DC modulation noise and demagnetizing fields in thin metallic media

Measurements of noise excited in magnetic recording media by a head energized by direct current yield a peak noise when the total field in the medium equals the remanent coercivity and opposes the original sense of medium saturation. The total field is the head field and a demagnetizing field stemming from a pseudotransition pinned under the head gap. This phenomenon has been studied experimentally by examining a thickness series of thin-film media with a wide range of demagnetizing ratios and by self-consistent calculations of this magnetizing process. Measurements of replay spectra were used to calibrate the record head. The demagnetizing field is about 19% of the remanent coercivity for a demagnetizing ratio of 767 nm. It is argued that careful calibration of this noise measurement yields an accurate in situ technique to monitor disk media coercivity or head efficiency variability