Magnetic and Structural Investigation of Highly Porous CoFe2O4

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J. Phys. Chem. C 2007, 111, 916-922

Magnetic and Structural Investigation of Highly Porous CoFe2O4-SiO2 Nanocomposite Aerogels Alberto Casu, Maria F. Casula, Anna Corrias,* Andrea Falqui, Danilo Loche, and Sergio Marras Dipartimento di Scienze Chimiche and INSTM, UniVersita` di Cagliari, S.P. Monserrato-Sestu Km 0.700, I-09042 Monserrato, Cagliari, Italy ReceiVed: August 4, 2006; In Final Form: October 17, 2006

The structural and magnetic properties of CoFe2O4-SiO2 nanocomposite porous aerogels have been systematically investigated by XRD, TEM, and SQUID magnetometry. The preparation of the samples was performed by a cogelation sol-gel route which allows one to obtain nanocomposites with high purity and homogeneity and to control the composition of the sample in terms of ferrite loading in the nanocomposite. In particular, the compositional effect on both the structural and magnetic properties of the nanocomposite was investigated in two nanocomposites with relative CoFe2O4 amounts of 5 and 10 wt %. The characterization shows both similarities and differences in the structural evolution of the nanophases within the amorphous porous matrix of the two nanocomposites as a function of the calcination temperature. In both samples the ferrite nanoparticles are formed above 750 °C; however, a slower structural evolution is observed in the nanocomposites with 5 wt % CoFe2O4. The nanocomposites display superparamagnetic behavior with blocking temperature that increases with both the ferrite amount and the calcination temperature. The importance of combining information from different techniques is also discussed, with particular reference to nanocrystal size determination. The results provide insights into the characterization of innovative materials and on how the compositional and microstructural features affect the properties of magnetic nanocomposites.

1. Introduction Recent advances in preparation procedures have led to innovative magnetic nanocomposites with more than one functionality by combining the properties of the dispersed magnetic phase and of the matrix.1,2 Sol-gel techniques3 can be successfully used for the preparation of nanocomposites with an accurate control over the composition, purity, and homogeneity at a microscopic level.4-12 In particular, the well-developed sol-gel silica chemistry has been widely applied for the preparation of silica-based nanocomposites with controlled textural and microstructural features.4-10 Silica aerogels provide unique features such as chemical inertness, lightness, transparency, and low dielectric constant, related to the porous, fractal-like silica matrix.13,14 Innovative magnetic nanocomposites such as high-coercivity NdFeB-SiO2 aerogels have been prepared by exploiting the large and accessible porosity of aerogels.15 Nanocomposites containing ferrite (general formula MFe2O4) nanoparticles are of particular interest,16,17 due to the broad range of applications of ferrites, including magnetic fluids,18 drug delivery,19 and magnetic high-density information storage.20 Pure iron oxide-silica aerogel nanocomposites have been obtained either by diffusion from the vapor phase,21 by impregnation from solution into a preformed porous silica aerogel matrix,22 or by cogelation of silica and iron oxide precursors.23 However, little work has been done so far on ferrite-silica nanocomposite aerogels. Cobalt ferrite-silica nanocomposites have high potential for applications due to the remarkable properties of bulk cobalt ferrite (large anisotropy, high saturation magnetization and * Corresponding author. E-mail: [email protected]. Phone: +39 070 675 4351. Fax: +39 070 675 4388.

coercivity, mechanical hardness, chemical stability),24 combined with the magnetic properties typical of the nanoparticles (superparamagnetism), which depend dramatically on particle size and shape, particle-matrix interactions, and degree of dispersion throughout the matrix. Hence, the design of such magnetic nanocomposites should involve a careful and complete characterization of both magnetic and structural properties using a multitechnique approach. In characterizing nanocomposites, several techniques might need to be used complementarily including well-established techniques such as powder X-ray diffraction (XRD), which however may be limited by severe line broadening due to size effects. Transmission electron microscopy (TEM) investigation is also required since it can provide information on the shape, size, and distribution of the nanoparticles within the matrix. Important complementary information about mean nanoparticle volume and its distribution can be extracted from magnetic measurements. This is because the magnetic behavior of systems consisting of isolated, single-domain ferro- or ferrimagnetic particles, depends on the relative magnitude of the thermal energy with respect to KV (V being the particle volume and K the magnetic anisotropy constant). When kT . KV, the magnetization vector of a particle can rotate in response to a change in temperature or applied field, whereas when kT , KV the presence of the anisotropy barrier impedes or blocks the rotation.25 Hence, studying the blocking of magnetization vectors of an ensemble of particles gives indirect information about its mean volume and distribution. It should be noted that the contribution of magnetic interactions among the particles has to be taken into account. The aims of the present work are the magnetic and structural characterization of CoFe2O4-SiO2 nanocomposite aerogels. The work investigates the effects on the magnetic and structural

