J. Phys. Chem. C 2010, 114, 1789–1795
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Magnetic Behavior of ZnFe2O4 Nanoparticles: Effects of a Solid Matrix and the Particle Size V. Blanco-Gutie´rrez, Marı´a J. Torralvo-Ferna´ndez, and R. Sa´ez-Puche* Departamento de Quı´mica Inorga´nica, Facultad de Ciencias Quı´micas, UniVersidad Complutense, Ciudad UniVersitaria, 28040 Madrid, Spain ReceiVed: August 31, 2009; ReVised Manuscript ReceiVed: NoVember 13, 2009
ZnFe2O4 nanoparticles with sizes between 3 and 20 nm have been prepared nonembedded and embedded in an amorphous SiO2 matrix. All the samples are ferrimagnetic below the blocking temperature that presents similar values for all of them, indicated by the maxima in the ZFC/FC curves. However, the embedded nanoparticles with sizes between 3 and 7 nm present much higher values of coercive field than the nonembedded particles. The fact that particles with different sizes present similar blocking temperature values suggests that they have similar anisotropy energy, and this has been justified using experimental anisotropy constants determined for different sized nanoparticles. Although the matrix can increase the surface anisotropy of the particles, it would not greatly affect the core anisotropy. Taking this into account, it can be justified that embedded particles present a higher coercive field but similar blocking temperature than nonembedded particles. From the variation of the coercive field as a function of the particle size, the limit between the single-domain and multidomain regions seems to be in the range of 15-18 nm. The range of temperature in which the samples behave as superparamagnetic has been estimated from the plots of the inverse susceptibility versus temperature. The particle size and the inversion parameter control the magnetization value that seems to determine the transition temperature from superparamagnetic to paramagnetic behavior. Introduction Ferrite compounds have been investigated for decades and present a growing interest due to their technological applications, such as magnetic and optical materials,1–3 semiconductors,4 pigments,5 catalysts,6,7 or materials for biomedical applications.8 Nowadays, spinel ferrite nanoparticles have attracted a lot of attention as they present different properties than their bulk counterparts.9,10 In this sense, Zn ferrite is found to be one of the most interesting spinel systems as its magnetic behavior depends on the particle size.11 Thus, the bulk ZnFe2O4 behaves as antiferromagnetic with a Ne´el temperature TN ) 10.5 K as a consequence of its normal spinel structure (Zn2+)Td[Fe3+Fe3+]OhO2-4 in which all the Zn2+ cations are located in tetrahedral sites (A) and Fe3+ cations are in octahedral sites (B) with antiparallel moments.12,13 In contrast, Zn ferrite, in the nanometric range, presents ferrimagnetic behavior below a blocking temperature (TB) as it has a mixed spinel structure (Zn2+1-xFe3+x)Td[Fe3+2-xZn2+x]OhO2-4 in which both Zn2+ and Fe3+ cations are distributed along the A and B sites with “x” being the inversion parameter.14 Whereas in the normal spinel Zn ferrite, the antiferromagnetic superexchange interactions occur between Fe3+ located in B sites, in mixed spinel ZnFe2O4, the interactions occur between Fe3+ located in A and B sites being stronger due to the more effective overlap of the orbitals involved in the superexchange Fe-O-Fe pathway. This is the reason why the TB is higher than the TN.15 Above the TB, ZnFe2O4 nanoparticles behave as superparamagnetic9 with an absence of a hysteresis loop in the M(H) curves. It has been found that it is possible to tune up the magnetic behavior of the ZnFe2O4 compound by modifying its particle size and synthesis conditions.16–21 In single-domain nanopar* To whom correspondence should be addressed. E-mail: rsp92@ quim.ucm.es.
