Abstract:
A method for constructing a magnetoresistive sensor using a horizontally disposed superconducting magnetic tool. The superconducting magnetic tool is capable of generating very high magnetic fields for sustained periods of time to effectively set the magnetizations of magnetoresitive sensors having a very high pinning field. The supermagnetic tool has a ceramic tube surrounded by a superconducting coil. The tube has a longitudinal axis that is oriented horizontally, thereby providing numerous important benefits, such as: facilitating manipulation of the sensor containing wafer within the tool; facilitating loading of the wafer into the tool; preventing temperature and field gradients within the wafer during the anneal; and facilitating maintenance and storage of the tool by limiting the height of the tool.

Description:
BACKGROUND OF THE INVENTION  
     Field of the Invention  
       [0001]     The present invention relates to the construction of magnetoresistive sensors and more particularly to the use of a superconducting magnet to set the magnetization of magnetic layers in a magnetoresistive sensor.  
       BACKGROUND OF THE INVENTION  
       [0002]     The heart of a computer is an assembly that is referred to as a magnetic disk drive. The magnetic disk drive includes a rotating magnetic disk, write and read heads that are suspended by a suspension arm adjacent to a surface of the rotating magnetic disk and an actuator that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The read and write heads are directly located on a slider that has an air bearing surface (ABS). The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk. When the slider rides on the air bearing, the write and read heads are employed for writing magnetic impressions to and reading magnetic impressions from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.  
         [0003]     The write head includes a coil layer embedded in first, second and third insulation layers (insulation stack), the insulation stack being sandwiched between first and second pole piece layers. A gap is formed between the first and second pole piece layers by a gap layer at an air bearing surface (ABS) of the write head and the pole piece layers are connected at a back gap. Current conducted to the coil layer induces a magnetic flux in the pole pieces which causes a magnetic field to fringe out at a write gap at the ABS for the purpose of writing the aforementioned magnetic impressions in tracks on the moving media, such as in circular tracks on the aforementioned rotating disk.  
         [0004]     In recent read head designs a spin valve sensor, also referred to as a giant magnetoresistive (GMR) sensor, has been employed for sensing magnetic fields from the rotating magnetic disk. The sensor includes a nonmagnetic conductive layer, hereinafter referred to as a spacer layer, sandwiched between first and second ferromagnetic layers, hereinafter referred to as a pinned layer and a free layer. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. The magnetization of the pinned layer is pinned perpendicular to the air bearing surface (ABS) and the magnetic moment of the free layer is located parallel to the ABS, but free to rotate in response to external magnetic fields. The magnetization of the pinned layer is typically pinned by exchange coupling with an antiferromagnetic layer.  
         [0005]     The thickness of the spacer layer is chosen to be less than the mean free path of conduction electrons through the sensor. With this arrangement, a portion of the conduction electrons is scattered by the interfaces of the spacer layer with each of the pinned and free layers. When the magnetizations of the pinned and free layers are parallel with respect to one another, scattering is minimal and when the magnetizations of the pinned and free layer are antiparallel, scattering is maximized. Changes in scattering alter the resistance of the spin valve sensor in proportion to cos Θ, where Θ is the angle between the magnetizations of the pinned and free layers. In a read mode the resistance of the spin valve sensor changes proportionally to the magnitudes of the magnetic fields from the rotating disk. When a sense current is conducted through the spin valve sensor, resistance changes cause potential changes that are detected and processed as playback signals.  
         [0006]     The spin valve sensor is located between first and second nonmagnetic electrically insulating read gap layers and the first and second read gap layers are located between ferromagnetic first and second shield layers. In a merged magnetic head a single ferromagnetic layer functions as the second shield layer of the read head and as the first pole piece layer of the write head. In a piggyback head the second shield layer and the first pole piece layer are separate layers.  
         [0007]     Magnetization of the pinned layer is usually fixed by exchange coupling one of the ferromagnetic layers (AP1 ) with a layer of antiferromagnetic material such as PtMn. While an antiferromagnetic (AFM) material such as PtMn does not in and of itself have a magnetization, when exchange coupled with a magnetic material, it can strongly pin the magnetization of the ferromagnetic layer.  
