Magnetic properties of Co-impregnated zeolites

Article abstract of DOI:10.1016/j.jmmm.2009.07.046

The structure and magnetic properties of Co-containing zeolites prepared by wet impregnation were investigated. The samples were calcined and then reduced in flowing H₂. The samples studied have large saturation magnetization due to the presenc

Full text of DOI:10.1016/j.jmmm.2009.07.046

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Journal of Magnetism and Magnetic Materials 321 (2009) 3813–3820
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Magnetic properties of Co-impregnated zeolites
Ã
P.G. Bercoff a, , H.R. Bertorello a, C. Saux b, L.B. Pierella b, P.M. Botta c, Toshiyuki Kanazawa d, Ying Zhang e
a FaMAF, Universidad Nacional de Co´rdoba, and IFFAMAF, Conicet, Argentina
b CITeQ, Facultad Regional Co´rdoba, UTN, and Conicet, Argentina
c INTEMA, Conicet-UNMdP, Mar del Plata, Argentina
d JEOL USA, 11 Dearborn Road, Peabody MA 01960, USA
e Ames Laboratory, Materials Science and Engineering, Iowa State University, Ames IA 50011, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
The structure and magnetic properties of Co-containing zeolites prepared by wet impregnation were
investigated. The samples were calcined and then reduced in flowing H2. The samples studied have
large saturation magnetization due to the presence of cubic Co particles over a wide range of sizes. The
zero field cooling-field cooling curves show a sharp magnetization peak with a blocking temperature
around 7 K followed by an exponential decay and two other peaks—of much lower amplitude—around
160 and 280 K. The low temperature peak is analyzed considering first, at Tcrit, thermal relaxation
toward equilibrium over an energy barrier, with increasing viscosity S with T. Above Tcrit relaxation does
not occur and viscosity abruptly goes to zero. The behavior of the smallest Co particles is unusual above
the blocking temperature.
Received 7 May 2009
Received in revised form
17 July 2009
PACS:
75.75.+a
82.75.Vx
Keywords:
& 2009 Elsevier B.V. All rights reserved.
Co–zeolite
Wet impregnation
Magnetic property
Cubic Co
1. Introduction
porous materials as they can be potentially used in optics,
electronics, as sensors, as magnetic materials (small molecular
magnets) and in photocatalysis [5,6]. Transition metal ion-
containing molecular sieves are extensively investigated in
academic and industrial laboratories because of their promising
catalytic behavior in the oxidation of organic compounds and in
the removal of NOx [7–9]. In this respect, cobalt is an interesting
transition metal ion, not only because of its promising catalytic
activity, but also because of the rich spectroscopy of its divalent
state [10].
Nowadays, nanoscale magnetism has great scientific interest
and potential applications [1,2]. When the size of magnetic
particles is reduced to a few nanometers, they often exhibit a
number of outstanding physical properties such as giant magne-
toresistance, superparamagnetism, large coercivity, as compared
to the corresponding bulk values. Due to these unique physical
properties upon size reduction, magnetic nanoparticles contribute
to revolutionary changes in
a variety of applications from
In the case of zeolites of the type ZSM-5 (structure MFI: mirror
framework inversion; Mobil five) the diameter of the channels is
0:5120:56 nm [11]. A variety of Co species co-exists in Co–ZSM-5
prepared by ion-exchange or impregnation methods. Besides Co2þ
and ðCoOHÞþ at exchange sites, Co3O4 and Co oxo-ions have been
detected in the zeolite channels and/or on the external surface of
the zeolite microcrystals when the catalysts are heated in oxygen.
These cobalt species may exhibit different catalytic properties
[12,13].
On the other hand, cobalt–zeolites can be interesting materials
not only as catalysts, but also because of their magnetic proper-
ties. Through different thermal treatments it is possible that the
added cations keep their oxidation state or form different oxides
or hydroxides. The structure of zeolites with pores or channels of
well-defined size is particularly suitable for the study of magnetic
properties of magnetic cations introduced into the structure
biomedicine to spintronics [3,4]. In view of their technological
importance, the synthesis of magnetic systems with characteristic
nanoscale dimensions has attracted much attention in the
research field.