ticles, when the particle size increases, the magnetization and coercive field increase but the inversion degree decreases.22–26 The TB is controlled by the anisotropy energy that is the result of contributions from both the anisotropy constant and the particle size. The anisotropy constant is influenced by the inversion parameter that, as mentioned before, increases when the particle size decreases but also is affected by the synthesis conditions.17,27 Therefore, nanoparticles with similar sizes can present different magnetic behavior because many factors must be considered in order to control the properties. A remarkable fact is the trend of small particles to agglomerate in order to reduce the energy associated with their high surface area. For many applications, it is thus crucial to develop strategies to avoid or minimize the nanoparticles’ agglomeration. These strategies comprise coating with organic species19,21 or with an inorganic layer28–30 or dispersing or embedding the nanoparticles into a matrix, such as a polymer31,32 or silica33–35 to form a composite. Besides a prevention of nanoparticles agglomeration, the network matrix affects the magnetic behavior of the hosted nanoparticles. Thus, magnetic particles in the nanometric range firmly embedded in the matrix are prevented from any local movement when a magnetic field is applied, while those nanoparticles hosted in a structurally weak environment are free to rotate and align with the applied field.32 In this work, ZnFe2O4 nanoparticles with sizes from 3 to 20 nm have been obtained by the solvothermal method using different conditions, and their magnetic properties have been analyzed. Nanoparticles in the same range of sizes have also been prepared embedded in amorphous silica, and the effect of the matrix in the magnetic behavior and magnetic hardness of the composite has been investigated.
10.1021/jp908395v 2010 American Chemical Society Published on Web 01/07/2010
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TABLE 1: Treatment Conditions and Magnetic Parameters of the ZnFe2O4/SiO2 Nanocomposite with Different Zn Ferrite Particle Sizes sample
Ta (°C)
ta (h)
Da (nm)
HC,5a (Oe)
TB (K)
ZnM3 ZnM6 ZnM7 ZnM11 ZnM14 ZnM20
600 700 800 900 950 1000
12 12 12 12 15 48
3.0 ( 1.0 6.0 ( 1.5 7.0 ( 1.5 11.0 ( 3.0 14.0 ( 3.0 20.0 ( 5.0
1300 560 260 310 345 311
23 18 16 21 19 21
a T, treatment temperature; t, treatment time; D, particle size (major dimension); HC,5, coercive field at 5 K.
Experimental Section Preparation of the Samples. ZnFe2O4 nanoparticles with sizes from 3 to 20 nm have been prepared nonembedded and embedded in an amorphous silica matrix. All chemicals were from Aldrich Chemical Inc. and used without further treatment. Samples of embedded nanoparticles with a weight ratio of ZnFe2O4/SiO2 of 30/70 were obtained by dissolving stoichiometric amounts of zinc nitrate and iron nitrate in ethanol and adding later distilled water and tetraethylorthosilicate (TEOS) in a molar ratio of TEOS/EtOH/H2O of 1:4:11.67.33 After a gelling period of 4 days, the precursor of SiO2 (TEOS) polymerizes to give a solid silica network in which the metal nitrates are distributed. Different portions of the gel were subjected to thermal treatments in air atmosphere ranging from 500 to 1000 °C with a heating rate of 10 °C/min, during different times (Table 1). After these thermal treatments, ZnFe2O4 nanoparticles with different sizes embedded in the silica matrix were obtained. The nonembedded Zn ferrite nanoparticles were prepared by the solvothermal method. In this case, stoichiometric amounts of zinc and iron nitrates were dissolved in ethylene glycol or distilled water with different concentrations. After adding KOH (0.5 or 2 M) as precipitant agent until pH ) 11, the mixture was transferred into a Teflon stainless steel autoclave to be treated at 160 or 200 °C during different periods of time from 2 to 288 h. The synthesis conditions that were selected in order to obtain nanoparticles with different sizes are indicated in Table 2. Characterization Techniques. The crystalline phases were identified by X-ray diffraction using a Siemens D-5000 powder diffractometer (25 mW, 35 kV) with a Cu KR radiation. The morphology and size of the particles were characterized by transmission electron microscopy (TEM) using a JEOL-2000FX microscope working at 200 kV. Magnetic susceptibility and magnetization measurements were done in a Quantum Design XL-SQUID magnetometer in the temperature range of 4-300 K up to 5 T. Magnetic susceptibility was measured after cooling the sample at 4 K in zero-field cooling (ZFC). In the case of field-cooling measurements (FC), the sample was cooled in the presence of a 500 Oe field down to 4 K. Results and Discussion Figure 1 shows the X-ray diffraction patterns for ZnFe2O4/ SiO2 nanocomposites treated at different temperatures (Figure 1a) and nonembedded nanoparticles obtained in different conditions (Figure 1b). Pure spinel phase was obtained in all the cases as it can be observed from the maxima of the patterns, although the X-ray diffraction diagram corresponding to sample ZnM3 presents only a broad maximum of low intensity (Figure 1a). In the case of the embedded nanoparticles, it is appreciable
