Abstract:
Improved spatial resolution during TAMR has been achieved by shaping the write pole to have a lower surface that slopes away from the surface of the recording medium, starting at, or near, the pole&#39;s leading edge. The approach may be used for simple or compound (stitched) poles.

Description:
FIELD OF THE INVENTION 
     The invention relates to the general field of magnetic recording with particular attention to achieving high storage density through a local reduction of the recording medium&#39;s coercivity during recording, including improved alignment between the main pole and the plasmon generator. 
     BACKGROUND OF THE INVENTION 
     Perpendicular magnetic recording (PMR) heads, combined with a double-layered recording medium, have made it possible to extend the ongoing increase of the recording density in hard disk drives (HDD) beyond 100 Gb/in 2 . However as the track width shrinks, the write field decreases due to the small pole area and pole tip saturation if head-media spacing reduction cannot be suitably applied. This situation makes it difficult to achieve 1 Tb/in 2 . In addition, a small grain size for the recording media is required to achieve these higher recording densities. However, conventional solutions to this problem tend not to be compatible with thermal stability. Two anticipated obstacle to further areal density growth are lack of a proper head field and dealing with the media&#39;s super-paramagnetic limit. 
     Fortunately, new technology options are currently being explored that promise areal density growth beyond these limits. Thermally assisted magnetic recording (TAMR) is the most promising of these technologies. Bit patterned magnetic recording (BPMR) is too expensive and the manufacturing throughput is very slow. Microwave assisted magnetic recording (MAMR) is one of the candidates but its effect is too small to increase current recording densities. Additionally it is incompatible with high anisotropy media. 
       FIG. 1  shows a TAMR head configuration of the prior art. Laser diode  11  illuminates the inlet of optical waveguide  12 , sending light through it to couple with plasmon generator  13 . This enables light in plasmon mode to be emitted at the surface or edges of plasmon generator  13 . Finally, a near field spot appears at the tip of the plasmon generator&#39;s air bearing surface (ABS). This tiny near field spot induces a very localized temperature rise in the recording media. 
     Since the media coercivity field decreases with increasing temperature, TAMR enables magnetic recording to be achievable in a medium whose coercivity is too large for recording at room temperature. However, precise alignment of the thermal spot and the head field is critical for TAMR recording to be successful. 
       FIGS. 2   a - 2   c  are schematic illustrations of three conventional plasmon generator/main-pole configurations currently in use for TAMR. Plasmon generator  13  is located on the leading side of the main-pole  21 &#39;s leading edge at a distance of 10-60 nm therefrom. In this configuration, the center of the thermal spot is not directly under the main-pole. However the head field negative slope is inside of the main-pole. 
       FIG. 3  is a down-track profile of the head field in relation to main-pole  21 , optical spot  31 , and thermal spot  32 . The head field H eff  is defined in equation (1) below: 
     
       
         
           
             
               
                 
                   
                     H 
                     eff 
                   
                   = 
                   
                     
                       ( 
                       
                         
                           H 
                           in 
                           
                             2 
                             / 
                             3 
                           
                         
                         + 
                         
                           H 
                           y 
                           
                             2 
                             / 
                             3 
                           
                         
                       
                       ) 
                     