The uniform intracrystalline microporosity of zeolites provides
access to very large and well-defined surfaces with pore
structures which can act as a convenient compartment for a
desired reaction because, in many cases, they are able to induce
size control and in other cases they form the species encapsulated
in their cavities. Nowadays, there exists a considerable interest in
the study of nanoparticles or nanoclusters of both metal oxides
and other species, interacting with the lattice in micro- and meso-
Ã
Corresponding author. Tel.: +54 351433 4051; fax: +54 351433 4054.
E-mail address: bercoff@famaf.unc.edu.ar (P.G. Bercoff).
0304-8853/$ - see front matter & 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmmm.2009.07.046

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[6,14]. In particular, when Co cations are introduced into the
zeolite matrix by means of ionic interchange, the formation of
superparamagnetic clusters of Co cations is observed, with
blocking temperature below 7 K [14].
In this work, our aim is to study the magnetic properties of
Co–zeolite with Co cations introduced via wet impregnation,
calcined at 500 3C and reduced in H atmosphere. In this paper, we
show how the Co is associated with the zeolite matrix and
evaluate its magnetic response.
Fig. 1 shows the micrographs of sample Co2.8IR taken at 1.5 kx
(top) and at 6 kx (middle). The bright spots correspond to Co
particles of the order of 123 mm, as evidenced by the EDS
spectrum taken on a bright spot (Point 1 in Fig. 1), where
characteristic Co peaks are observed along with the Si, Al and O
peaks corresponding to the matrix. The spectrum corresponding
to Point 2 in Fig. 1 shows the characteristic energies of the matrix
and very small peaks that correspond to Co energies.
Fig. 2 shows secondary (top) and backscattered (middle)
electron images, SEI and BEI, respectively, taken at 150 kx. The
blocky faceted grains are parallelepipeds with edges of
2. Experimental
ZSM-5 zeolite (MFI structure) with Si=Al ¼ 17 ratio was
synthesized
by
hydrothermal
crystallization
in
a
Na2O2Al2O32SiO2 system using TPAOH (tetra-propyl ammonium
hydroxide) as structure-directing agent by well-known methods
[15] with some modifications. The template agent (TPA) was
removed in N2 flow (20 ml/min) from 110 to 520 3C at 10 3C=min
and calcined in air at 520 3C for 12 h to obtain the Na–ZSM-5
zeolite. The ammonium form of the catalyst (NH4–zeolite) was
prepared by ion-exchange with 1 M NH4Cl solution at 80 3C for
40 h. Co–HZSM-5 zeolites were prepared by wet impregnation of
NH4–MFI with aqueous solution of cobalt salt ðCoCl2 ꢀ 6 H2OÞ to
yield the desired wt% of the cation (2.8 Co wt%). Finally, the
samples were reduced in flowing H2 when heating from room
temperature to 500 3C at 5 3C=min and holding at 500 3C for 5 h.
The resulting samples were called Co2.8IR. The Co content was
determined by atomic absorption.
Co distribution over the matrix was studied at low magnifica-
tion with a LEO 1450 VP scanning electron microscope (SEM) and
an energy dispersive spectrometer Genesis 2000. High magnifica-
tion SEM micrographs in both secondary (SE) and transmission
(STEM) modes and energy dispersive spectroscopy (EDS) were
taken with JEOL JSM-7500F. The transmission electron microscopy
(TEM) was performed on a FEI Tecnai G2 F20 field emission
transmission electron microscope operated at 200 keV in both
scanning and transmission modes, with high angle annular dark
field (HAADF) detector for noticing Z contrast. Elemental analysis
was performed using energy dispersive spectroscopy in STEM
mode for point and line analysis. High resolution TEM (HRTEM)
imaging mode has point to point resolution o0:25 nm and line to
line resolution o0:10 nm.
