Abstract:
A perpendicular magnetic recording medium comprising a substrate, a soft underlayer, a seed layer, a non-magnetic FCC NiW alloy underlayer, a non-magnetic HCP underlayer, and a magnetic layer. We have discovered that the combination of a seed layer comprising Ta and a NiW alloy underlayer uniquely improves media recording performance and thermal stability by achieving excellent coercivity of the thin bottom magnetic recording layer and narrow C axis orientation distribution.

Description:
BACKGROUND OF THE INVENTION  
       [0001]    This invention pertains to perpendicular magnetic recording media and methods for making perpendicular magnetic recording media. 
         [0002]      FIG. 1  illustrates a prior art magnetic recording medium  10  used for perpendicular recording. Medium  10  comprises a substrate  11 , an adhesion layer  12 , a soft underlayer (“SUL”) structure  13 , a Ta seed layer  14 , a hexagonal close packed (“HCP”) RuCr 30  alloy layer  15 , a HCP Ru layer  17 , a bottom magnetic HCP CoCr 17 Pt 18 (SiO 2 ) 2  alloy layer  18 , a capping magnetic HCP CoCr 16 Pt 18 (TiO 2 ) 1.5  alloy layer  19 , and a carbon protective overcoat  20 . The &lt;0001&gt; axis (the C axis) of the HCP crystals of layers  18  and  19  preferentially orient vertically. Layers  14 ,  15  and  17  are provided to promote vertical orientation of the C axis and to enhance grain isolation in layers  18  and  19  when layers  18  and  19  are deposited which result in enhancing the coercivity Hc of magnetic layers  18  and  19 . 
         [0003]    Layers  18  and  19  store magnetically recorded data when the medium is in use. The Hc of layer  18  is greater than that of layer  19 . During reactive sputtering, amorphous oxide grain boundaries in layer  18  form to decouple the magnetic grains of layer  18  so that individual grains of layer  18  can magnetically switch independently, thereby reducing noise exhibited by layer  18 . The oxide content of layer  18  is controlled by both oxide content in a given target and degree of reactive sputtering. Unfortunately, formation of amorphous oxide grain boundaries can degrade the vertical orientation of the magnetization and cause broad switching field distribution in layer  18 , as discussed in H. S. Jung et al., “Effect of Oxygen Incorporation on Microstructure and Media Performance in CoCrPt—SiO 2  Perpendicular Recording Media”, IEEE Transactions on Magnetics, Vol. 43, No. 2, pp. 615-620, February 2007. Layer  19  (which has either no or reduced oxide content and more intergranular exchange interaction than layer  18 ) is used to tailor the magnetic characteristics of layer  18  and improve the vertical orientation of magnetization in the dual magnetic layers  18 ,  19 . 
         [0004]    SUL structure  13  consists of soft magnetic layers  13   a  and  13   c  separated by a thin Ru layer  13   b . Layers  13   a  and  13   c  are antiferromagnetically coupled to each other due to Ru layer  13   b . SUL structure  13  provides a magnetic return path from the write pole to the return pole of a read-write head (not shown). 
         [0005]    As mentioned above, layers  15  and  17  consist of RuCr 30  and Ru, respectively. In order to achieve narrow crystallographic C axis orientation distribution and excellent crystallinity, a thicker RuCr 30  underlayer  15  is needed. Unfortunately, Ru is expensive and in short supply. Accordingly, it would be desirable to reduce the number of Ru-containing layers in medium  10  while still achieving good vertical orientation of layers  18  and  19  and a high Hc. 
         [0006]    Other vertical magnetic recording media are discussed in U.S. Patent Application 2004/0247945, U.S. Pat. No. 7,067,206, U.S. Patent Application 2006/0093867, U.S. Pat. No. 6,902,835, U.S. Patent Application 2003/0170500, U.S. Patent Application 2004/0023074, and U.S. Patent Application 2006/0275629. 
       SUMMARY  
       [0007]    A magnetic recording medium comprises first, second and third underlayers and a magnetic recording layer. The magnetic recording layer is a HCP material typically comprising one or more magnetic Co alloy layers. The underlayers promote vertical orientation of the C axis of the magnetic layers and enhance grain isolation, resulting in an increase in the coercivity of the magnetic layers. The first underlayer is a seed layer that typically comprises amorphous Ta or a Ta alloy and is non-magnetic. 
