Coating processes – Electrical product produced – Metal coating
Reexamination Certificate
2000-08-07
2003-02-11
Barr, Michael (Department: 1762)
Coating processes
Electrical product produced
Metal coating
C427S126300, C427S126400, C427S404000, C427S419100, C427S419200, C427S376100, C427S128000
Reexamination Certificate
active
06517896
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to the general field of GMR recording heads for magnetic disk systems with particular reference to design of the free layer.
BACKGROUND OF THE INVENTION
Read-write heads for magnetic disk systems have undergone substantial development during the last few years. In particular, older systems in which a single device was used for both reading and writing, have given way to configurations in which the two functions are performed by different structures. An example of such a read-write head is schematically illustrated in FIG.
1
. The magnetic field that ‘writes’ a bit at the surface of recording medium
15
is generated by a flat coil, two of whose windings
14
can be seen in the figure. The magnetic flux generated by the flat coil is concentrated within pole pieces
12
and
13
which, while being connected at a point beyond the top edge of the figure, are separated by small gap
16
. Thus, most of the magnetic flux generated by the flat coil passes across this gap with fringing fields extending out for a short distance where the field is still powerful enough to magnetize a small portion of recoding medium
15
.
The present invention is directed towards the design of read element
20
which can be seen to be a thin slice of material located between magnetic shields
11
and
12
(
12
doing double duty as a pole piece, as just discussed). The principle governing the operation of read sensor
20
is the change of resistivity of certain materials in the presence of a magnetic field (magneto-resistance). Most magnetic materials exhibit anisotropic behavior in that they have a preferred direction along which they are most easily magnetized (known as the easy axis). The magneto-resistance effect manifests itself as a decrease in resistivity when the material is magnetized in a direction perpendicular to the easy axis, said decrease being reduced to zero when magnetization is along the easy axis. Thus, any magnetic field that changes the direction of magnetization in a magneto-resistive material can be detected as a change in resistance.
It is widely known that the magneto-resistance effect can be significantly increased by means of a structure known as a spin valve. The resulting increase (known as Giant magneto-resistance or GMR) derives from the fact that electrons in a magnetized solid are subject to significantly less scattering by the lattice when their own magnetization vectors (due to spin) are parallel (as opposed to anti-parallel) to the direction of magnetization of the solid as a whole.
The key elements of a spin valve structure are shown in FIG.
2
. In addition to a seed layer
22
on a substrate
21
and a topmost cap layer
27
, these key elements are two magnetic layers
24
and
26
, separated by a non-magnetic layer
25
. The thickness of layer
25
is chosen so that layers
24
and
26
are sufficiently far apart for exchange effects to be negligible (i.e. the layers do not influence each others magnetic behavior at the atomic level) but are close enough to be within the mean free path of conduction electrons in the material. If, now, layers
24
and
26
are magnetized in opposite directions and a current is passed though them along the direction of magnetization (such as direction
28
in the figure), half the electrons in each layer will be subject to increased scattering while half will be unaffected (to a first approximation). Furthermore, only the unaffected electrons will have mean free paths long enough for them to have a high probability of crossing over from
24
to
26
(or vice versa). However, once these electrons ‘switch sides’, they are immediately subject to increased scattering, thereby becoming unlikely to return to their original side, the overall result being a significant increase in the resistance of the entire structure.
In order to make use of the GMR effect, the direction of magnetization of one of the layers
24
and
26
is permanently fixed, or pinned. In
FIG. 2
it is layer
24
that is pinned. Pinning is achieved by first magnetizing the layer (by depositing and/or annealing it in the presence of a magnetic field) and then permanently maintaining the magnetization with an undercoat of a layer of antiferromagnetic material, or AFM, (layer
23
in the figure). Layer
26
, by contrast,, is a “free layer” whose direction of magnetization can be readily changed by an external field (such as that associated with a bit at the surface
15
of a magnetic disk).
The structure shown in
FIG. 2
is referred to as a bottom spin valve because the pinned layer is at the bottom. It is also possible to form a ‘top spin valve’ structure where the pinned layer is deposited after the pinning layer.
Ultra-thin free layers as well as MR ratio are very effective to obtain high output spin valve GMR heads for over 30 Gb/in
2
magnetic recording. In general, magneto-resistive devices have a characteristic response curve such that their sensitivity initially increases with the applied field, then is constant with applied field, and then decreases to zero for even higher fields. It is therefore standard to provide a biasing field to keep the sensor operating in the linear range where it is also at its most sensitive. However, as the free layer thickness decreases, it becomes difficult to obtain a controllable bias point, high GMR ratio and good magnetic softness all at the same time. Synthetic antiferromagnets (SyAF) are known to reduce magneto-static fields in a pinned layer, but a large bias point shift due to sense current fields remains a problem for practical use of an ultra-thin free layer. To overcome this problem, the spin-filter spin valve (SFSV) was invented.
In a SFSV, the free layer is placed between the Cu spacer and an additional high-conductance-layer (HCL). SFSV reduces sense current fields in the free layer by shifting the sense current center toward the free layer, resulting in a smaller bias point shift by sense current fields. High GMR ratio is maintained even in the ultra-thin free layer because the HCL improves the mean free path of a spin-up electron while maintaining the mean free path difference between spin-up and spin-down electrons.
As discussed earlier, spin valve GMR heads may be either top or bottom types. The GMR sensor track is defined by a patterned longitudinal biasing layer in the form of two bias stripes. These are permanently magnetized in a direction parallel to the surface. Their purpose is to prevent the formation of multiple magnetic domains in the free layer. The most commonly used longitudinal bias for the bottom spin valve is with contiguous (abutted) junction hard bias. The problem with the abutted junction is the existence of a “dead zone” at the sensor ends. A MR sensor track defined by continuous spacer exchange bias (similar to that for the DSMR) does not have the “dead zone”. This may be critical for a very narrow track for ultra-high density recording application.
A routine search of the prior art was performed. The following references of interest were found. U.S. Pat. No. 5,637,235(Kim et al.) shows a SV with a capping layer. U.S. Pat. No. 5,896,252 (Kanai) shows a SV with a free magnetic layer composed of a CoFe and NiFe sublayers. while U.S. Pat. No. 5,648,885 (Nishioka et al.) teaches a SV with CoFe free layer.
SUMMARY OF THE INVENTION
It has been an object of the present invention to provide a spin-filter synthetic antiferromagnetic bottom spin valve that is suitable for ultra-high density magnetic recording applications.
Another object of the invention has been to provide suitable longitudinal biasing leads for this structure.
A further object of the invention has been to provide processes for the manufacture of these structures.
These objects have been achieved in a structure made up the following layers:
NiCr/MnPt/CoFe/Ru/CoFe/Cu/(free layer)/Cu/Ta or TaO. A key feature is that the free layer is made of thin CoFe plus a CoFe/NiFe composite layer in which CoFe is thinner than NiFe. Experimental data confirming the effectiveness of this structure is provid
Chen Mao-Min
Horng Cheng T.
Li Min
Tong Ru-Ying
Ackerman Stephen B.
Barr Michael
Headway Technologies Inc.
Saile George O.
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