Influence of Processing Methodology on Magnetic Behavior of

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Influence of Processing Methodology on Magnetic Behavior of Multicomponent Ferrite Nanocrystals Sanjeev Kumar,† Vaishali Singh,*,‡ Saroj Aggarwal,‡ Uttam Kumar Mandal,§ and Ravinder Kumar Kotnala| Department of Chemistry, UniVersity of Petroleum & Energy Studies, Dehradun-248007, Uttarakhand, India, UniVersity School of Basic and Applied Sciences and UniVersity School of Chemical Technology, GGS Indraprastha UniVersity, Kashmere Gate, Delhi - 110403, India, and National Physical Laboratory, New Delhi - 110012, India ReceiVed: December 7, 2009; ReVised Manuscript ReceiVed: February 24, 2010

A novel and facile reverse microemulsion route has been developed for the synthesis of multicomponent ferrite nanocrystals, namely, Co0.5Zn0.5Fe2O4 and Ni0.5Zn0.5Fe2O4. In addition to microemulsions, these ferrite nanocrystals are also synthesized via a general chemical coprecipitation route. The crystals possess cubic spinel structure and spherical morphology as revealed by Fourier transform infrared (FTIR), X-ray diffraction (XRD), and field emission transmission electron microscopy (FETEM) analysis. The average diameters of Co0.5Zn0.5Fe2O4 and Ni0.5Zn0.5Fe2O4 nanocrystals obtained from chemical coprecipitation method are about 14 and 10 nm, respectively, and for those obtained from reverse microemulsion route they are calculated to be 5 and 2 nm, respectively. Vibrating sample magnetometery (VSM) reveals that the ferrite nanocrystals obtained from the reverse microemulsion route exhibit superparamagnetism whereas the ferrite nanocrystals obtained from chemical coprecipitation show ferromagnetism. The decrease in Curie (TC) and blocking (TB) temperature with size is attributed to the structural changes. Introduction Spinel ferrites are widely used magnetic materials in low cost and high performance devices. Nanocrystals of spinel ferrite, a class of complex oxides, are under intense investigation to design with improved properties and for widespread applications.1,2 The interest aroused due to the broad range of magnetic behavior that can be engineered into such oxides. The properties of ferrites are highly sensitive to the cation distribution, which in turn is controlled by preparation conditions and substitution of different metals. It is well established that zinc substitution plays a decisive role in determining the ferrite properties. Zn2+ ions have a strong preference for tetrahedral (A) sites3 and affect the lattice parameter (a0).4 In the case of NixZn1-xFe2O4 ferrite, it is found that, for x greater than 0.5, the tetrahedral and octahedral (B) site Fe moments have collinear arrangements, whereas for x less than 0.5 a noncollinear arrangement of B-site moments exists.5 The Ni0.5Zn0.5Fe2O4 composition is therefore interesting for studying the magnetic properties. This motivated us to carry out a similar study on Co0.5Zn0.5Fe2O4 composition. The common methodologies reported for the synthesis of ferrites are sol-gel pyrolysis method,6,7 hydrothermal technique,8 pulse laser deposition,9 autocombustion method,10 mechanical alloying,11 and so on. However, strict reaction conditions and complicated instrumentation in the aforementioned methods are necessary. Also the controllability of the nanoscale * To whom correspondence should be addressed. Mailing address: USBAS, GGS Indraprastha University, Delhi - 110403, India. Telephone: +91-9810118128.Fax:91-11-23865941.E-mail:[email protected], [email protected]. † University of Petroleum & Energy Studies. ‡ University School of Basic and Applied Sciences, GGS Indraprastha University. § University School of Chemical Technology, GGS Indraprastha University. | National Physical Laboratory.

