High Hc pinned self-pinned sensor

A self pinned magnetoresistive sensor having an anitparallel coupled pinned layer structure including a high coercivity (high Hc) layer of TbCo.

FIELD OF THE INVENTION

The present invention relates to giant magnetoresistive (GMR) sensors and more particularly to a novel pinning structure for a current perpendicular to plane (CPP) GMR sensor.

BACKGROUND OF THE INVENTION

The heart of a computer is an assembly that is referred to as a magnetic disk drive. The magnetic disk drive includes a rotating magnetic disk, write and read heads that are suspended by a suspension arm adjacent to a surface of a rotating magnetic 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 directly located 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. When the slider rides on the air bearing, the write and read heads are employed for writing magnetic impressions to and reading magnetic impressions from the rotating 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 gap is formed between the first and second pole piece layers by a gap layer at an air bearing surface (ABS) of the write head and the pole piece layers are connected at a back gap. Current conducted to the coil layer induces a magnetic flux in the pole pieces which causes a magnetic field to fringe out at a write gap at the ABS for the purpose of writing the aforementioned magnetic impressions in tracks on the moving media, such as in circular tracks on the aforementioned rotating disk.

In recent read head designs a spin valve sensor, also referred to as a giant magnetoresistive (GMR) sensor, has been employed for sensing magnetic fields from the rotating magnetic disk. The sensor includes a nonmagnetic conductive layer, hereinafter referred to as a spacer layer, sandwiched between first and second ferromagnetic layers, hereinafter referred to as a pinned layer and a 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 air bearing surface (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 by the interfaces of the spacer layer with each of the pinned and free layers. When the magnetizations of the pinned and free layers are parallel with respect to one another, scattering is minimal and when the magnetizations of the pinned and free layer are antiparallel, scattering is maximized. Changes in scattering alter the resistance of the spin valve sensor in proportion to cos θ, where θ is the angle between the magnetizations of the pinned and free layers. In a read mode the resistance of the spin valve sensor changes proportionally to the magnitudes of the magnetic fields from the rotating disk. When a sense current is conducted through the spin valve sensor, resistance changes cause potential changes that are detected and processed as playback signals.

A spin valve sensor is characterized by a magnetoresistive (MR) coefficient that is substantially higher than the MR coefficient of an anisotropic magnetoresistive (AMR) sensor. For this reason a spin valve sensor is sometimes referred to as a giant magnetoresistive (GMR) sensor. When a spin valve sensor employs a single pinned layer it is referred to as a simple spin valve. When a spin valve employs an antiparallel (AP) pinned layer it is referred to as an AP pinned spin valve. An AP spin valve includes first and second magnetic layers separated by a thin non-magnetic coupling layer such as Ru. The thickness of the spacer layer is chosen so as to antiparallel couple the magnetizations of the ferromagnetic layers of the pinned layer. A spin valve is also known as a top or bottom spin valve depending upon whether the pinning layer is at the top (formed after the free layer) or at the bottom (before the free layer).

The spin valve sensor is located between first and second nonmagnetic electrically insulating read gap layers and the first and second read gap layers are located between ferromagnetic first and second shield layers. In a merged magnetic head a single ferromagnetic layer functions as the second shield layer of the read head and as the first pole piece layer of the write head. In a piggyback head the second shield layer and the first pole piece layer are separate layers.

Sensors can also be categorized as current in plane (CIP) sensors or as current perpendicular to plane (CPP) sensors. In a CIP sensor, current flows from one side of the sensor to the other side parallel to the planes of the materials making up the sensor. Conversely, in a CPP sensor the sense current flows from the top of the sensor to the bottom of the sensor perpendicular to the plane of the layers of material making up the sensor. In a CPP sensor design, the magnetic shields usually double as electrical leads for supplying a sense current to the sensor. Therefore, in CPP sensor design, the shields/leads contact the top and bottom of the sensor, and the space between the shields defines the length of a bit of data.

