Superparamagnetism and Microstructural Properties of Carbon Encapsulated Ni nanoparticle Assemblies

  • PDF / 274,950 Bytes
  • 6 Pages / 612 x 792 pts (letter) Page_size
  • 32 Downloads / 250 Views

DOWNLOAD

REPORT


Superparamagnetism and Microstructural Properties of Carbon Encapsulated Ni nanoparticle Assemblies Xiang-Cheng Sun1,*, Xinglong Dong2 , J. A. Toledo1 and M. J. Yacaman3 1

Prog. Molecular Simulation, Instituto Mexicano del Petróleo, Lázaro Cárdenas 152#, 07730, D.F. México, México *E-mail: [email protected]; Fax: +525-3336239 2 Shenyang Polytechnic University, Shenyang P. R. China 3 Institute of Physics, National University of Mexico, México, D.F. México ABSTRACT Carbon encapsulated Ni nanoparticles ( Ni(C) ) were synthesized by modified arcdischarge reactor under methane atmosphere. The presence of carbon encapsulation is confirmed by HR-TEM imaging, and Nano-diffraction. The average particle radius is typically 10.5 nm with spherical shape. The intimate and contiguous carbon fringe around these Ni nanoparticles is good evidence for complete encapsulation by carbon shell layers. Superparamagnetic property studies were performed using SQUID magnetometer for the assemblies of Ni(C) nanoparticles. The blocking temperature (TB) is determined to around 115K at 1000Oe applied field. Above TB, the magnetization M (H, T) can be described by the classical Langevin function L using the relation, M/Ms(T=0) = coth(µH/kT)- kT/µH. The particle radius can be inferred from Langevin fit ( particle moment µ ) and blocking temperature theory (TB), which values are a little bigger than HR-TEM observations. It is suggested, these assemblies of carbon encapsulated Ni nanoparticles have been showed typical single-domain, field-dependent superparamagnetic relaxation properties. INTRODUCTION Magnetic properties of nanoparticles are of intense research interest both in fundamental science and potential application. Such as, superparamgnetic relaxation phenomena [1-3] has been exhibited many potential applications including ferrofluid technology [4], magnetocaloric refrigeration [5], and high-density information storage media[6]. According Stroner-Wohlfarth theory [7], when the magnetocrystalline anisotropy EA becomes comparable with thermal activation energy, KBT with KB as the Boltzmann constant. The anisotropy energy barrier is so small, the thermal activation energy and /or an external magnetic field can easily move the magnetic moments away from the easy axis. Consequently, the collective behavior of the magnetic nanoparticles system is the same as that as paramagnetic atom. As we know, suck like behavior is know as superparamagnetism (SPM). The temperature, at which the magnetic anisotropy energy barrier of the nanoparticles are overcome by thermal activation and the nanoparticle becomes superparamagnetically relaxed, is known as the blocking temperature (TB). For a non-interacting nanoparticles system, the SPM transition (blocking) temperature, TB, depends on particle size, anisotropy energies, and the time scale of the measuring techniques. At Y7.2.1

temperature above TB, the assemblies of non-interacting particle show zero coercivity because of thermal effect that allows the magnetization to flip between easy directions sur