Abstract:
By using a composite free layer of Fe25% Co/NiFe, an improved CPP GMR device has been created. The resulting structure yields a higher CPP GMR ratio than prior art devices, while maintaining free layer softness and acceptable magnetostriction. A process for manufacturing the device is also described.

Description:
FIELD OF THE INVENTION  
       [0001]     The invention relates to the general field of CPP GMR read heads with particular reference to the free layer sub-structure.  
       BACKGROUND OF THE INVENTION  
       [0002]     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.  
         [0003]     The key elements of a spin valve are illustrated in  FIG. 1 . They are seed layer  11  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 ).  
         [0004]     Next is a copper 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.  
         [0005]     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%.  
         [0006]     Earlier GMR devices were designed so as to measure the resistance of the free layer for current flowing parallel to its two surfaces. However, as the quest for ever greater densities has progressed, devices that measure current flowing perpendicular to the plane (CPP) have also emerged. CPP GMR heads are considered to be promising candidates for the over 100 Gb/in 2  recording density domain (see references 1-3 below).  
         [0007]     A routine search of the prior art was performed with the following references of interest being found:  
         [0008]     No references were found that disclosed a specific percentage of Fe in the free layer. In U.S. Pat. No. 6,680,831, Hiramoto et al. disclose a simplified SV structure with only a pinned layer and a free layer separated by an intermediate, non-magnetic, layer. The pinned layer could be FeCo containing at least 50% Fe or Co. In U.S. Pat. No. 6,529,353, Shimazawa et al. and in U.S. Pat. No. 6,519,124, Redon et al. teach that the free layer may be a laminate of FeCo and NiFe. U.S. Patent Application 2002/0048127, Fukuzawa et al. teach a CoFeNi free layer for a higher rate of change in MR than CoFe/NiFe. Both Redon and Shimazawa disclose a laminated CoFe/NiFe free layer. Unless otherwise specified, CoFe usually means Co90Fe10; CoFe/NiFe composited free layers of this type are well known for spin valve applications.  
         [0009]     An improved free layer in a CPP spin valve needs to achieve three objectives: 
    1) higher CPP GMR ratio;     2) low coercivity i.e., good magnetic softness; and     3) low positive magnetostriction. 
 
 None of the prior art inventions listed above achieve all three of these, particularly the low positive magnetostriction 
   
 
       REFERENCES  
       [0000]    
       
          [1] M. Lederman et al U.S. Pat. No. 5,627,704.  
          [2] J. W. Dykes et al U.S. Pat. No. 5,668,688  
          [3] Min Li et al patent application Ser. No. ______ dated ______ “Spin valve structure with enhanced CPP GMR, and process for making it” (HT03-043) NOTE: THIS DISCLOSURE IS AWAITING RESOLUTION OF A POSSIBLE PRIOR ART PROBLEM  
       
