Maghemite

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J. Phys. Chem. C 2009, 113, 12040–12047

Evolution of Morphology and Magnetic Properties in Silica/Maghemite Nanocomposites Dan Li,† Wey Yang Teoh,† Robert C. Woodward,‡ John D. Cashion,§ Cordelia Selomulya,| and Rose Amal*,† ARC Centre of Excellence for Functional Nanomaterials, School of Chemical Sciences and Engineering, UniVersity of New South Wales, Sydney, NSW 2052, Australia, School of Physics, UniVersity of Western Australia, Crawley, WA 6009, Australia, and School of Physics and Department of Chemical Engineering, Clayton, Monash UniVersity, Melbourne, VIC 3800, Australia ReceiVed: March 25, 2009; ReVised Manuscript ReceiVed: May 5, 2009

The morphological evolution of SiO2/γ-Fe2O3 nanocomposites was systematically synthesized in a one-step flame spray pyrolysis. Under these conditions, a gradual transformation from discrete γ-Fe2O3 nanoparticles to thin SiO2 coatings, segregated single γ-Fe2O3 core, and multiple γ-Fe2O3 cores within a SiO2 matrix was obtained as a function of SiO2 loading. The presence of SiO2 up to 13% has a pronounced effect on the γ-Fe2O3 crystallite structure (transforming from P4132 to Fd3jm space group) and its cationic vacancy ordering. Decrease in the latter was further reflected through the intrinsic magnetic properties of the γ-Fe2O3 cores (i.e., decreasing specific saturation magnetization and increasing coercivity and exchange bias). Deviation from the magnon-type thermal dependence, T3/2 Bloch law, was observed for nanocomposites with SiO2 content above 13%. The Ms vs T curves could be fitted with the sum of an exponential component and a Bloch law component, where the magnitude of the exponential component increased with increasing SiO2 content above 13% SiO2. The thermal dependence of the saturation magnetization for these samples could not be adequately explained by a finite size effect or via freezing of canted spins. 1. Introduction The advent of nanotechnology has introduced many new exciting applications using magnetic particles. These applications range across the areas of electronics, magnetic refrigeration, heavy metal recovery, bioseparation, biosensors, magnetic resonance imaging, hyperthermia treatment, drug delivery, and gene therapies.1-4 Nevertheless, direct application of bare magnetic nanoparticles is limited by various factors such as magnetically induced aggregation, surface oxidation, instability under physiological conditions,5 and leaching in acidic environments.6 As such, techniques to stabilize magnetic particles by encapsulating the magnetic core within surfactant layer,7 polymers,8 silica,2 or carbon9,10 matrixes have been reported. The dispersion of metallic or oxide iron nanoparticles in silica matrixes is one of the most intensively investigated systems.2,11 They exist in various forms and morphologies ranging from core-shell SiO2/Fe6,14 and SiO2/γ-Fe2O3,12,13 mesoporous SiO2/γ-Fe2O315-17 and SiO2/ε-Fe2O3,18 SiO2/γ-Fe2O3 nanorods19 and nanowires,20 to Fe3O4 within hollow silica,21 among others. The interfacing with SiO2 coating introduces an important biocompatible functionality,22 which is stable under a wide range of pH, temperature, physiological, and supraphysiological salt concentrations,23 while enhancing particle wear and corrosion resistance.14 Moreover, separation of the iron cores by SiO2 coatings reduces the magnetic dipolar attraction between particles,24 allowing better stability and dispersibility. The high concentration of surface hydroxyl groups on the SiO2 surface * To whom correspondence should be addressed. Telephone: +61 2 9385 4361. Fax: +61 2 9385 5966. E-mail: [email protected]. † University of New South Wales. ‡ University of Western Australia. § School of Physics, Monash University. | Department of Chemical Engineering, Clayton, Monash University.