         [0008]    The second underlayer is non-magnetic and typically comprises a NiW alloy and typically has a FCC crystal structure. In one embodiment, the second underlayer comprises NiW x , where x is between 6 and 15. The remainder of the alloy comprises Ni. In another embodiment, the remainder of the alloy contains other additives, but in other embodiments the remainder of the alloy is about 100% Ni. 
         [0009]    The third underlayer is typically a non-magnetic HCP material, and can comprise Ru (including a Ru-based alloy) or a Co-based alloy that can comprise one or more of Cr, Ta, W, Mo, Nb, Ti, Hf, Y, V, Sr, and Ni. We have discovered that by using these materials we can achieve good crystal growth (e.g. with vertical orientation of the C axis of the magnetic layer) and high magnetic coercivity while using less Ru than medium  10 . We have also discovered that we can achieve reduced transition noise and improved thermal stability. 
         [0010]    In one embodiment, the medium comprises two magnetic layers formed above the underlayers. 
         [0011]    In one embodiment, the medium comprises a substrate and a SUL formed underneath the underlayers. It is desirable to minimize the thickness of the layers between the SUL and the magnetic layers. Of importance, by using a seed layer comprising Ta and a second underlayer comprising a NiW alloy, we are able to achieve this objective. 
         [0012]    In one embodiment, the SUL comprises first and second soft magnetic layers separated by a thin Ru layer. The first and second soft magnetic layers are antiferromagnetically coupled to one another. However, in another embodiment, the SUL comprises only a single layer. 
         [0013]    As mentioned above, we can achieve a high Hc owing to the unique combination of underlayers comprising Ta and NiW, for the case of a single or a bottom magnetic layer, we can achieve a high Hc of about 7 kOe even when the bottom magnetic recording layer is thin, e.g. 7 nm, while simultaneously achieving excellent crystallographic C axis orientation. A benefit of the high Hc in the thin bottom magnetic recording layer is the reduction of transition noise and improved thermal stability in dual magnetic recording layers. We have been able to achieve a medium signal-to-noise ratio SNR me  improvement of 0.6 to 1.3 dB compared to conventional underlayer structures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0014]      FIG. 1  illustrates in cross section a magnetic recording medium constructed in accordance with the prior art. 
           [0015]      FIG. 2  illustrates in cross section a magnetic recording medium constructed in accordance with a first embodiment of the invention. 
           [0016]      FIG. 3  illustrates in cross section a magnetic recording medium constructed in accordance with a second embodiment of the invention. 
           [0017]      FIG. 4  illustrates the relationship between the thickness of various non-magnetic underlayers and the coercivity Hc of a bottom magnetic recording layer. 
           [0018]      FIGS. 5A and 5B  illustrate the relationship between the thickness of various non-magnetic underlayers and the crystal orientation of subsequently deposited Ru and Co alloy layers. 
           [0019]      FIG. 6  illustrates the relationship between the thickness of various non-magnetic underlayers and the coercivity Hc of dual magnetic recording layers. 
           [0020]      FIG. 7  illustrates the relationship between the thickness of various non-magnetic underlayers and the saturation field Hs of dual magnetic recording layers. 
           [0021]      FIG. 8  illustrates the relationship between the thickness of various non-magnetic underlayers and the nucleation field Hn of the dual magnetic recording layers. 
           [0022]      FIG. 9  illustrates the relationship between the thickness of various non-magnetic underlayers and the magnetic write width (“MWW”) of dual magnetic recording layers. 
           [0023]      FIG. 10  illustrates the relationship between the thickness of various non-magnetic underlayers and the medium signal-to-noise ratio SNR me  of dual magnetic recording layers. 
           [0024]      FIG. 11  illustrates the relationship between the thickness of various non-magnetic underlayers and the DC erase signal-to-noise ratio SNR DC  of dual magnetic recording layers. 
           [0025]      FIG. 12  illustrates the relationship between the thickness of various non-magnetic underlayers and the reverse overwrite performance OW 2  of dual magnetic recording layers. 
           [0026]      FIG. 13  illustrates the relationship between the thickness of a non-magnetic NiW 10  layer and the temperature coefficient of remanent coercivity dHcr/dT of dual magnetic recording layers. 
