Abstract:
A fully-pinned, flux-closed spin valve (SV) magnetoresistive sensor having a reference (pinned) layer with magnetization fixed by a first antiferromagnetic (AFM 1 ) layer, and a keeper layer with magnetization fixed by a second antiferromagnetic (AFM 2 ) layer. The magnetization of the keeper layer is saturated and fixed in an antiparallel orientation to the pinned layer magnetization by an exchange interaction with the AFM2 layer. The magnetic moments of the pinned layer and the keeper layer are approximately matched to form a flux-closed magnetic configuration wherein demagnetizing fields in the pinned layer are largely canceled and magnetostatic interaction with the free layer is reduced. Saturation of the keeper layer magnetization by exchange coupling with the AFM 2  layer eliminates or reduces magnetization canting at the edges of the keeper layer which can result in signal field shunting through the keeper layer. AFM 1  and AFM 2  layers may be formed of the same antiferromagnetic material, such as NiO, or alternatively may be formed of different antiferromagnetic materials, such as Ni—Mn and NiO, respectively.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates in general to spin valve magnetic transducers for reading information signals from a magnetic medium and, in particular, to a spin valve sensor with a pinned keeper layer, and to magnetic storage systems which incorporate such sensors. 
     2. Description of Related Art 
     Computers often include auxiliary memory storage devices having media on which data can be written and from which data can be read for later use. A direct access storage device (disk drive) incorporating rotating magnetic disks is commonly used for storing data in magnetic form on the disk surfaces. Data is recorded on concentric, radially spaced tracks on the disk surfaces. Magnetic heads including read sensors are then used to read data from the tracks on the disk surfaces. 
     In high capacity disk drives, magnetoresistive (MR) read sensors, commonly referred to as MR sensors, are the prevailing read sensors because of their capability to read data from a surface of a disk at greater track and linear densities than thin film inductive heads. An MR sensor detects a magnetic field through the change in the resistance of its MR sensing layer (also referred to as an “MR element”) as a function of the strength and direction of the magnetic flux being sensed by the MR layer. 
     The conventional MR sensor operates on the basis of the anisotropic magnetoresistive (AMR) effect in which an MR element resistance varies as the square of the cosine of the angle between the magnetization in the MR element and the direction of sense current flowing through the MR element. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in the direction of magnetization in the MR element, which in turn causes a change in resistance in the MR element and a corresponding change in the sensed current or voltage. 
     Another type of MR sensor is the giant magnetoresistance (GMR) sensor manifesting the GMR effect. In GMR sensors, the resistance of the MR sensing layer varies as a function of the spin-dependent transmission of the conduction electrons between magnetic layers separated by a non-magnetic layer (spacer) and the accompanying spin-dependent scattering which takes place at the interface of the magnetic and non-magnetic layers and within the magnetic layers. 
     GMR sensors using only two layers of ferromagnetic material (e.g., Ni—Fe) separated by a layer of non-magnetic material (e.g., copper) are generally referred to as spin valve (SV) sensors manifesting the SV effect. In an SV sensor, one of the ferromagnetic layers, referred to as the pinned layer (reference layer), has its magnetization typically pinned by exchange coupling with an antiferromagnetic (e.g., NiO or Fe—Mn) layer. The pinning field generated by the antiferromagnetic layer should be greater than demagnetizing fields (about 200 Oe) at the operating temperature of the SV sensor (about 120 C.) to ensure that the magnetization direction of the pinned layer remains fixed during the application of external fields (e.g., fields from bits recorded on the disk). The magnetization of the other ferromagnetic layer, referred to as the free layer, however, is not fixed and is free to rotate in response to the field from the recorded magnetic medium (the signal field). In the SV sensor, the SV effect varies as the cosine of the angle between the magnetization of the pinned layer and the magnetization of the free layer. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in direction of magnetization in the free layer, which in turn causes a change in resistance of the SV sensor and a corresponding change in the sensed current or voltage. IBM&#39;s U.S. Pat. No. 5,206,590 granted to Dieny et al., incorporated herein by reference, discloses a GMR sensor operating on the basis of the SV effect. 
     FIG. 1 shows a prior art SV sensor  100  comprising a free layer (free ferromagnetic layer)  110  separated from a pinned layer (pinned ferromagnetic layer)  120  by a non-magnetic, electrically-conducting spacer layer  115 . The magnetization of the pinned layer  120  is fixed by an antiferromagnetic (AFM) layer  130 . 
