Comparison of the magnetic properties of metastable hexagonal close-packed Ni nanoparticles with those of the stable face-centered cubic Ni nanoparticles

Article abstract of DOI:10.1021/jp054608b

We report the first magnetic study of pure and metastable hexagonal close-packed (hep) Ni nanoparticles (sample 1). We also produced stable face-centered cubic (fcc) Ni nanoparticles, as mixtures with the hcp Ni nanoparticles (samples 2 and 3). We compared the magnetic properties of the hcp Ni nanoparticles with those of the fcc Ni nanoparticles by observing the evolution of magnetic properties from those of the hcp Ni nanoparticles to those of the fcc Ni nanoparticles as the number of fcc Ni nanoparticles increased from sample 1 to sample 3. The blocking temperature (T_B) of the hcp Ni nanoparticles is ～12 K for particle diameters ranging between 8.5 and 18 nm, whereas those of the fcc Ni nanoparticles are 250 and 270 K for average particle diameters of 18 and 26 nm, respectively. The hcp Ni nanoparticles seem to be antiferromagnetic for T < T_B and paramagnetic for T > T_B. This is very different from the fcc Ni nanoparticles, which are ferromagnetic for T < T_B and superparamagnetic for T > T _B. This unusual magnetic state of the metastable hcp Ni nanoparticles is likely related to their increased bond distance (2.665 A), compared to that (2.499 A) of the stable fcc Ni nanoparticles.

Full text of DOI:10.1021/jp054608b

J. Phys. Chem. B 2006, 110, 1187-1191
1187
Comparison of the Magnetic Properties of Metastable Hexagonal Close-Packed Ni
Nanoparticles with Those of the Stable Face-Centered Cubic Ni Nanoparticles
Yoon Tae Jeon, Je Yong Moon, and Gang Ho Lee*
Department of Chemistry, College of Natural Sciences, Kyungpook National UniVersity,
Taegu 702-701, South Korea
Jeunghee Park
Department of Chemistry, Korea UniVersity, Jochiwon 339-700, South Korea
Yongmin Chang
Department of Diagnostic Radiology, School of Medicine, Kyungpook National UniVersity and Hospital,
Taegu 702-701, South Korea
ReceiVed: August 16, 2005; In Final Form: NoVember 28, 2005
We report the first magnetic study of pure and metastable hexagonal close-packed (hcp) Ni nanoparticles
(sample 1). We also produced stable face-centered cubic (fcc) Ni nanoparticles, as mixtures with the hcp Ni
nanoparticles (samples 2 and 3). We compared the magnetic properties of the hcp Ni nanoparticles with
those of the fcc Ni nanoparticles by observing the evolution of magnetic properties from those of the hcp Ni
nanoparticles to those of the fcc Ni nanoparticles as the number of fcc Ni nanoparticles increased from sample
1 to sample 3. The blocking temperature (TB) of the hcp Ni nanoparticles is ∼12 K for particle diameters
ranging between 8.5 and 18 nm, whereas those of the fcc Ni nanoparticles are 250 and 270 K for average
particle diameters of 18 and 26 nm, respectively. The hcp Ni nanoparticles seem to be antiferromagnetic for
T < TB and paramagnetic for T > TB. This is very different from the fcc Ni nanoparticles, which are
ferromagnetic for T < TB and superparamagnetic for T > TB. This unusual magnetic state of the metastable
hcp Ni nanoparticles is likely related to their increased bond distance (2.665 Å), compared to that (2.499 Å)
of the stable fcc Ni nanoparticles.
1. Introduction
than for fcc Ni nanoparticles at room temperature, implying that
the magnetization of the hcp Ni nanoparticles is lower than that
Face-centered cubic (fcc) Ni nanoparticles have been well
documented.1,2 For instance, size-dependent magnetic studies
on fcc Ni nanoparticles have shown that both the blocking
temperature and the magnetic moment per atom increase with
increasing particle size.2
of the fcc Ni nanoparticles.7
In this research, we report the first magnetic study of pure
and metastable hcp Ni nanoparticles. We also produced stable
fcc Ni nanoparticles and compared the magnetic properties of
the hcp Ni nanoparticles with those of the fcc Ni nanoparticles.
