Ferromagnetic resonance of nickel nanoparticles in an amorphous oxide matrix

Article abstract of DOI:10.1023/B:KICA.0000044987.66237.8d

The formation of a ferromagnetic phase in the thermal decomposition of a layered lithium-aluminum double hydroxide with intercalated nickel-EDTA complexes was studied using ferromagnetic resonance (FMR) spectroscopy. The FMR spectra were simulated based on a superparamagnetic narrowing model. Data on the size and shape of the resulting nickel nanoparticles were calculated.

Full text of DOI:10.1023/B:KICA.0000044987.66237.8d

Kinetics and Catalysis, Vol. 45, No. 5, 2004, pp. 735–738. Translated from Kinetika i Kataliz, Vol. 45, No. 5, 2004, pp. 776–779.
Original Russian Text Copyright © 2004 by Yulikov, Abornev, Mart’yanov, Yudanov, Isupov, Chupakhina, Tarasov, Mitrofanova.
SCHOOL
OF YOUNG SCIENTISTS
Ferromagnetic Resonance of Nickel Nanoparticles
in an Amorphous Oxide Matrix
M. M. Yulikov*, I. S. Abornev*, O. N. Mart’yanov*, V. F. Yudanov*,
V. P. Isupov**, L. E. Chupakhina**, K. A. Tarasov**, and R. P. Mitrofanova**
* Boreskov Institute of Catalysis, Siberian Division, Russian Academy of Sciences, Novosibirsk, 630090 Russia
** Institute of Solid-State Chemistry and Mechanochemistry, Siberian Division, Russian Academy of Sciences,
Novosibirsk, 630128 Russia
Received July 4, 2003
Abstract—The formation of a ferromagnetic phase in the thermal decomposition of a layered lithium–alumi-
num double hydroxide with intercalated nickel–EDTA complexes was studied using ferromagnetic resonance
(FMR) spectroscopy. The FMR spectra were simulated based on a superparamagnetic narrowing model. Data
on the size and shape of the resulting nickel nanoparticles were calculated.
INTRODUCTION
direction of a magnetization vector, the speed of mag-
netization rotation in an external magnetic field and a
magnetic anisotropy field should be averaged, as dis-
tinct from the above approach [7–10], according to
which a local anisotropy field is averaged.
The properties of nanosized particles are of consid-
erable interest in heterogeneous catalysis. In the last
few years, ferromagnetic resonance (FMR) spectros-
copy has been used in increasing frequency, along with
other techniques, for studies of magnetic oxide and
In this work, we used FMR spectroscopy to study
metal nanoparticles [1]. This technique is highly sensi- magnetic nickel nanoparticles that are formed in the
tive, and its sensitivity is higher than the sensitivity of thermal decomposition of the layered lithium–alumi-
EPR spectroscopy by two or three orders of magnitude, num double hydroxide with intercalated nickel–EDTA
all other factors being the same. This provides an complexes [LiAl2(OH)6]2[NiEDTA] · 4H2O. Previ-
opportunity to study early steps in the generation of a ously, it was found that the thermal decomposition of
ferromagnetic phase, and these studies may be very this compound at temperatures higher than 340°ë
useful for studying the synthesis of catalysts.
resulted in the formation of nanosized nickel particles
distributed in an X-ray amorphous matrix of lithium
aluminate and a carbon-containing phase [12, 13]. This
solid phase was studied by small-angle X-ray scattering
analysis and high-resolution electron microscopy. It
was found that the nickel particles exhibited a narrow
size distribution, and the average diameter was 3–5 nm.
The resulting composites containing nickel nanoparti-
cles were sufficiently stable against oxidation in air at
room temperature. Therefore, it was of interest to study
both the magnetic properties of nanosized nickel parti-
cles having a narrow size distribution and the formation
of these particles in the thermal decomposition of
[LiAl2(OH)6]2[NiEDTA] · 4H2O with the use of FMR
spectroscopy.
A so-called superparamagnetic effect is well
known for ferromagnetic nanoparticles of sizes of
about 1–10 nm [1, 2]. The magnetic moment of a par-
ticle is small, so that thermal changes in its direction
play an important role. The FMR spectrum of these par-
ticles depends on their size. The Brownian rotation of
the magnetization vector of a nanoparticle effectively
averaged magnetic anisotropy fields and, as a conse-
quence, narrowed the FMR line.
Previously, oxide and metal nanoparticles were
studied by FMR spectroscopy [1–6]. De Biasi and
Devezas [7] proposed a model; based on this model,
Berger and coauthors [8–10] calculated the FMR spec-
tra of iron oxide nanoparticles in silicate and borate
glass matrices. Calculations of this kind give informa-
tion on the shape and size of the ferromagnetic nano-
particles in a sample. However, the above model cannot
be used for calculations under conditions of a strong
EXPERIMENTAL
Samples of [LiAl2(OH)6]2[NiEDTA] · 4H2O synthe-
magnetic anisotropy, which is characteristic of non- sized by the ion exchange of chloride ions in
spherical particles or magnetically hard materials, such [LiAl2(OH)6]Cl · nH2O for [NiEDTA]2– [12, 13] were
as cobalt. Yulikov et al. proposed a model [11] in order used in the experiments. The process of thermal decom-
to extend the above approach [7–10] to systems with a position was performed in a tube reactor under condi-
strong magnetic anisotropy. The calculation is based on tions of continuous pumping. A temperature gradient
the idea that, in the occurrence of rapid changes in the across the sample and temperature fluctuations were no
0023-1584/04/4505-0735 © 2004 MAIK “Nauka /Interperiodica”

