Abstract:
An improved structure for the construction of perpendicular recording media is disclosed. The structure includes a perpendicular recording layer with at least two oxide sublayers or a lower sublayer of a non-oxide. One structure includes an upper sublayer comprised of a Silicon-oxide, while a lower sublayer is comprised of a Tantalum-oxide. The structures provide for increased coercivity and corrosion resistance.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates generally to perpendicular magnetic recording media, and more particularly to a disk with a perpendicular magnetic recording layer for use in magnetic recording hard disk drives. 
       BACKGROUND OF THE INVENTION 
       [0002]    Magnetic media are widely used in various applications, particularly in the computer and data storage industries, in devices such as hard disk drives and other recording devices. Efforts are continually being made with the aim of increasing the areal recording density, i.e., bit density of the magnetic media. In order to produce storage densities in excess of 200 Gb/in 2 , new recording media structures will be required. In this regard, perpendicular recording media structures (PMR) have been found to be superior to the more conventional longitudinal media in achieving very high bit densities. In perpendicular magnetic recording media, remanent magnetization is formed in a direction perpendicular to the surface of the magnetic medium. 
         [0003]    Perpendicular magnetic recording, wherein the recorded bits are stored in a perpendicular or out-of-plane orientation in the recording layer, is a promising path toward ultra-high recording densities in magnetic recording hard disk drives. A common type of perpendicular magnetic recording system is one that uses a “dual-layer” media. This type of system is shown in  FIG. 1  with a single write pole type of recording head. The dual-layer media includes a perpendicular magnetic data recording layer (RL) formed on a “soft” or relatively low-coercivity magnetically permeable underlayer (SUL). The SUL serves as a flux return path for the field from the write pole to the return pole of the recording head. In  FIG. 1 , the RL is illustrated with perpendicularly recorded or magnetized regions, with adjacent regions having opposite magnetization directions, as represented by the arrows. The magnetic transitions between adjacent oppositely-directed magnetized regions are detectable by the read element or head as the recorded bits. 
         [0004]      FIG. 2  is a schematic of a cross-section of a prior art perpendicular magnetic recording disk showing the write field H w  acting on the recording layer RL. The disk also includes the hard disk substrate, a seed or onset layer (OL) for growth of the SUL, an intermediate layer (IL) between the SUL and the RL, and a protective overcoat (OC). The IL is a nonmagnetic layer or multilayer structure, also called an “exchange break layer” or EBL, that breaks the magnetic exchange coupling between the magnetically permeable films of the SUL and the RL and facilitates epitaxial growth of the RL. While not shown in  FIG. 2 , a seed layer is typically deposited directly on the SUL to facilitate the growth of the IL. As shown in  FIG. 2 , the RL is located inside the gap of the “apparent” recording head (ARH), which allows for significantly higher write fields compared to longitudinal or in-plane recording. The ARH comprises the write pole ( FIG. 1 ) which is the real write head (RWH) above the disk, and an effective secondary write pole (SWP) beneath the RL. The SWP is facilitated by the SUL, which is decoupled from the RL by the IL and by virtue of its high permeability produces a magnetic mirror image of the RWH during the write process. This effectively brings the RL into the gap of the ARH and allows for a large write field H w  inside the RL. 
         [0005]    One type of material for the RL is a granular ferromagnetic cobalt alloy, such as a CoPtCr alloy, with a hexagonal-close-packed (hcp) crystalline structure having the c-axis oriented substantially out-of-plane or perpendicular to the RL. The granular cobalt alloy RL should also have a well-isolated fine-grain structure to produce a high-coercivity (Hc) media and to reduce intergranular exchange coupling, which is responsible for high intrinsic media noise. Enhancement of grain segregation in the cobalt alloy RL is achieved by the addition of oxides, including oxides of Si, Ta, Ti, and Nb. These oxides tend to precipitate to the grain boundaries, and together with the elements of the cobalt alloy form nonmagnetic intergranular material. A perpendicular magnetic recording medium with a RL of a CoPtCr granular alloy with added SiO 2  is described by H. Uwazumi, et al., “CoPtCr—SiO 2  Granular Media for High-Density Perpendicular Recording”,  IEEE Transactions on Magnetics,  Vol. 39, No. 4, July 2003, pp. 1914-1918. A perpendicular magnetic recording medium with a RL of a CoPt granular alloy with added Ta 2 O 5  is described by T. Chiba et al., “Structure and magnetic properties of Co—Pt—Ta 2 O 5  film for perpendicular magnetic recording media”,  Journal of Magnetism and Magnetic Materials,  Vol. 287, February 2005, pp. 167-171. 
