Dynamic magnetic information storage or retrieval – Head – Magnetoresistive reproducing head
Reexamination Certificate
2001-04-23
2003-09-02
Miller, Brian E. (Department: 2652)
Dynamic magnetic information storage or retrieval
Head
Magnetoresistive reproducing head
Reexamination Certificate
active
06614630
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to the fabrication of a giant magnetoresistive (GMR) magnetic field sensor in a magnetic read head, more specifically to a spin valve type of GMR sensor of the top spin valve type having an ultra-thin free layer.
2. Description of the Related Art
Early forms of magnetic read heads decoded magnetically stored data on media such as disks and tapes by making use of the anisotropic magnetoresistive effect (AMR) in magnetic materials such as permalloy. This effect is the change in the electrical resistance, r, of certain magnetic materials in proportion to the angle between the direction of their magnetization and the direction of the current through them. Since changing magnetic fields of moving magnetized media, such as magnetically encoded tapes and disks, will change the direction of the magnetization in a read head, the resistance variations of the AMR effect allows the information on such encoded media to be sensed and interpreted by appropriate circuitry.
One shortcoming of the AMR effect was the fact that it produced a maximum fractional resistance change, Dr/r also called MR, the magnetoresistive ratio, (where Dr is the change in resistance between the magnetic material subjected to its anisotropy field, H
k
, and the material subjected to zero field), which was only on the order of a few percent. This made the sensing process difficult to achieve with accuracy.
In the late 1980's and early 1990's the phenomenon of giant magnetoresistance (GMR) was discovered and soon applied to read head technology. The GMR effect derives from the fact that thin layers of ferromagnetic materials, between 20 and 80 Å (Angstroms) when separated by even thinner (20-30 Å) layers of electrically conductive but non-magnetic materials, will acquire ferromagnetic (parallel spin direction of the layers) or antiferromagnetic states (antiparallel spin direction of the layers) by means of exchange interactions between the layers. As a result of spin dependent electron scattering as electrons crossed the layers, the magnetoresistance of such layered structures was found to be significantly higher in the antiferromagnetic state than the ferromagnetic state and the fractional change in resistance was much higher than that found in the AMR of individual magnetic layers.
Shortly thereafter a version of the GMR effect called spin valve magnetoresistance (SVMR) was discovered and implemented. In the SVMR version of GMR, two ferromagnetic layers such as CoFe or NiFe are separated by a thin layer of electrically conducting but non-magnetic material such as Cu. One of the layers has its magnetization direction fixed in space or “pinned,” by exchange coupling with an antiferromagnetic layer directly deposited upon it. The remaining ferromagnetic layer, the unpinned or free layer, can rotate its magnetization vector in response to small variations in external magnetic fields such as are produced by moving magnetic media, (which do not affect the magnetization direction of the pinned layer). This rotation of one magnetization relative to the other then produces changes in the magnetoresistance of the three layer structure. Other forms of the spin valve utilize an antiferromagnetic layer for the pinned layer. Nepela et al. (U.S. Pat. No. 5,717,550) disclose a spin valve magnetoresistive sensor in which a buffer layer is deposited on a soft adjacent layer (ie. a layer with reduced magnetic coercivity), wherein the soft adjacent layer provides longitudinal biasing for the magnetoresistive sensor element whereas the buffer layer provides an enhanced exchange coupling to an antiferromagnetic layer deposited over the fabrication for the purpose of fixing the magnetization direction of the pinned layer. Barnard et al. (U.S. Pat. No. 5,919,580) provides a spin valve device containing a chromium and aluminum antiferromagnetic layer which serves as a pinning layer for a magnetoresistive ferromagnetic layer.
The combination of a free magnetic layer overlapping a pinned magnetic layer is utilized in forming magnetic tunnel junction (MTJ) devices, which are devices that can sense magnetic fields or can be used as memory cells in magnetic random access (MRAM) arrays. The similarities in structure, if not in function, between spin valve sensors and magnetic tunnel junction devices is worth noting. Gallagher et al. (U.S. Pat. No. 5,841,692) discloses an MTJ wherein a top electrode stack includes a free ferromagnetic layer which is not pinned by exchange coupling. The said top electrode stack is formed over a tunneling layer which, in turn, is formed over a pinned ferromagnetic layer whose magnetic moment is pinned by interfacial exchange coupling to a lower antiferromagnetic layer. The pinned electrode stack in the MTJ device is a laminate of 20 nm Pt/4 nm NiFe/10 nm MnFe/8 nm NiFe, wherein the MnFe serves to antiferromagnetically couple the NiFe layers. Parkin (U.S. Pat. No. 5,764,567) discloses an MTJ device wherein a laminate comprising a seed layer, an antiferromagnetic layer, a pinned ferromagnetic layer, a tunnel junction layer and a free ferromagnetic layer are sandwiched between a pair of electrical lead layers. Further, Parkin (U.S. Pat. No. 5,936,293) provides an MTJ device with “hard”/“soft” ferromagnetic layers (ie. high coercivity/low coercivity) wherein adjacent ferromagnetic structures provide a transverse magnetic bias for the ferromagnetic layers that inhibits their ultimate demagnetization by motion of domain walls.
The spin valve structure has now become the implementation of choice in the fabrication of magnetic read head assemblies. However, the development of ultra-high recording densities, typically now in the range of >60 Gb/in
2
, has placed stringent requirements on the GMR read head (unlike any such similar restrictions on the MTJ devices described above) for it to be capable of resolving very high linear bit density (bits per inch, or BPI) and track density (tracks per inch, or TPI). As a consequence, GMR read head design has been pushed in the direction of narrower trackwidths and thinner free layers in order to maintain high signal output in spite of thinner tracks and reduced gap length. The MTJ devices discussed above do not share the burden of GMR read heads that require their constant evolution to keep up with increasing recording density. Consequently, the MTJ devices do not provide ultra-thin free ferromagnetic layers and corresponding methods for increasing the magnetoresistive ratio and output levels. Similarly, the read heads provided by Barnard et al and Nepala et al. cited above do not have free layers of the requisite thinness required by the increased recording densities.
Ultra-thin free layers, which are here defined as layers whose magnetic moments are equivalent to those of a ferromagnetic layer whose moment is less than that of a 20 Å thickness of Co
90
Fe
10
or 36 Å of Ni
80
Fe
20
, as well as increased MR ratios are both effective paths to higher signal strengths when reading increased recorded densities. In a related patent application (Ser. No. 09/992,517) the present applicants have already disclosed a novel bias compensation layer (BCL) that allows thinner read head structures while not affecting their electrical output. In another related patent application (Ser. No. 09/570,017 now issued as U.S. Pat. No. 6,521,507), the present applicants have disclosed novel material compositions for a single top spin valve read head with a synthetic antiferromagnet pinned layer that provides an enhanced degree of specular reflection and a larger difference in the mean-free-paths of spin up and spin down electrons. The present invention provides novel and advantageous attributes that build upon the inventions disclosed in these related applications yet go appreciably beyond them.
SUMMARY OF THE INVENTION
A first object of this invention is to provide a method for forming a giant magnetoresistive (GMR) sensor element with a specularly reflecting top spin valve structu
Horng Cheng
Li Min
Tong Ru-Ying
Ackerman Stephen B.
Headway Technologies Inc.
Miller Brian E.
Saile George O.
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