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EffectofaSiO2coatingonthemagnetic
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Effect of a SiO2 coating on the magnetic properties of Fe3O4 nanoparticles

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IOP PUBLISHING

JOURNAL OF PHYSICS: CONDENSED MATTER

J. Phys.: Condens. Matter 24 (2012) 266007 (6pp)

doi:10.1088/0953-8984/24/26/266007

Effect of a SiO2 coating on the magnetic
properties of Fe3O4 nanoparticles
S Larumbe, C G´omez-Polo, J I P´erez-Landaz´abal and J M Pastor
Departamento de Fisica, Universidad P´ublica de Navarra, Campus de Arrosad´ıa, 31006 Pamplona, Spain
E-mail:

Received 16 December 2011, in final form 26 April 2012
Published 14 June 2012
Online at stacks.iop.org/JPhysCM/24/266007
Abstract
In this work the effect of a SiO2 coating on the magnetic properties of Fe3 O4 nanoparticles
obtained by the sol–gel method is analyzed. Two sets of samples were prepared: Fe3 O4
nanoparticles and Fe3 O4 @SiO2 core–shell composites. The samples display the characteristic
spinel structure associated with the magnetite Fe3 O4 phase, with the majority of grain sizes
around 5–10 nm. At room temperature the nanoparticles show the characteristic
superparamagnetic behavior with mean blocking temperatures around 160 and 120 K for
Fe3 O4 and Fe3 O4 @SiO2 , respectively. The main effect of the SiO2 coating is reflected in the
temperature dependence of the high field magnetization (µ0 H = 6 T), i.e. deviations from the
Bloch law at low temperatures (T < 20 K). Such deviations, enhanced by the introduction of
the SiO2 coating, are associated with the occurrence of surface spin disordered effects. The
induction heating effects (magnetic hyperthermia) are analyzed under the application of an AC

magnetic field. Maximum specific absorption rate (SAR) values around 1.5 W g−1 were
achieved for the Fe3 O4 nanoparticles. A significant decrease (around 26%) is found in the
SAR values of the SiO2 coated nanocomposite. The different heating response is analyzed in
terms of the decrease of the effective nanoparticle magnetization in the Fe3 O4 @SiO2
core–shell composites at room temperature.
(Some figures may appear in colour only in the online journal)

1. Introduction

biocompatible organic or inorganic coatings are employed
(polymers, dextran, chitosan or silica [2, 4, 5]). Silica coating
is relatively easy to prepare through a sol–gel method using
metal alkoxides as precursors. This synthesis procedure is
able to synthesize in one step the magnetite nanoparticles
with the silica coating and optimize the size of nanoparticles
and their structure through the synthesis conditions. However,
the immersion of the magnetic nanoparticles within the
silica matrix is usually associated with a decrease in the
net magnetization [6, 7]. This effect is correlated with the
occurrence of surface spin disorder induced by the silica
coating. Such a core–shell structure modifies the magnetic
response of the nanoparticles and determines their magnetic
characteristics.
In this work, the effect of the silica coating on the
magnetic properties of magnetite nanoparticles is studied.
Two samples of Fe3 O4 and Fe3 O4 @SiO2 were synthesized by
a sol–gel method. The magnetic properties of both samples

Ferrite nanoparticles have been extensively explored for
different applications, for example in magnetic hyperthermia

due to their biocompatibility and for their special magnetic properties as nanosized systems. Different synthesis
methods are employed in the preparation of magnetite
nanoparticles, including chemical methods (sol–gel, coprecipitation, hydrothermal. . . ) or physical methods (laser
ablation, mechanical ball milling, sputtering) [1–3]. For
biomedical applications, the nanoparticle systems should be
biocompatible and easily eliminated by the organism. In this
sense, it is of great importance to prevent their agglomeration
by reducing the particle diameter to the superparamagnetic
range [1]. Hence, two aspects are relevant: the size of
the magnetic nanoparticles and the magnetic interaction
between them. Furthermore, to combine the biocompatible
characteristics and reduce particle agglomeration, different
0953-8984/12/266007+06$33.00

1

c 2012 IOP Publishing Ltd Printed in the UK & the USA


J. Phys.: Condens. Matter 24 (2012) 266007

S Larumbe et al

were analyzed and their self-heating features (magnetic
hyperthermia) were evaluated in terms of the values of the
specific absorption rate (SAR).

