Noise-after-write has been reported in thin film heads, although its source or sources have not yet been identified. To discover these noise sources, I observed conversions of magnetic domain structure after writing with an electron microscope. The magnetic domain observations involved lock-in image processing for back-scattered electron contrast (type II), using a JEOL 2000FX II 200 kV electron microscope. Thin-film heads were excited by a 10 mAp-p, 100 kHz driving current. The magnetic domains in the upper yokes of the heads were observed after 60 mAp-p write operations. The head with the highest noise probability, at 5.2Ã10Â¿2, exhibited walls radiating outward from the back-gap closure, which were due to stress. The magnetic domain structure was severely deformed after writing. The head with the lowest noise probability, at 1.6Ã104, exhibited a normal closure domain configuration, with very little conversion. These results indicate that noise-after-write is generated mainly near the backgap closure.