γ -Fe 2 O 3 nanoparticles dispersed in porous Vycor glass: A magnetically dilutedintegrated systemDanielle Cangussu, Wallace Castro Nunes, Heberton Luis da Silva Corrêa, Waldemar Augusto de AlmeidaMacedo, Marcelo Knobel, Oswaldo Luiz Alves, Antônio Gomes Souza Filho, and Italo Odone Mazali Citation: Journal of Applied Physics 105, 013901 (2009); doi: 10.1063/1.3054173 View online: http://dx.doi.org/10.1063/1.3054173 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/105/1?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Physical limits of pure superparamagnetic Fe3O4 nanoparticles for a local hyperthermia agent in nanomedicine Appl. Phys. Lett. 100, 092406 (2012); 10.1063/1.3689751 Formation and evolution of magnetic nanoparticles in borate glass simultaneously doped with Fe and Mn oxides J. Appl. Phys. 104, 103917 (2008); 10.1063/1.3021289 Magnetic properties of ( γ Fe 2 O 3 ) 80 Ag 20 nanocomposites prepared in reverse micelles J. Appl. Phys. 97, 10G101 (2005); 10.1063/1.1847331 Fragmentation of Fe 2 O 3 nanoparticles driven by a phase transition in a flame and their magnetic properties Appl. Phys. Lett. 83, 4842 (2003); 10.1063/1.1632534 Magnetic nanoparticles of Fe 2 O 3 synthesized by the pulsed wire evaporation method J. Appl. Phys. 93, 7196 (2003); 10.1063/1.1558234

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�-Fe2O3 nanoparticles dispersed in porous Vycor glass: A magneticallydiluted integrated system

Danielle Cangussu,1 Wallace Castro Nunes,2 Heberton Luis da Silva Corrêa,3

Waldemar Augusto de Almeida Macedo,3 Marcelo Knobel,4 Oswaldo Luiz Alves,1

Antônio Gomes Souza Filho,5 and Italo Odone Mazali1,a�

1Laboratório de Química do Estado Sólido, Instituto de Química, Universidade Estadualde Campinas-UNICAMP, Campinas, P.O. Box 6154, 13083-970, São Paulo, Brazil2Departamento de Física, Centro de Física da Matéria Condensada, Universidade de Lisboa, 1749-016,Lisboa, Portugal3Laboratório de Física Aplicada, Centro de Desenvolvimento da Tecnologia Nuclear, Belo Horizonte,Minas Gerais, 31270-901, Brazil4Departamento de Física da Matéria Condensada, Instituto de Física Gleb Wataghin, UniversidadeEstadual de Campinas-UNICAMP, Campinas, P.O. Box 6165, 13083-970, São Paulo, Brazil5Departamento de Física, Universidade Federal do Ceará, P.O. Box 6030, 60450-900, Fortaleza, Ceará,Brazil

�Received 28 April 2008; accepted 11 November 2008; published online 5 January 2009�

An investigation of the effect of interparticle interaction and particle size distribution has beencarried out on iron oxide nanoparticles dispersed into porous Vycor glass. �-Fe2O3 nanoparticlesdispersed into monoliths of Vycor glass were obtained using impregnation-decomposition cyclesthrough the single-source metallo-organic decomposition process. Magnetic properties wereinvestigated by ac magnetic susceptibility measurements, as a function of temperature at differentfrequencies, by measuring zero-field-cooled and field-cooled magnetization curves and byconstructing hysteresis loops at different temperatures. A log-normal size distribution ofmonodomain nanoparticles has been deduced from the analysis of the magnetization curves. 57FeMössbauer spectroscopy was also employed for investigating the magnetic behavior as a function ofnanoparticle size. The systems exhibit typical superparamagnetic behaviors with a wide particle sizedistribution that can be changed without significantly affecting the interparticle interaction. Theexperimental data are discussed in terms of the evolution of the particle size distribution with thenumber of impregnation-decomposition cycles used for preparing the nanoparticles.© 2009 American Institute of Physics. �DOI: 10.1063/1.3054173�

I. INTRODUCTION

Research on magnetic nanoparticles �NPs� has grownsignificantly in recent years, owing to their unique physicalproperties and their potential for applications in future nano-based technologies. The goal of nanotechnology is to exploitchanges in a system induced by its low dimensionality, thusallowing the design of materials with superior performancescompared with their bulk counterparts. However, for theproper use of such systems in devices, a better understandingand modeling of the relationship between their magneticproperties and their nanoscopic structural features is needed.1

The preparation of magnetic NPs dispersed into solidmatrices is nowadays attracting much scientific and techno-logical interest. The physical and chemical properties of NPare sometimes dramatically different from those of their bulkcounterpart. These materials often exhibit size-induced ef-fects, for example, below a critical size magnetic particlesbecome single domain particles, as opposed to multidomainspecies in the bulk structure. In addition, small magneticparticles exhibit unique phenomena such as superparamag-netism and quantum tunneling of magnetization.2,3 Magnetic

NPs have been dispersed into polymer matrices,4,5 silicagels,6–8 and grown into porous colloidal silica particles.9

Nanocomposites consisting of iron oxide NPs dispersedinto polymer matrices10,11 and into porous Vycor glass�PVG�12–15 have also been prepared. The NPs have been pre-pared by using different methods where one of the maingoals is to achieve control of the particle size distribution aswell as the homogeneous dispersion of these NP into thematrices. In particular, an interesting synthesis technique fornanostructured materials has been labeled the single-sourceprecursor �SSP� approach and has been increasingly em-ployed in the preparation of semiconductor nanocrystals andthin films.16–18 A major challenge for this research field is toachieve a fine-controlled synthesis of nanostructures and toassemble them into chemically integrated systems wheretheir performance can be engineered.

In this work, we investigated the magnetic properties of�-Fe2O3 NPs dispersed into porous Vycor glass synthesizedby using the single-source metallo-organic decompositionprocess. Samples obtained with different impregnation-decomposition cycles �IDCs� were prepared in order tomodify the size distributions of the NP and to study theirinfluence on the macroscopic magnetic properties of the re-sulting �-Fe2O3/porous glass nanocomposites.

a�Author to whom correspondence should be addressed. Electronic mail:mazali@iqm.unicamp.br.

JOURNAL OF APPLIED PHYSICS 105, 013901 �2009�

0021-8979/2009/105�1�/013901/7/$23.00 © 2009 American Institute of Physics105, 013901-1

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II. EXPERIMENTAL PROCEDURE

A. Porous Vycor glass monolith

The PVG, code 7930, was purchased from CorningGlass Co. Glass plates were cut with dimensions of approxi-mately �10�10�1� mm3 and polished. The PVG wascleaned and activated as described by the manufacturer.19

PVG has an open porous structure of fundamentally puresilica with interconnecting pores ranging from 2 to 20 nm.The volume of pores represents approximately 28% of thetotal volume.20 The porous surface contains silanol groups,with acidic hydrogen atoms �pKa�9�, which can be used asactive sites for incorporation of several compounds,14–18

such as the metallo-organic compound employed in this pa-per.

B. Synthesis of �-Fe2O3 NPs dispersed in PVG

As the precursor of iron oxide the single-source metallo-organic compound iron �III� 2-ethylhexanoateFe�OOCCH�C2H5�C4H9�3 was employed and synthesized asdescribed elsewhere.21 The �-Fe2O3 /PVG nanocompositeswere prepared by impregnation of monoliths of PVG in ahexane solution of Fe�OOCCH�C2H5�C4H9�3 �25 ml,1.0 mol l−1� for 8 h at room temperature, followed by a ther-mal treatment at 873 K for 8 h under a synthetic air flow.�-Fe2O3 /PVG with different crystallite sizes were obtainedby employing different IDCs.16–18 We have prepared sampleswith 2, 4, 6, 8, and 10 IDCs. All the monoliths, independentof the number of IDC, were submitted to ten thermal treat-ments completing a total of 80 h at 873 K.