can be easily modified via silane chemistry11 to provide molecular anchorage for a wide range of biomolecules.19 Despite its versatility and importance, a thorough appreciation of the effect of SiO2 coating on the morphologies and magnetic behavior is still elusive. In many cases, it is assumed that the core magnet retains its properties and hence any losses in magnetic moment are due to the shielding effect of SiO2 coating. This is based on the assumption that SiO2, being a nonmagnetic material, does not influence the magnetic properties of the iron cores. However, a number of other factors may indeed affect the magnetic properties of the cores, such as the (1) variation in physicochemical structure of metallic or oxide iron developed during the nanocomposite synthesis, (2) dissolution of small amounts of Si in the magnetic core, and (3) interfacial energies, including stress, at the core-coating interface. The latter effect becomes significant especially for nanoparticles, where a large proportion of atoms exists at the particle surface. Hence, in this study, the evolution of the morphology and magnetic properties of SiO2/γ-Fe2O3 nanocomposites synthesized by rapid flame spray pyrolysis (FSP) is presented in a systematic manner. The synthesis technique not only allows for accurate doping of SiO2 content over a wide range, but also allows careful tuning of the nanocomposite morphology, a feature deemed important in the current study, to relate to its corresponding magnetic properties. For the first time, an inherently complex correlation between morphology-induced microstructural changes such as crystallinity, vacancy ordering, and particle size and distribution, with magnetic parameters including saturation magnetization, exchange bias, and coercivity is thoroughly investigated and discussed. 2. Experimental Methods 2.1. Particle Synthesis. A one-step FSP process was used to synthesize the SiO2/γ-Fe2O3 particles, as described previously.25

10.1021/jp902684g CCC: $40.75  2009 American Chemical Society Published on Web 06/11/2009

Morphology and Magnetic Properties in SiO2/γ-Fe2O3 Briefly, liquid precursors consisting of 0.34 M iron(III) acetylacetonate (Fe(acac)3, Aldrich, 97%) and tetraethylorthosilicate (TEOS, Aldrich, 98%) were codissolved in xylene (Riedel-de-Haen, analytical grade)/acetonitrile (JT Baker, HPLC grade) to give a final precursor combustion enthalpy of -26.7 kJ/mL. During FSP, the liquid precursor was delivered to the pressure-assisted nozzle (1.5 bar) at a flow rate of 10 mL/min (Inotech R233), where it was atomized by dispersant O2 (5 L/min). Combustion was ignited by a surrounding methane/oxygen (1.5 L/min/3.2 L/min) supporting flame. An additional 5 L/min of sheath O2 was provided through an outermost sintered metal ring. Since the flame is fully exposed to the ambient environment without enclosure, this is termed “open flame” synthesis. The aerosol nanoparticles leaving the flame were collected on glass fiber filter (Whatmann GF/D, 25.7 cm diameter) with the aid of a vacuum pump (Alcatel SD series). For the “enclosed flame” synthesis, a quartz tube (i.d. ) 6.6 cm and length ) 40 cm) was placed around the flame during FSP. In this case, the precursor was delivered at 8 mL/min and dispersed by 5 L/min of O2. The sheath O2 flow rate was set at 15 L/min. 2.2. Particle Characterization. The size and morphology of the nanoparticles were examined using a transmission electron microscope (TEM, Philips CM-200) at an accelerating voltage of 200 kV. The particle size distribution was determined on the basis of statistical counting of at least 150 particles. X-ray diffraction (XRD) of the powder samples was measured on a Philips PW3020 using Co KR radiation. The particles were scanned for 2θ ) 20-80° with step size of 0.03° and scan time of 2.5 s/step. Infrared spectra of the powder samples diluted in KBr were recorded from 1500 to 400 cm-1 at a resolution of 4 cm-1 for a collection of 100 scans (Nicolet Avatar 300). Chemical composition analysis was carried out by fully digesting the particles in aqua regia and/or hydrofluoric acid. The aliquot was analyzed for Si and Fe elemental concentrations on an induced coupled plasma-optical emission spectrometer (ICPOES, GBC Integra XMP). The magnetic properties were measured using a Quantum Design 7 T MPMS SQUID magnetometer between 5 and 400 K. Mo¨ssbauer spectra of the samples were taken at room temperature, 78 K, and 5 K using a conventional constant acceleration spectrometer. Calibration was carried out with R-Fe, and all isomer shifts were referenced against R-Fe at room temperature. 3. Results and Discussion 3.1. Morphologies and Cationic Vacancy Ordering. Morphological evolution of the SiO2/γ-Fe2O3 nanocomposites was obtained by varying the SiO2 content over a wide range, from 1 to 79 wt %. Discrete crystalline γ-Fe2O3 cores (cubic phase, JCPDS: 39-1346) were discernible in all cases. These cores were surrounded or embedded within an amorphous SiO2 matrix, as shown in Figure 1. No trace of other phases such as hematite or Fe silicates was detected by XRD (Figure 2) and further supported using 57Fe Mo¨ssbauer spectroscopy, the latter of which will be discussed in section 3.3. At low SiO2 loading of 1%, only single-phase γ-Fe2O3 could be seen (Figure 1a) and no sign of the amorphous SiO2 hump at Bragg angle of 20-40° in the XRD could be discerned. This is consistent with the reported 4 mol % solubility limit of SiO2 in Fe2O3 (equivalent to 1.5 wt %).26 However, as the amount of Si doping exceeds the solubility limit, a thin segregated SiO2 layer around γ-Fe2O3 becomes evident (Figure 1b,c). Despite the minimal effect of SiO2 on the nanocomposite morphologies at these relatively low loadings, the γ-Fe2O3 crystallite size (dXRD) decreases monotonically from 13 nm for bare γ-Fe2O3 to just 9 nm for 13% SiO2. The role of SiO2 as a growth suppression agent during FSP synthesis

