Synthetic anti-parallel/parallel/pinned layer spin valve

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.

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, in the SyAP SV, AP 1 and AP 2 (two anti-parallel layers with AP 2 being the layer close to the AFM) are 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, there are still loop opens in some cases (see later).

A routine search of the prior art was conducted. The following references of interest were found:

In U.S. Pat. No. 6,122,150, Gill shows a SV with two anti-parallel CoFe layers separated by a layer of Ru and capped with NIO. Iwasaki et al. in U.S. Pat. No. 5,738,946 show a SV MR with a NiCr protective film, while in U.S. Pat. No. 6,115,224, Saito discloses a SV MR process. Additionally, Iwasaki et al. (U.S. Pat. No. 5,702,832), Nakamoto et al. (U.S. Pat. No. 5,936,810), and Iwasaki et al. (U.S. Pat. No. 5,780,176) all show related MR processes and structures.

It has been an object of the present invention to provide a spin valve structure that has greater pinned layer robustness than is found in spin valves of the existing known art.

It has been an object of the present invention to provide a spin valve structure that exhibits a minimum amount of open looping in its hysteresis curve.

Another object of the invention has been to provide spin valve that is highly suitable for use in high density recording.

A further object of the invention has been to provide a process for the manufacture pf said spin valve and pinned layer.

These objects have been achieved by a using a modified pinned layer that is a laminate of five layers—a first layer of cobalt-iron, a layer of ruthenium, a second layer of cobalt-iron, a layer of nickel-chromium, and a third layer of cobalt-iron. The second layer of cobalt-iron should be about twice the thickness of the third cobalt-iron layer. The sum of the second and third cobalt-iron layer thicknesses may be greater or smaller than the thickness of the first cobalt-iron layer. A process for manufacturing the structure is also disclosed.

FIG.1 is a cross-section of a spin valve structure formed according to the teachings of the prior art.

FIG. 2 is a cross-section of a spin valve structure formed according to the teachings of the present invention.

FIGS. 3 a and 4 a are R-H plots for SV structures made according to our earlier practices.

FIGS. 3 b and 4 b are R-H plots for SV structures made according to the practices disclosed in the present invention.

In a recent application by Horng et al (application Ser. No. 09/495,348, filed Feb. 1, 2000), it is shown that single SV having pinned layer laminated with a very thin (3-5 Å) NiCr layer greatly reduces the loop opening. The two laminated pinned layers in the single SV were defined as P 1 and P 2 , in which P 2 is the layer closest to the AFM material. The coupling between P 1 and P 2 is ferromagnetic and the strength is in the order of 1 erg/cm 2 . This structure was named a parallel-pinned (PP) structure. The reason that this structure can reduce loop opens is similar to SyAP structure. The coupling between P 1 and P 2 makes the rotation coherent and hence the Hck from P 2 is reduced. However, the pinning field of the PP structure, as in the single spin valve, is not high enough for high-density recording heads.

The above two structures: SyAP and PP, show that separation of AP 1 (or P 1 ) from AP 2 (or P 2 ) by a nonmagnetic layer, either through antiferromagnetic coupling or ferromagnetic coupling, can greatly reduce the loop open (hysteresis). P 1 takes advantage of both the increased pinning strength and the loop open reduction. In the present invention, we disclose a structure that combines the strong points of these two structures.

The new structure, which we call SyAPP, is:

Seed/free/Cu/AP 1 /Ru/AP 2 a /NiCr/AP 2 b /AFM/CAP

For comparison, SyAP is:

Seed/free/Cu/API/Ru/AP 2 /AFM/CAP

and PP is:

Seed/free/Cu/P 1 /NiCr/P 2 /AFM/CAP

Referring now to FIG. 2, we show there a schematic cross-section of a top spin valve structure that embodies the present invention. A comparison with FIG. 1 of the prior art will reveal that the key novel feature of the invention is the replacement of the homogeneous pinned layer 15 with a multi-layer laminate 35 of 5 layers (layers 21 - 25 ).

We now provide a description of a process for manufacturing the spin valve of the present invention. It is important to note that, since the exchange forces that determine the behavior of structures such as spin valves operate over very short distances, the various thicknesses specified in the claims are critical, as opposed to optimal. A structure having the same sequence of layers, but one or more thicknesses outside the claimed ranges, will likely have quite different characteristics from the structures disclosed by the present invention.

The process begins with the provision of suitable substrate 11 (as seen in FIG. 2 ). Examples of suitable substrate materials include aluminum oxide and silicon oxide. Next, seed layer 12 of NiCr is deposited onto substrate 11 . This magneto-resistance enhancing layer has a thickness between about 45 and 60 Angstroms and a composition of between about 55 and 65 atomic % nickel and between about 35 and 45 atomic % chromium. Layer 12 , as well as all subsequent layers, was deposited by means of DC-magnetron sputtering. The use of this material (instead of, for example, tantalum) is essential if the structure is to exhibit specular reflection at the interfaces.

