Abstract:
By inserting a spin polarizing layer (typically pure iron) within the free layer of a MTJ or GMR memory cell, dR/R can be improved without significantly affecting other free layer properties such as Hc. Additional performance improvements can be achieved by also inserting a surfactant layer (typically oxygen) within the free layer.

Description:
This application is filed on Apr, 5, 2007 as application Ser. No. 11/784,076, and is herein incorporated, by reference, in its entirety. 
     FIELD OF THE INVENTION 
     The invention relates to the general field of magnetic memory cells including MTJ (magnetic tunnel junction) and GMR (giant magnetoresistance) devices, with particular reference to the structure of the free layer. 
     BACKGROUND OF THE INVENTION 
     A TMR (tunneling magneto-resistance) sensor (MTJ device) whose free layer is limited to FeCo (bcc) will have a large TMR ratio but other magnetic properties, such as Hc (coercive field), Hk (anisotropy field), and lambda (magnetostriction), will have values that fall well outside the usable range. In most current TMR manufacturing processes it is normal practice to deposit NiFe (fcc) as an additional component of the free layer in order to achieve a softer free layer. However, use of FeCo/NiFe as the free layer will also substantially reduce the TMR ratio (dR/R) compared to what may be achieved with a FeCo only free layer. 
     The present invention discloses a structure, and method for its manufacture, which makes it possible to achieve the high TMR ratio associated with a FeCo free layer without suffering an attendant degradation of other magnetic characteristics. 
     A typical MR memory cell of the prior art is illustrated in  FIG. 1 . Seen there are magnetic pinning layer  11  (normally an antiferromagnetic layer of a material such as IrMn or MnPt), magnetically pinned layer  12  (either a ferromagnetic layer or, more commonly, a synthetic antiferromagnetic trilayer), transition layer  13  (either copper for a GMR device or a thin insulating layer for a TMR device), CoFe layer  14 , NiFe layer  15  (which, together with layer  14 , makes up the free layer), and capping layer  16 . 
     A routine search of the prior art was performed with the following references of interest being found: 
     U.S. patent application 2005/0243477 (Gill) discloses a thin layer of Fe (on the order of several Angstroms) may be added to the free layer adjacent to the coupling layer to assure antiparallel coupling. U.S. Pat. No. 7,116,529 (Yoshikawa et al) is an example of many patents that disclose a free layer comprising FeCo/NiFe. U.S. Pat. No. 7,046,489 (Kamiguchi et al) shows a free layer of Fe/CoFe/Fe or Fe/NiFe/Fe. 
     SUMMARY OF THE INVENTION 
     It has been an object of at least one embodiment of the present invention to attain a high dR/R ratio in a TMR or GMR memory element without pushing other magnetic properties of said memory element outside their acceptable limits 
     Another object of at least one embodiment of the present invention has been to provide a structure that meets the preceding object along with a process for forming said structure. 
     Still another object of at least one embodiment of the present invention has been that the invention apply to TMR and also to CIP, CPP, and CCP type GMR devices. 
     A further object of at least one embodiment of the present invention has been that adoption of said process require that only minor changes be made to current processes for manufacturing said memory elements. 
     These objects have been achieved by inserting a spin polarizing layer (typically pure iron) within (as opposed to above or below) the free layer. Additional improvement can be obtained if, in addition to said spin polarizing layer, a surfactant layer (such as oxygen) is also inserted within the free layer. Data comparing the dR/R performance of prior art devices to devices made according to the teachings of the present invention is presented. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a typical memory element structure of the prior art. 
         FIG. 2  shows a memory element, similar to that of  FIG. 1 , modified according to an embodiment of the present invention wherein a spin polarizing layer has been inserted within the free layer. 
         FIG. 3  extends the example shown in  FIG. 2  by inserting within the free layer, in addition to a spin polarizing layer, a surfactant layer. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The key innovation introduced by the present invention is the insertion of a thin layer of pure iron between the FeCo and NiFe layers of a conventional free layer. When this is done, the TMR ratio can be improved 15˜20%, even with a very thin (2˜10 Å) Fe layer. This TMR ratio improvement is due mainly to two major factors: (1) Fe has high spin polarization and (2) the thin Fe layer reduces Ni diffusion from the NiFe into the FeCo, thereby enabling the higher TMR ratio to be maintained. In addition to the higher TMR ratio, a magnetically softer free layer (lower Hc, Hk, etc.) is also obtained with this free layer structure. 
     The modified structure is illustrated in  FIG. 2 . As can be seen, it is similar to the prior art structure seen in  FIG. 1  but with the key difference that, between layers  21  and  23  (which are equivalent to layers  14  and  15  in  FIG. 1 ), spin polarizing layer  22  has been inserted. Our preferred material for layer  22  has been pure iron, but any spin polarizing material, such as FeB, could be substituted for the invention to still show an improvement in dR/R. Other, similar structures are also possible, for example FeCo/Fe/FeCo/NiFe. 
     An important question that needed to be answered at the outset was what effect, if any, insertion of the spin polarizing layer would have on the key magnetic properties of the free layer—Hc (coercivity), Hk (anisotropy field), and lambda (λ—magnetostriction coefficient). Experimental results are summarized in TABLE I: 
     TABLE I compares free layer properties (other than the TMR ratio) with and without an inserted thin 4 Angstrom Fe layer. 
     Sample structure: Seed/AFM/outer pinned/Ru/inner pinned/MgOx/Free/Cap 
     
