ARTICLE pubs.acs.org/JPCC
€ssbauer Spectroscopy Studies X-ray Absorption Spectroscopy and Mo of Superparamagnetic ZnFe2O4 Nanoparticles V. Blanco-Gutierrez,† F. Jimenez-Villacorta,‡ P. Bonville,§ María J. Torralvo-Fernandez,† and R. Saez-Puche*,† †
Departamento de Química Inorganica, Facultad de Ciencias Químicas, Universidad Complutense, Ciudad Universitaria, 28040 Madrid, Spain ‡ SpLine, Spanish CRG beamline at the European Synchrotron Radiation Facility, 6 rue Jules Horowitz B.P.: 38043-Grenoble Cedex, France, and Instituto de Ciencia de Materiales de Madrid, Consejo Superior de Investigaciones Científicas, calle Sor Juana Ines de la Cruz, 3, 28049-Madrid, Spain § CEA, Centre d’Etudes de Saclay, IRAMIS/Service de Physique de l’Etat Condense, 91191 Gif-sur-Yvette, France
bS Supporting Information ABSTRACT: Zn ferrite nanoparticles ranging from 3 to 19 nm have been obtained under solvothermal conditions. In addition, two samples of ZnFe2O4/SiO2 nanocomposite of 11 and 20 nm ferrite particle size have been also synthesized by means of the sol-gel method. To evaluate the inversion degree of these ferrite samples with mixed spinel structure ((Zn1-xFex)[ZnxFe2-x]O4), X-ray absorption spectroscopy measurements at the Zn and Fe K-edge have been performed, and the obtained spectra along with the Fourier transform of the EXAFS signal have been compared to those corresponding to a ceramic sample with a lower inversion degree. All the samples composed by nanosized particles illustrate superparamagnetic behavior above the blocking temperature as it can be inferred from the high values of ZFC and FC magnetic susceptibility that they present. M€ ossbauer measurements done to the different nanosized particles obtained by the solvothermal method were useful to study the “frozen” to “superparamagnetic” crossover with the thermal variation and estimate the anisotropy constant that has been found to increase as the particle size decreases.
’ INTRODUCTION Magnetic nanoparticles have been subject of many studies in the past decade due to their technological importance. This can be attributed to the fact that they present one of the most interesting features concerned with magnetic systems, the superparamagnetic behavior.1-3 This behavior characterized by the absence of coercive field (HC) occurs above a certain temperature called the blocking temperature (TB) and is related to the single-domain regime for nanoparticles below a critical size.4 At temperatures higher than TB, the thermal energy is sufficiently effective to overcome the energy barrier created by the magnetocrystalline anisotropy, and therefore the moment of each particle is able to fluctuate. This contributes to the consideration of these materials as good candidates for potential applications such as ferrofluids, catalysts, and for biomedical purposes.5-7 ZnFe2O4 compound presents spinel structure (S.G. Fd3m) in which the O2- ions are arranged in a cubic close packing being occupied 1/8 of tetrahedral (A) and 1/2 of octahedral (B) interstitial sites. The stable zinc ferrite phase corresponds to the normal spinel structure (Zn2þ)[Fe3þ2]O2-4 in which the A sites are occupied by Zn2þ ions because of their favorable trend to assume sp3 hybridization.8 In this kind of spinel structure, the r 2011 American Chemical Society
superexchange interactions (JBB) take place through a 90° angle of the Fe-O-Fe pathway, giving as a result the antiparallel magnetic moments of the Fe3þ ions located in the B sites. This weak interaction fully justifies the low value of 10.5 K reported for the Neel temperature below which the ferrite presents zero net magnetic moment.9 However, nanostructured zinc ferrite compound, which is a metastable phase, presents mixed spinel structure (Zn2þ1-xFe3þx)[Fe3þ2-x]O2-4, where x is the socalled inversion degree. In this case, the Zn2þ and Fe3þ ions are distributed along the A and B sites, and the superexchange interaction (JAB) occurs between the Fe3þ ions located in tetrahedral and octahedral sites giving antiparallel moments. The JAB interaction is larger than the JBB due to the more effective overlap of the involved orbitals in the pathway Fe3þA-O-Fe3þB as the angle is around 130°.10 Zn-ferrite nanoparticles present ferrimagnetic behavior below the TB because the Fe3þ magnetic sublattices are decompensated. It has been found a fundamental relationship between the inversion Received: September 30, 2010 Revised: December 21, 2010 Published: January 14, 2011 1627
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Table 1. Synthesis Conditions of ZnFe2O4 Nanoparticles Obtained by the Solvothermal Method (Z3-Z19) and by Means of the Sol-Gel Method (ZM11 and ZM20)
a
sample
[nitrates] (mol/mL)
[KOH] (M)
Ta (°C)
ta (h)
Da (nm)
Z3 Z4 Z10 Z11 Z14 Z19 ZM11 ZM20
10-5 10-4 10-4 10-5 10-5 10-5
0.5 2 2 0.5 0.5 0.5
160 160 160 160 160 200 900 1000
2 2 168 24 168 288 12 48
3 4 10 11 14 19 11 20
T, treatment temperature; t, treatment time; D, mean ferrite particle size (major dimension).
