Influence of Protected Annealing on the Magnetic Properties of Î³

Article pubs.acs.org/JPCC

Influence of Protected Annealing on the Magnetic Properties of γ‑Fe2O3 Nanoparticles Charlotte Vichery,† Isabelle Maurin,*,† Pierre Bonville,‡ Jean-Pierre Boilot,† and Thierry Gacoin*,† †

Physique de la Matière Condensé, CNRS, Ecole Polytechnique, 91128 Palaiseau, France CEA, Centre d’Etudes de Saclay, DSM/IRAMIS/Service de Physique de l’Etat Condensée, 91191 Gif-sur-Yvette Cedex, France

‡

S Supporting Information *

ABSTRACT: It is usually considered that nanoparticles synthesized by low-temperature routes present structural disorder, from extended defects to local rearrangements (e.g., vacancy ordering or inversion in spinel ferrites), that may severely impact their magnetic properties. In the present work, we have investigated the influence of postsynthesis thermal treatments on 7-nm-sized γ-Fe2O3 nanoparticles prepared by room temperature coprecipitation of ferric and ferrous salts in alkaline medium, followed by the dispersion of the preformed particles in a sol− gel silica binder. Such protected annealing in a refractory matrix prevents coalescence and growth, thus preserving the mean size and size distribution of the pristine particles. Structural characterizations show that heat treatments up to 1000 °C turned the raw grains into wellcrystallized particles without transformation into hematite. This strategy thus allows accounting for the influence of structural rearrangements on magnetic properties at fixed particle size. For such 7 nm particles, postsynthesis heat treatments were found to mainly influence the shell of misaligned spins at the surface.

1. INTRODUCTION The main characteristic of nanomaterials is that their microstructure largely influences their macroscopic properties and may give rise to new phenomena which are not observed in the corresponding bulk phase. These features are mostly due to a strong interplay between intrinsic properties, size and shape distributions, finite size or surface effects, and interparticle interactions. More specifically, size reduction in magnetic materials leads to superparamagnetic behavior, enhanced magnetic anisotropy, and various surface effects,1,2 which have important impact on magnetic data recording and biotechnologies.3,4 In the field of biomedical applications, most attention has been paid to maghemite (γ-Fe2O3) and magnetite (Fe3O4) nanoparticles as contrast agent for magnetic resonance imaging5 or new vectors for targeted drug delivery,6 magnetically assisted chemical separation,7 and hyperthermia cancer treatment.8 Among others, saturation magnetization and coercivity values are known to show a marked domain size dependence,9 the former effect being ascribed to spin canting within the volume and/or at the surface of the particles.10−12 Because of this strong correlation between size and magnetic behavior, a low polydispersity is often required for optimized properties. Various chemical routes have thus been proposed to produce ultrafine γ-Fe2O3 particles,13 such as hot-injection routes based on the decomposition of organometallic precursors that lead to highly monodispersed colloids.14 Nevertheless, coprecipitation in water derived from the Massart protocol15 remains widely © 2012 American Chemical Society

used, as the reactant toxicity and costs are much reduced. In addition, this method has shown a large versatility for controlling the particle size by changing experimental conditions such as pH, concentration of Fe(III) and Fe(II) precursors, nature of the base used, or temperature.16 If size polydispersity is usually observed, it can be readily reduced by a postsynthesis size selection step involving differential centrifugations or selective flocculation by addition of salts.17 Nevertheless, different magnetic characteristics are often reported for a same mean particle size depending on the synthesis technique that is used.18,19 Recent results have shown that these discrepancies could be explained by the fact that geometric and magnetic size distributions may not coincide.20 Even for optimized chemical routes that yield uniform particle sizes in the single-domain regime, the mean size of the magnetic domains can be substantially lowered with respect to the geometric size and their distribution can be strongly broadened. For iron oxide nanoparticles, the presence of twins and/or of dislocations was found to disrupt magnetic couplings and to have a detrimental impact on magnetic properties.21 Magnetic studies of nanomaterials are thus complex, as one should take into account intrinsic phenomena but also extrinsic effects based not only on geometric size distribution but also on nonuniform chemical composition and lack of crystallinity due Received: May 24, 2012 Revised: July 5, 2012 Published: July 12, 2012 16311

