Browse Prior Art Database

Thermomagnetic Recording Medium for a Direct-Overwrite Recording System

IP.com Disclosure Number: IPCOM000036216D
Original Publication Date: 1989-Sep-01
Included in the Prior Art Database: 2005-Jan-28
Document File: 3 page(s) / 79K

Publishing Venue

IBM

Related People

Gambino, RJ: AUTHOR [+2]

Abstract

Direct-overwrite means that an individual data record can be changed without erasing and rewriting the entire track containing the record. Magneto-optic recording, as presently practiced, does not provide a means for direct overwrite. An external magnetic field is applied over a large area and a focused laser beam is used to heat a small area to lower the switching field. The material within the heated spot cools in the external field which determines its direction of remanent magnetization - down or up, representing a binary 1 or 0. The time required to switch the external field is too long to allow the polarity of the field to be used to determine the direction of magnetization when writing on any given track. Present practice is to leave the field in one direction (e.g.

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Thermomagnetic Recording Medium for a Direct-Overwrite Recording System

Direct-overwrite means that an individual data record can be changed without erasing and rewriting the entire track containing the record. Magneto- optic recording, as presently practiced, does not provide a means for direct overwrite. An external magnetic field is applied over a large area and a focused laser beam is used to heat a small area to lower the switching field. The material within the heated spot cools in the external field which determines its direction of remanent magnetization - down or up, representing a binary 1 or 0. The time required to switch the external field is too long to allow the polarity of the field to be used to determine the direction of magnetization when writing on any given track. Present practice is to leave the field in one direction (e.g., up) and the laser on CW to erase a track completely - magnetize it in one direction (up). This means that the track must pass under the head two times, once to locate and read the data and the second time to clear the track. Then, during the third pass, under the head, the laser is used in pulse mode with the field direction reversed (down) to generate the ones in the new data set.

This invention makes it possible to use the power and/or duration of the laser pulse to determine whether a one or a zero is written rather than the direction of the external field. This is accomplished by thermally controlling the exchange coupling between the storage layer and a permanently magnetized exchange bias layer. The power and duration of the laser pulse can be controlled by fast, low inductance semiconductor switching circuits. This makes it possible to use a "look ahead" means to determine the state of an approaching bit along the track and select the appropriate laser pulse to switch the bit when it passes under the focal point of the optical head. The medium consists of three thin film magnetic layers on a transparent substrate, as shown in Fig. 1. Layer 1, the storage layer is a ferromagnet with Curie temperature Tc1 . It is deposited directly on the transparent substrate for good thermal contact. Layer 2, called the exchange coupling layer, is a ferromagnet, or an antiferromagnet which is deposited directly on the storage layer for good exchange coupling. The coupling layer has a Curie (or Neel) temperature Tc2 . Layer 3, the bias layer, is deposited directly on layer 2 for exchange coupling. The bias layer is a ferromagnet with Curie Temperature Tc3 and coercive field Hc3 . The storage and exchange bias layers have uniaxial perpendicular easy axis anisotropy so that the magnetization is normal to the film plane. Ferromagnets can also be used for any of the three layers. The Curie temperatures have the relationship that Tc3> Tc2> Tc1> ambient temperature. Also, Hc3 must be much larger than any magnetic fields the medium will be subjected to during operation so that layer 3 acts like a permanent...