Abstract:
A tunneling magnetoresistive (TMR) sensor with a free layer made of a Co—Fe—B alloy is disclosed. After annealing at a temperature of less than 300° C., the Co—Fe—B free layer exhibits a negative or zero saturation magnetostriction, λ S , while the TMR sensor exhibits superior TMR properties. The Co—Fe—B free layer has an Fe content of not greater than 10 atomic percent, and a B content of not greater than 10 atomic percent. Alternatively, a free-layer structure is used in place of the Co—Fe—B free layer The free-layer structure includes a first free layer lying on a barrier layer and a second free layer lying on the first free layer. The first free layer is made of an alloy selected from Co—Fe, Co—B and Co—Fe—B alloys, while the second free layer is made of an alloy selected from Co—B and Co—Fe—B alloys. The first free layer has an Fe content of not greater than 10 atomic percent, and a B content of not greater than 10 atomic percent. The second free layer has an Fe content of not greater than 20 atomic percent, and a B content of not greater than 20 atomic percent. After annealing for 2 to 20 hours at a temperature ranging from 220° C. up to 300° C., the free-layer structure exhibits a negative saturation magnetostriction, λ S , while the TMR sensor exhibits a very high TMR coefficient at a very low junction resistance-area product. By adjusting the compositions and thicknesses of the first and second Co—Fe—B free layers, it is possible to “tune” to any desired value of saturation magnetostriction, λ S , in the range of −1×10 −5 &lt;λ S &lt;0.

Description:
RELATED APPLICATION 
       [0001]    The present application is related to a commonly assigned patent application Ser. No. 11/611,828, entitled A CURRENT-PERPENDICULAR-TO-PLANE SENSOR WITH DUAL KEEPER LAYERS, filed on Dec. 15, 2006, which is hereby incorporated by reference in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to a current-perpendicular-to-plane (CPP) tunneling magnetoresistive (TMR) sensors and more particularly to a TMR sensor having a Co—Fe—B free layer having a negative saturation magnetorestriction, λ S . 
       BACKGROUND OF THE INVENTION 
       [0003]    The heart of a computer is a non-volatile storage device that is referred to as a magnetic disk drive. The magnetic disk drive includes a magnetic disk, and write and read heads. The write and read heads are supported by a slider that is mounted on a suspension arm. When the magnetic disk rotates, an actuator swings the suspension arm to place the write and read heads over selected circular tracks on the surface of the rotating magnetic disk. An airflow generated by the rotation of the magnetic disk causes an air-bearing surface (ABS) of the slider to fly at a very low elevation (fly height) over the surface of the rotating magnetic disk. The write and read heads write magnetic transitions to and read magnetic transitions from the rotating magnetic disk, respectively. Processing circuitry connected to the write and read heads then operates according to a computer program to implement writing and reading functions. 
         [0004]    The write head includes a coil embedded in an insulation stack that is sandwiched between main and auxiliary poles. The main and auxiliary poles are magnetically coupled at a back gap and are coated with an overcoat. A write current conducted through the coil induces a magnetic flux in the main pole that causes a magnetic field to write the aforementioned magnetic transitions to the rotating magnetic disk. 
         [0005]    The read head has traditionally included a current-in-plane (CIP) giant magnetoresistive (GMR) sensor. The GMR sensor includes a magnetically pinned layer and a magnetically free layer separated by an electrically conductive nonmagnetic spacer layer. The relative orientations of the magnetizations of the pinned and free layers change the electrical resistance of the GMR sensor based on the spin-dependent scattering of conduction electrons in the GMR sensor. 
         [0006]    Recently, in order to improve the performance of read heads at very small track widths, researchers have focused on the development of current-perpendicular-to-plane (CPP) GMR and tunneling magnetoresistive (TMR) sensors. The TMR sensor also includes a magnetically pinned layer and a magnetically free layer, but both are separated by an electrically insulating nonmagnetic barrier layer. The relative orientations of the magnetizations of the pinned and free layers change the electrical resistance of the TMR sensor based on the spin-dependent tunneling of conduction electrons through the barrier layer. 