         [0006]    The cobalt alloy RL has substantially out-of-plane or perpendicular magnetic anisotropy as a result of the c-axis of its hcp crystalline structure being induced to grow substantially perpendicular to the plane of the layer during deposition. To induce this growth of the hcp RL, the IL onto which the RL is formed is also an hcp material. Ruthenium (Ru) and certain Ru alloys, such as RuCr, are nonmagnetic hcp materials that are used for the IL. 
         [0007]    The enhancement of segregation of the magnetic grains in the RL by the additive oxides is important for achieving high areal density and recording performance. The intergranular material not only effectively decouples intergranular exchange but also exerts control on the size and distribution of the magnetic grains in the RL. Current disk fabrication methods achieve this segregated RL by growing the RL on an IL that exhibits columnar growth of its grains. The columnar growth of the IL is accomplished by sputter depositing it at a relatively high sputtering pressure. However, growth of the RL on this type of IL leads to significant roughness and discontinuities in the RL, and consequently to reduced mechanical integrity of the protective OC. Poor OC coverage, roughness in the RL, and columnar growth of the IL provide a relatively easy path for water and corrosive agents to migrate through these layers and interact with the SUL. Formation of the IL at reduced sputtering pressure can reduce the RL roughness and improve the corrosion resistance of the disk. However, disks with ILs formed at lower sputtering pressure exhibit significantly reduced coercivity and thus poor recording performance. 
         [0008]    What is needed is a perpendicular magnetic recording disk that has a granular cobalt alloy RL with additive oxides and that exhibits good corrosion resistance without compromising recording performance. 
         [0009]    US patent application US20050153169A1 discloses in FIG. 1 a  a dual magnetic recording layer perpendicular media. The media is described in paragraphs 20 and 21. Paragraph 20 describes the top layer of the recording layer to possibly be comprised of an oxide. However, paragraph 21, which describes the bottom layer of the recording layer does not describe this layer as an oxide. 
         [0010]    US patent application US20040185308A1 discloses in FIG. 1 a  a dual magnetic recording layer perpendicular media. The application describes two oxide layers used as the recording dual layer. However, the application does not describe the use of Ta in the recording layer. Further, the application describes an O content higher in the top layer of the recording layer than the bottom layer. 
         [0011]    Two other references describe various recording media. US patent application 20020136929A1 describes a multi-layer structure for a magnetic recording layer. However, the application describes a recording layer and media structure for a longitudinal recording system. U.S. Pat. No. 5,516,547 describes a media with a single oxide recording layer. However, the media described in the reference does not include a dual layer recording layer. 
         [0012]    What is needed is a dual layer magnetic recording layer for perpendicular recording. The recording layer includes the advantages of the properties of Ta alloy Oxides. Further, the recording layer benefits from judicious selection of O levels in each of the two layers of the magnetic recording layer. These elements provide improvement in magnetic recording performance when compared to prior art PMR media employing another magnetic recording layer. 
       SUMMARY OF THE INVENTION 
       [0013]    Described herein is a media for perpendicular magnetic recording. The media includes in order a substrate, an adhesion layer, a soft under layer (SUL), an exchange break layer (EBL), a magnetic recording layer and an overcoat. Specifically, the magnetic recording layer includes two magnetic oxide layers. The dual layer magnetic recording layer includes an O content which is higher in the bottom layer than in the top layer. The bottom magnetic layer of the magnetic recording layer also includes Ta. 
         [0014]    A dual layer magnetic recording layer where the bottom layer includes Ta provides several additional advantages. These advantages include a thinner EBL and better corrosion resistance. 
         [0015]    In addition the bottom magnetic layer of the magnetic recording layer may be a non-oxide. This layer could be Co or CoCr. Also, the bottom magnetic layer of the magnetic recording layer can be CoCrOXY where X═Pt, Pd, Fe or Ni and Y=Ta, Nb, Hf or Si. 
