Journal of Alloys and Compounds 607 (2014) 125–131

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Journal of Alloys and Compounds

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c-Fe2O3 by sol–gel with large nanoparticles size for magnetichyperthermia application

http://dx.doi.org/10.1016/j.jallcom.2014.04.0020925-8388/� 2014 Elsevier B.V. All rights reserved.

⇑ Corresponding author. Tel.: +966 2586775; fax: +966 012581614.E-mail addresses: leminej@yahoo.com, mamamin@imamu.edu.sa (O.M. Lemine).

O.M. Lemine a,⇑, K. Omri b, M. Iglesias c, V. Velasco c, P. Crespo c,d, P. de la Presa c,d, L. El Mir a,b,Houcine Bouzid e,f, A. Yousif g, Ali Al-Hajry e

a Physics Department, College of Sciences, Al Imam Mohammad Ibn Saud Islamic University (IMSIU), Riyadh, Saudi Arabiab Laboratory of Physics of Materials and Nanomaterials Applied at Environment (LaPhyMNE), Faculty of Sciences in Gabes, Gabes, Tunisiac Instituto de Magnetismo Aplicado, UCM-ADIF-CSIC, Spaind Dpto. Física de Materiales, Universidad Complutense de Madrid, Spaine Promising Centre for Sensors and Electronic Devices (PCSED), Najran University, P.O. Box 1988, Najran 11001, Saudi Arabiaf Laboratoire des Matériaux Ferroélectriques, Faculté des Sciences de Sfax, Route Soukra Km 3 5, B.P. 802, F-3018 Sfax, Tunisiag Department of Physics, College of Science, Sultan Qaboos University, P.O. Box 36, Code 123, Al Khoud, Oman

a r t i c l e i n f o

Article history:Received 9 March 2014Received in revised form 28 March 2014Accepted 1 April 2014Available online 20 April 2014

Keywords:HyperthermiaIron oxides nanoparticlesSARSol–gelSQUID

a b s t r a c t

Iron oxides nanoparticles with different sizes are successfully synthesized using sol–gel method. X-raydiffraction (XRD) and Mössbauer spectroscopy show that the obtained nanoparticles are mainly com-posed of maghemite phase (c-Fe2O3). XRD and transmission electron microscopy (TEM) results suggestthat the nanoparticles have sizes ranging from 14 to 30 nm, which are indeed confirmed by large mag-netic saturation and high blocking temperature. At room temperature, the observation of a non-negligiblecoercive field suggests that the particles are ferro/ferrimagnetic. The specific absorption rate (SAR) underan alternating magnetic field is investigated as a function of size, frequency and amplitude of the appliedmagnetic field. A mean heating efficiency of 30 W/g is obtained for the smallest particles at 110 kHz and190 Oe, whereas further increase of particle size does not improve significantly the heating efficiency.

� 2014 Elsevier B.V. All rights reserved.

1. Introduction

Iron oxides nanoparticles attract increasing attention due totheir immense broad range of applications [1–4]. Among the appli-cations, magnetic hyperthermia has awaked particular interestduring the last years [5–9]. Magnetic hyperthermia is a cancertreatment method which uses the ability of magnetic nanoparticlesto generate heat when exposed to alternating magnetic field [10].Two different types of magnetic relaxation mechanisms areresponsible for the heat generation: the relaxation of magneticmoments inside the volume (Neel relaxation) or the relaxation ofmagnetic moments fixed to the particles (Brownian relaxations)[11]. Generally, the heating efficiency of the magnetic nanoparti-cles in the presence of an AC magnetic field is defined by specificabsorption rate (SAR) which is the amount of heat generated perunit gram of magnetic material and per unit time [6,12,13]. It isreported that the SAR could be affected by several parameters suchas sample preparation method, structural and magnetic propertiesof the nanoparticles, the amplitudes and frequency of the applied

field, among others, [5–7,12–15]. In addition, several works havereported that the SAR values of iron oxides depend on particle sizeand media viscosity [7,15,16]. Moreover, it has been observed thatlarge particles produce heat more efficiently in less viscous media,i.e., in water suspension, than in higher viscous media. De la Presaet al. [7] investigated the influence of the size, concentration,frequency and amplitude of applied magnetic field on SAR of c-Fe2O3 nanoparticles synthesized by co-precipitation method. Theyreported a critical size around 12 nm for obtaining the maximumheating efficiency in media with high viscosity.

