Abstract:
A giant magnetoresistive stack for use in a magnetic read head has a plurality of layers including at least one ferromagnetic layer which contributes to a giant magnetoresistive signal, a doped ferromagnetic pinned layer and a doped ferromagnetic underlayer which do not contribute to a giant magnetoresistive signal. The dopant in the doped ferromagnetic pinned layer and underlayer reduces parasitic shunting current through the giant magnetoresistive stack by providing an increase in resistivity without a decrease in magnetization.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application claims priority from Provisional Application No. 60/305,749, filed Jul. 16, 2001 entitled “Spin Valve with High-Resistive Magnetic Layers” by C. Hou and O. Heinonen. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to a giant magnetoresistive sensor for use in a magnetic read head. In particular, the present invention relates to a giant magnetoresistive read sensor having an enhanced giant magnetoresistive response. 
     Giant magnetoresistive (GMR) read sensors are used in magnetic data storage systems to detect magnetically-encoded information stored on a magnetic data storage medium such as a magnetic disc. A time-dependent magnetic field from a magnetic medium directly modulates the resistivity of the GMR read sensor. A change in resistance of the GMR read sensor can be detected by passing a sense current through the GMR read sensor and measuring the voltage across the GMR read sensor. The resulting signal can be used to recover the encoded information from the magnetic medium. 
     A typical GMR read sensor configuration is the GMR spin valve, in which the GMR read sensor is a multi-layered structure formed of a nonmagnetic spacer layer positioned between a synthetic antiferromagnet and a ferromagnetic free layer. The magnetization of the synthetic antiferromagnet is fixed, typically normal to an air bearing surface of the GMR read sensor, while the magnetization of the free layer rotates freely in response to an external magnetic field. The synthetic antiferromagnet includes a reference layer and a pinned layer which are magnetically coupled by a coupling layer such that the magnetization direction of the reference layer is opposite to the magnetization of the pinned layer. The resistance of the GMR read sensor varies as a function of an angle formed between the magnetization direction of the free layer and the magnetization direction of the reference layer. This multi-layered spin valve configuration allows for a more pronounced magnetoresistive effect, i.e. greater sensitivity and higher total change in resistance, than is possible with anisotropic magnetoresistive (AMR) read sensors, which generally consist of a single ferromagnetic layer. 
     A pinning layer is typically exchange coupled to the pinned layer of the synthetic antiferromagnet to fix the magnetization of the pinned layer in a predetermined direction. The pinning layer is typically formed of an antiferromagnetic material. In antiferromagnetic materials, the magnetic moments of adjacent atoms point in opposite directions and, thus, there is no net magnetic moment in the material. 
     An underlayer is typically used to promote the texture of the pinning layer consequently grown on top of it. The underlayer is typically formed of a ferromagnetic material and is chosen such that its atomic structure, or arrangement, corresponds with a desired crystallographic direction. 
     A seed layer is typically used to enhance the grain growth of the layers consequently grown on top of it. In particular, the seed layer provides a desired grain structure and size for the underlayer. 
     One principal concern in the performance of GMR read sensors is the ΔR (the maximum absolute change in resistance of the GMR read sensor), which directly affects the GMR ratio. The GMR ratio (the maximum absolute change in resistance of the GMR read sensor divided by the resistance of the GMR read sensor multiplied by 100%) determines the magnetoresistive effect of the GMR read sensor. Ultimately, a higher GMR ratio yields a GMR read sensor with a greater magnetoresistive effect which is capable of detecting information from a magnetic medium with a higher linear density of data. 
     A key determinant of the GMR ratio is the amount of parasitic shunting current flowing through the GMR read sensor. The GMR signal produced by the GMR read sensor is generated by the current flowing through the free layer, the spacer layer, and the reference layer of the synthetic antiferromagnet. Current flowing through any other layer is a parasitic shunting current, and reduces the GMR signal. As a result, the less parasitic shunting current that is present in the GMR read sensor, the greater the GMR ratio. Parasitic shunting current can be reduced by increasing the resistivity of the layers that do not contribute directly to the GMR signal. In particular, increasing the resistivities of the pinning layer and the underlayer is especially desirable because these layers are typically formed of magnetic materials with low resistivities. In these instances, however, it is important to ensure that the magnetic properties of these layers are maintained in order for the GMR read sensor to function properly. 
