The effect of the shell thickness and density on the magnetic properties of
composite colloidal particles consisting of a hematite (alpha -Fe2O3) core
and an yttrium oxide (Y2O3) layer is described. Pure iron oxide colloidal
spheres show two clearly different trends of variation of their magnetic su
sceptibility, chi (m), with temperature. Below T-M approximate to 220 K, ch
i (m) shows a slight increase when the particles are heated; a sharp transi
tion is observed at such a critical temperature, whereby chi (m) increases
almost 3 times in a very narrow temperature interval, decreasing slowly aft
erward. This is the result of a well-known transition from perfect to imper
fect antiferromagnetism (canted antiferromagnetism). Three types of core/sh
ell particles have been prepared, and a gradual change is observed in chi (
m) from that of hematite to that of pure Y2O3. Even the most efficiently co
vered particles still show a change in their chi (m)-T trends around T-M, a
nd are clearly distinct from Y2O3 particles. Magnetization curves show that
coating of hematite particles induces significant changes in the coercivit
y of the samples. The latter is always larger for composite than for core p
articles. (C) 2001 Academic Press.