The primary objectives of the present work are: (1) the synthe-sis of iron oxides nanoparticles with different size by a simpleprocess using modified sol–gel method and (2) study the influenceof the size, frequency and amplitude of the applied magnetic fieldon SARs values for the obtained nanoparticles.

2. Experimental

2.1. Synthesis of Fe2O3 nanoparticles

Iron oxide Fe2O3 nanoparticles samples are prepared by modified sol–gelmethod under supercritical conditions of ethyl alcohol (EtOH) based on El Miret al. protocol [17], where water for hydrolysis is slowly released by esterification

-12 -8 -4 0 4 8 12

Velocity mm/s

M14

M22

M30

Standard Fe2O3

126 O.M. Lemine et al. / Journal of Alloys and Compounds 607 (2014) 125–131

reaction to control the size of the formed nanoparticles. In this case, 6 g of iron (III)acetylacetonate [C15H12FeO6] used as precursor is dissolved in a 36 ml of methanol.After magnetic stirring, the solution is placed in an autoclave and dried undersupercritical conditions of EtOH. The supercritical conditions of ethanol wereTc = 243 �C and Pc = 63.3 bar. The heating of the autoclave was provided by a fur-nace controlled by a temperature programmer. The particles size can be tuned bytime agitation of magnetic stirring. Three agitation times of 15 min, 24 h and 48 hwere used to produce nanoparticles with average diameteres of 14 nm (M14),22 nm (M22) and 30 nm (M30) respectively.

2.2. Structural characterization

X-ray powder diffraction (XRD) measurements are performed by PANalyticalXpertPro diffractometer with Cu k radiation (k = 1.54178 ÅA

0

) in the range 2h = 20–70� at 0.02� step. The average crystallite size is calculated from the Scherrer’sformula [18]:

D ¼ KkB cos h

ð1Þ

where k = 1.54178 Å, K is a constant whose value is approximately 0.9 and B is thepeak full width at half height maximum (FWHM) expressed in radians. Particles sizeand shape are determined by transmission electron microscopy (TEM) by using JEOL(JEM-2100F) electron operated at 200 keV.

2.3. Magnetic characterization

Mössbauer spectra have been recorded at room temperature (RT) in standardtransmission geometry, using a constant acceleration signal spectrometer equippedwith 57Co source in a rhodium matrix. The measured data are analyzed using a non-linear least-squares fitting program assuming Lorentzian distribution. Isomer shiftsare referred to a-Fe at RT.

Magnetic characterization of powder samples and colloidal suspensions areperformed in a Quantum Design MPMS-5S SQUID magnetometer. For colloidalcharacterization, 50 lL iron oxide suspension with a concentration of 20 mgFe2O3

per mL of water is carried out in special closed sample holders. Hysteresis loopshave been measured, with a maximum applied field of 50 kOe, at 5 and 250 K. Itshould be noticed that within this temperature range the colloids freeze. Due tothe large iron oxide content, diamagnetic contribution arising from water can bediscarded. In the case of powder samples, the magnetization curves have been mea-sured up to 300 K, whereas thermal dependence of the magnetization have beenobtained under an applied field of 50 Oe after cooling the samples under zero fieldand under a field of 50 Oe. The blocking temperature TB was determined from themaximum of the ZFC curves.

2.4. Heating efficiency

The characterization of the heating efficiency of the samples was carried outwith a commercial system Magnetherm 1.5 (Nanotherics) working at 100 Oe fieldamplitude and at frequencies of 110 and 523 kHz in order to study the influenceof field frequency. For investigate the influence of field strength,magnetic fieldamplitudes of 110 Oe and 180 Oe have been also applied at 110 kHz. Mean mag-netic field inside the coil are measured with a secondary two turns coil with across-sectional value close to that of sample holder (6.2 mm diameter). The samplesare diluted with milli-Q water at 20 mg/ml (sample mass per volume) and

Fig. 1. XRD patterns of samples for different agitation.

sonicated during 15 min at RT. Thereafter, a microvial with 0.9 ml of suspensionis inserted inside the coils. A half-length of the sample is made to coincide withthe maximum value of the magnetic field. The temperature of the coils is controlledthrough a close circuit of water maintained at 16 �C with a cryostat bath. Underthese conditions, a water sample is measured under the same experimental condi-tions and checked that water temperature does not increase in sample space. Thetemperature of particle suspension is measured with a fiber optical thermometerand registered with a computer. The temperature of the sample is controlled andstabilized to RT before each measurement. Prior to turning the magnetic field on,sample temperature is recorded for about 60 s in order to ensure thermal stabilityand to have a baseline for the calculation of the SARs values. The temperatureincrease of the samples is measured either up to 80 �C or during 30 min.

