Abstract:
The present invention provides a method for forming a dual giant magnetoresistive sensor. First and second spin valves are first formed and arranged such that a dielectric layer is positioned between the first and the second spin valves. The first spin valve has a plurality of layers including a first antiferromagnetic layer and a first pinned layer. The second spin valve has a plurality of layers including a second antiferromagnetic layer and a second pinned layer. First and second currents are supplied respectively to first and second spin valves. The first current generates a first magnetic field on the first pinned layer that orients a magnetization of the first pinned layer in a first desired direction. The second current generates a second magnetic field on the second pinned layer that orients a magnetization of the second antiferromagnetic layer in a second desired direction. While continuing to supply the first and the second currents, the dual giant magnetoresistive sensor is cooled from a temperature greater than N{acute over (e)}el temperatures of both first and second antiferromagnetic layers to a temperature below the N{acute over (e)}el temperature of both first and second antiferromagnetic layers.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to magnetoresistive read sensors for use in magnetic read heads. In particular, the present invention relates to the simultaneous fixation of the magnetization direction in pinned layers of first and second spin valves of a dual giant magnetoresistive read sensor. 
     A magnetic read head retrieves magnetically-encoded information that is stored on a magnetic medium or disc. The magnetic read head is typically formed of several layers that include atop shield, abottom shield, and aread sensor positioned between the top and bottom shields. The read sensor is generally a type of magnetoresistive sensor, such as a giant magnetoresistive (GMR) read sensor. The resistance of a GMR read sensor fluctuates in response to a magnetic field emanating from a magnetic medium when the GMR read sensor is used in a magnetic read head and positioned near the magnetic medium. By providing a sense current through the GMR read sensor, the resistance of the GMR read sensor can be measured and used by external circuitry to decipher the information stored on the magnetic medium. 
     A common GMR read sensor configuration is the GMR spin valve configuration in which the GMR read sensor is a multi-layered structure formed of a ferromagnetic free layer, a ferromagnetic pinned layer and a nonmagnetic spacer layer positioned between the free layer and the pinned layer. The magnetization direction of the pinned layer is fixed in a predetermined direction, generally normal to an air bearing surface of the GMR spin valve, while a magnetization direction of the free layer rotates freely in response to an external magnetic field. An easy axis of the free layer is generally set normal to the magnetization direction of the pinned layer. The resistance of the GMR read sensor varies as a finction of an angle formed between the magnetization direction of the free layer and the magnetization direction of the pinned layer. This multi-layered spin valve configuration allows for a more pronounced magnetoresistive effect than is possible with anisotropic magnetoresistive (AMR) read sensors, which generally consist of a single ferromagnetic layer. 
     Typically, the magnetization of the pinned layer is fixed in the predetermined direction by exchange coupling an antiferromagnetic layer to the pinned layer. The antiferromagnetic layer is positioned upon the pinned layer such that the antiferromagnetic layer and the free layer form distal edges of the GMR spin valve. The spin valve is then heated to a temperature greater than a N{acute over (e)}el temperature of the antiferromagnetic layer. Next, a magnetic field oriented in the predetermined direction is applied to the spin valve, thereby causing the magnetization direction of the pinned layer to orient in the direction of the applied magnetic field. The magnetic field may be applied to the spin valve before the spin valve is heated to the temperature greater than the N{acute over (e)}el temperature of the antiferromagnetic layer. While continuing to apply the magnetic field, the spin valve is cooled to a temperature lower than the N{acute over (e)}el temperature of the antiferromagnetic layer. Once the magnetic field is removed from the spin valve, the magnetization direction of the pinned layer will remain fixed, as a result of the exchange with the antiferromagnetic layer, so long as the temperature of the spin valve remains lower than the N{acute over (e)}el temperature of the antiferromagnetic layer. 
