Hardware Reference
In-Depth Information
Figure 8.5. Cross-section of an MR head.
The read element, which is the actual magneto-resistive sensor, consists of a nickel-ferrite (NiFe)
film separated by a spacer from a magnetically soft layer. The NiFe film layer changes resistance in
the presence of a magnetic field. Layers of shielding protect the MR sensor read element from being
corrupted by adjacent or stray magnetic fields. In many designs, the second shield also functions as
one pole of the write element, resulting in what is called a merged MR head. The write element is not
of MR design but is instead a traditional TF inductive head.
IBM's MR head design employs a Soft Adjacent Layer (SAL) structure, consisting of the MR NiFe
film, as well as a magnetically soft alloy layer separated by a film with high electrical resistance. In
this design, a resistance change occurs in the NiFe layer as the MR sensor passes through a magnetic
field.
As areal densities have increased, heads have been designed with narrower and thinner MR elements.
More recent designs have reduced the film width between the side contacts to as little as half a
micron or less.
Giant Magneto-Resistive Heads
In the quest for even greater density, IBM introduced a new type of MR head in 1997. Called
giant
magneto-resistive (GMR) heads
, they are smaller than standard MR heads but are so named for the
GMR effect on which they are based. The design is similar; however, additional layers replace the
single NiFe layer in a conventional MR design. In MR heads, a single NiFe film changes resistance in
response to a flux reversal on the disk. In GMR heads, two films (separated by a very thin copper