     The present invention addresses these and other needs, and offers other advantages over current devices. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is a giant magnetoresistive stack for use in a magnetic read head. The giant magnetoresistive stack has a plurality of layers including at least one ferromagnetic layer which contributes to a giant magnetoresistive signal, and at least one doped ferromagnetic layer which does not contribute to a giant magnetoresistive signal. The dopant in the doped ferromagnetic layer reduces parasitic shunting current through the giant magnetoresistive stack by providing an increase in resistivity without a decrease in magnetization. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a layer diagram of a first embodiment of a giant magnetoresistive stack of the present invention. 
     FIG. 2 is a bar graph of the GMR ratio of the first embodiment of a giant magnetoresistive stack of the present invention. 
     FIG. 3 is a bar graph of the ΔR of the first embodiment of a giant magnetoresistive stack of the present invention. 
     FIG. 4 is a layer diagram of a second embodiment of a giant magnetoresistive stack of the present invention. 
     FIG. 5 is a layer diagram of a third embodiment of a giant magnetoresistive stack of the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 is a layer diagram of a first embodiment of a giant magnetoresistive (GMR) stack  10  of the present invention. GMR stack  10  is configured as a bottom spin valve and includes a seed layer  12 , an underlayer  14 , a pinning layer  16 , a synthetic antiferromagnet  18 , a spacer layer  20 , and a free layer  22 . Seed layer  12  is preferably NiFeCr or Ta. Underlayer  14  is a ferromagnetic material, preferably CoFeX or NiFeX, where X is selected from the group consisting of B, V, Cr, Mo, W and Ti, and is positioned adjacent to seed layer  12 . Pinning layer  16  is an antiferromagnetic material, preferably selected from the group consisting of PtMn, IrMn, NiMn, NiO and FeMn, and is positioned adjacent to underlayer  14 . Synthetic antiferromagnet  18  includes a ferromagnetic pinned layer  24 , a ferromagnetic reference layer  28 , and a coupling layer  26  positioned between pinned layer  24  and reference layer  28 , and is positioned such that pinned layer  24  is adjacent to pinning layer  16 . Coupling layer  26  is preferably ruthenium, reference layer  28  is preferably CoFe, and pinned layer  24  is preferably CoFeX, where X is selected from the group consisting of B, V, Cr, Mo, W and Ti. Free layer  22  is a ferromagnetic material, preferably CoFe or NiFe. Spacer layer  20  is a nonmagnetic material, preferably copper, and is positioned between synthetic antiferromagnet  18  and free layer  22 . 
     The magnetization of synthetic antiferromagnet  18  is fixed while the magnetization of free layer  22  rotates freely in response to an external magnetic field emanating from a magnetic medium. Reference layer  28  and pinned layer  24  are magnetically coupled by coupling layer  26  such that the magnetization direction of reference layer  28  is opposite to the magnetization direction of pinned layer  24 . The magnetization of pinned layer  24  is pinned by exchange coupling pinning layer  16  with pinned layer  24 . Underlayer  14  promotes the crystallographic texture of pinning layer  16 , and seed layer  12  enhances the grain growth of underlayer  14 . The resistance of GMR stack  10  varies as a function of an angle that is formed between the magnetization of free layer  22  and the magnetization of reference layer  28 . 
     The GMR signal produced by GMR stack  10  is generated by the current flowing through free layer  22 , spacer layer  20 , and reference layer  28 . It is therefore desirable to minimize the parasitic shunting current through the layers of GMR stack  10  that are not responsible for generating the GMR signal. As a result, underlayer  14  and pinned layer  24  are doped with X, where X is selected from the group consisting of B, V, Cr, Mo, W and Ti, while free layer  22 , spacer layer  20 , and reference layer  28  are not doped with X. By doping underlayer  14  and pinned layer  24  with X, the resistivities of underlayer  14  and pinned layer  24  are significantly increased from about 10 μΩ·cm (without X) to about 100 μΩ·cm (with X), while the magnetizations of underlayer  14  and pinned layer  24  are maintained at about 2.2 Tesla. In this way, the GMR signal produced by GMR stack  10  is enhanced and, in particular, the GMR ratio and the ΔR are increased. 
