Ferromagnetism discovered on heat-treating the aromatic polyimide film Kapton

A commercially available aromatic polyimide film Kapton H 25-μm thick was heat-treated at temperatures between 490 and 540 °C in a nitrogen flow. Magnetization was measured as a function of magnetic field at 5 and 300 K and function of temperature in a field of 1 T. Diamagnetic and paramagnetic components were observed for all heat-treated films. Ferromagnetism was discovered even at 300 K in the films heat-treated at 490–520 °C. The saturation magnetization, coercive force, and residual magnetization for the 520 °C treated film were 0.059 J T⁻¹ kg⁻¹, 0.004 T, and 9 × 10⁻⁴ J T⁻¹ kg⁻¹, respectively, at 300 K. The ferromagnetism has been maintained 5 months after. Original Kapton H and the heated films were found to contain no metallic elements. The ferromagnetism should be caused by a long-range magnetic spin ordering of unpaired electrons located on slightly decomposed imide molecules with defects or on intermediates with free radicals formed by thermal decomposition. The ordering is probably established three dimensionally throughout the heat-treated films with a structural regularity similar to that of the original Kapton H.