10.1021/jp0650230 CCC: $37.00 © 2007 American Chemical Society Published on Web 12/09/2006

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features of different amounts of cobalt ferrite at the different stages of preparation. The magnetic behavior is correlated with the structural properties as determined by XRD and TEM in order to interpret the magnetic properties in terms of size, amount of magnetic nanophase, and magnetic interactions. The study shows the importance of a combined, multitechnique approach for revealing the microstructural features, with particular emphasis on characterizing nanoparticle size. 2. Experimental Section 2.1. Preparation of Nanocomposite Aerogels. The samples were prepared according to the sol-gel procedure reported in ref 26. Briefly, tetraethoxysilane (Si(OC2H5)4, Aldrich 98%, TEOS) and iron(III) and cobalt(II) nitrates (Fe(NO3)3‚9H2O, Aldrich, 98%, and Co(NO3)2‚6H2O, Aldrich, 98%), were used as precursors for the silica and for the cobalt ferrite phases, respectively; absolute ethanol (purchased from Fluka) was used as mutual solvent. The alcogel was obtained by a two-step procedure, including the mixing of the ethanolic solution of the metal salts into the TEOS prehydrolyzed under acidic catalysis, followed by the addition of urea (NH2CONH2, Sigma-Aldrich, >99.0%) under reflux for 2 h at 85 °C. Gelation was performed at 40 °C in a closed container and occurred in less than 2 days. In order to obtain the aerogel samples, the alcogels were submitted to high-temperature supercritical drying in an autoclave (Parr, 300 cm3) flushed with N2, and filled with an appropriate amount of ethanol. After reaching 330 °C and 70 atm the autoclave was vented, and porous aerogels were obtained. After supercritical drying, the aerogel samples, which were almost monolithic and had an apparent density of about 0.07 g‚cm-3, were powdered and calcined at 450 °C in static air for 1 h in order to eliminate the organics. The weight loss after calcination at 450 °C was about 8%. The samples were further calcined at increasing temperature. Two samples were prepared, which differ for the relative amount of the ferrite phase in the nanocomposite: 5 and 10 wt % CoFe2O4/(CoFe2O4 + SiO2). The aerogel samples will be hereafter called AX_YYY, where X refers to the amount of CoFe2O4 and YYY to the calcination temperature. 2.2. Structural Characterization. X-ray diffraction (XRD) patterns were recorded on a D500 Siemens diffractometer equipped with a graphite monochromator on the diffracted beam. The scans were collected within the range of 4-45° (2θ) using Mo KR radiation. The crystallite average size was calculated using the Scherrer formula; instrumental broadening was determined on a standard Si sample.27 TEM bright-field (BF) and dark-field (DF) images and selected-area electron diffraction (SAED) patterns were obtained on a JEOL 200CX microscope equipped with a tungsten cathode operating at 200 kV. DF images were obtained by selecting the 311 ring of the electron diffraction pattern of the CoFe2O4 phase. The samples were dispersed in ethanol and dropped on a holey carbon-coated copper grid. 2.3. Magnetic Characterization. Measurements of static magnetizations and hysteretic behavior of the samples were performed on a Quantum Design MPMS SQUID magnetometer, equipped with a superconducting magnet producing fields up to 50 kOe. Zero-field-cooled (ZFC) magnetizations were measured by cooling samples in zero magnetic field and then by increasing the temperature in an applied field of 25 Oe, while field-cooled (FC) curves were recorded by cooling the samples in the same field of 25 Oe. The field dependence of the magnetization (hysteresis loop) was recorded up to (50 kOe, at T ) 4 K. Finally, thermoremanent magnetization (TRM) was

Figure 1. XRD patterns of the A5 (dotted line) and A10 (solid line) nanocomposites after calcination at 450, 750, and 900 °C.