morphologies and properties of ferrite crystals by these methods is limited. For these reasons, it is of great importance to develop an inexpensive method to control the crystal morphology and grain size. A low temperature technique such as chemical coprecipitation is expected to control the morphology under mild reaction conditions, typically at a temperature less than 100 °C. However, it leads to the precipitation of nanocrystals with a relatively broad size distribution.12 In this study, a simple and efficient technique, that is, microemulsion route with excellent synthetic tunability, has been proposed to achieve the desired size and magnetic properties. In the microemulsion technique, the surfactant assemblies control both the shape and size of the crystals.13-17 In order to produce the ferrite nanocrystals, two microemulsions carrying the stoichiometric amount of reactants are mixed. The chemical reaction starts when there are fusion-fission events between the droplets, a prerequisite for the mixing of the reactants. These droplets offer a unique environment for the formation of nanoparticles as well as inhibit the excess aggregation of particles, due to adsorption of surfactant molecules on the particle surface when the particle size approaches to that of the nanodomain (droplet).18 Collisions between micelles are frequent, and approximately one collision in every thousand results in the formation of a short-lived dimer with an approximate lifetime of 100 ns, formed by the expulsion of some surfactant molecules into the bulk oil phase.19,20 During its lifetime, two reverse micelles will exchange the contents of their aqueous cores before decoalescing by the fusion-fission process, resulting in the eventual equilibrium distribution of all contents.21 This model of reverse micelle interaction shows the suitability of reverse micelles as nanoreactors. In principle, one could control the shape, size, and chemical composition by changing reaction conditions and the nature of the surfactant to tune the properties of magnetic nanostructures.4,14,15 Few workers have reported the fabrication of monodisperse multicomponent

10.1021/jp911586d  2010 American Chemical Society Published on Web 03/19/2010

Magnetic Behavior of Ferrite Nanocrystals ferrite nanocrystals via reverse microemulsion.22-25 The chemical coprecipitation route has generally been used to fabricate Co, Zn-ferrite and Ni, Zn-ferrite nanocrystals.26-31 Although some work has been published on a self-assembly method for the preparation of magnetic metal oxide nanocrystals,22,23,32 to the best of our knowledge, none of these works have been devoted to the synthesis of Co0.5Zn0.5Fe2O4 and Ni0.5Zn0.5Fe2O4 nanocrystals by the reverse microemulsion route. The aim of the present work is to prepare nanoscale morphologies of Co0.5Zn0.5Fe2O4 and Ni0.5Zn0.5Fe2O4 ferrite nanocrystals and study their structural and magnetic properties. We selected a quaternary reverse microemulsion system to synthesize these nanocrystals, and cetyltrimethylammonium bromide (CTAB) was used as surfactant. The quaternary reverse microemulsion selected here is a four component system with the phases water, oil, surfactant, and cosurfactant. The presence of a fourth component, that is, the cosurfactant, in this system carries great importance because it plays a vital role in controlling the droplet size and hence the size of the crystal. Experimental Section Materials. All chemicals in this work, such as (i) FeCl3 (sd fine chemicals, India), (ii) NiCl2 · 6H2O (sd fine chemicals, India), (iii) ZnCl2 (sd fine chemicals, India), (iv) CoCl2 · H2O (sd fine chemicals, India), (v) isoamyl alcohol (SRL, India), and (vi) cetyltrimethylammonium bromide (Spectrochem, India) were of analytical grade and used as received without further purification. (vii) Kerosene with a specific gravity of 0.76 was purchased from a local market and used after double distillation. Synthesis of Co0.5Zn0.5Fe2O4 and Ni0.5Zn0.5Fe2O4 Ferrite Nanocrystals by Reverse Microemulsion Process. For the reverse microemulsion route, a quaternary system of kerosene/ CTAB/isoamyl alcohol/H2O was selected. For Co0.5Zn0.5Fe2O4 ferrite, an aqueous solution was prepared by mixing stoichiometric amounts of 0.5 M FeCl3, 0.125 M CoCl2 · 6H2O, and 0.125 M ZnCl2. All chemicals used were analytical grade. Two reverse microemulsions ME1 and ME2 were prepared. CTAB of 15.78 wt % was added to 42 wt % kerosene, giving a murky emulsion. An aqueous solution (31 wt %) containing the precursor salts and 10.52 wt % isoamyl alcohol were then added to the emulsion under constant magnetic stirring at a speed of 1000 rpm. The murky emulsion became transparent. The stirring was continued for 1 h, resulting in a stable reverse microemulsion (ME1). The reverse microemulsion (ME2) was prepared with 0.5 M aqueous solution of NaOH as water phase under similar conditions. The reverse microemulsion ME2 was then heated to 80 °C, and to this was added reverse microemulsion ME1 dropwise under constant magnetic stirring. The appearance of a blackish brown color after a few minutes marks the completion of the reaction and formation of the desired ferrite colloidal solution. The reaction mixture was further stirred for 4 h on a magnetic stirrer. The pH of the reaction was maintained at 12. The nanocrystals present inside the colloid were then collected by centrifugation (10 733g, 20 min). To take the surfactant completely away, the powder was subjected to several cycles of washing with methanol and double distilled water followed by centrifugation (10 733g, 20 min) and finally dried in vacuum oven at 100 °C for 48 h. To prepare Ni0.5Zn0.5Fe2O4 ferrite nanocrystals, the same procedure was followed except that the water phase used to prepare ME1 was a mixture of aqueous solution of 0.125 M NiCl2 · 6H2O, 0.125 M ZnCl2, and 0.5 M FeCl3. Reverse