We characterized the magnetic properties by recording both the
M-T and M-H curves. We observed that the M-T and M-H
curves of the hcp Ni nanoparticles were completely different
from those of the fcc Ni nanoparticles. This indicates that the
magnetic state of the hcp Ni nanoparticles is different from that
of the fcc Ni nanoparticles.
Magnetic states that are different from those of bulk metal
have been observed in Fe metal. In fcc Fe films prepared on
fcc Cu substrates, both antiferromagnetism and ferromagnetism
have been observed.12-15 Theoretical calculations have also
predicted that fcc Fe films could have antiferromagnetism,
ferromagnetism, and nonmagnetism, depending on the lattice
spacing of the substrates,16,17 even though the body-centered
cubic (bcc) Fe metal, the stable structure at room temperature
and 1 atm, is ferromagnetic. Also, note that both MnBi alloy
and Heusler alloys (i.e., Cu2MnSn and Cu2MnAl), in which none
of the elements are themselves ferromagnetic, are ferromagnetic.
Slater and Bethe roughly explained ferromagnetism in these
alloys such that the Mn atoms are farther apart in alloys than in
pure Mn metal and, as a result, spins in Mn atoms become
Studies of magnetic properties of hexagonal close-packed
(hcp) Ni nanoparticles, however, are rare. This is because of
the metastability of the hcp structure. Note that the fcc structure
is stable for Ni. Early magnetic studies on hcp Ni include those
on thin films.3-6 In those experiments, hcp Ni films were found
to be nonmagnetic with a zero magnetic moment at room
temperature,3,5 which is the opposite of ferromagnetic fcc Ni.
Recently, hcp Ni nanoparticles have been produced by reducing
Ni cations in the solution phase.7-10 A mixture of hcp and fcc
Ni nanoparticles has also been produced by the laser decom-
position of nickel carbonyl in the gas phase.11 The hcp structure
changed into the fcc structure upon the application of heat, thus
confirming the metastability of the hcp structure at room
temperature.3-5,10 However, note that the metastable hcp
structure did not spontaneously decay into the stable fcc structure
at room temperature unless the hcp Ni nanoparticles were heated.
It is interesting to note that Hinotsu et al. observed a lower
magnetization for a mixture of hcp and fcc Ni nanoparticles
* Corresponding author. Tel.: -82-53-950-5340. Fax: -82-53-950-6330.
E-mail: ghlee@mail.knu.ac.kr.
10.1021/jp054608b CCC: $33.50 © 2006 American Chemical Society
Published on Web 12/24/2005

1188 J. Phys. Chem. B, Vol. 110, No. 3, 2006
Jeon et al.
ferromagnetically ordered.18-20 These examples show that lattice
spacing (or bond distance) significantly affects the magnetic
state. Note that the bond distance (2.665 Å) of the metastable
hcp Ni nanoparticles is much larger than that (2.499 Å) of the
stable fcc Ni nanoparticles, as determined in the present work.
This indicates that the unusual magnetic properties of the hcp
Ni nanoparticles are due to their larger bond distance than that
of fcc Ni nanoparticles.
Figure 1. TEM micrographs of (a) sample 1, (b) sample 2, and (c)
We observed a reduced magnetization in the hcp Ni nano-
particles, compared to that of the fcc Ni nanoparticles. We also
observed a sharp phase transition at ∼12 K, which is much lower
than that of the fcc Ni nanoparticles at the same particle size.
We also observed paramagnetic behavior in the M-H curve at
300 K. These observations, together with nonmagnetism ob-
served in hcp Ni thin films,3,5 suggest that the hcp Ni
nanoparticles might be antiferromagnetic for T < TB and
paramagnetic for T > TB, as discussed in the text. This is very
different from the behavior of fcc Ni nanoparticles, which are
ferromagnetic for T < TB and superparamagnetic for T > TB.
sample 3. The scale bars represent 50 nm.
