Abstract:
A tunneling magnetoresistive (TMR) stack configured to operate in a current-perpendicular-to-plane (CPP) mode has a plurality of layers including a spin valve and a barrier layer. The spin valve is used to inject a spin polarized sense current into the barrier layer for increasing a magnetoresistive (MR) ratio of the TMR stack.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application claims priority from Provisional Application No. 60/311,728, filed Aug. 10, 2001 entitled “Tunneling Magnetoresistive Sensor with Spin Polarized Current Injection” by O. Heinonen and D. Macken. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to a magnetoresistive sensor for use in a magnetic read head. In particular, the present invention relates to a tunneling magnetoresistive (TMR) read sensor having an enhanced magnetoresistive response. 
     Magnetoresistive read sensors, such as 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 (SAF) and a ferromagnetic free layer. The magnetization of the SAF 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 SAF 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 of the SAF. 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 TMR read sensor is similar in structure to a GMR spin valve, but the physics of the device are different. For a TMR read sensor, rather than using a spacer layer, a barrier layer is positioned between the free layer and the SAF. Electrons must tunnel through the barrier layer. A sense current flowing perpendicularly to the plane of the layers of the TMR read sensor experiences a resistance which is proportional to the cosine of an angle formed between the magnetization direction of the free layer and the magnetization direction of the reference layer of the SAF. 
     A pinning layer is typically exchange coupled to the pinned layer of the SAF 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 atomic planes are aligned in alternating 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 underlayer consequently grown on top of it. In particular, the seed layer provides a desired grain structure and size. 
     One principal concern in the performance of TMR read sensors is the ΔR (the maximum absolute change in resistance of the TMR read sensor), which directly affects the magnetoresistive (MR) ratio. The MR ratio (the maximum absolute change in resistance of the TMR read sensor divided by the resistance of the TMR read sensor multiplied by 100%) determines the magnetoresistive effect of the TMR read sensor. Ultimately, a higher MR ratio yields a TMR 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 MR ratio is the spin polarization of the sense current passing through the barrier layer. The sense current consists of majority spin electrons (spin is in the same direction of the magnetization) and minority spin electrons (spin is in the opposite direction of the magnetization). A spin polarized current has an unequal population of majority and minority spin electrons. According to the Julliere model of the TMR read sensor, the magnetoresistive effect in a tunneling junction is significantly enhanced if the sense current is spin polarized. This is because the magnetoresistive effect is determined by ΔR/R=2PP′/(1−PP′), where ΔR/R is the MR ratio, and P and P′ are the spin polarization ratios of the effective tunneling density of states on each side of the barrier layer. The MR ratio reaches a maximum value for completely polarized tunneling density of states (P=P′=1). 
     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 tunneling magnetoresistive (TMR) stack configured to operate in a current-perpendicular-to-plane (CPP) mode. The TMR stack has a plurality of layers including a spin valve and a barrier layer. The spin valve is used to inject a spin polarized sense current into the barrier layer for increasing a magnetoresistive (MR) ratio of the TMR stack. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a layer diagram of a prior art tunneling magnetoresistive stack. 
     FIG. 2 is a layer diagram of a first embodiment of a tunneling magnetoresistive stack of the present invention. 
     FIG. 3 is a layer diagram of a second embodiment of a tunneling magnetoresistive stack of the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 is a layer diagram of a prior art tunneling magnetoresistive (TMR) stack  10 . TMR stack  10  includes a seed layer  12 , an underlayer  14 , a pinning layer  16 , a synthetic antiferromagnet (SAF)  18 , a barrier layer  20 , and a free layer  22 . Underlayer  14  is a ferromagnetic material and is positioned adjacent to seed layer  12 . Pinning layer  16  is an antiferromagnetic material and is positioned adjacent to underlayer  14 . SAF  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 . Free layer  22  is a ferromagnetic material. Barrier layer  20  is an insulating material and is positioned between SAF  18  and free layer  22 . 
     The magnetization of SAF  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 TMR 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 TMR signal produced by TMR stack  10  is generated by a sense current flowing perpendicularly through the layers of TMR stack  10  from free layer  22  to seed layer  12 . The sense current injected into free layer  22  is unpolarized. While it is possible for TMR stack  10  to exhibit a MR ratio greater than 15% by increasing the thickness of barrier layer  20  to 10 Å or more, the resistance-area (RA) product becomes too large (10-100 Ωμm 2 ) to be practically useful for devices for high areal density magnetic recording. The RA product can be reduced by decreasing the thickness of barrier layer  20 , but a corresponding decrease in the MR ratio inevitably occurs. 
