Patent Publication Number: US-8988834-B2

Title: Current perpendicular to plane magnetoresistive sensor employing half metal alloys for improved sensor performance

Description:
RELATED APPLICATIONS 
     The present Application is a Divisional Application of commonly assigned patent application Ser. No. 12/119,961, filed May 13, 2008 entitled CURRENT PERPENDICULAR TO PLANE MAGNETORESISTIVE SENSOR EMPLOYING HALF METAL ALLOYS FOR IMPROVED SENSOR PERFORMANCE. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to magnetoresistive sensors and more particularly to a sensor employing Mn containing Heusler alloys for improved magnetoresistive performance while also exhibiting high corrosion resistance and low Mn diffusion. 
     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 five 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. 
     SUMMARY OF THE INVENTION 
     Provided herein is a magnetoresistive sensor, that includes a magnetic free layer structure, and a magnetic pinned layer structure comprising a lamination of layers of a material selected from the group consisting of Co 2 [Mn 1−x Cr x ]Si, Co 2 [Mn 1−x Cr x ]Al or Co 2 [Mn 1−x Cr x ]Ge. The sensor also includes a non-magnetic layer sandwiched between the free layer structure and the pinned layer structure. 
     The sensor may also be a magnetoresistive sensor that uses Huesler alloys for improved magnetoresistive performance while also minimizing corrosion and Mn migration. The sensor includes a free layer structure and a pinned layer structure with a non-magnetic barrier or spacer layer sandwiched between the free and pinned layer. The pinned layer includes a lamination of layers of Co 2 MnX and CoFe (where X is Al, Ge or Si). 
     By forming the pinned layer with a lamination of Co 2 MnX and CoFe, the amount of Mn exposed at the air bearing surface is greatly reduced, thereby minimizing corrosion, while also allowing the advantages of the Co 2 MnX alloy for sensor performance improvement. 
     The lamination of layers can be configured so that the layer of Co 2 MnX is sandwiched between layers of CoFe, which advantageously prevents the migration of Mn into adjacent layers such as the spacer/barrier layer. 
     The free layer can also be constructed as a lamination of Co 2 MnX and CoFe layers, and may be configured with a layer of Co 2 MnX sandwiched between CoFe layers to prevent Mn migration. The free layer may also be constructed as an antiparallel coupled free layer structure or as a simple free layer structure. 
     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. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a fuller understanding of the nature and advantages of this invention, as well as the preferred mode of use, reference should be made to the following detailed description read in conjunction with the accompanying drawings which are not to scale. 
         FIG. 1  is a schematic illustration of a disk drive system in which the invention might be embodied; 
         FIG. 2  is an ABS view of a slider illustrating the location of a magnetic head thereon; 
         FIG. 3  is an enlarged ABS view of a magnetoresistive sensor of a magnetic head for use in disk drive system; and 
         FIGS. 4-8  are ABS views of a sensor stack of a magnetoresistive sensor according to various embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following description is of the best embodiments presently contemplated for carrying out this invention. This description is made for the purpose of illustrating the general principles of this invention and is not meant to limit the inventive concepts claimed herein. 
     Referring now to  FIG. 1 , there is shown a disk drive  100  embodying this invention. As shown in  FIG. 1 , at least one rotatable magnetic disk  112  is supported on a spindle  114  and rotated by a disk drive motor  118 . The magnetic recording on each disk is in the form of annular patterns of concentric data tracks (not shown) on the magnetic disk  112 . 
     At least one slider  113  is positioned near the magnetic disk  112 , each slider  113  supporting one or more magnetic head assemblies  121 . As the magnetic disk rotates, slider  113  moves radially in and out over the disk surface  122  so that the magnetic head assembly  121  may access different tracks of the magnetic disk where desired data are written. Each slider  113  is attached to an actuator arm  119  by way of a suspension  115 . The suspension  115  provides a slight spring force which biases slider  113  against the disk surface  122 . Each actuator arm  119  is attached to an actuator means  127 . The actuator means  127  as shown in  FIG. 1  may be a voice coil motor (VCM). The VCM comprises a coil movable within a fixed magnetic field, the direction and speed of the coil movements being controlled by the motor current signals supplied by controller  129 . 
     During operation of the disk storage system, the rotation of the magnetic disk  112  generates an air bearing between slider  113  and the disk surface  122  which exerts an upward force or lift on the slider. The air bearing thus counter balances the slight spring force of suspension  115  and supports slider  113  off and slightly above the disk surface by a small, substantially constant spacing during normal operation. 