         [0008]     The demand for ever increasing data rate and data density has led a push to develop magnetoresistive sensors having ever smaller size and ever increased performance. However, as sensors become smaller, a challenge that arises is that the strength of the pinning field decreases. The pinning field of the sensor can be understood as the strength of magnetic field that is needed to overcome the pinning of the magnetization of the pinned layer. For example, if the pinning field is very small, the magnetic pinning can be easily overcome, and the orientation of the magnetization of the pinned layer can easily switch from its desired orientation to an orientation that is 180 out of phase. This is known as “amplitude flipping” and results in catastrophic head failure. Events that can lead to amplitude flipping include heat spikes or mechanical stresses such as from head disk contact or electrostatic discharge. Therefore, in order for a sensor to be reliable and robust in use, the sensor must have a very strongly pinned pinned layer (ie. a high pinning field).  
         [0009]     Mechanisms and processes have been proposed to increase the pinning field of a sensor. However, increasing the pinning field of the pinned layer also means that an increased magnetic field is needed to set the pinned layer magnetization during manufacture. For example, in order to set the magnetization of a pinned layer, the sensor is heated above the blocking temperature of the AFM layer. The blocking temperature is the temperature at which the AFM layer ceases to be antiferromagnetic and at which exchange coupling with the pinned layer is lost. While the sensor is held at a temperature above the blocking temperature, a magnetic field is applied to the sensor. This field magnetizes the magnetic pinned layer closest to the AFM layer in a desired direction perpendicular to the air bearing surface (ABS). The application of this magnetic field continues while the sensor is cooled to a temperature below the blocking temperature, at which point exchange coupling between the AFM layer and its closest magnetic layer pins the pinned layer in the desired orientation.  
         [0010]     The magnetic field used to set the pinned layer has traditionally been supplied by a standard solenoid electromagnet. Such a magnetic has magnetic core with an electrically conductive wire wrapped around the core. The core forms first and second poles between which the wafer sits during application of the magnetic field. This form of electromagnet has been suitable for prior art sensors where a magnetic field on the order of only 1.3 Tesla has been needed to set the pinned layer. However, as mentioned above much larger fields are needed to set the pinned layers of future generation sensors. For example, magnetic fields of 4 Tesla and higher will be needed.  
         [0011]     Therefore, there is a strong felt need for a mechanism for setting pinned layers in sensors having very high pinning fields. Such a pinning mechanism will preferably include a means for producing very high magnetic fields, on the order of 4 Tesla or higher. A means for producing such a high magnetic field would also preferably be practical for the mass production of sensors, such as by the use of a tool that can be easily accessed and which can be housed and maintained in a standard building or clean-room. Such a tool for producing a wafer would also allow for convenient manipulation of the wafer within the area in which the magnetic field is maintained.  
       SUMMARY OF INVENTION  
       [0012]     This invention deals with a method and apparatus for setting a sensor AFM with superconducting magnet with 5 Tesla field at elevated temperature. The current designs orient the superconducting magnet/anneal vacuum chamber in a vertical direction. The problems with the vertical design are that the wafers have to “standup” as oppose to lying flat. “Standup” experiences more temperature gradient. In addition, vertical design makes wafer manipulation (rotating wafers) during anneal process very unreliable. The present invention can be embodied in a horizontal superconducting magnet design where the annealing chamber is horizontal and wafers can be annealed lying flat with uniform temperature/field and reliable rotation capability. Many modifications need to be made in order to rotate the magnets from the conventional way and handle the wafers horizontal as opposed to vertical.  
