International Journal of Advanced Engineering and Nano Technology (IJAENT) ISSN: 2347-6389, Volume-2 Issue-8, July 2015 1 Published By: Blue Eyes Intelligence Engineering & Sciences Publication Pvt. Ltd. Enhancement of Magnetic Properties of Nanocrystalline BiFeO 3 Synthesized by a Facile Sol-Gel Auto-Combustion Process Mehedi Hasan, Md. Fakhrul Islam Abstract— In this study a facile sol-gel auto-combustion methodology has been used to synthesize nearly pure BiFeO 3 (BFO) nanocrystals at relatively low temperature. An optimum synthesis condition has been established to obtain particles with spherical shape and uniform size distribution. Well crystallized BFO nanoparticles of average particle size 26 nm have been confirmed by X-ray diffraction analysis. Size and morphology of the synthesized materials are observed using Field emission scanning electron microscope (FESEM). Magnetic hysteresis loop measurement of BFO nanoparticles shows substantial improvement in saturation magnetization with a value of ∼ ∼ ∼ 6.5 emu/g compared to 0.1 emu/g for the bulk antiferromagnetic sample. The origin of the magnetic property can be attributed to the size confinement effect for the particles with size less than 62 nm, period of the spiral modulated spin structure. Index Terms— BiFeO 3 , ferromagnetism, nanoparticles, sol-gel auto-combustion. I. INTRODUCTION Recently there is intriguing interest in the emerging novel group of materials called multiferroics because of their promising applications in fundamental research and various possible technological schemes. Due to the simultaneous coexistence of ferroelectric, ferromagnetic and ferroelastic phases these materials exhibit collective responses to the electric, magnetic and stress fields. Therefore, these materials have potential applications in magnetic and ferroelectric devices [1]. At the same time, the coupling between two order parameters provides an addition degree of freedom in device design. The perovskite BFO is the most interesting in the family of very few single phase multiferroics because of its high phase transition temperatures (i.e. Curie temperature ̴ 830°C and Neel temperature ̴ 370°C) [2]. Since its discovery in the 1960s, difficult synthesis of BFO and its very low magnetic moment are the mostly reported weaknesses which have hampered its potential applications [3]. G type antiferromagnetism with a spiral spin structure having a period of 62 nm subdues any net magnetization in bulk BFO [4-7]. Several approaches to improve the magnetization in BFO ceramics have been reported in previous work [7]. A number of studies on different parameters such as A or B site substitution and co-doping have been investigated to improve magnetic properties [7-10]. Recent approaches have focused on developing novel structural formulations such as zero-, one-, and two-dimensional (0-D, 1-D, and 2-D) nanostructures of BFO materials [5, 6]. Revised Version Manuscript Received on June 20, 2015. Mehedi Hasan, Department of Glass and Ceramic Engineering, Bangladesh University of Engineering and Technology, Dhaka, Bangladesh. Dr. Md. Fakhrul Islam, Department of Glass and Ceramic Engineering, Bangladesh University of Engineering and Technology, Dhaka, Bangladesh. The antiferromagnetic ordering can be broken if the size of the BFO nanostructures be less than the spiral period order (62 nm). Enhanced magnetization for BFO has been reported in nanoparticles [11], nanowires [12] and thinfilms [13], which is thought to be originated from the destruction of the spiral spin structure and uncompensated spins at surface [5]. However there is intense study, a fundamental understanding of structure-property correlations for BFO is still lacking. Specifically, the nature of the magnetic response on particle size is of great interest. Thus, in this paper, we systematically investigate the effect of reduced particle size on magnetic properties of BFO powders. Up to now, several wet chemical routes have been developed for the synthesis of BFO powders [14-16]. In this work a facile sol-gel auto-combustion route is reported to prepare BFO nanoparticles at relatively low annealing temperature. II. EXPERIMENTAL A. Materials and Synthesis The BFO powders were prepared by sol-gel auto-combustion method as described below. Bismuth nitrate pentahydrate (Bi(NO 3 ) 3 ·5H 2 O) and iron nitrate nonahydrate (Fe(NO 3 ) 3 ·9H 2 O) were taken in a suitable stoichiometry and dissolved in 400 ml with individual concentration of 0.025 M. The citric acid as chelating agent was then added into the nitrate solution in such a manner that the molar amount of citric acid and metal nitrates was 1:1. The solution was then stirred and heated at 75-80°C for 6-8 hrs to form a sol. Aqueous ammonium hydroxide was used to adjust the pH between 1-2. Subsequently appropriate amount of ethylene glycol was added to the solution as polymerization agent and as well as surfactant agent. Afterwards the solution was gently evaporated at around 85°C to obtain a viscous gel. The resultant gel was still heated at that temperature to dry up and initiate sudden combustion reaction with vigorous fuming. After the completion of the combustion reaction the resultant powders were annealed at 400°C to obtain homogenous BFO nanoparticles. A portion of the powder was also annealed at temperature as high as 800°C to obtain bulk BFO sample. B. Characterizations and Measurements Crystalline structures of the BFO powders were examined using an X-ray diffractometer (XRD, model 3040-X’Pert PRO, Philips). The high-intensity X-ray beam was focused on the sample in the scanning range from 10° to 70°. The average crystallite size (d) was calculated from the XRD patterns using the Scherrer formula, d = kλ/βcosθ, where k is the dimensionless shape factor with a typical value of about 0.9, λ is the wavelength of Cu Kα radiation with the value of 1.5418 Å, θ is the Bragg angle for the (102) diffraction peak and β is