     The various components of the disk storage system are controlled in operation by control signals generated by control unit  129 , such as access control signals and internal clock signals. Typically, the control unit  129  comprises logic control circuits, storage means and a microprocessor. The control unit  129  generates control signals to control various system operations such as drive motor control signals on line  123  and head position and seek control signals on line  128 . The control signals on line  128  provide the desired current profiles to optimally move and position slider  113  to the desired data track on disk  112 . Write and read signals are communicated to and from write and read heads  121  by way of recording channel  125 . 
     With reference to  FIG. 2 , the orientation of the magnetic head  121  in a slider  113  can be seen in more detail.  FIG. 2  is an ABS view of the slider  113 , 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 of  FIG. 1  are for representation purposes only. It should be apparent that disk storage systems may contain a large number of disks and actuators, an each actuator may support a number of sliders. 
     With reference now to  FIG. 3 , a magnetic read head  302  is shown as viewed from the air bearing surface (ABS). The read head  300  includes a magnetoresistive sensor stack  302  that is sandwiched between first and second electrically conductive lead layers  304 ,  306 . The lead layers  304 ,  306  can be constructed of a magnetic material such as NiFe or CoFe so that they can function as magnetic shields as well as electrically conductive leads. 
     The sensor stack  302  includes a magnetic free layer structure  316  and a magnetic pinned layer structure  318 . A non-magnetic layer  320  is sandwiched between the free layer  316  and pinned layer structure  318 . It should be pointed out at this point that the invention can be embodied in a current perpendicular to plane giant magnetoresistive sensor (CPP GMR) or in a tunnel junction magnetoresistive sensor (TMR). If the read head  300  is a CPP GMR, then the non-magnetic layer  320  is an electrically conductive, non-magnetic spacer layer constructed of a material such as Cu or an oxide of Cu. On the other hand, if the read head  300  is a TMR sensor, then the non-magnetic layer  320  will be a thin, non-magnetic, electrically insulating barrier layer, constructed of a material such as MgO or AlO. 
     The pinned layer structure  318  includes a magnetically pinned layer  322  (AP2) and a reference layer  324  (AP2). The layers  322  and  324  are antiferromagnetically coupled across a non-magnetic antiparallel coupling layer  326 . The pinned layer  322  can be exchange coupled with a layer of antiferromagnetic material (AFM)  328  such as IrMn or PtMn which strongly pins the magnetization of the pinned layer  322  in a first direction perpendicular to the ABS as indicated by arrow-head symbol  330 . The antiparallel coupling between the layers  322 ,  324 , then strongly pins the magnetization of the reference layer  324  as indicated by arrow tail symbol  332 . 
     First and second hard bias layers  308 ,  310  can be provided at either side of the sensor stack  302 . The hard bias layers  308 ,  310  can be constructed of a hard magnetic material such as CoPt or CoPtCr, and provide a magnetic bias field that biases a magnetization of a magnetic free layer  316  in a direction parallel with the ABS as indicated by arrow symbol  334 . The hard bias layers  308 ,  310  are separated from the sensor stack  302  and at least one of the lead/shield layers  304  by non-magnetic, electrically insulating layers  312 ,  314 , which can be, for example, alumina. Various configurations of the sensor stack  302 , according to various possible embodiments of the invention, will be described in greater detail with reference to  FIGS. 4-8 . 
     The use of Heusler alloys in a pinned and free layer structures of a CPP magnetoresistive sensor (either tunnel or GMR) can provide significant performance improvements such as higher dR/R. However, the presence of Mn in these alloys poses corrosion and reliability problems. In addition, the materials used in spacer or barrier layers strongly attract the Mn used in such Heusler alloys, causing the Mn to diffuse into the spacer or barrier layer. This diffusion of Mn into the spacer or barrier layer has a disastrous affect on sensor performance. As a result, these materials have not been successfully used in commercial heads, and the potential performance benefits have not been realized. The present invention includes pinned layer and free layer structures that can allow the performance advantageous provided by these Heusler alloys to be realized, while avoiding the above mentioned corrosion and diffusion problems associated with such materials. Various embodiments for achieving this are described below with reference to  FIGS. 4-8 , which show ABS views of a sensor stack according to various possible embodiments of the invention. 
     With reference now to  FIG. 4 , a sensor stack  302  according to an embodiment of the invention is described. The sensor stack  302  can include a seed layer  402  formed at the bottom of the sensor stack for initiating a desired grain structure in the above layers, and a capping layer  404 , such as Ta, at the top of the sensor stack  302  to protect the layers of the sensor stack during manufacture. 