2. Experimental procedure
All reagents supplied by Sigma Aldrich were of analytical
grade and were used without any further purification. Ferrite

nanoparticles were synthesized via an autocombustion sol–gel
method. Citric acid was used as a fuel to promote the
crystallization of the spinel through its combustion and
ferric nitrate was employed as the chemical precursor
salt. After the hydrolysis of the nitrate in distilled water
with a hydrolysis rate of 130, citric acid with a molar
ratio of 1:2 and 1:1 (ferric nitrate:citric acid) for Fe3 O4
and Fe3 O4 @SiO2 nanoparticles, respectively, was added as
fuel. These molar ratios were found to be the optimum
values to obtain a single spinel phase. Afterwards, an
ethanolic solution of the silica precursor, TEOS (tetraethyl
orthosilicate), was dropped into the dark brown solution to
give the SiO2 composite. Taking into account the relative
molar concentrations (ferric nitrate:TEOS) employed during
the process, the SiO2 content was approximately 23% by
weight with respect to the magnetite weight. The reaction
was performed in an acid medium (pH ≈ 1) to reduce the
nanoparticle diameter (decrease in the condensation rate).
The calcination temperature leading to the decomposition
of the organic matrix was determined by thermogravimetric
analysis (HI-RES 2950 TA Instruments). The dark brown
as-synthesized gel was calcined in an inert atmosphere
to prevent the oxidation of the sample to maghemite
or hematite. The structure of the calcined samples was
analyzed by x-ray diffraction (Siemens D-5000) with Cu Kα
˚ and the mean diameter was estimated
radiation (1.5418 A)
through the Debye–Scherrer equation. Transmission electron
microscopy (TEM; JEM 2100 HT at the ICTS Centro
Nacional de Microscop´ıa Electr´onica UCM, Spain) confirms

the nanometric size of the calcined nanoparticles. Magnetic
measurements were carried out with a SQUID magnetometer
(Quantum Design MPMS XL7). The induction heating curves
under an AC field (magnetic hyperthermia) were measured
with a home-made set up, composed of a water refrigerated
coil with six turns (N = 100 turns m−1 ) connected to
a 2 kW RF power amplifier (Electronic and Innovation,
model 1240L). The temperature rise of the nanoparticles
(powder) was measured with a fiber optic thermometer
(Neooptix, model T1) under an AC magnetic field (amplitude
170–340 Oe, frequency 340 kHz).

Figure 1. TG thermograms (—) and derivative curves (◦) for (a)
Fe3 O4 and (b) Fe3 O4 @SiO2 gels.

the derivative curves a deeper analysis about the effect of
the silica matrix in the decomposition temperatures can
be performed. Both samples display a peak below 200 ◦ C
related to the evaporation of solvents. The second peaks
observed at 238 ◦ C and 218 ◦ C for Fe3 O4 and Fe3 O4 @SiO2 ,
respectively, correspond to the oxidation of citric acid by
nitrates, whose heat combustion leads to the crystallization
of the spinel phase [8, 9]. The third peak at 373 ◦ C and
334 ◦ C (Fe3 O4 and Fe3 O4 @SiO2 , respectively) is due to
the decomposition of the excess of citric acid added to
the initial solution, taking into account that the molar ratio
between the citric and iron salts was 2:1. Finally, the peak
at 635 ◦ C only observed in the precursor gel of the Fe3 O4
sample would correspond to the dehydroxylation of the
OH groups located at the surface of the nanoparticles [10].

In the Fe3 O4 @SiO2 nanoparticles, the covalent interaction
between the Si atoms and the magnetite surface decreases
the number of surface hydroxyl groups at the surface, giving
rise to a decrease in the final decomposition temperature
at which all the hydroxyl groups disappear [10]. Thus,
according to the present thermogravimetric characterization
(see figure 1), optimum calcination temperatures (complete
decomposition of the organic matrix) of 650 ◦ C and 400 ◦ C
are obtained for Fe3 O4 and Fe3 O4 @SiO2 , respectively.
Accordingly, the as-synthesized gels were calcined at the
indicated temperatures for 2 h. In order to prevent the

3. Results and discussion
Firstly, in order to estimate the optimum calcination
conditions associated with the decomposition temperature of
the solvents and the organic matrix, the thermograms (TG)
of the initial gels were analyzed employing heating and Ar
flow rates of 20 ◦ C min−1 and 60 ml min−1 , respectively.
Figure 1 shows the TG (weight loss) and the derivative
curves of the gels for both nanoparticle systems. Through
2