J. Phys. Chem. C, Vol. 113, No. 28, 2009 12041 without the formation of silicates has been documented recently in other systems such as SnO227 and BiVO4.28 At 43% SiO2 and above, two distinct morphologies were obtained: single γ-Fe2O3 core segregated to the edge of the SiO2 matrix (43-60% SiO2, Figure 1d,e) and multiple γ-Fe2O3 cores dispersed evenly within a SiO2 sphere (79% SiO2, Figure 1g).29 At intermediate SiO2 loading of 69%, however, a mixture of discrete particles consisting of a combination of the two morphologies resulted (Figure 1f). In addition to the presence of SiO2 as a growth suppressant, the γ-Fe2O3 crystallite size at such high SiO2 loading is now determined by the core morphologies, decreasing to just 6 nm for 79% SiO2/γ-Fe2O3. Erhman and Friedlander29 suggested that the nanocomposite morphologies were governed predominantly by the high temperature liquid-phase miscibility (i.e., Fe to Si ratio), which in turn may be a function of interfacial tensions between the liquid phases.30 For further conceptual comprehension, other factors such as the combination of high-temperature solid-state solubility and the diffusivity of γ-Fe2O3 and SiO2, their respective boiling/sublimation points, and the flame time-temperature profile should also be taken into consideration. With increasing SiO2 content in the nanocomposite, the γ-Fe2O3 cores gradually transformed from P4132 to Fd3jm space group, the latter of which was characterized by the disappearance of (210) and (211) crystal planes (Figure 2). Such transformation was previously observed on bare γ-Fe2O3 as a function of sintering-induced crystallite size (dXRD) increase.33 However, in the present case, only minor variations of core size between 6 and 10 nm were observed in all cases of “open aerosol flame” (i.e., absence of flame enclosure in a quartz tube). For comparison, quartz tube enclosure of the aerosol flame is adopted here to fend off quenching of the flame from the surrounding air and also to prolong the flame residence time.32 (For convenience, samples prepared via quartz tube enclosure will be denoted by “Q”.) As a result, significantly larger γ-Fe2O3 cores (Table 1) were obtained through exposure to a prolonged high-temperature zone during synthesis, without affecting the overall composite morphologies (Figure 1h,i). More importantly, the technique is analogous to our previous synthesis of large bare γ-Fe2O3 nanoparticles with the tetragonal P43212 space group.31 However, only cubic-phase γ-Fe2O3 consisting of Fd3jm space group could be obtained in this case in the presence of SiO2, hence ruling out that the crystallography is solely determined by the time-temperature flame profile and that the doping of SiO2 in γ-Fe2O3 seems to prevent the high-temperature transformation to the tetragonal phase. In stoichiometric terms, the γ-Fe2O3 crystal lattice can be written as (Fe3+8)Tetr[Fe3+40/308/3]OctaO32, where 0 denotes cationic vacancies. The degree of vacancy ordering in this structure can be exemplified by the stretching of the bond between the partially occupied octahedral Fe cation and its neighboring O as measured by FTIR (Figure 3).33 Well-defined bare γ-Fe2O3 crystal is characterized by the strong absorption bands at 729, 696, 640, 590, 563, 482, and 445 cm-1 (Figure 3a), all of which are characteristics of the aforementioned Fe-O stretching and indicative of a high degree of cationic vacancy ordering.31,33 The introduction of SiO2 in γ-Fe2O3 has a pronounced effect on its short-range structure by decreasing drastically the cationic vacancy ordering and reducing the heights of the characteristic FTIR peaks, even at 1% SiO2 loading (Figure 3). At a higher loading of 13% SiO2, these characteristic absorptions diminish, indicative of a sample in which cationic vacancies are nearly fully disordered. Further increase in SiO2 content up to 79% no longer affects the extent of disordering. The larger and more crystalline γ-Fe2O3 cores in 69% SiO2/γ-

