Dynamic magnetic information storage or retrieval – Head – Magnetoresistive reproducing head
Reexamination Certificate
1999-11-18
2002-10-08
Cao, Allen (Department: 2652)
Dynamic magnetic information storage or retrieval
Head
Magnetoresistive reproducing head
Reexamination Certificate
active
06462919
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to magnetoresistive read sensors for use in magnetic read heads. In particular, the present invention relates to a spin valve head with antiferromagnetic exchange stabilization and method for forming such a spin valve.
A magnetic read head retrieves magnetically-encoded information that is stored on a magnetic medium or disc. The magnetic read head is typically formed of several layers that include a top shield, a bottom shield, and a read sensor positioned between the top and bottom shields. The read sensor is generally a type of magnetoresistive sensor, such as a giant magnetoresistive (GMR) read sensor. The resistance of a GMR read sensor fluctuates in response to a magnetic field emanating from a magnetic medium when the GMR read sensor is used in a magnetic read head and positioned near the magnetic medium. By providing a sense current through the GMR read sensor, the resistance of the GMR read sensor can be measured and used by external circuitry to decipher the information stored on the magnetic medium.
A common GMR read sensor configuration is the GMR spin valve configuration in which the GMR read sensor is a multi-layered structure formed of a ferromagnetic free layer, a ferromagnetic pinned layer and a nonmagnetic spacer layer positioned between the free layer and the pinned layer. The magnetization direction of the pinned layer is fixed in a predetermined direction, generally normal to an air bearing surface of the GMR spin valve, while a magnetization direction of the free layer rotates freely in response to an external magnetic field. An easy axis of the free layer is generally set normal to the magnetization direction of the pinned layer. The resistance of the GMR read sensor varies as a function of an angle formed between the magnetization direction of the free layer and the magnetization direction of the pinned layer. This multi-layered spin valve configuration allows for a more pronounced magnetoresistive effect than is possible with anisotropic magnetoresistive (AMR) read sensors, which generally consist of a single ferromagnetic layer.
Typically, the magnetization of the pinned layer is fixed in the predetermined direction by exchange coupling an antiferromagnetic layer to the pinned layer. The antiferromagnetic layer is positioned upon the pinned layer such that the antiferromagnetic layer and the free layer form distal edges of the GMR spin valve. The spin valve is then heated to a temperature greater than a Néel temperature of the antiferromagnetic layer. Next, a magnetic field oriented in the predetermined direction is applied to the spin valve, thereby causing the magnetization direction of the pinned layer to orient in the direction of the applied magnetic field. The magnetic field may be applied to the spin valve before the spin valve is heated to the temperature greater than the Néel temperature of the antiferromagnetic layer. While continuing to apply the magnetic field, the spin valve is cooled to a temperature lower than the Neel temperature of the antiferromagnetic layer. Once the magnetic field is removed from the spin valve, the magnetization direction of the pinned layer will remain fixed, as a result of the exchange with the antiferromagnetic layer, so long as the temperature of the spin valve remains lower than the Neel temperature of the antiferromagnetic layer.
The free layer of a spin valve sensor must be stabilized against the formation of edge domain walls because domain wall motion results in electrical noise, which makes data recovery impossible. A common way to achieve stabilization is with a permanent magnet abutted junction design. Permanent magnets have a high coercive field (i.e., are hard magnets). The field from the permanent magnets stabilizes the free layer and prevents edge domain formation, and provides proper bias.
However, there are several problems with permanent magnet abutted junctions. To properly stabilize the free layer, the permanent magnets must provide more flux than can be closed by the free layer. This undesirable extra flux stiffens the edges of the free layer so that the edges cannot rotate in response to flux from the media, and may also cause shield saturation which adversely affects the ability of the sensor to read high data densities. The extra flux from the permanent magnets may produce multiple domains in the free layer and may also produce dead regions which reduce the sensitivity of the sensor. The junction between the permanent magnet and the various layers must be carefully engineered to minimize the stray flux as well as to minimize the junction resistance. Also, a junction of dissimilar metals can cause unwanted strain in the sensor. The free layer will respond to the strain unless the magnetostriction is exactly zeroed. Another disadvantage of permanent magnet abutted junctions is the nature of hard magnetic materials, which are multi-domained. Variation in grain size and shape leads to a distribution of domain coercivity. Lower coercivity domains may rotate when subjected to external fields. Such a grain near the edge of the free layer could cause domain wall formation and failure.
Tabs of antiferromagnetic material or “exchange tabs” have also been used to stabilize the free layer of magnetic sensors. Exchange tabs are deposited upon the outer regions of the free layer and are exchange coupled thereto. Functions of the exchange tabs include pinning the magnetization of the outer regions of the free layer in the proper direction, preventing the formation of edge domains and defining the width of an active area of the free layer by preventing free layer rotation at the outer regions of the free layer.
There are several advantages to the use of exchange tabs rather than permanent magnet abutted junctions. There is no junction to produce stray magnetic flux or junction resistance. Also, the lack of a junction of abutted, dissimilar metals makes it less likely that high strain will be produced within the sensor. And control is maintained over the anisotropy of the free layer regardless of how narrow the width of the active area is made.
The use of antiferromagnetic exchange tabs in AMR type sensors has been disclosed in U.S. Pat. No. 4,663,685, entitled “MAGNETORESISTIVE READ TRANSDUCER HAVING PATTERNED LONGITUDINAL BIAS”, U.S. Pat. No. 4,713,708, entitled “MAGNETORESISTIVE READ TRANSDUCER”, and U.S. Pat. No. 5,753,131, entitled “MAGNETORESISTIVE DEVICE AND MANUFACTURING METHOD THEREOF”. Longitudinal bias in AMR type sensors is also discussed in U.S. Pat. No. 4,785,366, entitled “MAGNETORESISTIVE READ TRANSDUCER HAVING PATTERNED ORIENTATION OF LONGITUDINAL BIAS”.
The use of antiferromagnetic exchange tabs in spin valve type sensors has been disclosed in U.S. Pat. No. 5,206,590, entitled “MAGNETO RESISTIVE SENSOR BASED ON THE SPIN VALVE EFFECT”, and U.S. Pat. No. 5,949,623, entitled “MONOLAYER LONGITUDINAL BIAS AND SENSOR TRACKWIDTH DEFINITION FOR OVERLAID ANISOTROPIC AND GIANT MAGNETORESISTIVE HEADS”.
Not all antiferromagnetic materials are suitable for use as exchange tabs in spin valve type sensors. Materials such as MnFe and TbCo are too corrosive for head production. MnFe and similar materials also have relatively low blocking temperatures, which is undesirable because the pinned regions of the free layer may become unpinned if the temperature of the sensor is raised above the blocking temperature of the exchange tabs during operation. MnFe and similar materials also have relatively low coupling constants, which results in weaker exchange coupling and higher side readings. Materials such as Fe
2
O
3
only increase the coercivity of the ferromagnetic layer instead of the pinning field.
The prior art does not disclose an exchange tab structure that makes use of a synthetic antiferromagnet, which provides an increased pinning field and reduced side reading. Furthermore, current methods for forming sensors with exchange tabs are inadequate. A major difficulty in manufacturing sensors with exchange tabs using conventional methods i
Amin Nurul
Gao Zheng
Li Jin
Mack Anthony M.
Mao Sining
Cao Allen
Kinney & Lange P. A.
Seagate Technology LLC
Watko Julie Anne
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