     The composition of underlayer  14  when CoFeX is used is preferably in the range of about [Co(90)Fe(10)]X(&gt;0) to about [Co(90)Fe(10)]X(10), and more preferably in the range of about [Co(90)Fe(10)]X(1) to about [Co(90)Fe(10)]X(3), where the numbers in parentheses represent atomic percentage, and where the atomic ratio of CoFe in brackets is maintained while the atomic percentage of X is varied. The composition of underlayer  14  when NiFeX is used is preferably in the range of about [Ni(80)Fe(20)]X(&gt;0) to about [Ni(80)Fe(20)]X(10), and more preferably in the range of about [Ni(80)Fe(20)]X(1) to about [Ni(80)Fe(20)]X(3). 
     The composition of pinned layer  24  of synthetic antiferromagnet  18  is preferably in the range of about [Co(90)Fe(10)]X(&gt;0) to about [Co(90)Fe(10)]X(10), and more preferably in the range of about [Co(90)Fe(10)]X(1) to about [Co(90)Fe(10)]X(3). 
     FIG. 2 is a bar graph comparing the GMR ratio of GMR stack  10  of the present invention to the GMR ratio of two similar GMR stacks. Bar  100  shows the GMR ratio (the maximum absolute change in resistance of the GMR read sensor divided by the resistance of the GMR read sensor multiplied by 100%) of GMR stack  10 , where underlayer  14  and pinned layer  24  of GMR stack  10  are both CoFeV. Bar  102  shows the GMR ratio of a GMR stack similar to GMR stack  10 , except pinned layer  24  is replaced with a CoFe layer (underlayer  14  remains CoFeV). Bar  104  shows the GMR ratio of a GMR stack similar to GMR stack  10 , except underlayer  14  and pinned layer  24  are both replaced by CoFe layers. Bar  100  shows that the GMR ratio of GMR stack  10  is 15.49%. Bar  102  shows that if pinned layer  24  is replaced with a conventional CoFe layer, the GMR ratio drops to 15.17%. Bar  104  shows that if both underlayer  14  and pinned layer  24  are replaced with conventional CoFe layers, the GMR ratio drops to 14.94%. 
     The bar graph of FIG. 3 corresponds to the bar graph of FIG. 2, and compares the ΔR of GMR stack  10  of the present invention to the ΔR of two similar GMR stacks. Bar  110  shows the ΔR (the maximum absolute change in resistance of the GMR read sensor) of GMR stack  10  where underlayer  14  and pinned layer  24  of GMR stack  10  are both CoFeV. Bar  112  shows the ΔR of a GMR stack similar to GMR stack  10 , except pinned layer  24  is replaced with a CoFe layer (underlayer  14  remains CoFeV). Bar  114  shows the ΔR of a GMR stack similar to GMR stack  10 , except underlayer  14  and pinned layer  24  are both replaced by CoFe layers. Bar  110  shows that the ΔR of GMR stack  10  is 3.22 Ω/sq. Bar  112  shows that if pinned layer  24  is replaced with a conventional CoFe layer, the ΔR drops to 3.05 Ω/sq. Bar  114  shows that if both underlayer  14  and pinned layer  24  are replaced with conventional CoFe layers, the ΔR drops to 2.76 Ω/sq. 
     FIG. 4 is a layer diagram of a second embodiment of a GMR stack  30  of the present invention. GMR stack  30  is configured as a top spin valve and includes a seed layer  32 , a free layer  34 , a spacer layer  36 , a synthetic antiferromagnet  38 , and a pinning layer  40 . Seed layer  32  is preferably NiFeCr or Ta. Free layer  34  is a ferromagnetic material, preferably CoFe or NiFe, and is positioned adjacent to seed layer  32 . Synthetic antiferromagnet  38  includes a ferromagnetic reference layer  42 , a ferromagnetic pinned layer  46 , and a coupling layer  44  positioned between reference layer  42  and pinned layer  46 . Reference layer  42  is preferably CoFe, coupling layer  26  is preferably ruthenium, and pinned layer  46  is preferably CoFeX, where X is selected from the group consisting of B, V, Cr, Mo, W, and Ti. Pinning layer  40  is an antiferromagnetic material, preferably selected from the group consisting of PtMn, IrMn, NiMn, NiO and FeMn, and is positioned adjacent to pinned layer  46  of synthetic antiferromagnet  38 . Spacer layer  36  is a nonmagnetic material, preferably copper, and is positioned between free layer  34  and synthetic antiferromagnet  38 . 
