Abstract:
An MR sensor, and a method for making it, is described. Part of the MR stack, from the free layer on up, is removed and then replaced by a flux guide. Additional stabilizing means for this flux guide are provided, either as hard bias or through exchange coupling.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to the general field of sensing magnetically recorded data with particular reference to very high data densities. 
       BACKGROUND OF THE INVENTION 
       [0002]    With an ever-increasing data areal density in hard disk drives (HDD), the magneto-resistive (MR) sensor that is used as the read-back element in HDDs is required to have correspondingly better spatial resolution while at the same time achieving reasonable signal-to-noise ratio (SNR).  FIG. 1  shows the structure of a generic TMR (tunneling-magneto-resistive) head which is the main MR sensor structure used in state-of-the-art HDD. 
         [0003]    As seen in  FIG. 1A , a generic TMR head has top and bottom reader shields  1  and  2  respectively, spaced distance  3  apart, hard bias (HB) magnets  5  on the sides and MR sensor stack  6  located between the reader shields.  FIG. 1B  shows conventional MR sensor stack  6  that includes free layer (FL)  8 , tunneling barrier  9 , reference layer  10 , anti-parallel coupling layer  11  of Ru, pinned layer  12 , and anti-ferromagnetic layer  13  beneath the pinned layer  12  to provide the pinned field on  12  and  10 . 
         [0004]    Between top shield  1  and free layer  8  is non-magnetic capping layer  7 . The longitudinal magnetization of HB  5  provides a biasing magnetic field within sensor stack  6  to bias the magnetization  81  of free layer  8  in the cross-track direction. In today&#39;s hard disk drive, to further increase area data density, increased data linear density along both the down-track and cross-track directions is being developed. For higher track density, read heads with higher spatial resolution in the cross-track direction are required and smaller sensor sizes are needed. However, with smaller sensor size, magnetic noise gets worse as does sensor stability. 
         [0005]    To overcome these magnetic’ noise and reduced stability problems, a stronger HB field is needed, but this also has the effect of making the sensor less sensitive. Furthermore, due to the smaller bit size within the medium, the field from the medium becomes smaller and so higher sensitivity sensors are required. 
         [0006]    Thus, a trade-off exists between lower noise, better stability and higher signal. When solving this problem it is always beneficial to further increase the dR/R of the TMR film. This is, however, very hard to achieve in existing state-of-the-art TMR sensors. An Improved MR sensor design that can enhance the read-back signal without increasing noise and instability, are therefore needed. 
         [0007]    A routine search of the prior art was performed with the following references of interest being found: 
         [0008]    R. Olivier, and A. Satoru, “Magnetic tunnel junction read head using a hybrid, low-magnetization flux guide” see U.S. Pat. No. 6,519,124 B1 (2003). In U.S. Pat. No. 6,873,499, Lee et al. teach that a flux guide abuts the back edge of a read sensor. Dovek et al. in U.S. Pat. No. 6,239,955, show a flux guide on the back end of a MR sensor where the flux guide overlaps the lead and hard bias layers while Wu (in U.S. Pat. No. 7,170,721) discloses a flux guide on the side of a GMR element with permanent magnets surrounding the flux guide. 
       SUMMARY OF THE INVENTION 
       [0009]    It has been an object of at least one embodiment of the present invention to provide a method for sensing magnetic data stored at densities of 450 TPI and track widths less than 56 nm without increasing noise and instability. 
         [0010]    Another object of at least one embodiment of the present invention has been to provide a device that achieves the foregoing objectives. 
         [0011]    Still another object of at least one embodiment of the present invention has been to also achieve an increased magneto-resistance ratio. 
         [0012]    These objects have been achieved by a partial etching away of the free layer, the removed material being replaced by a magnetic flux guide structure that reduces the free layer&#39;s demagnetization field. This in turn reduces the stripe height of the sensor so that the resolution and the read-back signal are enhanced without increasing noise and instability. 
         [0013]    Stabilization of the flux guide is achieved by providing it with its own longitudinal field generated by an additional pair of hard bias magnets or, alternatively, by an exchange structure. 
         [0014]    The resulting device exhibits an on-track signal increase over existing MR sensor structures, enabling less-dependent optimization of sensor stability and sensitivity as well as better performance in densely recorded environments. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIGS. 1A-1B  illustrate prior art devices 
           [0016]      FIGS. 2A-2E  show various views of a first embodiment of the invention 
           [0017]      FIG. 3  compares dR/R for the invention and for a prior art device as a function of magnetic field applied normal to the sensor&#39;s ABS. 
           [0018]      FIG. 4A-4C  contrast the down-track waveforms of a conventional sensor with those generated by the invention. 