         [0007]    In order for the TMR sensor to operate stably at very small track widths, it is desired that the free layer has a negative, or at least zero, saturation magnetostriction, λ S . After receiving compressive stresses induced by mechanical lapping in the fabrication process of the write and read heads, a free layer with a negative λ S  longitudinally biases its own magnetization in a longitudinal direction parallel to the ABS in the absence of an external magnetic field. Thus, a free layer only needs low longitudinal bias fields provided by neighboring hard-magnetic films for stable read performance, thereby causing high read sensitivity. In contrast, a free layer with a positive λ S  transversely biases its own magnetization in a transverse direction perpendicular to the ABS in the absence of an external magnetic field. Thus, a free layer with a positive λ S  requires high longitudinal bias fields for stable read performance, thereby causing low read sensitivity. 
         [0008]    Thus, it is important to use a free layer with a desired negative λ S  to ensure stable read performance. The most extensively explored TMR sensor with pinned and free layers made of ferromagnetic 60% Co-20% Fe-20% B (in atomic percent) alloys separated by an barrier made of a thin MgO X  film, exhibits superior TMR properties. However, its free layer exhibits very highly positive λ S  mainly due to the high Fe content. For example, in prior art, this TMR sensor exhibits a TMR coefficient, ΔR T /R J , (where R J  is a minimum junction resistance measured when the magnetizations of the pinned and free layers are parallel to each other, and R J +ΔR T  is a maximum junction resistance measured when the magnetizations of the pinned and free layers are antiparallel to each other) of as high as 138% at a junction resistance-area product, R J  A J , (where A J  is a junction area) of as low as 2.4 Ω-μm 2 , after annealing for 2 hour at 360° C. in 8,000 Oe in a high vacuum oven. 
         [0009]    It is believed that the high Fe and B contents cause two microstructural effects. First, during depositions on a wafer, the Co—Fe—B pinned layer grows with an amorphous phase, so that the Mg—O barrier layer can grow freely with its {001} crystalline planes in parallel to the wafer surface (or with a &lt;001&gt; crystalline texture). Subsequently, the Co—Fe—B free layer also grows with an amorphous phase. Second, during annealing, Co—Fe—B polycrystalline grains with a body-center-cubic (bcc) &lt;001&gt; crystalline texture nucleate at two Mg—O interfaces, and then grow in the entire Co—Fe—B pinned and free layers. This crystallization results in an epitaxial relationship among the pinned, barrier and free layers, which facilitates coherent spin polarization through the two Mg—O interfaces. As a result, this TMR sensor exhibits superior TMR properties. However, this TMR sensor exhibits a λ S  of more than 6×10 −6 , not only due to the high Fe content, but also due to the impractically high temperature which causes unwanted interfacial mixing and inevitably deteriorates overall ferromagnetic properties. Such an impractical high temperature is considered to be crucial for the desired transformation from the amorphous phase (formed after deposition due to the high B content) to the polycrystalline phase. 
         [0010]    Therefore, there is a need for the free layer to exhibit a negative λ S , while still facilitating the TMR sensor to exhibit superior TMR properties. In addition, to ensure manufacturability, this TMR sensor with such a free layer must be fabricated without using an impractical high temperature. 
       SUMMARY OF THE INVENTION 
       [0011]    The present invention provides a tunneling magnetoresistive (TMR) sensor with a free layer made of a Co—Fe—B alloy. After annealing at a temperature of less than 300° C., the Co—Fe—B free layer exhibits a negative or zero saturation magnetostriction, λ S , while the TMR sensor exhibits superior TMR properties. 
         [0012]    In a preferred embodiment of the present invention, the Co—Fe—B free layer has an Fe content of not greater than 10 atomic percent (preferably about 4 atomic percent), and a B content of not greater than 10 atomic percent (preferably about 10 atomic percent). After annealing for 5 hours at a temperature as low as 240° C. this free layer exhibits a negative saturation magnetostriction, λ S , while the TMR sensor exhibits a very high TMR coefficient, ΔR T /R J  (where R J  is a minimum junction resistance measured when the magnetizations of the pinned and free layers are parallel to each other, and R J +ΔR T  is a maximum junction resistance measured when the magnetizations of the pinned and free layers are antiparallel to each other) at a very low junction resistance-area product, R J A J  (where A J  is a junction area). 