           [0027]      FIGS. 14A and 14B  illustrate the effect of a Ta seed layer and the thickness of a non-magnetic NiW 10  layer on the crystallographic C axis orientation of a subsequently deposited Ru and Co alloy layer. 
           [0028]      FIG. 15A  illustrates the relationship between the thickness of a NiW 10  alloy layer and the SNR me  of a magnetic recording medium in the presence and absence of a Ta seed layer. 
           [0029]      FIG. 15B  illustrates the relationship between the thickness of a NiTi 10  alloy layer and the SNR me  of a magnetic recording medium in the presence and absence of a Ta seed layer. 
           [0030]      FIG. 16  illustrates in cross section a magnetic disk drive including a magnetic disk in accordance with our invention. 
       
    
    
     DETAILED DESCRIPTION  
       [0031]    Referring to  FIG. 2 , a magnetic recording medium  100  comprises a substrate  102 , an adhesion layer  104 , a SUL  106 , a seed layer  108 , a non-magnetic layer  110 , a HCP non-magnetic layer  112 , a bottom magnetic recording layer  114 , a capping magnetic recording layer  116  and a protective carbon overcoat  118 . A thin lubricant layer such as perfluoropolyether (not shown) can be applied to the top surface of overcoat  118 . Although  FIG. 2  only shows the various layers on one side of substrate  102 , typically, these layers are formed on both sides of substrate  102 . 
         [0032]    Substrate  102  can be glass, glass ceramic, a NiP-plated aluminum alloy substrate (e.g. an AlMg substrate), or other appropriate material. Substrate  102  can be either textured or non-textured. 
         [0033]    Adhesion layer  104  can be Cr, CrTi, Ti, or other material. In one embodiment, layer  104  is 5 nm thick Ti, although other thicknesses can be used. Alternatively, adhesion layer  104  can be omitted. 
         [0034]    SUL  106  can comprise Co-based magnetically soft materials, e.g. Co alloyed with one or more of Ta, Zr, Nb, Ni, Fe and B. Alternatively, SUL  106  can comprise a Co-based magnetically soft material containing an oxide and one or more of Ta, Zr, Nb, Ni, Fe and B. In another embodiment, SUL  106  can comprise first and second soft magnetic layers  106   a ,  106   c  separated by a thin Ru intermediate layer  106   b  (see  FIG. 3 ). In one such embodiment, layer  106   a  is a 40 nm thick CoTa 5 Zr 5  alloy, layer  106   b  is Ru between 6 and 9 angstroms thick (e.g. 8 angstroms), and layer  106   c  is 40 nm thick CoTa 5 Zr 5 . In the embodiment of  FIG. 3 , layers  106   a  and  106   c  are antiferromagnetically coupled due to the presence of Ru layer  106   b.    
         [0035]    Seed layer  108  is 3 nm thick amorphous Ta. However, in other embodiments, layer  108  can have other thicknesses, e.g. between 2 and 15 nm. Also, in other embodiments, layer  108  is a Ta alloy, e.g. comprising 90% to about 100% Ta. 
         [0036]    Layer  110  is a non-magnetic FCC NiW alloy such as NiW 10 , and can be between 1 and 15 nm thick, and preferably between 2 and 6 nm thick. 
         [0037]    Layer  112  is 15 nm thick HCP Ru. However, in other embodiments, layer  112  can have other thicknesses. e.g. between 10 and 30 nm, and can be another HCP material such as an Ru based alloy, or a Co based alloy comprising one or more of Cr, Ta, W, Mo, Nb, Ti, Hf, Y, V, Sr or Ni. 
         [0038]    Layer  114  can be CoCr 17 Pt 18 (SiO 2 ) 2  and  116  can be CoCr 16 Pt 18 (TiO 2 ) 1.5 . Each of layers  114  and  116  is 7 nm thick, although in other embodiments, layers  114  and  116  have other compositions and thicknesses. Addition of oxide, SiO 2  in layer  114  and TiO 2  in layer  116 , reduces intergranular exchange coupling between magnetic grains. 