2. Experimental Section
2.1. Preparation. The nanoparticles were prepared using a
method similar to that of Carturan et al.10 Instead of using K/B
alloy as a reducing agent, however, we used 1.0 M hydrazine
in a tetrahydrofuran solvent. We labeled the three samples as
samples 1-3, respectively. Two solutions in separate flasks were
initially prepared: One is a 8 mL of oleylamine solvent into
which 9 mL of hydrazine solution had been added (the reducing
agent solution), and the other is 0.15 g of Ni(acac)2 (acac )
acetylacetonate) in 10 mL of oleylamine solvent (the precursor
solution). All chemicals were purchased from Aldrich and were
used as received. During reaction, all solutions were magneti-
cally stirred under a flow of argon gas. Before the precursor
solution was injected into the reducing agent solution, both
solutions were heated to the same temperature. At a solution
temperature of 180 °C, the precursor solution was injected into
the reducing agent solution, and the mixture was maintained
for 3 h and 30 min (sample 1). For the other samples, the
precursor solution was injected into the reducing agent solution
at a solution temperature of 90 °C, and this mixture was
maintained for 4 h. Then, the solution temperature further
increased to 180 °C (sample 2) or 210 °C (sample 3) and
maintained for 3 h. All final product solutions were tinted black
as a result of nanoparticle formation. After being cooled to room
temperature, the solutions were diluted with a mixture of 15
mL of methanol and 15 mL of chloroform. Then, the nanopar-
ticles were separated from the rest of the solution by centrifuga-
tion. Note that the metastable hcp Ni nanoparticles did not
spontaneously decay into stable fcc Ni nanoparticles at room
temperature unless the hcp Ni nanoparticles were heated. This
eliminates any change of hcp Ni nanoparticles into fcc Ni
nanoparticles during characterization.
Figure 2. XRD patterns of (a) sample 1, (b) sample 2, and (c) sample
3. The assignments are the Miller indices (hkl).
3. Results and Discussion
3.1. TEM Micrographs. Figure 1a-c shows TEM micro-
graphs of samples 1-3, respectively. Here, samples 1 and 2
are nearly monodisperse in diameter with average particle
diameters of 8.5 and 18 nm, respectively. Sample 3, however,
shows a dual size distribution with average particle diameters
of 11 and 26 nm.
3.2. XRD Patterns. Figure 2a-c shows XRD patterns of
samples 1-3, respectively.22 The XRD pattern in Figure 2a
seems to show that sample 1 consists of only hcp Ni nanopar-
ticles. This is clearly confirmed by the temperature-dependent
magnetization curve (M-T curve), as shown later, in which only
the phase transition resulting from the hcp Ni nanoparticles is
observed. Although it is not clear for sample 2 from the XRD
pattern itself, both samples 2 and 3 consist of hcp and fcc Ni
nanoparticles. It is clear that fcc Ni nanoparticles also exist in
sample 3 because a separate fcc (200) peak is observed. As
confirmed in the M-T curves later, the number of fcc Ni
nanoparticles increases from sample 2 to sample 3. The cell
constants are estimated to be a ) 2.665 and c ) 4.300 Å for
the hcp structure and a ) 3.534 Å for the fcc structure. These
cell constants are consistent with the reported values.1,3-5,10 Note
that the bond distances (2r values) estimated from the corre-
sponding cell constants (a values) are 2.499 and 2.665 Å for
the fcc and hcp structures, respectively. Thus, the bond distance
of the metastable hcp Ni nanoparticles is much larger than that
of the stable fcc Ni nanoparticles.
2.2. Characterization. The diameters were measured with a
transmission electron microscope (TEM) (200 keV, Philips CM
200), the structures were examined with an X-ray diffractometer
(XRD) (Philips, X-PERT), and the magnetic properties were
evaluated with a Squid magnetometer (Quantum Design, MPMS
7). The average wavelength of the Cu KR lines, i.e., 1.5418 Å
was used for the XRD measurements. The nanoparticles were
dispersed in a benzene solvent, and then suspended on copper
grids for the TEM measurements. Capsules were sealed with
paraffin wax21 in order to prevent air oxidation after they were
filled with Ni nanoparticles for the magnetic property measure-
ments.
Many studies have shown that the structure of a variety of
metallic clusters changes with cluster size (n).23 This is due to
the evolution of stability with n. Although less common, the
cluster structure can be affected by production conditions such

Magnetic Properties of Metastable hcp Ni Nanoparticles
J. Phys. Chem. B, Vol. 110, No. 3, 2006 1189
Figure 4. Hysteresis loops of (a,b) sample 1, (c,d) sample 2, and (e,f)
sample 3 at 5 and 300 K, respectively. The insets at 5 K are expanded
around the origin.
