Patent Application
20110235216
The invention relates to an improved longitudinal bias stack for stabilizing the sense-layer structure of a current-perpendicular-to-plane (CPP) tunneling magnetoresistance (TMR) or giant magnetoresistance (GMR) read sensor in a read head.
One of many extensively used non-volatile storage devices is a magnetic disk drive. The magnetic disk drive includes a rotatable magnetic disk and an assembly of write and read heads. The assembly of write and read heads is supported by a slider that is mounted on a suspension arm. The suspension arm is supported by an actuator that can swing the suspension arm to place the slider with its air bearing surface (ABS) over the surface of the magnetic disk.
When the magnetic disk rotates, an air flow generated by the rotation of the magnetic disk causes the slider to fly on a cushion of air at a very low elevation (fly height) over the magnetic disk. When the slider rides on the air, the actuator moves the suspension arm to position the assembly of the write and read heads over selected data tracks on the magnetic disk. The write and read heads read and write data on the magnetic disk. Processing circuitry connected to the assembly of the write and read heads then operates according to a computer program to implement writing and reading functions.
The write head includes a magnetic write pole and a magnetic return pole, which are magnetically connected with each other at a region away from the ABS, and are surrounded by an electrically conductive write coil. In a writing process, the electrically conductive write coil induces a magnetic flux in the write and return poles. This results in a magnetic write field that is emitted from the write pole to the magnetic disk in a direction perpendicular to the surface of the magnetic disk. The magnetic write field writes data on the magnetic disk, and then returns to the return pole so that it will not erase previously written data tracks.
The read head includes a read sensor which is electrically separated by insulation layers from longitudinal bias stacks in two side regions, but electrically connected with lower and upper ferromagnetic shields. In a reading process, the read head passes over magnetic transitions of a data track on the magnetic disk, and magnetic fields emitting from the magnetic transitions modulate the resistance of the read sensor in the read head. Changes in the resistance of the read sensor are detected by a sense current passing through the read sensor, and are then converted into voltage changes that generate read signals. The resulting read signals are used to decode data encoded in the magnetic transitions of the data track.
A current-perpendicular-to-plane (CPP) tunneling magnetoresistance (TMR) or giant magnetoresistance (GMR) read sensor is typically used in the read head. The CPP TMR read sensor includes a nonmagnetic insulating barrier layer sandwiched between lower and upper sensor stacks, while the CPP GMR read sensor includes a nonmagnetic conducting spacer layer sandwiched between the lower and upper sensor stacks. The thickness of the barrier or spacer layer is selected to be less than the mean free path of conduction electrons passing through the CPP GMR or GMR read sensor. The lower sensor stack comprises a buffer layer, a seed layer, a pinning layer, a keeper layer structure, an antiparallel-coupling layer, and a reference layer structure, while the upper sensor stack comprises a sense layer structure and a cap layer. The keeper layer structure, the antiparallel-coupling layer, and the reference layer structure form a flux closure, where antiparallel coupling occurring across the antiparallel-coupling layer orients the magnetizations of the keeper and reference layer structures in opposite transverse directions, one away from and the other towards the ABS. On the other hand, the reference layer structure, the barrier or spacer layer, and the sense layer structure form a scattering zone, where ferromagnetic coupling occurring across the barrier or spacer layer counter-balances demagnetizing stemming from the flux closure, resulting in the orientation of the magnetizations of the sense layer structure in a longitudinal direction parallel to the ABS.
When passing a sense current through the CPP TMR or GMR read sensor, conduction electrons are scattered at lower and upper interfaces of the barrier or spacer layer. When receiving magnetic fields emitting from the magnetic transitions on the magnetic disk, the magnetization of the reference layer structure remains pinned while that of the sense layer structure rotates. Scattering decreases as the magnetization of the sense layer structure rotates towards that of the reference layer structure, but increases as the magnetization of the sense layer structure rotates away from that of the reference layer structure. These scattering variations lead to changes in the resistance of the CPP TMR or GMR read sensor in proportion to the magnitudes of the magnetic fields, or to cos θ, where θ is an angle between the magnetizations of the reference and sense layer structures. The changes in the resistance of the CPP TMR or GMR read sensor are then detected by the sense current and converted into voltage changes that are detected and processed as playback signals.
