  CoFe2O4/Co-Fe magnetic composites are usually prepared through partial
reduction of CoFe2O4, which often yields monoxides (i.e., FeO, CoO) as
secondary phases. Since these compounds are paramagnetic at ambient conditions,
the presence of a small amount of monoxide is generally downplayed in the
literature, and the possible effects on the magnetic properties are simply
ignored. However, the present study shows that even a low concentration of
monoxide results in decoupling of the soft and hard magnetic phases, which
inevitably leads to a deterioration of the magnetic properties. Additionally,
it is confirmed that a partial reduction of CoFe2O4 is a suitable method to
produce CoFe2O4/Co-Fe nanocomposites, provided that the treatment is well
controlled with respect to duration, temperature and flow of reductant. A
monoxide-free nanocomposite was produced and its magnetic properties evaluated
both at room and low temperature. Our model system exemplifies the potential of
exchange-coupling (and decoupling) as a tool to tune the magnetic properties of
a material within a relatively wide range of values, thus widening its spectrum
of potential applications.
