Magnetoresistive sensor with free layer bias adjustment capability

A magnetoresistive sensor having a free layer biased by an in stack bias layer that has a magnetic moment canted with respect to the ABS, such that the magnetic moment of the biasing layer has a longitudinal component in a direction parallel with the ABS and a component in a transverse direction that is perpendicular to the ABS. The transverse component of the bias layer moment creates a balancing field in the free layer that counterbalances the coupling field in the free layer generated by the pinned layer. The counterbalance field provided by the canted moment of the biasing layer is especially useful in a tunnel valve sensor, because the very thin barrier layer of the tunnel valve design generates a strong coupling field in the free layer and this coupling field cannot be offset by a field from the sensor current.

FIELD OF THE INVENTION

The present invention relates to free layer biasing, and more particularly to free layer biasing in a current perpendicular to plane magnetoresistive sensor such as a tunnel valve.

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 the 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.

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.

Magnetization of the pinned layer is usually fixed by exchange coupling one of the ferromagnetic layers (AP1) with a layer of antiferromagnetic material such as PtMn. While an antiferromagnetic (AFM) material such as PtMn does not in and of itself have a magnetization, when exchange coupled with a magnetic material, it can strongly pin the magnetization of the ferromagnetic layer.

In order to meet the ever increasing demand for improved data rate and data capacity, researchers have recently been focusing their efforts on the development of perpendicular recording systems. A traditional longitudinal recording system stores data as magnetic bits oriented longitudinally along a track in the plane of the surface of the magnetic disk. This longitudinal data bit is recorded by a fringing field that forms between a pair of magnetic poles separated by a write gap. A perpendicular recording system, on the other hand, records data as magnetic transitions oriented perpendicular to the plane of the magnetic disk. The magnetic disk has a magnetically soft underlayer covered by a thin magnetically hard top layer. The perpendicular write head has a write pole with a very small cross section and a return pole having a much larger cross section. A strong, highly concentrated magnetic field emits from the write pole in a direction perpendicular to the magnetic disk surface, magnetizing the magnetically hard top layer. The resulting magnetic flux then travels through the soft underlayer, returning to the return pole where it is sufficiently spread out and weak that it will not erase the signal recorded by the write pole.

The advent of perpendicular recording systems has lead to an increased interest in current perpendicular to plane (CPP) sensors, which are particularly suited to use in perpendicular recording systems, due to their ability to read signals from a high coercivity medium. This is in part due to the short gap height afforded by such CPP sensors which allows them to read a very short bit of data. A CPP sensor differs from a more conventional current in plane (CIP) sensor such as that discussed above in that the sense current flows through the CPP sensor from top to bottom in a direction perpendicular to the plane of the layers making up the sensor. Whereas the more traditional CIP sensor has insulation layers separating it from the shields, the CPP sensor contacts the shields at its top and bottom surfaces, thereby using the shields as leads. A CPP sensor can be in the form of a CPP GMR sensor which operates based upon the spin dependent scattering of electrons as described above.

Another type of CPP sensor is what has been referred to as a tunnel junction sensor or tunnel valve. A tunnel valve operates based upon the spin dependent tunneling of electrons through a thin electrically insulating barrier layer. The tunnel valve has a free layer and a pinned layer similar to a GMR sensor, however, the free layer and pinned layer are separated from one another by a thin electrically insulating barrier layer rather than by a conductive spacer layer. The barrier layer can be for example alumina. When the magnetic moments of the free layer and the pinned layer are parallel, the electrical resistance through the barrier layer is at a minimum and when the moments are antiparallel the resistance through the barrier is at a maximum.

A challenge to the development of CPP sensors has been that the coupling field from the pinned layer prevents the free layer from maintaining a desired neutral magnetic moment biased in a direction parallel with the ABS. Because the pinned layer has its magnetic moment pinned in a direction perpendicular to the ABS, the coupling field from the pinned layer acts on the free layer to cant the moment of the free layer in a direction that is not parallel with the ABS as would be desired. This problem is especially acute in a tunnel valve design, because the coupling field across the very thin barrier layer is very strong compared with that through a thicker electrically conductive spacer layer. Also, in prior art current in plane (CIP) sensors this coupling field was offset by an opposite magnetic field from the currently flowing through the sensor from one side to the other along the planes of the sensor. However, in a CPP sensor such as a tunnel valve, the sense current does not produce a magnetic field that can counteract the coupling field from the pinned layer.

Therefore, there is a strong felt need for a sensor design that can maintain balanced free layer biasing in a current perpendicular to plane sensor such as a CPP GMR or a tunnel valve. Such a sensor design would preferably incorporate an in stack bias layer since such bias structures are more suitable for use in a CPP sensor than are laterally disposed hard bias layers.

