Hardware Reference
In-Depth Information
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
electricalresistance.Inthisdesign,aresistancechangeoccursintheNiFelayerastheMR
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
,theyaresmallerthanstandardMRheadsbut
aresonamedfortheGMReffectonwhichtheyarebased.Thedesignissimilar;however,
additionallayersreplacethesingleNiFelayerinaconventionalMRdesign.InMRheads,
a single NiFe film changes resistance in response to a flux reversal on the disk. In GMR
heads, two films (separated bya very thin copper 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 metal-
lic elements. The key structure in GMR materials is a spacer layer of a nonmagnetic met-
al 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
inthesamedirection.Soifthespacerlayeristhinenough,thefreelayertakesonthesame
orientation as the pinned layer. What was discovered was that the magnetic alignment of
the free magnetic layer wouldperiodically swingback andforthfrombeing aligned inthe
same magnetic direction asthe pinned layer tobeing aligned inthe opposite magnetic dir-
ection. 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.