Point 1
Point 2
Magnetic measurements were performed in a commercial
SQUID (Quantum Design), varying the temperature from 5 to
300 K, with applied fields ranging from 0.0025 to 0.02 T, and at
constant temperature with applied fields up to 5 T. Also,
magnetization measurements as a function of time were carried
out at selected constant temperatures with an applied field of
0.0075 T.
Co
Co
Point 1
20000
15000
10000
5000
0
Si
O
Co
Al
0
0
2
4
6
8
Crystal structure determinations were performed by means of
Energy (keV)
X-ray diffraction patterns in a Philips diffractometer using CuK
radiation.
a
Si
Point 2
50000
40000
30000
20000
10000
0
Co
O
3. Results
Al
Co
Co
Co
3.1. Structural and microstructural characterization
2
4
6
8
Energy (keV)
The structural characterization of the prepared samples was
previously published [16]. Here we mention only those aspects
which are relevant to the magnetic properties.
Fig. 1. Low magnification SEM micrographs. Top: 1.5 kx; the bar corresponds to
20 m. Middle: 6 kx; the bar corresponds to 2 m. The bright spots on the zeolite
m
m
surface are Co particles; Point 1 marks a bright spot and Point 2 marks a dark zone.
Bottom: EDS spectra corresponding to Points 1 and 2. Both the characteristic lines
of Co (very intense) and the matrix are clearly seen in Point 1 spectrum, while the
other shows the characteristic lines of the matrix and very small peaks of Co
energies.
X-ray diffractograms of the samples confirm the orthorhombic
symmetry and crystallinity of the synthesized materials [16]. No
Co oxides have been detected and metallic Co appears in a cubic
phase, with mean crystallite size of 30 nm.

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co2.81R
SEI
co2.81R
Point 2
Point 1
BEI
Fig. 3. SEI and BEI taken at 200 kx. The bar corresponds to 100 nm. The particles
over the zeolite blocks are Co particles of around 25 nm and are noticed in the BEI
micrograph.
Full scale counts: 3725
Point 1
3000
Point 2
is observed in BEI) intense characteristic Co peaks occur. This Co
particle has a size of 60 nm. Associated with Point 2 there is only a
very small signal from Co, indicating a low Co concentration in
that position. Further comments will follow after the analysis of
Fig. 5.
O
2000
Si
Fig. 3 shows Co particles of 25 nm on top of a zeolite particle,
and the images of Fig. 4 (SEI, BEI and STEM from top to bottom)
can resolve several Co particles of 10 nm, all of them on the
surface of the zeolite grain.
Si
1000
C
C
Co
Co
High magnification TEM micrographs were taken in order to
investigate the existence of Co particles smaller than 10 nm.
Fig. 5a shows that there is a cluster of very fine metal particles,
clearly not in the grains but on the surfaces (inside the marked
square). The high magnification STEM image of Fig. 5b shows that
the Co particles lie on the surface and are only a few nanometers
across. This is the kind of particles which might originate
the small Co peak observed in the spectrum corresponding to
Point 2 in Fig. 2. Larger magnifications with TEM were not
possible to achieve because the samples are quickly damaged by
the electron beam.
Al
Al
CI
0
2
3
2
1
3
1
keV
keV
Fig. 2. SEI (top) and BEI (middle) taken at 150 kx. The bar corresponds to 100 nm.
The EDS spectra at the bottom correspond to Points 1 and 2, respectively, indicated
with white arrows in the BEI micrograph. The particle indicated in Point 1 is a Co
particle and has a size of 60 nm.
approximately 300 nm and correspond to the zeolite matrix. The Z
contrast in the BEI micrograph allows to identify small Co
particles over the surface of the zeolite. The EDS spectra of
Points 1 and 2 are shown at the bottom. In both spectra the
characteristic peaks corresponding to the matrix are seen, but
only in the spectrum obtained from Point 1 (where the bright spot
In summary, Co was introduced in ZSM-5 via wet impregna-
tion, heat-treatment and reduction in H atmosphere. Metallic Co
is found in a wide range of particle sizes over the surface of the
much larger zeolite particles. We have been able to detect groups

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Fig. 5. (a) STEM image of a cluster of small Co particles. Bar ¼ 100 nm. (b) STEM
image (acquired with HAADF detector) of the smaller features of the studied
sample. Particles smaller than 10 nm are noticed on the surface. Bar ¼ 10 nm.
3.2. MðTÞ and MðH=TÞ
Fig. 4. SEI and BEI taken at 300 kx (top and middle). The bar corresponds to 10 nm.
