| 摘要 |
A computer disk of the type having a central drive hub comprised at least in part of ferromagnetic metal to clamp the disk to a magnet spindle drive is provided with a ring-shaped amorphous ferromagnetic surveillance marker between the hub flange and the disk substrate. The hub metal has a high permeability, e.g. >/=2,000 to concentrate the drive magnet field in the hub, when the disk is in the disk drive, thereby to eliminate fringe fields that would magnetize the marker. This is to allow the use of a low magnetic field intensity sensor in the sensor to detect the presence of the marker to enable recording of data on the disk. The hub metal also has a coercivity between the coercivity of the marker and the intensity of the magnetic field employed in the interrogation zone of an area surveillance portal to eliminate the effect of any residual magnetism on the hub metal caused by the drive magnet thereby to enhance the detectability of the marker by the portal.
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