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A simple synthesis of Fe3O4 powders and their Structure, magnetic and electrical transport properties K.L. Liu* Corresponding author. Address: Department of Physics, Zhoukou Normal University, Zhoukou 466000, PR China. TelE-mail address:, Y. D. Du, C.Y.LiDepartment of Physics, Zhenzhou electicl power collage zhang haiyang Peoples Republic of China.ABSTRACT Fe3O4 powders with highly crystalline have been synthesized by a sol-gel auto combustion synthesis method accompanying thermal process. The phase structures, morphologies of Fe3O4 have been characterized by x-ray diffraction (XRD) and scanning electron microscopy (SEM) equipped with energy dispersive x-ray spectrometer (EDX). The results show that complete and highly crystalline nature magnetite powders can be obtained. Furthermore, the room-temperature magnetizations and the resistance of Fe3O4 powders were also measured. The maximum magnetization of sample S1, S2and S3 is 88.3, 87.32 and 89.4emu/g at the applied magnetic field 5kOe, respectively, which are larger than the saturation magnetization of the bulk -Fe2O3 (76emu/g). It is proved further that Fe3O4 powders can be synthesized by the citrate-nitrate sol-gel auto combustion reaction method. The preliminary reaction mechanism is also discussed.Keywords: Fe3O4; Magnetic materials ; Sol-gel preparation; Half metallic1. IntroductionMagnetite, Fe3O4, is ferromagnetic inverse spinel structure with an anomalous high Curie temperature of 860 K and an exceptionally high conductivity 1. In addition, the band-structure calculations for magnetite have indicated that Fe3O4 is half-metallic and the conduction electrons are 100% spin polarized 2. These properties make magnetite to be applied extensively in magnetoresistive devices, magnetic storage and biomedicine 3-7.The nanocrystalline magnetite with a uniform particle size has been successfully synthesized by chemical co-precipitation 8,9, thermal decomposition10, hydrothermal reaction 11 and solgel 12 and other method 13. In this work, uniform Fe3O4 powders were synthesized by the citrate-nitrate sol-gel auto combustion reaction method. The sol-gel auto combustion synthesis method for preparing nanoparticles of ferrite spinels such as MFe2O4 (M=Ni, Zn, Co) 14-18 has been developed successfully, which can be an alternative method to those listed above. In this method, intermediate decomposition and calcining steps are not involved. Moreover, it is easy to control precisely the stoichiometric composition and crystallite size, which have important effects on the magnetic properties and applications of the magnetite. Here, for the first time the synthesis of ferrite spinel Fe3O4 powders, using citric acid as reducing agent and iron nitrate as starting material, is reported, and the magnetizations and resistances of the samples are also carried out.2. Experimental procedureThe chemical materials Fe(NO3)39H2O and citric acid with analytical grade were employed as raw materials to prepare Fe3O4 ferrite. An aqueous solution containing ferric nitrate (0.8M) was prepared first. With constant stirring, an appropriate amount of citric acid was added to this solution to produce clear cationic solution. In order to prepare the perfect Fe3O4 powders, the different molar ratio of iron nitrate to citric acid was introduced. S1, S2, and S3 were used to present the samples synthesized at the different molar ratio of 1:1, 2:1 and 3:1, respectively. The resulting solution was evaporated at 348K and stirred to form a transparent sol. Then, the transparent sol was dried at 403K for half an hour to further remove water. The sol turned into a viscous brown gel. Finally, the obtained gel was not heated until ignited point was observed. The dried gel was burnt in a self-propagating combustion manner until all the gel was burnt out completely to swell into a fluffy mass, which transformed into powder at the slightest touch. The as-prepared chocolate brown powder was then calcined at 923K for 10 h in argon atmosphere and the resulting black Fe3O4 powders were obtained.Phase composition of the samples was performed by x-ray powder diffraction (XRD, Philips Xpertpro). The morphology and composition of the samples were examined by scanning electron microscopy (SEM) equipped with energy dispersive x-ray spectrometer (EDX). The resistances and the magnetizations of all samples were measured in a commercial Physical Property Measurements System (Quantum Design PPMS). 3. Results and discussionFig. 