     FIG. 2 is a layer diagram of a first embodiment of a tunneling magnetoresistive (TMR) stack  30  of the present invention. TMR stack  30  includes a seed layer  32 , an underlayer  34 , a first pinning layer  36 , a synthetic antiferromagnet (SAF)  38 , a barrier layer  40 , a free layer  42 , a spacer layer  44 , a reference layer  46 , and a second pinning layer  48 . Seed layer  32  is preferably NiFeCr or Ta. Underlayer  34  is a ferromagnetic material, preferably CoFe or NiFe, and is positioned adjacent to seed layer  32 . First pinning layer  36  is an antiferromagnetic material, preferably selected from the group consisting of PtMn, IrMn, NiMn and FeMn, and is positioned adjacent to underlayer  34 . SAF  38  includes a ferromagnetic pinned layer  50 , a ferromagnetic reference layer  54 , and a coupling layer  52  positioned between pinned layer  50  and reference layer  54 . SAF  38  is positioned such that pinned layer  50  is adjacent to first pinning layer  36 . Pinned layer  50  and reference layer  54  are preferably CoFe, and coupling layer  52  is preferably selected from the group consisting of Ru, Cu and Ag. Free layer  42  is a ferromagnetic material, preferably CoFe or NiFe. Barrier layer  40  is an insulating material, preferably selected from the group consisting of Al 2 O 3 , Y 2 O 3 , CeO 2 , TaO, SiN, AlN, CrO 2 , HfO 2 , and TiO 2 , and is positioned between SAF  38  and free layer  42 . Reference layer  46  is a ferromagnetic material, preferably CoFe. Second pinning layer  48  is an antiferromagnetic material, preferably selected from the group consisting of PtMn, IrMn, NiMn and FeMn, and is positioned adjacent to reference layer  46 . Spacer layer  44  is a nonmagnetic material, preferably Cu or Ag, and is positioned between free layer  42  and reference layer  46 . Free layer  42 , spacer layer  44 , reference layer  46 , and second pinning layer  48  together form a simple spin valve. 
     The magnetizations of SAF  38  and reference layer  46  are fixed while the magnetization of free layer  42  rotates freely in response to an external magnetic field emanating from a magnetic medium. The magnetization of reference layer  46  is pinned by exchange coupling second pinning layer  48  with reference layer  46  such that the magnetization direction of reference layer  46  is the same as the magnetization direction of reference layer  54 . Reference layer  54  and pinned layer  50  are magnetically coupled by coupling layer  52  such that the magnetization direction of reference layer  54  is opposite to the magnetization direction of pinned layer  50 . The magnetization of pinned layer  50  is pinned by exchange coupling first pinning layer  36  with pinned layer  50 . Underlayer  34  promotes the crystallographic texture of first pinning layer  36 , and seed layer  32  enhances the grain growth of underlayer  34 . The magnetization of reference layer  46  is pinned by exchange coupling second pinning layer  48  with reference layer  46 . The resistance of TMR stack  30  varies as a function of an angle that is formed between the magnetization of free layer  42  and the magnetizations of reference layers  54  and  46 . 
     The TMR signal produced by TMR stack  30  is generated by a sense current flowing perpendicularly through the layers of TMR stack  30  from second pinning layer  48  to seed layer  32 . The sense current injected into second pinning layer  48  is unpolarized. The simple spin valve formed by free layer  42 , spacer layer  44 , reference layer  46 , and second pinning layer  48  acts as an electrode to spin polarize the sense current. Because an external magnetic field from a magnetic medium rotates the magnetization of free layer  42 , majority and minority spin electrons scatter at different rates at the interfaces between reference layer  46 , spacer layer  44 , and free layer  42 . The resulting current emanating from free layer  42  to barrier layer  40  is spin polarized due to this spin selective scattering. As a result, the magnetoresistive effect of TMR stack  30  is significantly enhanced without appreciably increasing the RA product. Therefore, it is possible for TMR stack  30  to exhibit a MR ratio greater than 15% with an RA product of only 1-2 Ωμm 2 . 