         [0013]     Conventional electromagnets used in the production of giant magnetoresistive (GMR) and tunneling magnetoresistive (TMR) heads for the setting of the magnetization of pinned layer structures, such as antiparallel (AP) pinned structures, rely on large planar pole caps of large dimension made from the highest saturation magnetization materials, such as Fe or CoFe alloys. The fields in the air gap, or working space, between the pole caps of the electromagnet generated by these electromagnets are limited by the saturation magnetization of these alloys which for Fe is 21.5 KG or 2.15 T, and for Co50Fe50 alloy, about 23 KG or 2.3 T. Higher fields, on the order of 5 T, are required for setting the new thin Ru AP-pinned structures. Although high fields greater than 2 T can be obtained with conventional solenoidal electromagnets based on the Bitter magnet design, the size and bulk of such magnets, the non-uniformity of the fields generated, the short duration of sustained fields, the substantial cost of high current generation facilities, and cooling water requirements to dissipate the heat generated by Ohmic conductors makes such designs impracticable in a manufacturing environment. Designs based on superconducting magnets overcome these limitations: the size limitation, because superconducting magnets are small and relatively compact due to the higher current carrying capacity of superconductors; the field uniformity limitation, because superconducting magnets can be made with large diameters; the field duration limitation, because superconducting magnets can conduct a current for as long as their temperature is maintained at or below the critical superconducting temperature; the substantial cost of high current generation facilities, because, unlike Bitter magnets, superconducting magnets do not require Megawatt power generation facilities; and the cooling water requirements, because superconducting magnets do not generate Ohmic heat due to their negligible electrical resistance. The ability to generate large fields without the attendant costs and limitations of conventional solenoidal electromagnets makes superconducting magnets ideal for setting the magnetization of the thin Ru and thin Ru alloy AP-pinned structures in advanced GMR and TMR head wafers, which are  5 ″ or greater in diameter. Moreover, the ability to sustain high, uniform, magnetic fields over large areas provided by superconducting magnets is an absolute requirement for the long term, 2 hours or longer, magnetic anneals at 200 degrees C. or greater required to set the magnetization in thin Ru AP-pinned structures.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]     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.  
         [0015]      FIG. 1  is a schematic diagram of a disk drive system;  
         [0016]      FIG. 2  is a perspective view of a wafer on which a plurality of magnetic heads are formed;  
         [0017]      FIG. 3  is a cross sectional view of a wafer having a plurality of magnetoresistive sensors formed thereon;  
         [0018]      FIG. 4  is an ABS view of an example of a sensor that could be formed on the wafer of  FIG. 3 ;  
         [0019]      FIGS. 5-6  are schematic views of a superconducting magnetic tool in which sensors on a wafer can be annealed to set the magnetization of the pinned layer;  
         [0020]      FIG. 7  is a schematic view of a superconducting magnetic tool according to an alternate embodiment of the invention in which sensors can be annealed;  
         [0021]      FIG. 8  is an external view of a tool for annealing magnetoresitive sensors; and  
         [0022]      FIG. 9  is a flowchart illustrating a method of constructing a sensor.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0023]     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.  
         [0024]     Referring now to  FIG. 1 , there is shown a 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 on each disk is in the form of annular patterns of concentric data tracks (not shown) on the magnetic disk  112 .  
         [0025]     At least one slider  113  is positioned near the magnetic disk  112 , each slider  113  supporting one or more magnetic head assemblies  121 . As the magnetic disk rotates, slider  113  moves radially in and out over the disk surface  122  so that the magnetic head assembly  121  may access different tracks of the magnetic disk where desired data are written. 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 slider  113  against the disk surface  122 . Each actuator arm  119  is attached to an actuator means  127 . The actuator means  127  as shown in  FIG. 1  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 .  
         [0026]     During operation of the disk storage system, 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. The air bearing thus counter-balances the slight spring force of suspension  115  and supports slider  113  off and slightly above the disk surface by a small, substantially constant spacing during normal operation.  
         [0027]     The various components of the disk storage system are controlled in operation by control signals generated by 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 slider  113  to the desired data track on disk  112 . Write and read signals are communicated to and from write and read heads  121  by way of recording channel  125 .  
         [0028]     With reference now to  FIG. 2 , the magnetic head assemblies  121  ( FIG. 1 ) are manufactured on a wafer  202 , with thousands of such heads being manufactured on a single wafer  202 .  FIG. 3  shows an enlarged cross section of the wafer with several magnetic heads  121  formed thereon. The wafer includes a substrate  204 , which may be aluminum titanium carbide (AlTiC) or some other material. Each head  121  includes a magnetoresistive sensor  206  and an inductive write element  208 . For purposes of the clarity, the cross section shown in  FIG. 3  is taken at a location where an air bearing surface (ABS) would be located, so that only a first and second pole tip  210 ,  212  of each write element can be seen. The read and write elements  206 ,  208  are embedded within non-magnetic, electrically insulating material  214  such as alumina.  
