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MFe2O4 (M: Co2+, Ni2+) Nanoparticles: Mössbauer and X‑ray Absorption Spectroscopies Studies and High-Temperature Superparamagnetic Behavior V. Blanco-Gutiérrez,† J. A. Gallastegui,‡ Pierre Bonville,§ María J. Torralvo-Fernández,† and R. Sáez-Puche*,†

J. Phys. Chem. C 2012.116:24331-24339. Downloaded from pubs.acs.org by IOWA STATE UNIV on 01/29/19. For personal use only.

†

Departamento de Química Inorgánica, Facultad de Ciencias Químicas, Universidad Complutense de Madrid, 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 Ciencias de Materiales de Madrid, Consejo Superior de Investigaciones Científicas, calle Sor Juana Inés de la Cruz, 3, 28049 Madrid, Spain § ́ ́ CEA, Centre dEtudes de Saclay, IRAMIS/Service de Physique de lEtat Condensé, 91191 Gif-sur-Yvette, France ABSTRACT: MFe2O4 (M: Co2+, Ni2+) nanoparticles with different sizes and crystal-chemistry have been synthesized under solvothermal conditions. The inversion degree (x) of the mixed spinel structure (Fe1−xMx)[Fe1+xM1−x]O4 has been investigated by XAS and Mössbauer spectroscopies obtaining values between 0.20 and 0.30 in the case of Co-ferrite samples and between 0.00 and 0.16 for the Ni-ferrite ones. In order to have a better understanding of the superparamagnetic behavior of the nanosized samples, it was necessary to do hightemperature magnetic measurements. Although Co-ferrite samples present higher magnetization values, their effective superparamagnetic moment is similar to those found for the Niferrite ones. This suggests that the dipolar interactions may be stronger in the Ni-ferrite system, probably due to a less-effective anisotropy that can be considered an impediment to the particle interactions. This fact is supported by the effective volume/ single-particle volume ratio that presents similar values for both Co-ferrite and Ni-ferrite.

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INTRODUCTION Magnetic nanoparticles have attracted a great deal of attention for decades due to their promising potential applications in the field of ferrofluids, data storage, catalysis, and biomedicine. This is mainly due to the superparamagnetism,1,2 one of the most studied finite size effects that single-domain nanoparticles display in the blocking temperature (TB)−ordering temperature (TO) range and is characterized by an absence of coercivity, high-magnetization values, and low-saturation fields. In this sense, nanosized ferrite compounds, which are one of the most important magnetic materials, are studied in depth as a result of their different magnetic behavior compared with that of their bulk counterpart materials.3,4 Ferrites, MFe2O4 (M: Co2+, Ni2+), present a spinel-type structure (S.G.Fd3̅m) in which the O2− ions are arranged in a cubic with close packing being occupied one-eighth of tetrahedral (A) and one-half of octahedral (B) interstitial sites. An intriguing feature occurs by changing the scale of particle size in which the ferrites are obtained. When these materials are prepared in the microscopic range, they present an inverse spinel structure (Fe)[MFe]O4 where () and [ ] mean tetrahedral and octahedral sites, respectively. However, if they are prepared in the nanometric scale, due to the softening of the lattice vibrations, they present a structure that corresponds to the mixed spinel (Fe1−xMx)© 2012 American Chemical Society

[Fe1+xM1−x]O4 where x is the so-called inversion degree and indicates the cation distribution between the A and B sites. The internal magnetic order in MFe2O4 compounds occurs through a superexchange Fe−O−Fe pathway giving rise to two opposite magnetic sublattices that correspond to the A and B sites. Therefore, the Fe3+ occupancy in A and B sites, that is the inversion degree, fully determines the magnetic behavior of the ferrite material. It is thus crucial to determine the inversion degree, and for such purpose, several techniques can be employed. In this sense, XAS spectroscopy has been found to be a useful tool to evaluate the cation distribution, as in the case of the Fe−K edge spectrum; there is an energy region known as the pre-edge that offers very useful information about the Fe3+ occupancy in noncentrosymetric A-sites.3 Also, Mössbauer spectroscopy under an applied magnetic field offers the possibility to accurately determine the inversion degree by fitting the obtained spectra to two subspectra corresponding to the magnetic sublattices with opposite orientation.5,6 A great number of synthesis procedures of nanostructured magnetic materials have been proposed in recent times such as Received: July 25, 2012 Revised: October 28, 2012 Published: October 29, 2012 24331

