Tunneling magnetoresistive sensor with spin polarized current injection

[0002] 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.

[0003] 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.

[0004] 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.

[0005] 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.

[0006] 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.

[0007] 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.

[0008] 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.

[0009] 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.

[0010] 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.

[0011] 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).

[0012] The present invention addresses these and other needs, and offers other advantages over current devices.

[0013] 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.

[0014] FIG. 1 is a layer diagram of a prior art tunneling magnetoresistive stack.

[0015] FIG. 2 is a layer diagram of a first embodiment of a tunneling magnetoresistive stack of the present invention.

[0016] FIG. 3 is a layer diagram of a second embodiment of a tunneling magnetoresistive stack of the present invention.

[0017] 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.

[0018] 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.

[0019] 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 Ωμm²) 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.

[0020] 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 Al₂O₃, Y₂O₃, CeO₂, TaO, SiN, AlN, CrO₂, HfO₂, and TiO₂, 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.

[0021] 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.

[0022] 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 Ωμm².

[0023] 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 Al₂O₃, Y₂O₃, CeO₂, TaO, SiN, AlN, CrO₂, HfO₂, and TiO₂, 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.

[0024] 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.

[0025] 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 Ωμm².

[0026] 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.