     The AP coupled pinned layer structure includes a pinned layer  322  that, as mentioned above, is exchange coupled with the AFM layer  338 . The pinned layer  322  is preferably constructed of CoFe which exhibits good exchange coupling with IrMn. 
     The reference layer  324  is a multi-layer structure that provides the magnetoresistive enhancement benefits of a Heusler alloy without the corrosion and diffusion problems that have previously been associated with such alloys. The reference layer  324  includes a nano-layer of CoFe  406  adjacent to the Ru AP coupling layer  326 , and a nano-layer of Co 2 MnX  408  adjacent to the spacer/barrier layer  320 , where X can be Si, Ge or Al. A nano-layer of CoFeX  410  (where X is Si, Ge or Al) is sandwiched between the layers  406  and  408 . 
     With continued reference to  FIG. 4 , the free layer structure includes a nano-layer of Co 2 MnX  412  adjacent to the spacer/barrier layer  320  and a nano-layer of Co 2 FeX  414  away from the spacer/barrier layer  320 . Again the element X can be a material selected from the group consisting of Si, Ge and Al. 
     By using nano-layers of Co 2 FeX along with other layers, the amount of Mn that can be exposed at the air bearing surface (ABS) is greatly reduced. This allows the advantageous use of a Heusler alloy in the pinned and free layer structures  318 ,  316 , while greatly reducing the chance of corrosion associated with the use of such materials. 
     With reference now to  FIG. 5 , in another embodiment of the invention, the pinned layer structure includes a pinned layer  322  constructed of CoFe, and a Ru AP coupling layer  326  formed over the pinned layer  322 . The reference layer  324  is a multi-layer structure that includes a CoFe nano-layer adjacent to the Ru AP coupling layer. A tri-layer structure  503  is formed above the CoFe nano-layer  502 . The tri-layer structure  503  includes a layer of Co 2 MnX  506  sandwiched between first and second layers of Co 2 FeX  504 ,  508 . In other words, the reference layer includes consecutive layers of CoFe  502 , Co 2 FeX  504 , Co 2 MnX  506  and Co 2 FeX  508 . The element X can be Si, Al or Ge. 
     Similarly, the free layer  316  includes a layer of Co 2 MnX  512  sandwiched between first and second layers of Co 2 FeX  510 ,  514 , where X can be Si, Al or Ge. The free layer  316 , therefore includes consecutive layers of Co 2 FeX  510 , Co 2 MnX  512 , and CO 2 FeX  514 , with the first layer of Co 2 FeX  510  being located adjacent to the spacer/barrier layer  320 . As can be seen, then in each of the free and pinned layers  316 ,  318  has a Mn containing layer of Heusler alloy sandwiched between layers that don&#39;t contain Mn. 
     As with the previously described embodiment, the multi-layer structure reduces the amount of Mn exposed at the air bearing surface which reduces the chance of corrosion. The total thickness of the free layer  316  can be about 40 Angstroms, and the thickness of the Co 2 MnX layer  512  can be about 5-15 Angstroms. Therefore, as can be seen, the free layer  316  has a small amount of Mn containing material to be exposed at the ABS. 
     However, this embodiment has the added advantage that the Mn containing layers  504 ,  512  are removed from the spacer/barrier layer  320 . As mentioned above, the material making up the layer  320  (whether it is a spacer or barrier layer) tend to strongly attract and absorb Mn. Therefore, if a layer containing Mn is placed adjacent to the spacer/barrier layer  320 , the Mn can diffuse into the spacer/barrier layer  320 , which can seriously degrade magnetic performance. In the presently described embodiment, Mn diffusion is prevented by the layers  508 ,  510 . In addition, diffusion of Mn into the AP coupling layer  326  is prevented by the layer  502 . Similarly, diffusion of Mn into the capping layer  404  is prevented by the layer  514 . 
     With reference now to  FIG. 6 , another embodiment is described which further prevents corrosion. This embodiment is similar to that described above in  FIG. 5 , except that in the Mn containing layers, 5-10 atomic percent of the Mn has been replaced by Cr. In addition, this embodiment uses an AP coupled free layer for improved free layer sensitivity. However, it should be pointed out that the AP coupled free layer could be used with the other described embodiments as well. In addition, the embodiment described herein with reference to  FIG. 6 , could also be constructed with a simple (not AP coupled) free layer. 
