99 j phys cond matt 24(2012) 266007

Superconducting quantum interference device is used to measure extremely weak signals with a device called Josephson junction. Josephson junction made up of two superconductors lightly separated that pairs of electrons can tunnel through the junction in order to generate the current called Josephson current between 2 superconductors at applied voltage is zero.

Effect of a SiO 2 coating on the magnetic

properties of Fe 3 O 4 nanoparticles

Article in Journal of Physics Condensed Matter · June 2012

DOI: 10.1088/0953-8984/24/26/266007 · Source: PubMed

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

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J Phys.: Condens Matter 24 (2012) 266007 (6pp) doi:10.1088/0953-8984/24/26/266007

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:gpolo@unavarra.es

Received 16 December 2011, in final form 26 April 2012

Published 14 June 2012

Online atstacks.iop.org/JPhysCM/24/266007

Abstract

In this work the effect of a SiO2coating on the magnetic properties of Fe3O4nanoparticles

obtained by the sol–gel method is analyzed Two sets of samples were prepared: Fe3O4

nanoparticles and Fe3O4@SiO2core–shell composites The samples display the characteristic

spinel structure associated with the magnetite Fe3O4phase, 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

Fe3O4and Fe3O4@SiO2, respectively The main effect of the SiO2coating is reflected in the

temperature dependence of the high field magnetization (µ0H =6 T), i.e deviations from the

Bloch law at low temperatures (T< 20 K) Such deviations, enhanced by the introduction of

the SiO2coating, 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−1were

achieved for the Fe3O4nanoparticles A significant decrease (around 26%) is found in the

SAR values of the SiO2coated nanocomposite The different heating response is analyzed in

terms of the decrease of the effective nanoparticle magnetization in the Fe3O4@SiO2

core–shell composites at room temperature

(Some figures may appear in colour only in the online journal)

1 Introduction

Ferrite nanoparticles have been extensively explored for

different applications, for example in magnetic hyperthermia

due to their biocompatibility and for their special

mag-netic properties as nanosized systems Different synthesis

methods are employed in the preparation of magnetite

nanoparticles, including chemical methods (sol–gel,

co-precipitation, 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

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 Fe3O4and Fe3O4@SiO2were synthesized by

a sol–gel method The magnetic properties of both samples

1 0953-8984/12/266007+06$33.00 c 2012 IOP Publishing Ltd Printed in the UK & the USA

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 Fe3O4

and Fe3O4@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

radiation (1.5418 ˚A) and the mean diameter was estimated

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)

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

Figure 1 TG thermograms (—) and derivative curves (◦) for (a)

Fe3O4and (b) Fe3O4@SiO2gels

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 Fe3O4and Fe3O4@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 (Fe3O4 and Fe3O4@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 Fe3O4

sample would correspond to the dehydroxylation of the

OH groups located at the surface of the nanoparticles [10]

In the Fe3O4@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 Fe3O4 and Fe3O4@SiO2, respectively Accordingly, the as-synthesized gels were calcined at the indicated temperatures for 2 h In order to prevent the

2

Figure 2 X-ray diffraction patterns for the calcined samples.

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 Fe3O4 and Fe3O4@SiO2 nanoparticles A single

spinel phase (PDF card number 01-089-0691 from the

PDFWIN database), with lattice parameters close to the

reported bulk magnetite (a = 8.39 ˚A) is observed in both

calcined nanoparticles: a = 8.39 and 8.40 ˚A for Fe3O4

and Fe3O4@SiO2, respectively Lattice parameters were

esti-mated using the Bragg law reflection peaks A linear decrease

in the lattice parameter of the spinel is reported with the

oxygen vacancies (δ; Fe3 (1−δ)O4−δ) from 8.39 ˚A (magnetite:

Fe3O4) to 8.35 ˚A; maghemite: Fe2O3) [11] This result

confirms the Fe3O4composition 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 Fe3O4 and Fe3O4@SiO2, respectively, employing the

Debye–Scherrer equation, d = kλ

β cos θ, where k = 0.9, λ is the wavelength of the Cu Kα line (1.5418 ˚A), β is full width at

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 Fe3O4

and Fe3O4@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 Fe3O4 and

Fe3O4–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 Fe3O4@SiO2nanoparticles (see figure3(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, µ0H = 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

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 figure4(b)) Figure5displays the temperature dependence of HCfor 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 HCto the T1/2law of

uniaxial non-interacting single domain particles [12]:

HC=HK

"

1 − T

TB

1 /2#

(1)

where HK =2Keff/MS, with Keff the effective anisotropy constant and MSthe saturation magnetization The solid lines

in figure 5 represent the data fitting according equation (1) with the following fitting parameters: (i) Fe3O4, HK=(670±

50) Oe, TB=(160 ± 20 K); (ii) Fe3O4@SiO2, HK=(530 ±

40) Oe, TB=(120 ± 10 K) The slight increase in TB for the Fe3O4 sample would correspond to the higher value of the mean nanoparticle size with respect to the Fe3O4@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 figure6

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 Fe3O4 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 µ0H = 6 T) As figure 7 shows, at low temperatures the Bloch law, MS(T) = MS(0)(1 − BTα), 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]:

MS(T) = MS(0)[(1 − BTα) + A0e( −T

Tf )] (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

Figure 3 TEM images for (a) Fe3O4and (c) Fe3O4@SiO2nanoparticles EDX analyses are shown in (b) and (d) images.

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

nanoparticles

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 Table1

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,

4

Figure 6 ZFC–FC magnetization (M) curves (applied magnetic

field 5 mT)

Figure 7 Temperature (T) dependence of the high field

magnetization M (applied magnetic field 6 T)

Table 1 Parameters obtained from the fitting to the modified

Bloch’s law (see equation (2))

Sample

MS(0)

(emu g− 1) B×106(K− 2) A0 Tf(K)

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 figure7shows, 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 A0parameter 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)

Figure 8 Temperature rise,1T, versus time, t, under an AC magnetic field (initial temperature 18◦

C)

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 Fe3O4 system (MS(0) = 72 emu g−1; see table1)

With respect to the induction heating effects (magnetic hyperthermia), figure 8 shows the temperature rise, 1T, 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 the1T 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:

SARFe3O4 =cFe3O41T

1t SARFe3O4@SiO2 =

P

icimi

mFe3O4

1T 1t

(3)

with ci the heat capacity of each component (magnetite

0.937 J g−1K−1[23]; silica 0.713 J g−1K−1[24]),1T/1t the initial slope of the heating curve and mFe3O4 the mass

of magnetite in the samples [25,26] For the Fe3O4@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 Fe3O4 and Fe3O4@SiO2 nanoparticles However, the spin disorder surface effects and the fact that not all the Fe3O4nanoparticles 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−1for Fe3O4 and Fe3O4@SiO2 nanoparticles, respectively) and the mass correction of the silica in the Fe3O4@SiO2nanoparticles, just

a 67% of the mass of Fe3O4nanoparticles 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−1are obtained for the Fe3O4@SiO2sample

5

Figure 9 Dependence of the SAR on the amplitude of magnetic

field HAC The line represents the fitting to H2

AC

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

mFe3O4 mass correction was introduced in the Fe3O4@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

00(f )HAC2

with χ00 the imaginary component of the magnetic

susceptibility andρ 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

Fe3O4@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

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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