         [0039]    Carbon overcoat  118  can comprise a diamond-like hydrogenated carbon layer deposited by ion beam deposition covered by a flash layer of carbon. An example of an appropriate structure is discussed in U.S. Pat. No. 6,855,232, issued to Lairson et al., assigned to Komag, Inc. and incorporated herein by reference. Layer  118  can be 2.5 nm thick. However, other materials can be used in lieu of carbon, e.g. ZrO 2 . 
         [0040]    A magnetic disk in accordance with our invention can be manufactured by subsequently depositing layers  104 ,  106 ,  108 ,  110 ,  112 ,  114 ,  116  and  118  on substrate  102 , e.g. by a vacuum deposition process such as sputtering, evaporation or other technique. As mentioned above, layer  118  can comprise two carbon-based sublayers, the first sublayer deposited by ion beam deposition and the second sublayer deposited by sputtering. 
         [0041]    We have performed experiments that demonstrate the superiority of medium  100 .  FIG. 4  illustrates the relationship between the thickness of layer  110  (for the case in which layer  110  is nonmagnetic FCC NiW 10  and layer  108  is 3 nm thick amorphous Ta) and the Hc of bottom magnetic recording layer  114  (see curve  120 ) compared to media in which Pd, NiTi 10  and RuCr 30  were used in lieu of NiW 10  (see curves  121 ,  122  and  123 ). As can be seen, the disks comprising NiW 10  exhibited uniquely superior Hc, even when layer  108  was between 2.5 and 5 nm thick. The NiW 10  significantly increases Hc from 6 kOe for a thickness of 2.5 nm to about 7 kOe at a thickness of 5.0 nm even when the bottom recording layer  114  is only 7 nm thick. 
         [0042]    We have also demonstrated that the combination of a FCC nonmagnetic NiW alloy for layer  110  and amorphous Ta for layer  108  in accordance with our invention provides superior C axis crystal orientation in layers  112 ,  114  and  116 . In particular,  FIGS. 5A and 5B  illustrate the relationship between a figure of merit Δθ 50  and the thickness of layer  110 , as well as the corresponding relationships for Pd, NiTi 10  and RuCr 30  when layer  108  comprises Ta. Δθ 50  is a measure of variation in the orientation of the C axis as measured in degrees, determined by full width of the (0002) peak at half maximum in X-ray diffraction rocking curves. As can be seen, one can achieve a lower Δθ 50  of the (0002) planes for Ru and Co using NiW 10  (curves  124  and  128 ) than Pd (curves  125  and  129 ), NiTi 10  (curves  126  and  130 ) and RuCr 30  (curves  127  and  131 ). This means that advantageously, there is less variation in the alignment of the C axis in the Ru and Co magnetic layer when one uses a NiW 10  alloy in accordance with the present invention for layer  110 . 
         [0043]      FIG. 6  illustrates the relationship between the thickness of layer  110  and Hc of dual magnetic recording layers  114 ,  116  (see curve  134 ) for the case in which layer  110  is NiW 10  and the corresponding relationship in which Pd, NiTi 10  and RuCr 30  were used in lieu of NiW 10  (see curves  135 ,  136  and  137 ). A 2.5 nm thick NiW 10  layer provides Hc of about 5 kOe, comparable to a 10 nm thick RuCr 30  layer (compare curves  134  and  137 ). (Again, 3 nm thick amorphous Ta was used as layer  108  for the data of  FIG. 6  as well as  FIGS. 7-13 .) 
         [0044]      FIG. 7  illustrates the relationship between the thickness of layer  110  and the saturation field Hs of dual magnetic recording layers  114 ,  116  as well as the corresponding relationships for Pd, NiTi 10  and RuCr 30 . Once again, a 2.5 to 5 nm thick NiW 10  layer provides significantly increased Hs in the dual magnetic layers (curve  138 ) compared to Pd, NiTi 10  and RuCr 30  (curves  139 ,  140  and  141 ). Higher magnetic anisotropy constant Ku in bottom magnetic layer  114  providing higher Hc and Hs is important for reducing media transition noise but it limits media writeability. Values of Hs strongly affect media writeability. The role of top magnetic recording layer  116  helps minimize the side effects of well-isolated bottom magnetic recording layer  114  with high Ku by adjusting intergranular exchange interactions. The increase in Hc and Hs is caused by using NiW 10  but it provides more margins to control both composition and thickness in top magnetic recording layer  116  for further improvement of recording performance. 