     With this in mind, the sensor stack  302  of  FIG. 6  can include a pinned layer  318  having a first and second magnetic layer structures  322 ,  324 , which are antiparallel coupled across a Ru AP coupling layer  602 . The First magnetic layer  322  can be CoFe, which strongly exchange couples with the AFM layer  338 . The second magnetic layer  324  can include a layer of Co 2 [Mn 1−x Cr x ]Si, Co 2 [Mn 1−x Cr x ]Al or Co 2 [Mn 1−x Cr x ]Ge  606  (where x is 0.05 to 0.1) sandwiched between first and second layers of CoFe  604 ,  608 . Each of the layers  604 ,  606 ,  608  can have a thickness of 5-15 Angstroms. 
     With continued reference to  FIG. 6 , the free layer structure  316  includes a first magnetic layer structure  624  and a second magnetic layer structure  626 , which are antiparallel coupled across a second non-magnetic antiparallel coupling layer  618  that can be constructed of Ru. The first magnetic layer structure  624  can include a layer of Co 2 [Mn 1−x Cr x ]Si, Co 2 [Mn 1−x Cr x ]Al or Co 2 [Mn 1−x Cr x ]Ge  614  sandwiched between layers of CoFe  612 ,  616 . As with the above described layer, in the layer  614  X can be 0.05 to 0.1. Each of the layers  612 ,  614 ,  616  can have a thickness of 5-30 Angstroms. The second magnetic layer  626  can be constructed of a layer of CoFe  620  adjacent to the AP coupling layer  618  and a layer of NiFe  622  formed over the layer  618 . 
     With reference now to  FIG. 7 , another embodiment of the invention is described. This embodiment, can be considered to a preferred embodiment. The sensor stack  302  has a Heusler alloy only in the pinned layer structure. Therefore, the sensor stack  302  has a pinned layer structure  318  that has first and second magnetic layer structures  322 ,  324  that are AP coupled across an AP coupling layer  326 . The second magnetic layer structure  324 , the layer closest to the spacer layer  320  includes a layer of Co 2 [Mn 1−x Cr x ]Si, Co 2 [Mn 1−x Cr x ]Al or Co 2 [Mn 1−x Cr x ]Ge  704  (where x is 0.05 to 0.1) sandwiched between first and second layer of CoFe  702 ,  706 . 
     The free layer  316  can be an AP coupled structure including a first magnetic layer comprising CoFe  708  adjacent to the spacer/barrier layer  320 , and a second magnetic layer structure  710  that is AP coupled with the first layer  708  across an AP coupling layer  712 , such as Ru. The second magnetic layer  710  of the free layer structure  316  can include a layer of CoFe  714  adjacent to the AP coupling layer and a second layer comprising NiFe  716 . 
     Heusler alloys, such as the Co 2 [Mn 1−x Cr x ]Si, Co 2 [Mn 1−x Cr x ]Al or Co 2 [Mn 1−x Cr x ]Ge tend to have high magnetic coercivities (they are hard to make soft) and have a positive magnetostriction, which can be problematic in a free layer structure. The high coercivity makes the free layer less sensitive to magnetic fields. The positive magnetostriction (when combined with compressive stresses that are inevitably present in magnetic heads) produce a magnetic anisotropy that is perpendicular to the air bearing surface. This causes the free layer to be unstable and difficult to effectively bias. In the above described embodiments, these effects are mitigated to a large extend by the laminated structures that reduce the amount of Huesler alloy that is present in the free layer. In the embodiment described with reference to  FIG. 7 , however, the Fleusler alloy is completely removed from the free layer. This allows the advantages of the Heusler alloy to be realized in the pinned layer, without the negative effects of magnetostriction and coercivity described above affecting the free layer performance. 
     With reference to  FIG. 8  still another embodiment of the invention is described. This embodiment includes a pinned layer structure  318  having a first magnetic layer  322  constructed of  322  and a second magnetic layer structure  324  that is AP coupled with the first layer  322 . The second layer  324  includes alternating layers of CoFe  802 ,  806 ,  810  and Co 2 FeX  804 ,  808 ,  812 , where X is can be Si or Al. 
     Similarly, the free layer structure  316  is constructed of alternating layers of NiFe  816 ,  820  and Co 2 FeX  814 ,  818  where X is Al or Si. Preferably in both the free layer structure  316  and pinned layer  318 , a layer of Co 2 FeX  814 ,  812  is adjacent to the barrier/spacer layer  320 . Note that in this embodiment the Mn has been completely removed from the Huelser alloy. Therefore, this embodiment completely eliminates the corrosion, and diffusion problems associated with the use of Mn. 
     While various embodiments have been described above, it should be understood that they have been presented by way of example only and not limitation. Other embodiments falling within the scope of the invention may also become apparent to those skilled in the art. Thus, the breadth and scope of the invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.