Abstract:
Disclosed is a system and method for forming a current-perpendicular-to-plane (CPP) spin-valve sensor with one or more metallic oxide barrier layers in order to provide a low junction resistance and a high GMR coefficient. In disclosed embodiments, the metallic oxide barrier layers are formed with oxygen-doping/in-situ oxidation processes comprising depositing a metallic film in a first mixture of argon and oxygen gases and subsequent in-situ oxidization in a second mixture of argon and oxygen gases. The exposure to oxygen may be conducted at a low partial oxygen pressure and at a moderate temperature. Smaller, more sensitive CPP spin-valve sensors may be formed through the use of the oxygen-doping/in-situ oxidization processes of the present invention, thus allowing for greater densities of disk drive systems.

Description:
BACKGROUND OF THE INVENTION 
     1. The Field of the Invention 
     The present invention relates generally to spin-valve sensors for reading data from a magnetic media and, more particularly to novel structures and processes of spin-valve sensors, and to magnetic recording systems which incorporate such spin-valve sensors 
     2. The Relevant Art 
     Computer systems generally utilize auxiliary memory storage devices having magnetic media on which data can be written and from which data can be read for later uses. A direct access storage device, such as a disk drive, incorporating rotating magnetic disks, is commonly used for storing data in a magnetic form on the disk surfaces. Data are written on concentric, radially spaced tracks on the disk surfaces. Magnetic read/write heads are then used to read data from the tracks on the disk surfaces. 
     FIG. 1 shows one example of a disk drive  100  embodying the present invention. As shown in FIG. 1, the disk drive  100  comprises at least one rotatable magnetic disk  112  supported on a spindle  114  and rotated by a disk drive motor  118 . The magnetic medium on each magnetic disk  112  is in the form of concentric, annular data tracks (not shown). 
     At least one slider  113  is positioned on the disk  112 . Each slider  113  supports one or more magnetic read/write heads  121  incorporating one or more read sensors of the present invention. As the magnetic disk rotates, the slider  113  is moved radially in and out over the disk surface  122  so that the magnetic read/write heads  121  may access different portions of the magnetic disk  112  where desired data are written. Each slider  113  is attached to an actuator arm  119  by means of a suspension  115 . The suspension  115  provides a slight spring force which biases the slider  113  against the disk surface  122 . Each actuator arm  119  is attached to an actuator  127 . The actuator  127  as shown in FIG. 1 may be a voice coil motor (VCM). The VCM comprises a coil movable within a fixed magnetic field, and the direction and speed of the coil movements are controlled by the motor current signals supplied by a controller  129 . 
     During operation of the disk storage system, the rotation of the magnetic disk  112  generates an air bearing between the surface of the slider  113  (which includes the surface of the head  121 ) referred to as an air bearing surface (ABS), and the surface  122  of the disk  112 . This air bearing exerts an upward force or lift on the slider  113 , and thus counter-balances the slight spring force of the suspension  115  and supports the 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 the control unit  129 . The control signals include 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 a line  123  and head position and seek control signals on a line  128 . The control signals on the line  128  provide the desired current profiles to optimally move and position the slider  113  to the desired data track on the disk  112 . Read and write signals are communicated to and from the read/write heads  121  by means of a recording channel  125 . In the depicted embodiment, the read/write heads  121  incorporate the read sensor of the present invention. 
     Two types of read sensors have been extensively explored for magnetic recording at ultrahigh densities (≧20 Gb/in 2 ). One is a current-in-plane (CIP) spin-valve sensor  200  in which a sense current  218  flows in a direction parallel to interfaces of a plurality of films, as depicted in FIG.  2 . The other is a current-perpendicular-to-plane (CPP) magnetic-tunnel-junction sensor  300  in which a sense current  318  flows in a direction perpendicular to the interfaces of a plurality of films. Greater details will be given to the CPP read sensor of the present invention below with reference to FIG.  3 . 
     In high capacity disk drives, a giant magnetoresistance (GMR) head carrying the CIP spin-valve sensor is now extensively used to read written data from the tracks on the disk surfaces. This CIP spin-valve sensor typically comprises two ferromagnetic films separated by an electrically conducting nonmagnetic film. Due to a GMR effect, the resistance of this CIP spin-valve sensor varies as a function of the spin-dependent transmission of conduction electrons between the two ferromagnetic films and the accompanying spin-dependent scattering which takes place at interfaces of the ferromagnetic and nonmagnetic films. 