dx.doi.org/10.1021/jp307371q | J. Phys. Chem. C 2012, 116, 24331−24339

The Journal of Physical Chemistry C

Article

sol−gel,7 mechanosynthesis,8 or coprecipitation.9 However, the solvothermal method seems to be one of the most advantageous due to its simplicity and mild experimental conditions. In addition, the employment of economical reactants, along with the possibility of dealing with a large number of experimental parameters, makes this synthesis method one of the most used in the nanochemistry field.10,11 On the other hand, an exhaustive magnetic study of the superparamagnetic behavior implies determining the temperature range for which the material exhibits this behavior and the value of the superparamagnetic moment (μSP). As most of the Co-ferrite and Ni-ferrite nanoparticles behave as superparamagnetic above the room temperature, it is necessary to perform high-temperature magnetic measurements (above 300 K) in order to obtain information concerning this behavior. In this work, we describe the preparation of MFe2O4 (M: Co2+, Ni2+) nanoparticles by means of the solvothermal method and present the study of the crystal chemistry of the obtained ferrites through XAS and Mössbauer spectroscopies. The obtained inversion degrees were helpful in the understanding of the magnetic behavior of these materials. Moreover, hightemperature magnetic measurements were done to investigate the superparamagnetic behavior of each sample.

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Characterization Techniques. The structural characterization was carried out by X-ray powder diffraction employing a Siemens D-5000 diffractometer with Cu Kα radiation, and the microstructural characterization was done by means of transmission electron microscopy (TEM) using a JEOL2000FX microscope working at 200 kV. X-ray absorption spectroscopy measurements at the Co, Ni, and Fe K-edges were performed at the Spanish CRG beamline (SpLine) of the European Synchrotron Radiation Facility. 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, and Co, Ni, and Fe foils were measured for energy calibration purposes. Data treatment and normalization of the edge spectra were achieved using ATHENA.12 Mössbauer spectra on the isotope 57Fe were carried out at 4.2 K in a 7 T applied magnetic field perpendicular to the direction of propagation of the γ-ray using a commercial 57Co:Rh γ-ray source. Magnetic measurements were done in a Quantum Design XL-SQUID magnetometer in the temperature range of 4−700 K up to 5 T. Magnetic susceptibility was measured after cooling the sample at 5 K in zero-field cooling (ZFC) and in the case of fieldcooling measurements (FC), the sample was cooled in the presence of a 500 Oe magnetic field down to 5 K. In the case of high-temperature measurements, the samples were cooled at 300 K from 600 to 700 K in the ZFC process.

EXPERIMENTAL SECTION

All the chemical reactants were purchased from Aldrich Chemical Inc. and used without further purification. Co-ferrite and Ni-ferrite samples with micrometric particle size were prepared by the ceramic method employing the corresponding nitrates as precursors. Stoichiometric amounts of the nitrates were mixed by grinding, and the homogenized mixture was thermally treated at 950 °C for 12 h with a 10 °C/min heating rate; afterward, the solid was cooled in the furnace. The obtained CoFe2O4 and NiFe2O4 samples were labeled as CoC and NiC, respectively. The nanosized samples were obtained by the solvothermal method.10,11 Stoichiometric amounts of the nitrates were dissolved in ethylene glycol or water in a 10−5 and 10−4 mol/ mL concentration, respectively, and KOH was used as precipitant agent to obtain the hydroxides (pH = 11). The obtained brown mixture was transferred into a stainless steel autoclave to be treated under solvothermal conditions. In order to obtain samples with a different particle size and cation distribution, the synthesis parameters such as reaction temperature, reaction time, and solvent and KOH concentration were modified according to Table 1. The samples were labeled with the ferrite cation (Co or Ni) followed by the mean particle size in nm estimated from the TEM images.