         [0016]    The media for perpendicular recording can be combined with an appropriate magnetic head and other components to form a perpendicular magnetic recording disk drive. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    The present invention will be better understood when consideration is given to the following detailed description thereof. Such description makes reference to the drawings, wherein: 
           [0018]      FIG. 1  is a perpendicular magnetic recording system that uses a dual-layer media; 
           [0019]      FIG. 2  is a schematic of a cross-section of a prior art perpendicular magnetic recording disk showing the write field H w  acting on the recording layer RL; 
           [0020]      FIG. 3  is a media with a magnetic recording layer including a dual oxide layer; 
           [0021]      FIG. 4  is a graph of Kerr hysteresis loops for various media; 
           [0022]      FIG. 5  is a graph of SNR for media based on the thicknesses of the oxide layers in the magnetic recording layer; 
           [0023]      FIG. 6  is a graph of Byte Error Rate dependence on the thickness of the EBL and the pressure in which the EBL is sputtered; and 
           [0024]      FIG. 7  is a graph of corrosion current dependence of the media based on the CoPtCrTaO layer thickness ration in a CoPtCrSiO/CoPtCrTaO dual layer. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0025]    For purposes of this disclosure, the following terms are defined as follows: 
         [0026]    “Above” means above, but not necessarily in contact with; and 
         [0027]    “Alloy” means a composition of matter with two or more elements, wherein at least one of the elements is a metal. An alloy of a composition of matter can include itself (e.g. an FeCo alloy includes FeCo). 
         [0028]    A media  300  for a perpendicular recording system is generally described in  FIG. 3 . The perpendicular media described can be used for instance as a disk in a disk drive or as tape in a tape drive. The media includes a substrate  301  of glass, AlMg or any other substrate used as a substrate for magnetic media such as ceramics or glass/ceramic mixtures. Above the substrate is an adhesion layer  302 . Above the adhesion layer  302  is a soft under layer (SUL)  303 . Above the SUL  303  is an exchange break layer  305 . Above the exchange break layer  305  is a magnetic recording layer  306 . Above the magnetic recording layer  306  is an overcoat  307 . The layers above the substrate may be sputter deposited or platted onto the media. Below are described particular implementations of the perpendicular media  300 . 
         [0029]    The adhesion layer  302  is made of AlTi. The adhesion layer  102  is typically between 2 nm and 20 nm thick. 
         [0030]    The SUL  303  is made of an alloy of CoTaZr. The SUL  303  is typically between 20 nm and 100 nm thick. In addition the SUL  303  may be an antiferomagnetically coupled (AFC) structure including a first SUL  303   a  and a second SUL  303   c  coupled by an AFC coupling layer  303   b.  In this configuration the two SUL layers  303   a  and  303   c  are between 10 nm and 50 nm. Further, the AFC coupling layer may be made of Ru or RuCo and be between 6 angstroms and 10 angstroms thick. 
         [0031]    Exchange break layer (EBL)  305  is designed to separate the magnetic recording layer from the SUL and to control the growth orientation of the magnetic recording layer. The EBL is made of Ru but can also be formed of RuTi and Ti. The EBL can be between  9  nm and  40  nm. Further a layer 5 nm to 30 nm of Ni or NiX may optionally be inserted below the EBL and above the SUL. 
         [0032]    Magnetic recording layer  306  is where data is stored on the media. In one embodiment, the perpendicular magnetic recording layer is a dual oxide layer. The top oxide layer  106   b  of the magnetic recording layer is an alloy of CoPtCrSiO. The bottom oxide layer  106   a  of the magnetic recording layer is an alloy of CoPtCrTaO. The total thickness of the dual oxide layers is around 8-25 nm. The bottom TaO layer can be thinner than top SiO layer to increase the corrosion resistivity of the disk. Therefore, the top SiO layer thickness is around 5-20 nm, and bottom TaO thickness is around 1-13 nm. More preferably, the top magnetic layer is 6-10 nm and the bottom magnetic layer is 3-7 nm. In an alternative embodiment, the bottom oxide layer is an alloy of CoPtCrNbO. 
         [0033]    The preferred composition for CoPtCrSiO is Co 65 at. %, Pt 18 at. %, Cr 17 at. %, in the CoPtCr alloy. Further, the SiO is most preferred at 8 mol % with CoPtCr at 92 mol %. The preferred range for Pt and Cr from the preferred composition is ±2 at. % and for Co is ±4 at. %. The preferred deviation from the preferred SiO composition is ±6 mol % and more preferred at ±2 mol %. 