     In this CIP spin-valve sensor, one of the ferromagnetic films, referred to as a transverse pinned layer, typically has its magnetization pinned by exchange coupling with an antiferromagnetic film (e.g., Ni—Mn, Pt—Mn, Ir—Mn, etc.) used as a transverse pinning layer. The magnetization of the other ferromagnetic film, referred to as a sense or “free” layer, however, is not fixed and is free to rotate in response to the signal field from written data on the magnetic medium. In this CIP spin-valve sensor, the GMR effect varies as the cosine of the angle between the magnetizations of the sense and transverse pinned layers. The written data can be read from the magnetic medium because the external magnetic field from the written data causes a change in the direction of magnetization in of the sense layer, which in turn causes a change in the resistance of the CIP spin-valve sensor and a corresponding change in the sensed current or voltage. It should be noted that an anisotropy magnetoresistance (AMR) effect is also present in the sense layer and tends to reduce the overall GMR effect. 
     The CIP spin-valve sensor  200  is formed with deposition methods, such as DC magnetron sputtering, ion beam sputtering, etc, onto a wafer and is confined in a central region with two end regions (not shown) that abut the edges of the central region. Seed layers  202  are deposited on the wafer. These seed layers have a face-centered-cubic crystalline structure, which orients the crystalline structures of subsequently deposited films so that the closest packed planes of these films are parallel to the wafer surface. These closest packed planes are believed to play a crucial role in improving GMR properties of the CIP spin-valve sensor  200 . 
     A transverse pinning layer made of an antiferromagnetic film  204  is deposited above the seed layer  202 . A keeper layer made of a ferromagnetic film  206  is separated from a reference layer also made of a ferromagnetic film  210  by a ruthenium (Ru) spacer layer  208 . The magnetizations of the keeper layer  206  and the reference layer  210  (both of which are used as transverse pinned layers) are fixed through antiferromagnetic/ferromagnetic coupling between the transverse pinning layer  204  and the keeper layer  206 , and through ferromagnetic/ferromagnetic coupling across the Ru spacer layer  208 . The reference layer  210  is separated by a copper (Cu) spacer layer  212  from a sense layer also made of a ferromagnetic film  214 . The cap layer  216  is deposited above the sense layer  214 . 
     The other, more recently explored CPP magnetic-tunnel-junction sensor is shown in FIG.  3 . This CPP magnetic-tunnel-junction sensor  300  has a similar sensor structure as that of the CIP spin-valve sensor  200 . The primary difference between the two sensors is that the Cu spacer layer used in the CIP spin-valve sensor  200  is replaced by an Al—O barrier layer in the magnetic-tunnel-junction sensor  300 . 
     The disk drive industry has been engaged in an ongoing effort to increase the recording density of the disk drive, and correspondingly to increase the overall signal sensitivity to permit the currently used CIP spin-valve sensor in the disk drive to read smaller changes in magnetic fields. The major property relevant to the signal sensitivity of the CIP spin-valve sensor is its GMR coefficient. A higher GMR coefficient leads to higher signal sensitivity and enables the storage of more data in a unit area on a disk surface. The GMR coefficient of the CIP spin-valve sensor is expressed as ΔR G /R ∥ , where R ∥  is a resistance measured when magnetizations of the sense and reference layers are parallel to each other, and ΔR G  is the maximum giant magnetoresistance measured when magnetizations of the sense and reference layers are antiparallel to each other. 
     An additional property relevant to the performance of the CIP spin-valve sensor is ferromagnetic/ferromagnetic coupling between the reference and sense layers. This ferromagnetic/ferromagnetic coupling induces a ferromagnetic coupling field (H F ), which must be very well controlled for optimal sensor operation. 
     In order to achieve higher recording densities, the disk drive industry is constantly miniaturizing the CIP spin-valve sensor. Several challenges have arisen due to the miniaturization of the CIP spin-valve sensor. One area of difficulty has been finding an ideal insulating gap material for the use as thin top and bottom gap layers. To attain, for example, a 38.8 nm thick CIP spin-valve sensor with the sense layer located in the center of a 80 nm thick read gap, the top and bottom gap layers must have thicknesses of 34.4 and 6.8 nm, respectively. A 6.8 nm thick bottom gap layer is too thin to prevent electrical shorting between the bottom magnetic shield layer and the CIP spin-valve sensor. Consequently, there is a high possibility of electrical shorting, making the CIP spin-valve sensor non-functional. 
     The CPP magnetic-tunnel-junction sensor has been used to solve these issues. The CPP magnetic-tunnel-junction sensor is made of at least two ferromagnetic films separated by an insulating barrier layer. The tunnel magnetoresistance (TMR) coefficient is defined as ΔR T /R ∥ , where R ∥  is the resistance measured when the magnetizations of the two ferromagnetic films are parallel to each other, and ΔR T  is the maximum tunnel magnetoresistance measured when the magnetizations of the two ferromagnetic films are antiparallel to each other. Since the sense current must flow from a top magnetic shield layer, through the sensor, to a bottom magnetic shield layer, or vice versa, both top and bottom gap layers must be formed of conducting films. As a result, electrical shorting between the top and bottom magnetic shield layers and the sensor, and between the sensor and the top magnetic shield layer, is no longer a concern, and further decreasing of the read gap thickness to below 60 nm becomes feasible. 
