Measurement of out-of-plane demagnetization in thin film media

A method has been developed to measure the thickness of a magnetic coating on a particulate tape. This involves a determination of the demagnetizing field for the film by one of three methods including (1) transverse ac susceptibility (chi/_t), (2) switching field measurement perpendicular to the applied field direction after the technique of Flanders and Shtrikman (1962) and (3) in-plane and out-of-plane hysteresis loop closure. The question arises as to the applicability of these measurements to thin film media. In fact, although the same principles apply, only the second method will give a satisfactory result. In case (1), transverse susceptibility measurement for commercial cobalt based sputtered films show virtually no reversible peaks. This is because the general expression for the transverse susceptibility contains a term dependent on k₂ = K₂/K₁ and the orientation of grains. Since k₂ = 0.279 for cobalt, the grain orientation in a sputtered disk will cause a broadening of the transverse susceptibility peak to such an extent that it is immeasurable. This paper will further demonstrate the inapplicability of methods (1) and (3) for thin films and show a series of measurements using method (2) on a number of commercial disks to demonstrate its applicability.