SUMMARY OF THE INVENTION

The present invention provides a sensor design that overcomes the signal asymmetry effect of a pinned layer coupling field on the free layer of the sensor. The invention includes an in stack bias structure that includes a ferromagnetic bias layer that is exchange coupled to a layer of antiferromagnetic material and that is separated from the free layer by an antiparallel coupling layer. The magnetic moment of the bias layer is pinned in a direction that includes a longitudinal component that is parallel with the ABS and a transverse component that is perpendicular to the ABS.

The parallel, longitudinal component of the bias layer moment generates a bias field that biases the free layer in an antiparallel direction parallel to the ABS as desired. The angle of the bias layer moment is determined at wafer level manufacturing to balance the coupling field from the pinned layer. The angle required might be different from wafer to wafer. The transverse component of the magnetic moment of the bias layer generates a counter balancing field in the free layer that is preferably equal and opposite to a coupling field from the pinned layer in order to balance the transverse fields within the free layer.

The pinned layer structure may be a tri-layer structure having three ferromagnetic layers that are antiparallel coupled with one another. The first or outermost ferromagnetic layer can be exchange coupled with a layer of antiferromagnetic material which strongly pins its magnetic moment in a direction perpendicular to the ABS. The trilayer structure ensures that the transverse component of the bias layer moment will be oriented in the same direction as that of the first (exchange coupled) ferromagnetic layer. This advantageously ensures that when the bias layer moment is set by annealing, the transverse component of the anneal field will be in the same direction as the direction of the moment of the pinned layer, thereby preventing degradation of the previously set pinning of the pinned layer.

These and other features and advantages of the invention will be apparent upon reading of the following detailed description of preferred embodiments taken in conjunction with the Figures in which like reference numerals indicate like elements throughout.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

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 the suspension115and supports the 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, a magnetoresistive sensor300according to an embodiment of the invention includes a magnetoresistive sensor element or sensor stack302, sandwiched between first and second leads304,306. The first and second leads304,306can be constructed of an electrically conductive, magnetic material such as NiFe and can thereby serve as magnetic shields as well as leads. Non-magnetic, electrically insulating gap material305fills the space between the shields304,306outside of the sensor stack302.

The sensor stack302includes a magnetic free layer structure308and a magnetic pinned layer structure310. The free layer308can be constructed as a single layer of, for example Co or CoFe or could be multiple layers such as a layers including two or more layers of Co, CoFe or NiFe. A thin, non-magnetic, electrically insulating spacer layer312such as Alumina (Al2O3) is sandwiched between the free and pinned layers308,310. It should be pointed out that the present invention is being described as a tunnel junction sensor or tunnel valve, however, the present invention could also be embodied in a GMR sensor, in which case a non-magnetic, electrically conductive spacer layer such as Cu would be sandwiched between the free and pinned layers308,310rather than the electrically insulating barrier layer312. The sensor stack302may also include a seed layer312, formed at the bottom of the sensor stack302to promote a desired crystalline growth in the subsequently deposited layers, and may also include a capping layer314, such as Ta, formed at the top of the sensor stack to protect the sensor layers from damage during manufacturing.

With continued reference toFIG. 3, The pinned layer structure310is preferably a tri-layer structure for reasons that will become apparent upon further discussion below. The tri-layer structure of the pinned layer310includes first second and third magnetic layers316,318and320separated from one another by antiparallel coupling layers322,324. The magnetic layers316,318,320can be constructed of for example NiFe or CoFe or some other suitable magnetic material. The antiparallel coupling layers322,324may be constructed of, for example Ru and may each have a thickness ranging from 2 to 10 Angstroms or about 8 Angstroms. The material and thickness of the AP coupling layers322,324are chosen such that the magnetic layers316,318,320will have magnetic moments326,328,330that are antiparallel to that of the adjacent magnetic layer and oriented perpendicular to the ABS and in and out of the plane of the page as indicated by arrow head and arrow tail symbols326,328,330. The magnetic moments326,328,330of the pinned layer310may be pinned by an antiferromagnetic layer (AFM layer)332, or alternatively may be self pinned. The AFM layer332may be constructed of several ferromagnetic materials and is preferably constructed of PtMn. The AFM layer332is exchange coupled with the first magnetic layer316which strongly pins the magnetic moment326of that layer. Antiferromagnetic coupling across the coupling layers322,324pins the moments328,330of the other layers318,320. In order to ensure strong pinning of the magnetic layers316,318,320the net magnetic moment of the pinned layer310should be near zero. In the case of a three layer pinned layer structure310, this means that the magnetic thicknesses of the outermost magnetic layers316,320should be equal to the magnetic thickness of the middle layer318. Put another way, the sum of the magnetic thicknesses of all layers having a moment in a first direction should be equal to the sum of the magnetic thicknesses of the magnetic layers having a moment in the opposite direction. It should be pointed out that although the pinned layer310is described as a three layer structure, it could have any number of magnetic layers. Three magnetic layers is however the preferred embodiment.