C. Techniques

X-ray measurements were performed on a ShimadzuXRD-6000 powder diffractometer with Cu K� radiation, inthe step scan mode �0.01° with 10 s for counting per step�.Transmission electron microscopy �TEM� images were ob-tained using a Carl Zeiss CEM 902 microscope equippedwith a Castaing–Henry energy filter spectrometer within thecolumn and a Proscan Slow Scan charge-coupled devicecamera, both controlled by a microcomputer running theAnaluSis 3.0 system. �-Fe2O3 /PVG powdered samples weredispersed in de-ionized water by an ultrasonic process and adrop of the suspension was placed onto a carbon coated cop-per grid. The excess liquid was removed using a paper wickand the deposit was dried in the air prior to imaging.

The zero-field-cooled �ZFC� and field-cooled �FC� mag-netization measurements were performed in a superconduct-ing quantum interference device magnetometer �QuantumDesign MPMS XL7�. The dynamics of the magnetizationwas studied by measuring the temperature dependence ofboth in-phase ���� and out-of-phase ���� components of theac susceptibility on a Physical Properties Measuring System�Quantum Design PPMS� at different frequencies �from �=10 Hz to 10 kHz� of the alternating ac field. 57Fe Möss-bauer spectroscopy experiments were performed at low tem-perature using a conventional constant-acceleration spec-trometer with a 57Co /Rh source and a closed-cycle cryostat.

III. RESULTS AND DISCUSSION

A. Structural and morphological properties

The thermal decomposition of freeFe�OOCCH�C2H5�C4H9�3 �outside the porous glass matrix�under a synthetic air flow was performed at 873 K for 8 hand leads to the formation of �-Fe2O3 as evidenced by thepeaks in the powder x-ray diffraction �XRD� spectrum �seeFig. 1�b�� being indexed to those of �-Fe2O3.22 The averagecrystallite size of 47 nm is estimated from x-ray line broad-ening analysis using the standard Scherrer’s equation23 con-sidering the �104�, �110�, �024�, and �116� diffraction peaks.In comparison, the XRD pattern for the Fe2O3 /PVG com-posites as shown in Fig. 1�a� for the ten IDC sample is com-posed of a broad peak near 2�=22°, which arises from thenoncrystalline PVG matrix and three smaller broad peaks athigher angles that can be indexed to either Fe3O4

�magnetite24� or �-Fe2O3 �maghemite25� since their XRDpatterns are essentially indistinguishable. Unfortunately, nei-ther subsequent IR nor Mössbauer spectroscopy measure-ments on the nanocomposite samples were able to provideconclusive evidence as to the iron oxide phase. Due to thelow concentration of the iron oxide and its dispersion withinthe matrix, the IR spectrum of the composite was identical tothe IR spectrum of the pristine PVG. Similarly the Möss-bauer measurements were unable to resolve the tetrahedral�A� and octahedral �B� sites for the Fe atoms. Nevertheless, itis more probable that the resulting iron oxide phase is�-Fe2O3 since maghemite can form from hematite in the473–523 K temperature range26,27 well below the tempera-ture of 873 K used to decompose the organic compounds inthe starting materials. Moreover, the average crystallite di-ameter estimated to be 6 nm from the width of the �311� peakfor the ten IDC sample is much smaller than the diametersizes of thermally stable crystallites of hematite.

The maghemite to hematite phase transition �hereafterindicated as �→� transition� is strongly dependent on crys-tallite size.27–29 �-phase has a surface energy smaller than

�-phase.27 With decreasing D̄ values the �→� transition

FIG. 1. XRD patterns for �a� Fe2O3 /PVG obtained with ten IDCs and �b�free Fe2O3 �dotted line=pristine PVG�.