dx.doi.org/10.1021/jp305069a | J. Phys. Chem. C 2012, 116, 16311−16318

The Journal of Physical Chemistry C

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air at different temperatures, ranging from 140 to 990 °C, for 3 h. Hereafter, the crude Fe2O3 nanoparticles and the Fe2O3/ SiO2 composite samples will be labeled FeX, FeSiX, and FeSidX, respectively, where X denotes the annealing temperature while the d subscript refers to the lowest Fe/Si ratio (dilute composite). 2.2. Samples Characterization. Powder X-ray diffraction patterns were recorded using a PANalytical X’Pert diffractometer equipped with Cu Kα radiation (λ = 1.5418 Å). Peak positions were extracted using the Fit subroutine of the Fullprof suite of programs,29 and the lattice constant was determined by least-squares refinement of the corresponding d-spacings. Note that an internal silicon calibrant was used to obtain the lattice parameter value with accuracy. The structural coherence length was evaluated through Pseudo-Voigt peak profile analysis using the Langford method,30 as implemented in the Fullprof suite. Such an analysis allows a discrimination of the line broadening associated with finite size and strain effects, after correction for the instrumental resolution function. Additional X-ray diffraction patterns with higher statistics were collected at the Swiss-Norwegian BeamLines (BM1A workstation) of the European Synchrotron Radiation Facility using a monochromatic beam of λ = 0.721 08 Å wavelength. Samples were sealed in 0.4 mm diameter glass capillaries, and images of the Debye− Scherrer rings were recorded using a MAR345 image-plate detector positioned at 110 mm distance from the sample. The calibration of the wavelength, sample-to-detector distance, and resolution of the setup were carried out using a LaB6 (NIST standard) powder sample. The morphology and size distribution of the particles were also investigated by transmission electron microscopy using a JEOL 2100F microscope operating at 200 kV. Infrared spectra in KBr pellets were recorded from 4000 to 400 cm−1 using a Bruker (Equinox 55) FTIR spectrometer. Magnetization measurements were performed on powder samples using a SQUID magnetometer (Cryogenic SX600). Samples were mounted in polycarbonate capsules with the powder blocked by paraffin wax. Owing to dehydration of both the silica matrix and the iron oxide particles during annealing, the fraction of magnetic material in each sample was derived from the iron content measured by inductively coupled plasma atomic emission spectroscopy. Data were systematically corrected for the diamagnetic contributions of the silica matrix, wax, and polycarbonate capsule. Additional measurements were performed up to 14 T using a Cryogenic Vibrating Sample Magnetometer. Mössbauer spectra were recorded in transmission geometry using a spectrometer operating in constant acceleration mode with a 57Co:Rh γ-ray source.

to extended defects or local atomic rearrangements. To tackle this problem, we have investigated the influence of postsynthesis heat treatments carried out up to 1000 °C on iron oxide nanoparticles prepared by room temperature coprecipitation in water. However, for the specific case of maghemite, annealing at temperatures larger than 400 °C leads to grain growth and implies an irreversible transformation into hematite (αFe2O3).22 An efficient alternative to prevent both effects is to embed the particles in a refractory matrix prior to heat treatment.23,24 Such a strategy of protected annealing has already been used to stabilize the ordered L10 phase of FePt nanoparticles that exhibits particularly large magnetocrystalline anisotropy.25 It was also shown to improve the luminescent properties of various phosphors through increased quantum yield26 or enhanced photostability.27 The advantage of this protocol is that the size distribution of the original colloid is preserved, even after heat treatment up to 1000 °C.26 It is also possible to tune the particle loading so that dipole interactions can be neglected, making easier the interpretation of the magnetization data. In the present work, we chose a sol−gel silica binder to ensure the role of the refractory matrix. This strategy allowed us to account for the influence of structural rearrangements on the magnetic properties of γ-Fe 2 O 3 nanoparticles at fixed particle size.

2. EXPERIMENTAL SECTION 2.1. Synthesis of the γ-Fe2O3 Colloid and Dispersion in a Silica Matrix. In a first step, a colloidal solution of maghemite nanoparticles was prepared following a method derived from that of Massart based on the coprecipitation of Fe(II) and Fe(III) salts in alkaline medium.15 Concentrated ammonium hydroxide (11.4 mL, 13 mol·L−1) was quickly added to an acidic solution (HCl, 0.33 mol·L−1) of ferrous and ferric chlorides (36 mL, [Fe(II)] = [Fe(III)] = 0.55 mol·L−1) under vigorous stirring. A black precipitate of magnetite was then recovered by magnetic decantation and washed twice with deionized water. After addition of nitric acid (4.9 mL, 2 mol·L−1), the slurry was agitated for 30 min and magnetically decanted, and the supernatant was discarded. A solution of ferric nitrate (12 mL, 1.5 mol·L−1) was added to the asobtained precipitate, and the mixture was heated up to reflux for 30 min in order to fully oxidize and stabilize the ferrite particles against dissolution in acidic media.28 The resulting particles of maghemite were washed once with deionized water, twice with acetone, and finally dispersed by peptization in 30 mL of an acidic solution (pH 2) of nitric acid. The ferrofluid was then sonicated during 5 min. Two centrifugations (11 400 g, 15 min) were subsequently performed, and the supernatant recovered, in order to lower the size distribution. In a second step, the magnetic particles were embedded in a sol−gel SiO2 matrix following a protocol similar to the one reported in ref 26, except that no templating agent was used during the condensation of the silica sol, thus yielding a microporous Fe2O3/SiO2 composite powder after the drying stage. Eleven milliliters of a solution of tetraethoxysilane (TEOS) in ethanol (2.25 mol·L−1) was first hydrolyzed by adding 2.25 mL of an aqueous solution of HCl with pH 1.15. The resulting solution was reacted for 1 h at 60 °C under stirring, and 0.4 mL (respectively 8 mL) of the ferrofluid was added to the sol, leading to composite materials with a Fe/Si atomic ratio of 0.01 (respectively 0.25). The two mixtures were then dried in air at 90 °C until a xerogel was obtained. The Fe2O3/SiO2 composite powders were subsequently heated in