         [0013]    In an alternative embodiment of the present invention, a free-layer structure is used in place of the Co—Fe—B free layer. The free-layer structure includes a first free layer lying on a barrier layer and a second free layer lying on the first free layer. The first free layer is made of an alloy selected from Co—Fe, Co—B and Co—Fe—B alloy systems, while the second free layer is made of an alloy selected from Co—B and Co—Fe—B alloy systems. The first free layer has an Fe content of not greater than 10 atomic percent (preferably about 4 atomic percent), and a B content of not greater than 10 atomic percent (preferably about 10 atomic percent). The second free layer has an Fe content of not greater than 20 atomic percent (preferably about 8 atomic percent), and a B content of not greater than 20 atomic percent (preferably about 20 atomic percent). After annealing for 2 to 20 hours at a temperature ranging from 220° C. up to 300° C., the free-layer structure exhibits a negative saturation magnetostriction, λ S , while the TMR sensor exhibits a very high TMR coefficient, ΔR T /R J  at a very low junction resistance-area product, R J A J . By adjusting the compositions and thicknesses of the first and second Co—Fe—B free layers, it is possible to “tune” to any desired value of saturation magnetostriction, λ S , in the range of −1×10 −5 &lt;λ S &lt;0 (preferably −2×10 −6 &lt;λ S &lt;0). 
         [0014]    These and other features and advantages of the invention will be apparent upon reading of the following detailed description of preferred embodiments taken in conjunction with the figures in which like reference numerals indicate like elements throughout. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    For a fuller understanding of the nature and advantages of this invention, as well as the preferred mode of use, reference should be made to the following detailed description read in conjunction with the accompanying drawings, which are not to scale: 
           [0016]      FIG. 1  is a schematic illustration of a magnetic disk drive in which this invention might be embodied; 
           [0017]      FIG. 2  is an ABS view of a slider, taken from line  2 - 2  of  FIG. 1 , illustrating the location of write and read heads thereon; 
           [0018]      FIG. 3  is an ABS view of a TMR sensor according to an embodiment of the invention taken from circle  3  of  FIG. 2 ; 
           [0019]      FIG. 4  is a graph illustrating relationships between the B content of a Co—Fe—B free layer and R J A J  of a TMR sensor annealed at 240° C. and 285° C.; 
           [0020]      FIG. 5  is a graph illustrating relationships between the B content of a Co—Fe—B free layer and the TMR coefficient of a TMR sensor annealed at 240° C. and 285° C.; 
           [0021]      FIG. 6  is another graph illustrating relationships between the B content of a Co—Fe—B free layer and R J A J  of a TMR sensor annealed at 240° C. and 285° C.; 
           [0022]      FIG. 7  is another a graph illustrating relationships between the B content of a Co—Fe—B free layer and the TMR coefficient of a TMR sensor annealed at 240° C. and 285° C.; 
           [0023]      FIG. 8  is an ABS view of a TMR sensor according to an alternative embodiment of the invention; and 
           [0024]      FIG. 9  illustrates the relationships between R J A J  and the TMR coefficient for alternative embodiments of the present invention comparing TMR sensors having a Co—Fe—B free layer with TMR sensors having a free-layer structure comprising two Co—Fe—B free layers. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0025]    The following is a description of various embodiments presently contemplated for carrying out this invention. This description is made for the purpose of illustrating the general principles of this invention and is not meant to limit the inventive concepts claimed herein. 
         [0026]    Referring now to  FIG. 1 , there is shown a magnetic disk drive  100  embodying this invention. As shown in  FIG. 1 , at least one rotatable magnetic disk  112  is supported on a spindle  114  and rotated by a disk drive motor  118 . The magnetic recording is performed on each magnetic disk  112 . 
         [0027]    At least one slider  113  is positioned near the magnetic disk  112 , each slider  113  supporting write and read heads  121 . As the magnetic disk  112  rotates, the slider  113  moves radially in and out over the disk surface  122  so that the write and read heads  121  may access different tracks of the magnetic disk  112 . Each slider  113  is attached to an actuator arm  119  by way of a suspension  115 . The suspension  115  provides a slight spring force, which biases the slider  113  against the disk surface  122 . Each actuator arm  119  is attached to an actuator means  127 , which 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 current signals supplied by a control unit  129  to the coil. 
