Effect of Exchange Anisotropy on Magnetic Properties of Surface Nitrided and Oxidezed Nanocrystalline Cobalt Particles

• Main Authors: Tse-Ta Kuan, 關則達
• Other Authors: Hong-Ming Lin
• Format: Others
• Language: en_US
• Published: 1994
• Online Access: http://ndltd.ncl.edu.tw/handle/01220997646026423916

碩士 === 大同工學院 === 材料科學(工程)研究所 === 82 === Ultrafine particles of cobalt were produced by using gas condensation method. Pure oxygen and nitrogen were introduced into the as-evaporated particles before they were taken out from chamber. The different layer structures around cobalt produced in different atmosphere. The crystal structure was identified by using electron transmission microscope (TEM). Magnetic properties will be determined by VSM and SQUID. The results indicate that the mean particle size of cabalt increases as the helium pressure increases. When the pressure changed from 1 mbar to 100 mbar, the mean core size of cobalt varied from about 14 nm to 54 nm. The structures of cobalt core are h.c.p and f.c.c.. The nitrided cobalt particles shows an oxide/nitride/cobalt multi-layer structures. The layer structure of nitrided and oxidized cobalt particles was not easy to be distinguished by selected area diffraction of TEM. Particles subjected to nitriding and oxidizing had an amorphous layer structure on the surface. After annealing by condensed electron beam of TEM, the amorphous structure became Co2N and Co304 for nitrided particles, Co304 for cxidized ones. When field cooling, both nitrided and oxidized cobalt particles gad exchange isotropy, while nitrided ones had stronger exchange anisotrophy interaction. The maximum amount of shift is 3200 Oe for nitrided cobalt particles with core size of 15 nm at temperature 5 K. However, oxidized particle had a larger coercivity than that of nitrided ones. The shift above 220 K is disappeared which may be attribued to (1) the superparamagnetism of the antiferromagntic shell, and (2) the decrease of Neel temperature of the shell. At the same temperature, the extend of shift is proportional to iner- face area between core and shell, that is the shift is propor- tional to the inverse of core radius.