how the diffraction maxima become sharper as the treatment temperature increases, revealing the larger size of the embedded nanocrystals. The X-ray diffraction patterns corresponding to the nonembedded nanoparticles (Figure 1b) show broad reflection maxima when the synthesis temperature is 160 °C without dependence on the other experimental conditions. By contrast, the samples prepared at 200 °C during different periods of time present good crystallinity as the X-ray diffraction pattern for the Zn19 sample indicates. Representative TEM images corresponding to the nanocomposites and nonembedded nanoparticles are shown in Figures 2 and 3, respectively (image of samples ZnM6 and Zn16 are given in the Supporting Information). From TEM images, statistical analysis has been done for each sample by measuring between 150 and 250 particles. The particle size distribution for the smallest and the largest particles are shown as inset in Figures 2 and 3. The analysis for all the samples is given in the Supporting Information. The mean particle size (major dimension) for all the samples is collected in Tables 1 and 2. In the case of the nanocomposite (Figure 2), the good dispersibility of the nanoparticles in the matrix can be observed. The mean size in embedded nanoparticles is larger when the temperature of treatment increases. In the case of nonembedded particles, the size is larger when the concentration of the precursors, the temperature, and the reaction time increase and the concentration of KOH decreases. On the other hand, polyhedral morphology can be also observed for particles larger than 7 nm in embedded and nonembedded samples. The magnetic measurements that will be discussed below have been analyzed, taking into account the weight ratio of ZnFe2O4/ SiO2 of 30/70. Magnetic susceptibility versus temperature measurements for both zero-field-cooled (ZFC) and field-cooled (FC) processes are depicted in Figures 4 and 5. The TB values estimated from the maxima in the ZFC curves are collected in Tables 1 and 2. High values of susceptibility can be seen for embedded and nonembedded nanocrystals, which suggests that, in all the cases, the ferrite nanoparticles behave as superparamagnetic above the TB. At low temperature, the magnetic susceptibility increases when the particle size increases between 3 and 11 nm for the embedded nanoparticles. In the case of the nonembedded nanoparticles, the increasing of the magnetic susceptibility takes place when the particle size goes from 4 to 7 nm and decreases for larger particles (magnetic susceptibility data for Zn16, not-shown, are almost coincident with the data corresponding to the Zn19 sample). In single-domain nanoparticles, the magnetic susceptibility and, therefore, the magnetization are affected by the amount of the coupled moments carriers that increases when the particle size increases. This can be understood taking into account that, as the spins surface are canted, the proportion of coupled moment carriers decreases when the particle size becomes smaller due to the increment of the surface/volume ratio.9,22,23 Moreover, the magnetic susceptibility also depends on the inversion parameter, which depends on the synthesis conditions27 and increases when the particle size decreases. Therefore, for the smallest particle sizes, the predominant factor probably is the amount of coupled moment carriers increasing the susceptibility when the particle size increases as the surface/volume ratio decreases (compare samples Zn4 and Zn7 in Figure 5 or samples ZnM3, ZnM6, ZnM7, and ZnM11 in Figure 4). When the surface/volume ratio is low enough, as in the case of the largest nanoparticles, the effect of the inversion parameter predominates and the magnetic susceptibility is lower than that for the small nanoparticles. In this sense, the inversion parameter for sample Zn19 has been
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TABLE 2: Synthesis Conditions and Magnetic Parameters of ZnFe2O4 with Different Particle Sizes Obtained by the Solvothermal Method sample Zn4 Zn7 Zn11 Zn16 Zn19 a
[nitrates] (mol/mL)
[KOH] (M)
solventa
Ta (°C)
ta (h)
D (nm)
HC,5 (Oe)
TB (K)
2 2 0.5 0.5 0.5
EG W EG EG EG
160 160 160 200 200
2 15 24 168 288
4.0 ( 1.0 7.0 ( 2.0 11.0 ( 3.0 16.0 ( 4.0 19.0 ( 4.0
106 160 220 429 385
18 20 22 20 21
-4
10 10-4 10-5 10-5 10-5
EG, ethylene glycol; W, distilled water; T, synthesis temperature; t, synthesis time.