         [0028]    During operation of the magnetic disk drive  100 , the rotation of the magnetic disk  112  generates an air bearing between the slider  113  and the disk surface  122 , which exerts an upward force or lift on the slider  113 . The air bearing thus counter-balances the slight spring force of the suspension  115  and supports the slider  113  off and slightly above the disk surface  122  by a small, substantially constant spacing during operation. 
         [0029]    The various components of the magnetic disk drive  100  are controlled in operation by control signals generated by the control unit  129 , such as access control signals and internal clock signals. Typically, the control unit  129  comprises logic control circuits, storage means and a microprocessor. The control unit  129  generates control signals to control various operations such as drive motor control signals on line  123  and head position and seek control signals on line  128 . The control signals on line  128  provide desired current profiles to optimally move and position the slider  113  to the desired data track on the magnetic disk  112 . Write and read signals are communicated to and from write and read heads  121  by way of a recording channel  125 . 
         [0030]    With reference to  FIG. 2 , the orientation of the write and read heads  121  in the slider  113  can be seen in more detail.  FIG. 2  is an ABS view of the slider  113 , and as can be seen, the write and read heads  121  are located at a trailing edge  202  of the slider  113 . The above description of a typical magnetic disk drive  100  and the accompanying illustration of  FIG. 1  are for representation purposes only. It should be apparent that the magnetic disk drive may contain a large number of magnetic disks and actuators, and each actuator may support a number of sliders. 
       Embodiment Comprising a Free Layer 
       [0031]    With reference now to  FIG. 3 , a read head  300  including a tunneling magnetoresistive (TMR) sensor  302  according to an embodiment of the invention is shown. The read head  300  includes the TMR sensor  302  sandwiched between a pair of electrically conductive leads or electrodes  304 ,  306 . The electrodes  304 ,  306  can be made of 1 μm thick ferromagnetic Ni—Fe alloys that function as magnetic shields, as well as electrically conductive leads. 
         [0032]    For measuring TMR properties with a CIP technique, the TMR sensor  302  sandwiched between another pair of electrodes was fabricated on a SiO X -coated Si substrate (where the subscript, x, indicates that the oxide may be, but need not be stoichiometric). The first electrode may comprise a 3 nm thick Ta film under a 45 nm thick CuN X  film under a 3 nm thick Ta film capped by a 45 nm thick CuN X  film (where the subscript, x, indicates the presence of nitrogen in the film, but not necessarily in stoichiometric amounts). The second electrode may comprise a 45 nm thick Cu film under a 6 nm thick Ru film. Note that theses electrodes are too thick for incorporation into the read head  300 , but remain suitable only for measurements of the TMR properties with the CIP technique. In the following discussion, TMR properties illustrated in  FIGS. 4 through 7 , and  9 , were measured on the SiO X -coated Si substrate with these electrodes. 
         [0033]    The TMR sensor  302  includes a pinned-layer structure  308  and a free layer  310 . The free layer  310  is separated from the pinned-layer structure  308  by an electrically insulating nonmagnetic, barrier-layer structure  312 . A pinning layer  314  made of an antiferromagnetic alloy can be used beneath the pinned-layer structure  308  for pinning the magnetizations of the ferromagnetic layers in the pinned-layer structure  308 , and will be described in greater detail herein below. Seed layers may be used beneath the pinning layer  314  to initiate a desired grain growth in the layers above. For example, as shown in  FIG. 3 , the TMR sensor  302  can comprise first and second seed layers  316 ,  318 . The first seed layer  316  can also act as an adhesion layer made of, for example, a Ta film, and the second seed layer  318  can be made of, for example, a Ru film. Alternatively, the second seed layer  318  can be made of a Ni—Cr—Fe alloy. To facilitate the grain growth, a third seed layer (not shown) made of a Ni—Fe alloy can be used. Additionally, a cap layer  320  made of, for example, a Ta film, can also be used on top of the free layer  310  to protect the underlying layers of the TMR sensor  302  during the fabrication process of the write and read heads. 
