Abstract:
A giant magnetoresistance (GMR) head for magnetic storage systems, the GMR head having a free layer with improved soft magnetic properties while retaining giant magnetoresistance (GMR) effects. The free layer comprises an alloy comprising Co x , Fe y , and Cu z , wherein x, y, and z represent the atomic weight percentage of Co, Fe, and Cu, respectively.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates in general to giant magnetoresistance (GMR) heads for magnetic storage systems, and more particularly to a method and apparatus for improving soft magnetic properties of a spin valve free layer while retaining giant magnetoresistance (GMR) effects. 
     2. Description of Related Art 
     Magnetic recording systems that utilize magnetic disk and tape drives constitute the main form of data storage and retrieval in present-day computer and data processing systems. In the recording process, information is written and stored as magnetization patterns on the magnetic recording medium. Scanning a write head over the medium and energizing the write head with appropriate current waveforms accomplish this recording process. In a read-back process, scanning a magnetoresistive (MR) sensor over the medium retrieves the stored information. This MR read head sensor intercepts magnetic flux from the magnetization patterns on the recording medium and converts the magnetic flux into electrical signals, which are then detected and decoded. 
     However, limitations of MR sensor performance were drastically expanded by the discovery of the giant magnetoresistance (GMR) effect, also known as the spin-valve effect. In contrast to a conventional MR effect, which is based on homogeneous ferromagnetic metals or alloys, the GMR effect is present only in heterogeneous magnetic systems with two or more ferromagnetic components and at least one nonmagnetic component. Hence, a GMR head has a greater sensitivity to magnetic fields from a disk. 
     Accordingly, a spin valve sensor is employed by a GMR read head for sensing magnetic fields on a moving magnetic medium, such as a rotating magnetic disk. A typical spin valve sensor includes a nonmagnetic electrically conductive spacer layer between a ferromagnetic pinned layer structure and a ferromagnetic free layer structure. An antiferromagnetic pinning layer interfaces and is exchange coupled to the pinned layer structure for pinning a magnetic moment of the pinned layer structure 90° to an air bearing surface (ABS) where the ABS is an exposed surface of the sensor that faces the rotating disk. Leads are connected to the spin valve sensor for conducting a sense current. 
     A magnetic moment of the free layer structure is typically oriented parallel to the ABS in a quiescent condition, the quiescent condition being where the sense current is conducted through the sensor in the absence of any signal fields. The magnetic moment of the free layer structure is free to rotate from the parallel position in response to signal fields from the rotating magnetic disk. Changes in response to field signals from the rotating disk changes the resistance of the spin valve sensor due to the angle between the magnetic moments of the pinned and free layer structures. The sensitivity of the sensor is quantified by a magnetoresistive coefficient dr/R (ΔR/R) where dr is the change in resistance of the sensor between parallel and antiparallel orientations of the pinned and free layer structures and R is the resistance of the sensor when the moments are parallel. The GMR effect operates to produce a lower resistance for parallel alignment of the pinned and free layer structures, and a higher resistance for antiparallel alignment of the pinned and free layer structures. 
     Several classes of soft magnetic materials have evolved for use in the construction of spin valves. Permalloy, a general term that refers to alloys of Ni and Fe, is one class used in the fabrication of spin valves due to permalloy&#39;s very small anisotropy (i.e., varying of magnetic properties along different axis) and magnetostriction characteristics. Another important design feature for spin valves is to provide a magnetic material for the free layer structure that lowers coercivity, i.e., the magnetic field necessary to switch the direction of magnetization and decrease magnetic induction to zero. 
     Moreover, the success of hard disk drives (HDDs) originates from these successful design features and an ever-increasing demand for storage capacity coupled with a consistent reduction in price per megabyte. Areal density (expressed as billions of bits per square inch of disk surface area, Gbits/in 2 ) is the product of linear density (bits of information per inch of track) multiplied by track density (tracks per inch), and varies with disk radius. Improved areal density levels have been the dominant reason for the reduction in price per megabyte. High areal densities have been achieved by introducing new technology and by proportionally reducing certain key dimensions, such as the GMR head, within the HDD (“scaling”). Thus, there is a present need to reduce the free layer thickness in GMR spin valve sensor. 
