Abstract:
A tunneling magnetoresistive stack configured to operate in a current-perpendicular-to-plane mode has a plurality of layers including a barrier layer. The TMR stack has a plurality of layers including a barrier layer, wherein the barrier layer is comprised of an insulating material selected from a group consisting of HfO, HfAlO, ZrO, TiO, TaO or NdO. The TMR stack exhibits a low resistance-area (RA) product, a stable magnetoresistive (MR) ratio, a lower RA product, a higher breakdown voltage of the TMR stack and enhanced thermal stability.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application claims priority from and incorporates by reference Provisional Application No. 60/325,901, filed Sept. 28, 2001 entitled “HfO As Tunneling Barrier Material For TGMR Recording Head Applications” by Z. Gao, S. Mao, K. Tran, J. Nowak and J. Chen. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to a magnetoresistive sensor for use in a magnetic read head. In particular, the present invention relates to a tunneling magnetoresistive (TMR) read sensor having a low resistance-area (RA) product, yet maintaining a stable magnetoresistive (MR) ratio. 
     Magnetoresistive read sensors, such as giant magnetoresistive (GMR) read sensors, are used in magnetic data storage systems to detect magnetically-encoded information stored on a magnetic data storage medium such as a magnetic disc. A time-dependent magnetic field from a magnetic medium directly modulates the resistivity of the GMR read sensor. A change in resistance of the GMR read sensor can be detected by passing a sense current through the GMR read sensor and measuring the voltage across the GMR read sensor. The resulting signal can be used to recover the encoded information from the magnetic medium. 
     A typical GMR read sensor configuration is the GMR spin valve, in which the GMR read sensor is a multi-layered structure formed of a nonmagnetic spacer layer positioned between a synthetic antiferromagnet (SAF) and a ferromagnetic free layer. The magnetization of the SAF is fixed, typically normal to an air bearing surface of the GMR read sensor, while the magnetization of the free layer rotates freely in response to an external magnetic field. The SAF includes a reference layer and a pinned layer which are magnetically coupled by a coupling layer such that the magnetization direction of the reference layer is opposite to the magnetization of the pinned layer. The resistance of the GMR read sensor varies as a function of an angle formed between the magnetization direction of the free layer and the magnetization direction of the reference layer of the SAF. This multi-layered spin valve configuration allows for a more pronounced magnetoresistive effect, i.e. greater sensitivity and higher total change in resistance, than is possible with anisotropic magnetoresistive (AMR) read sensors, which generally consist of a single ferromagnetic layer. 
     A TMR read sensor is similar in structure to a GMR spin valve, but the physics of the device are different. For a TMR read sensor, rather than using a metallic spacer layer, an insulating barrier layer is positioned between the free layer and the SAF. Aluminum Oxide (AlO) is commonly used as a material for the barrier layer. Electrons must tunnel through the barrier layer. A sense current flowing perpendicularly to the plane of the layers of the TMR read sensor experiences a change in resistance which is proportional to the cosine of an angle formed between the magnetization direction of the free layer and the magnetization direction of the reference layer of the SAF. 
     A pinning layer is typically exchange coupled to the pinned layer of the SAF to fix the magnetization of the pinned layer in a predetermined direction. The pinning layer is typically formed of an antiferromagnetic material. In antiferromagnetic materials, the magnetic moments of adjacent atomic planes are aligned in alternating directions and, thus, there is no net magnetic moment in the material. 
     An underlayer is typically used to promote the texture of the pinning layer consequently grown on top of it. The underlayer is typically formed of a ferromagnetic material and is chosen such that its atomic structure, or arrangement, corresponds with a desired crystallographic direction. 
     A seed layer is typically used to enhance the grain growth of the underlayer consequently grown on top of it. In particular, the seed layer provides a desired grain structure and size. A seed layer may also be combined with the underlayer, in which combined layer performs the functions of both the underlayer and the seed layer. 
     One principal concern in the performance of TMR read sensors is the ΔR (the maximum absolute change in resistance of the TMR read sensor), which directly affects the magnetoresistive (MR) ratio. The MR ratio (the maximum absolute change in resistance of the TMR read sensor divided by the resistance of the TMR read sensor multiplied by 100%) determines the magnetoresistive effect of the TMR read sensor. Ultimately, a higher MR ratio yields a TMR read sensor with a greater magnetoresistive effect which is capable of detecting information from a magnetic medium with a higher linear density of data. 
     A key determinant of the MR ratio is the spin polarization of the sense current passing through the barrier layer. The sense current consists of majority spin electrons (spin is in the same direction of the magnetization) and minority spin electrons (spin is in the opposite direction of the magnetization). A spin polarized current has an unequal population of majority and minority spin electrons. According to the Julliere model of the TMR read sensor, the magnetoresistive effect in a tunneling junction is significantly enhanced if the sense current is spin polarized. This is because the magnetoresistive effect is determined by ΔR/R=2PP′/(1−PP′), where ΔR/R is the MR ratio, and P and P′ are the spin polarization ratios of the effective tunneling density of states on each side of the barrier layer. The MR ratio reaches a maximum value for completely polarized tunneling density of states (P=P′=1). 