         [0034]    With continued reference to FIG,  3 , the read head  300  can also include first and second longitudinal bias layers  322  made of a hard-magnetic Co—Pt or Co—Pt—Cr alloy. First and second electrically insulating nonmagnetic layers  324  separate the longitudinal bias layers  322  from the TMR sensor  302  and from at least one of the electrodes, such as the electrode  304 , in order to prevent portions of the sense current from being shunted through the longitudinal bias layers  322 . The longitudinal bias layers  322  can be magnetized to provide a longitudinal bias field that, through the mechanism of magnetostatic coupling with the free layer  310 , causes the magnetization  326  of the free layer  310  to be biased in a desired, longitudinal direction. 
         [0035]    With reference still to  FIG. 3 , the pinned-layer structure  308  can include a ferromagnetic, keeper layer  328  separated from a ferromagnetic, reference layer  330  by an nonmagnetic, antiparallel-coupling layer  332 . The keeper and reference layers  328 ,  330  can be made of, for example, a Co—Fe alloy and a Co—Fe—B alloy, respectively. The antiparallel-coupling layer  332  can be made of a Ru film with a thickness selected at the first or second peak of an RKKY (Ruderman-Kittel-Kasuya-Yosida) antiparallel-coupling curve, in order to facilitate strong antiparallel coupling between the keeper and reference layers  328 ,  330 . The pinning layer  314  can be made of, for example, an Ir—Mn or Ir—Mn—Cr alloy. The keeper layer  328  is exchange coupled with the pinning layer  314 , which strongly pins the magnetization of the keeper layer  328  in a desired transverse direction perpendicular to the ABS as indicated by an arrow-tail symbol  334 . The strong antiparallel coupling between the keeper and reference layers  328 ,  330  strongly pins the magnetization of the reference layer  330  in a direction, as indicated by arrowhead symbol  336 , antiparallel to the transverse direction. 
         [0036]    With continued reference to  FIG. 3 , the barrier-layer structure  312  may comprise a first barrier layer  338  and a second barrier layer  340 , both of which can be made of very thin MgO X  films, where x indicates that the oxide need not be stoichiometric. The total thickness of the barrier-layer structure  312  can be about 1 nm. The amounts of oxygen in the first and second MgO X  barrier layers are independently tuned to minimize the oxygen penetration into the reference layer  330  and the free layer  310 , thereby maximizing the TMR coefficient of the TMR sensor  302 . The MgO X  barrier layer may be deposited by a variety of methods, for example: by direct-current (DC) magnetron sputtering reactively in an oxygen atmosphere from a Mg target, or by radio-frequency (RF) magnetron sputtering from a MgO X  target. Embodiments of the present invention are by no means limited to any particular method of depositing the MgO X  barrier layer. 
         [0037]    For embodiments of the invention described herein, further oxygen treatments (OTs) are applied to the MgO X  barrier-layer structure  312 , after deposition of the first and second MgO X  barrier layers  338 ,  340 , but before the deposition of the free layer  310 . In these oxygen treatments (OTs), the first and second MgO X  barrier layers  338 ,  340  are exposed to an oxygen gas flowing at a constant rate for up to 4 minutes in a deposition module. These oxygen treatments (OTs) have been found to suppress oxygen activity during the subsequent deposition of the free layer, thus improving magnetic and TMR properties of the TMR sensor  302 . 
         [0038]    The free layer  310  is made of a Co—Fe—B alloy having Fe and B contents that are carefully controlled so that the free layer  310  has a zero or negative saturation magnetostriction, λ S , while being capable of facilitating the TMR sensor to exhibit superior TMR properties after annealing at a relatively low temperature of less than 300° C. The Co—Fe—B free layer  310  preferably has an Fe content of not greater than 10 atomic percent and a B content not greater than 10 atomic percent. 