         [0029]     With reference now to  FIG. 4 , the structure of a read sensor  206  can be seen in more detail.  FIG. 4  shows a view of a sensor as it would appear when viewed from the air bearing surface (ABS) of a finished head (ie. as viewed from the surface that would face the magnetic medium  122  ( FIG. 1 ) during use. The sensor  206  includes a sensor stack  402  sandwiched between first and second non-magnetic, electrically insulating gap layers  404 ,  406 . The sensor described herein is described as a current in plane sensor for purposes of illustration. However, if the senor were embodied in a current perpendicular to plane (CPP) sensor, the gap layers  404 ,  406  would be replaced with electrically conductive leads layers.  
         [0030]     The sensor stack  402  includes a free layer  408 , a pinned layer structure  410  and a non-magnetic, electrically conductive spacer layer  412  sandwiched between the free layer  408  and pinned layer  410 . The free layer may be constructed of magnetic material such as CoFe, NiFe or a combination of these. The spacer layer  412  may be constructed of, for example, Cu. Although described herein as a GMR sensor, if the sensor were a tunnel valve, the layer  412  would be a thin, non-magnetic, electrically insulating barrier layer. A capping layer  414  such as Ta may be provided at the top of the sensor stack  402  to prevent damage to the sensor layers during manufacture.  
         [0031]     The free layer  408  has a magnetization  416  that is biased in a desired direction parallel with the ABS. Biasing of the free layer may be provided by first and second hard bias layers  418 ,  420  formed at either side of the sensor stack  402 . The bias layers  418 ,  420  may be constructed of, for example CoPt or CoPtCr. First and second electrically conductive lead layers  422 ,  424  may be provided at the top of each bias layer. The leads  422 ,  424  may be constructed of, for example Cu, Au, Rh or some other electrically conductive material.  
         [0032]     With continued reference to  FIG. 4 , the pinned layer structure  410  includes first and second magnetic layers AP1  426  and AP2  428 , which are separated from one another by an antiparallel coupling layer  430 , which can be constructed of, for example, Ru. The first and second magnetic layers can be constructed of a material such as CoFe. The AP1 and AP2 layers are strongly antiparallel coupled so that they have magnetizations  432 ,  434  that are oriented antiparallel to one another. A layer of antiferromagnetic material (AFM layer)  436  is exchange coupled with the AP1 layer, which strongly pins the magnetic magnetization  432  of the AP1 layer  426 . The AFM layer  436  can be constructed of, for example, PtMn, IrMn or some similar material.  
         [0033]     Setting the magnetizations  432 ,  434  of the AP1 and AP2 layers  426 ,  428  can be accomplished by an annealing process. The annealing process may include raising the sensor  206  to a temperature that is close to the blocking temperature of the AFM layer  436 . The blocking temperature is the temperature at which exchange coupling between the AFM layer  436  and the AP1 layer  426  is lost. For example, the blocking temperature of PtMn is about 350 degrees C. When annealing sensors having PtMn AFM layers, the wafer is raised to a temperature greater than 200 degrees C., such as 215 to 315 degrees C. or about 265 degrees C. IrMn has a slightly lower blocking temperature. Therefore, when annealing sensors having IrMn AFM layers, the wafer is raised to a temperature that is also greater than 200 degrees C., such as 190 to 290 degrees or about 240 degrees C. While the sensor is held at this temperature, a magnetic field is applied to the sensor to orient the magnetizations  432 ,  434  of the AP1 and AP2 layers  426 ,  428  in a desired direction perpendicular to the ABS. While maintaining the magnetic field, the sensor is cooled to a temperature well below its blocking temperature, or to about room temperature (around 20 degrees C.). In one method of setting pinned layer  410 , the magnetic field used to orient the magnetizations, is sufficiently strong that it overcomes the antiparallel coupling between the AP1 and AP2 layers  426 ,  428 . This causes the magnetizations  432 ,  434  to point in the same direction while the sensor is held above the blocking temperature of the AFM layer  436 . When the sensor is cooled and the magnetic field is removed, the magnetization  434  rotates 180 degrees due to the antiparallel coupling between the layers  432 ,  434 , while the magnetization  432  of the AP1 layer  426  remains oriented in the direction that it was oriented during application of the magnetic field. Strong exchange coupling between the AFM and the AP1 layer  426  keeps the magnetization  432  strongly pinned in this direction.  