The ever increasing demand for data storage density and data rate have increasingly pushed the limits of data storage designs. Recently in efforts to overcome such limits, engineers and scientists have focused on the use of perpendicular recording. In a perpendicular recording system a write pole emits a highly concentrated magnetic field that is directed perpendicular to the surface of the medium (eg. the disk). This field in turn magnetizes a localized portion of the disk in a direction perpendicular to the surface of the disk, thereby creating a bit of data. The resulting flux travels through the disk to a return path having a much larger area than the area in which the bit was recorded. The increased interest in perpendicular recording has lead to an increased interest in current perpendicular to plane (CPP) sensors, which are particularly suited to use in perpendicular recording.

Ever increasing demands for increased data density and data rate have also pushed sensor designs to decrease the size of a bit of data in order to fit more bits onto a given length of data track. This requires shrinking the distance between the shields of the sensor to decrease the length of the data bits that can be read by the sensor. One method used to reduce this length between shields (or gap height) has been to eliminate the antiferromagnei (AFM) pinning layer used to maintain the magnetization of the pinned layer. As discussed above, sensor designs have used a layer of AFM material to set the pinning of the pinned layer of a sensor. This saves a great deal of gap budget, because in order for an AFM layer to effectively set the pinning of a pinned layer, the AFM must be constructed very thick. In fact the AFM is usually much thicker than many of the other layers of the sensor combined.

In order to eliminate the AFM layer, sensors have been recently designed as “self pinned” sensors, wherein a pair of antiparallel pinned layers having a strong positive magnetostriction are pinned by a combination of positive magnetostriction and compressive forces present in the sensor. One problem that has arisen as a result of such self pinning designs is that the pinned layers can be prone to flipping. The positive magnetostriction tends to keep the magnetization of AP pinned layers oriented in a desired orientation perpendicular to the ABS of the sensor. However, if the sensor undergoes a stress, such as a heat spike or a mechanical deformation during head disk contact, the pinned layers can momentarily loose their magnetostriction induced pinning and can change orientation, an event referred to as amplitude flipping. This renders the sensor unusable.

Therefore, there remains a need for a design that can reduce the gap height (distance between shields/leads) such as by eliminating the use of an AFM layer, while also achieving robust pinning. Such a design would preferably be useable in a CPP sensor design since such sensors have promising futures for use in future perpendicular recording systems.

SUMMARY OF THE INVENTION

The present invention provides a self pinned sensor having improved pinned layer robustness. The sensor includes a free layer, a pinned layer structure and a spacer or barrier layer disposed between the free layer and the pinned layer structure. The pinned layer structure includes a first magnetic layer (AP1) and a second layer (AP2), which are antiparallel coupled across a non-magnetic, antiparallel coupling layer. The AP1layer is constructed of a magnetic material, which can be for example CoFe. The AP2layer is constructed of a first sublayer comprising a magnetic material such as for example CoFe and a second sublayer that comprises TbCo.

The presence of the TbCo layer advantageously greatly increases the magnetic coercivity (Hc) of the AP2layer, providing substantial protection against amplitude flipping during a catastrophic event such as a head disk contact. As discussed above, in a self pinned head, pinning is maintained by a combination of high positive magnetostriction of the pinned layer materials and compressive stresses in the sensor which magnetize the magnetic layers of the pinned layer perpendicular to the ABS as desired.

During an event such as a head disk contact, the sensor can be momentarily strained (deformed), which might momentarily eliminate the compressive stresses that maintain the desired pinning. In such case a self pinned sensor could be prone to amplitude flipping during that momentary strain. The present invention advantageously prevents amplitude flipping during such an event by adding substantial coersivity to the AP2layer which prevents the magnetization of the pinned layer from moving even when the magnetostrictively induced pinning is temporarily removed.