     A second GMR read sensor configuration is a dual GMR sensor having a first spin valve, a second spin valve, and a spacer positioned between the first and second spin valves. Both the first and second spin valves are formed of a free layer, a spacer layer, a pinned layer, and an antiferromagnetic layer. The spacer layer is positioned between the free layer and the pinned layer. The pinned layer is positioned between the free layer and the antiferromagnetic layer. The magnetization direction in the pinned layer of the first spin valve is antiparallel to the magnetization direction in the pinned layer of the second spin valve. The prior art method of fixing the magnetization directions in the pinned layers of the dual GMR sensor requires that the antiferromagnetic layers have substantially different N{acute over (e)}el temperatures. 
     To fix the magnetization directions in the pinned layers of the dual GMR sensor, the multi-layered spin valve is first assembled. In the situation where the N{acute over (e)}el temperature of the antiferromagnetic layer of the first spin valve is substantially greater than the N{acute over (e)}el temperature of the antiferromagnetic layer of the second spin valve, the dual GMR sensor is heated to a temperature greater than the N{acute over (e)}el temperature of the antiferromagnetic layer of the first spin valve. The GMR spin valve is then subjected to a first magnetic field oriented such that the magnetization direction in the pinned layer of the first spin valve orients in a desired direction. The first magnetic field maybe applied to the dual GMR sensor before the dual GMR sensor is heated. While continuing to apply the magnetic field, the dual GMR sensor is cooled to a temperature lower than the N{acute over (e)}el temperature of the antiferromagnetic layer of the first spin valve, but greater than the N{acute over (e)}el temperature of the antiferromagnetic layer of the second spin valve. The first magnetic field is next removed and a second magnetic field is applied to the dual GMR sensor. The second magnetic field is directed such that the magnetization direction in the pinned layer in the second spin valve orients in the desired direction, which is generally antiparallel to the desired direction of the magnetization direction in the pinned layer ofthe first spin valve. While continuing to apply the second magnetic field to the dual GMR sensor, the temperature of the dual GMR sensor is cooled to a temperature lower than the N{acute over (e)}el temperature of the antiferromagnetic layer of the second spin valve. 
     The magnetization direction in the pinned layers of the first and second spin valve are now fixed, as a result of the exchange with the respective antiferromagnetic layers, so long as the temperature of the dual GMR sensor remains lower than the N{acute over (e)}el temperatures of the antiferromagnetic layers. In the case where the N{acute over (e)}el temperature of the antiferromagnetic layer of the second spin valve is greater than the N{acute over (e)}el temperature of the antiferromagnetic layer of the first spin valve, the dual GMR sensor is first heated to a temperature greater than the N{acute over (e)}el temperature of the antiferromagnetic layer of the second spin valve. The second magnetic field is then applied while the temperature ofthe dual GMR sensor is reduced to a temperature lower than the N{acute over (e)}el temperature of the antiferromagnetic layer of the second spin valve, yet greater than the N{acute over (e)}el temperature of the antiferromagnetic layer of the first spin valve. The second magnetic field is then removed and the first magnetic field applied while the temperature of the dual GMR sensor is reduced to a temperature less than the N{acute over (e)}el temperature of the antiferromagnetic layer of the first spin valve. 
     The first and second spin valves can be connected in either a differential configuration or a gradiometer configuration. In a differential configuration, the output ofthe dual GMR sensor represents the difference between the voltage across the first spin valve and the voltage across the second spin valve. This differential configuration results in a read sensitivity greater than provided by a single spin valve GMR sensor. In a gradiometer configuration, the voltage measured across the first spin valve would be compared to the voltage measured across the second spin valve to measure the gradient of the magnetic field emanating from the magnetic media. This gradiometer configuration is useful in detecting peaks and valleys in the magnetic fields. 