736
YULIKOV et al.
and rapidly cooled to 20°ë. The FMR spectrum was
recorded at 20°ë, and the procedure was repeated. The
above procedure was chosen because the temperatures
at which a ferromagnetic phase was formed in the test
sample were higher than the Curie temperature of
nickel; this fact did not allow us to perform in situ mea-
surements.
340°ë
350°ë
RESULTS AND DISCUSSION
Figure 1 demonstrates the magnetic resonance spec-
tra of samples prepared at various decomposition tem-
peratures of the parent substance.
375°ë
400°ë
A ferromagnetic signal was not detected in the FMR
spectrum up to í ≈ 340°C. In the sample decomposed
at 340°C, a weak absorption line with g ≈ 2.2 and a
width of about 650 Oe was detected. As the decompo-
sition temperature of the parent substance was
increased, the line width decreased to ~500 Oe; the res-
onance position was shifted upfield by 30–50 Oe, and
the intensity increased. In this case, over the range 340–
457°C, the line intensity was approximately propor-
tional to the temperature of decomposition (Fig. 1,
insert). The FMR line shape was the same at decompo-
sition temperatures of 350–375°C; this fact is indicative
of the same nickel particle size distribution. At a
decomposition temperature of ~400°C, the shape of the
absorption line changed: the low-field wing was dis-
torted, and the line width increased. This behavior of
the spectrum can be explained based on the assumption
that the average particle size began to increase at this
temperature. The FMR line width of a powder depends
on the effective magnetic anisotropy field [14]. As the
size of the ferromagnetic particles increases, the effec-
tive averaging of a magnetic anisotropy due to thermal
fluctuations of the direction of a magnetization vector
should weaken and, correspondingly, the experimen-
tally observed absorption line width should increase.
457°ë
20
15
10
5
0
350
400
450 T, °C
0
1000 2000 3000 4000 5000
Magnetic field, Oe
Fig. 1. FMR spectra of samples prepared by the decompo-
sition of [LiAl (OH) ] [NiEDTA] · 4H O at various tem-
peratures. Insert: the dependence of the FMR signal inten-
sity on the temperature of decomposition.
2
6 2
2
more than 5°ë. The temperature was controlled with a
programmed temperature regulator. A sample placed in
a quartz cell was evacuated to a pressure of about 1–4 Pa;
next, it was heated to a required temperature for 1 h at
a certain rate and exposed for 2 h at this temperature.
After completion of the decomposition process, the
sample was cooled in the reactor with continuous evac-
uation until the reactor temperature had decreased to
room temperature.
The FMR spectra were measured on a Bruker ER-
200D EPR spectrometer at room temperature. A Radio-
pan SE/X 2547 EPR spectrometer with a high-temper-
ature attachment, which made it possible to heat the
sample in the resonator to ~400°ë, was used for mea-
surements at elevated pressures.
The experimentally observed narrowing of an FMR
line as the decomposition temperature was increased
from 340 to 350°C suggests that irregularly shaped par-
ticles, probably with a defect crystal structure, were
formed at the early stage of ferromagnetic phase forma-
tion at a low decomposition temperature. All of the
above facts should result in the broadening of an
absorption line in the case of a lower decomposition
temperature, as well as in a downfield resonance shift.
A narrow (~10 Oe in width) low-intensity line with
a g-factor of ~2.00 can be seen in the magnetic reso-
nance spectra of some samples against the background
of an FMR line. This line was due to the EPR signal of
coke deposits at some distance from the ferromagnetic
When thermal decomposition was performed particles. For coke deposits close to nickel particles or
directly in the resonator of the spectrometer, an ampule on the surface of these particles, this line will be fully
with the sample was evacuated to a pressure of about 1– broadened by local magnetic fields. As estimated, the
4 Pa with the use of a fore pump. Next, the sample was regions that give the above signal should lie at a dis-
heated to a certain temperature with continuous evacu- tance of about four or five linear particle sizes from the
ation, exposed at this temperature for a required time, nearest ferromagnetic particles.
KINETICS AND CATALYSIS Vol. 45 No. 5 2004