degree parameter and the magnetic properties.11-13 To obtain information about the Fe3þ occupancy in both sites that could help in the interpretation of the magnetic behavior, some techniques can be employed such as EELS,14 nuclear magnetic resonance, or neutron diffraction.15,16 In this work, we present how useful as well the X-ray absorption measurements at the Zn and Fe K-edge can be to evaluate the local atomic environments in the case of nanocrystalline ZnFe2O4 particles that have been obtained under different synthesis methods and conditions, and how the 57Fe M€ossbauer spectroscopy helps in the understanding of the magnetic behavior of such nanoparticles.
’ EXPERIMENTAL SECTION ZnFe2O4 Particles Obtained by the Ceramic Method. Iron and zinc nitrate, this last one in 5% molar excess, were mixed homogenously by grinding. The mixture was treated in air atmosphere at 950 °C (with a heating rate of 10 °C/min) during 4 h. After the thermal treatment, the obtained solid was cooled slowly. The sample was named ZC. ZnFe2O4 Nanoparticles Obtained by the Solvothermal Method. The synthesis was carried out following the experimental details explained in a previous work.17 Stoichiometric amounts of the metal nitrates were dissolved in ethylene glycol. Afterward, KOH was added to the mixture as precipitant agent until reaching pH = 11. The obtained mixture was transferred into an autoclave to be treated under solvothermal conditions, and the obtained product was recovered after filtering and washing with distilled water. In Table 1 is collected the different synthesis conditions used to obtain particles with different size and crystal chemistry. The obtained samples were named as Z followed by the mean particle size of the samples. ZnFe2O4 Nanoparticles Embedded in Amorphous SiO2 Matrix. ZnFe2O4/SiO2 nanocomposites were obtained in a weight ratio of 30/70 following the experimental method indicated in a previous work.17 The metal nitrates were dissolved in ethanol with tetraethylorthosillicate (TEOS) as precursor of the silica matrix and distilled water to be added later. Portions of the obtained gel were subjected to different thermal treatments (see Table 1) to obtain embedded ferrite nanoparticles with different size. The two samples were named as ZM followed by the mean particle size of the samples. Characterization Techniques. The structural phases of the samples were identified by X-ray diffraction employing a Siemens D-5000 powder diffractometer (25 mW, 35 kV) with a Cu KR radiation. Microstructural characterization was evaluated by transmission electron microscopy (TEM) using a JEOL-2000FX microscope working at 200 kV. Magnetic measurements were done
Figure 1. TEM images of samples (a) ZC, (b) Z4, (c) Z10, (d) Z14, (e) Z19, (f) ZM11, and (g) ZM20.
