Magnetic properties of iron nanoparticles in mesoporous silica matrix

Article abstract of DOI:10.1016/j.jmmm.2005.10.116

Anisotropic iron nanoparticles were successfully synthesized within the pores of mesoporous silica matrix by intercalation of hydrophobic iron complex (Fe(CO)₅) into the hydrophobic part of the mesoporous silica-surfactant composite. It was shown that the particles are uniform and well ordered in the matrix and their shape and size are in good agreement with that of the pores. The samples demonstrate spin-glass-like behavior at low temperatures, which transforms to the superparamagnetic one with temperature increases. Nanoscale-related magnetic properties were investigated by polarized SANS. It was shown that hysteretic and the spin-glass-like behavior may be attributed to bunches of the iron nanowires with average radius of 4.7 nm.

Full text of DOI:10.1016/j.jmmm.2005.10.116

ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 300 (2006) e342–e345
www.elsevier.com/locate/jmmm
Magnetic properties of iron nanoparticles in mesoporous silica matrix
Ã
N.A. Grigorievaa, S.V. Grigorievb, H. Eckerlebec, A.A. Eliseevd, , K.S. Napolskiid,
A.V. Lukashind, Yu.D. Tretyakovd
aSt-Petersburg State University, St-Petersburg 198504, Russia
bPetersburg Nuclear Physics Institute, Gatchina, St-Petersburg 188300, Russia
cGKSS Forschungszentrum, 21502 Geesthacht, Germany
dDepartment of Materials Science, Moscow State University, Moscow 119992, Russia
Available online 15 November 2005
Abstract
Anisotropic iron nanoparticles were successfully synthesized within the pores of mesoporous silica matrix by intercalation of
hydrophobic iron complex (Fe(CO)5) into the hydrophobic part of the mesoporous silica-surfactant composite. It was shown that the
particles are uniform and well ordered in the matrix and their shape and size are in good agreement with that of the pores. The samples
demonstrate spin-glass-like behavior at low temperatures, which transforms to the superparamagnetic one with temperature increases.
Nanoscale-related magnetic properties were investigated by polarized SANS. It was shown that hysteretic and the spin-glass-like
behavior may be attributed to bunches of the iron nanowires with average radius of 4.7 nm.
r 2005 Elsevier B.V. All rights reserved.
PACS: 74.78.Na; 61.12.Ex; 61.46.+w; 81.07.ꢀb; 75.; 75.50.Ss
Keywords: Magnetic nanowires; Nanoreactors; Mesoporous silica; SAPNS
Colossal growth of digital information volume with the
evolution of information technologies resulted in the
necessity for development of novel storage schemes
providing higher capacities and response rates. The
industry of the information carriers today relies upon
magnetic information storage, and no other technology can
compete them in storage density and data access time. The
further improvement of the density requires the develop-
ment of new magnetic materials, and from a technological
point of view, the special role here belongs to high-quality
nanopatterns and nanocomposites [1]. The way for
preparation of ordered nanosystems usually involves the
use of the nanoreactors.
may be achieved by addition of shape anisotropy effect
to magnetocrystalline anisotropy, i.e. by the preparation of
the highly anisotropic nanostructures. From the other
hand, the use of the purely nanocrystalline systems is
limited because of their low stability and tendency to
form aggregates. These problems could be solved by
encapsulation of nanoparticles to a chemically inert
matrix. One of the most promising matrices for the
preparation of anisotropic magnetic nanoparticles is
mesoporous SiO2 with highly ordered uniform pore
structure [2]. Hexagonal mesoporous silica phases have
maximal anisotropy constants and could be considered as
one-dimensional.
However, very small (below 10 nm) magnetic nanopar-
ticles generally show para- or superparamagnetic proper-
ties, with a low blocking temperature and no coercivity
at the normal conditions. The increase of data stability
Therefore here we report synthesis of ordered magnetic
iron nanowires in the mesoporous silica matrix. The
method was based on the introduction of a hydrophobic
metal compound (Fe(CO)5), into the hydrophobic part of
the as-prepared mesoporous silica-surfactant composite
with the sequel decomposition to elemental iron by
annealing in the hydrogen flow at different temperatures
Ã
Corresponding author. Tel.: +7 095 939 5931; fax: +7 095 939 0998.
E-mail address: eliseev@inorg.chem.msu.ru (A.A. Eliseev).
0304-8853/$ - see front matter r 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmmm.2005.10.116

