Abstract:
In this invention, we replace low resistivity NiFe with high-resistivity FeNi for the FL 2  portion of a composite free layer in a CIP GMR sensor in order to minimize current shunting effects while still retaining both magnetic softness and low magnetostriction. A process for manufacturing the device is also described.

Description:
FIELD OF THE INVENTION 
     The invention relates to the general field of CIP GMR read heads with particular reference to the free layer sub-structure. 
     BACKGROUND OF THE INVENTION 
     The principle governing the operation of most magnetic read heads is the change of resistivity of certain materials in the presence of a magnetic field (magneto-resistance or MR). Magneto-resistance can be significantly increased by means of a structure known as a spin valve where the resistance increase (known as Giant Magneto-Resistance or GMR) derives from the fact that electrons in a magnetized solid are subject to significantly less scattering by the lattice when their own magnetization vectors (due to spin) are parallel (as opposed to anti-parallel) to the direction of magnetization of their environment. 
     The key elements of a spin valve are illustrated in  FIG. 1 . They are seed layer  11  (lying on lower conductive lead  10 ) on which is antiferromagnetic layer  12  whose purpose is to act as a pinning agent for a magnetically pinned layer. The latter is a synthetic antiferromagnet formed by sandwiching antiferromagnetic coupling layer  14  between two antiparallel ferromagnetic layers  13  (AP 2 ) and  15  (AP 1 ). 
     Next is a non-magnetic spacer layer  16  on which is low coercivity (free) ferromagnetic layer  17 . A contacting layer such as lead  18  lies atop free layer  17 . When free layer  17  is exposed to an external magnetic field, the direction of its magnetization is free to rotate according to the direction of the external field. After the external field is removed, the magnetization of the free layer will stay at a direction, which is dictated by the minimum energy state, determined by the crystalline and shape anisotropy, current field, coupling field and demagnetization field. 
     If the direction of the pinned field is parallel to the free layer, electrons passing between the free and pinned layers suffer less scattering. Thus, the resistance in this state is lower. If, however, the magnetization of the pinned layer is anti-parallel to that of the free layer, electrons moving from one layer into the other will suffer more scattering so the resistance of the structure will increase. The change in resistance of a spin valve is typically 8-20%. 
     GMR devices may be designed so as to measure the resistance of the free layer for current flowing parallel to its two surfaces. This is referred to as a CIP (current in plane) device. 
     Instead of being a single layer, free layers that are laminates of several layers have begun to be used in magnetic recording heads. For example, Co 90 Fe 10 /Ni 80 Fe 20 , A typical composite free layer usually consists of two magnetic layers, a first free layer (FL 1 ) and second free layer (FL2), which are directly magnetically coupled to one another. FL 1  (usually Co-rich alloys) provides strong spin dependent scattering, while FL 2  (usually permalloy-type (NiFe) material) provides magnetic softness (i.e. low coercivity). 
     When compared with a free layer of only CoFe, a composite free layer has the following advantages: 1) Better magnetic softness can reduce noise and enhance the sensitivity of GMR sensor. 2) Magnetostriction can be easily adjusted by changing the thickness ratio of Ni 80 Fe 20  to Co 90 Fe 10 . However, a major drawback of composite free layers of the current and prior art is their low dR and dR/R in a CIP configuration because Ni 80 Fe 20 , with relatively low spin polarization and low resistivity, significantly contributes to shunting effects while top specular (or spin filter) schemes, such as CoFe\Cu\Oxide or CoFe\Oxide, cannot be applied in this case. 
     A routine search of the prior art found the following references to be of interest: 
     In U.S. Pat. Nos. 6,614,630 and 6,517,896 (Horng et al) show conventional CoFe/NiFe free layers. Gill teaches alternating CoFe and NiFe films to form the free layer in U.S. Pat. No. 6,466,417. In U.S. Pat. No. 6,038,107 Pinarbasi discloses a composite Co/NiFe free layer while Den discloses FeNi in the ferromagnetic layer in U.S. Pat. No. 6,611,034. 
     Tanaka et al. describe a Co 70 Fe 15 Ni 15  free layer having a ratio of 70:15:15 U.S. Pat. No. 6,608,740. In U.S. Pat. No. 6,123,780, Kanai et al) show a FeNi/CoFeB free layer but give no details on the Fe composition of the layer. In U.S. Pat. No. 5,896,252, Kanai describes a spin valve that includes two sub-layers and, in U.S. Pat. No. 6,352,621, Saito et al. disclose a FeNi free layer but give no details on the Fe composition of the layer. 
     SUMMARY OF THE INVENTION 
     It has been an object of at least one embodiment of the present invention to provide a CIP GMR magnetic read head having improved performance. 
     Another object of at least one embodiment of the present invention has been to provide a process for manufacturing said read head. 
     Still another object of at least one embodiment of the present invention has been that said process be compatible with existing processes for the manufacture of CIP GMR devices. 
     These objects have been achieved by replacing the conventional free layer with a composite layer that includes at least two layers, one of which is CoFe while the other is a ferromagnetic material having at least 60 atomic percent of iron as well as a resistivity of at least 35 micro-ohm cm. Additional elements may be added to this layer in order to maximize this resistivity value. The result is an improved CIP GMR device that has a higher GMR ratio than prior art devices, while still maintaining free layer softness and acceptable magnetostriction. A process for manufacturing the device is also described. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a GMR stack of the prior art which has a conventional free layer. 
         FIG. 2  shows a GMR stack according to the teachings of the present invention. 
         FIG. 3  is a more detailed version of the structure shown in  FIG. 2 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In conventional (standard) CPP spin valve structures, composite free layers made of CoFe (10%)  and NiFe (19%)  have been used. Such films are supposedly non magnetostrictive (i.e. the magnetostriction coefficient is around 10 −7 . For CoFe films, magnetostriction increases with higher Fe composition while for NiFe films, negative magnetostriction is obtained at lower Fe concentrations. The present invention takes advantage of these characteristics by increasing both the percentage of iron in the NiFe portion of this laminate as well as its resistivity, thereby improving the CIP GMR while still maintaining free layer softness and acceptable magnetostriction. 
     Referring now to  FIG. 2 , we provide a description of the process of the present invention. In the course of this description, the structure of the present invention will also become apparent. 
     The process begins with the formation of seed layer  11  onto which is deposited pinning layer  12 . Layer  12  comprises a suitable antiferromagnetic material such as IrMn and it is deposited to a thickness between 20 and 100 Angstroms. Layer  13  (known as AP 2 ), the first of the two antiparallel layers that will form the synthetic AFM pinned layer, is then deposited onto layer  12 . This is followed by layer of AFM coupling material  14  and then AP 1  layer is deposited thereon. Next, non-magnetic spacer layer  16  is deposited on AP 1  layer  15 . 
     Now follows a key feature of the invention whereby free layer  23  is formed by successive deposition of at least two layers, one of which is CoFe (deposited to a thickness between about 5 and 30 Angstroms) while the other is a ferromagnetic material having a resistivity of at least 35 micro-ohm cm and containing at least 60 atomic percent of iron. It is deposited to a thickness between about 10 and 40 Angstroms. in addition to iron and nickel, this layer of ferromagnetic material may also include one or more additional elements such as B or V, that serve to increase the resistivity. The total thickness of free layer  23  should be between about 15 and 70 Angstroms. 
     The two layers that make up the free layer are schematically illustrated in  FIG. 3  as layers  31  and  32  respectively but it should be noted that the invention will function equally well if the order of their deposition is reversed (such as in top and bottom spin valves). It should also be noted that additional layers of CoFe and/or Fe rich NiFe could be added to the free layer to bring about further improvements in device performance. 
     The process concludes with the deposition of capping layer  18  on composite free layer  23 , thereby forming the read head. If the process described above was correctly used to form the read head it will be found to have a GMR ratio of at least 14%, a coercivity that less than about 4 Oe, and a magnetostriction constant that less than about 2×10 −6 . 
     Confirmatory Results 
     TABLE I below compares the properties of a conventional (reference) GMR structure with one whose free layer was made according to the teachings of this invention Except for FL 2 , the other parts of the GMR stack are kept the same. Also we intentionally matched the magnetic moments of the free layers in these two structures for a fair comparison. The number after each named layer is thickness in Angstroms: The basic structure, common to both A and B below, was: 
     Seed layer\Antiferromagnetic layer\CoFe\Ru\CoFe\Cu\CoFe(FL 1 )\FL 2 \Capping layer. In sample A, FL 2  is permalloy(Ni 80 Fe 20 ) while in sample B it is and Fe 88 Ni 32 : 
     
       
         
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Sample 
                 R 
                 dR/R 
                 dR 
                 Bs 
                 Hc 
                 He 
                 Lambda 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 A 
                 21.5 
                 14.6% 
                 3.13 
                 0.250 
                 11.7 
                 29.1 
                 2.0E−06 
               
               
                 B 
                 24.2 
                 14.6% 
                 3.54 
                 0.259 
                 3.1 
                 32.7 
                 3.0E−07 
               
               
                   
               
             
          
         
       
     
     It can be seen that, the advantages of the invention structure are:
     1) High dR   2) Low Hc   3) Low magnetostriction   

     Manufacture of the invented structure requires only a target of new material to replace the current NiFe target used for GMR stack sputtering and the annealing process can be kept the same. Therefore, there is no change of the current process flow and/or related processes.