The next layer to be laid down is free layer 13 . This is depicted in FIG.2 as a single layer but, in actuality, it is made up of a layer of nickel-iron, between about 20 and 60 Angstroms thick, and a composition of between about 77 and 83 atomic % nickel and between about 17 and 23 atomic % iron, in contact with layer 12 , followed by a layer of cobalt-iron which is between about 5 and 15 Angstroms thick and having a composition of between about 85 and 95 atomic % cobalt.

Non-magnetic layer 14 , typically copper between about 16 and 25 Angstroms thick, is then deposited over free layer 13 . Then follows a choice of two sub-processes:

1 st Sub-process (AP 1 is Thicker than Ap 2 )

On the non-magnetic layer 14 is deposited layer 21 of CoFe between about 20 and 30 Angstroms thick with 25 Angstroms being preferred. This corresponds to AP 1 . This is followed by ruthenium layer 22 , between about 6.5 and 8.5 Angstroms thick with 7.5 Angstroms being preferred. Layer 23 corresponds to AP 2 a and is CoFe deposited onto ruthenium layer 22 to a thickness between about 10 and 15 Angstroms, with 14 Angstroms being preferred. On layer 23 is deposited NiCr laminating layer 24 which has a thickness of between about 3 and 7 Angstroms, with 5 Angstroms being preferred. Finally (for the 1 st sub-process) layer 25 is deposited onto laminating layer 24 . Layer 25 corresponds to AP 2 b and is CoFe with a thickness between about 5 and 8 Angstroms, with 7 Angstroms being preferred. Note that AP 2 a is about twice as thick as AP 2 b.

2 nd Sub-process (AP 2 is Thicker than API)

On the non-magnetic layer 14 is deposited layer 21 of CoFe between about 15 and 23 Angstroms thick with 19 Angstroms being preferred. This corresponds to AP 1 . This is followed by ruthenium layer 22 , between about 6.5 and 8.5 Angstroms thick with 7.5 Angstroms being preferred. Layer 23 corresponds to AP 2 a and is CoFe deposited onto ruthenium layer 22 to a thickness between about 12 and 16 Angstroms, with 14 Angstroms being preferred. On layer 23 is deposited NiCr laminating layer 24 which has a thickness of between about 3 and 7 Angstroms, with 5 Angstroms being preferred. Finally (for the 2 nd sub-process) layer 25 is deposited onto laminating layer 24 . Layer 25 corresponds to AP 2 b and is CoFe with a thickness between about 6 and 10 Angstroms, with 7 Angstroms being preferred. Note that AP 2 a is also about twice as thick as AP 2 b here.

At the conclusion of either sub-process, the main process also concludes with the deposition of MnPt layer 16 onto layer 25 , to a thickness between about 70 and 250 Angstroms, with 200 Angstroms being preferred, followed with the deposition of NiCr capping layer 17 to a thickness between about 30 and 50 Angstroms, with 50 Angstroms being preferred.

Experiments Done to Confirm Performance of Invention

The following SV structures (pinned layer underlined) were made. 1.     Ap1 > Ap2     case  :     AP1     AP2    SyAP  :     NiCr55 / NiFe65 / CoFe10 / Cu20 / CoFe25 / Ru / CoFe20 _ / MnPt200 / NiCr50    AP1     AP2a     AP2b    SyAPP  :     NiCr55 / NiFe65 / CoFe10 / Cu20 / CoFe25 / Ru / CoFe14 / NiCr5 / COFe7 _ / MnPt200 / NiCr50

2. AP 1 <AP 2 case:

SYAP:NiCr55/Cu10/NiFe30/NiCr5/NiFe30/CoFe3/Cu21/ CoFe19/Ru7.5/CoFe21 /MnPt200/NiCr50

SYAPP: NiCr55/Cu10/NiFe30/NiCr5/NiFe30/CoFe3/Cu21/ CoFe19/Ru7.5/CoFe14/NiCr3/CoFe7 /MnPt200/NiCr50

It is noted AP 2 a is about twice as thick as AP 2 b.

FIGS. 3 b and 4 b show the experimental R-H curves for top SVs with AP 1 >AP 2 and AP 1 <AP 2 cases, respectively. For comparison, R-H curves for the “normal” SYAP were also measured and are shown in FIGS. 3 a and 4 a . From these figures it can be seen that the loop open was clearly reduced for the SyAPP SV. The AP 1 <AP 2 case shows more drastic effects of loop open reduction. This result is in good agreement with the coherent rotation model simulation results.

The above results demonstrate that SVs made of synthetic anti-parallel parallel-pinned layers yield much better pinned layer robustness. This improved pinned layer robustness will make the SyAPP spin valve most suitable for high density recording.

We note here that this type of SyAPP layer can also be applied to a bottom SV structure.

Finally, comparison with the SyAP SV showed that the GMR of the SyAPP SV did not degrade much.

While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention.