Research Journal of Recent Sciences _________________________________________________ ISSN 2277-2502 Vol. 1(8), 48-52, August (2012) Res.J.Recent Sci. International Science Congress Association 48 Magnetic Properties of Fe doped ZnO Nanosystems Synthesized by Solution Combustion MethodDhiman Pooja*1, Sharma S.K.1,2, Knobel M., Ritu Rani and Singh M.Department of Physics, Himachal Pradesh University, Shimla-5, INDIA Instituto de Fisica GlebWataghin, Universidade Estadual de Campinas (UNICAMP) Campinas,13.083-859, SP, BRAZILAvailable online at: www.isca.in Received 27th April 2012, revised 7th May 2012, accepted 18th May 2012Abstract Zn1-xFe O (0.010.05) nanosystems synthesized by a solution combustion method were characterized by different techniques. The structural characterization by XRD confirmed the phase purity of the samples. TEM measurement depicted the crystallinity of nanosystems prepared and EDS analysis conformed the elemental composition. The magnetic behaviour of the nanoparticles of ZnO with varying Fe doping concentration was investigated using a super conducting interference device (SQUID) which confirmed the ferromagnetic state for 5% Fe doped sample at room temperature. Local environment around Fe atoms has been probed by 57Fe Mossbauer spectroscopy and measured isomer shifts confirmed the charge state of iron is Fe3+. Keywords: ZnO, Fe-doping, solution combustion, mossbauer spectroscopy. Introduction Semiconductor nano-crystals have been extensively studied due to their size tunable optical and electrical properties and their potential applications to electronics and bio-technology. There is rapid growing interest in diluted magnetic semiconductors (DMS), where magnetic ions are doped into the semiconductors hosts, allowing us to design a new generation spin-tronic devices with enhanced functionalities2,3. Spintronics is expected to improve upon traditional electronic and photonic devices, allowing for enhancement in the form of reduced power consumption, faster device operation, and new forms of information computation. In the recent years the researchers have paid much attention to the synthesis and characterization of II-IV semiconductor materials at nanometer scale, due to their ability to test the fundamental concepts of quantum mechanics and because of their key role in various applications such as solid-state lighting devices and sensors. ZnO belongs to the list of most promising candidates for spintronics application due to friendly nature and also due to its potential as a suitable optoelectronic with a wide band gap (-3.3ev) and high exciton binding energy of 60meV. Earlier first- principle electronic structure calculations suggested that TM (TM = Ti, V, Cr, Mn, Fe, Co, Ni, Cu) doped ZnO are ferromagnetic provided the TM doping produces carriers forming a partially filled spin-split impurity band in the nanoparticle form6-11. The effect of Fe doping as a magnetism activator and as a compensator of n-type material is of great importance for II-VI semiconductors12-13. Earlier Fe doped ZnO nanosystems have been prepared by co-precipitation method4,9, solid state method14, and sol-gel technique15. We have here presented a simple technique to synthesize ZnO: Fe nanoparticles. Material and Methods All materials used in this study were of analytical grade. Zn1-Fex O (0.010.05) nanocrystalline samples with different fraction of iron were prepared by solution combustion of aqueous solutions containing required amounts of the corresponding metal nitrates, zinc nitrate, iron nitrate and glycine16. By merely changing the temperature conditions of previously described synthesis technique, O/F ratio was kept two using total oxidizing and reducing valencies of the oxidizer and the fuel .The resulting mixture was allowed to heat at 100C with constant stirring. Finally the dried powder was kept in a muffle furnace at temperature of 400±20C for 4 hours. The obtained product was yellowish in color and fluffy. X-Ray powder diffraction analysis was conducted on XPERT-PRO Diffractometer (XRD) using Cu K radiation. The phase structure of nano ferrites was analyzed by XRD. Simulation of crystal structure based on the measured X-ray diffraction (XRD) data was performed using Rietveld crystal structure