Dynamic magnetic information storage or retrieval – Head – Magnetoresistive reproducing head
Reexamination Certificate
2000-11-27
2002-09-24
Tupper, Robert S. (Department: 2652)
Dynamic magnetic information storage or retrieval
Head
Magnetoresistive reproducing head
Reexamination Certificate
active
06456467
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to the field of electronic data storage and retrieval systems. In particular, the present invention relates to a novel lamination of materials for use as a shield of a magnetoresistive element of a transducing head.
In an electronic data storage and retrieval system, a transducing head typically includes a reader portion having a magnetoresistive (MR) sensor for retrieving magnetically-encoded information stored on a magnetic disc. MR sensors fall generally into two broad categories: (1) anisotropic magnetoresistive (AMR) sensors and (2) giant magnetoresistive (GMR) sensors. AMR sensors generally having a single MR layer formed of a ferromagnetic material. The resistance of the MR layer varies as a function of cos
2
&agr;, where a is the angle formed between the magnetization vector of the MR layer and the direction of the sense current flowing in the MR layer.
GMR sensors have a series of alternating magnetic and nonmagnetic layers. The resistance of GMR sensors varies as a function of the spin-dependent transmission of the conduction electrons between the magnetic layers separated by the nonmagnetic layer and the accompanying spin-dependent scattering which takes place at the interface of the magnetic and nonmagnetic layers and within the magnetic layers.
GMR sensors using two layers of ferromagnetic material separated by a layer of nonmagnetic electrically-conductive material are generally referred to as spin valve (SV) sensors. The layers of a SV sensor include a nonmagnetic spacer layer positioned between a ferromagnetic pinned layer and a ferromagnetic free layer. A magnetization of the pinned layer is fixed in a predetermined direction, typically normal to an air bearing surface (ABS) of the SV sensor, while a magnetization of the free layer rotates freely in response to an external magnetic field. An antiferromagnetic material is typically exchange coupled to the pinned layer to fix the magnetization of the pinned layer in a predetermined direction, although other means of fixing the magnetization of the pinned layer are available.
GMR sensors using two layers of ferromagnetic material separated by a layer of nonmagnetic electrically-insulating material are generally referred to as spin-dependent tunnel junction (STJ) sensors. The layers within a STJ sensor include an ultra-thin tunnel barrier layer positioned between a ferromagnetic pinned layer and a ferromagnetic free layer. As in the SV sensor, a magnetization of the pinned layer is fixed in a predetermined direction, typically normal to an air bearing surface of the STJ sensor, while a magnetization of the free layer rotates freely in response to an external magnetic field. An antiferromagnetic material is typically exchange coupled to the pinned layer to fix the magnetization of the pinned layer in a predetermined direction, although other means of fixing the magnetization of the pinned layer are available.
Magnetic flux from the surface of the disc causes rotation of the magnetization vector of a sensing layer of the MR sensor, which in turn causes a change in electrical resistivity of the MR sensor. The change in resistivity of the MR sensor can be detected by passing a current through the MR sensor and measuring a voltage across the MR sensor. External circuitry then converts the voltage information into an appropriate format and manipulates that information as necessary.
A response curve of the MR sensor compares the voltage across the MR sensor to the magnetic flux received from the disc by the sensor. This response curve has both linear and non-linear portions, of which it is preferred that the MR sensor operate along the linear portions. To force the MR sensor to operate along the linear portions, the sensor is magnetically biased at a biasing point that is located along the linear portion of the response curve.
During a read operation, the top and bottom shields ensure that the MR sensor reads only the information stored directly beneath it on a specific track of the magnetic medium or disc by absorbing any stray magnetic fields emanating from adjacent tracks and transitions.
Within a typical shield exists a plurality of magnetic domains separated from each other by a plurality of magnetic domain walls. Each domain has a magnetization that is oriented in a direction different than the magnetization of all adjacent domains. The application of an external magnetic field, either during manufacture or from an adjacent track or transition of the magnetic storage medium during operation, to the shield can cause the magnetization of each of the domains within that shield to rotate, thereby causing the domains to move. Because of the random nature of the domain wall location, the domain walls generally do not return to their original location after the external magnetic field is removed.
The shields exert stray magnetic fields on the MR sensor. These stray fields are accounted for when the MR sensor is biased. As the domain walls move, however, these stray magnetic fields change, thus changing the bias point of the MR sensor, as well as the response of the MR sensor to signals emanating from the rotating disc. The overall result is noise during the read operation.
This noise due to movement of domain walls is particularly acute in bottom shields for GMR sensors. The processing of a GMR sensor, either a SV or a STJ sensor, typically requires the magnetic annealing of an antiferromagnetic layer to pin the magnetization of its pinned layer. This magnetic anneal is performed while the bottom shield is present and can cause a realignment of the bottom shield anisotropy, giving rise to a highly undesirable domain configuration.
A reduction of domain wall density from magnetic shields can be achieved by use of a lamination consisting of alternating ferromagnetic and nonmagnetic spacer films. In such shields, a coupling will occur between the ferromagnetic layers, providing an alternate flux closure path that minimizes domain wall formation. Others have proposed that hard bias or antiferromagnetic layers can be used to bias the shields of MR sensors, thus resulting in a controlled domain structure within the shield.
BRIEF SUMMARY OF THE INVENTION
The present invention introduces a novel thin film structure suitable for use as a shield for a read element of a transducing head. The thin film structure of the present invention has an first ferromagnetic layer, a second ferromagnetic layer and a first decoupling layer. The first decoupling layer is positioned between the first ferromagnetic layer and the second ferromagnetic layer. An easy axis of the first ferromagnetic layer is substantially parallel to an easy axis of the second ferromagnetic layer. The first decoupling layer causes a magnetization of the first ferromagnetic layer to be substantially antiparallel to a magnetization of the second ferromagnetic layer.
A first alternate embodiment of the present invention includes a plurality of thin film structures as described above and a plurality of spacer layers interspersed between each of the plurality of thin film structures. In this first alternate embodiment, pairs of ferromagnetic layers on opposite sides of each of the plurality of spacer layers are substantially antiparallel magnetizations.
A second alternate embodiment of the present invention includes a third ferromagnetic layer and a fourth ferromagnetic layer. The third ferromagnetic layer is positioned between the first ferromagnetic layer and the first decoupling layer, and has a magnetization substantially parallel to the magnetization of the first ferromagnetic layer. The fourth ferromagnetic layer is positioned between the second ferromagnetic layer and the first decoupling layer, and has a magnetization substantially parallel to the magnetization of the second ferromagnetic layer.
A third alternate embodiment of the present invention includes a plurality of thin film structures in accord with the second alternate embodiment of the present invention and a plurality of spacer layers int
Amin Nurul
Mao Sining
Kinney & Lange , P.A.
Seagate Technology LLC
Tupper Robert S.
Watko Julie Anne
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