in a Quantum Design XL-SQUID magnetometer in the temperature range of 4-300 K up to 5 T. Magnetic susceptibility was measured after cooling the sample at 4 K in zero-field cooling (ZFC) and in the case of field-cooling measurements (FC), the sample was cooled in the presence of a 500 Oe field down to 4 K. X-ray absorption spectroscopy measurements at the Zn and Fe K-edges were performed at the Spanish CRG beamline (SpLine) of the ESRF. Energy was set by using a double-crystal Si(111) monochromator. Spectra were collected in the transmission mode, using two gas ionization chambers filled with N2 and Ar to detect the incoming and the transmitted beam, respectively. Samples were carefully ground with cellulose in agate mortar for some minutes to form a homogeneous mixture that later was pressed using a standard die. Zn and Fe foils were used as a reference for energy calibration. Data treatment and normalization of the edge spectra was achieved using the ATHENA software.18 Extended X-ray absorption fine structure (EXAFS) analysis was performed using the VIPER program.19 Amplitude and phase functions were previously obtained with the FEFF8 code.20 EXAFS signal ranges for the analysis were [3.3, 12.5] Å-1 for the Zn K-edge and [3.3, 13.5] Å-1 for the Fe K-edge, weighted in all cases by a smooth Hanning window. Analysis for the pseudoradial Fourier transforms was performed collecting up to the fifth neighbor, at around 3.6 Å. M€ossbauer spectra on the isotope 57Fe were carried out for the samples in the temperature range 1.4-50 K in a zero applied magnetic field using a commercial 57Co:Rh γ-ray source. 1628
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Figure 2. ZFC and FC susceptibility measured at 500 Oe for the indicated samples.
’ RESULTS AND DISCUSSION The X-ray diffraction diagrams for all the samples show Bragg reflections that can be indexed to the spinel structure (see the Supporting Information). In the case of the ferrite nanoparticles embedded in silica matrix, the diagrams also present a broad reflection peak around 20° that corresponds to the amorphous SiO2. TEM images of the samples are shown in Figure 1. As it can be observed, the ferrite particles obtained by the ceramic method (ZC sample) present the largest size (Figure 1a) whose value is around 300 nm. The TEM images of the samples obtained by the solvothermal method Z4, Z10, Z14, and Z19 shown in Figure 1b-e display rounded polyhedral particles with homogeneous size. In Figure 1f and g are shown the TEM images of the nanocomposites ferrite/SiO2 ZM11 and ZM20, respectively, and it can be observed the good dispersibility of the ferrite particles within the amorphous matrix for both samples. The mean particle size for all the samples was obtained after statistical analysis from TEM images by measuring the major dimension of 150 particles. The obtained values were collected in Table 1. Magnetic susceptibility ZFC and FC was measured at 500 Oe in the 5-300 K temperature range for some of the samples (Figure 2), and in the case of the ZnFe2O4/SiO2 samples, the 30/ 70 weight ratio was taken into account. The low values of susceptibility in the case of the ZC sample confirm its paramagnetic character above the ordering temperature that corresponds to the ZFC maximum and takes a value of 16 K. The slightly higher value of ordering temperature with respect to the well-known TN = 10.5 K suggests that the sample may present a low inversion degree giving a nonzero net magnetic moment below 16 K. Unlike the ceramic sample, those composed by nanoparticles present high values of susceptibility revealing their superparamagnetic behavior above the TB. Furthermore, as it can be seen by the almost coincident in temperature ZFC maxima, these samples with nanometric particle size present very similar values of TB (around 20 K). This indicates that their anisotropy barrier U (dependent on the anisotropy constant (K), particle volume (V), and applied magnetic field (H)) must be very similar as it can be inferred by the expression TB = (U/25κB). When the external magnetic field is zero, the anisotropy energy for a noninteracting uniaxial nanoparticle can be described by the formula Ea = KV sin2 θ21 (θ, angle between the easy-magnetization axis and the magnetization moment), and when the anisotropy barrier (KV) is overcome by the thermal energy, the particle moment fluctuates (superparamagnetic regime). As it can be observed in Figure 2, the Z4 sample presents the highest values of susceptibility at low temperature values. It can be understood under the consideration that in the case of small nanoparticles,
Figure 3. Normalized Zn K-edge XANES spectra of the ceramic sample (ZC) along with the samples obtained by solvothermal method (a) and embedded in silica matrix (b).