         [0045]      FIG. 8  illustrates the relationship between the thickness of layer  110  and the nucleation field Hn of dual magnetic recording layers  114 ,  116  (curve  142 ) as well as the corresponding relationships for Pd, NiTi 10  and RuCr 30  (curves  143 ,  144  and  145 ). Hn relates to adjacent track erasure (“ATE”) and strongly depends on Hc and intergranular exchange interactions. Higher values of Hn provide superior ATE, but they limit SNR due to the increase in transition noise if the increase in Hn is mostly caused by enhancing intergranular magnetic interactions. The medium in use typically should have a Hn value greater than −2.0 kOe. In  FIG. 8 , the values of Hn greater than −2.0 kOe are maintained at a thickness of the NiW 10  greater than 2.5 nm, mostly due to the significant increase in Hc. 
         [0046]      FIG. 9  illustrates the relationship between the thickness of layer  110  and the relative magnetic write width (“MWW”) of dual magnetic recording layers  114 ,  116  (curve  150 ) as well as the corresponding relationships for Pd, NiTi 10  and RuCr 30  (curves  151 ,  152  and  153 ). (The relative MWW is obtained by comparing the write width of a magnetic medium, using a given read-write head and a given standard magnetic disk.) Narrower MWW is highly desirable for supporting higher linear recording density. Reduced MWW is obtained even at a thickness of 2.5-5 nm thick NiW 10  layer due to the contribution of the high Hc in the bottom magnetic recording layer  114 . 
         [0047]      FIG. 10  illustrates the relationship between the thickness of layer  110  and the medium signal-to-noise ratio SNR me  for the dual magnetic recording layers  114 ,  116  (curve  160 ) as well as the corresponding relationships for Pd, NiTi 10  and RuCr 30  (curves  161 ,  162  and  163 ). Superior SNR me  is achieved even at 2.5 to 5 nm thick NiW 10  due to the contribution of narrow MWW caused by high Hc in the bottom magnetic recording layer  114 . 
         [0048]      FIG. 11  illustrates the relationship between the thickness of layer  110  and the DC erase signal-to-noise ratio SNR DC  for dual magnetic recording layers  114 ,  116  (curve  165 ) as well as the corresponding relationships for Pd, NiTi 10  and RuCr 30  (curves  166 ,  167  and  168 ). SNR DC  is maintained at 2.5 nm thick NiW 10 . This is a good indication because the medium has relatively high Hc and Hs compared with the other media indicated in the figures. 
         [0049]      FIG. 12  illustrates the relationship between the thickness of layer  110  and the relative reverse overwrite for magnetic recording layers  114 ,  116  (curve  170 ) compared to Pd, NiTi 10  and RuCr 30  (curves  171 ,  172  and  173 ). Reverse overwrite (“OW 2 ”) is measured by a procedure where the short wavelength pattern ( 2 T) is overwritten by the long wavelength pattern ( 15 T), where T is the minimum transition spacing in the drive operation. For the case of the drive used to generate  FIG. 12 ,  1 T equals 966 kFCI (966 thousand flux reversals per inch). As can be seen, a 2.5 nm thick NiW 10  provides less OW 2  than Pd, NiTi 10  and RuCr 30  but the value is not worse when the high Hc and Hs are considered. 
         [0050]      FIG. 13  illustrates the effect of the thickness of layer  110  and the temperature coefficient of remanent coercivity dHcr/dT. As is known in the art, it is desirable to have a stable remanent coercivity Hcr that does not vary with respect to temperature. Values of dHcr/dT less than −15 Oe/° C. are highly desirable for current magnetic recording applications.  FIG. 13  shows that a thicker layer  110  significantly reduces temperature sensitivity of Hcr from −16 Oe/° C. at 0 nm to −14 Oe/° C. at 2.5 nm and −10 Oe/° C. at 15 nm. 
         [0051]      FIG. 14  illustrates the effect of the presence of Ta seed layer  108  and the crystal orientation of layers  112  ( FIG. 14A ) and layers  114 ,  116  ( FIG. 14B ). As can be seen, when Ta layer  108  is present (curves  180 ,  182 ), the Δθ 50  of the Ru and Co layers is lower, indicating more consistent vertical alignment, than when Ta layer  108  is absent (curves  181 ,  183 ). Use of Ta seed layer  108  achieves narrower C axis orientation of Ru and Co for further improvement of media performance. 