         [0039]    The TMR sensor  302 , as shown in  FIG. 3 , comprises 1 nm thick electrically insulating MgO X  barrier layer  312 . The Ta adhesion layer  316  can be 3 nm thick, and the Ru seed layer  318  can be 1.6 nm thick. The pinning layer  314  can be a 25.2% Ir-70.6% Mn-4.2% Cr alloy with a thickness of 6 nm. (Note that all compositions cited herein are in units of atomic percent.) The keeper layer  328  can be made of a 77.5% Co-22.5% Fe alloy with a thickness of 2 nm. The antiparallel-coupling layer  332  can be made of a Ru film with a thickness of 0.8 nm. The reference layer can be made of a 52.6% Co-33.1% Fe-14.3% B alloy with a thickness of 1.9 nm. As mentioned above, the free layer  310  can be made of a Co—Fe—B alloy with a thickness of 2.7 nm. The cap layer  320  can be made of a Ta film with a thickness of 6 nm. The read head  300  can be annealed for 5 hours at a temperature of less than 300° C. in a 50,000 Oe magnetic field in a high-vacuum oven. 
         [0040]    In order to prevent the Co—Fe keeper layer  328  from exhibiting unwanted hysteretic magnetization rotations caused by high coercivity. H C , a Pt—Mn pinning layer used in the prior art is replaced by the Ir—Mn—Cr pinning layer  314 . In order for the Ir—Mn—Cr pinning layer to exhibit strong antiferromagnetism, the Ru seed layer  318  is added to facilitate the formation of a preferred crystalline texture wherein the face-centered-cubic (fee) {111} crystalline planes in the Ir—Mn—Cr pinning layer  314  are oriented parallel to the interface between the pinning and reference layers  314 ,  318 . As a result, exchange coupling between the Ir—Mn—Cr pinning layer  314  and the Co—Fe keeper layer  328  induces a uniaxial anisotropy field, H UA , much higher, and coercivity. H C , much lower than those between the Pt—Mn pinning and Co—Fe keeper layers. The high H UA  ensures rigid pinning, while the low H C  minimizes unwanted hysteretic rotations of magnetization. 
         [0041]    In order to prevent the deterioration of the ferromagnetic properties of the Co—Fe keeper layer  328 , Co—Fe—B reference layer  330 , and Co—Fe—B free layer  310 , the annealing temperature as high as 360° C., used in the prior art, is substantially decreased to below 300° C. in the present invention. As a result, the crystallization of the amorphous Co—Fe—B alloy into a polyerystalline microstructure in the reference and free layers  330 ,  310  may not occur. Consequently, after annealing for 5 hours at 285° C., the TMR sensor  302  with 60% Co-20% Fe-20% B reference and free layer  330 ,  310  (used in the prior art) exhibits ΔR T /R J  of as low as 88.6% at R J A J  of 3.4 Ωμm 2 , while exhibiting λ S  of as high as +3.6×10 −6 . 
         [0042]    As mentioned above, a free layer having a positive saturation magnetostriction, λ S , would not be viable since its magnetization is biased in an undesirable transverse direction perpendicular to the ABS, after mechanical lapping. One way to reduce λ S  might be by depositing a Ni—Fe or Co—Fe—B—Ni alloy on top of the Co—Fe—B free layer  310 . However, it has been found that with such a second free layer containing Ni for reducing λ S , ΔR T /R J  substantially decreases from 88.6% to below 24% at R J A J  of approximately 3.4 Ωμm 2 . Thus, it is suspected that undesirable Ni diffusion into the Co—Fe—B first free layer substantially deteriorates the TMR properties. 
         [0043]    However, in the present invention, it has been found that the necessary negative λ S  and high ΔR T /R J  can be simultaneously attained through the use of the Co—Fe—B alloy as the free layer  310 . This Co—Fe—B free layer  310  has an Fe content of not greater than 10 atomic percent and a B content of not greater than 10 atomic percent. By substantially decreasing the Fe and B contents of the Co—Fe—B free layer  310  to far below 20 atomic percent, the Co—Fe—B free layer  310  can exhibit very negative λ S , as described below in greater detail. In addition, it is surprising to find that the annealing temperature can be substantially decreased to far below 300° C. for the Co—Fe—B free layer  310  to facilitate the TMR sensor  302  to still exhibit high ΔR T /R J . This result is not expected based on the teachings of prior art literature, and is, therefore, an unexpected result. 
