Dual, synthetic spin valve sensor using current pinning

Details Dual, synthetic spin valve sensor using current pinning Dual, synthetic spin valve sensor using current pinning

1999-01-21

2002-07-09

Cao, Allen (Department: 2652)

Dynamic magnetic information storage or retrieval

Head

Magnetoresistive reproducing head

Reexamination Certificate

active

06418000

ABSTRACT:

TECHNICAL FIELD
The present invention relates generally to magnetic sensors for reading information signals recorded in a magnetic medium and, more particularly, to a magnetoresistive read sensor based on the spin valve effect, and most particularly, to such a sensor which utilizes improved pinning in a dual spin valve configuration.
BACKGROUND ART
A large portion of the data storage in today's computers is done on magnetic media such as magnetic disks. Data is presented to a computer by huge quantities of bits (ones and zeroes) and stored on disks where each bit is represented by a transition which causes an applied magnetic field. In order to read the value of any given bit, a sensor able to detect changes in the applied magnetic field is required.
To this end, a sensor that changes electrical resistance in response to a magnetic field, called a magnetoresistive (MR) sensor, is employed. Most sensors utilize the anisotropic magnetoresistive (AMR) effect where a read element resistance varies in proportion to the square of the cosine of the angle between the magnetization in the read element and the direction of a sense current flowing through the read element. Data is read by the sensor from magnetic transitions recorded in the media. The magnetic field, resulting from a transition, causes a change in the direction of the magnetization in the read element. The new magnetization direction changes the resistance of the read element with a corresponding change in the sense current or voltage.
Newer sensors, which are more sensitive to smaller recorded transitions on higher density media, are starting to become more commonly used. These sensors use a larger form of magnetoresistance called the giant magnetoresistance (GMR) effect. The GMR effect produces a magnetoresistance that, for selected combinations of materials is greater in magnitude than that of the AMR effect. The GMR effect occurs in multilayer thin films of alternating ferromagnetic and nonferromagnetic metals. The resistance of a GMR film changes according to the cosine of the angle between the magnetization of the ferromagnetic (FM) layers.
A subset of the GMR devices is the spin valve in which two ferromagnetic layers, a “free” layer and a “pinned” layer, are used as explained in B. Dieny, et al. “Giant Magnetoresistance in Soft Ferromagnetic Multilayers”, Physical Review B, Vol. 43, No. 1, Jan. 1, 1991, pp. 1297-1300 and Dieny, et al. U.S. Pat. No. 5,206,590. When the magnetization in the two layers are aligned, the resistance is at a minimum. When the magnetization are anti-aligned, the resistance is at a maximum. The resistance varies as the cosine of the angle between the magnetizations and is independent of the direction of current flow. The magnetization of the pinned layer is held in place by depositing it next to a layer of antiferromagnetic (AFM) material with a resulting exchange coupling of the two layers. The free layer magnetization is free to rotate in response to the field from the disk. In this way, the magnetization swings between being parallel (low resistance state) to anti-parallel (high resistance state) as the head flies over recorded magnetic transitions on the disk. The resulting change in electrical resistance arising from the GMR effect is sensed and the magnetic information on the disk is transformed into electrical signals. Commonly used metallic AFM materials are platinum manganese (PtMn), iron manganese (FeMn), nickel manganese (NiMn), iridium manganese (IrMn), and nickel oxide (NiO).
In the past, GMR sensors using the spin valve effect had many problems. Some problems are directly related to the presence of the AFM material present in the pinned layers of the spin valve sensor. For example, when heated above a material dependent temperature (the blocking temperature), as occurs during surges in current flow through the sensor or a momentary contact of the read element with the media, the AFM material loses the ability to pin, or depins, the pinned layers. When the pinned layers lose their fixed orientation, the spin valve effect ceases to operate, and the sensor no longer functions. The same problem sometimes occurs because the AFM material has a tendency to corrode during manufacturing.
Additional problems occur because the magnetostatic field from the pinned layers tends to bias the free layer in an undesirable way, making readings from the sensor unreliable. It has also been found that the AFM material requires a buffer layer to maximize exchange coupling. However, the buffer layer and the AFM layer tend to shunt current which decreases the resistance and therefore the response of the device.
Thus, there have been many problems and solutions have been long sought. However, a solution has long eluded those skilled in the art.
DISCLOSURE OF THE INVENTION
The present invention provides a symmetric, or dual, spin valve sensor consisting of outer ferromagnetic pinned layers which are pinned by a current induced magnetic field. One of the outer pinned layers is a synthetic anti-ferromagnet consisting of two ferromagnetic layers separated by a thin space layer. The spacer thickness is set so the ferromagnetic layers are tightly anti-ferromagnetically exchange coupled; i.e. the magnetization of the layers are held in opposite directions. The free layer is separated from the pinned layers by conducting spacer layers. The current is centered on the free layer which minimizes the effect of the current induced field on the free layer.
The present invention further provides simplicity of design for a dual spin valve sensor.
The present invention further provides a reliable spin valve which is not subject to thermal asperities.
The present invention further provides a spin valve MR sensor in which the free layer is not subject to current induced magnetic fields.
The above and additional advantages of the present invention will become apparent to those skilled in the art from a reading of the following detailed description when taken in conjunction with the accompanying drawings.


REFERENCES:
patent: 5206590 (1993-04-01), Dieny et al.
patent: 5287238 (1994-02-01), Baumgart et al.
patent: 5465185 (1995-11-01), Heim et al.
patent: 6074743 (2000-06-01), Araki et al.
patent: 6175476 (2001-01-01), Huai et al.
patent: 6181533 (2001-01-01), Pokhil
patent: 6181534 (2001-01-01), Gill
Dieny, B. et al. “Giant Magnetoresistance in Soft Ferromagnetic Multilayers”, Physical Review B, vol. 43, No. 1, Jan. 1, 1991, pp. 1297-1300.

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