     Large signal output and reduced shield-to-shield spacing have made the TMR head attractive for high linear density application. However, one of the crucial factors strongly limiting the TMR sensor in recording head application is the large resistance at higher linear density, which results in large shot noise, thereby reducing the signal to noise ratio. Large resistance also increases the circuit response time since there is an equivalent capacitor in the reader circuit. Tremendous effort has been made to reduce the RA product of the TMR sensor in the past years. The lowest reported RA product of a given TMR sensor is 5 Ωμm 2 . Therefore, there is a need in the art for a TMR sensor with a low RA product. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is a tunneling magnetoresistive (TMR) stack configured to operate in a current-perpendicular-to-plane (CPP) mode. The TMR stack has a plurality of layers including a barrier layer. The TMR stack exhibits a lower resistance-area (RA) product, a stable magnetoresistive (MR) ratio, a higher breakdown voltage of the TMR stack and enhanced thermal stability. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a layer diagram of a tunneling magnetoresistive stack of the present invention. 
     FIG. 2 is a graph showing the RA product of the present invention in relation to the voltage bias of the present invention implementing the barrier material HfO. 
     FIG. 3A is graph showing the TMR ratio of the present invention in relation to the magnetic field from the head of the present invention implementing the barrier material HfO. 
     FIG. 3B is graph showing the TMR ratio of the present invention in relation to the magnetic field from the head of the present invention implementing the barrier material ZrO. 
     FIG. 3C is graph showing the MR of the present invention in relation to the magnetic field from the head of the present invention implementing the barrier material HfAlO. 
     FIG. 3D is graph showing the TMR ratio of the present invention in relation to the magnetic field from the head of the present invention implementing the barrier material TiO. 
     FIG. 3E is graph showing the TMR ratio of the present invention in relation to the magnetic field from the head of the present invention implementing the barrier material NdO. 
     FIG. 3F is graph showing the TMR ratio of the present invention in relation to the magnetic field from the head of the present invention implementing the barrier material TaO. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 is a layer diagram of a tunneling magnetoresistive (TMR) stack  10  according to the present invention. TMR stack  10  includes a seed layer  12 , a pinning layer  16 , a synthetic antiferromagnet (SAF)  18 , a barrier layer  26 , a free layer  28 , a cap layer  30 , and top/bottom leads  32 . The top and bottom leads  32  are preferably composed of Cu or other low resistive materials. 
     Seed layer  12  is preferably composed of Ta, Ta/NiFe, or NiFeCr/NiFe, and is positioned adjacent to bottom electrode  32 . Pinning layer  16  is an antiferromagnetic material, preferably PtMn, and is positioned adjacent to seed layer  12 . SAF  18  includes a ferromagnetic pinned layer  20 , a ferromagnetic reference layer  24 , and a coupling layer  22  positioned between the pinned layer  20  and reference layer  24 . SAF  18  is positioned such that pinned layer  20  is adjacent to pinning layer  16 . Pinned layer  20  and reference layer  24  are preferably CoFe, and coupling layer  22  is preferably Ru. Free layer  28  is a ferromagnetic material, preferably CoFe, NiFe, or CoFe/NiFe. Barrier layer  26  is an insulating material, preferably selected from the group consisting of HfO, HfAlO, TaO, NdO, TiO and ZrO, and is positioned between SAF  18  and free layer  28 . The thickness of barrier layer  26  is preferably 3.5 to 8 Å. The barrier layer metals are oxidized by a natural oxidation process, under 30 Torr for 20 minutes. Those skilled in the art would recognized that other oxidation processes, such as plasma and UV, may be used without departing from the present invention. Cap layer  30  is preferably Ta or Cu, and is positioned between the free layer  28  and the top electrode  32 . 
     The major advantage of using alternative junction barriers, such as the type disclosed in this application, is that a magnetic tunnel junction with a lower RA product and maintained large MR ratio can be achieved. With a traditional tunnel junction barrier material, such as AL 2 O 3 , the MR ratio decreases rapidly with small barrier thickness, and hence a small RA product. At a low RA product, tunnel junctions with barriers made of AL 2 O 3  typically have a low MR ratio, whereas tunnel junctions with barriers made of the materials disclosed herein result in a stable or high MR ratio. Therefore, the barrier materials disclosed herein allow reduction of the sensor resistance for high areal-density applications without sacrificing the output signal, which is proportional to the MR ratio. Table 1 illustrates the significance of using HfO, HfAlO, ZrO, or TiO as a barrier layer. Table 1 shows the RA product realized from the respective barriers with different thicknesses. One skilled in the art will appreciate that materials listed in Table 1, which have larger RA products, may have their RA products lowered by optimizing the oxidation level and thickness of the barrier. For instance, that implementation of NdO at approximately 8 Å produces an RA product of 100-150 Ωμm 2  and TaO produces an RA product of  8-12 Ωμm    2  These values may be lowered if the barrier layer implementing NdO or TaO is optimized by oxidation level and barrier thickness. In Table 1, HfO has been optimized, yet TiO, HfAlO, and ZrO have not been optimized. 
     