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Li et al.

Figure 1. Morphologies of SiO2/γ-Fe2O3 nanocomposites at different SiO2 loadings. (Q) in (h,i) denotes synthesis in enclosed flame to yield large particles.

Fe2O3 (Q) have an enhanced degree of vacancy ordering, as depicted by a weak but enhanced FTIR absorption band at 540-760 cm-1 relative to the corresponding sample prepared in the open flame. The FTIR absorption bands resembled a weak version of the well-ordered bare γ-Fe2O3. However, the effect for 79% SiO2/γ-Fe2O3 was less obvious because of the higher shielding effect of Si-O-Si and O-Si-O vibrations (V ) 461, 670, 798 cm-1). The absence of bands at 574, 680, and 900 cm-1, typical of Si-O-Fe bonding in all samples, corroborates the XRD and later Mo¨ssbauer spectroscopy results on the absence of iron silicate. As will be shown in the next section, the order of cationic vacancies has a direct impact on the magnetic properties of the γ-Fe2O3 cores. 3.2. Magnetic Properties and Deviation from the Bloch Law. All samples were predominantly superparamagnetic at 300 K, as shown in Figure S1 of the Supporting Information, with only a small ferromagnetic fraction (1-4%) for samples prepared in an open flame (Table 2). This small amount of ferromagnetic material is also evidenced in the small separation of the zero field cooled (ZFC) and field cooled (FC) curves (Figure S2 in the Supporting Information). The peak in the ZFC curves, which is related the average particle size, decreases with increasing SiO2 content. This decrease in the peak in the ZFC corresponds to a reduction of about 30% in the diameter of the particles between 1% SiO2 and 79% SiO2. For the samples prepared in the enclosed flame, the percent

Figure 2. XRD Co KR spectra of SiO2/γ-Fe2O3 nanocomposites and the corresponding characteristic peaks of cubic γ-Fe2O3 (JCPDS: 39-1346) as a function of SiO2 loading (0-100%).

Morphology and Magnetic Properties in SiO2/γ-Fe2O3

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TABLE 1: Composition and Size Variations of Bare and Composite SiO2/γ-Fe2O3 of Various SiO2 Loadings % SiO2 nominal (wt %)

Fe/Si nominal (mol/mol)

0 1 4 13 43 60 69 79 69 (Q)d 79 (Q)d

1:0.01 1:0.05 1:0.2 1:1 1:2 1:3 1:5 1:3 Q 1:5 Q

% SiO2ICP (wt %)

dXRDa (nm)

dTEMb (nm)

σgc

43 60 69 79 67 77

13 10 9 9 9 9 7 6 11 10

16 12 11 12 14 19 14 11 46 22

1.43 1.45 1.51 1.50 1.49 1.39 1.45 1.27 1.93 1.50

a Determined from Scherrer formula. b Determined from particle counting with sample size >150. c Number-based geometric standard deviation. d Synthesized by flame enclosure.