measured in zero field during the heating of the samples, after cooling them in a field of 25 Oe. 3. Results and Discussion 3.1. Structural Characterization. The A10 samples as obtained and after calcination at increasing temperatures were previously investigated by XRD, showing that cobalt ferrite formation occurs upon prolonged heat treatment at 750 °C and is complete after treatment at 900 °C.26 By further increasing the calcination temperature the average crystal size increases and the matrix undergoes partial condensation and crystallization.26 In Figure 1 the XRD patterns of the A5 and A10 samples are compared, after the initial calcination at 450 °C for 1 h and after further calcination at 750 °C for 6 h and at 900 °C for 1 h. All the patterns show the typical halo due to the amorphous silica matrix; in the A5 sample some extra broad and weak peaks which can be attributed to CoFe2O4 are detectable only after calcination at 900 °C.28 On the other hand, in the A10 sample some weak and broad peaks can be observed even in the sample treated at 450 °C. These peaks were attributed to two separate nanophases, containing iron and cobalt, respectively, on the basis of the comparison with the nanocomposites containing only iron or cobalt.26 After calcination at 750 °C the peaks attributable to CoFe2O4 are clearly evident,28 although a shoulder at about 27° indicates that one of the nanophases observed at 450 °C is still present. At 900 °C only the peaks due to the cobalt ferrite, which are more intense and sharper than in the sample treated at 750 °C, are present superimposed to the silica halo. The average crystallite size, estimated by diffraction line broadening, is 6 nm. More detailed information on the evolution of the cobalt ferrite nanoparticles during the calcination treatments can be obtained by extended X-ray absorption fine structure (EXAFS) spectroscopy,29 because it is an elemental specific technique which has recently shown to be a powerful tool for the structural study of metal oxide nanoparticles and nanocomposites prepared by the sol-gel process.30-36 An EXAFS investigation of the nanocomposites presented in this paper is currently underway and will be reported elsewhere.37 Preliminary results confirm

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Figure 2. Representative TEM bright-field and corresponding dark-field images of the A5 (left) and A10 (right) nanocomposites after calcination at 450 (A, D), 750 (B, E), and 900 °C (C, F).

that after calcination at 450 °C the iron and cobalt are still present as separate phases and further calcination promotes the formation of the desired ferrite nanoparticles. Information on nanocrystal size and dispersion within the matrix was obtained by transmission electron microscopy. Figure 2 compares representative TEM images of the A5 (left) and A10 (right) nanocomposite samples at the different stages of the calcination treatments. The bright field images of both the A5 and A10 samples show that all the samples exhibit a highly branched and very porous texture typical of aerogel materials, which is retained even after calcination up to 900 °C. These observations are in agreement with physisorption data, which indicated that the samples possess high surface areas (from 600 m2‚g-1 for A10_450 to 400 m2‚g-1 for A10_ 900, for example) due to the presence of meso- and macropores.26 From brightfield images, however, the accurate determination of nanocrystal size cannot be achieved due to the weak contrast difference between the dispersed phase and the matrix and due to the small nanocrystal size. On the other hand, nanocrystals can be imaged in dark-field mode as bright spots on the dark background of amorphous silica. In the A5_450 sample, however, no crystalline spots can be observed, and also in the A5_750 sample bright spots due to nanocrystals at the lower limit of the detectable size are barely visible and become just slightly more evident in the A5_900 sample. The presence and evolution of the nanocrystals is more clearly evidenced in the dark-field images of the A10 sample: around 2 nm nanocrystals are observed in the A10_450 sample, around 5 nm nanocrystals are visible in the A10_750, and after calcination at 900 °C the presence of nanocrystals between 4 and 7 nm is observed. It should be noted that due to the small size of the nanocrystals together with the high porosity of the sample, the nanocrystal size distribution determined by statistical analysis of TEM images is affected by a poor statistics. TEM results indicate that smaller nanocrystals are formed in the A5 samples with respect to the A10 samples calcined at the same temperature and that the nanophases undergo limited growth during calcination, especially in the A5 sample. More-

Figure 3. ZFC-FC magnetization curves (a) obtained by a magnetic field of 25 Oe and hysteresis loops (T ) 4 K) (b) of the A10 sample calcined at 450, 750, and 900 °C.