In order for the magnetization of the sense layer structure to rotate stably in the reading process, the longitudinal bias stack is used at two tails of the sense layer structure to induce magnetostatic interactions across the sense layer structure. The longitudinal bias stack typically comprises a seed layer, a longitudinal bias layer, and a cap layer. The longitudinal bias layer must exhibit a remnant moment (MR δLB, where MR is its remnant magnetization and δLB is its thickness) high enough to prevent reversals of the magnetization of the sense layer structure after various magnetic excitations through balancing of net charges at the two tails of the sense layer structure, and a coercivity (HC) high enough to pin the magnetization of the sense layer structure through magnetostatic interactions across the sense layer structure. This stabilization scheme is currently effective in stabilizing a sense layer structure with a saturation moment of 0.42 memu/cm2 (corresponding to that of a 6 nm thick ferromagnetic 80Ni-20Fe films sandwiched into two Cu films) in a read head with a width of 50 nm, but require improvements as the read head is progressively miniaturized for magnetic recording at higher recording densities.
The invention provides an improved longitudinal bias stack for stabilizing the sense-layer structure of a current-perpendicular-to-plane (CPP) tunneling magnetoresistance (TMR) or giant magnetoresistance (GMR) read sensor in a read head. The longitudinal bias stack is separated by an Al2O3 insulation layer from the CPP read sensor in each of two side regions, and is sandwiched together with the Al2O3 insulation layer and the CPP read sensor between lower and upper ferromagnetic shields in the read head.
In a preferred embodiment of the invention, the longitudinal bias stack mainly comprises an Fe—Pt longitudinal bias layer without any seed layers, and thus the thickness of the Al2O3 insulation layer, instead of the total thicknesses of the Al2O3 insulation and seed layers used in a prior art, defines a spacing between the longitudinal bias layer and the CPP read sensor. Since the Fe—Pt longitudinal bias layer without any seed layers exhibits good in-plane hard-magnetic properties after annealing and the spacing is narrow, the stabilization scheme is effective.
In an alternative embodiment of the invention, the longitudinal bias stack mainly comprises the Fe—Pt longitudinal bias layer with an insulating MgO seed layer that replaces a portion of the Al2O3 insulation layer. Since the Fe—Pt longitudinal bias layer with the MgO seed layer exhibits better in-plane hard-magnetic properties after annealing and the spacing remains narrow, the stabilization scheme is more effective.
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.
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.
The following description is of the best 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.
Referring now to
At least one slider 113 is positioned near the magnetic disk 112, each slider 113 supporting one assembly of 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 assembly of write and read heads 121 may access different data tracks on the magnetic disk 112. Each slider 113 is mounted on a suspension arm 115 that is supported by an actuator 119. The suspension arm 115 provides a slight spring force which biases the slider 113 against the disk surface 122. Each actuator 119 is attached to an actuator means 127 that 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 motor current signals supplied by controller 129.
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 arm 115 and supports the slider 113 off and slightly above the disk surface 122 by a small, substantially constant spacing during sensor operation.
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 system 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 the 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 the assembly of write and read heads 121 by way of the recording channel 125.
In a typical read head 200, as shown in
A typical CPP TMR read sensor 201 includes an electrically insulating barrier layer 210 sandwiched between lower and upper sensor stacks 212, 214. The barrier layer 210 is formed of a nonmagnetic oxygen-doped Mg (Mg—O), Mg oxide (MgO), or Mg—O/MgO/Mg—O (MgOx) film having a thickness ranging from 0.4 to 1 nm. When the sense current quantum-jumps across the Mg—O, MgO or MgOx barrier layer 210, changes in the resistance of the CPP TMR read sensor 201 are detected through a TMR effect.