         [0034]     As can be appreciated, a tool is required to supply the magnetic field for annealing the pinned layer as described above. Prior art sensors have been annealed in a magnetic field provided by a solenoid magnet, based on the Bitter magnet design. Such magnets include a ferromagnetic core that forms first and second poles and an electrically conductive coil wrapped around the core. The wafer on which the sensors are manufactured is placed between the poles of the magnet, where a magnetic field extending from one pole to the other sets the pinned layer magnetization.  
         [0035]     As discussed above in the Background of the Invention, sensor performance demands require ever increased pinning fields. These increased pinning fields, require higher magnetic fields for setting the pinned layer than were previously required. A conventional solenoid magnet such as that described can produce a magnetic field on the order of 1 to 3 Tesla or about 1.3 Tesla Tesla. Current and future generation sensors require fields on the order of 5 Tesla in order to effectively set the magnetizations of the pinned layer. Although high fields greater than 2 T can be obtained with conventional solenoidal electromagnets based on the Bitter magnet design, the size and bulk of such magnets, the non-uniformity of the fields generated, the short duration of sustained fields, the substantial cost of high current generation facilities, and cooling water requirements to dissipate the heat generated by Ohmic conductors makes such designs impracticable in a manufacturing environment. To set an AP pinned structure as described above, the wafer must be held within the magnetic field for 2 hours or greater at a temperature on the order of 200 degrees Celsius or greater.  
         [0036]     Superconducting magnetic tools have been developed that are capable of generating the high magnetic fields necessary to anneal current and future generation sensors. As mentioned above, in the Summary of the Invention, designs based on superconducting magnets overcome many of the limitations of conventional solenoid electromagnets. For example, the size limitation can be overcome, because superconducting magnets are small and relatively compact due to the higher current carrying capacity of superconductors. Superconducting magnets overcome field uniformity limitations, because superconducting magnets can be made with large diameters. The field duration limitation is overcome, because superconducting magnets can conduct a current for as long as their temperature is maintained at or below the critical superconducting temperature. Furthermore, the substantial cost of high current generation facilities is not an issue, because, unlike Bitter magnets, superconducting magnets do not require Megawatt power generation facilities, In addition, the cooling water requirements are virtually eliminated, because superconducting magnets do not generate Ohmic heat due to their negligible electrical resistance. The ability to generate large fields without the attendant costs and limitations of conventional solenoidal electromagnets makes superconducting magnets ideal for setting the magnetization of the thin Ru and thin Ru alloy AP-pinned structures in advanced GMR and TMR head wafers, which are 5″ or greater in diameter. Moreover, the ability to sustain high, uniform, magnetic fields over large areas provided by superconducting magnets is an absolute requirement for the long term, 2 Hr or longer, magnetic anneals at 200 C or greater required to set the magnetization in thin Ru AP-pinned structures.  
         [0037]     However, previously constructed superconducting magnets are unsuitable for use in annealing magnetoresistive sensors. Previously developed superconducting magnets include a ceramic tube oriented vertically with a superconducting coil surrounding the ceramic tube. A heating element wrapped around the ceramic tube is used to heat the wafer to the desired temperature during the anneal. In order to expose the wafer to a magnetic field, the wafer must be held within the ceramic tube. With currently available tools, this means that the wafer must be loaded into the tube through the bottom or top of the tube, making loading of the wafer extremely difficult.  
         [0038]     In addition, the vertical orientation of the tube makes manipulation of the wafer within the tube extremely difficult. The magnetic field within the tube is oriented along the length of the tube, which, when the tube is oriented vertically, means that the wafer must be held on its edge in order to correctly orient the sensors within the magnetic field. Such orientation requires that the wafer be held on some sort of complex clamping device that can hold and manipulate the wafer in a vertical position. Keeping in mind that the wafer must be maintained at a temperature greater than 200 degrees C. in the presence of a 5 Tesla magnetic field for a duration greater than 2 hours, any complex mechanism for manipulating the wafer would suffer from serious reliability and maintenance problems.  