over, the high porosity and homogeneity of the nanocomposite ensures that the nanocrystals are well distributed even after calcination at high temperature and no aggregates are observed. 3.2. Magnetic Characterization. The magnetic properties of each sample have been investigated by SQUID magnetom-

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TABLE 1: Parameters Obtained from the Hysteresis Loops (T ) 4 K) of the A10 Sample Calcined at 450, 750, and 900 °C sample

Hc (Oe)

Mr (emu/g)

Msat (emu/g)

Mr/Msat

CoFe2O4, calc wt %

A10_450 A10_750 A10_900

1666(15) 10062(15) 14286(15)

1.13(1) 2.09(1) 4.57(1)

7.80(5) 8.28(5) 9.95(5)

0.14(1) 0.25(1) 0.46(2)

8.9(5) 10.6(5)

etry. All the results have been normalized with respect to the total mass of the sample. Figure 3a reports the measurements of the magnetization versus temperature, according to the ZFC-FC procedure, for the A10_450, A10_750, and A10_900 samples. The ZFC-FC magnetization curves show a superparamagnetic behavior for all the systems. The ZFC curve of the A10_450 sample displays a sharp maximum in the range of 4-27 K. In particular, Tmax, i.e., the temperature corresponding to the maximum of the ZFC curve, is 13 K. This temperature is related in a complex way to the particle mean size. The temperature at which the ZFC and FC curves merge, Tcross, is around 15 K. Such a behavior corresponds to the superparamagnetic transition with a very sharp distribution of blocking temperatures38,39 due to an almost monodispersed nanoparticle population in the sample. On the other hand, the ZFC curves of A10_750 and A10_900 show a broad maximum in a thermal range starting at around 130 K for both samples. Tmax is around 273 and 303 K for the A10_750 and A10_900 samples, respectively, while their Tcross are out of the exploited thermal range, indicating that at T ) 325 K a fraction of the magnetic nanoparticles is still in the blocked state. These results show that the A10_450 sample is quite different with respect to the A10_750 and A10_900 samples. The behavior of A10_450 sample can be due to the occurrence of two different phenomena: (a) the formation of a magnetic compound with a lower magnetic anisotropy than CoFe2O4; (b) a very small nanoparticle mean size. On the basis of TEM and preliminary EXAFS results, we can affirm that CoFe2O4 is not present at this stage and that we are in the presence of very small particles. In order to gain more information, we have recorded the ZFC-FC curves for two composite samples containing either only iron or cobalt (total metal amount in the composite 10 wt %), as reported in the Supporting Information. The results (reported in Figure S1) indicate that the magnetic behavior observed for the composite containing only cobalt is analogous to the one observed for the A10_450 sample. These results further support that two separate nanophases containing iron and cobalt are present in the nanocomposite calcined at 450 °C, whose magnetic properties are related to the nanophase containing cobalt. Figure 3b shows the hysteresis loops measured at 4 K of the A10_450, A10_750, and A10_900 samples, and Table 1 summarizes the parameters obtained from these measurements. The saturation magnetization (Msat) was derived from a plot of M versus 1/H, extrapolating the M values for H f 0.40 Moreover, the wt % of cobalt ferrite contained in the samples was estimated by comparison with value reported in literature for bulk CoFe2O4 (Ms,bulk ) 93.9 emu/g),41 neglecting the small diamagnetic contribution due to the silica matrix. The data obtained from the hysteresis loops confirm the substantial difference among the A10_450 sample on one side and the A10_750 and A10_900 samples on the other side. First of all, the coercive field increases more than a factor 5 when passing from A10_450 to A10_750; these data further support that the observed differences should not be ascribed to a simple

Figure 4. ZFC-FC magnetization curves (a) of the A5_750 (multiplied by a factor 8) and A5_900 samples, obtained by a magnetic field of 25 Oe. (b) Hysteresis loops (T ) 4 K) of A5_750 and A5_900 samples.