     There are several inherent problems with the prior art method of fixing the magnetization direction of the pinned layers of a dual GMR sensor. First, the two antiferromagnetic layers must be annealed separately. For each layer, the annealing process can take hours, or even days. For a dual GMR sensor, this annealing process becomes twice as long as required for a single AMR sensor. Second, the temperature within an operating disc drive can reach fairly high temperatures. One of the antiferromagnetic layers of the dual GMR sensor has a N{acute over (e)}el temperature substantially lower than the other antiferromagnetic layer. It is, therefore, more likely that the temperature within the operating disc drive would exceed the lower N{acute over (e)}el temperature, causing the antiferromagnetic layer (and the pinned layer) associated with that spin valve to lose its fixed magnetization orientation. There is, therefore, a need for a better means of fixing the magnetization directions in pinned layers of a dual GMR sensor. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides a method for forming a dual giant magnetoresistive sensor. First and second spin valves are first formed and arranged such that a dielectric layer is positioned between the first and the second spin valves. The first spin valve has a plurality of layers including a first antiferromagnetic layer and a first pinned layer. The second spin valve has a plurality of layers including a second antiferromagnetic layer and a second pinned layer. First and second currents are supplied respectively to first and second spin valves. The first current generates a first magnetic field on the first pinned layer that orients a magnetization ofthe first pinned layer in a first desired direction. The second current generates a second magnetic field on the second pinned layer that orients a magnetization of the second pinned layer in a second desired direction. While continuing to supply the first and the second currents, the dual giant magnetoresistive sensor is cooled from a temperature greater than N{acute over (e)}el temperatures of both first and second antiferromagnetic layers to a temperature below the N{acute over (e)}el temperature of both first and second antiferromagnetic layers. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an expanded perspective view of a dual giant magnetoresistive read sensor designed according to the present invention. 
     FIG. 1A shows internally generated magnetic fields acting upon the dual giant magnetoresistive read sensor of FIG.  1 . 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 is an expanded perspective view of a dual giant magnetoresistive (GMR) read sensor  40  designed according to the present invention. GMR read sensor  40  includes first spin valve  42 , second spin valve  44 , and dielectric (D) layer  46  positioned between first spin valve  42  and second spin valve  44 . 
     First spin valve  42  includes first antiferromagnetic (Al) layer  48 , first pinned (P 1 ) layer  50 , first spacer (S 1 ) layer  52 , and first free (F 1 ) layer  54 . First spacer layer  52  is positioned between first free layer  54  and first pinned layer  50 , and first pinned layer  50  is positioned between first free layer  54  and first antiferromagnetic layer  48 . 
     Second spin valve  44  includes second antiferromagnetic (A 2 ) layer  56 , second pinned (P 2 ) layer  58 , second spacer (S 2 ) layer  60 , and second free (F 2 ) layer  62 . Second spacer layer  60  is positioned between second free layer  62  and second pinned layer  58 , and second pinned layer  58  is positioned between second free layer  62  and second antiferromagnetic layer  56 . 
     Each of the plurality of layers of dual GMR read sensor  40  are spaced apart in FIG. 1 for clarity only, and that in reality, the layers are actually adjacent to each other. 
     The planes of each of the layers included within first spin valve  42  and second spin valve  44  are each parallel to the plane of dielectric layer  46 . First antiferromagnetic layer  48  and second antiferromagnetic layer  56  are both positioned furthest from dielectric layer  46 ; whereas first free layer  54  and second free layer  62  are both positioned closest to dielectric layer  46 . Additional layers could be added to both first and second spin valve  42  and  44 . For example, a soft adjacent layer and/or a permanent magnet layer could be added to further fix the magnetization directions in pinned layers. 
     First free layer  54 , first pinned layer  50 , second free layer  62 , and second pinned layer  58  are each formed of ferromagnetic materials; whereas first spacer layer  52  and second spacer layer  60  are each formed of nonmagnetic materials. In a preferred embodiment, first free layer  54  and second free layer  62  each have a thickness in the range of 2 nanometers to 8 nanometers; first spacer layer  52  and second spacer layer  60  each have a thickness in the range of 2 nanometers to 4 nanometers; first pinned layer  50  and second pinned layer  58  each have a thickness in the range of 2 nanometers to 8 nanometers; and first antiferromagnetic layer  48  and second antiferromagnetic layer  56  each have a thickness in the range of 5 nanometers to 30 nanometers. Dielectric layer  46  preferably has a thickness in the range of 1 nanometers to 5 nanometers. 
     For first spin valve  42 , the magnetization M P1 , in first pinned layer  50  is fixed in a predetermined first direction, while the magnetization M F1  in first free layer  54  is allowed to rotate freely in response to external magnetic fields (not shown in FIG.  1 ). The resistance of first spin valve  42  varies as a function of an angle formed between magnetization M F1  of first free layer  54  and magnetization M P1 , first pinned layer  50 . 
