Size-dependence of Magnetic Properties of Bismuth Ferrite Nanopowders
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Size-dependence of Magnetic Properties of Bismuth Ferrite Nanopowders J. Li, H. He, F. Lü, Y. Duan and D. Song Department of Materials Science, Lanzhou University, Lanzhou 730000, China. ABSTRACT A series of BiFeO3 nanopowders were prepared by the sol-gel process. X-ray diffraction analysis shows that their rhombohedral crystal symmetry remains unchanged. However, as the particle size decreases, the edge length of the unit cell increases markedly and the angle between the edges deviates increasingly from 60°. Magnetic measurements show obvious weak ferromagnetism. The magnetization and magnetic susceptibility increase with decreasing particle size. Mössbauer studies reveal that the spin canting angles in the smaller particles are bigger and have a wider distribution. The magnetic structure in these particles is a complicated uncompensated antiferromagnetic spin arrangement. INTRODUCTION Perovskite-type bismuth ferrite, BiFeO3, is a distinctive magnetoelectric material because of its two types of long range orderings: antiferromagnetic ordering below 643 K [1] and ferroelectric ordering [2] with a high TC of about 1093-1123 K [3, 4]. Its crystal and magnetic structures have been fairly well studied. BiFeO3 is known to have a bimolecular rhombohedrally distorted perovskite structure in the space group R3c [5]. From the first neutron diffraction measurements by Kiselev et al, an antiferromagnetic G-type spin configuration was proposed [1]. Subsequently, Sosnowska et al. found a cycloidal spiral modulated magnetic structure in BiFeO3 using a high-resolution TOF neutron diffraction technique [6]. Surprisingly, the spiral period found in these coarse-grained BiFeO3 polycrystals is as long as 62 nm which has not been observed in any other orthoferrites previously. So it would be interesting to investigate the behavior if the crystal dimension is decreased to nanometer range, especially below 62 nm. As reported in previous studies [1-5], it is difficult to obtain the single phase BiFeO3. However, the experience in synthesizing nanopowders by chemical methods could help to overcome this problem. In our present work, a series of BiFeO3 nanopowders with different particle sizes were prepared, and the magnetic properties and their size-dependence were studied. EXPERIMENTAL DETAILS BiFeO3 nanopowders were prepared by sol-gel process. Bi2O3, Fe(NO3)3⋅9H2O, citric acid and a dispersion agent were dissolved in dilute nitric acid with the aid of an ultrasonic oscillator. The solutions were refluxed, dehydrated and dried to loose gels. Then these organic precursors were fired at various temperatures to get the desired products. Powder X-ray diffraction (XRD) data were collected at room temperature using a rotatingtarget X-ray diffractometer (Rigaku D/Max-2400) equipped with a graphite monochromator. Y7.7.1
Lattice parameters were obtained by means of extrapolation method. TEM observations were undertaken by a JEOL JEM-2000FX analytical microscope. Mössbauer effect measurements were performed at room temperature in standard transmis
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