Maghemite (γ-Fe₂O₃) is a very common mineral at the earth’s
surface and also an important material for making music and
video tapes. Maghemite is usually synthesized from magnetite
under oxidizing conditions after a few hours or a few days below
a temperature of 300°C. The magnetic property of thermal
instability and the chemical action after heating is an
important character for maghemite. That is, it will become
hematite in certain proportion after being heated above 250°C.
Maghemite is therefore actually unable to have its Curie
temperature measured. But late using synthetic sample, maghemite
was further found partially thermal stable with a measurable
Curie temperature ∼645°C. During our thermally magnetic
experiments for a set of synthetic magnetite, we found that
extra fined grain size (pseudo single domain (PSD) and small
multi-domain (MD), mainly 1–10 μm) magnetite was formed to a
completely thermally stable maghemite. This maghemite can also
be produced by heating the same powder up to 700°C in an ov