The present invention can be embodied in a current perpendicular to plane (CPP) sensor as well as a current in plane (CIP) sensor and even in a tunnel valve sensor. The invention provides the gap height reduction advantages of using a self pinned sensor (eliminating the AFM layer) while also providing pinned layer robustness, thereby providing a practical high performance self pinned read element. These and other advantages of the invention will be better understood by reading the following detailed description, in conjunction with the figures, which are not to scale and in which like numerals refer to like elements.

BEST MODE FOR CARRYING OUT THE INVENTION

During operation of the disk storage system, the rotation of the magnetic disk112generates an air bearing between the slider113and the disk surface122which exerts an upward force or lift on the slider. The air bearing thus counter-balances the slight spring force of suspension115and supports slider113off and slightly above the disk surface by a small, substantially constant spacing during normal operation.

With reference toFIG. 2, the orientation of the magnetic head121in a slider113can be seen in more detail.FIG. 2is an ABS view of the slider113, and as can be seen the magnetic head including an inductive write head and a read sensor, is located at a trailing edge of the slider. The above description of a typical magnetic disk storage system, and the accompanying illustration ofFIG. 1are for representation purposes only. It should be apparent that disk storage systems may contain a large number of disks and actuators, and each actuator may support a number of sliders.

With reference now toFIG. 3, the a magnetoresistive sensor300according to an embodiment of the present invention includes a sensor stack301having a free layer302, a pinned layer structure304and an electrically conductive spacer layer306disposed there between.

It should be pointed out that although the sensor is being described as a giant magnetoresistive sensor (GMR), the present invention could also be practiced in a tunnel valve, in which case the layer306would be a non-magnetic, electrically insulating barrier layer such as Al2O3. It should also be pointed out that the embodiment described herein is being described as a current perpendicular to plane (CPP) sensor. However, the present invention could just as easily be embodied in a current in plane (CIP) sensor, in which case electrical leads (not shown) would be disposed at left and right sides of the sensor to conduct sensor current through the sensor parallel with the planes of the layers.

With continued reference toFIG. 3, the sensor stack301is sandwiched between first and second magnetic, electrically conductive shields308,310, which serve as both magnetic shields and also as electrical leads. First and second hard bias layers312,314are disposed at either side of the sensor stack301. The hard bias layers312,314are constructed of a high coercivity (high Hc) magnetic material such as CoPtCr or some other hard magnetic material. The hard bias layers provide a magnetic field that biases the magnetitation of the free layer in a desired direction parallel to the ABS while allowing the magnetization of the free layer to rotate in the presence of a magnetic field such as from a nearby magnetic medium (eg. disk).

First and second electrically insulating layers316,318at either side of the sensor stack301, separating the hard bias layers312,314from the sensor stack301and separating the hard bias layers312,314from at least one of the shields310. The insulation layers316,318prevent current shunting through the hard bias layers312,314from one shield308to the other310.

With reference still toFIG. 3, the free layer is constructed of a low coercivity (low Hc) material, and may be constructed of multiple layers. For example, the free layer may be constructed of a first free layer sublayer320of NiFe, and a second sublayer322of CoFe. The free layer could also include a layer of pure Co (not shown). The spacer layer could be constructed of several non-magnetic, electrically conductive materials such as for example Cu.

A seed layer324can be provided at the base of the sensor stack301such as beneath the free layer320. The seed layer initiates a desired grain structure (such as face centered cubic (FCC)), which can then be carried though to the other subsequently deposited layers. The seed layer can be for example Ta, NiFeCr, Ru, PtMn or some combination of some or all of these or other materials. At the other top of the sensor stack301, a capping layer326can be provided, to protect the sensor materials from damage, such as by corrosion, during the manufacture of the sensor. The capping layer326can be constructed of for example Ta.

With continued reference toFIG. 3, the pinned layer304is constructed as an antiparallel pinned structure including a first magnetic layer (AP1)328. a second magnetic layer (AP2) and a non-magnetic electrically conductive antiparallel coupling layer332sandwiched therebetween. The antiparallel coupling layer332is constructed of a material such as Ru and is constructed of such a thickness as to antiparallel couple the AP1and AP2layers328,330. This thickness of the coupling layer332could be for example 3 to 9 Angstroms.