the magnetization and therefore the susceptibility gets increased when the particle size increases. Because the spins of the particle surface are canted, the amount of coupled moments-carriers increases when the particle size increases due to the decreasing of the surface/volume ratio. However, above a certain particle size, the surface/volume ratio is negligible, and the inversion degree is the only parameter that affects the susceptibility. According to this, Z19, ZM11, and ZM20 with larger particle sizes present lower values of susceptibility than does the Z4 sample as the inversion degree is the only parameter that influences the magnetization, which in addition seems to be lower than in the Z4 sample as it will be seen below. X-ray absorption measurements were done to the samples to evaluate their inversion degrees. The normalized experimental XANES spectra at the Zn K-edge, which are caused by electronic transitions from the Zn 1s core level to unoccupied p-type states, are depicted in Figure 3. The spectrum corresponding to the ZC sample was taken as reference to compare the rest of the samples that are supposed to have a higher inversion degree. Hence, together with the spectrum corresponding to the ceramic sample (ZC), the spectra of the samples obtained by the solvothermal method (Figure 3a) and those corresponding to the embedded ferrite particles (Figure 3b) are shown. The XANES spectra can be decomposed into three resolved peaks A, B, and C at 9664, 9668, and 9672 eV, and a hump D at 9677 eV plus additional structure at higher energies. It is worth noting in the case of the samples obtained by the solvothermal method, the clear evolution of the profile spectra and the shift of the peak positions to lower values of energy when the particle size gets reduced from hundreds of nanometres (ZC sample) to 4 nm (Z4 sample) (Figure 3a). In this sense, it can be observed an increasing of the B peak intensity and also a marked decreasing of the D hump intensity (see Z4 spectrum) because its features are originated by electronic transitions involving Zn atoms located in sites with tetrahedral symmetry.22 This fact clearly reveals the inversion degree dependence on the particle size for samples obtained by the same synthesis method. In the case of the embedded nanoparticles, it can be observed a less marked evolution of the peaks profile, suggesting a low inversion degree for them. 1629
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Figure 6. Fourier transform of EXAFS spectra Zn and Fe K-edge for the samples obtained by the solvothermal method.
Figure 4. Normalized Fe K-edge XANES spectra of the ceramic sample (ZC) along with the samples obtained by solvothermal method (a) and embedded in silica matrix (b).
Figure 7. Fourier transform of EXAFS spectra Zn and Fe K-edge for the ferrite particles embedded in silica matrix.
Figure 5. Enlarged view of the Fe K-edge XANES pre-edge corresponding to the ceramic sample (ZC) together with the samples obtained by solvothermal method (a) and embedded in silica matrix (b).
In Figure 4 are shown the normalized experimental XANES spectra at the Fe K-edge for all the samples taking as reference the ZC spectrum. They are mainly formed by four peaks labeled as E, F, G, and H at 7112, 7130, 7136, and 7143 eV, respectively. It should be noted the increasing of the G and H peaks intensities with respect to that of the F peak when the particle size is reduced. Also, a slightly broadening of the peaks can be seen in the spectra of the samples with nanosized particles as compared to the ZC spectrum. This is probably due to the disorder of the cation distribution whose energy transition depends on the bond length.22