3. RESULTS AND DISCUSSION 3.1. Protected Annealing of γ-Fe2O3 Nanoparticles in Silica Matrix. The γ-Fe2O3 nanoparticles (NPs) were prepared through standard coprecipitation of ferric and ferrous salts in aqueous solution followed by an oxidation treatment made by heating the colloid under reflux in presence of Fe(NO3)3.15,28 A representative image of the size distribution determined by transmission electron microscopy (TEM) is shown in the inset of Figure 1, indicating spherical-like particles. The size histogram (statistics over ca. 700 particles) fits well to a lognormal function with a mean diameter value dm = 7.1 nm and a standard deviation σd = 0.28, which corresponds to a size distribution σ = (eσd2 − 1)·dm2 of 2 nm (Figure 1). The X-ray diffraction (XRD) pattern of the powder sample is consistent with the Fd3̅m space group (Figure 2), which is adopted by 16312



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departs from that of bulk magnetite (a = 8.396 Å, JCPDS 190629), suggesting a complete oxidation of the magnetite particles formed after precipitation of the Fe(II) and Fe(III) salts in alkaline medium. A Mössbauer spectrum was measured at 4.2 K (see the Supporting Information). It shows the 6-line spectrum characteristic of maghemite nanoparticles in a blocked state, which has been fitted to the sum of two subspectra. Iron is only in the +III valence state as the isomeric shift values for the two subspectra are equal to 0.41 and 0.46 mm/s with respect to α-Fe, supporting the X-ray diffraction result of fully oxidized maghemite nanoparticles. To reduce magnetic dipole interactions and prevent coalescence and grain growth during thermal treatments, the γ-Fe2O3 NPs were dispersed in a microporous silica matrix prepared through sol−gel chemistry. Incorporation of the NPs in the SiO2 binder was achieved by addition of the ferrofluid into an acidic silica sol. Neither flocculation nor aggregation was observed after addition, even for the largest Fe loading (Fe/Si = 0.25), yielding homogeneous Fe2O3/SiO2 composite powders after silica gelation and drying. The good dispersion state of the colloid was preserved in the composite material as checked by TEM, similarly to what was reported by Chanéac et al. by direct addition of the triethoxysilane precursor into the acidic ferrofluid and subsequent gelation.31 A study was first conducted to monitor the evolution of the size and of the crystal structure of the Fe2O3 NPs upon annealing in the silica binder. Experiments were performed on the Fe2O3/SiO2 composite with Fe/Si = 0.25 (FeSiX) to obtain well-resolved diffraction patterns. The XRD profiles of the crude particles and those of the composite materials are shown in Figure 2, after the drying stage (90 °C) and after annealing at selected temperatures: 290, 540, and 840 °C. The corresponding lattice parameter, a, and coherence length, Lc, values are summarized in Table 1. These two parameters were system-

Figure 1. Histogram of the particle size distribution where the solid line is a fit to a log-normal function. Inset: TEM micrograph of the crude γ-Fe2O3 nanoparticles (Fe90 sample).

