Effects of SiO/sub 2/ on magnetic-properties of Al/sub 2/O/sub 3/-coated acicular /spl alpha/-Fe particles

(SiO/sub 2/, Al/sub 2/O/sub 3/)-coated acicular /spl alpha/-Fe particles were obtained by first coprecipitating Si and Al ions on the surface of /spl alpha/-FeOOH particles and next the /spl alpha/-FeOOH particles were dehydrated and reduced by hydrogen. The effects of Si ion on Al/sub 2/O/sub 3/-coated /spl alpha/-Fe particles are as follows: (1) it prevented the acicular /spl alpha/-Fe particles from particle growth and grain growth during reduction; (2) a suitable amount of Si ion (0.3 wt%) in the particles only slightly decreased the /spl sigma//sub s/ value but significantly increased the /sub i/H/sub c/ value of the particles; (3) 0.3 wt% of Si ion effectively prevented the /spl sigma//sub s/ value of the Al/sub 2/O/sub 3/-coated /spl alpha/-Fe particles from deteriorating with time, higher Si ion content was required to prevent the /sub i/H/sub c/ from deteriorating: and (4) Si ion could not prevent the magnetics of the Al/sub 2/O/sub 3/-coated /spl alpha/-Fe particle from deteriorating under severe surroundings.