Ferromagnetism discovered on heat-treating the aromatic polyimide film Kapton
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A commercially available aromatic polyimide film Kapton H 25-m 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.
I. INTRODUCTION
Ferromagnets of organic polymers containing only light elements (C, H, N, O, S, etc.) attract much attention from both physical and chemical viewpoints, and a movement to search for the ferromagnets of pure organic compounds has been quite active.1,2 However, most organic magnetic materials exhibit paramagnetic behavior at high temperatures where the correlation in the spin alignment is extended only within small regions of radical molecules in the materials. Therefore, reports on the polymers exhibiting ferromagnetism at room temperature were very few, and the stability and reproducibility have been important problems.3–5 Some carbonized materials showing ferromagnetism at temperatures up to room temperature have also been obtained by pyrolysis of organic polymers containing no metal elements at relatively low pyrolysis temperatures around 1000 °C.6–8 The pyrolyzed materials were regarded to be disordered carbons on the basis of microunits with low packing and many defects connected by a random network. Recently, we investigated the change in magnetization of Kapton H by heat treatment.9 Kapton H is a commercially available aromatic polyimide film, which exhibits
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2000
http://journals.cambridge.org
J. Mater. Res., Vol. 17, No. 8, Aug 2002
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the usual diamagnetism as a polymer.9,10 The heat treatments were made at temperatures of every 100 °C up to 1800 °C in inert atmosphere. A paramagnetic behavior was observed clearly for the 500–1200 °C treated films. Especially, a large magnetization due to Curie paramagnetism was found at 5 K for film heated at 500 °C. The total effective spin (spin quantum number) S of the paramagnetic component was evaluated to be about 5/2 for the 500 °C treated film and was about 1/2 for other heattreated ones by fitting the component t
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