         [0039]     In addition, in order to maintain such high magnetic fields using a superconducting magnet, the inside of the tube must be evacuated. This makes the use of a complex wafer clamping and manipulating device even more challenging, since the actuation mechanism must either be located within the harsh environment within the evacuated chamber or must pierce the chamber, making evacuation more difficult.  
         [0040]     In addition, housing and maintaining such a tool poses a great challenge. The ceramic tube of such a device has a length along its axis of about 6 feet. Since the tube is oriented vertically, the tool cannot be housed within a standard clean-room having a ceiling of only about 12 feet. For example, in order to maintain such a vertically oriented tool and access the inside of the tool to load a wafer, an operator would have to access the top of the tool at a height of about 14 feet.  
         [0041]      FIGS. 5 and 6  schematically illustrate a superconducting annealing tool  500  according to an embodiment of the invention. With reference to  FIG. 5 , at its most basic, the tool  500  includes a ceramic tube  502  which can be for example quartz, and a superconducting coil  504  forming a magnet wrapped around the ceramic tube  502 . The wafer  202 , held on a platter, table or tray  506  enters the tube  502  through a hole in an end of the tube.  
         [0042]     With reference now to  FIG. 6 , which shows a schematic view of the tool  500  in greater detail and in cross section, the tool  500  includes an evacuation chamber  508 , which can be formed by capping the ends of the ceramic tube  502  with caps  520  and providing a pump (not shown) to evacuate the tube. A magnetic shield  510  surrounds the magnet  504 , to protect the operators from exposure to the high magnetic fields produced by the tool  500 . At least one of the caps  520  at the end of the tube  502  is configured with a door for inserting the wafer  202 .  
         [0043]     An electrically conductive heating coil  511  surrounds the vacuum chamber  508 . This heating coil can be used to raise the temperature of the wafer  202  inside the chamber to a temperature that is necessary to anneal the sensors  206  as described above.  
         [0044]     The tube  502  has a longitudinal axis  512  that is oriented with relation to a horizontal plane  514  and a vertical plane  516 . The longitudinal axis  502  of the tool  500  is configured to be oriented substantially parallel with the horizontal plane  514  and substantially perpendicular to the vertical plane  516 . However, the axis  512  may be at an angle of, for example, 0-30 degrees with respect to horizontal  514 . Similarly, gravity in the environment of the tool (represented as a vector  518 ) is oriented in a vertical direction perpendicular to the longitudinal axis  512  of the ceramic tube  502 .  
         [0045]     Orienting the tool  500  so that the longitudinal axis is substantially parallel with the horizon (horizontal plane  514 ) provides numerous substantial advantages over prior art designs. For example, the wafer  506  can be easily loaded through an end of the tube  502  through a door or opening in one of the caps  520 . This makes loading of the wafer much easier, since the end of the tube  502  is located at an elevation that is accessible to an operator standing on the ground, as compared with requiring the operator to climb up a ladder or scaffold to reach the end of the tube if it were oriented vertically.  
         [0046]     Furthermore, the wafer can be easily held on the platter  506  without the use of any complex clamping device, because the wafer  202  can be held on the platter  506  with the assistance of gravity  518 . A support structure  522  may be provided to support the platter  506  within the housing. The support structure  522  may include an actuator mechanism  524  and servo device  526  to orient or rotate the platter  506  while it is within the tube  502 . Optionally, the actuator mechanism may be eliminated, simplifying the design and resulting in improved maintenance and reduced manufacturing cost. If the actuator  524  and servo  526  are not included, the proper orientation of the wafer can be ensured by placing the wafer on the platter in the desired orientation and then loading the platter into position within the tube  502 . Advantageously, since the wafer can be held on the platter  506  by gravity, the mechanism for supporting the wafer within the tube  502  can be greatly simplified.  
         [0047]     With reference now to  FIG. 7 , in another embodiment of the invention, a vacuum chamber  702  that is separate from the ceramic tube  502  can provided for evacuating the atmosphere surrounding the magnetic coil  504 . This vacuum chamber  702  can have a toroidal or doughnut shape, with the ceramic tube  502  extending through the hole in the center of the doughnut. This separate evacuation chamber  702  thermally isolates the magnetic coil  504 , and assists in keeping the coil  504  at the very low temperatures (around 9 degrees Kelvin) needed to maintain the coil in a solid state and enjoy the superconductive properties of the coil.  