J. Phys.: Condens. Matter 24 (2012) 266007

S Larumbe et al

At room temperature both samples display the characteristic
anhysteretic behavior of superparamagnetic nanoparticles.
The decrease of temperature promotes a noticeable increase

in the coercivity, HC , as a result of the blocking of the
nanoparticle magnetization (see figure 4(b)). Figure 5 displays
the temperature dependence of HC for both nanoparticles. The
coercive field displays the characteristic temperature decay
of superparamagnetic systems. In fact, a mean estimation of
the blocking temperature, TB , can be performed through the
fitting of the temperature dependence of HC to the T 1/2 law of
uniaxial non-interacting single domain particles [12]:
HC = HK 1 −
Figure 2. X-ray diffraction patterns for the calcined samples.

T
TB

1/2

(1)

where HK = 2Keff /MS , with Keff the effective anisotropy
constant and MS the saturation magnetization. The solid lines
in figure 5 represent the data fitting according equation (1)
with the following fitting parameters: (i) Fe3 O4 , HK = (670 ±
50) Oe, TB = (160 ± 20 K); (ii) Fe3 O4 @SiO2 , HK = (530 ±
40) Oe, TB = (120 ± 10 K). The slight increase in TB for
the Fe3 O4 sample would correspond to the higher value of
the mean nanoparticle size with respect to the Fe3 O4 @SiO2
sample. However, a clear dispersion in the coercivity fitting
for both nanoparticles for measuring temperatures above the
estimated TB should be noticed. This behavior could be
explained in terms of the contribution of the nanoparticles

with higher mean size. However, it should be kept in mind that
equation (3) is strictly valid for uniaxial non-interacting single
domain particles. Thus, the occurrence of magnetic (dipolar)
interactions would also contribute to the detected behavior.
In order to analyze this effect in further detail, the zero
field cool–field cooled (ZFC–FC) magnetization curves were
analyzed under an applied magnetic field of 5 mT. As figure 6
shows, the ZFC–FC magnetization curves do not display
the characteristic features of a defined superparamagnetic
blocking temperature. The irreversible behavior found for
temperatures above the estimated TB should be associated
with the magnetization contribution of the larger blocked
nanoparticles. In fact, some features of the Verwey transition
are detected around TV ≈ 120 K in the Fe3 O4 sample
with higher mean nanoparticle size [13]. Additionally, the
occurrence of strong interactions between the calcined
nanoparticles would also contribute to the anomalous shape
of the ZFC–FC curves.
However, the main effect of the silica coating is detected
in the temperature dependence of the high field magnetization
(measured at µ0 H = 6 T). As figure 7 shows, at low
temperatures the Bloch law, MS (T) = MS (0)(1 − BT α ), with
MS (0) the saturation magnetization at 0 K and B the Bloch
constant, is not followed and the experimental data can
be suitably fitted through the introduction of an additional
exponential term [14, 15]:

oxidation of the sample to maghemite or hematite an inert
atmosphere was employed during the calcination procedure.
Figure 2 shows the x-ray diffraction patterns of the

calcined Fe3 O4 and Fe3 O4 @SiO2 nanoparticles. A single
spinel phase (PDF card number 01-089-0691 from the
PDFWIN database), with lattice parameters close to the
˚ is observed in both
reported bulk magnetite (a = 8.39 A)
˚ for Fe3 O4
calcined nanoparticles: a = 8.39 and 8.40 A
and Fe3 O4 @SiO2 , respectively. Lattice parameters were estimated using the Bragg law reflection peaks. A linear decrease
in the lattice parameter of the spinel is reported with the
˚ (magnetite:
oxygen vacancies (δ; Fe3(1−δ) O4−δ ) from 8.39 A
˚
Fe3 O4 ) to 8.35 A; maghemite: Fe2 O3 ) [11]. This result
confirms the Fe3 O4 composition of the calcined nanoparticles
and therefore the effectiveness of the heating procedure in
an inert atmosphere to prevent the oxidation of the samples.
On the other hand, the large width of the diffraction peaks
indicates the nanometric size of the samples. Mean values
of crystallite size, d, of 12 and 11 nm were obtained
for Fe3 O4 and Fe3 O4 @SiO2 , respectively, employing the
kλ
Debye–Scherrer equation, d = β cos
θ , where k = 0.9, λ is the
˚ β is full width at
wavelength of the Cu Kα line (1.5418 A),
half maximum and θ is the diffraction angle corrected with
the instrumental width (βinst ); β = βexp − βinst ).
In order to check the morphology and mean crystallite
size, the calcined nanoparticles were analyzed by TEM.
Figure 3 shows the TEM micrographs for the Fe3 O4

