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

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 line iron particles as a catalyst, the Fe-PIF films heated in a temperature range from 700 to 1200 degreesC showed turbostratic structure having interlayer spacing d(002) of about 0.343-0.344 nm and mean crystallite size L-a ranging from 3.6 to 6.0 nm. The formation of alpha -Fe particles occurred mainly during carbonization process after pyrolysis of Fe-PIF. Mean crystallite size L-110 of the alpha -Fe particles was evaluated to be 16 nm for the film heated at 800 degreesC and 25 nm at 1200 degreesC. 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 degreesC and below were superparamagnetic (SP), while those at 900 degreesC 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 degreesC treated Fe-PIF, respectively. Irreversible behavior was observed between zero field cooled magnetization and field cooled fur 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. (C) 2001 Elsevier Science Ltd. All rights reserved.