DC erasure and demagnetizing fields on written bits in high density longitudinal media

The DC erasure process in a CoCrTa medium used for 0.8 Gbit/in/sup 2/ recording is investigated and compared with a CoCrPtTa medium capable of supporting 5 Gbit/in/sup 2/. All-ones patterns up to 250 kfci were imaged in the remanent state after the application of in situ magnetic fields parallel to the tracks. For CoCrTa, the averaged MFM signal drops faster with field compared to the hulk DC demagnetization (DCD) curve, This is different from CoCrPtTa where the MFM signal fits the DCD curve for fields applied along the track direction. For both media, the normalized MFM signal reduction with field is independent of recording density indicating reduced average demagnetizing fields.