     FIG. 3 is a layer diagram of a second embodiment of a tunneling magnetoresistive (TMR) stack  60  of the present invention. TMR stack  60  includes a seed layer  62 , an underlayer  64 , a first pinning layer  66 , a first synthetic antiferromagnet (SAF)  68 , a barrier layer  70 , a free layer  72 , a spacer layer  74 , a second SAF  76 , and a second pinning layer  78 . Seed layer  62  is preferably NiFeCr or Ta. Underlayer  64  is a ferromagnetic material, preferably CoFe or NiFe, and is positioned adjacent to seed layer  62 . First pinning layer  66  is an antiferromagnetic material, preferably selected from the group consisting of PtMn, IrMn, NiMn and FeMn, and is positioned adjacent to underlayer  64 . First SAF  68  includes a ferromagnetic pinned layer  80 , a ferromagnetic reference layer  84 , and a coupling layer  82  positioned between pinned layer  80  and reference layer  84 . First SAF  68  is positioned such that pinned layer  80  is adjacent to first pinning layer  66 . Pinned layer  80  is preferably CoFe, coupling layer  82  is preferably ruthenium, and reference layer  84  is preferably CoFe. Free layer  72  is a ferromagnetic material, preferably CoFe or NiFe. Barrier layer  70  is an insulating material, preferably selected from the group consisting of Al 2 O 3 , Y 2 O 3 , CeO 2 , TaO, SiN, AlN, CrO 2 , HfO 2 , and TiO 2 , and is positioned between first SAF  68  and free layer  72 . Second SAF  76  includes a ferromagnetic reference layer  86 , a ferromagnetic pinned layer  90 , and a coupling layer  88  positioned between reference layer  86  and pinned layer  90 . Second pinning layer  78  is an antiferromagnetic material, preferably selected from the group consisting of PtMn, IrMn, NiMn and FeMn, and is positioned adjacent to pinned layer  90  of second SAF  76 . Spacer layer  74  is a nonmagnetic material, preferably Cu or Ag, and is positioned between free layer  72  and reference layer  68  of second SAF  76 . Free layer  72 , spacer layer  74 , second SAF  76 , and second pinning layer  78  together form a SAF spin valve. 
     The magnetizations of first and second SAFs  68  and  76  are fixed while the magnetization of free layer  72  rotates freely in response to an external magnetic field emanating from a magnetic medium. Reference layer  84  and pinned layer  80  are magnetically coupled by coupling layer  82  such that the magnetization direction of reference layer  84  is opposite to the magnetization direction of pinned layer  80 . The magnetization of pinned layer  80  is pinned by exchange coupling first pinning layer  66  with pinned layer  80 . Underlayer  64  promotes the crystallographic texture of first pinning layer  66 , and seed layer  62  enhances the grain growth of underlayer  64 . Reference layer  86  and pinned layer  90  are magnetically coupled by coupling layer  88  such that the magnetization direction of reference layer  86  is opposite to the magnetization direction of pinned layer  90 . The magnetization of pinned layer  90  is pinned by exchange coupling second pinning layer  78  with pinned layer  90 . The magnetization direction of reference layer  84  is the same as the magnetization direction of reference layer  86 . Similarly, the magnetization direction of pinned layer  80  is the same as the magnetization direction of pinned layer  90 . The resistance of TMR stack  60  varies as a function of an angle that is formed between the magnetization of free layer  72  and the magnetizations of reference layers  84  and  86 . 
     The TMR signal produced by TMR stack  60  is generated by a sense current flowing perpendicularly through the layers of TMR stack  60  from second pinning layer  78  to seed layer  62 . The sense current injected into second pinning layer  78  is unpolarized. The SAF spin valve formed by free layer  72 , spacer layer  74 , second SAF  76 , and second pinning layer  78  acts as an electrode to spin polarize the sense current. Because an external magnetic field from a magnetic medium rotates the magnetization of free layer  72 , majority and minority spin electrons scatter at different rates at the interfaces between second SAF  76 , spacer layer  74 , and free layer  72 . The resulting current emanating from free layer  72  to barrier layer  70  is spin polarized due to this spin selective scattering. As a result, the magnetoresistive effect of TMR stack  60  is significantly enhanced without appreciably increasing the RA product. Therefore, it is possible for TMR stack  60  to exhibit a MR ratio greater than 15% with an RA product of only 1-2 Ωμm 2 . 
     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.