         [0044]    In particular, in order for the Co—Fe—B free layer  310  to exhibit overall good soft ferromagnetic properties (such as a low easy-axis coercivity, a nearly zero hard-axis coercivity, a low uniaxial anisotropy field, a ferromagnetic coupling field, etc.), in addition to the desired negative λ S , and for the TMR sensor to exhibit a high TMR coefficient, deposition parameters of the MgO X  barrier layer and Ta cap layers are substantially tuned. In this embodiment, the MgO X  barrier layer is formed by reactively sputtering in an oxygen atmosphere from a Mg target at a power of 100 W and by exposing the MgO X  barrier layer to an oxygen gas flowing at a constant rate for about four minutes, while the Ta cap layer is deposited at a power of 200 W. By ensuring oxygen saturation on the MgO X  barrier layer and minimizing mixing at the interface between the Co—Fe—B free layer and Ta cap layer, overall good soft ferromagnetic properties can be attained. 
         [0045]      FIGS. 4 and 5  show R J A J  and ΔR T /R J , respectively, versus the B content for a TMR sensor  302  with a Co—Fe—B free layer  310  having an Fe content ranging from 20 to 25 atomic percent, and a B content ranging from 0 to 20 atomic percent, the balance being Co, after annealing at 240° C. and 285° C. The Co—Fe—B free layer  310  of varying Fe and B contents were deposited by co-sputtering from two targets having the following compositions: 75% Co-25% Fe and 60% Co-20% Fe-20% B. Thus, the composition of the Co—Fe—B free layer  310  varied as (75%-0.75c B )Co-(25%-0.25c B )Fe-c B B, where c B  is the B content in atomic percent. For a B content of more than 14 atomic percent, annealing at a lower temperature leads to a higher R J A J  and a lower ΔR T /R J , consistent with those reported in the prior art. However, for the B content of less than 14 atomic percent, annealing at a lower temperature leads to a lower R J A J  and a higher ΔR T /R J . For example, after annealing at as low as 240° C., a TMR sensor with a 67.5% Co-22.5% Fe-10% B free layer exhibits ΔR T /R J  of as high as 127.8% at R J A J  of 3.3 Ωμm 2 . Thus, a high Fe content plays a more important role than a high B content and a high annealing temperature in exhibiting a high ΔR T /R J . However, due to the high Fe content, this Co—Fe—B free layer  310  still inevitably exhibits a high λ S . In addition, the addition of at least 4 atomic, percent B into the Co—Fe free layer changes its initially isotropic ferromagnetic properties to anisotropic. 
         [0046]      FIGS. 6 and 7  show R J A J  and ΔR T /R J , respectively, versus the B content for a TMR sensor  302  with the Co—Fe—B free layer  310  having an Fe content ranging from 0 to 8 atomic percent, and a B content ranging from 0 to 20 atomic percent, the balance being Co, after annealing at 240° C. and 285° C. The Co—Fe—B free layer  310  of varying Fe and B contents was deposited by co-sputtering from two targets having the following compositions: 100% Co and 72% Co-8% Fe-20% B. Thus, the composition of the Co—Fe—B free layer  310  varied as (100%-1,4c B )Co-(0.4c B )Fe-c B B, where c B  is the B content in atomic percent. Again, the TMR sensor with a low B content only requires annealing at a low temperature to attain good TMR properties. For example, after annealing at as low as 240° C., a TMR sensor with a 86% Co-4% Fe-10% B free layer exhibits ΔR T /R J  of as high as 88.1% at R J A J  of 3.1 Ωμm 2 . More importantly, with such a low Fe content, the Co—Fe—B free layer  310  exhibits λ S  of as low as − 3 . 6 × 10   −6 . 
         [0047]    In spite of the use of the Ta cap layer  320 , which in general causes a substantial increase in λ S  due to interfacial mixing with the Co—Fe—B free layer  310 , the TMR sensor  302  with specifically selected low Fe and low B contents, as described above, still exhibits an unexpected very low λ S , viz. −3.6×10 −6 . If the cap layer is made of a noble-metal film such as Pt, Rh, Ru, etc., it is expected that λ S  will be even more negative. Such a very negative λ S  can be finely tuned to a design value around −2.0×10 −6 , while substantially improving ΔR T /R J , as described below. 
