Microstructural and Magnetic Properties on Graphitic Encapsulated Ni Nanocrystals and Pure Ni Nanoparticles with NiO Lay
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ABSTRACT Two kinds of different nickel nanoparticles with distinct morphological properties, Ni(C) and Ni(O), are studied. Magnetization measurements for the assembly of two kinds of Ni nanoparticles show, a larger coercivity and remanence as well as the deviation between the zero field cooling (ZFC) and the field cooling (FC) magnetization have been observed in the Ni(O) particles. This deviation may be explained as a typical cluster glass-like behavior due to ferromagnetic interaction among the assembly of Ni(O) particles. However, Ni(C) particles exhibit superparamagnetism at room temperature. The average blocking temperature (TB) is determined to around 115K. We also observe gradual decrease in saturation magnetization, which is attributed to the nanocrystalline nature of the encapsulated particles.
INTRODUCTION Nickel (Ni) nanoparticles have been attracted considerable attention over the last decade because the many interesting optical, magnetic, chemical properties and by it's potentially technological applications [ 1-3 ]. In this work, we use the modified arc-discharge (carbon-arc) method to fabricate two types of nickel nanoparticles at a methane and a mixture of H2 and Ar atmospheres, respectively. Microstructural characteristics and magnetic properties at different temperatures and magnetic fields have been extensively studied and are reported here.
EXPERIMENT The detailed experimental apparatus (modified arc-discharge) is illustrated in reference [4]. JEOL-201 0EX high-resolution transmission electron microscope (HRTEM), equipped with energy dispersive X-ray spectroscopic (EDS) analysis, is used to determine the phase, and morphology of the particle, they also allow us to record selected area electron diffraction (SAED) patterns. The magnetization measurements are performed by using SQUID magnetometer in the temperature range from 2 to 300K at different applied magnetic fields. Measurements are performed by the following useful method: zero-field-cooling (ZFC) and field-cooling (FC).
535 Mat. Res. Soc. Symp. Proc. Vol. 581 © 2000 Materials Research Society
In the first method, the sample is cooled down to the target temperature, then the applied magnetic field is put on; the measurement is performed as the temperature is raised; in this case to room temperature. FC measurements are performed with the magnetic field applied at high temperature; and the measurement performed as the sample is cooled down. In measurements of M vs T, it is convenient to define the blocking temperature, TB; as the temperature at which the two curves, ZFC and FC, merge in one. The average blocking temperature is defined as the maximum of ZFC curve. Above TB, thermal fluctuations flips and scatters the direction of the magnetic moment; and no hysteresis is observed in the M(H) measurement. Magnetic properties of small ferromagnetic particles may present different features when the size is smaller than the exchange correlation length; i.e. in the superparamagnetic state. In this case for such an assembly of particles, the c
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