The bottom image is a scanning transmission electron micrograph (STEM) of the
same portion as shown above.
Magnetization vs. temperature measurements were performed
by means of the standard zero field cooling-field cooling (ZFC–FC)
procedure with an applied field of 0.01 T.
Fig. 6 shows the ZFC–FC curves for sample Co2.8IR. The FC and
ZFC curves do not coincide before 300 K. In the ZFC curve there is a
sharp maximum at T ¼ 7 K and two wider peaks around 160 and
280 K of much lower intensity (approximately 7% of the 7 K peak)
which are hardly noticed in Fig. 6 due to the scale of the y axis. The
inset in this figure shows an abrupt decrease of the first peak,
which cannot be fitted either with Brillouin or with Langevin
of Co particles ranging 223 nm (Fig. 5b), 10215 nm (Figs. 4 and
5a) and others of the order of 50 nm (Figs. 2 and 3). Also micron-
sized particles of about 1–3 mm have been observed (Fig. 1).
Although all the evidence indicates that Co is on the surface of the
zeolite, it is not possible to disregard that within the zeolitic
matrix small quantities of Co may be present, but are not resolved
in the micrographs.

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5.5
5.0
4.5
4.0
3.5
3.0
2.5
2.0
150
4.0
3.6
3.2
2.8
2.4
125
100
75
50
25
0
ZFC
10
20
30
T [K]
T=5K
T=10K
T=40K
T=160K
ZFC
FC
H=0.01 T
0.0
0.1
0.2
0.3
0.4
0
50
100
150
200
250
300
H/T [Tesla/K]
T [K]
Fig. 8. M vs. H=T. No universal curve is found, due to several contributions to the
Fig. 6. ZFC–FC magnetization curves. The inset shows in detail the low
temperature peak, with magnetization decreasing as expðꢁT=T1Þ, with a decay
constant T1 ¼ 3:5 K.
magnetization.
Magnetization measurements at constant temperature were
performed at 5, 10, 40 and 160 K.
6.9
6.8
6.7
6.6
6.5
6.4
6.3
The MðH=TÞ experimental data do not converge to a universal
curve (see Fig. 8) and the observed differences can be attributed to
several clusters contributions that add up as the temperature
increases. These contributions are due to the particle size
distribution of cubic metallic Co.
3.3. MðtÞ
Some measurements of magnetization relaxation at low
temperature were performed on the sample in order to clarify
the nature of the low temperature peak observed in the MðTÞ-ZFC
curve. All the relaxation measurements were performed after
demagnetizing at 40 K, in the following modes:
6.2
Linear fit: TB =6.83 - 33 x 10-3 H
(i) After ZFC from 40 K down to the selected temperature, a field
of 0.0075 T was applied and MðtÞ was registered (target
temperature reached on cooling).
(ii) After ZFC from 40 K down to a temperature below 10 K, a field
of 0.0075 T was applied. While measuring magnetization, the
temperature was raised above 10 K at a rate of 2 K/min up to
the selected temperature, where MðtÞ was registered (target
temperature reached on heating).
6.1
0
5
10
15
20
H [x 10-3 Tesla]
Fig. 7. Blocking temperature TB as a function of the applied field H.
(iii) After FC with a field of 0.0075 T, from 40 K down to the
selected temperature, MðtÞ was measured (target tempera-
ture reached on cooling).
(iv) After FC with a field of 0.0075 T, from 40 K down to 5 K, the
temperature was then raised up to the selected value and
then MðtÞ was measured (target temperature reached on
heating).
functions but an exponential of the type expðꢁT=T1Þ, with a decay
constant T1 ¼ 3:5 K. The largest contribution to the magnetization
is a high ferromagnetic constant background, which is also
observed at room temperature, due to large (micron-sized)
ferromagnetic particles. The contribution of two different kinds
of clusters is superimposed onto the background, with blocking
temperatures of 160 and 280 K, corresponding to the broad
maxima observed at these temperatures. The FC curve does not
coincide with the ZFC curve from 10 K up to room temperature
because of the ferromagnetic contribution of the micron-sized
particles.