Figure 1. X-ray diffraction patterns for (a) embedded and (b) nonembedded ZnFe2O4 nanoparticles.
calculated from neutron diffraction experiments,36 giving a result of x ) 0.126. This small value of x can justify the low susceptibility for the nanoparticles with a size around 20 nm. On the other hand, as it can be seen in Figure 4, the embedded nanoparticles present lower magnetic susceptibility than their counterparts nonembedded nanocrystals (Figure 5), this difference being higher for the smallest particles. When the particle size reaches a relatively high value, around 20 nm (samples ZnM20 and Zn19), the magnetic susceptibilities are almost coincident. All the samples show a higher TB than TN ) 10.5 K, which confirms that they present a nonzero inversion parameter. As it can be seen, the TB has similar values for embedded and nonembedded nanoparticles with different sizes, which suggests that all the samples present similar anisotropy energy (Ea). Considering the studied nanoparticles as uniaxial systems, the Ea can be described by the following formula:
Ea ) KV sin2 θ (K, anisotropy constant; V, particle volume; θ, angle between the easy-magnetization axis and the magnetization moment) For temperatures lower than TB, the thermal energy is lower than the Ea and, consequently, the individual particle moments are blocked. TB is controlled by Ea, which increases when Ea increases. Therefore, higher values of K or V would imply larger values of TB. K values between 7 ×105 and 4 ×103 erg/cm3 have been estimated from Mo¨ssbauer spectroscopy for particle sizes between 3 and 20 nm.37 The K value is affected by several anisotropy factors, but taking into account that the particles present similar morphology and that the intrinsic anisotropy due
Figure 2. TEM images of samples (a) ZnM3, (b) ZnM7, (c) ZnM11, (d) ZnM14, and (e) ZnM20. Particle size distributions are shown as insets for ZnM3 and ZnM20 samples.
to Fe3+ is low, probably the K variation mainly depends on the surface anisotropy that increases when the particle size decreases and the crystalline anisotropy that increases when the inversion parameter increases. The experimental K values lead to very similar Ea values for the samples in this range of particle sizes, justifying the similar values obtained for TB. In a previous X-ray diffraction and Mo¨ssbauer spectroscopy study, it has been reported that the TB can be more influenced by the synthesis method than by the particle size.27 In this sense, if the milling technique is used in the preparation of ZnFe2O4 nanoparticles, the high energy supplied to the nanocrystals’ surface makes them have a higher inversion parameter owing to the more random character of the cationic distribution, giving, as a result, a high value of TB.25 Hysteresis loops M(H) at 5 K for representative samples of particle sizes between 3 and 20 nm of embedded and nonembedded nanoparticles are shown in Figure 6. These hysteresis
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Figure 5. ZFC and FC magnetic susceptibility curves for nonembedded ZnFe2O4 nanoparticles.
Figure 3. TEM images of (a) Zn4, (b) Zn7, (c) Zn11, and (d) Zn19 samples. Insets show particle size distributions for Zn4 and Zn19 samples.
