  The inherent existence of multi phases in iron oxide nanostructures
highlights the significance of them being investigated deliberately to
understand and possibly control the phases. Here, the effects of annealing at
250 0C with a variable duration on the bulk magnetic and structural properties
of high aspect ratio bi-phase iron oxide nanorods with ferrimagnetic Fe3O4 and
antiferromagnetic alpha-Fe2O3 is explored. Increasing annealing time under a
free flow of oxygen enhanced the alpha-Fe2O3 volume fraction, and improved the
crystallinity of the Fe3O4 phase, identified in changes in the magnetization as
a function of annealing time. A critical annealing time of approximately 3
hours maximized the presence of both phases, as observed via an enhancement in
the magnetization and an interfacial pinning effect. This is attributed to
disordered spins separating the magnetically distinct phases which tend to
align with the application of a magnetic field at high temperatures. The
increased antiferromagnetic phase can be distinguished due to the field-induced
metamagnetic transitions observed in structures annealed for more than 3 hours
and was especially prominent in the 9-hour annealed sample. Our controlled
study in determining the changes in volume fractions with annealing time will
enable precise control over phase tunability in iron oxide nanorods, allowing
custom-made phase volume fractions in different applications ranging from
spintronics to biomedical applications.