J. Phys. Chem. C, Vol. 114, No. 14, 2010 6273 microemulsion ME2 was prepared in a similar way as described for the case of Co0.5Zn0.5Fe2O4. Synthesis of Co0.5Zn0.5Fe2O4 and Ni0.5Zn0.5Fe2O4 Ferrite Nanocrystals by Chemical Coprecipitation Process. Nanocrystals of Co0.5Zn0.5Fe2O4 were prepared by chemical coprecipitation of Fe3+, Co2+, and Zn2+ in an alkaline medium (pH 12). The stock solutions of all the precursors were prepared with the same concentrations as those used in microemulsion method. Stoichiometric amounts of FeCl3, CoCl2 · 6H2O, and ZnCl2 were mixed. Then this mixture was poured into boiling 0.5 M NaOH solution under stirring. The resultant mixture was kept at 80 °C for 4 h. The precipitates obtained were washed several times with methanol and double distilled water followed by drying in a vacuum oven at 100 °C for 48 h. Ni0.5Zn0.5Fe2O4 ferrite nanocrystals were prepared by chemical coprecipitation of Ni2+, Zn2+, and Fe3+ in identical conditions. The rest of the procedure was same as that described above. Characterization. The microstructure of the crystals was characterized by X-ray diffraction (XRD) with a Philips PW 3040/60 X’Pert PRO (PANalytical) diffractometer (The Netherlands) using nickel filtered Cu KR radiation at 1.54 Å. The resultant intensity data were processed using in-built PC-APD diffraction software to monitor the peak position and its corresponding intensity data correctly. The samples were placed on a slide, and the measurements were taken continuously from 10° to 70° angles at 0.02° intervals. The average diameter (D) of the ferrite nanocrystals has been calculated from the broadening of the XRD peak intensity after KR2 corrections using the Debye-Scherrer equation,27