Figure 3. FTIR spectra of bare γ-Fe2O3 and SiO2/γ-Fe2O3 at various SiO2 loadings.

of ferromagnetic material (∼20%) and peak in the ZFC curves was significantly higher, commensurate with the much larger γ-Fe2O3 core sizes for these materials. As evident in Figure 4, the maghemite specific saturation magnetization (Ms, emu/g of γ-Fe2O3) of SiO2/γ-Fe2O3 composites decreased sharply from 74 emu/g for bare γ-Fe2O3 to just 36 emu/g in the presence of 13% SiO2. The exact origin for this form of Ms reduction in nanoparticulate systems is somewhat contentious; some authors attributed it to surface effects,34,35 whereas others demonstrated it as originating from the internal crystalline disorder within the magnetic nanoparticles.36,37 In general, the reduction in magnetization in either case is a result of spin canting caused by (1) the reduced coordination and broken exchange at the surface or (2) the broken symmetry associated with crystalline disorder.38,39 While it is difficult to rule out contribution from surface effects in these samples, the large variation in specific Ms with relatively small change in crystallite size suggests that the dominant effect is due to changes in the degree of cationic vacancy ordering. This is supported by the XRD and FTIR measurements of a

decrease in the degree of vacancy ordering as SiO2 content is increased to 13%. Further increase in SiO2 loading (>13%) did not produce any further significant reduction in γ-Fe2O3 specific Ms. Note also that the measured specific Ms is consistently lower than that of similarly prepared bare γ-Fe2O3 of similar size.31 In other words, the specific Ms of γ-Fe2O3 was affected by the changes in vacancy ordering induced by the presence of SiO2. At SiO2 loading >13%, the overall nanocomposite Ms (emu/g sample) decreases linearly with increasing proportion of nonmagnetic SiO2. The nanocomposite samples with larger γ-Fe2O3 cores synthesized by the enclosed flame (i.e., 69 and 79% SiO2 (Q)) displayed higher specific Ms compared to their corresponding samples prepared in the open flame. This can again be traced to the greater vacancy ordering. More interestingly, these samples imply that for a given SiO2 content the vacancy ordering and hence their specific Ms (emu/g γ-Fe2O3) could be enhanced by improving the size of the γ-Fe2O3 cores and more importantly their crystallinity. To the same extent but with the opposite trend to the rapid decrease in specific Ms, the coercivity (Hc) and exchange bias (He) increased rapidly up to 13% SiO2 (Figure 5). This is strongly correlated with the cationic vacancy disordering which results in spin frustration and canting within the γ-Fe2O3 cores. It has been shown that spin frustration can lead to exchange bias and enhanced coercivity.40 Both Hc and He reach a steady state for SiO2 loading above 13%, again in agreement with the formation of cubic-phase γ-Fe2O3 with a virtually constant degree of cationic vacancy disorder. To better reflect these two effects, Hc was plotted against He, as shown in Figure 6. In principle, spin frustration in γ-Fe2O3 cores is directly related to the increase in He and effective anisotropy, the latter leading to further increase in Hc. To illustrate this conjecture, a series of similarly prepared bare γ-Fe2O3 of different sizes31 representing different degrees of cationic vacancy ordering (open squares) were plotted and found to conform to a strict linear relationship (Figure 6). In this plot, the magnetic effect of different γ-Fe2O3 size was assimilated in the more general and governing effect of cationic vacancy ordering. The set of SiO2/γ-Fe2O3 nanocomposites prepared in the open flame (solid circles) fall within the same variation of Hc and He as the bare γ-Fe2O3 nanoparticles. An anomaly, however, was observed for the large core size samples (i.e., SiO2/γ-Fe2O3 (Q)) (open circles), where relatively high coercivity was observed given the value of the exchange bias, suggesting that an additional source of effective anisotropy was present in these samples. This effect was substantiated through Mo¨ssbauer experiments (section 3.3). In principle, the Ms of ferri- or ferromagnetic materials such as γ-Fe2O3 (below its blocking temperature) should follow a magnon-type thermal dependence as defined by the Bloch T3/2 law:

Ms(T) ) (1 - BT3/2) Ms(0)

(1)

where Ms(T) is the temperature-dependent saturation magnetization, Ms(0) is the saturation magnetization at 0 K, and B is the Bloch constant. While the Ms(T) of bare γ-Fe2O3 and those with low SiO2 loadings (300 250 165 165 145 155 115 80 >350 >360

Based on remnant magnetization at 300 K.