     Current spin valve designs have free layers composed of a bilayer of CoFe and NiFe. A minimum thickness of CoFe in contact with a Cu spacer layer in the spin valve is necessary to achieve the highest GMR signal. In other words, sensitivity is increased with a reduction in the thickness of the free layer. However, to maintain acceptable sensor performance, and GMR, the bilayer material CoFe should not be reduced far below 15 Å. Hence, in reducing the bilayer structure of CoFe and NiFe to a thickness below 15 Å, the NiFe must be reduced to near zero. 
     The soft magnetic properties of CoFe are less attractive than NiFe and as the total thickness of the free layer is reduced, the ratio of CoFe to NiFe increases. As a result of the increased ratio, coercivity increases causing a strong resistance to change in magnetization of the bilayer structure. Thus, it is important to find a replacement for CoFe with improved soft magnetic properties, yet while retaining high GMR. 
     It can be seen that there is a need for providing a high quality soft magnetic material for the spin valve free layers of magnetic recording heads. 
     More particularly, it can be seen that there is a need for providing improved soft magnetic properties for free layers of spin valves while retaining giant magnetoresistance (GMR) effects. 
     SUMMARY OF THE INVENTION 
     To overcome the limitations in the prior art described above, and to overcome other limitations that will become apparent upon reading and understanding the present specification, the present invention discloses a method and apparatus for improving soft magnetic properties of the spin valve free layer while retaining giant magnetoresistance (GMR) effects. 
     The present invention solves the above-described problems by providing a high quality magnetic material, such as CoFeCu, as a replacement for the CoFe/NiFe bilayer spin valve structure. The CoFeCu free layer provides enhanced sensitivity by an improved magnetoresistive coefficient (dr/R) and increased sheet resistance coupled with a low uniaxial anisotropy field (Hk). The design of the present invention yields a high amplitude sensor with a desired magnetic stability. 
     A method for forming a spin valve sensor in accordance with the principles of the present invention includes forming a pinned layer, forming a spacer layer; and forming a free layer disposed on the spacer layer, the free layer comprising Co x , Fe y , and Cu z , wherein x, y, and z represent the atomic weight percentages of Co, Fe, and Cu, respectively. 
     A thin film magnetoresistive (MR) spin valve read sensor in accordance with the principles of the present invention includes a pinned layer, a spacer layer disposed on the pinned layer and a free layer being disposed upon the spacer layer, the free layer comprising Co x , Fe y , and Cu z , wherein x, y, and z represent the atomic weight percentages of Co, Fe, and Cu, respectively. 
     A magnetic storage system in accordance with the principles of the present invention includes at least one movable magnetic medium, a slider, an actuator for positioning the slider relative to the movable magnetic medium and a head coupled to the slider such that the head may be positioned relative to the at least one movable magnetic medium by the action of moving the slider with the actuator; wherein the head includes a GMR sensor having a pinned layer, a spacer layer disposed on the pinned layer and a free layer being disposed upon the spacer layer, the free layer comprising Co x , Fe y , and Cu z , wherein x, y, and z represent the atomic weight percentages of Co, Fe, and Cu, respectively. 
     These and various other advantages and features of novelty which characterize the invention are pointed out with particularity in the claims annexed hereto and form a part hereof. However, for a better understanding of the invention, its advantages, and the objects obtained by its use, reference should be made to the drawings which form a further part hereof, and to accompanying descriptive matter, in which there are illustrated and described specific examples of an apparatus in accordance with the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Referring now to the drawings in which like reference numbers represent corresponding parts throughout: 
         FIG. 1  illustrates a storage system according to the present invention. 