A typical CPP GMR read sensor 201 includes an electrically conducting spacer layer 210 sandwiched between lower and upper sensor stacks 212, 214. The spacer layer 210 is formed of a nonmagnetic Cu or oxygen-doped Cu (Cu—O) film having a thickness ranging from 1.6 to 4 nm. When the sense current flows across the Cu or Cu—O spacer layer 210, changes in the resistance of the CPP GMR read sensor is detected through a GMR effect.
The lower sensor stack 212 comprises a buffer layer 216 formed of a 2 nm thick nonmagnetic Ta film, a seed layer 218 formed of a 2 nm thick nonmagnetic Ru film, a pinning layer 220 formed of a 6 nm thick antiferromagnetic 23.2Ir-76.8Mn film (composition in atomic percent), a keeper layer structure 222, an antiparallel-coupling layer 226 formed of a 0.8 nm thick nonmagnetic Ru film, and a reference layer structure 224. The keeper layer structure 222 comprises a first keeper layer formed of a 1.6 nm thick 72.5Co-27.5Fe film and a second keeper layer formed of a 0.6 nm thick 64.1Co-35.9Fe film. The thickness of the keeper layer structure 222 is selected in order to attain a saturation moment of 0.32 memu/cm2 (corresponding to that of 4.6 nm thick ferromagnetic 80Ni-20Fe films sandwiched into two Cu films). The reference layer structure 224 comprises a first reference layer formed of a 0.6 nm thick 64.1Co-35.9Fc film, a second reference layer formed of a 0.6 nm thick 75.5Co-24.5Hf film, a third reference layer formed of a 1.2 nm thick 65.5Co-19.9 Fe-14.6 B film, and a fourth reference layer formed of a 0.3 nm thick 46.8Co-53.2 Fe film. The thickness of the reference layer structure 224 is selected in order to attain a saturation moment of 0.30 memu/cm2 (corresponding to that of 4.3 nm thick ferromagnetic 80Ni-20Fe films sandwiched into two Cu films).
The upper sensor stack 214 comprises a sense layer structure 228 and a cap layer 230 formed of a 6 nm thick nonmagnetic Ru film. The sense layer structure 228 comprises a first sense layer formed of a 0.4 nm thick ferromagnetic 87.5Co-12.5Fe film, a second sense layer formed of a 1.6 nm thick ferromagnetic 1.6 nm 79.3Co-4.0Fe-16.713 film, and a third sense layer formed of a 2.8 nm thick ferromagnetic 87.1Co-12.9Hf film. The thickness of the sense layer structure 228 is selected in order to attain a saturation moment of 0.42 memu/cm2 (corresponding to that of a 6 nm thick ferromagnetic 80Ni-20Fe film sandwiched into two Cu films).
A typical insulation layer 202 in each side region is formed of a 3 nm thick nonmagnetic, amorphous Al2O3 film. A typical longitudinal bias stack 204 in each side region comprises a seed layer 232 formed of a 4 nm thick nonmagnetic Cr film, a longitudinal bias layer 234 formed of a 24 nm thick hard-magnetic 82Co-18Pt film, and a cap layer 236 formed of a 10 nm thick nonmagnetic Cr film. The thickness of the Co—Pt longitudinal layer 234 is selected in order to attain a MRδLB of 2.1 memu/cm2 (corresponding to that of a 30 nm thick ferromagnetic 80Ni-20Fe film sandwiched into two Cu films, or five times of the saturation moment of the sense layer 228).