Since the Fe concentrations are in the range 1–5 wt%, the SAR values for themagnetic hyperthermia experiments can be calculated as SAR ¼ Cw

cFeðDT

DtÞ, where Cw

is the specific heat capacity of water and cFe the Fe weight concentration in the col-loid [7,13]. By performing a linear fit of temperature increase versus time at theinitial time interval the slope DT/Dt is obtained. The measurements are performedin non adiabatic conditions; thus, the curves slopes are fitted only in the first fewseconds after magnetic field is turned on, and the range is selected at the maximumslope (1 to �30 s) [19]. Finally, the SARs values are obtained by fitting the experi-mental heating curves normalized to iron mass (W/gFe).

-12 -8 -4 0 4 8 12

Velocity mm/s

Fig. 2. Mössbauer spectra at RT of the synthesized iron oxide nanoparticles M14,M22 and M30 compared with bulk c-Fe2O3.

Table 1Hyperfine parameters for the samples and for bulk Fe2O3.

Sample Isomer shift (mm/s) Hyperfine field (T) Intensity (%)

M14 0.34 49.1 760.37 44.5 24

M22 0.34 48.4 100M30 0.34 48.9 100Bulk Fe2O3 0.37 51.5 100

Fig. 3. TEM images and the distribution the size for the samples with different agitation times: (a–a1) 15 min, (b–b1) 24 h and (c–c1) 48 h.

O.M. Lemine et al. / Journal of Alloys and Compounds 607 (2014) 125–131 127

All the SARs values are obtained under experimental condition H0f < 5 � 109 A/m�1s�1, a limit at which unwanted nonselective heating of both cancerous as wellas healthy tissue due to generation of body eddy currents may occur [20].

3. Results and discussion

3.1. Structural and magnetic properties

Fig. 1 shows the XRD pattern of samples prepared with differentagitation time. The peaks are very broad indicating the formationof very fine particles in the nanoscale range. The average crystallitesize obtained by using the Debye–Scherrer formula confirms thisresult. The most intense peak corresponding to (311) reflectionwas used and the crystallite sizes of the synthesized samples at15 min, 24 and 48 h are 14, 22 and 30 nm, respectively. The XRDpatterns show peaks that correspond to an inverse spinel structure,

which could be either magnetite (Fe3O4) or maghemite (c-Fe2O3).No additional peaks have been observed suggesting that thesynthesis procedure leads to the formation of a phase withoutimpurities that remain from the un-reacted precursors of the for-mation of other phases. However, due to their structural similari-ties, it is difficult to differentiate between the two phases formthe XRD patterns. Thus, Mossbauer spectroscopy studies have beencarried out for distinguish between the two phases since thehyperfine parameters of both Fe-oxides are different: the spectrumcorresponding to maghemite is characterized by a single wellresolved sextet for the Fe3+, whereas magnetite spectrum showstwo well resolved sextets corresponding to Fe3+ at the tetrahedralsites (33%) and the Fe3+and Fe2+ at the octahedral sites [21], respec-tively. Fig. 2 shows the RT Mössbauer spectra of the as-preparedM14, M22 and M30 samples compared with the spectrum of stan-dard c-Fe2O3.

Fig. 4. Hysteresis loops for the powders samples (a) M14, (b) M22, (c) M30 and ZFCFC at 100 Oe.

Table 2Magnetic saturation, remnant ratio Mr/Ms and coercive field Hc of samples M14, M22and M30 measured in powders and colloids.