and Fe3 O4 @SiO2 nanoparticles (figures 3(a) and (c),
respectively). As can be seen, low particle size dispersion
is detected with mean sizes close to the estimated values
by x-ray diffractometry (d = 5 and 7.5 nm for Fe3 O4 and
Fe3 O4 –SiO2 , respectively). Thus, the close match between
the estimated x-ray and TEM sizes indicates the single
crystalline structure of the nanoparticles. Energy dispersive
x-ray analysis (EDX) confirms the presence of the SiO2 in
the Fe3 O4 @SiO2 nanoparticles (see figure 3(d)).
With respect to the magnetic behavior of the samples,
the hysteresis loops were analyzed in the temperature range
from 5 to 300 K using a maximum applied magnetic
field, µ0 H = 6 T. As an example, figures 4(a) and (b)
display the hysteresis loops of the calcined nanoparticles
at high (300 K) and low (5 K) temperatures, respectively.

( −T
T )

MS (T) = MS (0)[(1 − BT α ) + A0 e

f

].

(2)

This deviation is explained by the presence of spin
surface disordered effects and the occurrence in the
nanoparticles of a core–shell structure. Thus, the spins

3


J. Phys.: Condens. Matter 24 (2012) 266007

S Larumbe et al

Figure 3. TEM images for (a) Fe3 O4 and (c) Fe3 O4 @SiO2 nanoparticles. EDX analyses are shown in (b) and (d) images.

Figure 5. Coercive field, HC , versus temperature, T.

at the surface will display a disordered state mainly due
to the broken exchange bonds at the nanoparticle surface
(shell) area [16–20]. Accordingly, the constant A0 in
equation (2) would represent a measurement of the fraction
of the disordered surface and Tf the characteristic freezing
temperature below which the deviations are observed. Table 1
displays the obtained fitting parameters for both samples.
It should be remarked that the estimated saturation
magnetization MS (0) is below the reported value in bulk
magnetite (MS (0) = 92 emu g−1 ) [21]. Such a decrease
is inherent in nanoparticle systems and is correlated to
the existence of spin disordered effects [17]. Moreover,
the best fitting is obtained in both samples for α = 2,

Figure 4. Hysteresis loops (M–H) at (a) 300 K and (b) 5 K for both
nanoparticles.

4

J. Phys.: Condens. Matter 24 (2012) 266007

S Larumbe et al

Figure 8. Temperature rise, T, versus time, t, under an AC
magnetic field (initial temperature 18 ◦ C).

Figure 6. ZFC–FC magnetization (M) curves (applied magnetic
field 5 mT).

and subtracting the 23% weight of the SiO2 coating, the
saturation magnetization in this sample would reach a value
of 48 emu g−1 . This value is still far below the estimated
magnetization in the Fe3 O4 system (MS (0) = 72 emu g−1 ;
see table 1).
With respect to the induction heating effects (magnetic
hyperthermia), figure 8 shows the temperature rise, T,
versus time, t, in both nanoparticle systems under the
application of an AC magnetic field (amplitude, HAC =
340 Oe and frequency, f = 340 kHz). The samples were in
powder form (negligible Brownian contribution) and the T
versus t curves were registered five times in order to estimate
the mean value and its dispersion.
The SAR was calculated through the initial slope of the
heating curve:

Figure 7. Temperature (T) dependence of the high field
magnetization M (applied magnetic field 6 T).

Sample

MS (0)
(emu g−1 )

B×106 (K−2 )

A0

Tf (K)

Fe3 O4
Fe3 O4 @SiO2

72
37

1.1
3.2

3.3 × 10−3
0.116

8
15

T
t
c
m

i
i
i

SARFe3 O4 = cFe3 O4

Table 1. Parameters obtained from the fitting to the modified
Bloch’s law (see equation (2)).