D ) kλ/β cos θ where k is the Scherrer factor (0.89), λ is the X-ray wavelength (1.54 Å), and β is the line broadening of a diffraction peak at angle θ. Fourier transform infrared (FTIR) spectroscopic measurements were taken on samples using a Shimadzu Japan FTIR8700 spectrophotometer. Samples of isolated particles of prepared nanostructures were mixed with KBr, homogenized, and converted into pellets under a pressure of 8 tons, and the spectra (transmittance % with wavenumber) were taken thereafter. The prepared nanoparticles were then characterized by the FTIR spectra. Field emission transmission electron microscopy (FETEM) measurements of the samples were taken on a JEOL JEM 2100 F instrument with a 200 kV accelerating voltage. The dispersions of nanocrystals in water were placed on carbon-coated 400 mesh copper grids and allowed to dry at room temperature before taking measurements. The obtained micrographs were then examined for particle shape and size. Magnetic characterization of the powder samples were recorded in a Lakeshore 7304 vibrating sample magnetometer (VSM) at room temperature using a maximum field of 5000 Oe, and parameters such as magnetization (M), coercive force (HC), and remanence magnetization (Mr) were evaluated for each sample. The zero field cooled (ZFC) magnetization versus temperature measurements were carried out by cooling the sample from 300 to 5 K in zero field then applying a field of 60 Oe and measuring the magnetization while the sample was heated from 5 to 300 K. The field cooled (FC) measurements were performed in the same manner with the difference being that the field was applied before cooling. All magnetic measurements were carried out with the field applied parallel to the substrate.

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Figure 1. XRD patterns of (a) Co0.5Zn0.5Fe2O4 and (c) Ni0.5Zn0.5Fe2O4 ferrite nanocrystals obtained from chemical coprecipitation and XRD patterns of (b) Co0.5Zn0.5Fe2O4 and (d) Ni0.5Zn0.5Fe2O4 ferrite nanocrystals obtained from reverse microemulsion. All samples were sintered at 600 °C for 4 h.

Results and Discussion Structural and Morphological Analysis. Figure 1 depicts the X-ray diffractographs of Co0.5Zn0.5Fe2O4 and Ni0.5Zn0.5Fe2O4 ferrite nanocrystals by reverse microemulsion and chemical

Kumar et al. coprecipitation routes, respectively. The peak position and relative intensity of all diffraction peaks observed for both samples matched well with standard powder diffraction data.4,22,33,34 The diffraction peaks at 2θ values of 29.9, 35.3, 43.0, 53.1, 56.8, and 62.4° could be ascribed to Bragg reflections of (220), (311), (400), (422), (511), and (440) planes which can be readily indexed to the spinel phase (Figure 1). No characteristic peaks of impurities were detected, confirming the formation of the cubic spinel structure of ferrite nanocrystals. The reflections are comparatively broader, revealing the nanosize of the crystals. According to the Debye-Scherrer equation,27 the average crystallite size was determined from the half-width of the most intense peak (311). The Co0.5Zn0.5Fe2O4 and Ni0.5Zn0.5Fe2O4 ferrite nanocrystals obtained from the chemical coprecipitation method possess an average crystallite size of about 14 and 10 nm, respectively, whereas for the Co0.5Zn0.5Fe2O4 and Ni0.5Zn0.5Fe2O4 ferrite nanocrystals obtained from the reverse microemulsion method the average diameter calculated was 5 and 2 nm, respectively, which is in close agreement with the TEM results. The difference in crystallite size was probably due to the different preparation conditions followed here which gave rise to different rates of ferrite formation, favoring the variation in crystallite size. The X-ray diffractograph of Ni0.5Zn0.5Fe2O4 ferrite nanocrystals obtained from the chemical coprecipitation method shows the highest intensity, indicating their highest crystallinity among all the synthesized nanocrystals (Figure 1c). Further the lattice constants of ferrite nanocrystals were computed using the “d” value and their respective (hkl) parameters. The lattice constants for Co0.5Zn0.5Fe2O4 and Ni0.5Zn0.5Fe2O4 ferrite nanocrystals prepared by chemical coprecipitation and reverse microemulsion were found to be 8.3827, 8.3561 Å and 8.3510, 8.3460 Å, respectively.

Figure 2. FETEM image of (a,b) Co0.5Zn0.5Fe2O4 ferrite nanocrystals obtained from reverse microemulsion and (c) their magnified lattice and (d) Co0.5Zn0.5Fe2O4 ferrite nanocrystals obtained from chemical coprecipitation.