Powders Colloidal suspensions

Ms (emu/g) Mr/Ms Hc (Oe) Mr/Ms Hc (Oe)

5 K 300 K 5 K 5 K 300 K 5 K 5 K

M14 82 67 0.30 450 40 0.43 450M22 80 75 0.31 430 40 0.39 400M30 79 74 0.31 370 70 0.38 300

128 O.M. Lemine et al. / Journal of Alloys and Compounds 607 (2014) 125–131

It is observed that the sample M14 is composed mainly byc-Fe2O3, 76% relative intensity correspond to a site with hyperfineparameters very close to maghemite phase; however, there is a24% of iron atoms under a well-resolved hyperfine magnetic fieldof 44.5 T, which is characteristic of the Fe at octahedral sites inmagnetite. Therefore, we can conclude that sample M14, obtainedwith the shorter agitation time, is composed mainly of maghemitewith 24% of magnetite. The presence of magnetite could indicatethat for short agitation times the oxidation is not complete and aremaining magnetite phase is still present after 15 min agitation.Table 1 summarizes the hyperfine parameters of the three samples.For samples M22 and M30, the best fit was obtained by consideringa single sextet whose hyperfine parameters are close to that ofmaghemite. The hyperfine magnetic field is about 49 T, slightlylower than bulk samples (51.5 T). The difference can be attributed

to particle size and/or crystallinity degree [22]. Thus, it can be con-cluded from XRD and Mössbauer results that the synthesis proce-dure produces, for high enough agitation times, single phase iron

Fig. 5. Hysteresis loops for the colloidal suspension samples (a) M14, (b) M22, (c) M30 and ZFC-FC at 100 Oe.

O.M. Lemine et al. / Journal of Alloys and Compounds 607 (2014) 125–131 129

oxides which could be indexed on the basis of c-Fe2O3 with spacegroup P4132 (JCPDS No. 39-1346).

TEM images of the obtained nanoparticles with different agita-tion times are shown in Fig. 3. It can be seen that the particles arevery small (nanometric size) and the size increases with increasingagitation times. Moreover, it can be observed also that the particleshave high crystallinity and tend to aggregate, due probably to themagneto dipole interparticles interactions. The average particlessize is in the range of 14–30 nm and the values are close to thoseobtained from XRD patterns (Fig. 3a1–c1).

Hysteresis loops and ZFC-FC curves of the powders samples areshown in Fig. 4. The values of the saturation magnetization (Ms),remnant to saturation magnetization ratio (Mr/Ms) and coercivefield (Hc) values of the three samples are presented in Table 2.Low temperature Ms of sample M14 is 82 emu/g, which is higherthan the standard value of maghemite c-Fe2O3 (Ms = 74 emu/g inbulk. This difference confirms the presence of a magnetite phase,which has a higher magnetic saturation than maghemite. As agingtime and particle size increase, low temperature Ms slightlydecreases to 79 emu/g, indicating that magnetite oxidizes tomaghemite due to aging treatment, in agreement with Mössbauerresults. The samples exhibit a non-negligible coercive field at roomtemperature, indicating that the systems do not behave assuperparamagnetic. Room temperature Hc slightly increases with

particle size (see Table 1). This is in agreement with Mössbauerspectroscopy that clearly shows a ferro/ferrimagnetic contributionat room temperature.

The ZFC-FC (Fig. 4d–f) curves indicate that blocking tempera-ture (TB) is above RT for the three samples, in agreement withthe non-vanishing coercive field at RT in hysteresis loop curves.The high Ms and TB values also suggest a high crystallinity degreeof the nanoparticles which is confirmed by TEM images shown inFig. 3.

It’s important to notice that around 20 K there is a kink in themagnetization at ZFC which is barely perceptible for the smallestparticles size (M14 – Fig. 4(d)) and become more noticeable as sizeincreases (Fig. 4(f)). This kink can be associated to a spin-glass liketransition, a change from a state of disordered frozen spins at par-ticle surface that undergoes to a ferrimagnetic state with orderedmagnetic spins; this yields to an abrupt increase of magnetic sus-ceptibility. A spin-glass like transition is normally related to anamorphous shell at particle surface [23] and Mössbauer spectrashow in this case broad distribution. It can be confirmed fromour Mössbauer results that there are any spin glass transition.

Hysteresis loop and ZFC-FC curves for the colloidal suspensionsare shown in Fig. 5. It can be seen that Mr is higher in the colloidthan in powder samples (see Table 2). For the smallest particlesM14, remnant ratio is 0.42, suggesting that dispersion of particles

Fig. 6. Temperature increases of the magnetic colloidal suspension as a function of:(a) H0 = 100 Oe and f = 110 kHz, (b) H0 = 100 Oe and f = 523 kHz and (c) H0 = 180 Oeand f = 110 kHz.

Table 3SARs values for the samples.