013901-2 Cangussu et al. J. Appl. Phys. 105, 013901 �2009�

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rate will increase because of the increase in the specific sur-face area. Consequently the transformation rate will increase,therefore decreasing the temperature of the �→� transition.According to Multani,30 �-Fe2O3 is a thermodynamicallystable phase up to diameters of 30 nm. For larger sizes, thethermodynamically stable phase is �-Fe2O3. The �→� tran-sition temperature occurs in the 523–873 K temperaturerange, depending on the prior history.28 Gnanaprakash etal.29 obtained �-Fe2O3 powders through the coprecipitation

method with D̄�5 nm and verified that after thermal treat-ment at 873 K for 1 h, the conversion of �→� transition was

complete and the crystallites of �-Fe2O3 presented D̄�35 nm. The stabilization of the �-Fe2O3 phase needs thecoalescence of the �-Fe2O3 particles that is induced by thediffusion of cations during the thermal treatment, resulting inincreasing the average crystallite size.27,28 The critical size

for the �→� transition to take place is around D̄�15 nm.

Despite the reduction in D̄ to increase the growth rate and

decrease the �→� transition temperature, smaller D̄ valuesinside the pores were found here. This result is significant inthe sense that the final �-Fe2O3 /PVG was submitted to tenIDCs, completing a total of 80 h of thermal annealing at 873K. The inhibition of crystallite growth and the consequentdecrease in the �→� transition was associated with the pres-ence of crystallites dispersed in the porous structures thatprevent their growth through coalescence processes.29

The IDC methodology showed a linear mass incrementinside the PVG �Fig. 2� leading to �-Fe2O3 /PVG with dif-ferent crystallite sizes �see TEM data below� but keeping theaverage distance between the NP almost constant. The linearmass increase in Fig. 2 is the result of a mean diameter sizeincrease in the spherelike shaped NP via layer-by-layer as-sembly. The IDC methodology has been shown to be a ver-satile bottom-up nanofabrication technique. This statement isconfirmed by the TEM results and magnetic analysis in thispaper and by our other manuscripts about CeO2,18 TiO2,17,31

CdS,16 and PbS �Ref. 16� in PVG that show particle sizeincrements as a function of the number of cycles. However,

the correlation between the mass increase and particle sizeincrease with different numbers of the IDCs is not linearbecause the particle size distribution width increases with thenumber of IDC. For low diameter nanocrystals �low numbersof IDCs� both the poor signal-to-noise ratio and the overlapwith the broad peak from PVG make the analysis of crystal-lite size very difficult.

TEM bright field images of the �-Fe2O3 /PVG obtainedwith two and eight IDCs are shown in Fig. 3. An estimate ofnanocrystal size was performed by using the image toolpackage where the size of the nanocrystals is measured by ascale on the computer screen. Typical histograms are shownin the inset of Fig. 3. From TEM images spherelike shapedNP with large size distributions can be inferred and the av-

FIG. 2. Cumulative mass gain of �-Fe2O3 /PVG as a function of the numberof IDC.

FIG. 3. TEM bright field image of �-Fe2O3 /PVG obtained with �a� two and�b� eight IDCs. The insets show histograms of the particle size distributions.

013901-3 Cangussu et al. J. Appl. Phys. 105, 013901 �2009�

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erage nanocrystal sizes were found to be about 3.1�0.7 and5.2�1.1 nm for two and eight IDCs, respectively. The crys-tallinity degree of the �-Fe2O3 nanocrystals is further con-firmed through dark field imaging �not shown here� fromwhich it was possible to observe images from the diffractedelectrons. Therefore, the TEM images show the presence of�-Fe2O3 NPs finely dispersed into porous Vycor glass, thusindicating that the applied synthesis procedure inhibits theirgrowth through coalescence processes.