Table 1. Structural Characterization of the Fe2O3/SiO2 Composite Samples (Fe/Si = 0.25)

Figure 2. (Color online) X-ray diffraction profiles (λ = 1.5418 Å) of Fe2O3 NPs dried at 90 °C and annealed at 290, 540, and 840 °C: (a) crude particles and (b) particles embedded in silica (Fe/Si = 0.25). Indexing refers to the Fd3̅m space group of maghemite. The peaks related to the Si calibrant are indicated by open circles. Insets: synchrotron X-ray diffraction profiles (λ = 0.721 08 Å) of the corresponding samples. The P4332 extra reflections are marked by asterisks, whereas the peaks corresponding to ε-Fe2O3 are indicated by arrows. The broad band centered at 2θ ∼ 11° is mainly related to the amorphous silica matrix.

sample

aa (±0.003 Å)

Lcb (±0.5 nm)

FeSi90 FeSi290 FeSi540 FeSi840

8.349 8.346 8.346 8.356

8.4 8.3 8.3 9.3

a Lattice parameter value determined with a Si calibrant. bCoherence length value.

atically determined assuming the Fd3̅m space group of maghemite, which corresponds to a disordered distribution of the Fe vacancies. The cell parameters for the composite samples annealed at 90, 290, 540, and 840 °C are very close to that of natural maghemite (see Table 1). Both the constant value of the lattice parameter and the absence of secondary phase involving mixed silica and iron oxides in the FeSiX samples confirm the chemical inertness of the host matrix, which remains amorphous up to 990 °C. Additional XRD measurements were performed between 140 and 990 °C with a 50 K step. A complete γ-Fe2O3 → α-Fe2O3 (hematite) transformation was observed between 390 and 440 °C for the crude particles. In addition, the coherence length value, calculated using the Scherrer formula on the (311) peak for maghemite and on the (104) line for hematite, increased exponentially with annealing temperature (Figure 3). In contrast, for the FeSiX powders, the coherence length did

both magnetite (Fe3O4) and maghemite (γ-Fe2O3) when iron vacancies are randomly distributed over the octahedral sites of the fcc structure. The coherence length value, Lc, evaluated using the Langford method30 is 7.1 ± 0.5 nm, i.e., close to the mean particle size derived from the TEM observations indicating single-crystalline particles. This conclusion is also supported by high-resolution TEM images (Supporting Information). The lattice parameter, a = 8.353 ± 0.003 Å, was determined using an internal Si calibrant. Its value is close to that of natural γ-Fe2O3 (a = 8.352 Å, JCPDS 39-1346), while it very strongly 16313



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Figure 3. Structural coherence length, Lc, as a function of the annealing temperature for crude Fe2O3 NPs (open and full circles) and for NPs embedded in silica, Fe/Si = 0.25 (triangles). Coherence length values were calculated using Scherrer formula.

Figure 4. (Color online) Infrared spectra for Fe2O3 NPs embedded in silica (Fe/Si = 0.25) and annealed at different temperatures. L (respectively T) stands for longitudinal (respectively transverse) modes, whereas S and AS refer to symmetric and antisymmetric modes, respectively.

not change with heating, and no significant differences in the diffraction patterns were reported after heat treatments performed between 90 and 990 °C (Figures 2 and 3). Only higher statistics data using a synchrotron source allowed us to detect a minority ε-Fe2O3 phase formed at 840 °C for the FeSiX samples (diffraction lines marked by arrows in the inset of Figure 2b). As previously reported by Jolivet and coworkers,23,31 dispersion of γ-Fe2O3 particles within a silica matrix precluded their coalescence thus preventing their structural transformation into hematite. Indeed, according to literature, the free energy of Fe2O3 nanoparticles is successively minimized for the γ-, ε-, β-, and α-polymorphs upon crystallite growth.32 The synchrotron X-ray diffraction data gave also some additional insight into the Fe vacancy distribution. The diffraction patterns of the Fe90 and FeSi90 samples obtained after the drying stage were consistent with a randomized distribution of Fe vacancies (Fd3̅m space group). Upon thermal annealing, additional reflections became visible at low 2θ angles, due to a partial vacancy ordering and symmetry lowering to the P4332 space group.33 These extra peaks, marked by asterisks in the insets of Figure 2, were observed for heat treatments above 290 °C for the crude γ-Fe2O3 NPs whereas they were detected at higher temperatures for the FeSiX composite samples. In that latter case, the broad band characteristic of amorphous silica strongly hinders the observation of these peaks, so that complementary experiments using infrared spectroscopy were undertaken to better assess the vacancy distribution and its evolution upon annealing (Figure 4). The fact that the Fe cations are randomly distributed over the interstitial sites of the fcc structure results in a large distribution of Fe−O distances, which in turn leads to multiple wavenumber values for the Fe−O−Fe vibrational modes. In the disordered form of maghemite, IR bands are consequently broad and weak. On the contrary, when cations are well ordered within the lattice, the distribution of Fe−O distances sharpens and additional bands at specific wavenumbers appear.18,34 In Figure 4, the low-frequency bands at 565 and 640 cm−1, which correspond to Fe−O−Fe stretching modes, are assigned to the spinel γ-Fe2O3 structure.35 Two extra bands at 694 and 727 cm−1, also ascribed to Fe−O−Fe stretching