The Fe K-edge XANES spectra present an energy region that offers very useful information to evaluate not only the oxidation state of the Fe atom but the inversion degree of the samples as well. It corresponds to the so-called pre-edge (named as E, see Figure 5) whose features stem from 1s to 3d quadrupole transitions forbidden by dipole selection rules and thus exhibiting weak intensity. However, an enhancement of the pre-edge intensity is observed when part of the Fe atoms occupies noncentrosymmetric A-sites instead of the centrosymmetric octahedral ones, due to 3d-4p hybridization.23 Moreover, the pre-edge position for all the samples is compatible with the Fe3þ oxidation state, which agrees well with the ferrite compound.24 By comparison of the Fe-K pre-edge corresponding to the spectra of the nanosized particles Z4, Z10, Z14, and Z19 with that corresponding to the ZC sample, it can be seen that the Z19 spectrum resembles the ZC one, indicating that it may present low inversion degree. In this sense, neutron diffraction measurement done to this sample reveals after Rietveld refinement of the diagram an inversion degree parameter of x = 0.126,25 which is consistent with the qualitative information obtained from the pre-edge intensity. As the particle size is reduced (from Z19 to Z4 samples), more Fe atoms occupy A-sites, inducing an enhancement of the pre-edge intensity. Thus, Z4 sample with the lowest particle size presents the highest intensity pre-edge, evidencing that is the sample with the highest random cation distribution. 1630
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Table 2. Structural Parameters Obtained by EXAFS Zn K-Edge Analysisa Zn K-edge ZC
Z19
Z14
Z10
Z4
ZM20
ZM11
shell
Nj
Rj (Å)
Table 3. Structural Parameters Obtained by EXAFS Fe K-Edge Analysis
σ12 (Å-2)
Fe K-edge ZC
shell
Nj
Rj (Å)
σ12 (Å-2)
O
4.0
1.98
0.0050
O
6.0
2.01
0.0085
Fe O
12.0 12.0
3.52 3.40
0.0043 0.0017
Fe O
6.0 1.5
2.97 3.36
0.0054 0.0104
Zn
4.0
3.68
0.0072
Zn
6.0
3.49
0.0085
O
4.0
2.00
0.0052
O
6.0
3.66
0.0104
Fe
11.3
3.51
0.0048
O
6.0
2.01
0.0075
O
12.0
3.39
0.0016
Fe
6.0
2.97
0.0050
Zn
4.0
3.65
0.0070
O
1.5
3.36
0.0090
Fe
1.2
3.07
0.0048
Zn
6.0
3.48
0.0075
O Fe
4.0 11.4
2.00 3.54
0.0065 0.0053
O O
6.0 6.0
3.60 2.01
0.0090 0.0086
O
12.0
3.36
0.0020
Fe
6.0
2.97
0.0066
Zn
4.0
3.72
0.0080
O
1.5
3.43
0.0030
Fe
1.3
3.10
0.0053
Zn
6.0
3.45
0.0084
O
3.9
1.99
0.0064
O
6.0
3.64
0.0050
Fe
10.7
3.54
0.0046
O
5.8
1.99
0.0081
O
10.7
3.40
0.0015
Fe
5.6
2.97
0.0061
Zn Fe
3.1 1.3
3.59 3.13
0.0058 0.0046
O Zn
1.3 5.0
3.43 3.47
0.0050 0.0079
O
3.5
2.01
0.0073
O
4.6
3.64
0.0050
Fe
9.0
3.45
0.0110
O
4.9
1.97
0.0097
O
9.7
3.38
0.0040
Fe
4.9
2.98
0.0067
Zn
2.7
3.53
0.0067
O
0.8
3.36
0.0050
Fe
1.3
3.08
0.0110
Zn
4.7
3.42
0.0074
O
4.0
1.98
0.0045
O
3.2
3.58
0.0011
Fe O
11.8 12.0
3.52 3.37
0.0060 0.0070
O Fe
6.0 6.0
2.00 2.98
0.0084 0.0047 0.0050
Z19
Z14
Z10
Z4
ZM20
Zn
4.0
3.67
0.0070
O
1.5
3.33
Fe
0.4
3.13
0.0075
Zn
6.0
3.48
0.0090
O
4.0
1.99
0.0060
O
6.0
3.60
0.0124
Fe
11.8
3.52
0.0085
O
6.0
2.00
0.0084
O
11.8
3.40
0.0115
Fe
6.0
2.98
0.0050
Zn
3.8
3.60
0.0101
O
1.5
3.38
0.0050
Fe
0.5
3.15
0.0085
Zn O
6.0 6.0
3.48 3.60
0.0080 0.0124
ZM11
a
Added contributions to the average coordination from Fe3þ cations in the tetrahedral sites are represented in bold.