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RESULTS AND DISCUSSION The X-ray diffraction patterns shown in Figure 1 indicate that both Co-ferrite and Ni-ferrite samples are single phases and have crystallized with the spinel-type structure. Moreover, broader diffraction maxima from the CoC to Co4 and NiC to

Table 1. Experimental Conditions to Obtain MFe2O4 (M: Co2+, Ni2+) Nanoparticles sample

Sva

KOH (M)

Ta (°C)

ta (h)

Co4 Co9 Co16 Ni3 Ni7 Ni12

EG EG W EG EG EG

0.5 2.0 2.0 0.5 2.0 0.5

160 200 200 180 200 200

18 168 168 3 48 92

a

Sv: solvent, EG: Ethylene glycol, W: water, T: treatment temperature, and t: treatment time.

Figure 1. X-ray diffraction patterns corresponding to the (a) CoFe2O4 samples and (b) NiFe2O4 samples. 24332



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Ni3 samples can be seen, thus revealing decreasing particle size. In this sense, the particle size for each sample has been calculated from the Scherrer equation13,14 D=

0.9λ β cos θ

(1)

where λ corresponds to the Cu Kα radiation, and β is the full width at half-maximum for a reflection maximum located at 2θ). It was obtained from a particle size of 4.0, 9.0, 15.0, and 100 nm for Co4, Co9, Co16, and CoC samples, respectively, and 2.0, 7.0, 12.0, and 50 nm for Ni3, Ni7, Ni12, and NiC, respectively. Furthermore, the slightly higher intensity ratio of the 220/400 reflection maxima in the case of the NiC sample compared with that corresponding to the CoC sample, indicates that the NiC sample presents a spinel structure closer to the inverse one than does CoC sample.15 Representative TEM images of the Co-ferrite and Ni-ferrite samples are shown in Figures 2 and 3, respectively. The Figure 3. TEM images of NiFe2O4 samples obtained by the (a−c) solvothermal and (d) ceramic method.

peaks A, B, C, D, and E appearing at 7705, 7719, 7725, 7730, and 7740 eV in agreement with the reported values for Co(II)oxides,16,17 and with the increasing cationic disorder from the CoC to Co16 peaks, a decrease in the B and D peaks can be seen. Also, an observed broadening of the white-line (C peak) from the CoC to nanosized samples is associated with the decrease in particle size. This broadening is related to the disorder in the nearest neighbor distances.17 In a micrometric particle, nearly all of the absorbent atoms are in the bulk, and so the tetrahedral and octahedral Co atoms types can be distinguished. However, for smaller particle sizes there are bulk atoms and surface atoms, which present different near neighbor distances compared with those of the bulk atoms. This way, with the reduction of the particle size, the disorder in the nearest neighbor distances increases, and so does the broadening of the main peak related to the disorder. The Fe−K edge XANES spectra are mainly described by four peaks at 7112, 7130, 7136, and 7143 eV, respectively, named as F, G, H, and I, and a decrease in the H and I peak intensities with respect to the G peak can be seen when the cation disorder is increased.3 The Fe K-pre-edge XANES spectra provides very useful information to evaluate not only the oxidation state of the Fe atom but also the inversion degree of the samples as well.3,18 Its features stem from the 1s to 3d quadrupole transitions forbidden by dipole selection rules and thus exhibits weak intensity. However, an enhancement of the pre-edge intensity is observed when part of the Fe atoms occupy noncentrosymmetric A-sites instead of the centrosymmetric octahedral ones, due to 3d−4p hybridization.19 Therefore, an increment of the inversion degree that implies a decrease of the Fe3+ population in the A-sites, results in a decrease in the pre-edge intensity. The pre-edge position for all of the samples is compatible with the Fe3+ oxidation state, which agrees well with the ferrite compound.20 Moreover, the pre-edge intensity corresponding to the CoC sample is the highest one, revealing that it presents a high proportion of Fe atoms in the tetrahedral sites, which means that it may present a spinel structure close to the inverse one, (Fe)[CoFe]O4. In the case of the nanosized samples that present the mixed spinel structure, (Fe1−xCox)[Co1−xFe1+x]O4, the increasing inversion

