, D. E. J. Magn. Magn. Mater. 1999, 193, 29.
(4) White, R. L. J. Magn. Magn. Mater. 2000, 209, 1.
(5) Huh, S. H.; Oh, S. J.; Kim, Y. N.; Lee, G. H. ReV. Sci. Instrum.
1999, 70, 4366.
(6) Villars, P.; Calvert, L. D. Pearson’s Handbook of Crystallographic
Data for Intermetallic Phases; American Society for Metals: Metal Park,
1985; Vol. 3, p 2162.
(7) Villars, P.; Calvert, L. D. Pearson’s Handbook of Crystallographic
Data for Intermetallic Phases; American Society for Metals: Metal Park,
1985; Vol. 2, p 1736.
(8) Cullity, B. D. Elements of X-ray Diffraction; Addison-Wesley:
Reading, 1978; p 102.
(9) Chikazumi, S. Physics of Ferromagnetism, 2nd ed.; Clarendon
Press: Oxford, 1997; pp 11-17.
(10) To measure both H| and H hysteresis loops, the NCWs were
collected simply by using a tape while a pair of permanent disc magnets
were applied in order to maintain the original straightness of NCWs as
much as possible. It is important to note that both actual coercivities and
remanences should be all higher than the measured values because it is
impossible to collect all of NCWs with their original straightness maintained.
At 300 K, the coercivities and the remanences of the iron NCWs were in
the range from 70 to 248 oersteds and from 34 to 61%, respectively, for
the H| and from 170 to 318 oersteds and from 10 to 13%, respectively, for
the H , and the coercivities and the remanences of the cobalt NCWs were
in the range from 578 to 964 oersteds and from 68 to 71%, respectively,
for the H| and from 673 to 690 oersteds and from 20 to 28%, respectively,
for the H .
Acknowledgment. We thank the Korea Basic Science
Institute (KBSI) for allowing us to use their SEM and XRD at
a membership rate and the Material Science Group at the KBSI
for allowing us to use their MPMS through a cooperational
research project. This work was supported by the KNU Research
Fund (2000) and the KOSEF (2001-1-12100-005-1).