     FIG.  2   a  shows another prior art SV sensor  200  with a flux keepered configuration. SV sensor  200  is substantially identical to the SV sensor  100  shown in FIG. 1 except for the addition of a keeper layer  206  formed of ferromagnetic material separated from the free layer  110  by a non-magnetic, spacer layer  208 . The keeper layer  206  provides a flux closure path for the magnetic field from the pinned layer  120  resulting in easier pinned layer saturation and reduced magnetostatic interaction of the pinned layer  120  with the free layer  110 . U.S. Pat. No. 5,508,867 granted to Cain et al., incorporated herein by reference, discloses an SV sensor having a flux keepered configuration. 
     FIG.  2   b  shows a perspective view of the SV sensor  200 . This view shows the multilayer structure of SV sensor  200  as a ribbon-like sheet (stripe) extending away (downward in FIG.  2   b ) from the air-bearing surface (ABS). The keeper layer  206  is formed as a keeper layer stripe  260  having a front edge  270  at the ABS and extending away from the ABS to a rear edge  272 . In SV sensors having a flux keepered configuration, the keeper layer  206  maintains its magnetization due to magnetostatic interaction with the pinned layer and to sense current induced fields. These forces are generally sufficient to properly orient the magnetization of a central portion  280  of the keeper layer stripe  260  as indicated by arrows  282 , but usually leave a substantial region at the front and rear edges  270 ,  272  of the keeper layer stripe  260  with magnetization canted relative to the central portion  280  as indicated by arrows  274 ,  276 . This means that only the central portion of the keeper layer stripe is providing flux keepering. In order to completely cancel the pinned layer moment a thicker keeper layer must be used. The thicker keeper layer increases the amount of sense current shunted through the keeper reducing the magnetoresistance signal. The magnetization in the canted regions at the front and rear edges  270 ,  272  of the keeper layer stripe  260  can also rotate in the presence of a signal field, thus shunting flux away from the free layer  110  resulting in a smaller detected signal. 
     Therefore there is a need for an SV sensor that provides improved orientation of the magnetization of the keeper layer in order to enhance the effectiveness of the keeper layer to form a flux closure path for the pinned layer magnetization. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to disclose a keepered SV sensor with improved orientation of the magnetization of the keeper layer. 
     It is another object of the present invention to disclose a keepered SV sensor wherein the magnetization direction of the keeper layer is fixed (pinned) by an exchange interaction with an antiferromnagnetic (AFM) layer. 
     It is a further object of the present invention to disclose a keepered SV sensor wherein the magnetic moments of the pinned layer and the keeper layer may be closely matched while still having a predictable GMR signal polarity. 
     In accordance with the principles of the present invention, there is disclosed a preferred embodiment wherein an SV sensor has a pinned layer with its direction of magnetization fixed (pinned) by exchange coupling with a first AFM layer, and a keeper layer formed of ferromagnetic material with its direction of magnetization fixed (pinned) by exchange coupling with a second AFM layer. An electrically conducting spacer layer, a ferromagnetic free layer and a non-magnetic, low conductivity spacer layer are disposed between the pinned layer and the keeper layer. The directions of magnetization of the pinned layer and the keeper layer are fixed in an antiparallel orientation with respect to each other in order to reduce or cancel their net magnetic moment. Pinning of the keeper layer by exchange coupling with an AFM layer results in fully saturated keeper layer magnetization in the transverse direction, eliminating or substantially reducing canting of the magnetization at the keeper layer edges. Having saturated keeper layer magnetization allows a thinner keeper layer to provide the magnetic moment needed to cancel the magnetic moment of the pinned layer. Saturation of the magnetization of the keeper layer also results in decreased shunting of the signal field through the keeper layer. 
     The first and second AFM layers used to fix the magnetization of the pinned and keeper layers, respectively, may be formed of the same material, for example NiO, and the proper magnetic state may be achieved by pulsed current initialization, or alternatively, the AFM layers may be formed of different materials and the proper magnetic states may be set at different temperatures for each AFM material. Sense current shunting by the first and second AFM layers is reduced by using high electrical resistivity AFM materials to form both layers. 