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Figure 3. FETEM image of (a,b) Ni0.5Zn0.5Fe2O4 ferrite nanocrystals obtained from reverse microemulsion and (c) their magnified lattice and (d) Ni0.5Zn0.5Fe2O4 ferrite nanocrystals obtained from chemical coprecipitation.

TABLE 1: Size and Magnetic Parameters for As-Synthesized Co0.5Zn0.5Fe2O4 and Ni0.5Zn0.5Fe2O4 Ferrite Nanoparticles at 300 K methods reverse microemulsion

chemical coprecipitation

parameters

Co0.5Zn0.5Fe2O4

Ni0.5Zn0.5Fe2O4

Co0.5Zn0.5Fe2O4

Ni0.5Zn0.5Fe2O4

particle size DTEM [nm] particle size DXRD [nm] lattice constant [Å] magnetization M [emu/g] coercivity Hc [Oe] blocking temperature TB [K] Curie temperature TC [°C]

5-6 5 8.3561 11.14

1-2 2 8.3460 1.14

165 195

130 210

16-18 14 8.3827 30.550 5.744 230 275

10-b12 10 8.3510 25.62 6.65 180 300

Figures 2 and 3 depict TEM micrographs of Co0.5Zn0.5Fe2O4 and Ni0.5Zn0.5Fe2O4 ferrite nanocrystals obtained from both methods. Reverse microemulsion resulted into the narrower and monodisperse particle size distribution with reduced average diameter in the ranges 5-6 nm and 1-2 nm, respectively, for Co0.5Zn0.5Fe2O4 and Ni0.5Zn0.5Fe2O4 ferrite nanocrystals, as shown in Figures 2a-c and 3a-c. The single crystal nature of the Ni0.5Zn0.5Fe2O4 ferrite is revealed by HRTEM analysis (Figures 2b and 3b), and the good lattice fringes (Figures 2b,c and 3b,c) further illustrate that these nanocrystals are single crystalline. The lattice fringes in Figures 2c and 3c show interplanar spacing of about 2.54 Å that corresponds to the (311) crystal planes of the spinel phase. Whereas the spherical Co0.5Zn0.5Fe2O4 and Ni0.5Zn0.5Fe2O4 ferrite nanocrystals of about 16-18 nm and 10-12 nm in size, respectively, were obtained by the chemical coprecipitation method (Figures 2d and 3d). These results corroborate XRD analysis results. The average

crystallite size DXRD, DTEM and the lattice constant ao of Co0.5Zn0.5Fe2O4 and Ni0.5Zn0.5Fe2O4 nanocrystals are summarized in Table 1. Chemical precipitation method although is the most widely used process for the synthesis of magnetic nanocrystals but leads to the precipitation of nanocrystals with a relatively broad size distribution, whereas nanocrystals prepared by the microemulsion technique have well-defined boundaries and narrow size distribution.14,15 The microreactors in reverse microemulsion not only control shape and size but also reduce the aggregation process of crystals because the surfactants could adsorb on the crystal surface when the crystal size approaches to that of the water pool. As a result, the crystals obtained via reverse microemulsion are generally very fine, monodisperse, morphologically controlled, and highly crystalline as compared to those from other processes.15

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τ ) τ0 exp(EA /kBT) where τ is the superparamagnetic relaxation time, τ0 is a time relaxation constant (∼10-9 s), and T is the temperature.38 For an arbitrary measurements time of say 100 s,

EA ) ln(τ/τ0)kBT ) 25kBT For a particle with uniaxial anisotropy, EA ) KV, and the condition for superparamagnetism becomes KV ) 25kBT; K is the magnetocrystalline anisotropy. Therefore, the blocking temperature should roughly satisfy the relation

TB ) KV/25kB The anisotropy is larger for nanocrystals than the bulk value and increases with the decrease in particle size. The nanocrystals display superparamagnetic relaxation when the energy barriers of magnetic anisotropy are overcome. The blocking temperatures for Co0.5Zn0.5Fe2O4 and Ni0.5Zn0.5Fe2O4 nanocrystals synthesized via chemical copreFigure 4. FTIR spectra of (a,b) Co0.5Zn0.5Fe2O4 and (c,d) Ni0.5Zn0.5Fe2O4 ferrite nanocrystals obtained from chemical coprecipitation and reverse microemulsion, respectively. All samples were sintered at 600 °C for 4 h.