Figure 2. TEM images of CoFe2O4 samples obtained by the (a−c) solvothermal and (d) ceramic method.

estimated mean particle sizes (indicated in the sample label) obtained after measuring 100 particles of each sample, agree well with those determined from the X-ray diffraction patterns. As it can be seen in Figures 2 and 3, while samples prepared by the ceramic method (CoC and NiC) present heterogeneous particle sizes, those prepared in the nanometric range show more homogeneous particle sizes. On the other hand, all the samples with nanometric particle sizes present rounded polyhedral morphology, although in the case of the Co16 sample it can be considered to be composed by well-defined polyhedral particles, probably as a consequence of employing water as the solvent instead of ethylene glycol. Figure 4 shows the normalized Co and Fe K-edge XANES spectra (panels a and b, respectively) along with the pre-edge of the Fe K-edge spectra (Figure 4c) for the CoFe2O4 samples, from the CoC to Co16 samples that corresponds to an increasing cation disorder sequence, as will be discussed below. The Co K-edge XANES spectra can be decomposed into five 24333



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Figure 4. Normalized (a) Co−K and (b) Fe−K edge XANES spectra of the CoFe2O4 samples. (c) The pre-edge corresponding to the Fe−K XANES spectra.

Figure 5. Normalized (a) Ni−K and (b) Fe−K edge XANES spectra of the NiFe2O4 samples. (c) The pre-edge corresponding to the Fe−K XANES spectra.

structure, the Hext tends to align the majority of the Fe3+ sublattice along its direction and the minority sublattice opposite to it. As the hyperfine field, Hhf, is opposite to the Fe3+ magnetic moment, the nuclei in the majority sublattice must experience an effective field (Heff = Hext + Hhf)22 smaller than Hhf, the hyperfine field in a zero-applied field and the minority sublattice of Heff larger than that of Hhf. Because of the different effective fields corresponding to the two magnetic sublattices, the subspectra are not overlapping and a weight percentage can be assigned for each one. The determination of the α ratio of the relative weights of the minority and majority subspectra between the weight percentages

degree sequence deduced from the pre-edge intensities corresponds to CoC < Co9 < Co4 ≈ Co16. In Figure 5, the normalized Ni and Fe K-edge XANES spectra (a and b, respectively) along with the pre-edge of the Fe K-edge spectra (Figure 5c) corresponding to the NiFe2O4 samples can be seen. The Ni K-edge XANES spectra can be described by four peaks: A, B, C, and D located at 8340, 8345, 8350, and 8360 eV21, and a decreasing intensity of the C and D peaks with respect to the B peak can be seen when the particle size decreases from ∼100 nm (NiC sample) to 3 nm (Ni3 sample) (Figure 5a). Also, a gradual broadening of the white line (C peak) can be observed from the NiC to Ni3 samples associated with the decrease in particle size. On the other hand, the decrease in the pre-edge intensity (F peak), observed when the particle size shrinks, reveals an augment of the cation disorder with the decrease in particle size. Thus, the intense pre-edge corresponding to the NiC sample indicates that this sample presents the highest Fe atoms proportion on the A-sites, which agrees with a spinel structure close to the inverse one, (Fe)[NiFe]O4. Taking into account the mixed spinel structure, (Fe1−xNix)[Ni1−xFe1+x]O4, corresponding to the samples with nanosized particles, the following cation disorder sequence can be established: NiC < Ni7 < Ni3. The Fe3+ proportion corresponding to the magnetic sublattices with an opposite net magnetic moment can be determined by Mössbauer spectroscopy under an applied magnetic field.5,22 The Mössbauer spectra were recorded under a 7 T magnetic field, Hext, applied perpendicularly to the propagation direction of the γ-rays. In the ferrimagnetic