ARTICLE IN PRESS
N.A. Grigorieva et al. / Journal of Magnetism and Magnetic Materials 300 (2006) e342–e345
e343
103
102
101
T ¼ 260, 300, 350, 375 and 400 1C. Obtained samples
were denoted as Fe-SiO2-260, Fe-SiO2-300, Fe-SiO2-350,
Fe-SiO2-375 and Fe-SiO2-400, respectively.
To provide the preliminary attestation of the samples the
transmission electron microscopy (TEM) and electron
diffraction were performed. It was observed the formation
of small diameter (ꢁ1 nm) iron particles at low annealing
temperatures (260–350 1C). An increase of the annealing
temperature to 375–400 1C results in increase of the particle
size up to 2 nm in diameter and 100 nm in length [3].
The standard bulk magnetization measurements were
performed by S600 APD Cryogenics SQUID magnetometer.
Temperature dependence of magnetization was measured in
the ZFC regime (Fig. 1). A very low magnetization was
obtained for the unannealed sample. The process of
annealing leads to the appearance of the magnetization. Its
temperature variation in ZFC regime demonstrates a broad
maximum. A position and a width of this maximum depend
on the annealing temperature. The maximum in the ZFC
curve is field dependent as shown in Fig. 2. There are two
features in this dependence. First one is the cross point with
the T axis at the TE80 K, which is associated with the
transition of the sample from the superparamagnetic state at
high T to the dipole-glass state at low T. Second one is the
break at haE110 G, which means that the dipole–dipole
interaction between iron nanowires becomes negligible as
compared with the interaction of the particle with the
external magnetic field. In order to determine the saturation
magnetization Ms and coercivity of iron nanoparticles we
have measured hysteresis curves for all samples at T ¼ 4 and
300 K. The typical behavior of the magnetization vs. H is
shown in Fig. 3. The magnetic hysteresis measurements of
samples indicate that the coercivity of samples increases with
the increasing of the annealing temperature (Table 1), which
indicates the growth of nanowires.
3x101
102
Tmax,
K
Fig. 2. ha vs. Tmax, the maxima position in the ZFC curve, for the sample
Fe-SiO2-375.
6
4
2
-0.5
0.5
ha,T
-2
-4
0.15
UV-irradiated
Annealed at 260°C
Annealed at 375°C
-6
T = 300K
T = 4 K
Annealed at 400°C
0.10
Fig. 3. Hysteresis curve at T ¼ 300 and 4 K for the sample Fe-SiO2-375
obtained by field cooling.
0.05
0.00
Table 1
Magnetic data for Fe-SiO2 nanocomposites
Sample
Tmax (K)
Coercive force (Oe)
Ms at 300 K
(emu/g)
4 K
300 K
0
100
200
T, K
300
Fe-SiO2-260 50.8
Fe-SiO2-375 100.5
Fe-SiO2-400 89.1
405
560
680
11
35
95
1.52
3.66
3.82
Fig. 1. Temperature dependence of magnetization for Fe-SiO2 nanocom-
posites measured in ZFC regime at ha ¼ 100 G.