     The above, as well as additional objects, features, and advantages of the present invention will become apparent in the following detailed written description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a fuller understanding of the nature and advantages of the present 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. In the following drawings, like reference numerals designate like or similar parts throughout the drawings. 
     FIG. 1 is an air bearing surface view, not to scale, of a prior art SV sensor; 
     FIG.  2   a  is an air bearing surface view, not to scale, of a prior art keepered SV sensor; 
     FIG.  2   b  is a perspective view, not to scale, of a prior art keepered SV sensor; 
     FIG. 3 is a simplified drawing of a magnetic recording disk drive system; 
     FIG. 4 is an air bearing surface view, not to scale, of the preferred embodiment of an SV sensor according to the present invention; 
     FIG. 5 is an air bearing surface view, not to scale, of an alternate embodiment of an SV sensor according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The following description is the best embodiment presently contemplated for carrying out the present invention. This description is made for the purpose of illustrating the general principles of the present invention and is not meant to limit the inventive concepts claimed herein. 
     Referring now to FIG. 3, there is shown a disk drive  300  embodying the present invention. As shown in FIG. 3, at least one rotatable magnetic disk  312  is supported on a spindle  314  and rotated by a disk drive motor  318 . The magnetic recording media on each disk is in the form of an annular pattern of concentric data tracks (not shown) on the disk  312 . 
     At least one slider  313  is positioned on the disk  312 , each slider  313  supporting one or more magnetic read/write heads  321  where the head  321  incorporates the MR sensor of the present invention. As the disks rotate, the slider  313  is moved radially in and out over the disk surface  322  so that the heads  321  may access different portions of the disk where desired data is recorded. Each slider  313  is attached to an actuator arm  319  by means of a suspension  315 . The suspension  315  provides a slight spring force which biases the slider  313  against the disk surface  322 . Each actuator arm  319  is attached to an actuator means  327 . The actuator means as shown in FIG. 3 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 a controller  329 . 
     During operation of the disk storage system, the rotation of the disk  312  generates an air bearing between the slider  313  (the surface of the slider  313  which includes the head  321  and faces the surface of the disk  312  is referred to as an air bearing surface (ABS)) and the disk surface  322  which exerts an upward force or lift on the slider. The air bearing thus counter-balances the slight spring force of the suspension  315  and supports the slider  313  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 the controller  329 , such as access control signals and internal clock signals. Typically, controller  329  comprises logic control circuits, storage means and a microprocessor. The controller  329  generates control signals to control various system operations such as drive motor control signals on a line  323  and head position and seek control signals on a line  328 . The control signals on the line  328  provide the desired current profiles to optimally move and position the slider  313  to the desired data track on the disk  312 . Read and write signals are communicated to and from read/write heads  321  by means of a recording channel  325 . 
     The above description of a typical magnetic disk storage system, 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 each actuator may support a number of sliders. 
     FIG. 4 shows an air bearing surface (ABS) view of an SV sensor  400  according to the preferred embodiment of the present invention. The SV sensor  400  comprises end regions  404  and  406  separated from each other by a central region  402 . The substrate  450  can be any suitable substance, including glass, semiconductor material, or a ceramic material, such as alumina (Al 2 O 3 ). The seed layer  440  is a layer deposited to modify the crystallographic texture or grain size of the subsequent layers, and may not be needed depending on the substrate and AFM materials. If used, the seed layer may be formed of tantulum (Ta), zirconium (Zr), nickel-iron (Ni—Fe), or Al 2 O 3 . In the preferred embodiment, the first AFM (AFM 1 ) layer  430  is formed of nickel-oxide (NiO) and is deposited directly over the substrate  450  by ion beam sputtering a nickel (Ni) target in the presence of an oxygen-containing gas to the thickness at which the desired exchange properties are achieved, typically in the range from 200 Å to 500 Å. A laminated pinned layer  420  comprising a second sublayer  424  formed of Ni—Fe (permalloy) having a thickness of about 20 Å deposited on the AFM 1  layer  430 , and a first sublayer  426  formed of cobalt (Co), or alternatively of Co—Fe, having a thickness of about 25 Å deposited over the second sublayer  424 . A first spacer (spacer 1 ) layer  415  formed of copper (Cu) having a thickness of about 20 Å is deposited on the laminated pinned layer  420 . A free layer  410  formed of Ni—Fe having a thickness of about 55 Å is deposited on the spacer 1  layer  415 . A second spacer (spacer 2 ) layer  408  formed of a tantalum (Ta) film having a thickness of about 50 Å deposited on the free layer  410  separates the free layer  410  from a keeper layer  406  formed of a Ni—Fe—Cr film having a thickness in the range from 50 Å to 100 Å. The spacer 2  layer  408  is made of a non-magnetic, high electrical resistivity material to provide magnetic isolation of the keeper layer  406  from the free layer  410 . A second AFM (AFM 2 ) layer  432  formed of NiO having a thickness in a range from 200 Å to 500 Å is deposited on the keeper layer  406 . A cap layer  405  formed of a tantalum (Ta) film having a thickness of about 50 Å completes the structure of the central portion  402  of the SV sensor  400 . 