Figure 3. Zero-field-cooled (ZFC) (-b-) and field cooled (FC) (-O-
EnDash) magnetization curves versus temperature of (a) sample 1, (b)
sample 2, and (c) sample 3.
decreases from sample 1 to sample 3, with respect to the
magnetization above 20 K arising from the fcc Ni nanoparticles.
Second, in the M-H curves, the loop shape changes from that
of the hcp Ni nanoparticles to that of the fcc Ni nanoparticles
from sample 1 to sample 3. Thus, both the M-T and M-H
curves clearly show that the number of fcc Ni nanoparticles
increases from sample 1 to sample 3.
as temperature, pressure, and kinetics. The hcp structure of the
Ni nanoparticles is related to the latter case. That is, the hcp Ni
nanoparticles were formed under rapid formation conditions,
as found in the thermal decomposition of precursors in the polyol
process7 and in laser ablation deposition.3 In the present
experiments, the number of fcc Ni nanoparticles increased with
increasing reaction time, thus confirming the metastability of
the hcp Ni nanoparticles.
3.3. Magnetic Properties. The magnetic properties of the
hcp Ni nanoparticles were studied by recording both the
temperature-dependent magnetization (M-T) and hysteresis
(M-H) curves, as shown in Figures 3a-c and 4a-f, respec-
tively. As previously mentioned in the XRD patterns, a single
phase transition in the M-T curve for sample 1 clearly shows
that this sample consists of only hcp Ni nanoparticles (Figure
3a), whereas two phase transitions in the M-T curves for
samples 2 and 3 indicate that both samples consist of hcp and
fcc Ni nanoparticles.
From the M-H curves, we estimated the coercivity values
(Hc) to be 302, 353, and 546 Oe at 5 K for samples 1-3,
respectively. The Hc values, however, are zero at 300 K for all
three samples. The M-T curves are composed of zero-field-
cooled (ZFC) and field-cooled (FC) curves. From the ZFC
curves, we estimated the blocking temperatures (TB). The TB
values of the hcp Ni nanoparticles were estimated to be ∼12 K
for all three samples within the experimental limit. On the other
hand, those of the fcc Ni nanoparticles were estimated to be
220 and 270 K for samples 2 and 3, respectively. It is known
that TB generally increases with increasing particle size. This
was observed in the fcc Ni nanoparticles.2 As can be seen in
Figure 3, however, TB of the hcp Ni nanoparticles is nearly the
same for average particle diameters ranging between 8.5 and
18 nm. This might indicate that the TB value of the hcp Ni
nanoparticles does not increase much with increasing particle
size.
As shown in Figures 3 and 4, both the M-T and M-H curves
of the hcp Ni nanoparticles are quite different from those of
the fcc Ni nanoparticles. The differences in magnetic properties
between the hcp and fcc Ni nanoparticles include the follow-
ing: First, in the M-T curves, the TB value of the hcp Ni
nanoparticles is much lower than that of the fcc Ni nanoparticles
at the same particle size. The blocking temperature (TB) peak
of the hcp Ni nanoparticles is very sharp, whereas that of the
fcc Ni nanoparticles is broad. Also, the TB value of the hcp Ni
nanoparticles does not much increase with increasing particle
size, whereas that of the fcc Ni nanoparticles does. Second, in
the M-H curves, the hcp Ni nanoparticles do not show
hysteretic behavior typical for ferromagnetic (or superparamag-
netic) materials, whereas the fcc Ni nanoparticles do. Third, in
the magnetization curves, the magnetization of the hcp Ni
nanoparticles is much smaller than that of the fcc Ni nanopar-
ticles, as shown in Figure 5. Here, the magnetizations of samples
1 and 2 are nearly the same within the experimental limit
because sample 2 contains only a small number of the fcc Ni
nanoparticles, as revealed from the XRD pattern of sample 2.
These results indicate that the magnetic state of the hcp Ni
nanoparticles is different from that of the fcc Ni nanoparticles.