     The magnetization of synthetic antiferromagnet  38  is fixed while the magnetization of free layer  34  rotates freely in response to an external magnetic field emanating from a magnetic medium. Reference layer  42  and pinned layer  46  are magnetically coupled by coupling layer  44  such that the magnetization direction of reference layer  42  is opposite to the magnetization direction of pinned layer  46 . The magnetization of pinned layer  46  is pinned by exchange coupling pinning layer  40  with pinned layer  46 . Seed layer  32  promotes the crystallographic texture and enhances the grain growth of free layer  34 . The resistance of GMR stack  30  varies as a function of an angle that is formed between the magnetization of free layer  34  and the magnetization of reference layer  42 . 
     The GMR signal produced by GMR stack  30  is generated by the current flowing through free layer  34 , spacer layer  36 , and reference layer  42 . It is therefore desirable to minimize the parasitic shunting current through the layers of GMR stack  30  that are not responsible for generating the GMR signal. As a result, pinned layer  46  is doped with X, where X is selected from the group consisting of B, V, Cr, Mo, W and Ti, while free layer  34 , spacer layer  36 , and reference layer  42  are not doped with X. By doping pinned layer  46  with X, the resistivity of pinned layer  46  is significantly increased from about 10 μΩ·cm (without X) to about 100 μΩ·cm (with X), while the magnetization of pinned layer  46  is maintained at about 2.2 Tesla. In this way, the GMR signal produced by GMR stack  30  is enhanced and, in particular, the GMR ratio and the ΔR are increased. 
     The composition of pinned layer  46  of synthetic antiferromagnet  38  is preferably in the range of about [Co(90)Fe(10)]X(&gt;0) to about [Co(90)Fe(10)]X(10), and more preferably in the range of about [Co(90)Fe(10)]X(1) to about [Co(90)Fe(10)]X(3). 
     FIG. 5 is a layer diagram of a third embodiment of a giant magnetoresistive (GMR) stack  50  of the present invention. GMR stack  50  is configured as a dual spin valve and includes a seed layer  52 , an underlayer  54 , a first pinning layer  56 , a first synthetic antiferromagnet  58 , a first spacer layer  60 , a free layer  62 , a second spacer layer  64 , a second synthetic antiferromagnet  66 , and a second pinning layer  68 . Seed layer  52  is preferably NiFeCr or Ta. Underlayer  54  is a ferromagnetic material, preferably CoFeX or NiFeX, where X is selected from the group consisting of B, V, Cr, Mo, W, and Ti, and is positioned adjacent to seed layer  52 . First pinning layer  56  is an antiferromagnetic material, preferably selected from the group consisting of PtMn, IrMn, NiMn, NiO and FeMn, and is positioned adjacent to underlayer  54 . First synthetic antiferromagnet  58  includes a ferromagnetic pinned layer  70 , a ferromagnetic reference layer  74 , and a coupling layer  72  positioned between pinned layer  70  and reference layer  74 , and is positioned such that pinned layer  70  is adjacent to first pinning layer  56 . Coupling layer  72  is preferably ruthenium, reference layer  74  is preferably CoFe, and pinned layer  70  is preferably CoFeX, where X is selected from the group consisting of B, V, Cr, Mo, W, and Ti. Free layer  62  is a ferromagnetic material, preferably CoFe or NiFe. First spacer layer  60  is a nonmagnetic material, preferably copper, and is positioned between first synthetic antiferromagnet  58  and free layer  62 . Second synthetic antiferromagnet  66  includes a ferromagnetic reference layer  76 , a ferromagnetic pinned layer  80 , and a coupling layer  78  positioned between reference layer  76  and pinned layer  80 . Reference layer  76  is preferably CoFe, coupling layer  78  is preferably ruthenium, and pinned layer  80  is preferably CoFeX, where X is selected from the group consisting of B, V, Cr, Mo, W, and Ti. Second pinning layer  68  is an antiferromagnetic material, preferably selected from the group consisting of PtMn, IrMn, NiMn, NiO and FeMn, and is positioned adjacent to pinned layer  80  of second synthetic antiferromagnet  66 . Second spacer layer  64  is a nonmagnetic material, preferably copper, and is positioned between free layer  62  and second synthetic antiferromagnet  66 . 