α=

%min ority %majority

(2)

along with the consideration of the magnetic sublattices arrangement in the mixed spinel structure ( ↑ Fe1 − xMx)[ ↓ Fe1 + xM1 − x]O4

allows the inversion degree from the equation to be obtained x=

1−α 1+α

(3)

Furthermore, in the case of the Mössbauer measurements under the applied magnetic field, perpendicular to the γ-ray propagation, the canting angle between the hyperfine field corresponding to each magnetic sublattice and the external field from the ratio between the intensities can be calculated 24334



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A 2,5

r=

together with the cation distribution. The obtained inversion degree values are in agreement with the XAS analyses. Moreover, as the A2,5/A3,4 ratio is close to 4 in both cases, the CoFe2O4 and NiFe2O4 samples can be considered at an almost negligible canting angle between the hyperfine field of both sublattices and the external field.5,23 On the other hand, it should be noted that the Co-ferrite samples present a higher inversion degree than do the Ni-ferrite ones, which may be explained by the considerations of the crystal field formalism. In this sense, the 10Dq values corresponding to the M2+ cation in the octahedral and tetrahedral geometries are closer in the case of the CoFe2O4 compound than in the case of the Ni-ferrite compound. This suggests that in energy terms, it is easier for Co2+ to exchange with Fe3+ occupying tetrahedral sites. The ZFC and FC magnetic susceptibility measured at 500 Oe up to 600 and 700 K corresponding to the CoFe2O4 and NiFe2O4 nanoparticles, respectively, are shown in Figure 7

A3,4

that corresponds to5 ⎛ 1 − cos2 θ ⎞ r = 4⎜ ⎟ ⎝ 1 + cos2 θ ⎠

(4)

The Fe Mössbauer spectra at 4.2 K with a 7 T applied field perpendicular to the propagation direction of the γ-ray corresponding to the CoFe2O4 and NiFe2O4 samples with different particle sizes are shown in Figure 6. The spectra were 57

Figure 6. Mössbauer spectra measured at 4.2 K under a 7 T applied field corresponding to (a) CoFe2O4 and (b) NiFe2O4 samples.

fitted to two subspectra providing values for Heff of 57.6(3) and 47.5(3) T for the A and B sublattices, respectively, in the case of the Co-ferrite samples and Heff of 57.5(6) and 48.3(3) T for the A and B sublattices, respectively, in the case of the Ni-ferrite samples. In the case of the CoC sample and the Ni-ferrite samples, the small nonresolved central signal has not been considered in the fitting. Table 2 shows the α and x values

Figure 7. Magnetic susceptibility (M/H) measured at 500 Oe up to 650 and 700 K, corresponding to (a) CoFe2O4 and (b) NiFe2O4 samples.

(panels a and b, respectively). The blocking temperature (TB) has been estimated for each sample from the ZFC maximum and the obtained values collected in Table 3. Both Co-ferrite and Ni-ferrite samples show an increase in the magnetic susceptibility values with the increase in particle size due to the augment of the proportion of coupled moment carriers. A displacement of the TB to higher values is also observed. In the case of SQUID magnetic measurements, the TB depends on the anisotropy barrier U as follows

Table 2. Weight Percentage Ratio (α), Inversion degree (x), and Mixed-Spinel Formula Corresponding to the CoFe2O4 and NiFe2O4 Samples Obtained from the Fitting of Mössbauer Spectra to Two Subspectrum sample

α = A/B

x

mixed-spinel formula

CoC Co4 Co9 Co16 NiC Ni7 Ni12

0.63(9) 0.58(9) 0.59(4) 0.54(5) 1.00 0.72(2) 0.72(5)

0.23(7) 0.28(6) 0.26(6) 0.30(4) 0.00 0.16(2) 0.16(4)