     After deposition of the layers forming the central portion  402 , the SV sensor  400  was annealed in a high vacuum oven for  20  minutes at 200° C. with a magnetic field applied in a transverse direction (perpendicular to and away from the ABS) to set the direction of the exchange interaction of the pinned and keeper layers with the AFM 1  and AFM 2  layers, respectively. 
     After annealing, bias layers  452  and  454  are formed in the end regions  404  and  406 , respectively, for providing a longitudinal bias field to the free layer  410  to ensure a single magnetic domain state in the free layer oriented in the direction indicated by arrow  412 . Lead layers  460  and  465  are also deposited in the end regions  404  and  406 , respectively, to provide electrical connections for the flow of the sensing current I s  from a current source  470  to the SV sensor  400 . A signal detector  480 , which is electrically connected to leads  460  and  465 , senses the change in the resistance due to changes induced in the free layer  410  by the external magnetic field (e.g., field generated by a data bit stored on a disk). The external magnetic field acts to rotate the direction of magnetization of the free layer  410  relative to the direction of magnetization of the pinned layer  420  which is preferably pinned perpendicular to the ABS. The signal detector  480  preferably includes a digital recording channel such as a PRML channel as is known to those skilled in the art. The signal detector  480  also includes other supporting circuitries such as a preamplifier (electrically placed between the sensor and the channel) for conditioning the sensed resistance changes as is known to those skilled in the art. 
     After the fabrication process of SV sensor  400 , the magnetizations of the pinned layer  420  and the keeper layer  406  are oriented perpendicular to and away from the ABS. To form the fully-pinned, flux closed configuration of the present invention, the magnetizations of the pinned layer  420  and the keeper layer  406  must be fixed in an antiparallel orientation by an initialization process. A current pulse is applied by the sense current source to the SV sensor  400 . The current pulse resistively heats the SV sensor  400  causing the temperature of the AFM 1  and AFM 2  layers to exceed the blocking temperature (temperature at which the AFM pinning field reaches zero Oe) of NiO (approximately 220° C.). The applied current pulse also induces magnetic fields in the pinned layer  420  and the keeper layer  406  directed perpendicular to the ABS. Since most of the current pulse flows through the spacer 1  and free layers,  415  and  410 , respectively, which lie between the pinned layer  420  and the keeper layer  406  the current pulse induced magnetizations of the pinned layer  420  and the keeper layer  406  are antiparallel as indicated by arrows  407  and  422 , respectively (arrow heads directed into and out of the plane of the paper, respectively). When the current pulse is removed, the SV sensor  400  cools below the NiO blocking temperature leaving the pinned and keeper layer magnetizations fixed in the desired antiparallel fully-pinned, flux-closed orientation. 
     Alternative AFM materials for the AFM 1  layer  430  or the AFM 2  layer  432  such as Fe—Mn, Pd—Mn, Pt—Mn, Pd—Pt—Mn, Ir—Mn, Rh—Mn,and Ru—Mn may also be used to fabricate SV sensors according to the present invention. However, the use of electrically insulating AFM materials, such as NiO or α—Fe 2 O 3 , is advantageous in reducing sense current shunting through the AFM 1  and AFM 2  layers. 
     Alternative spacer 1  layer  415  materials such as gold and silver, and alternative spacer 2  layer  408  materials such as tantalum oxide, Al 2 O 3  and SiO 2  may also be used to fabricate SV sensors according to the present invention. 
