Abstract:
High sensitivity in a CPP spin valve has been achieved by providing an extended free layer while maintaining good bias point control and edge domain control through use of exchange coupling with the whole free layer. In a second embodiment of the invention, a second spin valve is added so that the free layer receives filtered electrons from two directions. Processes to manufacture both embodiments are also described.

Description:
FIELD OF THE INVENTION 
     The invention relates to the general field of magnetic disks with particular reference to their read heads. 
     BACKGROUND OF THE INVENTION 
     As recording densities beyond 100 Gb per sq. in. and track widths below 0.1 microns become feasible, the traditional CIP (current in film plane) spin valve head can no longer meet the required signal level because the signal amplitude of a CIP head scales with track width. CPP (current perpendicular to plane) heads then become potential candidates. CPP mode has many advantages over the CIP mode, such as higher GMR ratio (by using multilayers), good joule heat dissipation (conductor as gap material), and a signal amplitude that is independent of track width. 
     Pillar-type spin valve CPP heads have been built and tested [1,2]. In the pillar-type design, a contiguous hard bias scheme is needed to stabilize the free layer edge domain. However, this reduces the head sensitivity dramatically for track widths below 0.1 microns, just as in the CIP spin valve abutted junction design. It is therefore not suitable for densities beyond 100 Gb psi. 
     A modified pillar-type design has been proposed [3] and is shown in FIG.  1 . Shield  19  sits atop antiferromagnetic layer  18  which serves to pin synthetic antiferromagnetic trilayer  15 / 16 / 17 . Copper spacer layer  14  is immediately below. In this design, continuous free layer  13  remains but is exchange coupled with antiferromagnetic film  12 . Micro-magnetic simulation shows that the free layer sensitivity drops only about 10% for a pinning field of about 150 Oe [3]. This sensitivity is much higher than for the abutted junction case. However, in the CPP mode, the current-induced magnetic field has a circular direction within the GMR film plane. This circular field yields a buckling domain in the free layer which may cause instability and noise during head operation. 
     Another proposed design is the synthetic pattern exchange structure [4] seen in FIG.  2 . In this scheme, the entire GMR stack, which includes free layer  13 , Cu spacer layer  14 , AFM pinned AP1 layer  15 , and AP2 layer  17  (layer  16  being ruthenium), all extend outward to a significant extent. The track width is defined by conductor  25  at the center of the stack. The sides of the free layer are pinned by a synthetic pattern exchange layer  24 / 23 / 22 . The advantage of this design is its high sensitivity and good track width definition. Due to the strong synthetic pinning strengths of the free layer side regions, the effects of circular field on the free layer can be largely eliminated. Layer  26  is the top shield. 
     However, this design also has several drawbacks:
         (1) the extended Cu spacer layer  14  causes current leakage which results in signal loss.       

     (2) the large shape anisotropy in the AP layer due to the large aspect ratio results in significant deviation of the AP layer from a transverse direction; strong AP coupling is needed for this design at small dimensions. 
     (3) there is difficulty with bias point control. In the CIP spin valve head, there are two magnetic fields along the transverse direction (one is the stray field from the AP layers and the other is a current induced field) which counterbalance each other. In the CPP mode, however, the current field in this direction vanishes and this stray field becomes the only transverse field. 
     REFERENCES 
     [1] A. Matsuzono et al, “A Study on Requirements for Shielded CPP Spin Valve Heads based on Dynamic Read Tests”, CB-02, 46h N&gt;Iv&gt;M conference abstract, November, 2001. (2) TDK internal presentation. 
     [3] Simon Liao et al, “Magnetic Tunneling Junction &amp; CPP Reader Design With Continuous Exchange-coupled Free Layer” Headway disclosure, December, 2001. 
     [4] Simon Liao et al, “CPP GMR Reader With Synthetic Pattern Exchange Stabilization”, Headway disclosure, December 2001. 
     A routine search of the prior art was performed with the following references of interest being found: 
     In U.S. Pat. No. 5,627,704, Lederman et al. show a MR CCP transducer structure. Dykes et al. (U.S. Pat. No. 5,668,688) show a Current Perpendicular to the plane SV MR. U.S. Pat. No. 5,731,937 (Yuan) describes a CPP GMR Transducer while in U.S. Pat. No. 6,347,022 B1, Saito shows a DSV. U.S. Pat. No. 6,219,205 B1 (Yuan et al.), U.S. Pat. No. 5,880,912 (Rottmayer) and U.S. Pat. No. 6,317,297 B1 (Tong et al.) are all related patents. 
     SUMMARY OF THE INVENTION 
     It has been an object of at least one embodiment of the present invention to provide a CPP spin valve that exhibits a minimal current induced circular domain effect. 
     Another object of at least one embodiment of the present invention has been to provide a CPP spin valve that has good bias point control. 
     Still another object of at least one embodiment of the present invention has been to provide a CPP spin valve having the higher sensitivity that is associated with an extended free layer. 
     A further object of at least one embodiment of the present invention has been to combine all of the improvements listed above in a single unit. 
     A still further object of at least one embodiment of the present invention has been to provide a process for manufacturing said CPP spin valve. 
     These objects have been achieved through a CPP spin valve that reduces circular field effects by means of synthetic pattern exchange pinning. The device achieves high sensitivity by providing an extended free layer while maintaining good bias point control and edge domain control through use of exchange coupling with the whole free layer. In a second embodiment of the invention, this device is provided with a second spin valve so that the free layer can receive filtered electrons from two directions. Processes to manufacture both embodiments are described. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1 and 2  show CPP spin valves of the prior art 
         FIG. 3  illustrates an early stage in the process of the present invention. 