       Alternative Embodiment Comprising a Free-Layer Structure 
       [0048]    With reference to  FIG. 8 , a read head  800  comprising a TMR sensor  802  according to an alternative embodiment of the present invention is shown. The TMR sensor  802  includes a bilayer, free-layer structure  810 , which differs from the TMR sensor  302  in that the free layer  310  of the TMR sensor  302  is replaced by a free-layer structure  810  comprising a first free layer  804  and a second free layer  806 . The first free layer  804  has an Fe content of not greater than 10 atomic percent and a B content of not greater than 10 atomic percent. The second free layer  806  has an Fe content of not greater than 20 atomic percent and a B content of not greater than 20 atomic percent. 
         [0049]      FIG. 9  shows R J A J  versus ΔR T /R J , after annealing for 5 hours at 285° C., for the TMR sensor  302  with the free layer  310  made of a 2.7 nm thick 86% Co-4% Fe-10% B alloy and the TMR sensor  802  with the free-layer structure  810  comprising a first free layer  804  made of a 1.7 nm thick 86% Co-4% Fe-10% B alloy, and the second free layer  806  made of 1.2 nm thick 72% Co-8% Fe-20% B alloy. While λ S  increases from −3.6×10 −6  to −2.0×10 −6 , ΔR T /R J  increases from 78.3% to 87.9% at R J A J  of about 2.7 Ωμm 2 . With such a desired negative λ S , the free-layer structure  810  can stabilize itself in a longitudinal direction parallel to the ABS. As a result, only a low longitudinal bias field provided by the neighboring hard-magnetic films is needed for stable read performance; and thus, read sensitivity can be improved. 
         [0050]    The saturation magnetostriction λ S  of the Co—Fe—B free layers as described in this invention is originally expected to decrease with decreasing the Fe content of the Co—Fe—B free layer, but is not expected to substantially decrease to a very negative value. Since interfaces of a very thin ferromagnetic film have been known to dominantly contribute to λ S , it is speculated that the oxygen exposure on the MgO X  barrier layer and the use of a low target power for the deposition of the Ta cap layer may dominantly lead to the unexpected very negative λ S  through minimizing oxygen activity at a lower interface and minimizing interfacial mixing at an upper interface. As a result, in addition to adjusting the Fe content of the Co—Fe—B free layer, it is very crucial to apply these interface engineering techniques to the Co—Fe—B free layer for attaining and tuning the unexpected very negative λ S  to a design value. 
         [0051]    The annealing temperature used in this invention is much lower than that used in the prior art. An annealing temperature of as high as 360° C. has been considered to be very crucial in developing superior TMR properties through a transformation of an amorphous phase into a polycrystalline phase in the 60% Co-20% Fe-20% B free layer. In contrast, in the present invention, an annealing temperature of as low high as 240° C. also surprisingly causes the Co—Fe—B with Fe and B contents of much lower than 20 atomic percent to facilitate the TMR sensor to exhibit reasonably good TMR properties. Since the desired phase transformation may not occur during annealing at such a low temperature, it is speculated that a polycrystalline phase in the as-deposited Co—Fe—B free layer might be still functional in exhibiting reasonably good TMR properties, and the low annealing temperature may be just used to thermally set the magnetizations of the pinned-layer structure for measuring the TMR properties, instead of to induce the phase transformation. 
         [0052]    TMR effects have been considered to be an interlace phenomenon in the prior art. However, it is surprising to find in the alternative embodiment that the second Co—Fe—B free layer, which is separated from the MgO X  barrier layer by the first Co—Fe—B free layer, caused a substantial improvement in the TMR properties, it is thus speculated that the as-deposited, free-layer structure, comprising the first Co—Fe—B free layer with a polycrystalline phase and the second Co—Fe—B free layer with a nanocrystalline or amorphous phase, has specific tunneling scattering characteristics in maximizing the TMR effect. 
         [0053]    While various embodiments have been described above, it should be understood that they have been presented by way of example only, and not limitation. Other embodiments falling within the scope of the invention may also become apparent to those skilled in the art. Thus, the breadth and scope of the invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents