US Patent 7630154 Hard disk inspection method and system

A method an apparatus for testing the surface of hard disk platters having vertically oriented magnetic domains is disclosed. According to the method of the present invention, all of the magnetic domains on the surface of the disk to be tested are oriented in the same direction, so that the magnetic field intensity adjacent to the surface is ideally uniform. The surface is then scanned using a read head to identify perturbations in the magnetic field intensity which correlate to surface defects.