Spectral Analysis. Typical FTIR spectra of Co0.5Zn0.5Fe2O4 and Ni0.5Zn0.5Fe2O4 ferrite nanocrystals obtained from reverse microemulsion and chemical coprecipitation routes are shown in Figure 4. The spectra presented are of the crystals sintered at 600 °C for 4 h. Initially, the size of the nanocrystals was so small that extremely weak intensity peaks due to the intrinsic vibrations of the tetrahedral and octahedral sites were observed, so the nanocrystals were sintered at 600 °C for 4 h. The FTIR spectra of both ferrite nanocrystals exhibit peaks around 3444 and 1600 cm-1 attributed to the stretching vibrations of hydrogen-bonded surface water molecules and hydroxyl groups.35 The peak near 1600 cm-1 could be ascribed to inplane O-H bending of adsorbed water.4 In ferrites, metal ions are situated in two different sublattices, designated as tetrahedral and octahedral according to the geometrical configuration of the oxygen nearest neighbors. The higher frequency band ν1, that is, around 580 cm-1, is attributed to the intrinsic vibration of the tetrahedral sites, and the low frequency band ν1 around 420 cm-1 is ascribed to the intrinsic vibration of the octahedral sites, thus confirming that samples prepared are spinel in structure.36,37 Magnetic Properties. Figures 5 and 6 shows the ZFC-FC curves recorded under an applied field of 60 Oe for Co0.5Zn0.5Fe2O4 and Ni0.5Zn0.5Fe2O4 nanocrystals synthesized via reverse microemulsion and chemical coprecipitation. As expected, the ZFC magnetization increases before reaching a maximum value, that is, the blocking temperature (TB).38 It is interesting to note that the curves diverge below a certain temperature, marked as the blocking temperature. At temperatures above TB, the thermal energy, characterized by kBT, is larger than the magnetic energy barrier and thus the materials become superparamagnetic following the Curie-Weiss law. The blocking temperatures of Co0.5Zn0.5Fe2O4 and Ni0.5Zn0.5Fe2O4 nanocrystals synthesized by reverse microemulsion are found to be approximately 165 K and 130 K, respectively (Figure 5). The behavior of superparamagnetic nanocrystals can be described by the Neel-Brown equation as38

Figure 5. ZFC-FC curve for (a) Co0.5Zn0.5Fe2O4 and (b) Ni0.5Zn0.5Fe2O4 ferrite nanocrystals obtained from reverse microemulsion in an applied field of 60 Oe.



Figure 7. Magnetization curves of (a) Co0.5Zn0.5Fe2O4 and (b) Ni0.5Zn0.5Fe2O4 ferrite nanocrystals obtained from reverse microemulsion. Figure 6. ZFC-FC curve for (a) Co0.5Zn0.5Fe2O4 and (b) Ni0.5Zn0.5Fe2O4 ferrite nanocrystals obtained from chemical coprecipitation in an applied field of 60 Oe.