SAR (W/g)

Size (nm) 523 kHz 100 Oe 110 kHz 100 Oe 110 kHz 180 Oe

Fe2O3 14 12(1) 6(1) 29(1)Fe2O3 agi 24 22 9(1) <2 20(1)Fe2O3 agi 48 30 9(1) 5(1) 35(1)

12 16 20 24 28 320

50

100

150

200

H0 (O

e)

d (nm)

Fig. 7. Area under the curve represents the magnetic field amplitudes that satisfythe condition l0MsVH0 < kBT of the LRT at room temperature for different particlesizes. (The full line represents l0MsVH0 < kBT). The field amplitude in this work100 Oe (red line) and 180 Oe (blue line). (For interpretation of the references tocolour in this figure legend, the reader is referred to the web version of this article.)

130 O.M. Lemine et al. / Journal of Alloys and Compounds 607 (2014) 125–131

in water resemble an ensemble of non-interacting magnetic parti-cles in single-domain (SD) regime which easy axis are randomlydistributed. Stoner–Wolfahrt model [24] for an ensemble of non-interacting SD magnetic particles randomly distributed givesMr = 0.5 Ms at 0 K, which is very close to our experimental value.The deviation from theoretical value Mr/Ms = 0.5 could beattributed either to thermal processes that reduce Mr at 5 K or to

a system of slightly interacting magnetic particles. The remnantratio decreases as size increases, indicating that the particle inter-actions become more significant with size.

3.2. SAR measurements

The study of heating efficiency is investigated for each particlesize as a function of field amplitude and frequency. Fig. 6 showsthe temperature increase due to the nano-ferrites when submittedto an alternating magnetic field and Table 3 summarizes the SARsvalues. As can be seen from Fig. 6(a and b), temperature increase ofcolloids enhances as field frequency increases from 110 to 523 kHzfor a 100 Oe field amplitude. For this set of measurements, the bestheating efficiency correspond to as prepared sample M14, whereassample M30-containing the largest particles – do not show anyimprovement in the heating efficiency regarding M14 (see Table 2).By fixing the frequency to 110 kHz and increasing the field ampli-tude from 100 to 180 Oe the SAR values increases for all set of sam-ples, and sample M30 shows the higher SARs.

For each sample, SAR increases as field strength or frequencyincrease, as expected; however, it does not follow linear responsetheory (LRT) in which SAR depends on square of field amplitude[7,11]. LRT models the relaxation mechanisms when the maximumapplied field amplitude H0 satisfies l0MsVH0 < kBT, where Ms is themagnetic saturation, V the nanoparticle volume, and T the absolutetemperature. This condition implies that the sample magnetizationdepends linearly on the applied field; thus, the maximum fieldamplitude is limited by particle size and saturation. In this model,SAR varies linearly with field frequency and quadratic with fieldamplitude. As can be seen from Fig. 7, it is not expected that theheating efficiency of this particles ensemble follows LRT underthe ac applied fields in this experiment (100 Oe and 180 Oe).

In addition, the magnetic moment of a particle suspended in afluid can relax after magnetic field removal by two different mech-anisms: the Neel sN ¼ s0eKeff V=kBT and Brown sB ¼ 3gVh

kBT relaxations[25,26], where s is the relaxation time. Although both mechanismsdepend on intrinsic properties, like particle size and magneticanisotropy, and on an extrinsic property (media viscosity), it isexpected that large particles in aqueous colloid relaxes mainly bya Brownian mechanism.

Due to SAR dependence on parameters which are extrinsic tothe NPs properties, as ac field amplitude and frequency, the com-parison of reported SAR values does not give much informationon the NPs heating efficiency. An intrinsic loss power parameterdefined as ILP ¼ SAR=fH2

0 has been proposed in order to compareheating efficiencies obtained under different experimentalconditions [27]. The definition of ILP parameter assumes the

Table 4ILP parameters for the three samples.

ILP (nH m2/kg)

Size (nm) 523 kHz 100 Oe 110 kHz 100 Oe 110 kHz 180 Oe

Fe2O3 14 0.4(1) 0.8(1) 1.3(1)Fe2O3 agi 24 22 0.3(1) <0.3 0.9(1)Fe2O3 48 30 0.3(1) 0.68 1.6(1)

O.M. Lemine et al. / Journal of Alloys and Compounds 607 (2014) 125–131 131

magnetization losses depend linearly on field frequency andquadratic with field amplitude (as expected by LRT); this parame-ter allows direct comparison of the particle-heating capabilitiesindependently of the applied ac fields. However, this approxima-tion is not always satisfied by particles in suspension; a nonlinearbehavior of imaginary part of susceptibility in MNPs is reported bydifferent authors [25,28].