refinement software FULLPROF 2007. Crystal structure, morphology and quantitative composition of the nanocrystals were obtained using a transmission electron microscope (model PHILIPS CM 200) operated at 200KV accelerating voltage by dissolving the synthesized powder sample in the ethanol. Magnetization measurements were carried out using a super conducting interference device (SQUID) upto a field of 5000 Oe. Mössbauer spectra were recorded by using 57Fe spectroscopy using cobalt as radioactive source with pc-based spectrometer equipped with weissel velocity at the room temperature and the experimental results were fitted with the NORMOS-SITE program. Research Journal of Recent Sciences ______________________________________________________________ ISSN 2277-2502Vol. 1(8), 48-52, August (2012) Res. J. Recent Sci. International Science Congress Association 49 Results and DiscussionThe XRD patterns of the Fe doped ZnO nano-crystalline samples with different atomic fraction of dopant ion (00.05) which are denoted hence onwards as ZF, ZF1, ZF2, ZF3, ZF4, ZF5 respectively as shown in figure-1. It reveals that, there is no change in the wurtzite structure of ZnO after Fe doping. The average particle size calculated is found to be in the range of 10-21nm.Since the difference in radii between Zn2+ (0.60A) and Fe3+ (0.63 A) is considerable, significant changes in the lattice constants are expected for Fe doped samples. The crystal structure of the sample ZF1 at room temperature is determined to be wurtzite structure of P6mc with its lattice constant a0 =3.253A and c = 5.210 A by Rietveld refinement (figure-2). The determined Bragg constant R and R have values 2.43% and 1.93% respectively. The lattice constant a is found to increase from 3.250 to 3.253 Aand c is found to decrease from 5.211 to 5.210 A0 with increase of Fe concentration. Low resolution Transmission electron micrographs of the sample ZF1 is presented in figure-3 (a-b). The particles are more or less spherical having a distribution of size between 8-21 nm. The average particle size obtained matches well with size estimated from XRD. The elemental composition of the sample was checked by energy dispersive X-ray analysis (figure-3(c)), which confirms the presence of only Zn, Fe and O in the sample. The selected area electron diffraction (SAED) pattern shows the crystalline nature of the sample (figure-3(d)). Figure-4 shows the results of the M-H measurements of the samples ZF1 to ZF5 at room temperature. A clear hysteresis loop has been observed for sample ZF5. From the loop remanant magnetization and the coercive field were estimated to be 0.002 emu/g and 162 Oe, where it is 0.0002 emu/g and 5 Oe (bulk sample) under the application of field of 20,000 Oe but the saturation didn’t take place10Whereas M and H hasfound to be 0.008 and 98Oe (nano-particles) by applying high field upto 20,000Oe. The narrow hysteresis implies a small amount of dissipated energy in repeatedly reversing the magnetization which is important for quick magnetization and demagnetization of the samples synthesized. The ferromagnetic behaviour can be attributed the presence of small magnetic dipoles located at the surface of nanocrystals, which interacts with their nearest neighbours inside the crystal. As a result of which the interchange energy in these magnetic dipoles making other neighbouring dipoles oriented in the same direction, so the population of magnetic dipoles oriented in the same direction will increase at the surface. Thus, the sum of the total amount of dipoles oriented along the same direction will increase subsequently. In short, the crystal surface will be usually more magnetically oriented. Since the particle size play a crucial role in magnetism, so it can be expected for zf1 that somekind of superparamagneic character (almost zero coercive field) exists in the sample which is expected because of smaller size of particles (less than 10 nm from TEM). As the size of particles increase this superparamagnetic character begin to cease and paramagnetic type of character appears. However it is still unclear what factor is playing the role for ferromagnetic state for zf5 at room temperature. Temperature dependent magnetization [M (T)] curves in field cooled (FC) and zero field (ZFC) conditions in the presence of a dc magnetic field of 100 Oe for ZF5 sample is shown in figure-5. The outward concave nature of magnetization curve confirms the low carrier density and the localized nature of the carrier as well. The strong irreversibility persists even above the room temperature, signifying the surface effect of nanoparticles. However the spectra for ZF1 is quiet typical as supported by M-H spectra. Figure-6 shows the Mossbauer spectra for all the samples for ZF1-ZF5 shows doublet for all cases. Isomer shift (IS) ranges from 0.09mm/s to 0.12mm/s, and Quadrupole splitting (QS) from .68mm/s to 0.90 mm/s indicating Fe3+ nature of the Fe atom in ZnO. No signature of Fe2+ has been found in any of these spectrum. Similar kind of behavior had been found in Fe doped ZnO nanoparticles synthesized in using chemical pyrophoric reaction method with IS ~ 0.56mm/s and QS~0.73mm/s reported recently17. Whereas coexistences of Fe3+(QS ~ 0.81mm/s) and Fe2+ (QS ~ 2.00 mm/s state has been found in the Fe-doped ZnO polycrystalline sample prepared by solid state reaction method14. Lower value of IS in our case indicates lower s electron density in the sample. The increasing QS indicate the enhancement of electric field gradient around Fe probe atoms as more Fe atoms are introduced in the ZnO lattice. In a Fe-doped ZnO system, the valence state of Fe is expected to be +2 if Fe simply substitutes for Zn. Fe3+ nature may be due to the presence of nearby cationic vacancies at the substitutional sites, which trigger the conversion of Fe2+ to Fe3+ to neutralize the charge imbalance. In view of this, the substitution of Fe3+ for Zn2+ Carrier-mediated ferromagnetism has a possibility to explain the observed magnetic behavior in Fe doped ZnO. The theory based on interaction between magnetic polarons is another candidate. Disorder defects may also be an important factor for the observed ferromagnetism in Zn1-xFeO nanoparticles. Formation of Fe cluster can be other candidate. A detailed research is required on this problem to clarify the doubt. Figure-1 XRD patterns of ZF, ZF1, ZF2, ZF3, ZF4 and ZF5 Research Journal of Recent Sciences ______________________________________________________________ ISSN 2277-2502Vol. 1(8), 48-52, August (2012) Res. J. Recent Sci. International Science Congress Association 50 Figure-2 Refined XRD pattern of ZF1 Figure-3 (a-b) TEM images of ZF1, (c) EDS spectrum of ZF1, (d) SAED pattern of ZF1sample Figure-4 (a) Magnetization spectras of ZF1, ZF2, ZF3, ZF4 and ZF5, (b) of ZF1 near origin Research Journal of Recent Sciences ______________________________________________________________ ISSN 2277-2502Vol. 1(8), 48-52, August (2012) Res. J. Recent Sci. International Science Congress Association 51 Figure-5 FC and ZFC curve of ZF5, the inset displays curve for ZF1 Figure-6 Mossbauer spectra of Zn1-xFex O nanoparticles (ZF0, ZF1, ZF2, ZF3, ZF4 and ZF5) Research Journal of Recent Sciences ______________________________________________________________ ISSN 2277-2502Vol. 1(8), 48-52, August (2012) Res. J. Recent Sci. International Science Congress Association 52 ConclusionWe have successfully synthesized Fe doped ZnO nanosystems by solution combustion method using glycine as fuel. XRD spectra for all samples shows the wurtzite structure. TEM and SAED measurements confirmed the crystallinity and particle size determined is in good agreement with those calculated from XRD. EDS analysis showed that elemental composition contains only Fe, Zn and O. M-H studies of the samples confirmed the ferromagnetic state for the 5% Fe doped sample. Measured isomer shifts confirmed the Fe3+charge state of iron using Mossbauer spectroscopy. AcknowledgementThe authors would like to thank UNICAMP for work which was supported by FAPESP and CNPq, Brazil. SAIF- IIT Bombay is highly acknowledged for TEM measurements. Pooja Dhiman also acknowledges financial support from UGC New Delhi in the form of Rajiv Gandhi Junior Research fellowship. References 1.Taguchi S., Tayagaki T. and Kanemitsu Y., Luminescence and magnetic properties of Co doped ZnO nanocrystals Mater, Sci. Eng.,, 012029 (2009) 2.Furdyna J.K., Diluted magnetic semiconductors, J.Appl. 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