modes, grow upon annealing indicating some ordering of the Fe vacancies even though it could not be detected by XRD. The maximum intensity of these two bands increases upon heating, from 290 to 840 °C, suggesting a progressive rearrangement of the crystal structure associated with vacancy ordering, annealing of volume defects, and possible surface reconstruction related to the removal of surface hydroxyl groups. In parallel, Figure 4 shows a modification of the absorption bands at 459, 801, 948, 1084, and 1223 cm−1 which are characteristic of the silica network.36 A quasi-complete disappearance of the 948 cm−1 band, corresponding to the stretching frequency of silanol groups, is observed when increasing the annealing temperature, together with an increased intensity of the 801 cm−1 line attributed to the formation of Si−O−Si bridges. These results indicate a gradual polycondensation of the xerogel from 90 to 840 °C. Interpretation of the changes observed in the IR bands between 1050 and 1250 cm−1 is less straightforward, as the intensity of these lines, assigned to antisymmetric vibrations of the Si−O−Si linkages, is strongly affected by modifications of the polymeric structure of the silica network.37 A chemical modification of the particle surface state due to reaction with the SiO2 matrix cannot be inferred from these FTIR experiments as the stretching mode of Fe−O−Si bonds, expected at 1016 cm−1,38 could be hidden by the 1084 cm−1 band. The FeSi290 and FeSi540 spectra clearly display a weak band at 584 cm−1 which can be assigned to Fe−O stretching in Fe−O−Si bonds,36,39 but also in Fe−O−Fe linkages.40 3.2. Evolution of the Magnetic Properties upon Annealing. 3.2.1. Analysis of the M(H) Curves in the Superparamagnetic Regime. As the γ-Fe2O3 NPs dispersed in SiO2 do not show any significant coalescence or growth upon thermal annealing, even at relatively high Fe loading (Fe/Si = 0.25), the impact of structural rearrangements on magnetic properties could be studied at fixed particle size. Magnetic measurements were carried out on the diluted FeSid composite samples (Fe/Si = 0.01), so that magnetic dipole interactions and their possible change upon sintering of the SiO2 matrix could be neglected due to the low volume fraction in particles (∼0.5%).20 Magnetization reversal curves were first recorded at 300 K (Figure 5). They display an anhysteretic behavior, consistent with a superparamagnetic regime. At high magnetic 16314



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the SiO2 matrix as reported by Ortega and co-workers.46 This latter assumption was checked by analyzing the Mössbauer spectrum recorded for the FeSi90 sample (see the Supporting Information). At 4.2 K, γ-Fe2O3 particles of 7 nm in diameter should be in a blocked state18 and exhibit a 6-line magnetic hyperfine spectrum that differs from that of free Fe(III) ions characterized by a quadrupolar doublet. The fact that no doublet was detected in the present case definitely ruled out the presence of large amounts of Fe(III) ions (3% at maximum) trapped in the matrix as a result of a partial redissolution of the particles before or during silica condensation. The depressed magnetization values thus reflect noncollinear spin arrangements, which are known to be related to local changes of magnetic anisotropy at the surface or in the volume.10−12 At room temperature, these weakly coupled spins are unblocked and fluctuate between different local minima,47,48 giving rise to an additional contribution to the high-field susceptibility χ. The origin of misaligned spins at the particle surface is the presence of broken exchange bonds, which leads to a reduced coordination of Fe surface ions and to magnetic frustration.49 As previously reported, canted spins can also be present in the volume of the particles because of the presence of Fe vacancies.12 For instance, Morales and co-workers50 showed that the level of vacancy ordering has a large influence on spin canting. γ-Fe2O3 particles with random distribution of Fe vacancies (Fd3̅m space group) exhibit mean canting angles of the magnetic moments for Fe atoms in tetrahedral sites and in octahedral sites which are almost twice those of particles that present partial or full vacancy ordering corresponding to the P4332 or P43212 space groups, respectively. As postsynthesis heat treatments were found to induce Fe vacancy ordering and the possible removal of extended defects in the volume, the evolution of the magnetic features upon heating should allow discriminating between surface and volume effects. Table 2 shows a slight decrease in the saturation magnetization, Ms, and a much larger increase in the high-field susceptibility χ as the annealing temperature increases. These two trends are opposed to the ones expected from a vacancy ordering, as it should have led to an increased saturation magnetization.50 Defect annealing and vacancy ordering have thus a weak impact on the saturation magnetization values, and we shall rather consider that the enhanced paramagnetic-like contribution, indicated by the increased high-field slope on heating, is merely driven by surface effects. 3.2.2. Increased Surface Anisotropy upon Annealing. To get further insight into the evolution of the magnetic properties upon thermal annealing, zero-field cooled (ZFC)−field cooled (FC) measurements were carried out (Figure 6), as the effective anisotropy constant K can be approximated as K = 25kBTB/V, where kB is the Boltzmann constant, V the mean volume of the particles, and TB the blocking temperature. For an assembly of particles with a narrow size distribution, TB would correspond to the sharp maximum of the ZFC curve (hereafter labeled Tpeak). However, when the width of the size distribution increases, this maximum broadens, and TB strongly deviates from Tpeak and can be more than 1.5 times smaller.51,52 To correctly evaluate the anisotropy constant, the value of the ratio between Tpeak and TB is required. This quantity can be derived from simulations of the ZFC curves in the case of noninteracting NPs,53−55 considering log-normal size distributions with a mean diameter of 7.1 nm and different widths (σd). In that case, we found that Tpeak/TB can be expressed as