Figure 4. Correlation of composite saturation magnetization (Ms per gram of sample at 5 K) and specific saturation magnetization (Ms per gram of γ-Fe2O3 at 5 K) as function of SiO2 loadings. Open symbols are those prepared in enclosed flame. Unless otherwise stated, all samples (solid symbols) are prepared without flame enclosure.

Figure 5. Effect of coercivity and exchange bias of SiO2/γ-Fe2O3 at 5 K, as a function of SiO2 loadings. Open symbols are those prepared in enclosed flame. Unless otherwise stated, all samples (solid symbols) are prepared without flame enclosure.

in these nanostructures result in a temperature-dependent chemical potential that permit Bose-Einstein condensation of magnons,41 freezing of canted spins at the nanoparticle surface,42 and finite size quantization of the spin wave spectrum,43 the latter of which has been disputed.44 In the present study, the significant deviation from the Bloch law behavior for nanocomposites with SiO2 loading >13% can

Figure 6. Correlation of exchange bias and coercivity of SiO2/γ-Fe2O3 at 5 K (circle symbols), benchmarked against bare γ-Fe2O3 of different sizes possessing different cationic vacancy ordering.30 (Q) denotes composites prepared in enclosed flame.

Figure 7. Temperature dependence of Ms of SiO2/γ-Fe2O3 composites as function of SiO2 loadings. The lines are fits to the sum of Bloch T3/2 law and an additional exponential component representing the contribution of freezing of disordered spins.

be described by a modified Bloch law incorporating an exponential component.

Ms(T) T ) (1 - BT 3/2) + A exp Ms(0) C

( )

(2)

Morphology and Magnetic Properties in SiO2/γ-Fe2O3

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Figure 8. Mo¨ssbauer spectra of composite (a) 79% SiO2/γ-Fe2O3, (b) 79% SiO2/γ-Fe2O3 (Q), and bare γ-Fe2O3 of (c) 6 nm and (d) 8 nm. The measurements were collected at (i) room temperature, (ii) 78 K, and (iii) 5 K.

where A is a scaling parameter related to the magnitude of the upturn in Ms at low temperature and C is a parameter that is related to the temperature at which the upturn manifests. This equation is similar to that used to describe the behavior in terms of freezing of canted surface spins,42,45 but as will be shown, the experimental results reported here do not support this theory. Obviously, eq 2 converges to the original Bloch law when A ) 0. The lines in Figure 7 are the fits of modified Bloch law (eq 2) to the data, with the parameters of the fit given in Table 2. For low SiO2 contents below 13%, the temperature-dependent specific Ms can be fitted without the exponential component (i.e., A ) 0) and are well described by the Bloch law. Above 13% SiO2, the exponential component increases with increasing SiO2 content (Table 2). This is true despite the similar magnetizations and crystallite sizes of the γ-Fe2O3 cores (dXRD ) 6-9 nm, Table 1) of the samples with more than 13% SiO2. Preparing samples within the quartz tube lowered the magnitude of the exponential component compared to equivalent compositions prepared without the quartz tube. Additional measurement for similarly prepared but smaller bare γ-Fe2O3 (dXRD ) 6 nm) also found strict conformation to the original Bloch law (not shown), implying that the Bloch law deviation seen in the nanocomposites is not merely a function of finite size effects. Nor does it appear to be related to freezing of canted spins, in terms of either surface spins or spins within the particle, as samples with