     
       SUMMARY OF THE INVENTION  
       [0016]     It has been an object of at least one embodiment of the present invention to provide a CPP GMR magnetic read head having improved stability and performance.  
         [0017]     Another object of at least one embodiment of the present invention has been to provide a process for manufacturing said read head.  
         [0018]     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 CPP GMR devices.  
         [0019]     These objects have been achieved by replacing the conventional free layer with a Fe25% Co/NiFe composite free layer for CPP GMR enhancement. The resulting CPP spin valve structure yields higher CPP GMR ratios, while maintaining both free layer softness and an acceptable magnetostriction constant. It is important to control the layer thicknesses so the FeCo layer is between about 5 and 15 Angstroms thick and the NiFe layer is between about 15 and 50 Angstroms thick. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]      FIG. 1  shows a GMR stack of the prior art in which has a conventional free layer.  
         [0021]      FIG. 2  shows a GMR stack according to the teachings of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0022]     It is well known that besides the requirement of a reasonable RA (resistance-area product) and higher CPP GMR, the free layer of the CPP GMR structure has to be magnetically soft and its magnetostriction constant needs to be within the desirable range (positive 1-3×10 −6 ). The present invention describes a new free layer design for a spin valve having enhanced CPP GMR.  
         [0023]     While it is known that Fe rich CoFe can be used in CPP GMR spin valve structures for CPP GMR ratio improvement, this is offset by the fact that Fe rich CoFe also has too large an Hc (coercivity) value, as well as undesirable magnetostriction, to be useful as a free layer. To overcome this difficulty we have made use of the fact that the magnetic properties of a composite free layer made of CoFe and NiFe can be adjusted through control of the thickness ratio between the NiFe and the CoFe.  
         [0024]     In conventional (standard) CPP spin valve structures, composite free layers made of CoFe(10%) and NiFe(19%) have been used. Single ferromagnetic films made of CoFe(10%) and NiFe(19%) 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 laminating Fe(min. 25%)Co with NiFe(17%) to provide a replacement for CoFe(10%)/NiFe thereby improving the CPP GMR while still maintaining free layer softness and acceptable magnetostriction.  
         [0025]     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.  
         [0026]     The process begins with the formation of lower lead  10  onto which is deposited seed layer  11  followed by pinning layer  12 . Layer  12  comprises a suitable antiferromagnetic material such as IrMn and it is deposited to a thickness between 45 and 80 Angstroms. Layer  13  (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, copper spacer layer  16  is deposited on AP 1  layer  15 .  
         [0027]     Note that although layer  16  is referred to simply as a “copper spacer” layer, in practice it is a multilayer structure that includes Cu/AICU/PIT/IAO/Cu, AICU is a discontinuous layer of alumina having Cu in the holes, PIT is an abbreviation for pre-ion treatment and IAO stands for ion assisted oxidation. For the sake of simplification, we will continue to refer to ‘copper spacers’ but it should be borne in mind that they are actually the more complicated structures described above.  
         [0028]     Now follows a key feature of the invention which is the formation of the free layer as a bilayer of cobalt iron, containing at least 25 atomic percent iron, between about 5 and 15 Angstroms thick, and a layer of nickel iron (containing, typically, between about 15 and 20 atomic % iron), between about 15 and 50 Angstroms thick. These are shown as layers  21  and  22  in  FIG. 2 . The order in which these two layers that make up the free layer are deposited is a matter of designer&#39;s choice but, in practice, for bottom spin valves we prefer to deposit the FeCo first, while for top spin valves we prefer to deposit NiFe first.  
         [0029]     The resulting free layer has a magnetostriction constant that is between 1 and 3×10 −6  (positive) and a coercivity between about 5 and 10 Oe. Similar results are obtained with even greater iron concentrations, such as 50 and 75%, in the CoFe layer.  
         [0030]     The process concludes with the deposition of upper lead layer  18 , the completed structure being now ready to serve as a CPP GMR read head having a GMR ratio of at least 5.9%.  
         [0000]     Confirmatory Results  
         [0031]     To confirm the effectiveness of the invention, the following structures were formed and then evaluated as CPP GMR readers. The number after each named layer is thickness in Angstroms: 
    A. (prior art) Ta5/NiCr45/lrMn70/Fe(25%)Co36/Ru7.54Fe(25%)Co12/Cu3]2/Fe(25%)Co12/Cu2.6/AICu8.0/Cu2.0/Co(90%)Fe12/NiFe35/Cu30/Ru200.     B. Ta5/NiCr45/IrMn70/Fe(25%)Co36/Ru7.5/[Fe25Col 2/Cu3]2/Fe(25%)Col2/Cu2.6/AICu8.0/Cu2.0/Fe(25%)Col 0/NiFe35/Cu30/Ru200    
 
         [0034]     The results are summarized in TABLE I below:  
                                                                         TABLE I                                           DR/R   Hc   Hn   Magneto-           free layer structure   RA (ohm.μm 2 )   (%)   (Oe)   (Oe)   striction                                    A   Co(90%)Fe12/NiFe25   0.5   5.48   7.7   1.3   1.20 × 10 −6         B   Fe(25%)Ci10/NiFe35   0.5   5.9   7.5   1.5   2.30 × 10 −6                    
 
         [0035]     It can be seen that structure B with the Fe 25% Co10/NiFe35 free layer showed higher CPP GMR ratio than reference structure A. The free layer coercivity (Hc) and interlayer coupling (Hin) are similar between structure A and B and the magnetostriction of structure B is higher than that of reference structure A but is still within the desirable range.