     Similarly for second spin valve  44 , the magnetization M P2  in second pinned layer  58  is fixed in a predetermined second direction, while the magnetization M F2  in second free layer  62  is allowed to rotate freely in response to external magnetic fields (not shown in FIG.  1 ). The resistance of second spin valve  44  varies as a function of an angle formed between the magnetization M F2  of second free layer  62  and the magnetization M P2  of second pinned layer  58 . 
     An easy axis of first free layer  54  is preferably oriented perpendicular to the direction of magnetization M P1  of first pinned layer  50 , and an easy axis of second free layer  62  is preferably oriented perpendicular to the magnetization M P2  of second pinned layer  58  and either parallel or antiparallel to the easy axis of first free layer  62 . In absence of an external magnetic field acting on GMR read sensor  40 , the direction of magnetization M F1  of first free layer  54  and the direction of magnetization M F2  of second free layer  62  will be in the easy axis direction. 
     In a preferred embodiment, the sheet resistance of first pinned layer  50  and the sheet resistance of first antiferromagnetic layer  48  are both preferably substantially greater than either the sheet resistance of first free layer  54  or the sheet resistance of first spacer layer  52 . Similarly, the sheet resistance of second pinned layer  58  and the sheet resistance of second antiferromagnetic layer  56  are both preferably substantially greater than either the sheet resistance of second free layer  62  and second spacer layer  60 . Thus, current flowing through first spin valve  42  will remain mostly within first free layer  54  and first spacer layer  52 , and current flowing through second spin valve  44  will remain mostly within second free layer  62  and second spacer layer  60 . 
     Once the structure of dual spin valve  40  is formed, as described with reference to FIG. 1, the directions of the magnetization M P1  of first pinned layer  50  and the magnetization M P2  of the second pinned layer  58  each are fixed in the desired directions. More specifically, the direction of magnetization M P1  of first pinned layer  50  is fixed in a predetermined first direction, generally normal to air bearing surface  64  of GMR read sensor  40 . Similarly, the direction of magnetization M P2  of second pinned layer  58  is fixed in a predetermined second direction, preferably antiparallel to the predetermined first direction. 
     To initially achieve the direction of magnetization M P1  of first pinned layer  50 , a first current I 1 , indicated in FIG. 1 by an arrow, is supplied to first spin valve  42 . First current I 1  is directed parallel to air bearing surface  64  and along the plane of each of the layers of first spin valve  42 . As previously discussed, the sheet resistance of each layer of first spin valve  42  is preferably selected so that most of first current I 1  flowing through first spin valve  42  will be in first free layer  54  and first spacer layer  52 . Accordingly, first free layer current I F1  which is the portion of first current I 1  flowing through first free layer  54 , and first spacer layer curtent I S1 , which is the portion of first current I 1  flowing through first spacer layer  52 , both generate a magnetic field H 1  upon first pinned layer  50 . First pinned layer current I P1  which is the portion of first current I 1  flowing through first pinned layer  50 , and first antiferromagnetic layer current I A1 , which is the portion of first current I 1  flowing through first antiferromagnetic layer  48 , are both small in comparison to first free layer current I F1  and first spacer layer current I S1 . 
     The direction of magnetic field H 1  depends upon the flow direction of first current I 1 . The first desired direction of magnetization M P1  of first pinned layer  50  is upward (as shown in FIG.  1 A). To induce magnetic field H 1  to flow downward (as shown in FIG. 1A) through both first free layer  54  and first spacer layer  52  and upward (as shown in FIG. 1A) through both first pinned layer  50 , the right-hand rule dictates that the flow direction of first current I 1  be from the right to the left (as shown in FIG.  1 A). In such a configuration, the magnetic field H 1  causes the direction of magnetization M P1  of first pinned layer  50  to orient in the first desired direction (upward, as shown in FIG.  1 A). If the first desired direction were downward in FIG. 1A, the flow direction of first current I 1  would be from the left to the right. 