With reference still toFIG. 3, the free layer308has a magnetic moment334that is biased parallel with the ABS and perpendicular to the moments330,328,326of the pinned layer structure310. The free layer308is biased such that it is free to rotate in response to a magnetic field. The moment334of the free layer is biased by an in stack bias structure336that includes a magnetic pinning layer338, a ferromagnetic, pinned biasing layer340and an AP coupling layer342that is constructed of, for example, Ru and is of such a thickness as to cause an antiparallel coupling between the free layer308and bias layer340that is sufficiently weak to maintain free layer sensitivity and strong enough to provide sufficient free layer stability. The AFM pinning layer338may be constructed of several antiferromagnetic materials and is preferably constructed of a material having a blocking temperature that is different from (preferably lower than) the blocking temperature of the AFM layer332used to pin the pinned layer. The bias structure pinning layer338is preferably constructed of IrMn. The pinned biasing layer340is can be constructed of several magnetic materials and is preferably constructed of CoFe.

The bias structure pinning layer338strongly fixes the magnetic moment of the bias layer340by exchange coupling with the bias layer340. The bias layer340has a pinned magnetic moment344that is canted with respect to the ABS, having a longitudinal component346that is parallel with the ABS and a transverse component348that is perpendicular to the ABS and in the same direction as the moment330of the nearest magnetic layer320of the pinned layer structure310.

The magnetic layer330of the of the pinned layer structure310imposes a coupling field350on the free layer308, which pushes the moment334away from its desired direction parallel with the ABS. If left unchecked, this coupling field350would cant the biased magnetic moment334away from its desired direction, resulting in an asymmetric signal being produced by the sensor. In order to counterbalance this coupling field, the moment344of the pinned bias layer340is canted as described above so that the component348is parallel with the moment330of the pinned layer320, and also with the coupling field350. Antiparallel coupling across the AP coupling layer342results in a balancing field352that counterbalances the coupling field, resulting in a balanced moment334that is parallel with the ABS. Therefore, the transverse component348of the bias layer moment344is of such a strength to produce an antiparallel balancing field352that has a strength equal and opposite to the strength of the coupling field350. While in prior art sensors, a field from the sense current was used to counterbalance this coupling field350, no such field is available in a CPP sensor design. What's more, the very thin (3 to 5 Angstroms) barrier layer generates a much stronger coupling field than would be the case in a GMR sensor having a thicker, electrically conductive spacer layer. The balancing field352provided by the present invention allows the CPP sensor (whether tunnel valve or GMR) to have a desired symmetric signal.

The magnetic orientations of the pinned, free, and biasing layers308,310,338can be set by a series of anneal procedures. First to set the pinned layer moments326,328,330, the sensor300is exposed to a strong magnetic field (13 to 15 KOe) while the sensor300is heated to a temperature above the blocking temperature of the AFM pinning layer332(265 degrees C. for CoPt). This magnetic field is maintained while the sensor300is cooled to a temperature below its blocking temperature. The magnetic field used to pin the pinned layers must be strong enough to overcome the AP coupling of the magnetic layers316,318,320. This will cause all of the moments of these layers to be oriented in the direction in which the first magnetic layer316adjacent to the AFM332will be oriented. When the sensor300is cooled and the field is removed, the first magnetic layer316will be strongly pinned by exchange coupling with the AFM layer and the moments328,330of the other layers318,320will orient themselves antiparallel to one another.

After the pinned layer magnetization has been set, a separate anneal can be performed to set the moment344of the bias layer340. In order to set the moment of the biasing layer340, the sensor300is heated to a temperature that is above the blocking temperature of the AFM layer338. Preferably, the AFM layer338is constructed of a material having a lower blocking temperature than that of the AFM pinning layer332. In this way, the sensor300can be raised to a temperature that is between the blocking temperatures of the biasing AFM layer338and that of the pinning AFM layer332. This will help prevent the bias anneal from negatively affecting the previously performed pinned layer anneal.

With the sensor raised to a temperature above the blocking temperature of the biasing AFM layer338, a field is applied in the desired canted direction, having the desired component in parallel with the ABS and a desired component in the direction perpendicular to the ABS as described above. The sensor can be cooled while this magnetic field is maintained, and once cooled below the blocking temperature of the biasing AFM338, the moment344will be strongly pinned in the desired canted direction.

It should be pointed out that, since the transverse component of the field used to set the biasing layer340is in the same direction as the moment326of the previously set AFM pinned layer316, the setting of the bias layer moment344will not degrade the previously set pinning of the pinned layer316. It is for this reason that a tri-layer structure310is preferred. Alternatively, if careful attention is paid to the strength magnetic field strength and temperature used to set the biasing layer340, the biasing anneal can be performed without damaging the pinning of the pinned layer308even if a traditional two layer AP pinned structure is used for the pinned layer.