B. Magnetic properties

Figure 4 shows the ZFC-FC curves for the samples withdifferent numbers of IDCs. At higher temperatures ZFC andFC curves coincide, suggesting that the samples behave astypical superparamagnetic NP. The study of such system hasbeen performed based on the Néel relaxation time,32

� = �0eKV/kBT, �1�

where the characteristic time constant �0 is normally taken inthe range of 10−11–10−9 s, kB is the Boltzmann constant, Kis the uniaxial anisotropy constant, and V is the particle vol-ume. KV represents the energy barrier between two easy di-rections. According to Bean and Livingston,33 at a given ob-servation time �obs there is a critical temperature called theblocking temperature �TB� above which the system behavesas a superparamagnet. On the contrary, below TB the systemsare said to be blocked.33 However, in real systems there is adistribution of particle sizes, which gives rise to a distribu-tion of TB. This distribution is characterized by an averagevalue and width. Therefore, the ZFC curve of an assembly ofmonodomain NP exhibits a maximum at the average block-ing temperature �TB� as a consequence of the progressiveunblocking of NP as the temperature is increased, while theirmagnetization decreases due to thermal fluctuations. TheZFC and FC curves converge at the temperature correspond-ing to the blocking of the biggest NP that herein will becalled the irreversibility temperature �Tirr�. For our samples,the irreversibility temperatures are rather higher than thetemperature where ZFC curves exhibit the maximum, indi-

cating the existence of a broad distribution of NP �Fig. 4�. Itis also remarkable that the peak in the ZFC curves, related tothe average blocking temperature, increases with an increas-ing number of IDC. Such increasing of blocking temperature

can be related to increases in D̄ �Ref. 34� and/or increases inmagnetic interactions between the particles �due to particleconcentration�, as observed in other experimental and theo-retical papers.34–36 In order to distinguish between these twopossibilities, other experimental studies were carried out.

The ZFC curve of a noninteracting system constituted bysingle domain NPs can be calculated considering the distri-bution of particle volumes �f�V�� as37,38

MZFC�H,T�MS

=MS

CH

3K

1

�

Vb�T�

f�V�dV

+MS

CH

3kBT

1

�

0

Vb�T�

Vf�V�dV , �2�

where =�0Vf�V�dV, Vb�T�=25kBT /K, H is the applied

field, MSC is the saturation magnetization of the NP, MS is the

sample saturation magnetization, and T is the absolute tem-perature. The first term in Eq. �2� comes from the blockedparticle contributions, while the second comes from the su-perparamagnetic ones. The FC magnetization curve can becalculated by using an expression similar to Eq. �2�.37 Wehave calculated the ZFC and FC curves of the samples stud-ied by considering a log-normal distribution of volumesMS

C=420 emu /cm3 �bulk value� and using the value of Kdetermined by a procedure described elsewhere36 �K=5.9�105 erg /cm3�. Figure 4 shows, using solid and dottedlines, respectively, the calculated magnetization curves forZFC and FC for different samples. The parameters of thelog-normal distribution resulting in the best fits to the dataare shown in Table I.

As can be seen in Fig. 4, the ZFC curves are well de-scribed by superparamagnetic behavior, even for the samplewith a large number of IDC; thus suggesting that particleagglomeration is not significant in such samples. We find arather large particle size distribution width ��� �Table I� and,

as expected, that the D̄ value increases with the increasingnumber of IDC in agreement with the previous analysis by

TEM. For �-Fe2O3 /PVG with eight IDCs, the D̄ determinedby TEM and magnetic analysis shows good agreement withboth around 5.2 nm. Furthermore, the results of the simula-tion show that the main effect of the number of IDC occursin the width of particle size distribution, with the exceptionof the �-Fe2O3 /PVG with ten IDCs �Table I� where a morepronounced change in magnetic behavior occurs, as will bediscussed later.