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N.A. Grigorieva et al. / Journal of Magnetism and Magnetic Materials 300 (2006) e342–e345
e344
To determine magnetic structure of the sample SAPNS
appears to be negligibly small (ꢁ1% of the nuclear one).
The polarization dependent part, attributed to the inter-
ference scattering, is determined as the difference DIðqÞ ¼
1=2ðIðq; P0Þ ꢀ Iðq; ꢀP0ÞÞ of the intensities for neutrons
polarized parallel (+) and anti-parallel (ꢀ) to the magnetic
field. The neutron scattering intensity for Fe-SiO2-375 and
pure mesoporous silica is presented in Fig. 4. There are two
features in the q- dependence of the neutron intensity I(q):
(i) the Bragg peak at qffi1.5 nmꢀ1 and (ii) an additional
scattering in the range 0.2 nmꢀ1oqo1 nmꢀ1. The inclusion
of the iron particles into the SiO2 matrix leads to an
increase of the diffraction peak intensity and to its small
shift towards smaller q. This demonstrates that iron does
not destroy matrix. We interpret an additional scattering as
the scattering of compact bunches of nanowires with radius
Rb ¼ 4:7 nm, spread DR ¼ 0:9 nm, and length L41/q. The
results of the fit are shown in Fig. 4 as solid curves. In this
case a bunch consists of six nanowires forming primitive
hexagonal structure [5,6].
experiments have been performed. The cross-section of
polarized neutrons consists of the nuclear and magnetic
contributions, as well as a nuclear-magnetic interference of
the scattering. The nuclear and magnetic cross-sections are
Fourier images of the corresponding density correlation
functions. The nuclear and magnetic scattering intensities
are proportional to the square of the density contrasts
A22hP0m iAnAmReðFnmðqÞÞ, where m is perpendicular to
?
?
n
the scattering vector component of the vector m ¼ Mp=mp
directed along the particle magnetization Mp. or A2m,
respectively. In case when AnbAm, the pure magnetic
contribution is small as compared with nuclear one and it is
difficult to extract any information on structural organiza-
tion of the samples unless there is a space correlation
between the magnetic and nuclear density correlation
functions. In this case the interference term appears as:
sintðq; P0Þ ¼ FnmðqÞ is a Furrier image of the cross
correlator of the nuclear-magnetic density [4]. As the
interference scattering sint is linear with respect magnetic
and nuclear scattering contrasts (ꢁAmAn), then it may be
much more sensitive to the parameters of the magnetic
subsystem as compare to the pure magnetic scattering (ꢁA2m).
The SAPNS experiments were carried out at the SANS-2
scattering facility of the FRG-1 reactor in Geesthacht. The
beam of polarized neutrons with initial polarization
P0 ¼ 0:96, wavelength l ¼ 0:58 nm (Dl=l ¼ 0:1) and di-
vergence of 1.5 mrad was used.
Since the magnetic scattering in these samples are small,
the magnetic amplitude can be studied through the
interference term: DIꢁAm à An. If we normalize DI by
the total scattering, we get polarization PðqÞ ¼ DI=Iꢁ
ðAm à AnÞ=A2n ¼ Am=An. In rough approximation the
polarization gives a Fourier image of the correlation
function between magnetic and nuclear density profiles.
No correlation was observed for pure SiO2 matrix, while
the function P(q) for Fe-SiO2-375 sample has a maximum
at qmaxE0.2C0.3 nmꢀ1 or Rꢁ1/qmaxE3–5 nm, which
coincide with radius Rb in the bunch model (see inset on
Fig. 4). Thus, the SAPNS data demonstrate the coexistence
of the long-range non-magnetic structure represented by
the diffraction peak at qE1.55 nmꢀ1 and of the short-range
both nuclear and magnetic ordering represented by the
An external magnetic field of 1–350 mT was applied in
the horizontal plane and perpendicularly to the incident
beam in order to reveal nuclear magnetic cross-correlator
of the system. We determine the nuclear scattering as:
IðqÞ ¼ 1=2ðIðq; P0Þ þ Iðq; ꢀP0ÞÞ. The magnetic scattering
scattering at 0.2 nmꢀ1oqo1 nmꢀ1
.
105
P = I / I
104
30
25
20
0.05
103
102
1
0.1
q, nm-1
101
100
15
ZFC (100 G)
FC (100 G)
FeSiO2-375
SiO2 empty matrix
ZFC (300 G)
FC (300 G)
10
10-1
5
0.1
1
q (nm-1)
0
100
200
300
T, K
Fig. 4. q-dependence of the full scattering intensity I(q) and the
polarization P(q) for pure mesoporous silica SiO2 and for the sample
Fe-SiO2-375 at ha ¼ 0:35 T.
Fig. 5. The temperature dependence of the integral DIint for ZFC and for
FC regimes at the magnetic field 100 and 300 G.

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e345
Thus, iron penetrates into the pores forming bunches of
the magnetic nanowires with the average radius Rb ¼
4:7 nm and spread DR ¼ 0:9 nm. The Nanoparticles have
anisotropic shape, which creates an easy axis for magnetic
moment. During the heating of ZF cooled sample the
interaction energy and the average magnetic moment of
the overall system grows up to some critical value. At the
temperature of ꢁ100 K the system transforms from the
dipole glass to the superparamagnetic state caused by
interplay of the magnetic field, dipole–dipole, and thermal
interactions. The composites represent high coercive force
up to 95 Oe (at 300 K) and saturation magnetization of
3,82 emu/g.
50
40
30
20
10
T = 8 K
The PNPI-team thanks the administration of FZC
GKSS for kind hospitality during its stay. The authors
thank RFBR (SS-1671.2003.2; 04-02-17509; 03-03-32182)
and INTAS (01-204; 03-55-1523).
0
100
200
300
ha, mT
Fig. 6. The intensity DIint as a function of field for T ¼ 8 K.
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field dependence of DIint at T ¼ 8 K presented on the Fig. 6
demonstrates the same hysteresis behavior as in SQUID
measurements (Fig. 3). But from the neutron scattering we
can conclude that magnetic properties of the system are
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