cipitation are found to be 230 K and 180 K, respectively (Figure 6). ZFC-FC curves reveal that the blocking temperature increases as the particle diameter increases. The ZFC-FC curve separation at lower temperature for all the ferrites can be described as high field irreversibility and implies a “freezing” of the disordered surface spins.39 The high reversibility associated with the blocking process of the magnetic crystals is expected to disappear for applied fields of a few kOe when the anisotropy field of the crystals is surpassed and crystals reach a state of saturation. However, it is suggested that high field reversibility is a strong function of particle size for crystals less than 20 nm.39 Figures 7-10 show the magnetization versus applied field curve for all ferrite samples. The M-H hysteresis loops indicating superparamagnetic property at a temperature of 300 K for Co0.5Zn0.5Fe2O4 and Ni0.5Zn0.5Fe2O4 ferrite nanocrystals synthesized via reverse microemulsion are presented in Figure 7. The absence of hysteresis, negligible coercivity and remanance, and the nonattainment of saturation even at high magnetic field are the characteristics of superparamagnetic behavior.40 It was found that at 300 K the hysteresis loops of the Co0.5Zn0.5Fe2O4 and Ni0.5Zn0.5Fe2O4 ferrite nanocrystals obtained from the reverse microemulsion route could not be saturated

with the available maximum field. Figure 8 shows that the hysteresis loop is closed for both ferrites obtained from reverse microemulsion. The nonsaturation of the M-H loop and absence of hysteresis, remanance, and coercivity at 300 K are indicative of the presence of superparamagnetic and single domain crystals. The values of magnetization for Co0.5Zn0.5Fe2O4 and Ni0.5Zn0.5Fe2O4 ferrite nanocrystals obtained from reverse microemulsion are 11.14 and 1.14 emu/g, respectively. On the other hand, the magnetic saturation values for the corresponding crystals obtained via chemical coprecipitation were found to be 30.550 and 25.62 emu/g, respectively. The magnetization curves shown in Figure 9 for these nanocrystals get saturated with the available maximum field, indicating their ferromagnetic behavior. It may also be noted from Figure 10 that the hysteresis loop is open. It is well-known that the magnetic properties of nanocrystals are predominantly dictated by the intrinsic properties of materials such as the anisotropy and saturation magnetization. The saturation magnetization values as small as those reported in some previous studies (5 emu/g) are difficult to justify considering surface effects only.41,42 The existence of some degree of spin canting in the whole volume of the particle, in addition to the disordered surface layer, could be an alternative explanation of this additional decrease of the saturation magnetization. In this sense, we observe a strong decrease in the magnetization for crystals smaller than 5 nm (about six cubic unit cells) which

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Figure 8. Hysteresis loops of (a) Co0.5Zn0.5Fe2O4 and (b) Ni0.5Zn0.5Fe2O4 ferrite nanoparticles obtained from reverse microemulsion measured at 300 K in an applied field of 450 Oe.

only occurs when the sample manifests vacancy disorder. Thus, it seems that, in addition to the surface effect, the order-disorder characteristic of the samples has also a strong influence on the final value of the saturation magnetization. It should be mentioned that similar evolution in the magnetization with the particle size has been found in γ-Fe2O3 nanocrystals prepared from micelles.43 The magnetic behavior of nanocrystals has a marked dependence on the decrease in particle size, where the surface effects start to dominate. Nanocrystals with a large surface/volume ratio show enhanced spin disorder relative to large crystals when measured with the same values of applied field and temperature.44 The surface layer magnetic moment anomalies may be due to broken exchange bonds, a high anisotropy layer on the surface, or a loss of the long-range order in the surface layer. These effects are more intense in the case of ferrites because of the superexchange interactions through the oxygen ions. The presence of another atom (ion) in the form of impurity or an absence of the oxygen ions at the surface leads to the breakage of the superexchange bonds between the magnetic cations, inducing a large surface spin disorder.45 The superexchange interaction depends on the bond angles and the bond lengths,

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Figure 9. Magnetization curves of (a) Co0.5Zn0.5Fe2O4 and (b) Ni0.5Zn0.5Fe2O4 ferrite nanocrystals obtained from chemical coprecipitation measured at 300 K.