         [0048]     As mentioned above, in order to maintain the superconductive properties of the magnetic coil  504 , the coil must be kept at a very low temperature. For example, the coil  504  can be constructed of NbTi, which must be kept at a temperature of about 9 degrees Kelvin. This low temperature can be maintained by a process that includes cooling the coil  504  using a cooling system having refrigerant conduit coil (not shown) and compressor (not shown) and using a material such as He a refrigerant. Cooling can be further improved by keeping the coil evacuated, as discussed with reference to  FIG. 7 .  
         [0049]      FIG. 8  shows a perspective view of a tool  800  according to an embodiment, shown from the outside.  FIG. 8  shows a stack of wafers  802  outside of the tool  800 , illustrating the ease of access of the end of the tool  800  for loading wafer into the tool  800 .  
         [0050]     With reference now to  FIG. 9 , a method  900  for manufacturing a magnetoresistive sensor is described. The method  900  begins with a step  902  of providing a substrate. The substrate can be a wafer constructed of, for example aluminum titanium carbide (AlTiC) or could be some other material such as Si. Then, in a step  904  a plurality of sensors are formed on the substrate (wafer). The sensors may include a pinned layer structure, a free layer structure and a non-magnetic spacer or barrier layer sandwiched between the free layer and the pinned layer. The pinned layer structure may include first and second magnetic layers AP1 and AP2 separated from one another by a coupling layer such as Ru. One of the magnetic layers AP1 may be exchange coupled with a layer of antiferromagnetic material (AFM) layer. The AFM layer has a blocking temperature, which is the temperature at which the AFM layer loses its antiferromagnetic properties and loses exchange coupling with the AP1 layer. In a step,  906  a horizontally disposed superconductive magnetic tool is provided. The tool includes a ceramic tube, which may be constructed of quartz and which is surrounded by a superconductive coil that is wrapped around the tube. The tube has a longitudinal axis that is oriented substantially horizontally. The tool may also include a platter connected with a support structure, the support structure being configured to move the platter laterally into the tube along a direction parallel with the longitudinal axis of the tube. The support structure may also be configured to rotate the platter about an axis that is substantially vertical (ie. rotate the platter in a horizontal plane), or may be configured so that the platter is fixed so that it does not rotate. In a step,  908  the wafer (substrate and sensors) is placed into the superconducting magnetic tool. The wafer can be loaded into the tool, by placing it on the platter, where the wafer can be held by gravity (rather than clamped) due to the horizontal orientation of the tube.  
         [0051]     In a step,  910 , the wafer is heated to a temperature near the blocking temperature of the AFM layers of the sensors formed on the substrate. This temperature may be 215-315 degrees C. or about 265 degrees C., if a PtMn AFM layer is used in the sensor. The annealing temperature may be about 190-290 degrees C. or about 240 degrees C. if an IrMn AFM layer is used. Then, in a step  912 , the tool is activated to generate a magnetic field within the tube, where the wafer is located. This magnetic field may be 4-6 Tesla or about 5 Tesla. The magnetic field is generated by conducting a current through the superconductive coil surrounding the tube. Since the superconducting coil generates negligible Ohmic heat, a large current can be supplied for a prolonged amount of time.  
         [0052]     With reference still to  FIG. 9 , in a step  914  the magnetic field and temperature of the wafer are both maintained for a desired duration. This duration is preferably greater than 1 hour can be, for example 1-3 hours or about 2 hours, or could be 5 or more hours. Then, in a step  916 , the wafer is cooled well below the blocking temperature, such as to a temperature below 100 degrees C. or to room temperature. The magnetic field is maintained while cooling the wafer to the desired temperature, in order to ensure that when the AFM layer becomes anti-ferromagnetic and exchange couples with the AP1 layer of the pinned layer, the AP1 layer will be magnetized in the desired direction perpendicular to the plane in which the air bearing surface (ABS) will be. After the wafer has been brought down to the desired temperature (ie. below 100 degrees C., or to room temperature), the magnetic tool can be deactivated to terminate the generation of the magnetic field. The wafer can then be easily removed from the tool through the end of the horizontally disposed tube.  
         [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.