     Issues are also encountered when attempting to use the CPP magnetic-tunnel-junction sensor to increase magnetic recording densities. These issues originate mainly from difficulties in attaining a high TMR coefficient and a low junction resistance simultaneously. For instance, in a typical oxidation process used for the CPP magnetic-tunnel-junction random access memory, where a 1.2 nm thick Al film is exposed for 1 hour in air, a TMR coefficient of 29.3% and a junction resistance of 5714 Ω-μm 2  are attained. This junction resistance is much higher than a most preferred junction resistance of 0.4 Ω-μm 2 . With this most preferred junction resistance, a CPP magnetic-tunnel-junction sensor with a width of 0.1 μm and a height of 0.1 μm will exhibit an optimal sensor resistance of 40 Ω. With the optimal resistance of 40 Ω, high signal amplitudes can be attained without concerns on electrostatic discharge. 
     To substantially reduce the junction resistance to a value in a preferred range of between 0.1 and 10 Ω-μm 2 , an in-situ oxidation process, where a 0.54 nm thick Al film is exposed for 4 min in an oxygen gas of 2 Torr, is applied. After this in-situ oxidation process, a TMR coefficient of 18.5% and a junction resistance of 8 Ω-μm 2  are attained. Hence, the TMR coefficient and junction resistance of the CPP magnetic-tunnel-junction sensor substantially depend on the thickness, oxidation pressure, and oxidation time of the Al—O barrier layer. As seen from the above discussion, the state-of-the-art CPP magnetic-tunnel-junction sensor is still not viable for the use for magnetic recording. 
     The difficulty in attaining a low junction resistance originates from the high electrical resistivity of the Al—O film (≧10 8  μΩ-cm). Hence, to attain a low junction resistance, a barrier layer with a low electrical resistivity must be selected. A CIP spin-valve sensor with a Cu spacer layer may be implemented into the CPP magnetic-tunnel-junction sensor structure, and used as a CPP spin-valve sensor. A GMR effects, instead of the tunneling effects, will occur in the CPP spin-valve sensor. Its GMR coefficient is typically higher by approximately 40% than that of the CIP spin-valve sensor. However, this CPP spin-valve sensor with the Cu barrier layer is also not viable due to a low electrical resistivity of the Cu film (˜3 μΩ-cm), which will lead to a junction resistance of as low as below 0.001 Ω-μm 2 . 
     Thus, it can be seen from the above discussion that there is a need existing in the art for an improved CPP spin-valve sensor exhibiting a high GMR coefficient and a low junction resistance simultaneously. Particularly, it would be advantageous to provide a CPP spin-valve sensor exhibiting a junction resistance controlled to be much higher than that of a CPP spin-valve sensor with a Cu barrier layer, but significantly lower than that of a CPP magnetic-tunnel-junction sensor with an Al—O barrier layer. 
     OBJECTS AND BRIEF SUMMARY OF THE INVENTION 
     The apparatus of the present invention has been developed in response to the present state of the art, and in particular, in response to the problems and needs in the art that have not yet been fully solved by currently available current-in-plane (CIP) spin-valve sensors. Accordingly, it is an overall object of the present invention to provide a current-perpendicular-to-plane (CPP) spin-valve sensor that overcomes many or all of the above-discussed shortcomings in the art. 
     To achieve the foregoing object, and in accordance with the invention as embodied and broadly described herein in the preferred embodiments, novel CPP spin-valve sensor is provided. The CPP spin-valve sensor of the present invention in one embodiment comprises a metallic oxide barrier layer interposed between the sense layer and the reference layer. Under a preferred embodiment of the present invention, the metallic oxide barrier layer is formed substantially of a Cu—O film with an oxygen content in the range of between about 12 and about 24 at %, with a thickness in the range of between about 2 and about 6 nm, and with an electrical resistivity in the range of between about 100 and about 1600 μΩ-cm. The CPP spin-valve sensor also comprises a reference layer formed of a second ferromagnetic film disposed to one side of the sense layer, a keeper layer disposed to one side of the reference layer, a transverse pinning layer disposed to one side of the keeper layer, and longitudinal pinned and pinning layers disposed to another side of the sense layer. 
     In an alternative embodiment, the CPP spin-valve sensor comprises a plurality of alternating metallic oxide barrier and sense layers. In one embodiment the plurality of alternating metallic oxide barrier and sense layers comprises 3 metallic oxide barrier and 3 sense layers. 