In order to obtain more information on the magneticproperties of these systems, the field dependence of the mag-netization up to 60 kOe at different temperatures was alsoinvestigated. The coercive fields obtained from M�H� curvesmeasured at different temperatures are shown in Fig. 5. Theinset in Fig. 5 shows the magnetization loop measured at 2 Kfor the �-Fe2O3 /PVG with ten IDCs, which displays a nar-row hysteresis with a coercive field of 931 Oe.

FIG. 4. Temperature dependence of the ZFC �solid lines� and FC �dottedlines� magnetization curves for �-Fe2O3 /PVG with different numbers ofIDC.

013901-4 Cangussu et al. J. Appl. Phys. 105, 013901 �2009�

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The temperature dependences of coercive field HC forsystems of identical noninteracting particles with random an-isotropic axes follow the well known relation,39

HC = 0.96K

MS�1 − T

TB1/2� , �3�

where TB is the average blocking temperature. It can be in-ferred that the temperature behavior of the samples studiedfollows Eq. �3�, at least over the low temperature range.These results further confirm the existence of single-particleblocking behavior in these systems. An increase in the coer-cive field with increasing particle size of a single-magneticdomains is expected, starting from zero when the particlesize becomes so small that the thermal energy is sufficient toflip the magnetization direction over the energy barrier asso-ciated with the anisotropy of the particle, and reaching amaximum value �2K /MS� when the particle size approachesthe single-domain limit.38 Table I shows that, as the numberof IDC increases from four to eight, a gradual increase in thecoercive field at low temperature is observed. Further in-creases in the IDC induce a decline in the coercive value.This result can be related to a considerable demagnetizingrole played by dipolar interactions.36 Dipolar interaction canmake a collection of magnetic moments of individual par-

ticles behave like a collective magnetic system. Such collec-tive magnetic systems have some magnetic properties similarto those observed in noninteracting superparamagnetic mate-rials but with an effective volume and anisotropy determinedby interplay between the individual particles and theirinteraction.40 Therefore, although we have observed, for thesample with ten ICDs, a good agreement between the ZFCand FC experimental data and that calculated based on theframework of noninteracting superparamagnetic approach,

the value of D̄ obtained from this procedure might be ratherlarger than the actual value.

Magnetic susceptibility is strongly sensitive to interac-tions between particles and thus can be used to better under-stand the magnetic behavior of the �-Fe2O3 /PVG compos-ites prepared with ten IDCs. Figure 6 shows the thermaldependence of the in-phase ���� and out-of-phase ���� acmagnetic susceptibility measured for different frequenciesunder a zero dc field and an ac field of 4 Oe for�-Fe2O3 /PVG with eight IDCs. For both �� and ��, the peakposition shifts toward higher temperatures as the frequencyis increased. Curves with similar profiles were measured forthe other samples studied �data not shown�. The calculatedrelaxation time ��=1 /�� at TB of the samples follows thecharacteristics of a thermally activated mechanism given by

TABLE I. Best parameters of the size analysis distribution used in the ZFC and FC simulation of the experi-

mental data, irreversibility temperature �Tirr�, and the coercive field �HC� values at 3 K and D̄ determined byTEM for the �-Fe2O3 /PVG samples studied.

No. of IDC

Best parameter of ZFC simulation

Tirr

�K�HC

�Oe� at 3 KD̄

�nm� by TEM TM /TM log ��V

D̄�nm�

2 ¯ ¯ ¯ ¯ 3.1�7� ¯

4 0.70�10� 4.7�3� 180�10� 746�5� ¯ 0.090�3�6 0.69�5� 4.8�2� 170�10� 843�5� ¯ 0.087�3�8 0.62�5� 5.2�2� 125�10� 873�5� 5.2�11� 0.083�3�10 0.58�5� 6.1�2� 150�10� 848�5� ¯ 0.082�3�

FIG. 5. Coercive field vs T1/2 for �-Fe2O3 /PVG with four, eight, and tenIDCs. The inset shows the hysteresis loop of �-Fe2O3 /PVG with ten IDCs atT=2 K.