The results of measurements (i) are shown in Figs. 9a and 10b,
while measurements (ii) and (iii) are shown in Figs. 10a and 9b,
respectively.
Some remarks are worth mentioning:
Measurements of the low-temperature peak in the ZFC curve
were performed with different applied fields in order to study the
blocking temperature dependence with the field. There is a
decrease of the blocking temperature with increasing applied
field (see Fig. 7). For H ¼ 0:1 T the blocking temperature decreases
ꢂ
ZFC, To10 K, measurements in mode (i), Fig. 9a: The relaxation
expected over an energy barrier is observed. The total
relaxation for a fixed time is higher at lower temperatures
and consistent with the lowering of the ‘‘driving force’’ ðM ꢁ
MeqÞ as we approach T ¼ 10 K, where MZFC ¼ MFC. It may be
below 5 K.
A line can be fitted with these data and the
corresponding analysis is left to the discussion section.

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0.30
0.25
0.20
0.15
0.10
0.05
0.00
0.05
0.04
0.03
0.02
0.01
T=5 K
T=6 K
T=7 K
T=8 K
T=5 K
T=6 K
T=7 K
T=8 K
0.00
ZFC
FC
6
0
2
4
6
8
10 12 14 16
0
2
4
8
10 12 14 16
t [min]
t [min]
Fig. 9. M vs. t for different temperatures. (a) After ZFC in mode (i). (b) After FC in mode (iii).
on heating
on cooling
6.45
6.40
6.35
6.30
6.25
6.20
6.15
6.10
6.05
6.00
5.95
5.90
5.85
6.10
6.08
6.06
T=12 K ZFC
T=15 K ZFC
T=17 K ZFC
T=12 K FC
T=15 K FC
T=17 K FC
5.68
5.66
5.64
5.62
5.60
5.58
5.56
T=12 K ZFC
T=15 K ZFC
T=17 K ZFC
0
2
4
6
8
10 12 14 16
0
2
4
6
8 10 12 14 16
t [min]
t [min]
Fig. 10. M vs. t for sample Co2.8IR, for different temperatures, reached on heating (modes (ii) and (iv) in part (a)) and on cooling (mode (i) in part (b)).
noticed that the total relaxation in 15 min is 324% of the total
which is completed in 223 min and the other one exhibits a
similar behavior to that observed at 5 K in the ZFC measure-
ments. We think this is due to an experimental effect, because
the equipment took longer when trying to stabilize the
temperature at 5 K and magnetization relaxed while the
temperature stabilized, so that fast initial relaxation could
not be recorded.
relaxation needed to reach the equilibrium state. Initially, there
is a fastest change of magnetization with time and after some
minutes a lnðtÞ regime is reached.
ꢂ
FC, To10 K, measurements in mode (iii), Fig. 9b: The results
are similar to those for ZFC measurements but a much smaller
1
20
relaxation is observed, about
of the value obtained for the
ZFC relaxation. This is an indication that the equilibrium state
is close to the measured value. Except for T ¼ 5 K, the
relaxation occurs in two stages—the first one at a high rate
ꢂ
ZFC and FC, T410 K, measurements in modes (ii) and (iv), Fig.
10a: When the target temperature is reached on heating, both
the ZFC and the FC measurements are coincident and do not

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show any relaxation, which clearly means that the obtained
states are the equilibrium ones. The same is observed for
measurements in mode (iv), FC, for To10 K, (not shown): in
this case, no relaxation was observed for any temperature and
the M values correspond to the MðTÞ-FC curve. This is the
reason why we take the MðTÞ-FC curve (measured on heating)
as the equilibrium curve MeqðT; HÞ, for all temperatures.
ZFC, T410 K, measurements in mode (i), Fig. 10b: A relaxation
is observed. The initial magnetization is about 95% of the
equilibrium values observed in Fig. 10a. It is intriguing that the
equilibrium value at T410 K is reached instantly on heating
but not on cooling.
H=0.0075 T
1.0
0.5
0.0
ꢂ
0.1
0.0
-0.1
-0.2
4. Discussion and comments
Tcrit
TB
According to the results obtained by XRD, EDS, Raman
spectroscopy and TPR experiments (which confirmed the absence
of cobalt oxide in the reduced samples) [16] we find that cubic Co
is responsible for the high magnetization of the studied samples,
amounting 135 emu/gCo at a field of 2 T and at 5 K. This value of M
is close to the saturation magnetization value of cubic Co at 0 K
(166 emu/g) [23].
