Stock material or miscellaneous articles – Composite – Of inorganic material
Reexamination Certificate
2001-01-26
2003-09-16
Kiliman, Leszek (Department: 1773)
Stock material or miscellaneous articles
Composite
Of inorganic material
C428S690000, C428S690000, C428S900000, C427S058000, C427S123000, C427S126300, C427S126400, C427S128000, C427S130000, C427S131000, C427S132000, C427S404000, C427S419100, C427S419200, C427S376100, C360S110000, C360S112000, C360S122000, C360S125330, C360S128000, C338S03200R, C324S252000
Reexamination Certificate
active
06620530
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to the general field of magnetic disk systems with particular reference to GMR based read heads and the stability of pinned layers therein.
BACKGROUND OF THE INVENTION
The principle governing the operation of magnetic read heads is the change of resistivity of certain materials in the presence of a magnetic field (magneto-resistance). In particular, 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 an increase in resistivity when the material is magnetized in a direction perpendicular to the easy axis, said increase 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.
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.
1
. In addition to a seed layer
12
on a substrate
11
and a topmost cap layer
17
, the key elements are two magnetic layers
13
and
15
, separated by a non-magnetic layer
14
. The thickness of layer
14
is chosen so that layers
13
and
15
are sufficiently far apart for exchange effects to be negligible (the layers do not influence each other's 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
13
and
15
are magnetized in opposite directions and a current is passed though them along the direction of magnetization (such as direction
18
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
13
to
15
(or vice versa). However, once these electron ‘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 the layers
13
and
15
is permanently fixed, or pinned. In
FIG. 1
it is layer
15
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 by over coating with a layer of antiferromagnetic material, or AFM, (layer
16
in the figure). Layer
13
, 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 of a magnetic disk).
The structure shown in
FIG. 1
is referred to as a top spin valve because the pinned layer is at the top. It is also possible to form a ‘bottom spin valve’ structure where the pinned layer is deposited first (immediately after the seed and pinning layers). In that case the cap layer would, of course, be over the free layer.
As discussed above, the pinned layer (typically CoFe or similar ferromagnetic material) in the spin valve structures has to be exchange-biased by an AFM material. When pinned by MnPt or NiMn (AFM materials with high blocking temperature), the pinned layers usually display large anisotropy. The anisotropy field, Hck, is comparable to the pinning field Hpin, both these parameters being distributed over a range of values. These features result in pinned layer loop open and instability. This problem is more severe for the NiCr or NiFeCr seeded SVs in comparison to Ta seeded SVs.
It is also known that SVs made of a synthetic anti-parallel pinned layer (SyAP) can significantly reduce the loop open in the pinned layer. The pinning strength of a SyAP SV is much higher than that of the regular single SV. Typically, the device contains two anti-parallel layers AP
1
and AP
2
(AP
2
being the layer closest to the AFM). These two layers are then coupled together through a layer of Ru and rotate coherently. This causes the Hck effect from AP
2
to be greatly reduced. While this approach is a definite improvement on the state of the art, the devices tend to exhibit loop opens (hysteresis) and are susceptible to damage from soft ESD (electrostatic discharges).
It is possible for a device to be subjected to an ESD event during manufacturing. During such an event, the sensor temperature rises and there is also an induced magnetic field acting on the pinned layer, due to the large ESD current which is often as high as 10-50 mA. ESD damage can be categorized as:
a. Excessive temperature rise during the ESD event—the head resistance increases and is permanently damaged due to inter-diffusion and cannot be recovered. We refer to this as “hard” ESD
b. The temperature rise is too low for significant inter-diffusion to occur and head resistance does not increase. However, the ESD induced magnetic field may be counter to the pinned layer magnetization and cause pinned layer magnetization rotation, resulting in signal loss, scattering of device properties etc. For most of these cases, it is possible to recover layer rotation. We refer to this as “soft” ESD. A key aspect of this problem is that if there is no loop open in the R-H curves, “soft” ESD will cause less damage to the head.
A routine search of the prior art was conducted. The following publications of interest were found:
1. S. Mao et al, “NiMn-pinned spin valves with high pinning field made by ion beam deposition” Appl. Phys. Lett 69(23)(1996)3593.
2. H. Kishi et al, “Study of exchange-coupled bias field in NiFe/PdPtMn thin films” IEEE Trans. Magnetics. V32(5)(1996)3380
3. M. Saito et al, “PtMn single and dual spin valves with synthetic ferrimagnet pinned layers”, J. Appl. Phys. V85(8)(1999)4928
4. M. Saito et al, “PtMn spin valve with synthic ferrimagnet free and pinned layers” J. Appl. Phys. 87(2000)6974
5. C. Horng et al.“Low field annealing for the spin valves biased by synthetic antiferromagnets”. Application Ser. No. 09/458,727, filed Dec. 13, 1999.
And the following patent references of interest:
U.S. Pat. No. 5,751,521 (Gill) shows a synthetic ferrimagnetic layers (e.g., Ru spacer). U.S. Pat. No. 5,856,897 (Mauri; shows a dual SV sensor with Ru spacers. U.S. Pat. No. 5, 408,377 (Gurney et al. and U.S. Pat. No. 6,134,090 (Mao et al.) show related sensors.
SUMMARY OF THE INVENTION
It has been an object of the present invention to provide a spin valve structure that has greater pinned layer stability and reduced pinning reversal relative to similar devices of the prior art.
A further object of the present invention to provide a spin valve structure that exhibits a minimum amount of loop opening in its hysteresis curve.
Another object of the invention has been to provide a spin valve that is highly suitable for use in high density recording.
Still another object of the invention has been to provide a process for the manufacture of said spin valve and pinned layer.
These objects have been achieved by a using a modified pinned layer that consists of two cobalt iron layers separated by a layer of ruthenium, iridium, or rhodium. A key feature of the invention is that this spacer layer is significantly thinner (typically 3-4 Angstroms) than similar layers in prior art structures. Normally, when such thin spacer layers are used, annealing fields in excess of 20,000 Oersted are needed to cause the two cobalt iron layers to become antiparallel. The present invention, howev
Horng Cheng T.
Ju Kochan
Li Min
Liao Simon H.
Tong Ru Ying
Ackerman Stephen B.
Headway Technologies Inc.
Kiliman Leszek
Saile George O.
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