           [0019]      FIG. 5A-5C  compares the simulated mag-noise spectrum of a prior art sensor with the invented device for two distances between the free layer and the flux guide. 
           [0020]      FIG. 6  shows the relationship between signal-to-noise ratio and longitudinal bias for the invented device as well as for several prior art designs. 
           [0021]      FIGS. 7 and 8  illustrate two additional embodiments of the invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0022]      FIGS. 2A-2E  show various schematic views of the invention which introduces a novel back edge flux guide (FG) sensor design. The views provided in  FIGS. 2A-2E  are, respectively, ABS, top-down, cross-sectional (taken at sensor width center), MR stack close-up (similar to the prior art), and three-dimensional. 
         [0023]      FIG. 2A  shows that the FG sensor has a conventional MR stack structure  6  including a pair of conventional hard bias magnets (HB  5 ) for biasing free layer  8 , as seen in prior art  FIG. 1 . In an important departure from the prior art, a second pair of hard bias magnets (HB  4 ) is provided for biasing FG  14 , the latter being located along the back edge of the sensor stack as shown in  FIG. 2B . 
         [0024]      FIG. 2C  further details how the FG is located along the back edge of the sensor stack. In prior art designs free layer  8  extends almost all the way to the back edge which results in the large stripe height (SH)  22 . In the present invention, however, FL  8  has been subjected to controlled etching (which may also involve full or partial removal of the MR junction layer  9 ). Consequently, the initially large SH of the FL has been reduced to the much smaller SH of  21 , also as shown in  FIG. 2C . This is another important novel feature of the invention. 
         [0025]    After etching at the back end of the sensor stack, tunneling barrier  9  or reference layer  10  is exposed. A thin non-magnetic insulation layer such as alumina is then deposited on this exposed surface, followed by the deposition, and patterning, of the thin FG layer  14  on this thin non-magnetic insulation layer. The edge of FG layer  14  that faces the FL must be separated from the FL back-edge by a distance that does not exceed the thickness of the FL. 
         [0026]    The flux guide&#39;s thickness should be similar to the free layer thickness of from 2 to 10 nm with from 4 to 8 nm being preferred. Other properties of the flux guide include: 
         [0027]    a. Hk&lt;˜50 Oe and Hc&lt;˜5 Oe. 
         [0028]    b. Preferred material is Permalloy with Ni(81%)Fe(19%) or CoNiFe alloys with appropriate oftness as the permalloy. 
         [0029]    When FL magnetization  81  rotates in plane, it generates a magnetic field in FG layer  14  which causes magnetization  141  of the FG layer to rotate correspondingly. This magnetostatic interaction is the basic mechanism behind the magnetic flux guide effect since it enables the free layer to undergo a larger magnetization rotation when exposed to the same medium magnetic field it normally experiences. Additionally, as mentioned above, etching the FL also removes the top layer of HB  5  thereby leaving a cavity within which a large FG layer may be located. 
         [0030]    After a second isolation layer has been deposited on FG layer  14 , outer hard bias magnets HB  4  are formed to stabilize the FG layer magnetization. This is followed by the formation of top shield  1 . 
         [0031]    Once fabrication of the sensor is completed, a single HB initialization field is used to orient both the HB  4  and HB  5  magnetizations along the same direction. This will also orient the FL and FG layer magnetizations to be in the same direction once the initialization field has been removed. HB  4  serves mainly to stabilize the FG magnetization but it can also stabilize the HB  5  at the same time. Thus, the sensor may have a thick HB  4  and much thinner HB  5  which is an advantage in narrow read gap applications.  FIG. 2E  is a schematic three dimensional view of the completed FG sensor. 
         [0032]    Benefits of the Invention 
         [0033]      FIG. 3  shows a simulated transfer curve for comparison between conventional and FG sensor structures. The x-axis is a magnetic field applied normal to the sensor&#39;s ABS and y-axis is the sensor&#39;s output expressed as % dR/R. Curve  31  is the transfer curve for the conventional sensor shown in  FIG. 1  while curves  32  and  33  are transfer curves for the invented FG sensor, with the gap between the ABS and the free layer&#39;s back edge being 5 nm and close to 0 nm respectively. The corresponding amplitude gains by the FG structure are 50% and 90% respectively. 
         [0034]      FIGS. 4A-4D  shows a simulated on-track read-back signal comparison between a prior art sensor and the invented FG sensor. 
         [0035]      FIG. 4A  shows the 1T and 4T down-track waveforms from a conventional sensor ( FIG. 1 ). The percentage numbers above the figure are 1T signal, 4T signal peak-to-peak amplitudes and 1T/4T resolution, the latter quantity being a measure of the sensor response difference between the high and the low frequency regions. 