The MðTÞ-ZFC curve (Fig. 6) shows a prominent feature—the
appearance of a well-defined magnetization peak at low tem-
perature followed by an exponential decay. The small width of
this peak indicates that the size distribution of the particles which
originate it is also narrow. The contribution of this peak to the
total magnetization is comparable to the background, indicating
that there is a very large number of the smaller particles that are
responsible for this peak. Two smaller peaks are also observed at
around 160 and 280 K, which we assign to cubic Co particles of
larger size.
4
3
2
1
0
5
10 15 20 25 30 35 40
T [K]
Fig. 11. MðTÞ vs. T and dM=dT vs. T, ZFC curve. (a) MðTÞ corresponding to the low
temperature peak after background substraction. (b) dM=dT, where the blocking
temperature TB is defined as the temperature where dM=dT ¼ 0 and the
temperature Tcrit where d2M=dT2 ¼ 0. (c) Viscosity constant SðTÞ.
magnetic viscosity which is independent from the initial and
final states is [27]
In Fig. 11, part (a) shows the low temperature peak appearing
in the ZFC curve, after background subtraction. The background is
constant and produced by other ferromagnetic contributions due
to large particles of cubic Co. Part (b) shows dM=dT, where the
blocking temperature TB is defined as the temperature where
dM=dT ¼ 0, and the temperature Tcrit where d2M=dT2 ¼ 0.
We consider that two processes take place at low temperature,
which give rise to this peak:
1
dM
S ¼ ꢁ
;
ð1Þ
M0 ꢁ Meq dðlnðtÞÞ
where M0 ¼ MðT; HÞ is the value of magnetization at the beginning
of the relaxation process and MeqðT; HÞ is the final state.
Using the results given in Figs. 9a and 10a, S values are
calculated for the ZFC measurements with Eq. (1) and are plotted
in Fig. 11c. For ToTB the behavior of S vs. T is similar to the results
(1) For temperatures ToTcrit the obtained states in ZFC experi-
ments are not equilibrium states (see Fig. 9a) but evolve
toward the equilibrium state with magnetization MeqðT; HÞ
given by the FC measurements. It is this process that produces
a blocking temperature TB.
observed for ferritin protein samples [28] and
[29]. For T4Tcrit the magnetic viscosity vanishes, demonstrating
that the achieved states are indeed equilibrium states.
aꢁFe203 particles
It can be seen that the magnetic viscosity S rises monotonically
with temperature for ToTcrit
. This is consistent with the
assumption that relaxation is produced by thermal activation
over an energy barrier.
(2) For T4Tcrit the obtained states in ZFC experiments are
equilibrium states (see Fig. 10a). The magnetization of the
particles which originate this peak decreases exponentially
with T, down to Meq ¼ 0.
At T ¼ Tcrit an abrupt change toward S ¼ 0 occurs and for
T4Tcrit, no relaxation is observed. The measured magnetization is
fitted by
Both types of behavior are clearly distinguished when plotting
dM=dT as a function of temperature, as shown in Fig. 11, for an
applied field H ¼ 0:0075 T.
MðT; HÞ ¼ M0 expðꢁT=T1Þ;
ð2Þ
T1 being a decay constant. This behavior differs from a Curie-type
law, which would correspond to paramagnetic particles, and
needs further study.
The appearance of a blocking temperature TB is interpreted as
thermal activation over an energy barrier. In the low applied field
approximation, the energy barrier that the magnetic system must
overcome by thermal activation is
The ZFC–FC curves depend on the magnetic relaxation and the
magnetization is the result of cumulated relaxation effects up to
the time when the magnetization is measured at temperature T.