Figure 6. (a) Hysteresis loops at 5 K for embedded and nonembedded nanoparticles. (b) Enlarged views of the hysteresis loops.
Figure 4. ZFC and FC magnetic susceptibility curves for embedded ZnFe2O4 nanoparticles.
loops reveal important differences in the magnetic behavior between both embedded and nonembedded nanoparticles. As in the case of the magnetic susceptibility, ZnFe2O4/SiO2 nanocomposites present lower values of magnetization than the nanoparticles with similar sizes but without matrix, and the difference between both magnetization values becomes smaller
when the size of the nanoparticle increases. However, one of the most remarkable facts is the higher value of coercive field (HC) that the ZnFe2O4/SiO2 nanocomposites present for small particle size with respect to the nonembedded nanoparticles (Tables 1 and 2). In this sense, it can be seen in Figure 6b that embedded nanoparticles of about 3 nm present a coercive field value of 1300 Oe, whereas nonembedded nanoparticles of a similar size present a 10 times lower value (106 Oe) at the same temperature. The difference in the coercive field for embedded and nonembedded nanoparticles would be attributed to the following factors: (i) interparticle interaction, (ii) inversion degree, and (iii) surface anisotropy energy. (i) The matrix minimizes the particle agglomeration and the dipole-dipole interaction, which can affect the coercive field value. However, our magnetic data indicate a high coercive field (1200 Oe) for a ZnFe2O4/SiO2 nanocomposite with a weight ratio of 50/50 in which the particles are agglomerated despite the matrix. (TEM
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Figure 7. Coercive field vs particle size for embedded and nonembedded ZnFe2O4 nanoparticles.
image and the hysteresis loop at 5 K for this sample are reported in the Supporting Information). In this sense, it has been reported for Mn ferrite and Co ferrite nanoparticles covered with SiO238 that the increasing in the interparticle distances caused by the SiO2 covering is not large enough to greatly modify the coercive field value with respect to the noncovered particles. (ii) On the other hand, taking into account that the inversion degree can be affected by the synthesis method, embedded and nonembedded nanoparticles with similar sizes can present different inversion parameters. In this sense, X-ray diffraction data could be a useful tool to estimate in a qualitative way the inversion degree. When the X-ray diffraction patterns for particles of similar sizes obtained by solvothermal, sol-gel, and microwave methods were compared, we have found a similar intensity ratio between the (220) and (400) reflections for the particles obtained by solvothermal and sol-gel methods and a higher ratio for those obtained by microwave. This seems to indicate that the embedded and nonembedded particles studied in this work present a similar value of inversion degree. (iii) The surface anisotropy is supposed to be higher for embedded nanoparticles due to the strain imposed by the rigid silica matrix.32 As it was reported,39 the coercive field seems to be governed by the surface anisotropy energy, which justifies the higher HC values in the case of the embedded particles. The effect that the matrix plays in the coercive field will increase when the particle size of the Zn ferrite nanocrystals decreases due to the increment of the surface/volume ratio of these nanoscale particles. However, as the core anisotropy that affects the TB seems not to be greatly affected by the presence of a matrix, the blocking temperature remains almost the same for embedded and nonembedded nanoparticles. The matrix effect can be also observed in the susceptibility. As the surface anisotropy is higher for embedded nanoparticles, the proportion of coupled moments carriers would be higher for nonembedded particles, giving higher magnetization and susceptibility (Figures 4 and 5). The variation of the matrix effect with the particle size is shown in Figure 7. Coercive field values of both embedded and nonembedded nanoparticles with different particle sizes are depicted. High HC values for the embedded ZnFe2O4 nanoparticles with smaller sizes can be observed. After this curve reaches a minimum, both curves keep on increasing until they remain almost together, reflecting a similar magnetic behavior, which suggests that the matrix effect is not important anymore. A maximum can be seen in the graph in the range of 15-18 nm drawn by both curves that probably corresponds to the critical particle size that marks the frontier between the singledomain region, at lower values of particle size, and the
Figure 8. Hysteresis loops at 250 K for (a) embedded and (b) nonembedded nanoparticles.