In the case of the embedded nanoparticles, both ZM11 and ZM20 samples seem to present very similar inversion degrees, although the pre-edge intensity is slightly higher for ZM11. Yet the interesting comparison performed is between the pre-edge intensities of Z19 and ZM20 samples. Both with very similar particle sizes, however they present slightly different pre-edge intensities, with it being a little bit higher in the second one. This makes clear that not only the particle size but also the synthesis method determines the inversion degree of the samples. Fourier transform (FT) of the κ3-χ(κ) signal at the Zn and Fe K-edges without phase corrections is shown in Figure 6 (Z4, Z10, Z14, Z19, and ZC samples) and Figure 7 (ZM11, ZM20, and ZC samples). The experimental and filtered Fe and Zn EXAFS signals for all the samples, with their corresponding fittings, are shown in the Supporting Information. A complete summary of the structural parameters obtained in the EXAFS analysis is presented in Tables 2 and 3. It is well-known that the FT peaks amplitude depends on the coordination number of the central
cation. At first glance, a slight reduction in the average coordination number can be observed as the particle size shrinks, as corresponds to the intrinsic loss of neighbors of increasing surface atoms in nanosized particles with average dimensions below 5 nm.26 This reduction is more noticeable in the further coordination shells. All Fourier-transformed EXAFS profiles at the Zn and Fe K-edge show large peaks for the r-space values below 4 Å and smaller peaks at higher r values that correspond to photoelectron backscattering for the most distant atoms shells. It can be observed a large peak around 1.5 Å that emerges from backscattering from the nearest-neighbor atomic shell of oxygen anions and that is present for cations occupying both A (four oxygens) and B sites (six oxygens). In the r-space range of 2-4 Å, the peaks are caused by backscattering from subsequent nearestneighbors, and the peak structure depends on whether the central atom occupies an octahedral or tetrahedral site. Attributing conventional positions for Zn and Fe cations, which are located in tetrahedral and octahedral sites, we can observe in the 1631
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Figure 8. M€ossbauer spectra obtained at different temperatures for Z3 (a), Z11 (b), and Z19 (c) samples.
FT figures that, while the amplitude of the peak positioned around 2.7 Å is proportional to the coordination of central cation located in octahedral B sites with octahedral B cations, the amplitude of the peak positioned around 3.2 Å depends on the amount of pair distributions A-A, B-A, and also A-B. In the Fe K-edge, a visible reduction of the peak at around 2.7 Å (accompanied by a broadening of the intense peak at 3.2 Å) suggests a different contribution from other geometries around Fe cations. This evolution is more remarkable in the Zn K-edge FT, in which the appearance of an increasing peak at 2.7 Å is followed by a subsequent reduction in the intensity of the large peak at 3.2 Å, from Z19 to Z4. In this case, the shell appearing at 2.7 Å can be associated with Fe neighbors located at around 3.1 Å, as an added contribution from Zn2þ cations in the octahedral sites. As shown in Tables 2 and 3, an additional peak is introduced, corresponding to the shell of Fe neighbors at around 3-3.1 Å of such enhanced contribution of Zn atoms located in the B sites. There is a visible reduction of the number of neighbors identified in the bulk, due to decreasing particle size, whereas the Fe shell, added contribution from Zn atoms in octahedral sites, remains almost constant even with the shrinking of the particle dimensions. Thus, the ratio between the coordination number of this Fe shell, indicative of ZnB cations, and the number of neighbors of the shells corresponding to ZnA geometry increases as the particle size decreases. The so evolved FT profiles corresponding to Z4 sample reveal not only the high cation exchange between FeB and ZnA, but the reduction in the average coordination number due to its small particle size as well. In Figure 8 are shown the zero field M€ossbauer spectra at different temperatures for Z3, Z11, and Z19 samples (Figure 8a-c, respectively). At 1.4 K (Z3 sample) and 4.2 K (Z11 and Z19 samples), the spectra can be described by two broadened sextets where the non-Lorentzian shape of the lines
Figure 9. Thermal dependence of the superparamagnetic fraction determined from the M€ ossbauer spectroscopy measurements.