similar magnetizations (which is itself a measure of the number of canted spins), for example, 13 and 43% SiO2 show vastly different temperature-dependent magnetizations. The clear correlation of the magnitude of the exponential component with the degree of dilution of the cores may be indicative of the mechanism that is driving this behavior and is the subject of ongoing investigations. 3.3. Mo¨ssbauer Spectroscopy. The structural-magnetic specificity of SiO2/γ-Fe2O3 was further investigated by recording the 57 Fe Mo¨ssbauer spectra of a selection of samples at ambient and cryogenic (liquid N2 and He) temperatures. In particular, the magnetic anomalies seen in samples with 79% SiO2 and 79% SiO2(Q), despite their similar chemical composition, deserve further elucidation. As evident from Figure 8a(i), the room temperature Mo¨ssbauer spectrum for 79% SiO2 consisted predominantly of a quadrupole split doublet, typical of superparamagnetic γ-Fe2O3 cores. Similarly, bare γ-Fe2O3 of the same crystallite size as that of the γ-Fe2O3 cores (dXRD ) 6 nm, Figure 8c(i)) and that having the same specific Ms (dXRD ) 8 nm, Figure 8d(i))31 also showed a predominant doublet feature at room temperature. The isomer shifts and quadrupole splittings (IS, QS) of these three doublets were (0.33(1) mm/s, 0.80(1) mm/s), (0.34(1) mm/s, 0.75(1) mm/s), and (0.34(1) mm/s, 0.76(1) mm/s), respectively, in agreement with the γ-Fe2O3 literature values reported for

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superparamagnetic samples for the IS (0.34-0.39 mm/s)14 and QS (0.66-0.81 mm/s).19,46 In our three samples, however, there exist weak but apparently reasonably static sextet subspectra, corresponding to the minor presence of room temperature ferromagnetism as seen in the magnetic measurements (Table 2). The mean sextet fields for these samples were 44.7(9) T for 79% SiO2/γ-Fe2O3, 49.9(6) T for 6-nm bare γ-Fe2O3, and 49.9(9) T for 8-nm γ-Fe2O3. While the latter two were within uncertainty of the value of 50.0 T for bulk γ-Fe2O3,47 the lower value for the composite suggests the existence of some form of structural modification in the γ-Fe2O3 cores induced by the presence of SiO2. Similarly, sample 79% SiO2/γ-Fe2O3(Q) exhibited a mean hyperfine field of 43.5 T (maximum ≈ 46.9 T). Unlike the corresponding sample prepared in the open flame, however, its Mo¨ssbauer spectrum is dominated by the sextet or ferromagnetic feature (Figure 8b(i)) as a result of the larger core size and possible increase in anisotropy. Upon cooling to 78 K, the majority of the γ-Fe2O3 cores in 79% SiO2 had passed through the superparamagnetic blocking temperature, thus giving rise to a dominant sextet spectrum (Figure 8a(ii)). It is interesting to note that the Mo¨ssbauer spectra were very similar to that of 79% SiO2/γ-Fe2O3 (Q) at room temperature. This implies that the value of KV/kBT (where K is the effective anisotropy constant, V is the volume of the particles, kB is the Boltzmann constant, and T is the measurement temperature) must be approximately the same for both samples despite the difference in measuring temperature and suggests that the product of the effective anisotropy constant (K) and mean particle volume (V) in 79% SiO2 must be four times smaller than that of 79% SiO2 (Q). This enabled us to estimate that the effective increase in anisotropy in the latter was between a factor of 2.6 (using dXRD) and 1.2 (using dTEM). This is particularly important as it not only explains the reason for the increased Hc observed earlier for the sample synthesized in an enclosed flame (Figure 6), but also suggests an additional anisotropy mechanism other than vacancy disorder and consequent spin frustration model, since the latter mechanism would have also led to an increase in exchange bias, which was not seen in Figure 6. Except for the larger particle size, there was substantially little difference in terms of particle morphology between the enclosed and open flame samples for the given composition (Figure 1), hence ruling out shape anisotropy as the main factor. At 5 K, all of the spectra were fitted with three nested sextets (Figure 8iii) with no trace of SPM doublet or relaxation. The IS were in the range 0.42-0.48 mm/s, in good agreement with the expected thermal shift from the room temperature doublets. All the spectra had the characteristic γ-Fe2O3 structure, with line 6 being shorter and broader than line 1, and this resulted in the largest field always having the most positive IS and vice versa. We note that this is consistent with the expectation that the tetrahedral site would have both the smaller IS and hyperfine field. While the maximum fitted hyperfine fields were similar for 79% SiO2/γ-Fe2O3 (51.8 T) and bare γ-Fe2O3 of 6 nm (51.6 T) and 8 nm (51.7 T), the value for 79% SiO2/γ-Fe2O3 (Q) was significantly higher (52.9 T). Although within uncertainty of the octahedral field for bulk γ-Fe2O3, the higher hyperfine field emphasizes the effect of higher crystallinity in the latter. More importantly, the low-temperature Mo¨ssbauer spectra collected for both 79% SiO2/γ-Fe2O3 and 79% SiO2/γ-Fe2O3 (Q) samples showed purely the characteristics of γ-Fe2O3 domains, with no indication of any component corresponding to isolated Fe ions in a silicate structure or other iron oxide phases, such as hematite