increase in the average size and size distribution of nanoparticles with the calcination temperature. In fact, following the StonerWohlfart theory,25 a coercive field around 4000 Oe or higher would be expected for the A10_450 sample if CoFe2O4 nanoparticles were present. At the same time, even though the extrapolated saturation magnetization would be compatible with the presence of 8 wt % of cobalt ferrite, the magnetic moment of the A10_450 sample can be ascribed to the presence of a compound with uncompensated total magnetic moment and with very small particles. The evidence of very small mean nanoparticle size is confirmed by the shape of ZFC-FC curves and by the very low Mr/Msat value, the latter indicating that a large amount of nanoparticles are still superparamagnetically fast relaxing at 4 K. These results are also supported by TEM and XRD analysis which show that nanocrystalline particles are not easily detectable at this stage, the nanocrystal size being at the limit of detection for these techniques. On the contrary, the ZFC and FC magnetization curves of the A10_750 and A10_900 samples, together with the parameters obtained from the corresponding hysteresis loops, are consistent with the presence of cobalt ferrite nanoparticles. It should be noted that the increase of both coercive field and Tmax value going from A10_750 to A10_900 indicates a slight TABLE 2: Parameters Obtained by the Hysteresis Loops (T ) 4 K) of the A5 Sample Calcined at 750 and 900 °C sample

Hc (Oe)

Mr (emu/g)

Msat (emu/g)

Mr/Msat

CoFe2O4, calc wt %

A5_ 750 A5_900

3206(15) 11531(15)

0.23(1) 1.87(1)

2.47(5) 4.83(5)

0.09(1) 0.46(2)

2.6(1) 5.1(2)

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Figure 5. (a) TRM of the A10_750 and A10_900 samples. (b) TRM of the A5_750 and A5_900 samples. In the inset, f(T) defined by eq 1 and obtained from each TRM curve is reported.

increase of cobalt ferrite nanoparticle mean size; at the same time the CoFe2O4 wt % amount also increases with the temperature of treatment, in accordance with XRD, which indicates that a fraction of the phase present at 450 °C is still present at 750 °C. The nominal value greater than 10 wt % for the A10_900 sample can be due to the error related to the extrapolation of Msat and also to the fact that this value can be affected by local variations of inversion degree and by variation in stoichiometry.42 Due to the very low magnetic response obtained for the sample A10_450, the study of the magnetic properties of the more diluted A5 samples was carried out only on those treated at 750 and 900 °C.

In Figure 4a, the ZFC-FC magnetization versus temperature curves of the A5_750 and A5_900 samples are reported. Tmax and Tcross of the samples are, respectively, 22 and 152 K for A5_750 and 49 and 252 K for A5_900. The ZFC-FC magnetizations also confirm for the A5 samples that the magnetic behavior is typical of superparamagnetic systems. Moreover, the ZFC and FC curves indicate that the mean nanoparticle diameter and the width of the size distribution is smaller for the A5_750 sample than for the A5_900 sample. In Figure 4b the hysteresis loops measured at T ) 4 K for the A5 samples are shown, and in Table 2 the parameters derived from the loops are reported. The coercive field of the A5_750 indicates that a fraction of cobalt ferrite nanoparticles is formed;

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TABLE 3: Mean Size Values, TRM, as Determined by Means of TRM Curves sample

TRM/(nm)

A10_750 A10_900 A5_750 A5_900

9.0 9.4 2.5 4.2

as inferred by the very low Mr/Msat value together with the low Tmax, the mean size of the particles is very small. The comparison of the extrapolated Msat values with that of bulk CoFe2O4 allows one to estimate that around half of the nominal composition is converted to cobalt ferrite in the A5_750, if we assume that Msat is not influenced by a surface spin canting effect, while the conversion is complete for A5_900. The comparison between A5 and A10 samples indicates that the formation of cobalt ferrite is somewhat slower in the 5 wt % sample since at 750 °C only 50% of the expected CoFe2O4 is formed. Finally, measurements according to the TRM protocol were carried out in the temperature range of 4-400 K for the A10 (Figure 5a) and A5 (Figure 5b) samples treated at 750 and 900 °C. The sample were cooled from 325 to 4 K with an external static magnetic field of 25 Oe; then, the field was turned off, and the magnetization was measured while warming up to 400 K. The magnetization measured with TRM protocol decreases with the temperature increase, and when all the nanoparticles go to the superparamagnetic state, MTRM falls to zero. From TRM magnetization information about the distribution of magnetic anisotropy energy of the nanoparticles can be obtained. In fact, as reported in ref 43, the derivative of the magnetization decay plot represents the distribution of anisotropy energy barriers:

f(T) )