     The CPP spin-valve sensor of the present invention may be incorporated within a disk drive system comprising a magnetic disk, a CPP spin-valve sensor configured in the manner discussed above, an actuator for moving the CPP spin-valve sensor across the magnetic disk so that the CPP spin-valve sensor may access different regions of written data on the magnetic disk, and a detector. The detector may be electrically coupled to the CPP spin-valve sensor for detecting changes in resistance of the sensor caused by the rotation of the magnetization of the sense layer relative to the fixed net magnetizations of the reference and keeper layers in response to magnetic fields from the written data. 
     A method of fabrication of the present invention is also presented for forming a metallic oxide barrier layer of a CPP spin-valve sensor. In one embodiment the fabrication method comprises forming the afore-mentioned layers of the CPP spin-valve sensor, and forming a metallic oxide barrier layer to one side of the reference layer by depositing a metallic film using reactive DC-pulsed sputtering in a first mixture of argon and oxygen gases and subsequent in-situ oxidizing in a second mixture of argon and oxygen gases. 
     The thickness of the oxygen-doped/in-situ oxidized film is in the range of between about 2 nm and about 6 nm. In one embodiment, the oxygen doping process is conducted preferably in the first mixture of argon and oxygen gases of 2.985 and 0.015 mTorr, respectively. The in-situ oxidation process is conducted preferably in the second mixture of argon and oxygen gases of 2.94 and 0.06 mTorr, respectively. 
     These and other objects, features, and advantages of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In order that the manner in which the advantages and objects of the invention are obtained will be readily understood, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments thereof which are illustrated in the appended drawings. Understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered to be limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which: 
     FIG. 1 is a schematic block diagram illustrating one embodiment of a magnetic recording disk drive system; 
     FIG. 2 is a cross-sectional view illustrating the structure of a CIP spin-valve sensor of the prior art; and 
     FIG. 3 is a cross-sectional view illustrating the structure of a CPP magnetic-tunnel-junction sensor of the prior art; and 
     FIG. 4 is a cross-sectional view illustrating the structure of a CPP spin-valve sensor of the present invention; and 
     FIG. 5 is a cross-sectional view illustrating the structure of an alternative embodiment of a CPP spin-valve sensor of the present invention; and 
     FIG. 6 is a schematic block diagram illustrating one embodiment of an integrated DC magnetron/ion beam sputtering system suitable for use with the present invention; and 
     FIG. 7 is a schematic flow chart illustrating a method of fabrication of a CPP spin-valve sensor of the present invention; and 
     FIG. 8 is a schematic flow chart illustrating a method of fabrication of a metallic oxide barrier layer of the present invention. 
     FIG. 9 is a chart depicting the ferromagnetic coupling field and GMR coefficient versus the oxygen partial pressure used in the oxygen-doping process. 
     FIG. 10 is a chart depicting the oxygen content and electrical resistivity of an approximately 100 nm thick Cu—O film versus the oxygen partial pressure used in the oxygen-doping process. 
     FIG. 11 is a chart depicting the ferromagnetic coupling field and GMR coefficient versus the oxygen partial pressure used in the in-situ oxidation process. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 4, shown therein is one embodiment of a CPP spin-valve sensor  400  of the present invention. The depicted embodiment is a bottom-type current-perpendicular-to-plane (CPP) spin-valve sensor (where the transverse pinning layer is located below the sense layer), but of course, other types of CPP spin-valve sensors may also be formed using the novel metallic oxide barrier layer and processes of the present invention, such as top and dual CPP spin-valve sensors. In the depicted embodiment, the CPP spin-valve sensor  400  is formed by a suitable deposition method such as DC magnetron or ion beam sputtering onto a wafer (not shown). 
     Under a preferred embodiment of the present invention, a seed layer  402  is formed on a wafer. The seed layer  402  may be configured in a number of different ways according to the invention, and functions primarily to form a foundation for the growth of the layers on top of it. In one embodiment, the seed layer  402  comprises a Ni—Cr—Fe film and a Ni—Fe film. The seed layer  402  preferably has a thickness ranging from about 2 nm to about 5 nm and a more preferred thickness of about 4 nm. In this embodiment, the Ni—Cr—Fe film may have a thickness of about 3 nm, and the Ni—Fe film may have a thickness of about 1 nm. 
     In the following description, “above” is intended to refer to a direction further away from the seed layer  402  and “below” is intended to refer to a direction closer to the seed layer  402 . Likewise, “bottom” layers are those closest to the seed layer  402 , and the “top” layers are those furthest from the seed layer  402 . Above the seed layer  402  are shown a transverse pinning layer  404 , a keeper layer  406 , a spacer layer  408 , a reference layer  410 , a barrier layer  412 , a sense layer  414 , a decoupling layer  416 , a longitudinal pinned layer  418 , a longitudinal pinning layer  420 , and a cap layer  422 . 