Li et al. (R-Fe2O3). The result corroborates very nicely with our XRD and FTIR results and earlier discussions. It is interesting to compare the results of SQUID measurements and the Mo¨ssbauer measurements at 5 K. A Mo¨ssbauer spectrum is basically a microscopic magnetization measurement with the magnitude of the hyperfine field being proportional to 〈Jz〉 on each atom, where the average is taken over the nuclear Larmor precession time. In contrast, bulk magnetization measurements, as measured by a SQUID magnetometer, are the vector sum of all the Jz values taken along the measurement direction. At 5 K, the Mo¨ssbauer hyperfine fields were, at most, 3-5% lower than the value expected for bulk maghemite. However, the magnetization measurements were 55% lower for 79% SiO2 and 41% lower for 79% SiO2 (Q) than the bulk value. This implies a high degree of spin canting in which the direction and local z-component of the spins are fixed, but in which the spins are not collinear even in the maximum SQUID field of 7 T. Note also that the difference in real time spin fluctuations that appear static on the Mo¨ssbauer time scale (∼10-8 to 10-9 s) but fluctuating on the magnetometer time scale (∼10 to 100 s) is ruled out here since the 7 T field in the SQUID magnetometer is sufficient to completely suppress any thermally induced magnetic relaxation. 4. Conclusions This work presents a systematic investigation on the morphological evolution of SiO2/γ-Fe2O3 nanocomposites and their corresponding magnetic properties. By controlling the SiO2 content during flame spray pyrolysis, a gradual transformation from discrete γ-Fe2O3 particles (1% SiO2) to thin segregated SiO2 coating (4-13% SiO2), then segregated γ-Fe2O3 core within every SiO2 matrix (43-60% SiO2) and finally multiple γ-Fe2O3 cores (79% SiO2) could be obtained. Despite the onestep synthesis, no silicates or other impurities were formed. The extent of cationic vacancy ordering decreased as SiO2 content was increased up to 13% SiO2. This has a pronounced effect on the magnetic properties of the γ-Fe2O3 cores as clearly reflected in the drastic drop in specific Ms and increase in coercivity and exchange bias. Comparison of the small reduction in the Mo¨ssbauer saturation hyperfine field from the value for bulk γ-Fe2O3 with the very large reduction in Ms suggests a model involving a noncollinear arrangement of spins with a very high anisotropy. For SiO2 contents greater than 13%, the values for specific Ms, Hc, and He are relatively constant. Improvement in cationic vacancy ordering (for nanocomposites of the same SiO2 loading) and hence specific Ms was possible through synthesis in an “enclosed” flame. Deviation from the Bloch T3/2 law was observed at low temperatures for composites >13% SiO2. A modified Bloch law containing an exponential component was used to fit the data. The magnitude of the exponential component increased with increasing SiO2 content. Comparisons of samples with different exponential components suggest that the behavior is not associated with finite size effects nor with the freezing of canted spins as has been proposed by other authors.41-44 The work identified for the first time an intimate and yet extensive relationship between physicochemical characteristics (morphology, size, cationic vacancy ordering) and the magnetic properties of SiO2/γ-Fe2O3 nanocomposites. The results provide fundamental insights to such composite materials that increasingly find important applications in the field of bionanotechnology. Supporting Information Available: Hysteresis loops of SiO2/γ-Fe2O3 nanocomposites at 5 and 300 K. ZFC and FC

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