Furthermore, our results show a field dependence of ILP (seeTable 4). Due to the large size of the particles, deviation from LRTimplies a more intricate dependence of heating efficiency withfield strength and frequency. For ac magnetic field of 100 Oe and110 kHz, the ILP parameter decreases as particle size increase.Since mean relaxation time sR of larger particles is longer thanthe smaller ones, magnetic moments of large particles relax slowerthan the smaller ones, thus hindering the heat production of sam-ple M30. As field frequency increases from 110 to 523 kHz, ILPvalue also decreases for sample M30 probably because magneticmoments of the bigger particles could not follow the faster varia-tion of alternating magnetic field, impacting again on heat produc-tion of particles.

A very different behavior is observed as field amplitude isincreased. At 110 kHz, a field amplitude increase from 100 to180 Oe makes sample M30 more effective for heating than M14.This is because the deviation from superparamagnetic behaviorof the larger particles: as field strength increases there are moreferro/ferrimagnetic particles that can contribute to heatproduction.

ILP values for commercial ferrofluids are reported in the range0.2–3.1 nH m2/kg [27]. The calculated ILP values for samples M14and M30 vary from 0.3 to 1.58 nH m2/kg (see Table 4), valueswhich are high enough to make these magnetic particles applicablefor hyperthermia treatment.

In addition, these ILP values are obtained under experimentalcondition H0f < 5 � 109 Am�1 s�1, a safe limit at which unwantednon selective heating of healthy tissues due to generation of bodyeddy currents are avoided [20].

Regarding sample M22, its heating efficiency deviatessignificantly from the other two despite their good magneticproperties, probably due to the difficulties to obtain a very stablecolloidal suspension of this sample.

4. Conclusion

Sol–gel method allows us to synthesize a large amount ofc-Fe2O3 nanoparticles in single step with high crystallinity. Theresults showed that the obtained nanoparticles are mainly com-posed of maghemite phase (c-Fe2O3) with presence of magnetitefor M14. M22 and M30 samples transform to maghemite withaging time. The heating efficiency of the obtained nanoparticlesis studied under an alternating magnetic field and as a functionof size, frequency and amplitude of the applied magnetic field.Our results show that the SARs values differ considerably betweensamples. For fixed field at 100 Oe and increasing the frequencyfrom 110 to 523 kHz, it is found that the best heating efficiencyis obtained for the smallest particles size M14, whereas sample

M30-containing the largest particle sizes- do not show any signif-icant improvement in the heating efficiency regarding M14. Byfixing the frequency to 110 kHz and increasing the field amplitudefrom 100 to 180 Oe the SAR values increase, and sample M30shows the highest value. The dependence of SARs with field ampli-tude and frequency deviate from linear response theory (LRT) dueto the particle size and magnetic properties or probably due tomagnetic clustering.

Acknowledgments

The authors acknowledged Al Imam Mohammad Ibn SaoudIslamic University for the finical support. This work was also par-tially supported by grants from the Spanish Ministry of Scienceand Innovation MAT2011-23641, MAT2012-37109-C02-01 andMadrid regional government CM (S009/MAT-1726).

References

[1] M. Colombo, S. Carregal-Romero, M.F. Casula, L. Gutierrez, M.P. Morales, I.B.Bohm, J.T. Heverhagen, D. Prosperi, W.J. Parak, Chem. Soc. Rev. 41 (11) (2012)4306–4334.

[2] A. Boal, in: V. Rotello (Ed.), Nanoparticles, Springer, US, 2004, pp. 1–27.[3] Q.A. Pankhurst, J. Connolly, S.K. Jones, J. Dobson, J. Phys. D: Appl. Phys. 36 (13)

(2003) R167.[4] P. Crespo, P. de la Presa, P. Marín, M. Multigner, J.M. Alonso, G. Rivero, F.

Yndurain, J.M. Gonzalez-Calbet, A. Hernando, J. Phys.: Condens. Matter 25 (48)(2013) 484006.

[5] E. Kita, S. Hashimoto, T. Kayano, M. Minagawa, H. Yanagihara, M. Kishimoto, K.Yamada, T. Oda, N. Ohkohchi, T. Takagi, T. Kanamori, Y. Ikehata, I. Nagano, J.Appl. Phys. 107 (9) (2010).