(Fe0.77Co0.23)[Co0.77Fe1.23]O4 (Fe0.72Co0.28)[Co0.72Fe1.28]O4 (Fe0.74Co0.26)[Co0.74Fe1.26]O4 (Fe0.70Co0.30)[Co0.70Fe1.30]O4 (Fe1.00Ni0.00)[Ni1.00Fe1.00]O4 (Fe0.84Ni0.16)[Ni0.84Fe1.16]O4 (Fe0.84Ni0.16)[Ni0.84Fe1.16]O4

TB =

U 25kB

(5)

where kB is the Boltzmann constant. In addition, with an external magnetic field, the anisotropy barrier is modified as follows 24335



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Table 3. Magnetic Parameters Corresponding to the Nanosized CoFe2O4 and NiFe2O4 Samples

a

sample

TB (K)

CoC Co4 Co9 Co16 NiC Ni3 Ni7 Ni12

− 235 350 560 − 16 40 37

μSP (μB) − 9.5 2.7 4.0 − 2.4 2.7 1.7

× 103 × 104 × 103 × 103 × 104 × 105

TO (K)a

HC (Oe)

MS(emu/g)

MS (μB)

− 550 650 >700 − 200 550 600

4445 7516 12513 12313 203 290 228 196

88.7 84.6 83.3 80.2 47.5 21.9 65.8 61.4

3.7 3.5 3.5 3.4 2.1 0.9 2.7 2.6

K (erg/cm3)

HK (Oe)

× × × × × × × ×

3150 3324 1995 1886 1977 2634 1097 882

7.4 7.4 4.4 4.1 2.6 1.6 2.0 1.5

105 105 105 105 105 105 105 105

μHT (μB) − 3.3 1.4 7.9 − 1.4 6.4 3.1

× 103 × 104 × 103 × 103 × 103 × 104

HS,HT (Oe) − 1805 532 1178 − 2737 581 121

Estimated values from the inverse of susceptibility. 2 ⎛ H⎞ U = KV ⎜1 − ⎟ HK ⎠ ⎝

(6)

where K is the effective anisotropy, V the particle volume, H the external magnetic field, and

HK =

2K MS

is the anisotropy field corresponding to crystals with cubic symmetry24 that reflects the magnetic hardness of the system.25 Therefore, an increasing of the particle volume implies an increase in the TB as well. It should be noted in the case of the NiFe2O4 samples that there exists a broadening of the ZFC maximum with the increase in particle size (Figure 7b). This can mainly be related to two factors: a heterogeneous particle size distribution or interparticle interactions,26 but taking into account that the TEM images show a similar particle size homogeneity for all of them, the dipolar interactions, that seem to occur through bulk particle moments,25 may be the main reason for the ZFC maximum broadening. In Figure 8, the inverse of the magnetic susceptibility versus temperature for (a) Co-ferrite and (b) Ni-ferrite nanoparticles is depicted in the TB−T range, and in some cases the superparamagnetic (SP) and paramagnetic (P) regimes that present different slopes can be distinguished. The transition from the superparamagnetic to the paramagnetic regime occurs gradually, and the estimation of the order temperature (TO) has been considered as the temperature at which this transition starts (values collected in Table 3). The magnetic susceptibility corresponding to the superparamagnetic regime can be described as χ=

Nμ2 kBT

Figure 8. Inverse of the magnetic susceptibility vs temperature corresponding to (a) CoFe2O4 and (b) NiFe2O4 samples.