Above TB (≈12 K), the magnetization of the hcp Ni
nanoparticles in the M-T curve at an applied field of 100 Oe
is nearly zero (Figure 3a). This negligible magnetization can
also be observed in the magnetization curve at 300 K (Figure
5b). These are consistent with the zero magnetic moment
observed in hcp Ni films at room temperature.3,5 That is, the
hcp Ni films were found to be nonmagnetic.3,5 In fact, the hcp
Ni nanoparticles were not attracted by a laboratory magnet of
∼500 G at room temperature. This indicates that the hcp Ni
nanoparticles do not have a ferromagnetic spin structure. This
is in contrast to the fcc Ni nanoparticles, which showed a large
magnetic moment. Also note that large magnetic moments have
also been reported for both icosahedral Ni clusters up to n )
74024 and tetragonal Ni nanoparticles with interstitial oxygens.25
As mentioned with respect to the XRD patterns, the number
of fcc Ni nanoparticles increases from sample 1 to sample 3.
This can be clearly observed in both the M-T and M-H curves,
being more noticeable in the former than in the latter. First, in
the M-T curves, the intensity of a sharp peak at ∼12 K,
corresponding to the TB value of hcp Ni nanoparticles, rapidly

1190 J. Phys. Chem. B, Vol. 110, No. 3, 2006
Jeon et al.
TABLE 1: Magnetic States, Structures, Cell Constants (a),
and Bond Distances (2r) of Fe, Co, and Ni Bulk Metals,
Films, and Nanoparticles
metal
Fe
size
bulk
magnetic state
structure
a (2r) (Å)
ferromagnetic
antiferromagnetic
antiferromagnetic
nonmagnetic
bcc
fcca
fcca
fcca
fcca
bcc
fcca
hcp
hcp
fcc
2.866 (2.482)26
3.666 (2.592)26
3.615 (2.556)12
3.439 (2.432)16
3.62 (2.56)14
film
ferromagnetic
nanoparticle ferromagnetic
paramagnetic
2.87 (2.49)27
3.615 (2.556)28
2.507 (2.507)29
2.49 (2.49)30
Co bulk
film
ferromagnetic
ferromagnetic
ferromagnetic
3.531 (2.496)31
3.52 (2.49)32
nanoparticle ferromagnetic
ferromagnetic
bulk
film
fcc
hcp
fcc
2.49 (2.49)33
Ni
ferromagnetic
nonmagnetic
ferromagnetic
3.523 (2.491)34
2.67 (2.67)3
hcpa
fcc
3.524 (2.491)3
3.534 (2.499)b
2.665 (2.665)b
nanoparticle ferromagnetic
fcc
antiferromagneticc
hcpa
Figure 5. Magnetization curves of sample 1, sample 2, and sample 3
at 5 and 300 K.
a Metastable structures at room temperature. b Present research.
c Suggestion of our research.
From the M-T, M-H, and magnetization curves, it is clear
that the fcc Ni nanoparticles have a ferromagnetic spin structure.
That is, the high and zero coercivity values at 5 and 300 K,
respectively; the high saturation magnetization; and the hyster-
etic behavior in the M-H curves clearly show that the fcc Ni
nanoparticles are ferromagnetic for T < TB and superparamag-
netic for T > TB, as observed by others.1,2 On the other hand,
we speculate that the hcp Ni nanoparticles are antiferromagnetic
for T < TB and paramagnetic for T > TB, for the following
reasons: (1) hcp Ni films were found to be nonmagnetic by
other researchers,3,5 implying that hcp Ni nanoparticles can also
be nonmagnetic or antiferromagnetic; (2) the hcp Ni nanopar-
ticles were not drawn by a laboratory magnet of ∼500 G,
whereas the fcc Ni nanoparticles were strongly drawn, indicating
that the hcp Ni nanoparticles are not ferromagnetic; (3)
magnetization of hcp Ni nanoparticles was reduced compared
to that of fcc Ni nanoparticles (see Figure 5); and (4) in the
M-H curve of hcp Ni nanoparticles, antiferromagnetic or
paramagnetic behavior was observed except at a low applied
field around H ) 0 Oe (Figure 4a,b), where the low-field region
is likely due to uncanceled surface spins. On the basis of these
observations, we speculate that the hcp Ni nanoparticles might
be antiferromagnetic with TB at ∼12 K. Another point of support
for this speculation is the increased bond distance of the hcp
Ni nanoparticles, compared to that of the fcc Ni nanoparticles,
because at such an enlarged bond distance, a different magnetic
state is often observed, as in Fe, where antiferromagnetism is
observed.12,26 Recall that the magnetic state strongly depends
on bond distance. The additional hysteretic behavior for a low
applied field around H ) 0 Oe in the M-H curves for the hcp
Ni nanoparticles might be due to uncanceled surface spins.