which would obviously be different at the surface due to termination of the bonds.46 The relatively higher saturation values for Co0.5Zn0.5Fe2O4 and Ni0.5Zn0.5Fe2O4 ferrite nanocrystals obtained from the chemical coprecipitation method may be due to the fact that the anisotropic features of these nanocrystals have enhanced dipole-dipole interaction, favoring a head-totail orientation, thus resulting in a relatively higher saturation value.47 The Curie temperature (TC) of Co0.5Zn0.5Fe2O4 and Ni0.5Zn0.5Fe2O4 ferrite nanocrystals obtained from reverse microemulsion was found to be 195 and 210 °C (Figure 11). The Curie temperature of Co0.5Zn0.5Fe2O4 and Ni0.5Zn0.5Fe2O4 ferrite nanocrystals obtained from chemical coprecipitation was found to be 275 and 300 °C (Figure 12). Analysis of thermomagnetic plots of Co0.5Zn0.5Fe2O4 and Ni0.5Zn0.5Fe2O4 nanocrystals synthesized via reverse microemulsion and chemical coprecipitation reveals that TC decreases as the particle size decreases. This behavior can be ascribed to the variation in inversion degree of cations. In nanoscale ferrite, cations occupy lattice sites by a certain degree against their preferences in bulk materials and this degree (of inversion) is dependent on particle size. Therefore, for Co0.5Zn0.5Fe2O4 ferrite nanocrystals prepared by the reverse microemulsion



Figure 10. Hysteresis loops of (a) Co0.5Zn0.5Fe2O4 and (b) Ni0.5Zn0.5Fe2O4 ferrite nanoparticles obtained from chemical coprecipitation measured at 300 K in an applied field of 450 Oe.

process, there are more Co2+ ions occupying A-sites and also more Zn2+ ions occupying octahedral B-sites, again resulting in lower TC values compared to Co0.5Zn0.5Fe2O4 ferrite nanocrystals prepared by chemical coprecipitation.48 The general formula for the mixed Ni, Zn ferrite is ZnA2+FeA3+[NiB2+FeB3+]O4, where the cations within the brackets are located at octahedral (B) sites. The Zn incorporation reduces the number of Fe3+ ions on A-sites, which in turn weakens the superexchange interaction between the A and B sublattices. As the crystal size comes down below a critical range, there is a change in the degree of inversion parameters; that is, there are more Ni2+ ions occupying A-sites and also more Zn2+ ions occupying octahedral B-sites which reduces the A-B and B-B interactions, resulting in lower TC values for Ni0.5Zn0.5Fe2O4 ferrite nanocrystals synthesized by reverse microemulsion as compared to those prepared by chemical coprecipitation.49,50 Conclusions The study demonstrates that reverse microemulsion could be used for the production of controlled architectures of the ferrite nanocrystals. The reverse microemulsion route gives a successful way for preparing Co0.5Zn0.5Fe2O4 and

Figure 11. Magnetization versus temperature graph for (a) Co0.5Zn0.5Fe2O4 and (b) Ni0.5Zn0.5Fe2O4 ferrite nanoparticles obtained from reverse microemulsion.

Ni0.5Zn0.5Fe2O4 ferrite nanocrystals with diameters in the size range of 2-5 nm at low temperature (80 °C). The synthetic procedure developed in the present study offers several advantageous features for the synthesis of magnetic nanocrystals. First, the synthetic process is economical and environmentally friendly, because it involves inexpensive and less toxic iron salts and a reduced amount of organic solvent. Second, monodisperse and highly crystalline nanocrystals were produced without going through any size-selection process or postsynthetic heat treatment. As the particle size is below the critical value, the coercivity and remanance become negligible, leading to a superparamagnetic state, which could be characteristically described as a transformation from multidomain nature to single domain nature. The reverse microemulsion route has shown a significant effect on blocking as well as Curie temperature of the ferrite crystals. Furthermore, these studies provide an excellent superparamagnetic nanoparticulate system for developing contrast enhancement agents for magnetic resonance imaging and

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Figure 12. Magnetization versus temperature graph for (a) Co0.5Zn0.5Fe2O4 and (b) Ni0.5Zn0.5Fe2O4 ferrite nanoparticles obtained from chemical coprecipitation.

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