     The transverse pinning layer  404  is deposited above the seed layer  402 . The transverse pinning layer  404  is preferably formed of a Pt—Mn film with a Mn content preferably in the range of between about 47 and about 53 at %, and a most preferred Mn content of about 50 at %. The transverse pinning layer  404  has a thickness preferably in the range of between about 15 and about 25 nm, and a most preferred thickness of about 20 nm. The composition and thickness of the Pt—Mn film are optimally selected in order for the Pt—Mn film to become antiferromagnetic and strongly exchange-couple with an adjacent ferromagnetic film after appropriate annealing. 
     A keeper layer  406  is deposited above the transverse pinning layer  404 . The keeper layer  406  is preferably formed of a Co—Fe film with an Fe content preferably in the range of between about 50 and about 0 at %, and a most preferred Fe content of about 10 at %. The Co—Fe film has a thickness preferably in the range of between about 1.6 nm and about 2.4 nm, and a most preferred thickness of about 2 nm. A spacer layer  408  is deposited above the keeper layer  406 . The spacer layer  408  is preferably formed of a Ru film with a thickness preferably in the range of between about 0.6 nm and about 1 nm, and a most preferred thickness of about 0.8 nm. The thickness of the Ru film must be very well selected in order to induce very strong antiparallel exchange-coupling between the keeper layer  406  and the reference layer  410 . 
     The reference layer  410  is deposited above the spacer layer  408 . The reference layer  410  is preferably formed of a Co—Fe film with an Fe content preferably in the range of between about 50 and about 0 at %, and a most preferred Fe content of about 10 at %. The Co—Fe film has a thickness preferably in the range of between about 1.8 nm and about 2.6 nm, and a most preferred thickness of about 2.2 nm. The thickness of the reference layer  410  is slightly higher than that of the keeper layer  406 , in order to attain designed small net magnetic moments of the reference and keeper layers, which induce desired small demagnetizing fields after sensor lapping. 
     A metallic oxide barrier layer  412  is shown deposited over and adjacent to the reference layer  410 . In order to provide the advantages of the present invention, the barrier layer  412  is preferably formed of a metallic oxide film that has an optimal electrical resistivity much higher than that (˜3 μΩ-cm) of the Cu spacer layer used in the CIP spin-valve sensor, but much lower than that (≧10 8  μΩ-cm) of the Al—O barrier layer used in the CPP magnetic-tunnel-junction sensor. In one embodiment, the barrier layer  412  is formed of an oxygen-doped/in-situ oxidized Cu (Cu—O) film with an oxygen content preferably in the range of between about 12 and about 24 at %, and a most preferred oxygen content of about 20 at %. This composition range is selected since a transformation from a Cu face-centered-cubic phase to a Cu—O amorphous phase occurs at the oxygen content of about 12 at %, and another transformation from the Cu—O amorphous phase to a CuO 2  face-centered-cubic phase occurs at the oxygen content of about 24 at %. In this composition range, the Cu—O film is thus amorphous, and this amorphous phase exhibits an optimal electrical resistivity in the range of between about 100 and about 1600 μΩ-cm. 
     The Cu—O film  412  has a thickness preferably in the range of about 2 nm and about 6 nm, and a most preferred thickness of about 2.2 nm. The thickness and the oxygen-doping/in-situ oxidation processes are carefully selected in order to attain a slightly positive ferromagnetic coupling field (≦20 Oe) between the reference and sense layers for balancing the demagnetizing fields (induced from the net magnetic moments of the reference and keeper layers after sensor lapping), thereby orienting the magnetization of the sense layer  414  in a longitudinal direction parallel to an air bearing surface for optimal sensor operation. The oxygen-doping/in-situ oxidation processes will be discussed in greater details below with reference to FIG.  8 . 
     In one embodiment, a sense layer  414  is formed above the barrier layer  412 . The sense layer  414  is preferably formed of a Co—Fe film with an Fe content preferably in the range of between about 10 and about 18 at %, and a most preferred Fe content of about 14 at %. The Fe content of the Co—Fe film is optimally selected in order for the sense layer with a designed magnetic moment to exhibit a negative saturation magnetostriction in the range of between −1×10 −6  and −2×10 −6 . The sense layer  414  has a thickness preferably in the range of between about 0.6 nm and about 3.6 nm, in order to provide a designed magnetic moment in the range of between about 0.08 and about 0.48 memu/cm 2 , and a most preferred thickness of about 1.8 nm (corresponding to a magnetic moment of about 0.24 memu/cm 2 ). 
     Preferably, a decoupling layer  416  is formed above the sense layer  414 . Under a preferred embodiment of the present invention, the decoupling layer  416  comprises bilayer films of Cu—O and Ru films. The bilayer films are more effective than a single-layer film in diminishing exchange coupling between the sense layer  414  and the longitudinal pinned layer  418 . Hence, the decoupling layer can be as thin as possible to ensure strong magnetostatic interaction through a flux closure formed by the sense and longitudinal pinned layers, thereby achieving good sensor stability. The Cu—O film is used as a lower portion of the decoupling layer in order to protect the underlying sense layer, thereby facilitating it to exhibit anisotropic and good soft magnetic properties. 