FIG. 6. Thermal dependence of in-phase ���� and out-of-phase ���� ac mag-netic susceptibility measured for different frequencies under a zero dc fieldand an ac field of 4 Oe for �-Fe2O3 /PVG with eight IDCs.

013901-5 Cangussu et al. J. Appl. Phys. 105, 013901 �2009�

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the Arrhenius law �Eq. �1��, with the pre-exponential factorvalues being in the range from �0=1.6�10−14 to 5�10−13 s and the energy barriers for the reversal of the mag-netization in the range of KV /kB=950–1800 K �Fig. 7�.Such magnitudes of �0 and KV /kB are in line with observedvalues for other NP system presenting interactions betweenparticles.35

A useful parameter that can be obtained from ac suscep-tibility is the measurement of the temperature shift in TB,usually obtained from TB /TB per decade of frequency �,i.e., TM /TM log �. This parameter has been used as a cri-teria to distinguish a spin-glass-like transition from a single-particle blocking effect. For our samples this parameter is inthe range of 0.082–0.090 �Table I�, which falls within therange observed for interacting magnetic NP.35,41 Therefore,although the dipolar interactions play an important role in themagnetic behavior of the �-Fe2O3 /PVG with ten IDCs, theac susceptibility results suggest that the ZFC and FC curvesof such samples are dominated by single-blocking of NP.

Moreover, these results support the analysis of the ZFC andFC curves for most of our samples based on the superpara-magnetic approach, indicating nucleation and coarsening of�-Fe2O3 NP for increasing numbers of IDCs.

Figure 8 shows the 57Fe Mössbauer spectra at 23 K forsamples with different numbers of IDC. The measurementsdo not result in any well resolved spectra, but show the su-perposition of a superparamagnetic component �broad dou-blet� with a broad magnetically ordered component, behaviortypical of superparamagnetic �-Fe2O3 NPs distributed insize.42 The broad doublets, attributed to the smallest particlesin the samples, dominate the spectra for two and four IDCs,and the broad six-line component, due to larger magneticallyordered particles, is dominant starting from six IDCs.

The superparamagnetic behavior of our samples is fur-ther illustrated in Fig. 9 where Mössbauer spectra for thesample �-Fe2O3 /PVG prepared with six IDCs at 23, 35, and50 K are shown. The fast collapse in the magnetic order�decrease in the broad six-line components� of the NP with arelatively small increase in the temperature is quite evident.

IV. CONCLUSION

In conclusion, we have shown that the SSP approach isan appropriate method for preparing �-Fe2O3 NP dispersedin porous Vycor glass with suitable superparamagnetic prop-erties. All experimental results suggest that the particle sizedistribution is the parameter that most significantly changesas a function of different numbers of IDC. The magneticproperties of the samples with different particle size distribu-tions follow well-established models for noninteracting par-ticle systems. Although the interparticle interactions start tobe important to the magnetic behavior of the nanocompositeafter a large number of IDC, progressive blocking of indi-vidual NP is observed for all samples studied.

FIG. 8. 57Fe Mössbauer spectra at 23 K for �-Fe2O3 /PVG with differentnumbers of IDC.

FIG. 9. 57Fe Mössbauer spectra of �-Fe2O3 /PVG with six IDCs as a func-tion of temperature.

FIG. 7. The log�1 /�� vs 1 /TB plot for a wide range of measured frequenciesof the �-Fe2O3 /PVG samples studied.

013901-6 Cangussu et al. J. Appl. Phys. 105, 013901 �2009�

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ACKNOWLEDGMENTS

The authors are grateful to FAPESP, CNPq, and CAPESfor financial support and to Professor C. H. Collins �IQ-UNICAMP, Brazil� for English revision.

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