GMR Read-Write Heads Yield Data Storage Record
Staff -- Semiconductor International, 2/1/1998
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Scientists at IBM's Almeden Research Center (San Jose, Calif.)
recently achieved a data storage density of 11.6Gb/in.2 in a
laboratory demonstration using giant magnetoresistive (GMR) heads.
According to IBM, this doubles the density it achieved in December
1996.
Producing GMR heads capable of handling this density requires submicron lithography and uniform sputtering of thin films that are tens of atomic layers thick. Producing a merged read-write head for this density also requires that the lithography have a significant depth of field. GMR read heads are made of four thin films (Fig. 1), a sensing layer, a conducting spacer, a pinned layer and an exchange layer. The first three of these layers are all very thin, and all but the spacer are magnetic materials. The magnetic orientation of the pinned layer is fixed, held constant by the exchange layer. The sensing layer takes on the magnetic orientation that it senses from the disk. The conducting spacer is thin enough to allow a weak magnetic coupling between the pinned and sensing layers. The coupling is weak enough to allow the sensing layer to take the magnetic orientation that it senses from the disk, but strong enough to force the sensing layer orientation to be either parallel or anti-parallel to that in the pinned layer. The GMR read head acts as a spin valve for electrons, controlling their flow according to their quantum spin states (Fig. 2). In a magnetized material, conduction electrons have spins that are either parallel or anti-parallel to the magnetic orientation of the material. The electrons with the parallel spin have a higher mobility in the material than the ones with anti-parallel spin, so they experience a smaller resistance. |
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| When the magnetic orientations of the sensing and pinned layers oppose each other, conduction electrons will experience a higher resistance in one of the layers, resulting in a higher resistance for the device. When the orientations are in the same direction, some electrons experience a lower resistance in both layers, leading to a lower overall resistance. |
1. GMR head structure uses three thin films. |
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According to IBM's calculations, 10Gb/in.2 applications require a
read track width (on the disk) of 0.68 µm and a sensing layer
thickness of 0.040 µm. Submicron lithography is required to define a
read head small enough to read only one track at a time. Tight
control of the sputtering process is required not only to make the
sensor layer thickness uniform, but also to maintain compositional
uniformity.
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2. The changing magnetic orientation (M) in the sensing layer operates the "spin valve". |
| To achieve the proper level of magnetic coupling between the pinned and sensing layers, the thickness of the conducting layer must be between 10 and 30 atomic layers, with a precision of one atomic layer. | |
