Hardware Reference
In-Depth Information
conducting layer) perform this function.
The GMR effect was discovered in 1988 in crystal samples that were exposed to high-powered
magnetic fields (1,000 times the fields used in hard disk drives). Scientists Peter Gruenberg of Julich,
Germany, and Albert Fert of Paris discovered that large resistance changes were occurring in
materials composed of alternating thin layers of various metallic elements. The key structure in GMR
materials is a spacer layer of a nonmagnetic metal between two layers of magnetic metals. One of the
magnetic layers is
pinned
, which means it has a forced magnetic orientation. The other magnetic layer
is
free
, which means it is free to change orientation or alignment. Magnetic materials tend to align
themselves in the same direction. So if the spacer layer is thin enough, the free layer takes on the same
orientation as the pinned layer. What was discovered was that the magnetic alignment of the free
magnetic layer would periodically swing back and forth from being aligned in the same magnetic
direction as the pinned layer to being aligned in the opposite magnetic direction. The overall
resistance is relatively low when the layers are in the same alignment and relatively high when in
opposite magnetic alignment.
Figure 8.6
shows a GMR read element.
Figure 8.6. Cross-section of a GMR head.
When a weak magnetic field, such as that from a bit on a hard disk, passes beneath a GMR head, the
magnetic orientation of the free magnetic layer rotates relative to that of the other and generates a
significant change in electrical resistance due to the GMR effect. Because the physical nature of the
resistance change was determined to be caused by the relative spin of the electrons in the different
layers, GMR heads are often referred to as spin-valve heads.
IBM announced the first commercially available drive using GMR heads (a 16.8GB 3 1/2-inch drive)
in December 1997. Since then, GMR heads have become standard in most 3.5-inch and 2.5-inch
drives.
In 2007 Hitachi developed a giant magneto-resistive head using perpendicular current that supports
areal densities of up to 1Tbits/sq. inch or more. These are called current perpendicular-to-the-plane
giant magneto-resistive heads or CPP-GMR for short, and began appearing in drives starting in 2011.
Head Sliders
The term
slider
describes the body of material that supports the actual drive head. The slider is what
actually floats or slides over the surface of the disk, carrying the head at the correct distance from the
medium for reading and writing. Older sliders resemble a trimaran, with two outboard pods that float
along the surface of the disk media and a central “hull” portion that actually carries the head and
read/write gap.
Figure 8.7
shows a typical mini slider. Note that the actual head, with the read/write