[6] I. Hilger, K. Frühauf, W. Andrä, R. Hiergeist, R. Hergt, W.A. Kaiser, Acad. Radiol.9 (2) (2002) 198–202.

[7] P. de la Presa, Y. Luengo, M. Multigner, R. Costo, M.P. Morales, G. Rivero, A.Hernando, J. Phys. Chem. C 116 (2012) 25602–25610.

[8] A. Jordan, R. Scholz, P. Wust, H. Fahling, R. Felix, J. Magn. Magn. Mater. 201(1999) 413–419.

[9] G.F. Goya, L. Asin, M.R. Ibarra, Inter. J. Hyperther. 29 (8) (2013) 810–818.[10] A. Jordan, P. Wust, H. Fahling, W. John, A. Hinz, R. Felix, Inter. J. Hyperther. 9 (1)

(1993) 51–68.[11] J. Carrey, B. Mehdaoui, M. Respaud, J. Appl. Phys. 109 (8) (2011).[12] N.K. Prasad, K. Rathinasamy, D. Panda, D. Bahadur, J. Mater. Chem. 17 (48)

(2007) 5042–5051.[13] M.A. Gonzalez-Fernandez, T.E. Torres, M. Andres-Verges, R. Costo, P. de la

Presa, C.J. Serna, M.R. Morales, C. Marquina, M.R. Ibarra, G.F. Goya, J. Solid StateChem. 182 (10) (2009) 2779–2784.

[14] C. Martinez-Boubeta, K. Simeonidis, A. Makridis, M. Angelakeris, O. Iglesias, P.Guardia, A. Cabot, L. Yedra, S. Estrade, F. Peiro, Z. Saghi, P.A. Midgley, I. Conde-Leboran, D. Serantes, D. Baldomir, Sci. Rep. 3 (2013).

[15] Y. Pineiro-Redondo, M. Banobre-Lopez, I. Pardinas-Blanco, G. Goya, M.A.Lopez-Quintela, J. Rivas, Nanoscale Res. Lett. 6 (1) (2011) 383.

[16] M. Levy, C. Wilhelm, J.-M. Siaugue, O. Horner, J.-C. Bacri, F. Gazeau, J. Phys.:Condens. Matter 20 (20) (2008) 204133.

[17] L. El Mir, A. Amlouk, C. Barthou, S. Alaya, Physica B: Condens. Matter 388 (1–2)(2007) 412–417.

[18] B.D. Cullity, C.D. Graham, Introduction to Magnetic Materials, Wiley, 2009.[19] E. Natividad, M. Castro, A. Mediano, J. Magn. Magn. Mater. 321 (10) (2009)

1497–1500.[20] R. Hergt, S. Dutz, J. Magn. Magn. Mater. 311 (1) (2007) 187–192.[21] M.A. Verges, R. Costo, A.G. Roca, J.F. Marco, G.F. Goya, C.J. Serna, M.P. Morales, J.

Phys. D-Appl. Phys. 41 (13) (2008).[22] M. Starowicz, P. Starowicz, J. Zukrowski, J. Przewoznik, A. Lemanski, C.

Kapusta, J. Banas, J. Nanopart. Res. 13 (12) (2011) 7167–7176.[23] K.L. Lopez Maldonado, P. de la Presa, E. Flores Tavizon, J.R. Farias Mancilla, J.A.

Matutes Aquino, A. Hernando, J.T. Elizalde Galindo, J. Appl. Phys. 113 (17)(2013) 17E132.

[24] E.C. Stoner, E.P. Wohlfarth, IEEE Trans. Magn. 27 (4) (1991) 3475–3518.[25] G. Glockl, R. Hergt, M. Zeisberger, S. Dutz, S. Nagel, W. Weitschies, J. Phys.-

Condes. Matter 18 (38) (2006) S2935–S2949.[26] S. Laurent, S. Dutz, U.O. Häfeli, M. Mahmoudi, Adv. Colloid Interfac. 166 (1–2)

(2011) 8–23.[27] M. Kallumadil, M. Tada, T. Nakagawa, M. Abe, P. Southern, Q.A. Pankhurst, J.

Magn. Magn. Mater. 321 (10) (2009) 1509–1513.[28] B. Fischer, B. Huke, M. Lücke, R. Hempelmann, J. Magn. Magn. Mater. 289

(2005) 74–77.