Microstructure and Magnetic Properties of Fe(C) and Fe(O) Nanoparticles
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Microstructure and Magnetic Properties of Fe(C) and Fe(O) Nanoparticles Xiang-Cheng Sun*,1, N. Nava1, and J. Reyes-Gasga2 1
Prog. Molecular Engineer, Instituto Mexicano del Petróleo (IMP), Central Lázaro Cárdenas 152, 07730, D. F. México *Fax: +52-55-5333-6239, *E-mail: [email protected] 2 Institute of Physics, National University of Mexico, D.F. México ABSTRACT: Two types of iron (Fe) nanoparticles, carbon-coated Fe nanoparticles (Fe(C)) and pure αFe nanoparticles that coated with oxide layers (Fe(O)), have been successfully synthesized using modified graphite arc-discharge method. X-ray diffraction (XRD), high-resolution transmission electron microscopy (HREM) and electron diffraction (SAED) analysis have been used to characterize these distinct structural morphologies. It is indicated that those two Fe nanoparticles have an average grain size of 15-20nm. The presence of carbon encapsulated αFe, γ-Fe and Fe3C phases are clearly identified by X-ray diffraction and SAED patterns in those Fe(C) particles. However, the evidence of pure α-Fe nanocrystal coated with oxide layer is also revealed by HR-TEM images and SAED patterns in these Fe(O) particles. Mössbauer spectra and hyperfine magnetic fields at room temperature for the assemblies of Fe(C) and Fe(O) nanoparticles further confirm their distinct nanophases that detected by XRD analysis and HRTEM observation. Specially, the assemblies of Fe(O) nanoparticles exhibit ferromagnetic properties at room temperature due to the stronger interparticle interaction and bigger magnetocrystalline anisotropy effects among these Fe(O) nanoparticles. Moreover, modified superparamagnetic relaxation is observed in the assemblies of Fe(C) nanoparticles, which is attributed to the nanocrystalline nature of the carbon-coated nanoparticles.
INTRODUCTION Nano-scale magnetic materials show novel properties that are markedly different from those of the bulk due to their very small sizes and fundamental change in the coordination, symmetry and confinement [1]. Research on nanoscale magnetic Fe, Co, and Ni particles/clusters has been fairly active in the last decades because of both the fundamental properties and potential application to high density magnetic recording media, color imaging, ferrofluids, and magnetic refrigeration [2-5]. As a matter, the intrinsic magnetic properties are strongly influenced by the particle size; nano-scale magnetic particles usually exhibit specific properties such as superparamgnetism and quantum tunneling of magnetization [6-7], which is regarded as a unique feature of magnetic nanoparticles. However, the formation of an antiferromagnetic oxide surface shell around ferromagnetic metal nanoparticles causes the magnetic exchange interaction that influences significantly the magnetic behavior of these particles [8]. Moreover, some novel magnetic system of carbon encapsulated ferromagnetic materials, e.g., Fe, Co, and Ni had successfully been obtained by a number of researchers [9W9.5.1
11]. Interest was paid not only to the properties of the encapsu
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