Unlike in the case of the paramagnetic behavior, the inverse of susceptibility in the superparamagnetic regime is described by a curve whose change of slope is associated with a variation in the effective magnetic moment with the temperature. This is due to the decrease in the ordered moments as the temperature approaches TO.25 The thermal energy action is less effective with the increase in the dipolar interactions, resulting in a broadening of the ZFC maximum that reveals a smooth magnetization loss with the increase in temperature.25 The effective μSP values (Table 3) were calculated in a TB−T range for which it has been found to be an acceptable linear fitting and corresponds to the effect of several particle moments. The μSP values provided in Table 3 tend to be higher in the case of the Ni-ferrite nanoparticles. On the other hand, from eqs 5 and 6, the effective volume (V) in the case of noninteracting systems can be calculated to correspond to the single-particle volume.25 In these studied ferrite systems, this parameter takes higher values as it reflects the coupling between several particle moments. Therefore, the dipolar interactions can roughly be analyzed through the V/VTEM ratio, where VTEM refers to the single-particle volume estimated from the TEM images. A

(7)

where N is the particle density per volume unit, and μ is the particle moment (μ = MSV). The Curie law can be written then as 1

χ=

Mμ Nμ2 C = = S T 3kBT 3kBT

(8)

And from the inverse susceptibility versus temperature, the effective superparamagnetic moment (μSP) can be calculated from the equation25 μSP (μB ) =

3kB 1 · MSμB p

(9)

with MS in emu/g and the slope, p, in (emuK/Oeg)−1 24336



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observed in other studies,23 may be due to a different magnetic sublattices arrangement with respect to the expected one (↑ Fe1−xMx)[↓ Fe1+xM1−x]O4. The low K and HK values obtained in the case of Ni-ferrite samples indicate that these materials are magnetically softer than Co-ferrite nanoparticles.25 This feature can be considered as promoter of the dipolar interactions justifying the broad ZFC maxima and the higher μSP values found in Ni-ferrite samples. The coercive field at 5 K (HC) has been estimated for each sample from its corresponding M versus H curve, and the values are collected in Table 3. The CoC sample presents a lower HC value than the rest of the Co-ferrite samples formed by nanoparticles. This may be explained under the consideration of the multidomain particles in the case of the CoC sample that requires lower energy to align the moment carriers with the external field as the domain-wall movement is easier than the reversal of the single-domain particles moment. As can be seen from the HC values provided in Table 3, while the coercive field dependence with the particle size corresponding to the CoFe2O4 samples describe the characteristic behavior of a single-domain regime (increasing HC values), in the case of the NiFe2O4 nanoparticles, a decreasing HC value with the increment of particle size is observed. As has been previously reported,28 the interparticle interactions seem to modify the anisotropy of the system. An example was found in the ZnFe2O4 nanoparticles hosted in porous structures that minimize the particle interaction.28 These ferrite nanopoarticles present higher HC values than those corresponding to particles of similar size without a hostage matrix. This fact was realized by considering that the interparticle interactions in the case of the nonencased particles tend to reduce the anisotropy of the system, and therefore the coercive field is also decreased. Thus, in the case of the Ni-ferrite samples, an increment of the dipolar interactions effect with the particle size can be considered from the broadening of the ZFC maximum, suggesting that the particle interactions are more important in the case of the sample with larger particle sizes probably because they present higher bulk moments.25 The more intense the interactions are, the higher the value obtained for the modification of anisotropy, and therefore the HC values are lower. In Figure 10, it is shown that the M versus H curves corresponding to the Co-ferrite and Ni-ferrite samples (a and b, respectively) measured at 400, 500, and 650 K, in the case of Co-ferrite samples, and 250 K, in the case of Ni-ferrite ones. These measurement temperatures (named as HT) belong to the temperature range for which the sample illustrates superparamagnetism. When the potential magnetic energy (μHT) and the thermal energy (kBT) are of the same magnitude, the saturation magnetization is reached, and therefore the saturation field at HT (HHT) can be calculated through the equation24

similar ratio has been found for both systems. Although the Coferrite samples present higher MS values than those of Niferrite, they present a similar effective superparamagnetic moment. This fact can be explained taking into account that the effective anisotropy that is higher in Co-ferrite would act as an impediment for a proper dipolar interaction, as will be explained below. These analyses are in agreement with the broad ZFC maximum observed in Ni-ferrite particles that indicates that the dipolar interactions are intense. In Figure 9, the hysteresis loops measured at 5 K corresponding to the Co-ferrite and Ni-ferrite samples (panels