multidomain region at higher values. In similar studies concerning NiFe2O4 nanoparticles,23 critical particle sizes of 12 and 15 nm were estimated for 80 and 300 K, respectively. The curve that corresponds to the nonembedded particles in Figure 7 shows, in the single-domain region, an augmentation of the coercive field with the increasing of the particle size. This is due to the fact that the magnetic moment becomes larger when the particle size is increased owing to the major concentration of magnetic carriers, and therefore, a higher magnetic field is needed to reverse the magnetic moment. The multidomain region is reached if the particle size continues increasing until a critical value for which the single domain splits up into smaller domains in order to minimize the energy of the system; then, as it is observed, the coercive field decreases with the increasing of the particle size.22 Magnetization (M) versus magnetic field (H) curves were measured at 250 K for embedded and nonembedded nanoparticles with different sizes. In Figure 8, M(H) curves for representative samples with particle sizes between 3 and 20 nm have been depicted. The absence of a coercive field in all the samples reveals that, at this temperature, the reversal of the magnetization is not prevented. Moreover, the shape of the M(H) curves seems to indicate that, in some cases, the particles are paramagnetic. For example, samples ZnM3, ZnM6, ZnM7 (not shown), Zn4, Zn11, Zn16 (not shown), and Zn19 present low magnetization even at the highest applied magnetic field. The difference in magnetization compared with the rest of the samples can be clearly seen in the insets of Figure 8a,b. To estimate the range of temperatures in which the samples behave as superparamagnetic, the 1/χ versus T curves were depicted (Figure 9). The reciprocal susceptibility can be fitted to a straight line corresponding to the Curie-Weiss law from about the TB to a certain temperature at which upward deviations are observed. The observed inflection points (marked with arrows) suggest a change from the low-temperature superparamagnetic to the high-temperature paramagnetic behavior. For samples
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Blanco-Gutie´rrez et al. On the other hand, samples with different particle sizes have similar values of TB as they may present a similar Ea. The increasing in the V values when the particle size increases from 3 to 20 nm seems to be compensated by the decrease in the K values. Above the TB, all the samples behave as superparamagnetic as it can be deduced by the high values of magnetic susceptibility in the ZFC/FC curves. At 250 K, the samples that present higher magnetization, due to larger particle size or probably higher inversion degree, have more superparamagnetic behavior. Acknowledgment. The authors are grateful to Dr. Pierre Bonville for fruitful discussions and help. Thanks are also due to Nubiola Inorganic Pigments and to the Spanish Ministerio de Ciencia e Innovacio´n for financial support under project MAT 2007-63497. Supporting Information Available: TEM and particle size distributions for ZnM6 and Zn16 samples; the particle size distributions for ZnM7, ZnM11, ZnM14, Zn7, and Zn11 samples; and a TEM image and the hysteresis loop at 5 K corresponding to a ZnFe2O4/SiO2 nanocomposite with a weight ratio of 50/50. This material is available free of charge via the Internet at http://pubs.acs.org.
Figure 9. Inverse of susceptibility vs temperature for (a) embedded and (b) nonembedded nanoparticles.