1 displays XRD patterns of the three samples prepared by the combustion reaction method at the different molar ratio of iron nitrate to citric acid. The position and relative intensity of all diffraction peaks match well with the cubic phase magnetite of Fe3O4 (JCPDC 19-0629) with lattice parameter of a=8.3960 . No other phase is detected in Fig. 1, which indicates that the final products consist of highly crystalline and single-phase Fe3O4. But the maghemite ( -Fe2O3) is easily formed due to oxidation during the formation of Fe3O4 and it has almost the same XRD pattern of Fe3O4, which gives rise to difficulty to identify the kind of the oxide only by the XRD patterns due to the similarity in lattice constant value between them. In order to clarify the phases exhibited in the powders, spot energy dispersive X-ray spectroscopy (EDX) analysis (Fig. 2(a) was used to confirm the pure nature of the Fe3O4 product: only Fe and O elements were detected from the product, the atom ratio of these elements Fe: O is 45.91:54.09, very similar to the theory ratio 3:4. The EDX date is consistent with the XRD analysis and further proves the formation of pure cubic phase of Fe3O4 product.The morphologies and microstructures of the Fe3O4 were further characterized by SEM. Fig. 2 shows a typical overall morphology of Fe3O4 powders synthesized with this method. From the images, one can see that the product is consist of irregular and rough spherical morphologies and that the average size of particle is about 300-500 nm, suggesting that the grain aggregation exists. In addition, the extremely compact aggregations of grains in sample S1 synthesized at the molar ratio of 1:1 occur. Comparing with S1 sample, the distribution of the grains in the sample S2 and S3 is just mildly aggregate. Especially, the grains in the sample S3 disperse uniformly. Based on these phenomena, the molar ratio of iron nitrate to citric acid can be selected reasonably.On the other hand, room temperature magnetizations and resistance of Fe3O4 powders were also measured by PPMS. The hysteresis loops of all Fe3O4 powders at room temperature show a ferromagnetic behavior with high magnetization as shown in Fig. 3. The maximum magnetization of sample S1, S2and S3 is 88.3, 87.32 and 89.4emu/g at the applied magnetic field 5kOe, respectively, which are larger than the saturation magnetization of the bulk -Fe2O3 (76 emu/g) 19, suggesting that the maximum magnetization of the resulting powders showed a strong independence on the fuel-to-oxidant ratio employed. Magnetization remanence and coercivity presented in the enlarged part are about 5emu/g and 50Oe, respectively. Compared to the corresponding magnetization value of bulk magnetite (90100emu/g) and the coercivity value of the bulk Fe3O4 (115150Oe), Fe3O4 powders exhibit a smaller values, which may be attributed to the nanosize effect and their irregular and rough spherical structures, respectively. The temperature dependence of resistance of the sintered samples is shown in Fig.4. Resistance was measured by a conventional four-probe technique. It is obvious that the sample exhibits semiconductor behavior in the whole chosen temperature range without any transition. With decreasing temperature, the resistance increases monotonically from 300 to 110 K, and then increases markedly with decreasing temperature below 110 K, which may be ascribed to the high degree of spin polarization at 110K for half metallic Fe3O420,21. Both of them are strong indicators of good stoichiometry and high crystalline phase of the Fe3O4 powders 22. In the chemical reaction process, citric acid, acting as reducing agent, plays an important role. When the dried gel was burnt in a self-propagating combustion manner, part of citric acid radical ion was used as the fuel of combustion reaction to decompose nitric acid radical ion 23 and another citric acid radical ion was used as reducing agent to reduce Fe (III) to Fe3O424. In a word, the reaction mechanism is complex, so further investigation is needed to better understand the formation of these unique materials.4. Conclusions In summary, a complete and uniform magnetite powder with particle size within 300-500 nm has been successfully synthesized through citrate-nitrate sol-gel combustion reaction method. The results of the XRD patterns, the EDX map, the hysteresis loops and the resistance versus temperature curves show that these samples are entirely Fe3O4 single phase with cubic spinel structure. Therefore, this synthetic method may be a promising technique for preparation of Fe3O4. AcknowledgementsThis work was supported by the Science Foundation from the HeNan province department of education (Grant No. 2008B430029).References1 Coey JMD, Berkowitz AE, Balcells Ll, Putris FF, Parker FT. Appl. Phys. Lett 1998; 72:734 -736.2 Zhang Z, Satpathy S. Phys. Rev. B 1991; 44: 13319-13331.3 Shen W, Shi MM, Wang M. Mater. Chem. Phys. 2010; 122:588-594.4 Park JO, Rhee KY, Park SJ. Appl. Surf. Sci. 2010; 256:6945-6950.5 Wang H, Hu P, Pan DA et al.J. Alloys Compd. 2010; 502:338-340.6 Zhang JC, Shen WQ, Zhang ZW, et al. Mater. Lett. 2010; 64:817-819.7 Mi WB, Ye TY, Jiang EY, Bai HL. Thin solid films. 2010; 518:4035-4040.8 Luo WS, Kelly SD, Kemner K M, et al. Environ. Sci. Technol. 2009;43: 75167522.9 Gee SH, Hong YK, Erickson DW, Park MH, Sur JC. J. Appl. Phys. 2003; 93:7560-7562.10 Sapieszako RS, Matijevic E. J.Colloid Interface Sci. 1980; 74:405-422.11 Li Y, Liao H, Qian Y. Mater. Res. 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