     Alternative keeper layer  406  materials such as Ni—Fe—Nb, Ni—Fe—Rh, Ni—Fe, Co—Fe—Nb, Co—Fe—Nb—Hf, Co—Zr—Mo, Fe—Hf—N, Fe—Y—N Fe—Zr—N, Fe—Hf—O, Fe—Y—O and Fe—Zr—O may be used to fabricate SV sensors according to the present invention. 
     Having the AFM 2  layer fixing (pinning) the magnetization of the keeper layer  406  by exchange pinning results in a predictable and stable magnetization configuration providing optimum flux closure for the SV sensor  400 . With a pinned keeper layer, the magnetization of the keeper layer  406  is saturated in the desired antiparallel direction to the magnetization of the pinned layer  420 , greatly reducing or eliminating canting of the magnetization direction at the keeper layer  406  edges. The improved and predictable keeper layer magnetization improves consistency in achieving nearly zero net magnetic moment of the pinned and keeper layers and allows a keeper layer having reduced thickness to be used. Achieving near zero net magnetic moment will improve SV sensor performance by reducing demagnetizing fields in the pinned layer  420  and by reducing magnetostatic coupling of the pinned and keeper layers  420 ,  407  to the free layer  410 . For optimum SV sensor biasing, it is preferred to have the keeper layer magnetic moment slightly larger than the pinned layer magnetic moment in order to compensate for the sense current induced bias field. 
     Saturation of the keeper layer  406  magnetization by exchange interaction with the AFM 2  layer  432  reduces the permeability of the keeper layer  406  resulting in reduced signal magnetic flux shunting from the free layer  410  through the keeper layer  406 . Reduced shunting of the signal magnetic flux will increase the sensitivity of SV sensor  400 . 
     FIG. 5 shows an air bearing surface (ABS) view of an SV sensor  500  according to an alternate embodiment of the present invention. In this embodiment, the second AFM (AFM 2 ) layer  532  is formed of nickel-oxide (NiO) having a thickness in a range from 200 Å to 400 Å deposited directly onto the substrate  450 . 
     A keeper layer  506  formed of Ni—Fe—Cr having a thickness of in a range from 50 Å to 100 Å is formed on the AFM 2  layer  532 . A second spacer (spacer 2 ) layer  508  formed of Ta having a thickness of about 50 Å is deposited on the keeper layer  506 . The spacer 2  layer  508  is a non-magnetic, high electrical resistivity material layer separating the keeper layer  506  from a free layer  510  formed of Ni—Fe having a thickness of about 55 Å. A first spacer (spacer 1 ) layer  515  formed of a copper (Cu) film having a thickness of about 20 Å is deposited on the free layer  510  and a pinned layer  520  formed of a cobalt (Co) film having a thickness in a range from 20 Å to 50 Å is deposited on the spacer 1  layer  515 . An AFM 1  layer  530  formed of Ni—Mn having a thickness in the range from 100 Å to 400 Å is deposited on the pinned layer  520 . The preferred composition of the Ni—Mn AFM layer is a Mn content in the range between  46  and  60  atomic percent. A cap layer  405  formed of a tantalum (Ta) film having a thickness of about 50 Å completes the structure of the central portion  402  of the SV sensor  500 . 
     After deposition of the layers forming the central portion  402 , the SV sensor  500  was annealed in a high vacuum oven for  2  hours at 280° C. with a magnetic field applied in a transverse direction (perpendicular to and toward the ABS) to set the direction of the exchange interaction of the pinned layer with the Ni—Mn AFM 1  layer. 
     After the fabrication process of SV sensor  500 , the magnetizations of the pinned layer  520  and the keeper layer  506  are oriented perpendicular to and toward the ABS. An initialization process is needed to set the magnetization direction of the keeper layer  506  antiparallel to the magnetization direction of the pinned layer  520 . Since the AFM 2  layer  532  has a lower blocking temperature than the AFML layer  530 , the initialization can be done by a low temperature anneal process (200° C. for 20 minutes) in an orienting magnetic field of about 100 Oe to cause the NiO AFM 2  layer to exceed its blocking temperature resulting in reorientation of the magnetization of the keeper layer  506  perpendicular to and away from the ABS. 
     While the present invention has been particularly shown and described with reference to the preferred embodiments, it will be understood by those skilled in the art that various changes in form and detail may be made without departing from the spirit, scope and teaching of the invention. Accordingly, the disclosed invention is to be considered merely as illustrative and limited in scope only as specified in the appended claims.