Figure 5. (Color online) Magnetization vs applied magnetic field at 300 K for Fe2O3 NPs embedded in silica (Fe/Si = 0.01) and annealed at selected temperatures: 90 °C (black, circles), 290 °C (red, triangles), 540 °C (purple, crosses), and 840 °C (blue, diamonds). Experimental data are compared with a curve calculated using a Langevin approximation for an assembly of particles with a log-normal distribution (dm = 7.1 nm, σd = 0.28) assuming a saturation magnetization of 75 emu/g identical to that of the bulk phase (orange, solid line).

fields, magnetization reversal proceeds through coherent rotation for both single- and polydomain particles.9,41,42 In this case, the saturation magnetization (Ms) and the high-field susceptibility (χ) can be estimated using a modified law of approach to saturation, which writes as follows for an assembly of noninteracting particles:43,44 ⎛ b ⎞ M = M s ⎜1 − 2 ⎟ + χ · H ⎝ H ⎠

(1)

Table 2 summarizes the data obtained from fits to eq 1 performed between 1.5 and 5 T for FeSidX composite samples Table 2. Magnetic Characterization of the Fe2O3/SiO2 Composite Samples (Fe/Si = 0.01) sample FeSid90 FeSid290 FeSid540 FeSid840

Ms (300 K)a (emu/gFe2O3) 55 57 53 53

± ± ± ±

2 2 2 2

χ (300 K)a

μ0Hcb (G)

Tpeakc (K)

± ± ± ±

160 170 200 260

75 80 84 102

0.33 0.29 0.36 0.49

0.02 0.02 0.03 0.03

Kd (erg/cm3) 4.2 4.6 4.8 5.7

× × × ×

105 105 105 105

a Saturation magnetization (Ms) and high-field susceptibility (χ) derived from fits to eq 1. bCoercive field value measured at 10 K. c Temperature of the maximum in the ZFC curve. dEffective anisotropy constant; see the main text for calculation details.

annealed at 90, 290, 540, and 840 °C. Note that both Ms and χ values were normalized to the Fe2O3 mass content. As reported in Table 2, the saturation magnetization value of the assynthesized particles was found to be 27% lower than the one expected for bulk γ-Fe2O3 (75 emu/g at 300 K).45 It is worth noticing that this reduced Ms is not a consequence of the presence of small particles in the assembly, as a theoretical curve calculated using a Langevin approximation and the size histogram of Figure 1 gives a magnetization value at 5 T of 74 emu/g. The strongly depressed Ms value rather arises from the presence of a large amount of paramagnetic-like species, either weakly coupled surface and/or volume spins as mentioned by various authors,10−12 or else free Fe(III) ions dispersed within 16315



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expansion of the shell made of noncollinear spins that would be driven by an increased surface anisotropy. It is worth mentioning that the propagation of these noncollinear spins in the volume could have been favored by a possible reinforcement of the antiferromagnetic interactions associated with the annealing of crystal defects and vacancy ordering. 3.2.3. Expansion of the Shell Made of Noncollinear Spins upon Annealing. For a further description of the canted spin shell, additional magnetization measurements were carried out at temperatures ranging from 5 to 300 K (see Figure 7 for the

Figure 6. (Color online) ZFC curves for γ-Fe2O3 NPs embedded in silica (Fe/Si = 0.01) dried at 90 °C and annealed at 290, 540, and 840 °C. Measurements were performed under 25 G magnetic field. Inset: magnetization versus magnetic field measured at 10 K.