-dMTRM dT

(1)

The TRM curves provide a useful indirect method to determine the cobalt ferrite mean size of the samples, something that was not available by direct TEM observations for the A5 samples and that was also uncertain for the A10 sample. In fact, assuming that the anisotropy constant Keff of the cobalt ferrite nanoparticles is equal to that of the bulk cobalt ferrite (1.81.9 106 erg cm-3),44,45 the equation46

KeffV ) 25kBTB

(2)

where V is the mean nanoparticle volume and kB the Boltzmann constant can be used to obtain the mean size values TRM. TB, as often reported in the literature, can be taken equal to Tmax. However, this is an approximation that does not take into account both the shape of the particle size distribution and the contribution to Tmax that can arise from the presence of magnetic interactions among the particles. For these reasons, the real TB of each sample was considered to correspond to the temperature of the maximum of the TRM derivative, as indicated in refs 43 and 47, by assuming that the size distribution of the particles is symmetric. Moreover, although some weak interparticle magnetic interactions can be present, the thermal magnetization decay is not affected by them.47 In Table 3 the TRM obtained as described above are reported. It has to be specified that the obtained values are likely slightly overestimated. In fact, it cannot be excluded that the real anisotropy constant, which was used in the calculation of

the values reported in Table 3, is enhanced with respect to that of bulk ferrite due to the surface contribution that usually increases as the nanoparticle mean size decreases. The slight increase of the value observed for the A10 samples is in agreement with the trend observed by TEM analysis. On the other hand, for the A5 samples the magnetic measurements are the only way to obtain an indirect determination of the particle mean diameter. The results are consistent with the common feature that smaller mean nanoparticle size is observed in samples with a lower amount of cobalt ferrite. Furthermore, the absolute difference between the Tmax and TB values obtained from TRM curves can be considered as a qualitative measure of the strength of interparticle magnetic interactions. This difference is 18 K for the A5_750 sample, 29 K for the A5_900 sample, 70 K for the A10_750 sample, and 73 K for the A10_900 sample. This seems to indicate that the strength of the interactions is increasing with both the ferrite content and the temperature of calcination treatment. This fact is not surprising considering that the strength of magnetic dipolar interactions, the most probable interactions in our samples, increases with the magnetic moment of the interacting nanoparticles (proportional to their volume) and decreases with their separation distance, the first effect being largely predominant in our samples. 4. Conclusions A suitable sol-gel procedure, based on urea-assisted cogelation, supercritical drying, and calcination treatments, was successful in obtaining highly porous CoFe2O4-SiO2 nanocomposite aerogels with different content of nanophase in the composite (5 and 10 wt %). The formation of CoFe2O4 nanocrystals occurs upon calcination of the starting aerogels in which two poorly crystalline separate phases of iron and cobalt are present. XRD, TEM, and magnetic characterization indicates that the structural evolution of the nanocomposites as a function of calcination is affected by the amount of nanophase present in the sample. In particular, in the A10 aerogel a significant amount of CoFe2O4 is formed at 750 °C, whereas in the A5 sample only about half of the dispersed phase is present as CoFe2O4. On the other hand, in both samples the formation of the cobalt ferrite is complete at 900 °C, although the average crystallite size of the nanoparticles is different in the two samples, i.e., at the limit of TEM detectability for the A5_900 sample and about 5 nm for the A10_900 sample, respectively. As a consequence of the compositional effect and consequent different microstructural features, significant differences are observed in the collective magnetic behavior. A superparamagnetic behavior was observed for all the CoFe2O4-SiO2 samples, but the magnetic state varies noticeably in the A5 and A10 samples; for example, a blocking temperature of 49 and 303 K was determined for the A5_900 and A10_900 samples, respectively. The magnetic and hysteretic behavior of the samples is in agreement with the structural characterization, in terms of the amount and crystallinity of the nanophase and of the mean and distribution of the nanocrystal size at the different calcination temperatures. In particular, the extensive magnetic characterization has provided indirect microstructural data which could not be achieved by the other structural techniques due to the small size and poor crystallinity of the nanophase. Acknowledgment. Funding was provided by the Italian Ministero dell’Istruzione, Universita` e Ricerca (MIUR PRIN). Supporting Information Available: ZFC-FC magnetization curves of samples containing only either Fe or Co. This

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