In the fabrication process of the read head 200, the CPP TMR or GMR read sensor 201 is deposited on a wafer with a lower ferromagnetic shield 206 formed of a 1 μm thick ferromagnetic 80Ni-20Fe film in a sputtering system, and is annealed in a magnetic field of 50,000 Oe for 5 hours at 280° C. in a high-vacuum oven. The CPP TMR or GMR read sensor 201 is patterned in a photolithographic process to produce sensor front and rear edges, and then patterned again in another photographic process to produce sensor tails at the two side regions. The Al2O3 insulation layer 202, the Cr seed layer 232, the Co—Pt longitudinal bias layer 234, and the Cr cap layers 236 are then sequentially deposited into the two side regions. Then, the photoresist is removed and a chemical-mechanical-polishing process is performed. As can be seen in
The Al2O3 insulation layer 202 must uniformly cover the tail of the CPP read sensor 201 in each side region and be thick enough to ensure full electrical isolation between the CPP read sensor 201 and the Co—Pt longitudinal bias stack 204. On the other hand, the Al2O3 insulation layer 204 must be also thin enough to minimize a spacing between the CPP read sensor 201 and the Co—Pt longitudinal bias stack 204, in order to enhance magnetostatic interactions between the CPP read sensor 201 and the Co—Pt longitudinal bias stack 204. With an optimal thickness of about 3 nm, the Al2O3 insulation layer 202 can fully confine the sense current in the CPP read sensor 201, and facilitate the Co—Pt longitudinal bias layer 234 to stabilize the sense layer structure 228.
The Cr seed layer 232 must uniformly cover the tail of the CPP read sensor 201 in each side region and be thick enough to ensure the free growth of its polycrystalline grains with body-centered-cubic (bcc) {011} planes lying in parallel to an interface between the Al2O3 insulation layer 202 and the Cr seed layer 232. This preferred crystalline texture will facilitate polycrystalline grains of the Co—Pt longitudinal bias layer 234 to grow with hexagonal-cubic-packed (hcp) {0110} or {0111} planes lying in parallel to an interface between the Cr seed layer 232 and the Co—Pt longitudinal bias layers 234, thereby orienting its c-axis (the easy axis of magnetization) to lie nearly in the interface and improving its in-plane hard-magnetic properties such as MR and HC. On the other hand, the Cr seed layer 232 must be also thin enough to minimize a spacing between the CPP read sensor 201 and the Co—Pt longitudinal bias layer 234, in order to enhance magnetostatic interactions between the CPP read sensor 201 and the Co—Pt longitudinal bias layer 234. With an optimal thickness of about 4 nm, the Cr seed layer 232 can improve the in-plane hard-magnetic properties of the Co—Pt longitudinal bias layer 234, and facilitate the Co—Pt longitudinal bias layer 234 to stabilize the sense layer structure 228.
In addition to high MRδLB and high HC needed for stabilizing the sense layer structure 214, the Co—Pt longitudinal bias layer 234 must uniformly cover the tail of the CPP read sensor 201 in each side region and be thick enough to provide MR δLB high enough to eliminate side reading caused by the reversals of the magnetization of the sense layer structure 214. On the other hand, the longitudinal bias layer 234 must be also thin enough to provide MR δLB low enough to attain high read sensitivity. With an optimal thickness of about 24 nm, the longitudinal bias layer 234 exhibits MR δLB of 2.1 memu/cm2 (corresponding to that of a 30 nm thick ferromagnetic 80Ni-20Fe film sandwiched into two Cu films, or five times of the saturation moment of the sense layer 228), and thus stabilize the sense layer structure 214 while maintaining high read sensitivity.
However, the total thickness of the insulation layer 202 and the longitudinal bias stack 204 (41 nm) is much larger than the thickness of the CPP read sensor 201 (27.3 nm), and thus a topography, on which the upper ferromagnetic shield 208 will be deposited, will be too steep. This steep topography will decrease read resolution. Therefore, it is crucial to minimize the optimal thicknesses of the insulation layer 202, the seed layer 232, and the longitudinal bias layer 234. Particularly for the longitudinal bias layer 234, since MR δLB must remain unchanged, δLB can be minimized only by maximizing MR. Since MR═S MS, where S is a squareness and MS is a saturation magnetization, MR can be maximized by maximizing S or MS.