Figure 9. Hysteresis loops measured at 5 K for (a) CoFe2O4 and (b) NiFe2O4 samples.

a and b, respectively) can be seen. From the fitting of the data on the high field region to the equation27 ⎛ M (T ) = M S ⎜1 − ⎝

b=

a b ⎞ − 2 ⎟ + cH H H ⎠

(10)

8 K2 105 MS2

where a is a constant, and c corresponds to the susceptibility in the high field region. The magnetization to saturation (MS) and anisotropy constant (K) can be obtained.25 The obtained values are collected in Table 3 together with the HK values for both the Co-ferrite and Ni-ferrite samples. It can be seen that the Co-ferrite samples present a higher anisotropy; their high values of K and HK reveal an intrinsic magnetic hardness in these materials. On the other hand, the MS values obtained from SQUID measurements are in agreement with the inversion degree determined by Mössbauer spectroscopy in the case of the Ni-ferrite samples. In the case of Co-ferrite samples, the calculated magnetic moment taking into account the inversion degrees obtained from Mössbauer spectroscopy are higher than the experimental ones. This fact, previously

HHT =

kBT μHT

(11)

where μHT corresponds to the effective magnetic moment at that temperature. μHT has been calculated from eq 9, considering the slope in the (HT ± 5) K temperature range. The μHT values are presented in Table 3 and are lower than those corresponding to μSP. Although both values, μSP and μHT, belong to the superparamagnetic regime, μSP (obtained in the TB−T range) presents a higher value as a consequence of the coupling of a larger number of particle moments. This is due to 24337



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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank MICIN and Comunidad de Madrid for financial support under Grants MAT 2010-19460 and S-2009/ PPQ-1626, respectively.

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REFERENCES

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Figure 10. Magnetization curves corresponding to the superparamagnetic behavior.

a more important action of the thermal energy at T = HT that manages to overcome most of the interparticle interactions. The M versus H curves at these high measurement temperatures show the characteristic superparamagneitc behavior with high magnetization values, low saturation fields, and noncoercivity which gives the recognizable S-shaped curve. On the other hand, the Ni3 sample presents a higher value of HHT at 250 K (Table 3) because at that temperature the sample is in the superparamagnetism−paramagnetism transition.

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CONCLUSIONS Spinel MFe2O4 (M: Co, Ni) nanoparticles have been easily prepared under solvothermal conditions, and by modification of the experimental parameters it was possible to obtain samples with different particle sizes and crystal chemistry. The inversion degree of the mixed spinel structure (Fe1−xMx)[Fe1+xM1−x]O4 has been investigated through XAS and Mössbauer spectroscopies, yielding a low value for both ferrite systems. The high temperature magnetic measurements indicate that all of the nanosized samples behave as superparamagnetic above room temperature, and the effective superparamagnetic moment has been calculated for each sample from the inverse of susceptibility versus temperature. The μSP are affected by the interparticle interactions that are minimized by the thermal energy. Thus, the effective superparamagnetic moment is the result of the coupling of a number of particle moments that increase with the interparticle interactions. The Ni-ferrite samples present more intense dipolar interactions than those of Co-ferrite as the broader ZFC maxima and the similar V/VTEM ratio indicate. The dipolar interactions are probably favored in the Ni-ferrite system due to the lower anisotropy of the system according to the low values of K and HK compared with those obtained for the Co-ferrite samples. 24338



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(26) Carpenter, E. E.; Long, J. W.; Rolison, D. R.; Logan, M. S.; Pettigrew, K.; Stroud, R.; Kuhn, L. T.; Hansen, B. R.; Morup, S. J. Appl. Phys. 2006, 99, 08N711. (27) Mitra, S.; Mandal, K.; Kumar, P. A. J. Magn. Magn. Mater. 2006, 306, 254. (28) Blanco-Gutiérrez, V.; Urones-Garrote, E.; Torralvo-Fernández, M. J.; Sáez-Puche, R. Chem. Mater. 2010, 22, 6130.

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