Magnetic states of both metastable and stable structures that
have been observed in Fe, Co, and Ni bulk metals, films, and
nanoparticles are listed in Table 1. In the case of Co, both hcp
and fcc structures are stable at room temperature and 1 atm.
Both structures are also ferromagnetic. Note that the bond
distances of both structures in Co are nearly the same. This
might indicate that the magnetic state is affected not by structure
but by bond distance. The magnetic states of metastable
structures in both Fe and Ni, however, are quite different from
those of the corresponding stable structures. Also, the bond
distances of the metastable structures are larger than those of
the corresponding stable structures. This shows that the magnetic
state is significantly affected by the bond distance. We include
the magnetic state of the hcp Ni nanoparticles in Table 1, as
speculated from the present and other works.
4. Conclusion
We produced pure and metastable hcp Ni nanoparticles. We
also produced stable fcc Ni nanoparticles, as mixtures with the
hcp Ni nanoparticles. The magnetic properties of the metastable
hcp Ni nanoparticles were investigated, and then they were
compared to those of the stable fcc Ni nanoparticles by
observing the evolution of magnetic properties from those of
hcp Ni nanoparticles into those of fcc Ni nanoparticles as the
number of fcc Ni nanoparticles increased. The bond distance
(2.665 Å) of the hcp Ni nanoparticles was found to be much
larger than that (2.499 Å) of the fcc Ni nanoparticles, which is
due to the metastability of the hcp Ni nanoparticles. We
observed that the magnetic properties of the hcp Ni nanoparticles
were entirely different from those of the fcc Ni nanoparticles.
These include (1) the reduced magnetization of the hcp Ni
nanoparticles, compared to that of the fcc Ni nanoparticles; (2)
the sharp and nearly size-independent blocking temperature (TB)
of the hcp Ni nanoparticles, occurring at ∼12 K for particle
diameters ranging between 8.5 and 18 nm, whereas those of
the fcc Ni nanoparticles are 250 and 270 K for average particle
diameters of 18 and 26 nm, respectively; and (3) the antifer-
romagnetic or paramagnetic behavior of the hcp Ni nanoparticles
in the M-H curves, whereas the fcc Ni nanoparticles show
ferromagnetic or superparamagnetic behavior. On the basis of
these observations and the nonmagnetism observed in the hcp
Ni films by other researchers, we speculate that the hcp Ni
nanoparticles might be antiferromagnetic for T < TB and
paramagnetic for T > TB. This is very different from the
behavior of fcc Ni nanoparticles, which are ferromagnetic for
T < TB and superparamagnetic for T > TB. This unusual
magnetic state of metastable hcp Ni nanoparticles is likely
related to their increased bond distance (2.665 Å), compared to
that (2.499 Å) of the stable fcc Ni nanoparticles, considering
that the magnetic state often depends on the bond distance.
Acknowledgment. This work was supported by a Korea
Research Foundation Grant funded by the Korean Government
(MOEHRD) (KRF-2004-015-C00240).
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Figure 2a: hcp (010) ) 39.13°, hcp (002) ) 42.15°, hcp (011) ) 44.61°,
hcp (012) ) 58.74°, hcp (110) ) 70.68°, hcp (103) ) 78.42°, hcp (112) )
85.70°, and hcp (201) ) 87.14°. Figure 2b: hcp (010) ) 39.01°, hcp (002)
) 42.10°, hcp (011) + fcc (111) ) 44.49°, hcp (012) ) 58.64°, hcp (110)
) 70.56°, hcp (103) ) 78.44°, hcp (112) ) 85.64°, and hcp (201) ) 87.14°.
Figure 2c: hcp (010) ) 39.06°, hcp (002) ) 42.06°, hcp (011) + fcc (111)
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fcc (220) ) 76.35°, hcp (103) ) 78.66°, hcp (112) ) 85.80°, and hcp (201)
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