ZnM3, ZnM6, ZnM7 (not shown), Zn4, Zn11, Zn16 (not shown), and Zn19, the inflection points correspond to temperatures lower than 200 K. Therefore, these samples should have more paramagnetic behavior at 250 K than ZnM11, ZnM14 (not shown), ZnM20, and Zn7 samples. The change from superparamagnetic to paramagnetic behavior occurs at higher temperature when the value of magnetization increases. Therefore, at 250 K, the samples with larger particle sizes (ZnM11, ZnM14, and ZnM20) present less paramagnetic behavior than the samples with smaller size. However, in the case of the nonembedded nanoparticles, the Zn7 sample shows a more superparamagnetic behavior than the particles with smaller size but also than the largest nanoparticles, probably owing to a higher inversion degree. Conclusions ZnFe2O4 nanoparticles have been prepared as nonembedded and embedded in an amorphous silica matrix. For temperatures lower than the TB, the nanoparticles present a ferrimagnetic behavior that indicates a nonzero inversion degree due to the mixed distribution of Zn2+ and Fe3+ cations along the A and B sites in the spinel structure. At 5 K, the nanoparticles with sizes lower than 15-18 nm are single-domain, as the maximum in the coercive field versus temperature plot suggests. This maximum probably corresponds to the boundary between the single-domain and multidomain regions. For particles with a size lower than 7 nm, the strain imposed by the matrix seems to increase the surface anisotropy energy of the embedded nanoparticles, causing higher values of coercive field than those corresponding to the nonembedded nanoparticles. Moreover, as the core anisotropy remains unaffected, the TB for both embedded and nonembedded particles presents similar values. The matrix effect appears not to be important for particle sizes larger than 7 nm since the coercive field becomes similar for both embedded and nonembedded particles.
References and Notes (1) Yao, C.; Zeng, Q.; Goya, G. F.; Torres, T.; Liu, J.; Wu, H.; Ge, M.; Zeng, Y.; Wang, Y.; Jiang, J. Z. J. Phys. Chem. C 2007, 111, 12274. (2) Mathew, D. S.; Juang, R. S. Chem. Eng. J. 2007, 129, 51. (3) Sultan, M.; Singh, R. J. Phys. D: Appl. Phys. 2009, 42, 115306. (4) Chen, Y. F.; Spoddig, D.; Ziese, M. J. Phys. D: Appl. Phys. 2008, 41, 205004. (5) Xavier, C. S.; Candeia, R. A.; Bernardi, M. I. B.; Lima, S. J.G.; Longo, E.; Paskocimas, C. A.; Soledade, L. E. B.; Souza, A. G.; Santos, I. M. G. J. Therm. Anal. Calorim. 2007, 87, 709. (6) Lee, H.; Jung, J. Ch.; Kim, H.; Chun, Y. M.; Kim, T. J.; Lee, S. J.; Oh, S. H.; Kim, Y. S.; Song, I. K. Catal. Lett. 2008, 122, 281. (7) Toledo-Antonio, J. A.; Nava, N.; Martinez, M.; Bokhimi, X. Appl. Catal., A 2002, 234, 137. (8) Jang, J. T.; Nah, H.; Lee, J. H.; Moon, S. H.; Kin, M. G.; Cheon, J. Angew. Chem., Int. Ed. 2009, 48, 1234. (9) Lu, A. H.; Salabas, E. L.; Schu¨th, F. Angew. Chem., Int. Ed. 2007, 46, 1222. (10) Li, F. S.; Wang, L.; Wang, J. B.; Zhou, Q. G.; Zhou, X. Z.; Kunkel, H. P.; Williams, G. J. Magn. Magn. Mater. 