         [0036]      FIGS. 4B and 4C  show the same plots as in  FIG. 4A  but with the invented FG structure having FG-FL gaps of 5 nm and close to 0 nm respectively. The presence of the FG also enhances the read-back signal by 13% and 30% respectively; for this case the invented sensor has basically same structure as the sensor of  FIG. 4A , except for the addition of the FG. 
         [0037]      FIG. 4E  shows the off-track amplitude profiles for the prior art sensor and for the invented FG sensor with a FG-FL gap of 5 nm or 0 nm. The profiles have been normalized to the on-track signal amplitude. The width of the profiles is a measure of the cross-track resolution of the sensor. The full-width at half-maximum is ˜16 nm for all three conditions, indicating that the invented FG sensor has the same cross-track resolution as a conventional prior art sensor having no FG. 
         [0038]      FIG. 5A  shows the simulated mag-noise spectrum of a prior art sensor while  FIGS. 5B and 5C  show the same spectrum from the invented FG sensor with FG-FL gaps of 5 nm and 0 nm respectively. As can be seen, relative to the prior art sensor, the invented FG sensor&#39;s major mag-noise peaks have moved to lower frequency, indicating an effectively lower hard bias field. Also, the secondary lower amplitude peaks that appear at the lower frequency of 6-7 GHz derive from a FG magnetization resonance mode. However, the overall SNR, calculated from the ratio of 1T signal power as in  FIG. 4  divided by the integrated mag-noise power in the 0-2 GHz range still shows an increase over the conventional sensor case for a FG-FL gap of 0 nm. 
         [0039]    For a more realistic comparison,  FIG. 6  shows the simulated SNR for a prior art sensor and for several FG sensors having different structural and HB conditions. The x-axis shows the sensor signal amplitude increase over that of a conventional sensor with HB Ms=700 emu/cc, track width (TW)=30 nm and SH=30 nm. The y-axis is the SNR calculated by using 1T signal power as in  FIG. 4 , mag-noise power integrated from 0-2 GHz of spectra in  FIG. 5  and Johnson white electrical noise within the same frequency range. 
         [0040]    Discussion 
         [0041]    For a conventional sensor, an amplitude increase can be the result of a HB strength reduction, i.e. lower HB Ms as in various cases in  FIG. 6 , and also from larger SH used to enhance SH direction sensitivity. However, in  FIG. 6  curve  61  (conventional sensor) shows the SNR saturating at ˜33.5 dB due to a strong mag-noise increase at low HB and large SH which offsets the amplitude gain. For the invented FG sensor structure, amplitude is by reduced SH, reduced FG-FGL gap and lower HB Ms. Curve  62  in  FIG. 6  corresponding to SNR vs amplitude increase of FG sensors breaks through the dashed line of curve  61  for an effective SNR gain over a conventional sensor. 
         [0042]    Note that the prior art [1] also mentions a FG type of sensor structure that utilizes a large flux guide layer, which either also serves as the free layer or is exchange coupled to the free layer, while positioning the reference layer and pin layer structure at the back-end of this FG layer. The draw-back of this prior art design is the lower SNR when compared with the FG sensor design of the present invention. Flux leakage while traveling along the prior art FG is major source of signal loss. Additionally, for the narrower FG structure of the prior art, the weak stabilization of the ABS end FG magnetization by HB will lead to large mag-noise from the FG structure as well. 
       In Summary 
       [0043]    The advantages of the disclosed FG MR sensor are:
       1. An on-track signal increase over existing MR sensor structures   2. Enabling less-dependent optimization of sensor stability and sensitivity   3. Better performance for denser MR sensor.       
 
       EMBODIMENTS 
     Embodiment 1 
       [0047]    The structure shown in  FIG. 2 . 
       Embodiment 2 
       [0048]    The same as Embodiment 1 except that HB  4  as in  FIG. 2  is in physical contact with HB  5  and FG layer  14 . In this way, HB  4  stabilizes FG layer and HB  5  through direct exchange coupling. 
       Embodiment 3 
       [0049]    The same as Embodiment 1, except that HB  4  is no existent and FG  14  edge magnetizations are stabilized by synthetic-anti-ferromagnetic (SAF) structures. Layer 21 is Ru layer and layer  22  is another magnetic layer with opposite magnetization to FG  14  and forms SAF structure with FG  14  edge magnetization 
       Embodiment 4 
       [0050]    The same as Embodiment 3, except that another anti-ferromagnetic layer (AFM)  23  exists on top of layer  22 . AFM layer  23  stabilizes SAF structure composed of layer  14 ,  21  and  22  through exchange coupling at the two edges of FG  14 .