                     
                       3 
                       / 
                       2 
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     where H in  is the in-plane field and H y  is the perpendicular component of the head field at 17.5 nm from the ABS. 
     The thermal spot diameter should be less 100 nm, with 50 nm or less being preferred. Even when the spot center is located only 40 nm from the main-pole, the thermal spot&#39;s trailing edge will still be outside or, at best, barely at the main-pole edge. At this point, the recording transition takes place since the head field gradient now turns negative. As a result, even in TAMR recording, the transition quality is very poor and shows severe transition curvature in conventional TAMR heads. This is because the transition is basically defined by the media anisotropy gradient (which follows the temperature profile) rather than by the head field gradient. 
       FIG. 4  is a computed media-recording pattern for a conventional TAMR head at 2000 kFCI (Kilo Flux Changes per Inch) linear density. The transition shows severe curvature and the signal to nose ratio (SNR) is poor at only 5.52 dB. Because of this, the behavior of TAMR, as currently implemented, is dominated by thermal factors. 
     A routine search of the prior art was performed with the following references of interest being found: 
     Tapered poles of various shapes are described in the prior art. Some examples are: U.S. Pat. No. 7,532,433 (Kawato et al), U.S. Patent Application 2009/0207525 (Guan et al—Headway), U.S. Patent Application 2009/0116145 (Guan et al—Headway), U.S. Patent Application 2004/0233578 (Gao), and U.S. Pat. No. 7,038,881 (Ito et al). None of these show the particular features, that we will disclose below, that would render them suitable for plasmon-based TAMR. 
     SUMMARY OF THE INVENTION 
     It has been an object of at least one embodiment of the present invention to increase storage density on a magnetic hard drive. 
     Another object of at least one embodiment of the present invention has been to temporarily reduce the coercivity of the recording medium in a spot close to the main pole. 
     Still another object of at least one embodiment of the present invention has been to reduce the separation between this spot and the main pole. 
     A further object of at least one embodiment of the present invention has been that the point where the head field is a maximum be inside this spot. 
     These objects have been achieved by shaping the write pole to have a lower surface that slopes away from the surface of the recording medium, starting at, or near, the pole&#39;s leading edge. The approach may be used for simple or compound (stitched) poles. The net effect of using this shape is that the peak of the head field is now located within the hot spot generated by the plasmon generator (instead of just outside it). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1 . Conventional TAMR head configuration. 
         FIGS. 2   a - c . Conventional TAMR head configurations for optical plasmon generator and magnetic main-pole. 
         FIG. 3 . Down-track profile of head field for conventional TAMR head 
         FIG. 4 . 2000 kFCI TAMR recording pattern on TAMR media by a conventional TAMR head. 
         FIG. 5 . A commonly used embodiment of the Invention. 
         FIG. 6 . A more general version of the embodiment shown in  FIG. 5 . 
         FIG. 7 . Head field gradient as a function of the taper angle. 
         FIG. 8 . Head field peak position from the main-pole leading edge as a function of the taper angle. 
         FIG. 9 . Head field gradient as a function of the thickness t 1 . 
         FIG. 10 . Temperature and anisotropy field of the recording medium during plasmon irradiation. 
         FIG. 11 . Down-track profiles of conventional and invented TAMR heads. 
         FIG. 12 . 2000 kFCI TAMR recording pattern by the invented TAMR head. 
         FIGS. 13   a - b . Examples of stitched main-poles based on the invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The key features of the TAMR head of the present invention are embodied in the example shown in  FIG. 5 . In particular, starting at, or very close to, its leading edge the main-pole is tapered at angle  51  so that its distance from the ABS gradually increases as the trailing edge is approached. 
     In order to determine just how close to the leading edge tapering should begin, the structure illustrated in  FIG. 6  was investigated. The dimensions of interest in  FIG. 6  are distance t 1  from the main pole&#39;s leading edge at which tapering begins and the taper angle itself. 
       FIG. 7  displays the head field gradient in Oe/nm as a function of the of the taper angle, for t 1 =0 and t 1 =10 nm. The data indicates that the minimum head field gradient is obtained for t 1 =0 for a taper angle between about 55 and 65 degrees. 
       FIG. 8  displays the distance from the main pole&#39;s leading edge at which the head field peaks, as a function of the taper angle, for t 1 =0 and t 1 =10 nm. Ideally, this peak field will occur at the center of the thermal spot produced by the plasmon generator. The data indicates that, starting at an angle of about 20 degrees, the field&#39;s peak distance increases in correspondence with the taper angle but is about 2 nm greater for the t 1 =0 case at a taper angle of about 60 degrees. 
       FIG. 9  plots the head field gradient as a function of the thickness t 1 , for the taper angle of 45 degrees. When t 1  is 20 nm or below, the gradient does not change and provides very large magnitude of −175 Oe/nm but if t 1  increases above 30 nm, the head field gradient degrades. Even if t 1  is 100 nm or more, the field gradient is still better than in a conventional TAMR head of 50 Oe/nm. 
       FIG. 10  plots the temperature of the recording medium (curve  81 ) as well as its anisotropy field H k  (curve  82 ) during TAMR. A region where the media has zero anisotropy field (corresponding to a temperature of about 700 K or greater) can be seen to begin at about 20 nm ahead of the main pole. At this distance the head field of the invented main pole can be seen to be greater negative field gradient than that of a conventional TAMR head, this difference remaining essentially the same until the media&#39;s anisotropy field begins to increase again about 30 nm further up track. 
       FIG. 11  shows how, in the present invention, the peak of the head field lies within the optical (i.e. thermal) spot, which is not the case for the prior art design. This feature thus provides better alignment between the thermal spot and head field. Accordingly, the head field gradient is negative at the main-pole leading edge. In addition, the slope is much sharper than the conventional TAMR head. 
       FIG. 12  shows a computed media-recording pattern for the invented TAMR write head. The conditions are the same as for  FIG. 4  except for the main-pole structure. As can be seen, the transition areas are not as sharply curved as those of the prior art design, implying better transition quality. This is reflected in the signal-to-noise ratio (SNR), which is 11.93 dB, while the SNR of the conventional TAMR head is 6.41 dB. Note the improvement over the 5.52 dB FIGURE quoted earlier. This is because of the reduced noise associated with the sharper transition. 
     Finally, we note that the above description of the invention has been limited to one-piece write heads. It is, however, possible to apply the principles of the invention to stitched poles. In general, a stitched pole is more expensive to fabricate but it offers the following advantages over a non-stitched pole: 
     In TAMR heads, the main-pole dampens the transmission of plasmons at the plasmon generator and degrades the light propagation of the waveguide. So, to maximize optical efficiency, the main-pole should be located at a sufficient distance (100 nm or more) from the waveguide and the plasmon generator. However, the plasmon generator must be set as close as possible to the main-pole tip at the ABS in order to optimize thermal and magnetic field alignment during TAMR recording. A stitched-pole enables the write field and the thermal spot to be set close to one another while still retaining adequate separation between the main pole body and the waveguide and plasmon generator. 
     Two examples of stitched poles shaped according to the teachings of the present invention are shown in  FIGS. 13   a  and  13   b  (designated  91  and  92  respectively). In both examples (as was the case for one-piece poles), tapering away from the ABS (i.e. increasing the pole&#39;s distance from the ABS) begins where the pole is closest to the plasmon generator.