2004, 268, 332. (11) Roy, M. K.; Haldar, B.; Verma, H. C. Nanotechnology 2006, 17, 232. (12) Hastings, J. M.; Corliss, L. M. ReV. Mod. Phys. 1953, 25, 114. (13) Schiessl, W.; Potzel, W.; Karzel, H.; Steiner, M.; Kalvius, G. M.; Martin, A.; Krause, M. K.; Halevy, I.; Gal, J.; Scha¨fer, W.; Will, G.; Hillberg, M.; Wa¨ppling, R. Phys. ReV. B 1996, 53, 9143. (14) Morrirsh, A. H.; Li, Z. W.; Jiang, J. Z. Int. J. Mod. Phys. B 2001, 15, 3312. (15) Goya, G. F.; Rechenberg, H. R. J. Magn. Magn. Mater. 1999, 196197, 191. (16) Li, F.; Wang, H.; Wang, L.; Wang, J. J. Magn. Magn. Mater. 2007, 309, 295. (17) Atif, M.; Hasanain, S. K.; Nadeem, M. Solid State Commun. 2006, 138, 416. (18) Nalbandian, L.; Delimitis, A.; Zaspalis, V. T.; Deliyanni, E. A.; Bakoyannakis, D. N.; Peleka, E. N. Microporous Mesoporous Mater. 2008, 114, 465. (19) Limaye, M. V.; Singh, S. B.; Date, S. K.; Cothari, D.; Reddy, V. R.; Gupta, A.; Sathe, V.; Choudhary, R. J.; Kulkarni, S. K. J. Phys. Chem. B 2009, 113, 9070. (20) Valde´s-Solı´s, T.; Tartaj, P.; Marba´n, G.; Fuertes, A. B. Nanotechnology 2007, 18, 145603. (21) Lattuada, M.; Hartton, T. A. Langmuir 2007, 23, 2158. (22) Hadjipanayis, G. C. J. Magn. Magn. Mater. 1999, 200, 373. (23) George, M.; John, A. M.; Nair, S. S.; Joy, P. A.; Anantharaman, M. R. J. Magn. Magn. Mater. 2006, 302, 190. (24) Chinnasamy, C. N.; Narayanasamy, A.; Ponpandian, N.; Chattopadhyay, K.; Guerault, H.; Greneche, J. M. Scr. Mater. 2001, 44, 1407.
Magnetic Behavior of ZnFe2O4 Nanoparticles (25) Chinnasamy, C. N.; Narayanasamy, A.; Ponpandian, N.; Chattopadhyay, K.; Guerault, H.; Greneche, J. M. J. Phys: Condens. Matter 2000, 12, 7795. (26) Ehrhardt, H.; Campbell, S. J.; Hofmann, M. J. Alloys Compd. 2002, 339, 255. (27) Stewart, S. J.; Figueroa, S. J. A.; Sturla, M. B.; Scorcelli, R. B.; Garcı´a, F.; Requejo, F. G. Phys. B 2007, 389, 155. (28) Lu, A. H.; Li, W. C.; Matoussevitch, N.; Spliethoff, B.; Bo¨nnemann, H.; Schu¨th, F. Chem. Commun. 2005, 98. (29) Lin, J.; Zhou, W.; Kumbhar, A.; Wiemann, J.; Fang, J.; Carpenter, E. E.; O’Connor, C. J. J. Solid State Chem. 2001, 159, 26. (30) Yi, D. K.; Selvan, S. T.; Lee, S. S.; Papaefthymiou, G. C.; Kundaliya, D.; Ying, J. Y. J. Am. Chem. Soc. 2005, 127, 4990. (31) Lepkova´, K.; Clohessy, J.; Cunnane, V. J. J. Phys.: Condens. Matter 2007, 19, 375106.
J. Phys. Chem. C, Vol. 114, No. 4, 2010 1795 (32) Del Barco, E.; Asenjo, J.; Zhang, X. X.; Pieczynski, R.; Juli, A.; Tejada, J.; Ziolo, R. F.; Fiorani, D.; Testa, A. M. Chem. Mater. 2001, 13, 1487. (33) Garcia Cerda, L. A.; Montemayor, S. M. J. Magn. Magn. Mater. 2005, 294, e43. (34) Zhou, Z. H.; Xue, J. M.; Chan, H. S. O.; Wang, J. Mater. Chem. Phys. 2002, 75, 181. (35) Vejpravova`, J. P.; Sechovsky, V.; Niznansky, D.; Plocek, J.; Hutlova´, A.; Rehspringer, J. L. WDS 2005 Proceedings of Contributed Papers, Part III, 2005, p 518. (36) Unpublished results. (37) Blanco-Gutie´rrez, V.; Torralvo, M. J.; Sa´ez-Puche, R.; Bonville, P. J. Phys. Proceedings of ICM, 2009. (38) Vestal, C. R.; Zhang, Z. J. Nano Lett. 2003, 3, 1739. (39) Vestal, C. R.; Zhang, Z. J. J. Am. Chem. Soc. 2003, 125, 9828.
JP908395V