         FIG. 2  illustrates one example of a magnetic disk drive storage system; 
         FIG. 3  is a top view of a magnetic disk drive; 
         FIG. 4  illustrates one example of a magnetic sensor; 
         FIG. 5  illustrates an air bearing surface (ABS) of a slider; 
         FIG. 6  illustrates an air bearing surface view of a GMR sensor according to the present invention; and 
         FIG. 7  is a table comparing the properties of a free layer formed by a CoFeCu alloy with other free layer compositions according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following description of the exemplary embodiment, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration the specific embodiments in which the invention may be practiced. It is to be understood that other embodiments may be utilized as structural changes may be made without departing from the scope of the present invention. 
     The present invention solves the above-described problems by forming a GMR sensor including a spin valve free layer formed of CoFeCu or a CoFeCu alloy. The free layer according to the present invention has soft magnetic properties that reduce at least the magnetic field needed to switch the direction of magnetization in a spin valve structure (i.e., coercivity). 
       FIG. 1  illustrates a storage system  100 . In  FIG. 1 , a transducer  110  is under control of an actuator  120 . The actuator  120  controls the position of the transducer  110 . The transducer  110  writes and reads data on magnetic media  130 . The read/write signals are passed to a data channel  140 . A signal processor  150  controls the actuator  120  and processes the signals of the data channel  140 . In addition, a media translator  160  is controlled by the signal processor  150  to cause the magnetic media  130  to move relative to the transducer  110 . The present invention is not meant to be limited to a particular type of storage system  100  or to the type of media  130  used in the storage system  100 . 
       FIG. 2  is an illustration of one example of a magnetic disk drive storage system  200 . As shown in  FIG. 2 , at least one rotatable magnetic disk  220  is supported on a spindle  222  and rotated by a disk drive motor  224 . The magnetic recording media on each disk  220  is in the form of an annular pattern of concentric data tracks (not shown). 
     At least one slider  226  is positioned on the disk  220 , each slider  226  supporting one or more magnetic read/write heads  228  where the heads  228  incorporate a giant magnetoresistive (GMR) sensor of the present invention. As the disk(s)  220  rotate, slider  226  is moved radially in and out over disk surface  230  so that heads  228  may access different portions of the disk  220  where desired data is recorded. Each slider  226  is attached to an actuator arm  232  by means of a suspension  234 . The suspension  234  provides a slight spring force, which biases slider  226  against the disk surface  230 . Each actuator arm  232  is attached to an actuator  236 . The actuator  236  may be a voice coil motor (VCM). The VCM has a coil movable within a fixed magnetic field, the direction and speed of the coil movements being controlled by motor current signals supplied by a control unit  240 . 
     During operation of the disk drive  200 , the rotation of the disk  220  generates an air bearing between slider  226  and the disk surface  230 , which exerts an upward force or lift on the slider  226 . The surface of the slider  226 , which includes head  228  and faces the surface of disk  220  is referred to as an air-bearing surface (ABS). The air bearing thus counter-balances the slight spring force of suspension  234  and, during normal operation, supports the slider  226  off of, and slightly above, the disk surface  230  at a small, substantially constant spacing. 
     The various components of the disk drive  200  are controlled in operation by control signals generated by a control unit  240 , such as access control signals and internal clock signals. Typically, control unit  240  has logic control circuits, storage apparatus, and a microprocessor. The control unit  240  generates control signals to control various system operations such as drive motor control signals on line  242  and head position and seek control signals on line  244 . The control signals on line  244  provide the desired current profiles to optimally move and position the slider  226  to the desired data track on the disk  220 . Read and write signals are communicated to and from read/write heads  228  through recording channel  246 . 
     The above description of a typical magnetic disk drive storage system  200 , and the accompanying illustration of  FIG. 3  are for representation purposes only. It should be apparent that disk storage systems may contain a large number of disks and actuators, and that each actuator may support a number of sliders. Many other variations of the basic typical magnetic disk drive storage system  200  may be used in conjunction with the present invention while keeping within the scope and intention of the invention. However, those skilled in the art will recognized that the present invention is not meant to be limited to magnetic disk drive storage systems as illustrated in FIG.  2 . 