The observed relaxation both in the ZFC and in the FC
measurements is neither purely exponential nor logarithmic in
time—there is an initial fast relaxation that is completed in
324 min and a lnðtÞ behavior is observed afterwards. We think
this is due to the existence of non-interacting particles with some
particle-size distribution. Considering the narrow peak observed
in the ZFC curve, this distribution must also be narrow. In the limit
H5Ha, Ha being the anisotropy field, the definition of the
DE ¼ K1V ꢁ MH;
ð3Þ
K1 being the anisotropy constant, V the volume, M the total
magnetization of volume V and H the applied field.

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Then, when the relaxation is produced by thermal activation
over an energy barrier given by Eq. (3), using that
[25], where kB is Boltzmann constant, the following relation can
paramagnetic in nature. Larger nanometric particles with particle
DE ꢃ 25kBTB
size of the order of 10 nm make a smaller contribution at higher
temperatures. Particles larger than 15 nm have blocking tempera-
ture above room temperature and contribute to the large
ferromagnetic background. A magnetization peak is observed in
the ZFC curve, which is interpreted as the result of two relaxation
mechanisms: below Tcrit a relaxation toward Meq given by the FC
curve and governed by a thermal activation over an energy barrier.
Above Tcrit an exponential dependence of MðTÞ toward Meq ¼ 0 is
observed. This is consistent with a spin reorientation mechanism.
be obtained:
K1V
M
TB
¼
ꢁ
H:
ð4Þ
25kB 25kB
From the fitting of the experimental data of TB vs. H (Fig. 7) the
slope and y intercept can give us K1V and M. We estimated the
value of V by considering the responsible of this low-temperature
behavior is cubic Co. We were led to this by the evidence given in
the previous characterization study [16] and the results of Section
3.1. So, if we use the atomic volume of Co vat ¼ 1:11 ꢄ 10ꢁ29 m3/at
and the number of Bohr magnetons per atoms for the metallic
Acknowledgments
state of Co n0 ¼ 1:714
mB/at [23] we end up with 869 atoms per
This work was partially funded by Consejo Nacional de
cluster (V ¼ 9:6 ꢄ 10ꢁ27 m3; dꢅ2:6 nm). With these values, and
using the independent term of Eq. (4), we find K1ꢅ3 ꢄ 105 J=m3.
This anisotropy is somewhat lower than the value expected for
bulk metallic Co (5:3 ꢄ 105 J=m3 [23]), but we must keep in mind
that here we are dealing with small volume clusters, in which the
ratio of atoms at the surface with respect to the bulk is very high
and this leads to canting of atomic magnetic moments at the
surface, thus reducing magnetic anisotropy. This effect has already
been observed in different systems of nanometric particles [26]. It
is remarkable that in our case such small particles are not
superparamagnetic above TB.
ꢀ
´
Investigaciones Cientıficas y Tecnicas (CONICET), Argentina, PIP
´
no. 6452 and PIP no. 6313/05; Agencia Nacional de Promocion
´
´
´
Cientıfica y Tecnologica, Argentina, PICT no. 12-14657; Secretarıa
´ ´
y Tecnologıa, Universidad Nacional de Cordoba,
de Ciencia
Argentina; UTN-PID 25E092; the Ministry of Science and Educa-
tion of Spain (MEC) under Project MAT 2004-05130-C02-01. The
work at the Ames Laboratory was supported by the Department of
Energy, Office of Basic Energy Sciences, under Contract no. DE-
AC02-07CH11358. P.G. Bercoff is indebted to M.J. Kramer, from
Ames Laboratory, for his invaluable help with TEM/STEM images.
From the blocking temperatures that correspond to the peaks
of 160 and 280 K we estimate the sizes of the particles that
originate these peaks. The result is d160ꢅ7 nm and d280ꢅ9 nm for
the lower and higher temperatures, respectively.
The larger particles that are visible in our SEM micrographs
have sizes of the order of 1 mm and their blocking temperature is
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The magnetic behavior of Co impregnated ZSM-5 zeolites was
studied. Reduced samples were prepared and their magnetic
properties were measured as a function of temperature and
applied magnetic field.
Co particles were detected by SEM and TEM with a significant
size distribution, which are responsible for the different magnetic
contributions. We observed that micron-sized particles of metallic
Co are responsible for the high observed values of saturation
magnetization and nanoparticles of ꢅ2:6 nm give rise to a large
magnetic contribution at low temperature, which is not super-