US 4,843,316 AGranted

Nondestructive M-H hysteresis testers for magnetic discs for computer disc drives

AssigneeHewlett-Packard Company

InventorVictor W. Hesterman

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Abstract

The manufacture of digital magnetic recording discs for computer disc drives requires the monitoring and control of the M-H hysteresis loop properties of the magnetic film deposited on the disc substrate. Several methods exist for measuring this M-H hysteresis loop, but they all have serious disadvantages. One method consists of cutting samples and measuring them with a vibrating sample magnetometer. This is a destructive test and requires a lot of time per sample. Another method uses the Kerr-rotation of polarized light. However, it samples only the mangetization of the surface and cannot determine the magnetic thickness of the film. A third method magnetizes the entire disc and samples a large region along a diameter. This method cannot distinguish between the top and bottom films of the disc, and cannot resolve circumferential variations of th M-H loop properties. The method of this invention solves these problems by measuring a small part of one surface of the disc nondestructively using a special balanced head having two air gaps. It is fast, requiring only about 2 minutes per measurement and has good reproducability.

Claims5

Dependent claims

An apparatus for measuring the M-H magnetic hysteresis loop characteristics of a magnetic data storage disc under test including a substrate and a magnetic film layer mounted to the substrate, where M is a magnetization characteristic of the magnetic film and H is a magnetic field intensity characteristic of a magnetic field inducing the magnetization M of the magnetic film, the apparatus comprising:

a. a generator means,
i. for generating and applying a magnetic field having a known intensity H to a magnetic disc to be characterized,
ii. positioned close enough to the disc so that the generated magnetic field H induces a desired magnetization M in the disc, the magnetization including
(1) a desired component induced in the magnetic film, and
(2) an undesired component in the form of a plurality of eddy currents induced in the substrate, the eddy currents in turn inducing undesirable eddy magnetic fields that interefere with a correct determination of M;
b. a sensor means
i. for determining the magnetization M of the magnetic film alone,
ii. including a cancelling means for removing the effects of the eddy magnetic fields,
iii. positioned to magnetically interact with the magnetic field associated with the magnetization of the magnetic film, and
iv. magnetically isolated from interacting with the magnetic field produced by the generator means;
c. a regulating means, coupled for varying the applied magnetic field intensity H generated by the generator means, the generator means, the sensor means and the regulator means thereby cooperating to determine the M-H data points in sufficient number to define the hysteresis loop associated with the particular magnetic disc under test, thereby characterizing the M and the H characteristics of the disc.

8 more elements

Dependencies are inferred from the claim text.