Tpeak TB

= 0.98 + 0.21e8.5σd

(2)

For a standard deviation σd of 0.28, as determined from the size histogram derived from the TEM observations, this ratio corresponds to 3.25. According to the previous section, the size distribution does not change upon annealing at least up to 840 °C. The same Tpeak/TB ratio should thus be considered for particles annealed between 90 and 840 °C. The as-obtained values of the anisotropy constant are reported in Table 2. These K values are about 1 order of magnitude larger than that of bulk maghemite (Kbulk = 4.7 × 104 erg/cm3).44 This phenomenon is commonly reported in nanoparticulate systems, for which surface anisotropy (Ks) becomes much larger than the volume one (Kv).56 According to the expression K = Kbulk+6 Ks /d,57 where d stands for the particle diameter, the values of the effective anisotropy reported in the present work are mostly representative of a surface contribution. As shown in Table 2 and Figure 6, Tpeak increases with the annealing temperature. The corresponding increase in K would thus indicate a modification of Ks, from 0.044 erg/cm2 after synthesis up to 0.067 erg/cm2 after heat treatment at 840 °C. This trend is confirmed by the M(H) curves measured at 10 K, i.e., in the blocked regime, which show a significant increase in the coercive field value μ0Hc upon thermal annealing (Table 2 and inset of Figure 6). Such an increase in magnetic anisotropy could be induced by a modification of the surface state of the particles, e.g., related to the removal of surface hydroxyl groups. Coordination number and bond lengths would then be modified leading to local changes of anisotropy. For maghemite nanoparticles, Monte Carlo simulations have shown that the presence of a shell of noncollinear spin arises from the difference between core (Kc) and surface anisotropy (Ks).58 The larger the Ks/Kc ratio is, the deeper the shell of misaligned spins propagates toward the center of the particle via antiferromagnetic exchange interactions between Fe atoms. In ideally crystalline particles, noncollinear spins are thus restricted to the surface, in a shell of variable thickness depending on the relative value of surface anisotropy with respect to that of the core. It was also reported that an increased Ks/Kc ratio is associated with an increased coercivity. All our experimental findings are thus consistent with an

Figure 7. (Color online) Magnetization versus magnetic field measured at different temperatures for the FeSid290 sample, expressed in emu per g of Fe2O3 or in Bohr magneton per Fe atom.

FeSid290 sample). Note that M(H) curves recorded at 5 K up to 14 T using a vibrating sample magnetometer showed that the magnetization value under 5 T corresponds to ca. 98% of the saturation value. As shown in Figure 7, where magnetization is expressed per unit weight of Fe2O3, there appears to be an inconsistency as data below 15 K led to a saturation magnetization value which is actually larger than the one expected, ca. 80 emu/g at 10 K. In the following, magnetization values will systematically refer to iron atoms, thus making no assumption of the local oxygen stoichiometry that may be different in the volume and at the surface of the particles. Saturation magnetization, here approximated by the magnetization value under 5 T, is plotted as a function of temperature in Figure 8 for the FeSid290 sample. Ms values do not follow a

Figure 8. (Color online) Magnetization value under 5 T as a function of temperature, fitted to eq 3, for the FeSid290 sample. 16316



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classical T3/2 Bloch law but exhibit a low-temperature exponential variation that was shown to be related to the contribution of disordered surface spins.59,47,48,60 Nanoparticles could thus be described as a two-component system involving (i) a core part with ferrimagnetic behavior, for which the strength of the exchange and anisotropy interactions are identical to those of the bulk material, and (ii) a shell part with smaller exchange and larger anisotropy interactions,58−60 which mainly arise from lower coordination of the Fe surface atoms and surface roughness. As a consequence, the saturation magnetization could be expressed as the sum of two terms Ms(T ) = M sshell(0) ·e−T / Tf + [M score(0) + a ·T ]

two independent measurements using different modeling of the experimental data (respectively the law of approach to saturation of eq 1 and the modified Bloch law of eq 3) have concluded a decreased saturation magnetization of the γ-Fe2O3 NPs under study after postsynthesis heat treatments.

