| Summary: | 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.
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