Slope-intercept analysis of M-H curves for samples with mixed ferromagnetic and paramagnetic regions

Rigid disks for magnetic recording usually employ an aluminum alloy substrate coated with a Ni-P film of 10–15 μm thickness prepared by electroless chemical deposition, followed by the ferromagnetic recording film deposited chemically or by sputtering. Depending on the bath composition and the method of preparation, it is possible for the Ni-P underlayer to consist of intermixed ferromagnetic and paramagnetic regions. Furthermore, the magnetic structure of the Ni-P underlayer may change significantly with subsequent heat treatments experienced during the sputter deposition of the ferromagnetic layer and other protective layers such as chromium and carbon. The presence of a ferromagnetic component in the Ni-P underlayer is, of course, undesirable and detrimental to the recording performance of the disk. Furthermore, in order to measure correctly the magnetic characteristics of the recording films, it is necessary to subtract out all the magnetic effects of the substrate and underlayer. Subtracting out a straight line M vs H curve can lead to incorrect results if the Ni-P underlayer has a significant ferromagnetic component. In this paper, we show that a good estimate of the magnitude of the ferromagnetic part of the underlayer can be obtained by extrapolating the slope of the raw M vs H curve of the Ni-P plus aluminum composite to the intercept at H=0 (slope intercept). Several examples of this are given for Ni-P films exposed to different heat treatments.