       
         
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Barrier Material 
                 HfO 
                 TiO 
                 HfAlO 
                 ZrO 
               
               
                   
                   
               
             
             
               
                   
                 Metal thickness (A) 
                 4.5 
                 8 
                 5.5 
                 5 
               
               
                   
                 RA (Ohm.um2) 
                 0.3˜1.2 
                 1˜2 
                 2˜3 
                 2˜5 
               
               
                   
                   
               
             
          
         
       
     
     The magnetization of SAF  18  is fixed while the magnetization of free layer  28  rotates freely in response to an external magnetic field emanating from an external field. Reference layer  24  and pinned layer  20  are magnetically coupled by coupling layer  22  such that the magnetization direction of reference layer  24  is opposite to the magnetization direction of pinned layer  20 . The magnetization of pinned layer  20  is pinned by exchange coupling the field between pinning layer  16  and pinned layer  20 . Seed layer  12  promotes the crystallographic texture of pinning layer  16 . The resistance of TMR stack  10  varies as a function of an angle that is formed between the magnetization of free layer  22  and the magnetization of reference layer  24 . 
     The TMR signal produced by TMR stack  10  is generated by a sense current flowing perpendicularly through the layers of TMR stack  10  from cap layer  30  to seed layer  12  and from seed layer  12  to cap layer  30 . By passing a bias current through two metallic leads that are placed in direct contact with the top and bottom leads  32 , conducting electrons are forced to tunnel through the insulating barrier layer  26 . The output signal that results from the change of the tunneling conductance is maximum when the alignment of the of the reference layer  24  and free layer  28  are anti-parallel and minimum when the alignment is parallel. TMR stack  10  optimally exhibits an RA product of 1 Ωμm 2 , a MR ratio of 8%, a breakdown voltage of over 300 mV, and survival of a thermal anneal of 290° C. for 2 hours, all of which is more fully shown and described in FIGS. 2-4. 
     FIG. 2 is a graph showing the RA product of the present invention in relation to the voltage bias of the present invention, which implements the barrier layer material HfO. The graph in FIG. 2 shows the results of testing the breakdown voltage for 64 TMR stacks in accordance with the present invention. Each line on the graph represents a TMR stack with a different thickness of barrier layer in accordance with the present invention. An important issue when operating TMR stack  10  is to maximize voltage breakdown. The graph of FIG. 2 illustrates that TMR stack  10  can withstand a post anneal temperature of up to 290° C. for 2 hours, can withstand a voltage of over 300 mV, and still maintain a RA product of 1 Ωμm 2 . Normally, anneal will make a RA product increase, but the overlapping of values in the graph of FIG. 2 exemplify that the RA product is very stable after the various thermal anneals. The thin barrier layer  26 , as disclosed, is thermally stable, which is vital for head operation. 
     FIGS. 3A-3F are graphs showing a MR value of the present invention in relation to the magnetic field from the head of the present invention. FIGS. 3A-3F represent barrier materials HfO, ZrO, HfAlO, TiO, NdO, and TaO, respectively. The apparatus in FIG. 3A was post annealed for 2 hours at 270° C. The apparatus in FIGS. 3B-3F were AS Finished for 4 hours at 250° C. The graphs of FIGS. 3A-3F illustrate that TMR stack  10  will operate efficiently, showing a stable MR ratio, and a high pinning field. 
     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. For instance, workers skilled in the art may change the thickness of the barrier layers disclosed herein to create lower RA values, without departing from the present invention.