     For example, when a 1.8 nm thick 86Co-14Fe film (composition in atomic percent) is sandwiched between two Cu—O films, its easy- and hard-axis coercivities can reach as low as below 6 and 0.2 Oe, respectively. The Cu—O film has a thickness preferably in the range of between about 0.5 and about 1.5 nm, with a most preferred thickness of about 1 nm. The Ru film is used as an upper portion of the decoupling layer in order to facilitate the overlying longitudinal pinned and pinning layers to exhibit a high pinning field. The Ru film has a thickness preferably in the range of between about 1 and about 3 nm, with a most preferred thickness of about 2 nm. 
     A longitudinal pinned layer  418  is formed above the decoupling layer  416 . The longitudinal pinned layer  418  is preferably formed of a Co—Fe film with an Fe content preferably in the range of between about 10 and about 18 at %, and a most preferred Fe content of about 14 at %. The longitudinal pinned layer  418  has a thickness preferably in the range of between about 0.9 and about 5.4 nm (to provide a designed magnetic moment in the range of between about 0.12 and about 0.72 memu/cm 2 ), with a most preferred thickness of about 2.7 nm (corresponding to a magnetic moment of about 0.36 memu/cm 2 ). The magnetic moment of the longitudinal pinned layer  418  is preferably 1.5 times of that of the sense layer in order to achieve sensor stability. 
     A longitudinal pinning layer  420  is formed above the longitudinal pinned layer  418 . The longitudinal pinning layer  420  is preferably formed of an Ir—Mn film with a Mn content preferably in the range of between about 75 and about 85 at %, and a most preferred Mn content of about 80 at %. The longitudinal pinning layer  420  has a thickness preferably in the range of between about 4.5 and about 9 nm, with a most preferred thickness of about 6 nm. 
     A cap layer  422  is formed above the longitudinal pinning layer  420 . The cap layer  422  is preferably formed of a Ru film with a thickness preferably in the range of between about 3 and about 9 nm, and a most preferred thickness of about 6 nm. 
     Referring now to FIG. 5, shown therein is a schematic block diagram illustrating an alternative embodiment of the present invention. A CPP spin-valve sensor with multiple barrier and sense layers  500  preferably comprises a seed layer  502 , a transverse pinning layer  504 , a transverse pinned layer  506 , a plurality of barrier layers  508 , a plurality of sense layers  510 , a decoupling layer  512 , a longitudinal pinned layer  514 , a longitudinal pinning layer  516 , and a cap layer  518 . 
     The compositions and thicknesses of the seed layer  502 , the transverse pinning layer  504 , the decoupling layer  512 , the longitudinal pinning layer  516 , and the cap layer  518  are substantially the same as those of the corresponding seed layer  402 , transverse pinning layer  404 , decoupling layer  416 , longitudinal pinning layer  420 , and cap layer  422  of FIG.  4 . The compositions of the transverse pinned layer  506 , the sense layers  510 , and the longitudinal pinned layer  514  are substantially the same as those of the corresponding reference layer  410 , sense layer  414 , and longitudinal pinned layer  418  of FIG.  4 . 
     The composition and thickness ranges of the barrier layers  508  are substantially equivalent to those of the corresponding barrier layer  412  of FIG.  4 . However, the preferred composition and thickness of the barrier layers  508  are slightly different from those of the corresponding barrier layer  412  of FIG.  4 . The key difference is that the oxygen-doping/in-situ oxidation processes are slightly modified in order to attain a negative ferromagnetic coupling field (≦−5 Oe) between the transverse pinned and lowest sense layer, as well as between any two adjacent sense layers. 
     The transverse pinned layer  506  and the sense layers  510  have thicknesses preferably in the range of between about 1 and about 1.4 nm, with a most preferred thickness of about 1.2 nm. The thickness of the transverse pinned layer  506  is basically identical to that of either one of the sense layers  510 , in order to provide demagnetization fields (induced from the magnetic moment of the transverse pinned layer  506  after sensor lapping) high enough to rotate the magnetization of either one of the sense layers  510  by ˜45° from a longitudinal direction parallel to the air bearing surface. The negative ferromagnetic coupling field between the transverse pinned and lowest sense layers, as well as between any two adjacent sense layers, also facilitate this magnetization rotation. Due to antiparallel exchange coupling between the transverse pinned and lowest sense layers, as well as between any two adjacent sense layers, this magnetization rotation causes the magnetizations of any two adjacent sense layers to be oriented in directions perpendicular to each other. This perpendicular orientation relationship is desired for good signal linearity and high signal sensitivity. 
