Read head with dual tunnel junction sensor

Details Read head with dual tunnel junction sensor Read head with dual tunnel junction sensor

1999-07-23

2001-08-14

Tupper, Robert S. (Department: 2652)

Dynamic magnetic information storage or retrieval

Head

Magnetoresistive reproducing head

Reexamination Certificate

active

06275363

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a read head with a dual tunnel junction sensor and more particularly to a dual tunnel junction sensor that produces a double tunnel junction effect and has a ferromagnetic free layer structure that has improved linear bit density.
2. Description of the Related Art
The heart of a computer is an assembly that is referred to as a magnetic disk drive. The magnetic disk drive includes a rotatable magnetic disk, write and read heads that are suspended by a suspension arm above the disk and an actuator that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The read and write heads are mounted on a slider that has an air bearing surface (ABS). The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk adjacent the ABS to cause the slider and the write and read heads to ride on an air bearing a slight distance from the surface of the rotating disk. During rotation of the disk the write head writes magnetic bits of information (signal fields) to the disk and the read senses the magnetic bits (signal fields) from the disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
The write head includes a coil layer embedded in first, second and third insulation layers (insulation stack), the insulation stack being sandwiched between first and second pole piece layers. A magnetic gap is formed between the first and second pole piece layers by a write gap layer at an air bearing surface (ABS) of the write head. The pole piece layers are connected at a back gap. Current conducted to the coil layer induces a magnetic field across the gap between the pole pieces. This field fringes across the gap at the ABS for the purpose of writing the aforementioned magnetic bits in circular tracks on the rotating disk.
A typical sensor employed by recent read heads for sensing signal fields from the rotating magnetic disk is a spin valve sensor. The spin valve sensor includes a nonmagnetic spacer layer sandwiched between a ferromagnetic pinned layer and a ferromagnetic free layer. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. The magnetization of the pinned layer is pinned perpendicular to the ABS and the magnetic moment of the free layer is located parallel to the ABS but free to rotate in response to external magnetic fields. The magnetization of the pinned layer is typically pinned by exchange coupling with an antiferromagnetic layer. The thickness of the spacer layer is chosen to be less than the mean free path of conduction electrons through the sensor. With this arrangement, a portion of the conduction electrons is scattered in phase by the interfaces of the spacer layer with the pinned and free layers. When the magnetizations of the pinned and free layers are parallel scattering is at a minimum and when the magnetizations of the pinned and free layers are antiparallel, scattering is at a maximum. Changes in scattering alter the resistance of the spin valve sensor in proportion to cos &thgr;, where &thgr; is the angle between the magnetizations of the pinned and free layers. When a sense current is conducted through the spin valve sensor in a direction parallel to surface planes of the layers resistance changes cause potential changes that are detected and processed as playback signals by the processing circuitry.
Another type of sensor is a tunnel junction sensor which receives a tunneling current perpendicular to the surface planes of the layers. The tunneling junction sensor includes a nonmagnetic nonconductive spacer layer between a ferromagnetic pinned layer and a ferromagnetic free layer. The spacer layer, which is an oxide, is thin enough that electron tunneling occurs between the free and pinned layers. The resistance of the sensor is spin dependent which means that the resistance of the sensor changes as a function of the relative orientation of the magnetic moments of the free and pinned layers. The pinned layer is located on and exchanged coupled to an antiferromagnetic pinning layer which pins a magnetic moment of the pinned layer in a first direction which is typically perpendicular to the ABS. The free layer has a magnetic moment which is free to rotate in response to signal fields from the rotating disk. A tunneling current I
T
tunnels through the oxide spacer layer as a function of sin
2
&thgr;, where &thgr; is the angle between the magnetic moments of the pinned and free layers. When the magnetic moments of the free and pinned layers are parallel the resistance to the tunneling current is at a minimum, and when these moments are antiparallel the resistance to the tunneling current is at a maximum. Accordingly, as the tunneling current I
T
is conducted through the tunnel junction sensor increases and decreases in the resistance of the sensor causes potential changes that are processed by the aforementioned processing circuitry as playback signals. The processing circuitry employs these potential changes to produce readback signals. The details of tunnel junction are described in a commonly assigned U.S. Pat. No. 5,650,958 to Gallagher et al., which is incorporated by reference herein.
Efforts continue to increase the magnetoresistive coefficient dr/R of spin valve and tunnel junction sensors. One way to increase the magnetoresistive coefficient dr/R is to increase the linear bit density of the sensor. A need exists to provide a tunnel junction sensor that produces asymmetric read back signals.
SUMMARY OF THE INVENTION
I have provided a tunnel junction sensor which produces a dual tunnel junction effect for increasing the magnetoresistive coefficient dr/R of the sensor. This has been accomplished by locating first and second pinned layer structures on each side of a free layer structure wherein the first pinned layer structure is separated from the free layer structure by an oxide first barrier layer and the second pinned layer structure is separated from the free layer structure by an oxide second barrier layer. Accordingly, the tunneling current is modified twice as it tunnels through the first and second barrier layers in response to resistance changes caused by orientations of the first and second pinned layers with respect to the free layer structure. Because of the additive effect of the resistance changes on each side of the free layer the present tunnel junction sensor is referred to herein as a dual tunnel junction sensor. In the several embodiments of the read head the free layer structure is an antiparallel (AP) coupled free layer structure with an AP coupling layer located between first and second AP free layers. Since the first and second AP free layers are strongly antiparallel coupled they rotate together in response to signal fields from a rotating magnetic disk.
The linear bit density of the AP coupled free layer structure is improved over the typical single free layer. In the AP coupled free layer structure one of the AP free layers is thicker than the other AP layer resulting in a net magnetic moment of the free layer structure which is designed to match the magnetization of the signal field from the rotating magnetic disk. This permits the thicknesses of the AP free layers of the AP coupled free layer structure to be optimized for in-phase scattering of the conduction electrons through the sensor. In contrast, a single free layer with a thickness that matches the magnetization of the free layer with the magnetization of high density signal fields from the rotating magnetic disk will be too thin to provide a thickness which optimizes in-phase scattering of conduction electrons through the sensor.
In a tunnel junction sensor the free layer structure is influenced by demagnetization and ferromagnetic coupling fields from the pinned layer structures.

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