         FIG. 4  shows an intermediate stage of the process of the present invention. 
         FIG. 5  shows a first embodiment of the present invention after said process has been completed. 
         FIG. 6  shows a second embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention discloses two designs that solve the problems associated with the prior art designs discussed above. 
     A schematic drawing of the first embodiment is shown in FIG.  5 . In this scheme, the bottom AFM layer  12  is used to help the bias point control. The synthetic pattern exchange layer  22 / 23  at the side wings of free layer  13  provides a stabilization scheme for the free layer. The synthetic pinning at the side region can significantly reduce the current induced circular field effect, thereby enabling large current input. No current leakage occurs since the Cu layer is patterned. Transverse pinning of the AP layer is more relaxed compared to that in the second prior art design since the AP layer shape anisotropy is much smaller than in the previous case. Also, due to the pinning of the free layer side wings, the required bottom AFM pinning strength can then be reduced compared to that of the first design. 
     All of the above designs have some degree of bias point control problems since none of them has another transverse field to cancel the stray field from the AP layer: The design shown in  FIG. 6 , referred to as a dual spin valve CPP head, can achieve an optimal bias state by adding another identical AP unit at the other side of the free layer. For example, the configuration: 
     AP1up13/Ru/AP2down15/Cu/free/Cu/AP3down13/Ru/AP4up15 
     can achieve zero stray field at the free layer and can therefore obtain an ideal bias state. For the same reason as in the design of  FIG. 2 , the bottom Cu spacer layer will have current leakage problems and cause signal loss. However, the GMR ratio in this dual spin valve structure is expected to be higher than that of a single spin valve structure. The signal loss due to current leakage will be more than compensated by this. 
     We now disclose the present invention in further detail through a description of a process for manufacturing it. In the course of so doing the structure of the present invention will also become apparent. 
     Referring now to  FIG. 3 , the process of the present invention begins with the provision of lower magnetic shield  11 . In addition to being ferromagnetic, the shield comprises conductive material over the ferromagnetic material so that it may also serve as a connecting lead for the device. 
     Bottom antiferromagnetic layer  12  is then deposited onto layer  11 , followed by the deposition of free layer  13  which is between about 20 and 100 Angstroms thick. Suitable materials include, but are not limited to, NiFe, CoFe, and NiFeCo. This is followed by copper spacer layer  14  (between about 20 and 50 Angstroms thick), AP1 layer  15  (between about 15 and 30 Angstroms thick of materials such as CoFe or Co, antiferromagnetic coupling layer  16  (between about 5 and 10 Angstroms thick of materials such as Ru or Rh), AP2 layer  17  (between about 15 and 30 Angstroms thick of materials such asCoFe or Co), top antiferromagnetic layer  18  (between about 50 and 150 Angstroms thick of PtMn, IrMn, or NiMn), and conductive layer  25  (between about 100 and 300 Angstroms thick of copper or gold. Note that all of the above listed layers were deposited during a single pumpdown. 
     With these layers in place, the structure is ion milled (with a suitable photoresist mask, not shown, in place) to produce the pillar structure shown in FIG.  3 . Pillar  31  will determine the shape and location of the spin valve stack. It has surface dimensions of about 0.1 by about 0.3 microns. End point detection during the ion milling step (at the copper-free layer interface) was achieved by means of SIMS (Secondary Ion Mass Spectrometry). 
     Next is the deposition of ferromagnetic bias layer  22  to a thickness between about 20 and 100 Angstroms. Suitable materials for this layer include, but are not limited to, NiFe and CoFe. This is followed by antiferromagnetic bias layer  23  to a thickness between about 50 and 150 Angstroms, suitable materials for this including PtMn, IrMn, and NiMn. Non-magnetic insulating layer  24 , of materials such as aluminum oxide or tantalum oxide, is then deposited. 
     At this stage the structure has the appearance seen in FIG.  4 . The structure is then planarized so all traces of material on layer  24  over pillar  31  are removed, following which upper magnetic shield layer  26  is laid down so the completed structure is as seen in FIG.  5 . The upper magnetic shield  26  is between about 1 and 5 microns thick and is, for example, of NiFe, CoFe, CoNiFe, CoFeN. 
     We refer now to  FIG. 6  for a description of the second embodiment of the invention which was mentioned earlier. The process and structure are similar to that seen in  FIG. 5  except that, the bottom antiferromagnetic layer (layer  12 ), is replaced by a second spin valve. This second spin valve is, in effect, a mirror reflection of layers  14  through  17 . 
     Thus, bottom antiferromagnetic layer  68  is deposited onto lower conductive magnetic shield  11 , whose conductance is enhanced by a layer of copper and/or gold and which is between about 100 and 300 Angstroms thick. This is followed by a ferromagnetic layer which is then magnetized in a first direction to become AP4 layer  65 . AP4 is between about 10 and 30 Angstroms thick and is of CoFe, Co, or CoFeB. Then antiferromagnetic coupling layer  66  (between about 3 and 10 Angstroms thick and of Ru or Rh) is deposited on AP4 layer  65 , followed by another ferromagnetic layer which is then magnetized in a second direction, antiparallel to the first direction, to become AP3 layer  67 . AP3 is between about 15 and 30 Angstroms thick and is also of CoFe, Co, or CoFeB. 
     An additional copper spacer layer  64  is then deposited onto AP3 layer  67 . Layer  64  is between about 20 and 50 Angstroms thick. From here on the process proceeds as described above for the embodiment illustrated in FIG.  5 —ion milling in the presence of a suitable photoresist mask to pattern the CPP structure into a pillar. The end point for etching was also controlled by SIMS to be at the interface between copper apacer  14  and the free layer.