     The longitudinal pinned layer  514  has a thickness preferably in the range of between about 4.5 and about 6.3 nm, with a most preferred thickness of about 5.4 nm. The thickness of the longitudinal pinned layer  514  is selected in order to exhibit a magnetic moment 1.5 times of the magnetic moments of the plurality of the sense layers  510  in order to achieve sensor stability. 
     Referring now to FIG. 6, shown therein is one embodiment of an integrated DC magnetron/ion beam sputtering system  600  suitable for fabricating a CPP spin-valve sensor and for conducting the oxygen doping/in-situ oxidation processes of the present invention. The sputtering system  600  of FIG. 6 is sold by the Veeco Corporation of Plainview, N.Y. The sputtering system  600  as depicted comprises a transport module  602  surrounded by a first single-target DC magnetron sputtering module  604 , a multi-target DC magnetron sputtering module  606 , a multi-target ion beam sputtering module  608 , and a second single-target DC magnetron sputtering module  610 . Loadlocks  616  allow the ingress and egress of wafers. 
     A control panel  614  controls the parameters and processes of the sputtering system  600 . The depositions of the seed and transverse pinned layers are preferably conducted in the multi-target ion beam sputtering module  608  in order to provide a flat surface, thereby attaining a reasonably low ferromagnetic coupling field. The depositions of all the other layers and the oxygen-doping/in-situ oxidation processes are preferably conducted in the DC magnetron sputtering modules  604 ,  606 ,  610 , in order to prevent interface mixing, thereby attaining a high GMR coefficient. 
     FIG. 7 illustrates one embodiment of a method  700  of fabricating a CPP spin-valve sensor of the present invention. The method  700  starts  702  and a wafer is provided  704 . Under a preferred embodiment of the present invention, a seed layer is formed  706  on the wafer and configured in the manner described above with reference to the seed layer  402  of FIG. 4. A transverse pinning layer is formed  708  above the seed layer and configured in the manner described with reference to the transverse pinning layer  404  of FIG.  4 . In one embodiment, a keeper layer is formed  710  above the transverse pinning layer and is preferably configured in the manner described with reference to the keeper layer  406  of FIG. 4. A spacer layer is formed  712  above the transverse pinning layer and is preferably configured in the manner described with reference to the spacer layer  408  of FIG.  4 . 
     A reference layer is formed  714  above the spacer layer and is configured in the manner described with reference to the reference layer  410  of FIG. 4. A barrier layer is formed  716  above the reference layer, the method of which will be described in greater details below with reference to FIG.  8 . In one embodiment, a sense layer is formed  718  above the barrier layer and configured in the manner described with reference to the sense layer  414  of FIG. 4. A decoupling layer is formed  720  above the sense layer and configured in the manner described with reference to the decoupling layer  416  of FIG.  4 . 
     A longitudinal pinned layer is formed  722  above the decoupling layer and configured in the manner described with reference to the longitudinal pinned layer  418  of FIG. 4. A longitudinal pinning layer is formed  724  above the longitudinal pinned layer and configured in the manner described with reference to the longitudinal pinning layer  420  of FIG. 4. A cap layer is formed  726  above the longitudinal pinning layer and configured in the manner described with reference to the cap layer  422  of FIG.  4 . The fabrication method  700  ends  728 . 
     The major merit of the Cu—O film used as the barrier layer of the CPP spin-valve sensor is a feasibility of attaining an optimal junction resistance, while still maintaining a reasonably low ferromagnetic coupling field and a high GMR coefficient. Two major approaches are used in the formation of this Cu—O film, one oxygen doping during the reactive DC-pulsed magnetron sputtering of a Cu film and the other the in-situ oxidation after the reactive DC-pulsed magnetron sputtering. Both the oxygen doping and in-situ oxidation must be very well optimized in order to ensure the viability of the CPP spin-valve sensor. 
     Referring now to FIG. 8, the oxygen-doping/in-situ oxidation method starts  802  for the formation of the barrier layer  716 . The oxygen doping process is conducted  804  in a first mixture of argon and oxygen gases during the reactive DC-pulsed magnetron sputtering, preferably in the multiple-target DC-magnetron sputtering module  606 , such as that described with reference to FIG.  6 . Subsequently, the in-situ oxidization process is conducted  806  in a second mixture of argon and oxygen gases immediately after the reactive DC-pulsed magnetron sputtering in the same module, after which the oxygen-doping/in-situ oxidation method ends  808 . 
