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Browse Prior Art Database


IP.com Disclosure Number: IPCOM000029780D
Original Publication Date: 2004-Jul-12
Included in the Prior Art Database: 2004-Jul-12
Document File: 4 page(s) / 272K

Publishing Venue



A non-evasive method of using both the magnetic and thermal component of the readback signal of the magnetoresistive (MR) head to separate radial motion from head-to-disk clearance motion is disclosed.

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As the requirements for magnetoresistive (MR) recording heads to fly even closer to the disk surface in high performance disk drives, the need for reliable head-to-disk clearance (flyheight) estimation methods, pending head crash warning, and testing are becoming increasingly important. Current methods for estimating flyheight are based in magnetically writing a special signal on a track and analyzing the readback signal using Wallace's spacing loss formula. It has become clear that the magnetic flyheight estimations are especially sensitive to radial off-track. Thus there is the possibility that the magnetic flyheight estimation is contaminated by radial head motion on the disk surface.

       Lately, it has also been discovered that the thermal response of a MR head, using a fixed bias current, is a nonlinear function of the flyheight. Here, the cause of variation in the MR resistance is due to changes temperature in the MR element. The temperature changes are being caused by changes in spacing between the MR head and the disk surface. If the head-disk spacing decreases below nominal, the head cools and its resistance decreases (assuming a positive temperature coefficient) causing the voltage across the MR element to decrease. If the head-disk spacing increases above nominal, the head heats up and its resistance increases causing the voltage across the MR element to increase. The thermal response is a valuable property of the MR element, since it can be used to detect surface defects. A surface bump underneath the MR element will cause a cooling of the MR element and a negative-going peak in the thermal response, while a positive going peak in the thermal response indicates a pit in the disk surface. A thermal asperity (TA), which is a disk surface defect that physically strikes the MR element, will generate heat ad cause a large positive thermal response.

       It has also been recognized for some time that the magnetic and thermal spacing measurements contain features that are both synchronous and non-synchronous with respect to the disk rotation. Synchronous features, some of which may contribute to magnetic and thermal spacing measurements, are typically associated with characteristics on the disk like defects, surface waviness, coercivity, bad servo patterns, spindle and disk runouts. These events are occurring regularly with disk rotation and can be considered stationary in nature. There are also features that are not synchronous with respect to the disk rotation. These non-synchronous features are mainly caused by more randomly occurring resonances in the mechanical actuator structure, air turbulence on the disk surface, actuator servo instabilities, debris, etc.

       Present magnetic and thermal glide methods look at these synchronous and non-synchronous features collectively and make specific analysis on the manufacturing line and predictive failure analysis (PFA) more difficul...