The first magnetic layer AP1328is preferably constructed of a high magnetostriction magnetic material such as CoFe. This AP1layer could be 15 to 25 Angstroms thick or about 20 Angstroms thick.

The second magnetic layer AP2330includes first and second sublayers334,336. The first comprises a magnetic material, which is preferably CoFe. This first sublayer334of the AP2layer330could be a CoFe layer having a thickness of about 15 Angstroms or 10 to 20 Angstroms.

The second sublayer336of the AP2layer330comprises TbCo. The second sublayer can be a layer of TbCo having about 25 atomic percent Tb and about 75 atomic percent Co. The second sublayer336could be for example 20 to 30 atomic percent Tb and 70 to 80 atomic percent Co. The second sublayer336can have a thickness of 20 to 60 Angstroms.

TbCo is a ferrimagnetic material, which means that the Tb atoms tend to align magnetically antiparallel to the Co atoms within the material. The result of this is that TbCo has a very high coercivity Hc, and a low magnetic moment. The magnetic moment of the material is the difference between the magnetic moments of the two materials Tb and Co within the material. This high coercivity of TbCo in the AP2layer330, advantageously increases the coercvity of the pinned layer304, which prevents amplitude flipping as discussed.

As mentioned above, the magnetic thicknesses of the AP1and AP2layers should be substantially equal, or nearly so. Magnetic thickness is the product of the magnetic moment of a layer and the physical thickness. As mentioned, TbCo has a low moment, much lower than that of CoFe. Therefore, the TbCo can layer can be very thick to equal a given thickness of CoFe. In fact the magnetic moment of TbCo is about 1/10 that of CoFe. This is the reason that, as discussed above, if the AP1layer328is made 20 Angstroms thick, and the first sublayer334of the AP2layer330is made 15 Angstroms thick, the TbCo second sublayer would be 5×10 or 50 Angstroms thick.

TbCo is amorphous and therefore it may not be desirable to place this material under the sensor stack301. For this reason it is preferable that the TbCo second sublayer336be constructed at the top of the sensor rather than at the bottom. If the pinned304were disposed at the bottom of the sensor (ie. deposited before the free layer), and the TbCo layer were deposited at the bottom of the pinned layer302, an undesirable epitaxial growth would initiate in the other layers of the sensor stack301. This the reason that the presently described embodiment illustrates the pinned layer304being at the top of the sensor rather than the bottom as is commonly done in sensor design. However, TbCo may be fine for use under a Tunnel Valve stack for providing pinning to the pinned layer.

With reference now toFIG. 4, a cross section perpendicular to the ABS of the sensor300can be seen. As can be seen the sensor stack301has a front edge402that is recessed from the ABS. The shields308,310extend beyond the front edge402of the sensor stack301and extend to the ABS. A fill layer404of non-corrosive, non-magnetic, dielectric material such as Al2O3fills the area in front of the sensor301between the shields. TbCo is a corrosive material. Similarly, other materials making up the sensor stack are corrosive as well, although to a somewhat lesser degree. By recessing the sensor stack301and filling the space in front of the sensor with a dielectric material as described above, the sensor will be protected from corrosion, such as by might otherwise occur from atmospheric exposure. Prior art sensors have been constructed by lapping the sensor stack at the ABS surface until a desired stripe height is achieved. However, to construct the recessed sensor300of the presently described embodiment, the front edge of the sensor stack404(and therefore the stripe height) must be defined photolithographically. The dielectric fill material402is deposited and planarized before forming the second shield310. A lapping procedure is then performed to remove shield material301and fill material404, thereby defining the ABS. The amount of recess is controlled by monitoring sensor resistance (in the presence of a magnetic field) while performing the lapping procedure and terminating the lapping when a predetermined resistance is reached.