       
         
               
               
               
               
               
             
           
               
                 TABLE I 
               
               
                   
               
               
                 Sample 
                 Free 
                 Hc 
                 Hk 
                 Lambda 
               
               
                   
               
             
             
               
                   
                 FeCo/4Fe/NiFe 
                 3.03 
                 11.08 
                 2.0E−06 
               
               
                 Reference 
                 FeCo/NiFe 
                 4.81 
                 13.11 
                 1.6E−06 to 2.5E−06 
               
               
                   
               
             
          
         
       
     
     It can be seen in TABLE I that a magnetically softer free layer is obtained when a thin Fe layer is inserted into the FeCo/NiFe free layer. Additionally, magnetostriction exhibits almost no change relative to the prior art structure. 
     TABLE II presents experimental data for the R.A (resistance area product in ohms·μm 2 ) and the TMR ratio for a TMR device with a MgOx barrier and with Fe inserted into the FeCo/NiFe free layer. 
     Sample structure: Seed/AFM/outer pinned/Ru/inner pinned/MgOx/Free/Cap 
     
       
         
               
               
               
               
               
             
           
               
                   
                 TABLE II 
               
               
                   
                   
               
               
                   
                 Sample 
                 Free 
                 RA 
                 dR/R 
               
               
                   
                   
               
             
             
               
                   
                   
                 FeCo/4Fe/NiFe 
                 3.48 
                 58% 
               
               
                   
                 Reference 
                 FeCo/NiFe 
                 3.50 
                 48% 
               
               
                   
                   
               
             
          
         
       
     
     From Table 2 we can see higher TMR ratio was obtained by thin Fe insertion in FeCo/NiFe free layer. With Fe inserted in between FeCo/NiFe free layer, a higher TMR ratio with reasonable free layer properties can still be expected. 
       FIG. 3  illustrates how, in addition to iron (i.e. spin polarizing) layer  22  being inserted within the free layer, further performance improvement can be obtained by also inserting a surfactant layer within the free layer (which is now made up of layers  21 ,  23 , and  34 ). This is shown as layer  33  in the figure but it should be noted that, as long as the surfactant and iron layers are inserted somewhere within the free layer, their exact locations within the free layer are not critical. Layer  33  may comprise any of several known surfactant materials, such as oxygen, or oxygen mixed with argon, krypton, xenon, or neon, with oxygen being preferred.