points to the presence of a slight distribution of Fe3þ hyperfine parameters. The mean hyperfine field is 47.2, 50.2, and 49.9 T for the Z3, Z11, and Z19 samples, respectively. The fits shown in Figure 8 are phenomenological fits with two or three components. When the measurement temperature is progressively increased, the intensities of the sextet gradually diminish to start to visualize the doublet characteristic of the superparamagnetic behavior. As temperature increases, there is an augment of the weight of particles whose magnetization fluctuates faster than the hyperfine Larmor period associated with 57Fe τL 5 10-9 s. By comparison of the three samples’ M€ossbauer spectra, it can be seen that the onset of the doublet occurs at lower temperatures as the particle size is smaller. Thus, while the Z3 sample presents a relatively marked doublet at 15 K, Z11 and Z19 samples still display the sextet spectrum characteristic of the “frozen” state at 20 K (better visualized in Z11 than in Z19 sample). This fact reflects the trend of increasing M€ossbauer TB (TBM), which 1632
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The Journal of Physical Chemistry C corresponds to the temperature at which the “frozen” and “superparamagnetic” fractions are equal to 50%, with the augment of the particle size. The spectra above 4.2 K were fitted to a superposition of a quadrupolar doublet and of a histogram of hyperfine fields, required to account for the very broad spectrum of the “frozen” fraction. In Figure 9 is depicted the thermal evolution of the superparamagnetic fraction fp(T) for Z3, Z11, and Z19 samples. The curves fitted to the experimental data were calculated according to the expression: Z Vb ðTÞ pðV ÞV dV fp ðTÞ ¼ hV i 0 where p(V) is the volumic distribution function, taken as lognormal, and Vb(T) is the blocking volume, which bears the dependence on the anisotropy density K: kB T τm ln Vb ðTÞ ¼ K τ0 where τm is the characteristic time of the measuring technique (∼10-8 s for 57Fe M€ossbauer spectroscopy), and τ0 is a microscopic “trial time” of about 10-10 s.27 The experimental points were then fitted to the above formula assuming a volumic distribution as determined using the TEM images. As it can seen, the Z3 sample presents a TBM around 20 K, and Z11 and Z19 samples present values of 38 and 34 K, respectively. The unexpected slightly lower value of TBM for the Z19 sample may be due to the broader particle size distribution as it is reflected in its curve. The fitting of these curves also provides an estimation of the anisotropy constant that for Z3, Z11, and Z19 samples takes values of 7(3) 105, 4(2) 104, and 4(2) 103 erg/cm3 respectively. This is consistent with the fact that when the particle size is reduced, the surface/volume ratio increases, with the surface phenomena gaining more importance. Thus, as the particle size gets reduced, the surface anisotropy increases, making higher the effective anisotropy value.
’ CONCLUSIONS Different ZnFe2O4 samples were prepared ranging from 3 to 19 nm particle size by means of the solvothermal synthesis (Z3-Z19 samples). Also, 11 and 20 nm sized ferrite particles were obtained embedded in amorphous silica matrix using the sol-gel method (ZM11 and ZM20 samples). To have a reference ferrite sample to evaluate the inversion degree of the samples with nanosized particles, a ceramic one (ZC) was synthesized with high cooling time after the thermal treatment to achieve a very low inversion degree. The high values of ZFC and FC magnetic susceptibility curves reveal the superparamagnetic behavior above the TB in the case of the samples with nanometric particles. All of them present similar TB, suggesting that their anisotropy barrier must be very similar. On the other hand, the slightly higher value of ordering temperature of the ceramic sample seems to indicate it may present a low inversion degree. It can be inferred from the evolution of the XANES spectra profiles at Zn and Fe k-edges with the decreasing of the particle size a higher cation exchange between Zn in A-sites and Fe in B-sites with the particle size reduction. Similarly, the gradual variation of FT of the EXAFS signal indicates an augment of the inversion degree with the decreasing of the particle size. In the case of the pre-edge intensities for Z19 and ZM20 samples, it can be considered that despite the similar particle size, free of matrix nanoparticles seem to present a slightly lower inversion
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degree, which indicates the influence of the synthesis method in the cation distribution. On the other hand, the M€ossbauer spectra collected for three of the solvothermally obtained samples (Z3, Z11, and Z19) show a thermal evolution from the characteristic sextet of the “frozen” state to the “superparamagnetic” doublet. The anisotropy constant of these samples was estimated from the fitting of the curves of the superparamagnetic fraction versus T to the experimental data obtaining values of 7(3) 105, 4(2) 104, and 4(2) 103 erg/cm3 for the Z3, Z11, and Z19 samples, respectively. This increment in magnitude of the effective anisotropy constant has been attributed to the surface anisotropy enhancement when the particle size decreases.
’ ASSOCIATED CONTENT
bS
Supporting Information. X-ray diffraction diagrams corresponding to the solvothermally synthesized, the ceramic, and Zn-ferrite/SiO2 samples; and experimental and filtered κ3 3 χ(κ) weighted Fe and Zn EXAFS for the solvothermal and ferrite/ SiO2 samples together with the ceramic one. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
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