The longitudinal bias stack 604 comprises a longitudinal bias layer 634 formed of a 20 nm thick hard-magnetic Fe—Pt film with a Pt content ranging from 44 to 50 at %, and a cap layer 636 formed of a 60 nm thick nonmagnetic Ru film. The thickness of the Fe—Pt longitudinal layer 634 is selected in order to attain MR δLB of 1.68 memu/cm2 (corresponding to that of a 24 nm thick ferromagnetic 80Ni-20Fe film sandwiched into two Cu films). By eliminating the seed layer 232, the thickness of the Al2O3 insulation layer 202 (3 nm) alone defines the spacing. Since the Fe—Pt longitudinal bias layer 634 without any seed layers exhibits HC of as high as beyond 2,400 Oe after annealing, as described below, and the spacing becomes as narrow as 3 nm, the improved longitudinal bias stack 604 will be more effective in stabilizing the sense layer structure of the CPP read sensor 201 progressively miniaturized for magnetic recording at ever higher recording densities.
The insulation layer 1102 is formed of a 1.4 nm thick nonmagnetic, amorphous Al2O3 film. The longitudinal bias stack 1104 comprises a insulating seed layer 1132 formed of a 1.6 nm thick nonmagnetic, polycrystalline MgO film, a longitudinal bias layer 634 formed of a 20 nm thick hard-magnetic Fe—Pt film with a Pt content ranging from 44 to 50 at %, and a cap layer 636 formed of a 60 nm thick nonmagnetic Ru film. The thickness of the Fe—Pt longitudinal layer is selected in order to attain MR δLB 1.68 memu/cm2 (corresponding to that of a 24 nm thick ferromagnetic 80Ni-20Fe film sandwiched into two Cu films). The MgO seed layer 1132 replaces a portion of the Al2O3 insulation layer 1102 and also acts as another insulation layer. As a result, the total thickness of the Al2O3 insulation layer 1102 and the MgO seed layer 1132 (3 nm) defines the spacing, and thus the spacing remains narrow. In addition, with an identical thickness, a spacing formed of two dissimilar oxide films generally exhibits a breakdown voltage (characterizing an insulating property) higher than that formed of an oxide film only, since the growth of pinholes in the lower oxide film during the deposition is interrupted at interfaces between the two dissimilar oxide films. Since the Fe—Pt longitudinal bias layer 634 with the MgO seed layer 1132 exhibits HC as high as beyond 3,200 Oe after annealing, as described below, and the spacing remains as narrow as 3 nm, the further improved longitudinal bias stack 1104 is expected more effective in stabilizing the sense layer structure 228 of the CPP read sensor 201 progressively miniaturized for magnetic recording at ever higher recording densities.
In addition, in both the preferred and alternative embodiments of the invention, an additional conducting seed layer formed of a 1.8 nm thick nonmagnetic, polycrystalline Ru film can be used beneath the Fe—Pt longitudinal bias layer 634. While the spacing increases unwantedly by 1.8 nm, the Fe—Pt longitudinal bias layer 634 with the MgO seed layer 1106 and the additional Ru seed layer exhibits HC of as high as beyond 4,000 Oe after annealing, as described below. Hence, although this approach violates the principle of minimizing the spacing in the invention, it is still acceptable and attractive due to the substantially high HC.
It should be noted that the cap layer also plays an important role in achieving high HC. Table 1 lists HC values for the Fe—Pt films with various seed layers. HC substantially increases as the Ru cap layer replaces a Ta cap layer. As a result, the cap layer formed of a nonmagnetic Ru film is proposed.
In addition, a method of activating the stabilization scheme is also proposed. After completing the read head fabrication process, the read head 600 is annealed in a magnetic field of 50,000 Oe perpendicular to sensor front and rear edges for 5 hours at 280° C. in a high-vacuum oven, and is then re-magnetized in a magnetic field of 50,000 Oe parallel to the sensor front and rear edges at room temperature. After the annealing and re-magnetization steps, the magnetizations of the keeper and reference layers 222, 224 of the CPP read sensor 201 are oriented in opposite directions perpendicular to sensor front and rear edges, while the magnetization of the Fe—Pt longitudinal bias layer 434 parallel to the sensor front and rear edges.
While various embodiments have been described, 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.