     To initially achieve the direction of magnetization M P2  of second pinned layer  58 , a second current I 2 , indicated in FIG. 1 by an arrow, is supplied to second spin valve  44 . Second current I 2  is directed parallel to air bearing surface  64  and along the plane of each of the layers of second spin valve  44 . As previously discussed, the sheet resistance of each layer of second spin valve  44  is preferably selected so that most of second current I 2  flowing through second spin valve  44  will be in second free layer  62  and second spacer layer  60 . Accordingly, second free layer current I F2 , which is the portion of second current I 2  flowing through second free layer  62  and second spacer layer current I S2 , which is the portion of second current  2  flowing through second spacer layer  60 , generate a magnetic field H 2  upon second pinned layer  58 . Second pinned layer current I P2 , which is the portion of second current I 2  flowing through second pinned layer  58 , and second antiferromagnetic layer current I A2 , which is the portion of second current I 2  flowing through second antiferromagnetic layer  56 , are both small in comparison to second free layer current I F2  and second spacer layer current I S2 . 
     The direction ofmagnetic fields H 2  depends upon the flow direction of second current I 2 . The second desired direction of magnetization M P2  of second pinned layer  58  is downward (as shown in FIG.  1 A). To induce magnetic field H 2  to flow upward (as shown in FIG. 1A) through both second free layer  62  and second spacer layer  60  and downward (as shown in FIG. 1A) through second pinned layer  58 , the right-hand rule dictates that the flow direction of second current I 2  be from the right to the left (as shown in FIG.  2 A). In such a configuration, magnetic fields H 2  causes the direction of magnetization M P2  of second pinned layer  58  to orient in the second desired direction (downward, as shown in FIG.  1 A). If the second desired direction were upward in FIG. 1A, the flow direction of second current I 2  would be from the left to the right. 
     First current I 1  flowing through first spin valve  42  will also generate magnetic field H 1  upon second pinned layer  58 , serving to further enforce the desired direction of magnetization M P2  of second pinned layer  58 . Similarly, second current  12  flowing through second spin valve  44  will generate magnetic field H 2  upon first pinned layer  50 , serving to further enforce the desired direction of magnetization M P1  of first pinned layer  50 . 
     Once the magnetization directions in pinned layers ofboth first and second spin valves  42  and  44  are oriented in desired directions, dual GMR read sensor  40  is heated to a temperature in excess of the Neel temperature of both first and second antiferromagnetic layers  48  and  56 . Dual GMR read sensor  40  is held at that first temperature for a time sufficient to permanently fix the magnetization directions of first and second pinned layers  50  and  58  in the desired directions. Dual GMR read sensor  40  may be heated to the first temperature before first current I 1  is supplied to first spin valve  42  and second current I 2  is supplied to second spin valve  44 . 
     Dual GMR read sensor  40  is then cooled to a temperature lower than the N{acute over (e)}el temperatures of both first and second antiferromagnetic layers  48  and  56  while first current I 1  and second current I 2  are continuously supplied. Once dual GMR read sensor  40  has cooled, the direction of magnetization M P1  of first pinned layer  50  and the direction of magnetization M P2  of second pinned layer  58  are permanently fixed. 
     In a preferred embodiment, first antiferromagnetic layer  48  has a N{acute over (e)}el temperature substantially equal to a Neel temperature of second antiferromagnetic layer  56 . It is further preferred that first antiferromagnetic layer  48  and second antiferromagnetic layer  56  each have aNeel temperature in the range of 100° C. to 300° C. 
     In summary, the present invention is advantageous over the prior art in that it eliminates the need to separately anneal two separate antiferromagnetic layers. By simultaneously providing magnetic fields on both first and second pinned layers  50  and  58  by means of first and second currents I 1  and I 2  flowing through first and second spin valves  42  and  44 , it is no longer necessary to separately anneal first and second antiferromagnetic layers  48  and  56 . In the prior art, the magnetic fields used to fix the directions of magnetization M P1  of first pinned layer  50  and magnetization M P2  of second pinned layer  58  were applied externally. It would be much more difficult to separately isolate the first and the second magnetic fields to apply separately to first and second spin valves  42  and  44  than is possible with the present invention. 
     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.