SARFe3 O4 @SiO2 =

mFe3 O4

T
t

(3)

with ci the heat capacity of each component (magnetite
0.937 J g−1 K−1 [23]; silica 0.713 J g−1 K−1 [24]), T/ t
the initial slope of the heating curve and mFe3 O4 the mass
of magnetite in the samples [25, 26]. For the Fe3 O4 @SiO2
nanoparticles the average heat capacity is calculated taking
into account the relative mass of silica in the sample (23%).
Thus, SAR values of 1.5 ± 0.1 and 1.08 ± 0.04 W g−1
were obtained, respectively, for Fe3 O4 and Fe3 O4 @SiO2
nanoparticles. However, the spin disorder surface effects and
the fact that not all the Fe3 O4 nanoparticles mass contribute
to the magnetic heating around room temperature should
be taken into account. In fact, considering the high field

magnetization values at 300 K (65 and 32 emu g−1 for Fe3 O4
and Fe3 O4 @SiO2 nanoparticles, respectively) and the mass
correction of the silica in the Fe3 O4 @SiO2 nanoparticles, just
a 67% of the mass of Fe3 O4 nanoparticles in this silica coated
system would be magnetically active for the heating process
at room temperature. If this mass correction is introduced
in the SAR estimation through equation (3), values around
1.6 W g−1 are obtained for the Fe3 O4 @SiO2 sample.

indicating a faster decrease of magnetization with temperature
than in the bulk state. This increase in the α exponent
has been previously reported in other ferrite nanoparticles
and is associated with finite size effects and a lack of
magnetic coordination at the surface [14, 22]. With respect
to the disordered surface contribution, as figure 7 shows, the
silica coating gives rise to a clear enhancement of the low
temperature magnetization deviations. This enhancement is
directly reflected in the highest value of the A0 parameter for
the magnetite nanoparticles dispersed in the silica matrix (see
table 1) [6, 7]. Besides this, the reduction in magnetization
with respect to the bulk value is also enhanced with the
introduction of the silica coating. The surface spin disorder
contribution is enhanced for thicker silica shells, giving rise
to an increase in the effective anisotropy constant of the
nanoparticles [6, 7]. Taking the estimated value of MS (0)
5


J. Phys.: Condens. Matter 24 (2012) 266007

S Larumbe et al

field indicates the main contribution of the N´eel relaxation to
the heating process.

Acknowledgment
This work was has been performed within the framework of
the project MET-NANOEFA17/08 (POCTEFA).

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Figure 9. Dependence of the SAR on the amplitude of magnetic
2
field. HAC . The line represents the fitting to HAC
.

In order to evaluate the main relaxation mechanisms
associated with the heating process, the heating curves were
determined as a function of the amplitude of the applied
AC magnetic field, HAC . Figure 9 displays the estimated
SAR values as a function of HAC , where the described
mFe3 O4 mass correction was introduced in the Fe3 O4 @SiO2
system. As figure 9 shows, both samples show similar
SAR values in the range of the applied HAC field taking
into account the mass correction. Moreover, the samples
display the characteristic quadratic field dependence of the
superparamagnetic nanoparticles (N´eel relaxation) [24, 27]:
SAR(HAC , f ) ∝

χ (f )HAC 2
ρ

(4)

with χ the imaginary component of the magnetic
susceptibility and ρ the electrical resistivity. Similar quadratic
dependences are obtained within the framework of a general
hysteresis model, taking into account the linear response
theory [28]. It should be noted that in spite of the wide size
distribution in both nanoparticle systems, the contribution
of the largest nanoparticles to the self-heating effects can
be disregarded. In this case, the field dependence of the
SAR should depart from the quadratic field dependence
experimentally found in the analyzed magnetite nanoparticles.

4. Conclusions
The effect of the silica matrix on the magnetic properties
and on the induction heating (magnetic hyperthermia) of
magnetite nanoparticles was evaluated. At room temperature
the samples display the characteristic superparamagnetic
behavior of magnetite nanoparticles. Surface spin disorder is
evidenced by the deviations in the temperature dependence
of the saturation magnetization in the low temperature range.
These disordered effects are greatly enhanced with the silica
coating of the nanoparticles. As a result of a lower fraction of
Fe atoms being magnetically active at room temperature, the
Fe3 O4 @SiO2 nanoparticles display lower SAR values with
respect to the sample without silica. Moreover, the quadratic
dependence of the SAR on the amplitude of the AC magnetic
6