4. CONCLUSION Postsynthesis heat treatments were performed on 7-nm-sized γFe2O3 nanoparticles prepared by room temperature coprecipitation of iron salts in water, followed by the dispersion of the preformed particles in a refractory silica matrix. Thermal annealings up to 840 °C were shown to afford a control of the crystalline state of the particles, preventing both grain growth and transformation into hematite. Absence of large-scale disorder in the original γ-Fe2O3 particles was derived from the analysis of the X-ray diffraction line widths. Upon annealing, the main structural changes consisted of a partial ordering of the Fe vacancies above 290 °C. Regarding the magnetic properties, postsynthesis heat treatments led to counterintuitive findings with a decreased saturation magnetization value and an increased high-field slope χ of the M(H) curve in the superparamagnetic regime. These two observations would agree with an enhanced contribution upon annealing of the weakly coupled and misaligned spins located at the surface of the particles. A gradual increase in magnetic anisotropy was also reported, which should be ascribed to a modification of the surface state upon heating involving the removal of surface hydroxyl groups. All experimental data were actually consistent with a propagation of the shell of noncollinear spins toward the center of the particles. Comparison with Monte Carlo simulations suggested that this expansion was mainly driven by the increased surface anisotropy. We conclude from this study that in the case of 7-nm-sized γFe2O3 particles, the strong reduction in saturation magnetization is mostly ascribed to surface effects. Significant influence of volume defects (dislocations, twins, or vacancy order) on magnetic properties may be expected in the case of larger particles. However, investigations are limited by the structural transformation into hematite. Preliminary studies have been achieved on 14 nm γ-Fe2O3 nanoparticles which retained the maghemite crystal structure up to 540 °C. In this case, the opposite trend seems to be observed, as compared to the one reported for the 7 nm particles, with a reduction of the number of misaligned spins on thermal treatment. Effects associated with annealing are nevertheless rather small, due to the fact that particles obtained through aqueous coprecipitation appear as already well-crystallized after synthesis. In this context, similar investigation would deserve to be conducted in the case of iron oxide nanoparticles prepared by thermal decomposition of iron oleate or of iron pentacarbonyl, considering recent reports on the large impact of their reduced crystallinity on their magnetic properties.20,21

(3)

that fully describes the temperature dependence of Ms (see the Supporting Information for details). A fit to the experimental data allows the determination of the freezing temperature, Tf, and of the relative weight of the core and shell contributions. If we assume that in the core part, the Ms value at 0 K is 2.5 μB per Fe2O3 formula unit, e.g., 1.25 μB per Fe atom, it is possible to derive the fraction of Fe atoms that would contribute to the corelike behavior: nFecore = Mscore(0)/1.25. The residual Fe fraction would then be ascribed to the shell part. The fitted parameters obtained for the different samples are summarized in Table 3 (Ms versus T curves are reported in the Table 3. Magnetic Parameters Derived from the Analysis of Ms versus T Curves for Fe2O3/SiO2 Composite Samples (Fe/ Si = 0.01) sample

Ms core(0) (±0.01 μB/Fe)

nFecore (±2%)

nFeshell (±2%)

Tf (K)

FeSid90 FeSid290 FeSid540 FeSid840

1.08 1.07 1.03 1.03

86.4% 85.6% 82.4% 82.4%

13.6% 14.4% 17.6% 17.6%

12.6 12.6 12.6 12.7

Supporting Information). Note that the freezing temperature corresponds to 13 K for all samples, which is consistent with the values found in several studies on ferrite nanoparticles.47,48 According to the M(H) curves measured at 300 K, postsynthesis heat treatments performed on γ-Fe2O3 NPs embedded in silica led to a larger contribution of misaligned spins as evidenced by the decreased saturation magnetization value accompanied by the larger slope at high magnetic fields. By fitting the temperature dependence of Ms for all samples, we confirmed that this decreased saturation magnetization is associated with an expansion of the canted spin shell, from 14% to 18% of the overall Fe content, which roughly corresponds to a 0.2 nm shell thickness. If the saturation magnetization of the core part is actually less than 1.25 μB per Fe atom, which would be realistic at low annealing temperature due to the presence of residual defects and randomly distributed vacancies, the fraction of spins in the canted shell would be even overestimated. This result is in good accordance with the reduction of saturation magnetization at 300 K as compared to 75 emu/g (Figure 5) that would correspond to a magnetic domain size of 6.4 nm and a dead magnetic layer of 0.35 nm thickness. It is worth mentioning that the rather large error bars on the magnetic parameters in Tables 2 and 3 mainly originate from the uncertainty in the iron content determined by elemental analysis in the composite materials. In spite of these error bars,

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ASSOCIATED CONTENT

S Supporting Information *

High-resolution TEM image of the crude Fe90 particles; Mössbauer spectrum of the FeSi90 composite sample and saturation magnetization versus temperature curves for composite samples annealed at 90, 290, 540, and 840 °C; full description of the Ms versus T law corresponding to eq 3. This material is available free of charge via the Internet at http:// pubs.acs.org. 16317



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Article

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (I.M.); thierry. [email protected] (T.G.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are indebted to the ESRF for provision of synchrotron X-ray beam time. They also thank D. Chernyshov for valuable discussions on the XRD data. J.-M. Guigner and E. Larquet from Institut de Minéralogie et Physique des Milieux Condensés (Université Pierre et Marie Curie, Paris) are acknowledged for the TEM observations. The research described here has been supported by Triangle de la Physique (contract 2008-051T).

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