       FIG. 3  is a top view  300  of a magnetic disk drive. The magnetic disk drive  300  includes a spindle  332  that supports and rotates a magnetic disk  334 . A combined read and write magnetic head  340  is mounted on a slider  342  that is supported by a suspension  344  and actuator arm  346 . The present invention is not limited to a single unit, and a plurality of disks, sliders and suspensions may be employed in a large capacity direct access storage device (DASD). The suspension  344  and actuator arm  346  position the slider  342  so that the magnetic head  340  is in a transducing relationship with a surface of the magnetic disk  334 . When the disk  334  is rotated by a motor, the slider is supported on a thin cushion of air (air bearing) between the surface of the disk  334  and the air-bearing surface (ABS) (FIG.  4 — 448 ). The magnetic head  340  may then be employed for writing information to multiple circular tracks on the surface of the disk  334 , as well as for reading information therefrom. 
       FIG. 4  illustrates one example of a magnetic sensor  400  according to the present invention. As shown in  FIG. 4 , first and second solder connections  404  and  416  connect leads from the slider  442  to a suspension (FIG.  3 — 346 ). Third and fourth solder connections  418  and  406  connect leads from a coil in the magnetic head  440  to the suspension (FIG.  3 — 346 ). However, one of ordinary skill in the art will realize that the present invention is not meant to be limited to the magnetic sensor configuration shown in  FIG. 4 , but that other magnetic sensor configurations may be used in the present invention. 
       FIG. 5  illustrates an air bearing surface (ABS) of a slider  500 . The slider  500  has a center rail  556  that supports the magnetic head  540 , and side rails  558  and  560 . The rails  556 ,  558  and  560  extend from a cross rail  562 . With respect to rotation of the magnetic disk (FIG.  3 — 334 ), the cross rail  562  is at a leading edge  564  of the slider and the magnetic head  540  is at a trailing edge  566  of the slider. However, one of ordinary skill in the art will realize that the present invention is not limited to the above ABS configuration. 
       FIG. 6  illustrates an air bearing surface view of a GMR sensor  600  according to the present invention. GMR heads are very attractive for use as high density recording magneto resistive (MR) heads because of their high readback output voltages, linear response, and symmetrical read sensitivity profiles. 
     In  FIG. 6 , an air bearing surface view of a GMR sensor  600  including end regions  612  and  614  separated by a central region  616  is shown. A free layer (free ferromagnetic layer)  618  is separated from a pinned layer (AP-pinned ferromagnetic layer)  620  by a non-magnetic, electrically-conducting spacer layer  622  (typically, primarily copper). In one embodiment of the present invention, the free layer  618  includes, for example, CoFeCu or a CoFeCu alloy. The magnetization of the pinned layer  620  is fixed through exchange coupling with an antiferromagnetic (AFM) layer  624 . The magnetization of the free layer  618 , however, is free to rotate in the presence of an external field. Free layer  618 , spacer layer  622 , pinned layer  620  and the AFM layer  624  are all formed in the central region  616 . 
     Hard bias layers  626  and  628  formed in the end regions  612  and  614 , respectively, provide longitudinal bias for the free layer  618 . Leads  630  and  632  formed over hard bias layers  626  and  628 , respectively, provide electrical connections for the flow of the sensing current I s , from a current source  634  to the GMR sensor  600 . A signal detector  640 , which is electrically connected to the leads  630  and  632 , senses the change in resistance of the GMR sensor  600  due to changes induced by the external magnetic field (e.g., the field generated when a field transition on a disk is moved past the GMR sensor  600 ). A cap (not shown) is optionally provided on the free layer  618 . 
     During the manufacturing of a read/write head for magnetic recording media, the write head may be formed adjacent to the GMR sensor  600 . One skilled in the art will realize that during the manufacture of the write head, and during some of the processes involved in manufacturing the GMR sensor  600 , itself, high temperature processes have inevitably been involved. (Examples are the photoresist baking of the write head, the annealing of the AFM layer  624  materials on a substrate  610 , which is required for some materials, and resetting of the pinned layer  620 ). At these temperatures, the grain boundaries of adjacent materials tend to become aligned, notably at the junction of the spacer  622  and the free layer  618  and/or at the boundary of the spacer  622  and the pinned layer  620 . In this condition, it is very easy for diffusion between such layers to occur. This results in a degradation of the output signal amplitude produced by the GMR sensor  600 . 