         [0052]    Ta seed layer  108  also improves the Δθ 50  of layer  110 . We have found that the Δθ 50  of NiW layer  110  is 2.3 when Ta seed layer  108  is present, and 3.0 when Ta seed layer  108  is absent. 
         [0053]      FIG. 15A  illustrates the relationship between the thickness of layer  110  and the SNR me  in the presence and absence (curves  190  and  191 , respectively) of Ta seed layer  106 . As can be seen, Ta improves the SNR me  of the medium.  FIG. 15B  illustrates the relationship between the SNR me  of a medium when NiTi 10  is used in lieu of NiW 10  both in the presence and absence (curves  192  and  193 , respectively) of seed layer  106 . 
         [0054]    A magnetic medium in accordance with the invention is typically incorporated into a magnetic disk drive such as disk drive  200  ( FIG. 16 ). Drive  200  comprises medium  100  rotated by a motor  202 . A pair of read-write heads  204   a ,  204   b  are coupled via arms  206   a ,  206   b  to an actuator  208  which in turn positions heads  204   a ,  204   b  over selected tracks of medium  100 . Heads  204   a ,  204   b  write data to and read data from medium  100 . Although  FIG. 16  shows only one medium in drive  200 , drive  200  can comprise more than one medium and more than one pair of read-write heads. 
         [0055]    While the invention has been described with respect to specific embodiments, those skilled in the art will recognize that modifications can be made in form and detail without departing from the spirit and scope of the invention. For example, seed layer  108  can be amorphous and consist essentially of Ta or an amorphous alloy of predominantly Ta, e.g. any additives in the alloy do not have a major impact on the properties of the alloy. In one embodiment, layer  108  is 90 to 100% Ta (although as used herein, a layer consisting of 100% Ta does not exclude those impurities typically found in layers formed by sputtering from commercially available Ta sputtering targets, e.g. targets of 99.9% purity or better). 
         [0056]    Layer  110  can be NiW x , where x is between 6 and 15, and preferably between 6 and 12. The remainder of layer  10  can be or consist essentially of Ni. 12% is the solid solubility limit for W in Ni. At concentrations exceeding 15%, W causes the NiW crystallinity to deteriorate and finally become amorphous, whereas it is desirable to use FCC material for layer  110 . In one embodiment, one provides a W concentration to increase the lattice spacing of the NiW to match the lattice spacing of the magnetic layers. In some embodiments, for a concentration below 6%, the effect of W on the lattice spacing of layer  110  may be insufficient. In one embodiment, layer  110  consists essentially of Ni and W, and in another embodiment, layer  110  consists of Ni and W (although as used herein, a layer consisting of materials, e.g. Ni and W, does not exclude impurities that are generally found in layers that are sputtered from commercially available sputtering targets, e.g. targets of about 99.9% purity or better). 
         [0057]    Alternatively, layer  110  can be NiCuW x , where x is between 1 and 15 or NiCoW x , where x is between 6 and 15. In the case of an alloy comprising Ni, Cu and W, the Cu content can be from 0 to an amount equal to the Ni content. (This is because such a composition will not adversely affect the FCC crystal structure of layer  110 .) For the case of an alloy comprising Ni, Co and W, the Co content can be from 0 to 30%. In other embodiments additives other than (or in addition to) Cu and/or Co may be present in the NiW alloy of layer  110 . In some embodiments, Ni is the predominant component in the alloy. Again, such embodiments are FCC non-magnetic alloys. 
         [0058]    Layer  112  can be Ru, a Ru-based alloy, or a Co-based alloy, e.g. comprising one or more of Cr, Ta, W, Mo, Nb, Ti, Hf, Y, V, Sr or Ni. A disk in accordance with the invention can include other layers (including other magnetic layers) in addition to the ones described herein. Also, layers having different thicknesses can be used. For example, in some embodiments, the total thickness of the magnetic recording layers can be 10 to 18 nm thick, e.g. between 14 and 16 nm thick. Accordingly, all such changes come within the present invention.