     The magnetizations of first and second synthetic antiferromagnets  58  and  66  are fixed while the magnetization of free layer  62  rotates freely in response to an external magnetic field emanating from a magnetic medium. Reference layer  74  and pinned layer  70  are magnetically coupled by coupling layer  72  such that the magnetization direction of reference layer  74  is opposite to the magnetization direction of pinned layer  70 . The magnetization of pinned layer  70  is pinned by exchange coupling first pinning layer  56  with pinned layer  70 . Underlayer  54  promotes the crystallographic texture of first pinning layer  56 , and seed layer  52  enhances the grain growth of underlayer  54 . Reference layer  76  and pinned layer  80  are magnetically coupled by coupling layer  78  such that the magnetization direction of reference layer  76  is opposite to the magnetization direction of pinned layer  80 . The magnetization of pinned layer  80  is pinned by exchange coupling second pinning layer  68  with pinned layer  80 . The resistance of GMR stack  50  varies as a function of the angles that are formed between the magnetization of free layer  62  and the magnetizations of reference layers  74  and  76 . 
     The GMR signal produced by GMR stack  50  is generated by the current flowing through free layer  62 , spacer layers  60  and  64 , and reference layers  74  and  76 . It is therefore desirable to minimize the parasitic shunting current through the layers of GMR stack  50  that are not responsible for generating the GMR signal. As a result, underlayer  54  and pinned layers  70  and  80  are doped with X, where X is selected from the group consisting of B, V, Cr, Mo, W and Ti, while free layer  62 , spacer layers  60  and  64 , and reference layers  74  and  76  are not doped with X. By doping underlayer  54  and pinned layers  70  and  80  with X, the resistivities of underlayer  54  and pinned layers  70  and  80  are significantly increased from about 10 μΩ·cm (without X) to about 100 μΩ·cm (with X), while the magnetizations of underlayer  54  and pinned layers  70  and  80  are maintained at about 2.2 Tesla. In this way, the GMR signal produced by GMR stack  50  is enhanced and, in particular, the GMR ratio and the ΔR are increased. 
     The composition of underlayer  54  when CoFeX is used is preferably in the range of about [Co(90)Fe(10)]X(&gt;0) to about [Co(90)Fe(10)]X(10), and more preferably in the range of about [Co(90)Fe(10)]X(1) to about [Co(90)Fe(10)]X(3). The composition of underlayer  54  when NiFeX is used is preferably in the range of about [Ni(80)Fe(20)]X(&gt;0) to about [Ni(80)Fe(20)]X(10), and more preferably in the range of about [Ni(80)Fe(20)]X(1) to about [Ni(80)Fe(20)]X(3). 
     The composition of pinned layer  70  of first synthetic antiferromagnet  58  is preferably in the range of about [Co(90)Fe(10)]X(&gt;0) to about [Co(90)Fe(10)]X(10), and more preferably in the range of about [Co(90)Fe(10)]X(1) to about [Co(90)Fe(10)]X(3). Similarly, the composition of pinned layer  80  of second synthetic antiferromagnet  66  is preferably in the range of about [Co(90)Fe(10)]X(&gt;0) to about [Co(90)Fe(10)]X(10), and more preferably in the range of about [Co(90)Fe(10)]X(1) to about [Co(90)Fe(10)]X(3). 
     In summary, the present invention introduces a GMR read sensor with at least one doped ferromagnetic layer which does not contribute to a GMR signal. The doped ferromagnetic layer reduces parasitic shunting current, and thus enhances the GMR response of the GMR read sensor. The dopant in the doped ferromagnetic layer is preferably selected from the group consisting of B, V, Cr, Mo, W, and Ti. The doped ferromagnetic layer may be a pinned layer, an S underlayer, or some other layer which does not contribute to a GMR signal. As a result, the present invention allows the resistivities of the ferromagnetic layers which do not contribute to a GMR signal to be increased without increasing the resistivities of the ferromagnetic layers which do contribute to a GMR signal. Furthermore, the present invention allows the resistivities of the ferromagnetic layers which do not contribute to a GMR signal to be increased without decreasing the magnetizations of these layers. 
     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.