Basic properties of AC noise

The noise characteristics of ac demagnetized particulate media with special emphasis on correlation function analysis are discussed. The recording samples were specially prepared iron-oxide media coated on glass substrates by means of a spin coating technique. The media were nonoriented, which meant that the particles in the dried coating constituted a virgin state which is different from the ac erased state. The noise spectra for the virgin state and the ac demagnetized state appear to be equal for wavelengths longer than about 20 μm; for shorter wavelengths, the difference is about 2 dB. A correlation function measurement was made for the long wavelength region. The noise voltage was digitized and stored in a computer. After the measurements, the correlation between different noise voltages was computed. As a result, no correlation was found at all (ρ = 0) between virgin noise and ac noise. When the sample was ac demagnetized again, there was a well-defined correlation (ρ = 0.32) between successive ac noise levels. This can be explained on the basis of the ac demagnetization process.