     Other constructions of the GMR sensor  600  are possible, and one skilled in the art could readily adapt the present invention for use with such alternative constructions. For example, where pinned layers  620  having multiple layers are used, multiple iterations of the spacer  622  (and diffusion barrier) could also be employed. It is important to note that in order to illustrate the present invention, the inventive free layer  618  is shown in the context of the GMR sensor  600 . However, the invention is by no means limited to such constructions. Indeed, it is intended that the free layer be incorporated, as described herein, into more sophisticated constructions (perhaps containing additional material layers, or the like), both those presently in existence and those to be developed in the future. 
       FIG. 7  is a table  700  comparing the properties of a free layer formed by a CoFeCu alloy with other free layer compositions according to the present invention. Magnetic properties vary as a function of the composition of an alloy. According to the present invention, a varying of the Cu content in a composition can cause a decrease in the coercivity of the composition. For example, anisotropy values increase with the Cu content of a CoFeCu film  710 . Also, for example, anisotropy values higher than 11 Oersteds (Oe) can be achieved in the alloys by Cu enrichment of a CoFeCu film  710 . Hence, the anisotropy for a CoFeCu film  710  in the 3 to 6 atomic % Cu range is 13-14 Oe, and increases to about 16 to 20 Oe for films with about 14 to 20 atomic % Cu. 
     In  FIG. 7 , a comparison of various magnetic properties of a CoFeCu alloy  710 , CoFe  720 , and a standard bilayer (CoFe 15 Å/NiFe 25 Å)  730  free layer is examined. The various properties include the sheet resistance of a spin valve (Rsheet)  740 , sensitivity (ΔR/R)  750  and the coupling layer between a pinned layer and the free layer (He)  760 . Also, the coercivity, or resistance of a magnetic material to magnetization, is shown. The coercivity is displayed for both the easy axis (Hce)  770  (magnetism of a material in a favorable direction), coercivity hard axis (Hch)  780  (magnetism of a material in an unfavorable direction), uniaxial anisotropy (Hk)  790 , thickness  792  and lambda (i.e., the magnetostriction constant)  795 . 
     As GMR heads are made smaller, the standard bilayer free layer structure formed from CoFe and NiFe is also reduced. However, to maintain acceptable free layer properties, the CoFe should not be reduced to a thickness below 15 Å. Thus, the NiFe must be reduced to near zero as the bilayer structure reaches 15 Å in thickness. Accordingly, the ratio of CoFe to NiFe in the bilayer structure increases. However, the soft magnetic properties of CoFe are less attractive than NiFe. 
     Hence, the table of  FIG. 7  compares properties for spin valves including a 40 Å free layers of CoFeCu (81/11/8 atomic %)  710 , a CoFe (90/10 atomic %)  720  and NiFe bilayer structure with NiFe reduced to zero, and a standard bilayer structure (CoFe 15 Å/NiFe 25 Å)  730 . According to the present invention, although the sensitivity (ΔR/R)  750  is slightly reduced for CoFe  720  and CoFeCu  710 , the coercivity, Hce  770  and Hch  780 , for CoFeCu  710  is much improved over CoFe  720  as seen by these reduced values. This improved coercively (Hce  770 , Hch  780 ) coupled with a low uniaxial anisotropy field, Hk  790 , yields a high amplitude sensor (ΔR/R  750 ), and thus, a desired magnetic softness and stability. 
     In one embodiment of the present invention, a free layer  797  for Co x , Fe y , and Cu z  may have, for example, the atomic weight percentages wherein x is substantially equal to 81%, y is substantially equal to 11% and z is substantially equal to 8%. However, other percentages may be possible and yet provide a high amplitude sensor with desired magnetic stability. 
     The foregoing description of the exemplary embodiment of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not with this detailed description, but rather by the claims appended hereto.