     The oxygen-doping process is preferably conducted in the first mixture of argon and oxygen gases with a total pressure in the range of between about 2 and about 4 mTorr, and a most preferred total pressure of about 3 mTorr. The oxygen gas has a partial pressure preferably in the range of between about 0.005 and about 0.025 mTorr, and a most preferred partial pressure of 0.015 mTorr. Referring now to FIG. 9, in this preferred partial pressure range, the ferromagnetic coupling field, ranging from −10 to 20 Oe, is reasonably low, while the GMR coefficient, ranging from 12.4% to 13.2%, is very high. It should be noted that these magnetic properties can only be measured from a CIP spin-valve sensor with the same structure. When used as the CPP spin-valve sensor, the ferromagnetic coupling field is expected to be the same, while the GMR coefficient is expected to be higher by 40%. 
     In the range of the preferred partial oxygen pressure used in the oxygen-doping process, the Cu—O film appears to exhibit an amorphous phase. Referring to FIG. 10, as the oxygen partial pressure increases from 0 to 0.005 mTorr, a Cu face-centered-cubic phase with an electrical resistivity of 3 μΩ-cm is transformed into a Cu—O amorphous phase with an oxygen content of ˜12 at % and with an electrical resistivity of 100 μΩ-cm. As the oxygen partial pressure further increases to 0.025 mTorr, a Cu—O amorphous phase with an oxygen content of ˜24 at % and with an electrical resistivity of 1600 μΩ-cm is transformed into a CuO 2  face-centered-cubic phase. 
     In one embodiment, the in-situ oxidation process is preferably conducted in the second mixture of argon and oxygen gases with a total pressure in the range of between about 1 and about 100 mTorr, with a most preferred total pressure of about 3 mTorr. The oxygen gas has a partial pressure preferably in the range of between about 0.01 and about 10 mTorr, and a most preferred partial pressure of about 0.06 mTorr. Referring to FIG. 11, in this preferred partial pressure range, the ferromagnetic coupling field, ranging from 0 to 10 Oe, is reasonably low, while the GMR coefficient of ˜13.2% is very high. It should also be noted that these magnetic properties can only be measured from a CIP spin-valve sensor with the same structure. When used as the CPP spin-valve sensor, the ferromagnetic coupling field is expected to be the same, while the GMR coefficient is expected to be higher by 40%. The total pressure of argon and oxygen gases are preferably maintained for a period in a range of between about 1 and about 120 minutes, and for a preferred period of about 4 minutes. The temperature is preferably maintained at about room temperature (i.e., about 70° F.). 
     After the depositions of the CPP spin-valve sensor and an additional deposition of 3 nm thick Ta film (used as a protection layer during subsequent annealing processes and as an adhesion layer during patterning processes) on a wafer, the wafer is annealed for 120 minutes at 280° C. in a magnetic field of 10 kOe perpendicular to an alignment mark, and then annealed again in a magnetic field of 200 Oe parallel to the alignment mark for 120 minutes at 240° C. After these two anneals, bilayer photoresists are applied and exposed in a photolithographic tool to mask the CPP spin-valve sensor in a central region, and then developed in a solvent to form an undercut. The CPP spin-valve sensor in two unmasked side regions is removed by ion milling until a Ni—Fe bottom shield layer is exposed, and a 90 nm thick Al 2 O 3  film is deposited on the exposed Ni—Fe bottom shield layer. Following this Al 2 O 3  deposition, the bilayer photoresists are lifted off and the additional Ta film is removed by reactive ion etching. A 1 μm thick Ni—Fe film to be used as a top magnetic shield layer is deposited on the wafer. Bilayer photoresists are applied and exposed in a photolithographic tool to define the shape of the top magnetic shield layer, and then developed in a solvent to form an undercut. The Ni—Fe film in unmasked regions is then removed by selective chemical etching until the Al 2 O 3  film is exposed. 
     A CIP spin-valve sensor with a spacer layer formed of an oxygen-doped/in-situ oxidized film, fabricated as described in this invention, has been found to exhibit much better magnetic properties than a conventional CIP spin-valve sensors with a metallic spacer layer. For example, as a Cu spacer layer is replaced by a Cu—O spacer layer, the ferromagnetic coupling fields decreases from 17 to −10 Oe, the GMR coefficient increases from 12.4% to 13.2%. When converting these CIP spin-valve sensor structures into CPP spin-valve sensor structures, the ferromagnetic coupling field is expected to be the same, the GMR coefficient is expected to be higher by 40%, and most importantly, the junction resistance is expected to increase from 0.001 to 0.1 Ω-μm 2 . In order to achieve a higher junction resistance, the partial pressure used for the in-situ oxidation may be as high as 10 mTorr, as shown in the graph of FIG.  11 . If the oxygen partial pressure is greater than 10 mTorr, a full in-situ oxidation may lead to an unwanted high ferromagnetic coupling field. 
     The oxygen-doping/in-situ oxidation processes of the present invention may also be applied to other layers of the CPP spin-valve sensor for further increasing the junction resistance while still maintaining a low ferromagnetic coupling field and a high GMR coefficient. 
     The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.