Growth of iron clusters and change of magnetic property with carbonization of aromatic polyimide film containing iron complex Original Research Article

Carbonization process of a polyimide film containing iron of 0.6 at% (Fe-PIF) was investigated in detail by the measurements of XRD and mass magnetization. Because of the existence of fine iron particles as a catalyst, the Fe-PIF films heated in a temperature range from 700 to 1200°C showed turbostratic structure having interlayer spacing d002 of about 0.343–0.344 nm and mean crystallite size La ranging from 3.6 to 6.0 nm. The formation of α-Fe particles occurred mainly during carbonization process after pyrolysis of Fe-PIF. Mean crystallite size L110 of the α-Fe particles was evaluated to be 16 nm for the film heated at 800°C and 25 nm at 1200°C. Magnitudes of the magnetization for Fe-PIF and the heat-treated films were independent of magnetic field direction applied, indicating random orientation of the crystallographic directions of iron particles. Films heated at 800°C and below were superparamagnetic (SP), while those at 900°C and above ferromagnetic (FM). However, all the films were found to have both magnetic components, SP and FM ones; 2.55 wt% (0.58 at%) and 0.0007 wt% (0.00016 at%) for Fe-PIF and 6.56 wt% (1.48 at%) and very small, but still present, for the 1200°C treated Fe-PIF, respectively. Irreversible behavior was observed between zero field cooled magnetization and field cooled for the heat-treated films, but not for original Fe-PIF because of fine clusters of iron. The irreversible effect are attributed to freezing of spin directions of iron particles at low temperatures.