FERROMAGNETIC RESONANCE OF NICKEL NANOPARTICLES
737
Intensity, arb. units
Intensity, arb. units
25
25
20
15
10
5
20
15
10
5
0
50
100 150 200 250 300 350
T, °C
Fig. 3. Dependence of the intensity of the FMR line of a
sample prepared by the decomposition of
0
50
100
150
200
t, min
LiAl (OH) ] [NiEDTA] · 4H O at 400°C on the tempera-
2
6 2
2
ture at which the spectrum was measured.
Fig. 2. Dependence of the intensity of the FMR line of a
sample of [LiAl (OH) ] [NiEDTA] · 4H O subjected to
with a deviation from sphericity of about 20%. In our
calculation, we also considered the fact that a portion of
the nickel particles can be much coarser and have an
irregular shape. The insert in Fig. 4 demonstrates the
particle-size distribution obtained from calculations.
The fraction of coarser “ferromagnetic” particles is
shown separately. According to our calculations, the
coarse particles are film fragments with a great perime-
ter-to-thickness ratio. Note that, although the weight
fraction of these particles is significant, their number
should be very small, and a detailed microscopic study
is required for detecting them in a sample. The resulting
2
6 2
2
thermolysis (385°C; ~1–4 Pa) on the time of decomposi-
tion.
Figure 2 demonstrates the dependence of FMR sig-
nal intensity on the time of vacuum decomposition at
385°C. It can be seen in Fig. 2 that the FMR signal
intensity continuously increased with decomposition
time and did not reach saturation. We can conclude that
a considerable portion of nickel was not transformed
into a ferromagnetic state at the decomposition temper-
ature and in the allotted time. The shape of the absorp-
tion line remained practically unchanged with decom-
position time. At decomposition times longer than
100 min, an additional low-field feature, which corre-
sponded to the formation of a small amount of coarser
particles in a “pure” ferromagnetic state, appeared in
the spectrum.
SPR
FMR
Figure 3 demonstrates the dependence of the inten-
sity of absorption on the temperature at which the spec-
trum was measured for a sample prepared at a decom-
position temperature of 400°C. This dependence is
characteristic of particles that exhibit superparamag-
netic properties [1], and it suggests that the particle size
was no greater than 10–12 nm. The Curie temperature
of the resulting nanoparticles (600–620 K) measured
using FMR spectroscopy was close to the correspond-
ing value for bulk nickel (627.4 K [15]). It is well
known that the Curie point for nickel particles 1–2 nm
in size can be shifted toward lower temperatures by
more than 100 K [16].
2
4
6
8
10
2
1
d, nm
1000
2000
3000
Magnetic field, Oe
4000
5000
Fig. 4. (1) FMR spectrum of a sample prepared by the
decomposition of [LiAl (OH) ] [NiEDTA] · 4H O at T =
400°C and (2) the result of a numerical simulation of the
above spectrum. Insert: particle size distribution by weight
corresponding to the numerical calculation (SPR); the frac-
tion of coarse particles in a pure ferromagnetic state is
shown separately.
We performed a numerical calculation of the spec-
trum of a sample prepared at a decomposition tempera-
ture of 400°C. The simulation in terms of an approach
proposed previously [11] allowed us to describe ade-
quately the line shape (Fig. 4). In this case, we found
that the particles should be oblate ellipsoids in shape
2
6 2
2
KINETICS AND CATALYSIS Vol. 45 No. 5 2004

738
YULIKOV et al.
distribution, which has a maximum at 4–5 nm, corre-
sponding to superparamagnetic particles, is consistent
with published data [13].
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CONCLUSIONS
In this work, we studied the formation of ferromag-
netic nickel nanoparticles in the course of thermolysis
of [LiAl2(OH)6]2[NiEDTA] · 4H2O using FMR spectros-
copy. Data on the early stage of formation of a ferro-
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ACKNOWLEDGMENTS
This study was supported by the Russian Founda-
tion for Basic Research (grant nos. 02-03-33322 and
02-03-32066) and by the Siberian Division of the Rus-
sian Academy of Sciences (complex integration project
no. 33).
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KINETICS AND CATALYSIS Vol. 45 No. 5 2004