State-of-the-Art Magnetic Hard Disk Drives
there is noise originating from the upper cular to the direction of current flow. By layer, typically a cobalt alloy; a nonmag-
and lower layers, given by NU and NL , re- 1997, this technology was replaced by the netic spacer layer, typically copper; and a
spectively. The composite noise power N2 spin valve, or giant magnetoresistive second magnetic free layer, typically also a
may be expressed as (GMR), head. The term “spin valve” refers cobalt alloy. Current flowing in the mag-
to a structure where the relative orienta- netic layers becomes spin-polarized, and
N2 NU2 NU2 2cNUNL , (2) tion of the spins, or magnetization, in two the probability of electrons scattering as
adjacent layers controls the flow of current they move between the magnetic layers
where c describes the degree of cross- through the device. The name GMR refers depends on the relative orientation of the
correlation between the magnetization to the effect in general, since the magneto- magnetization of these layers. The resis-
in the layers.12 In the simplest case, where resistance was considered “giant” by com- tance is a minimum, R0, when the free
the two layers are antiparallel grain by parison with MR sensors. Early GMR layer and reference layer moments are
grain such that c –1, then the noise heads achieved 4% MR, but this value has parallel. Spin-dependent scattering in-
N NU – NL ; the noise of the lower layer steadily climbed to 15% today. The GMR creases as the layers deviate from parallel.
similarly subtracts from the upper layer, head is a complicated stack of ferromag- In general, if θ is the angle between the
giving a composite SNR that is independ- netic, antiferromagnetic, and nonmag- free and reference layer moments, the re-
ent of the lower-layer thickness. If the noise netic metals, the details of which we will sistance follows the form
of the two layers is uncorrelated (c 0), now discuss.
then the signals would subtract while the If one were able to look up at the read 1
R R0 R 1 cos , (3)
noise would add, yielding poor SNR. head from the media, the view would look 2
The implications of Equations 1 and 2 like Figure 3. The GMR sensor is sand-
can at times be counterintuitive. If the wiched between micrometer-thick mag- where ∆R is the maximum additional re-
lower layer could be designed to have no netic shield layers. These shields provide sistance due to GMR. The maximum resis-
noise (NL 0), then the SNR of the com- down-track spatial resolution by absorbing tance, R0 ∆R, is obtained when the
posite would be worse than the upper the magnetic flux from nearby media tran- moments are antiparallel (θ 180). Obvi-
layer on its own, since you subtract signal sitions. The shield-to-shield spacing is cur- ously, increasing the value of ∆R is a pri-
without subtracting noise. Conversely, the rently about 50 nm. This puts a significant mary concern in the development of spin
SNR of the composite structure can, in constraint on the materials that can be used valves.
principle, be improved by having poor in the GMR sensor, as will be discussed In a spin-valve read head, the magnetic
SNR media in the lower layer. That is, you later in this section. The sensor itself is moment of the reference layer points per-
can possibly subtract more noise than lithographically patterned to a width of pendicular to the medium surface. With-
signal, yielding an improved SNR for the approximately half the track width, W, out any field from the medium, the free
composite. This is the basic principle of which is about 100 nm today. As the track layer moment points perpendicular to this
acoustic noise-canceling headphones and width scales to smaller dimensions, this is direction (θ 90). As the head passes
partly explains the excellent performance pushing magnetic recording past semi- over a magnetic transition in the medium,
of AFC media where the lower layer is ex- conductor processing in terms of the the direction of the free layer moment ro-
pected to be quite noisy on its own.12 More smallest feature size. The third dimension tates, causing a change in resistance. The
generally, these ideas point toward general of the sensor, the stripe height, is dis- moment of the free layer is chosen such
directions where multiple coupled layers cussed at the end of this section. that θ makes only 10 deviations from
are used to design media that go beyond The basis of the GMR effect is contained 90 in response to the transition field. The
simple statistical averaging of grains. An in only three layers: a magnetic reference output signal is, then, fairly linear with
example of this approach is described in
References 5 and 6 for laminated AFC
media that combine both antiferromag-
netic and uncoupled layers.
GMR Heads
Read-back head technology13 has also
changed markedly since the original disk
drive, in which the same head was used
for both inductive writing (time-varying
current producing a time-varying field)
and inductive reading (changing flux in
the yoke due to the motion of the medium
past the head, resulting in an induced cur-
rent in the conductor wound around the
yoke). As areal density increased, the signal
from the recorded transitions decreased,
and a more sensitive detection scheme was
required. In the early 1990s, the magneto-
resistive, or MR, head was introduced. This
Figure 3. Transmission electron micrograph of a giant magnetoresistive spin-valve read
head is based on anisotropic magneto-
head, viewed as if looking up at the head from the media. The 120-nm-wide sensor is a
resistance in a single Permalloy (Ni80Fe20) multilayer stack. In addition to providing the sense current, the leads contain a magnetically
layer, which provides a change in resis- hard bias layer that applies a small magnetic field to the sensor. The magnetic shields
tance of about 2% as the magnetization in ensure that the sensor detects only the field from a single transition at a time. (Image
the layer rotates from parallel to perpendi- courtesy of P. Shang and W. Legg.)
MRS BULLETIN • VOLUME 31 • MAY 2006 381