         [0034]    For both CoPtCr—TaO and —NbO, the preferred composition is Co 65 at. %, Pt 16 at. %, Cr 19 at. %, in CoPtCr alloy, while the TaO or NbO is 3.75 mol % and the CoPtCr alloy is 96.25 mol %. The preferred range for Pt and Cr from the preferred composition is ±2 at. % and for Co is ±4 at. %. The preferred deviation from the preferred TaO or NbO range is ±2.25 mol % and more preferably ±1 mol %. 
         [0035]    In a second alternative embodiment, the bottom magnetic layer  306   a  may be a non-oxide, specifically Co or CoCr. An example of such a bottom layer is CoCr 14 . The Cr can range from 0 to 25 at. % in the CoCr alloy of layer  306   a  and preferably, the at. % or Cr is less than 20. Further, the Co or CoCr layer  306   a  may be sputtered in an Ar/N 2  gas mixture in order to dope N 2  in the layer. The non-oxide can help to inhibit corrosion of the disk. This layer may be from 2 to 10 nm and preferably from 3 to 6 nm. 
         [0036]    In a third alternative embodiment, the lower magnetic layer  306   a  is a weakly magnetic layer comprising CoCrOXY where X═Pt, Pd, Fe or Ni and Y═Ta, Nb, Hf or Si. The Co is between 35 at. % and 70 at. % and preferably 50 at. % to 60 at. %. The Cr is between 15 at. % and 30 at. % and preferably 20 at. % to 25 at. %. The YO is between 5 at. % and 20 at. % and preferably 8 at. % to 12 at. %. The X is between 2 at. % and 25 at. % and preferably 10 at. % to 15 at. %. The weakly magnetic layer helps to limit the amount of exchange between the grains in the magnetic layer. This layer may be from 0.5 to 10 nm and preferably from 1 to 7 nm. 
         [0037]      FIG. 4  is a graph of Kerr hysteresis loops for three types of media. Loop  401  is a hysteresis loop for a media as described in  FIG. 3  except wherein the magnetic recording later is made of a single layer of TaO. Loop  402  is a hysteresis loop for a media as described in  FIG. 1  except wherein the magnetic recording later is made of a single layer of SiO. Loop  403  is a hysteresis loop for a media as described in  FIG. 1 , wherein the thicknesses of the upper SiOx layer  306   b  and the lower TaO layer  306   a  are equal. As can be seen from the hysteresis loops, the coercivity of the media is maximized with a recording layer which includes the CoPtCrSiO/CoPtCrTaO bi-layer. Further, loop  402  shows a gentler slope which indicates more grain separation. The negligible nucleation field indicates the media includes small and unstable grains. Loop  401  being more square with sharper transitions demonstrates larger inter-granular-exchange coupling of media with a Ta-oxide recording layer. However, neither of these conditions is optimal for recording. By combining the two layers in a bi-layer several advantages occur such as an increase in coercivity, increase in nucleation field, and increase in thermal stability. It appears that the small isolated grains in the upper layer  306   b  are stabilized by the larger exchange coupling of the lower layer. 
         [0038]      FIG. 5  shows a graph of the recording layer of the media  300  against SNR. The x-axis is the CoPtCrSiO thickness ratio (%) of the media. Thus, the left end of the graph is a single CoPtCrTaO recording layer  306  while the right end of the graph is a single CoPtCrSiO recording layer  306 . As can be seen from the graph, the SNR is optimal when the bi-layer is approximately CoPtCrSiO and CoPtCrTaO. 
         [0039]      FIG. 6  is a graph of Byte Error Rate (BER) dependence on the thickness of the EBL  305  and the pressure in which the EBL  305  is sputtered. The EBL  305  is compromised of two Ru layers deposited at different conditions—a lower layer deposited at lower pressure and upper layer deposited at higher pressure. The curve  602  shows changes to the upper Ru thickness while keeping lower Ru thickness constant. The curve  601  show changes to the lower Ru thickness while keeping upper Ru thickness constant. The vertical lines indicate the standard thickness of each Ru layer for optimized single CoPtCrTaO media. The BER remains good even after reducing the Ru thickness from the standard thickness. In this b-layer case, since the recording layer is more exchange coupled layer than a single layer CoPtCrTaO media, a thinner Ru underlayer which provide less grain segregation than thick Ru layer without degrading the performance can be used. Thinner EBLs yield sharp write field gradient, and result in higher recording density by higher resolution, higher writability, and higher amplitude. 
         [0040]    Tests, such as electrochemical corrosion current, Co-extraction and condensation, show the corrosion resistivity is also improved by the EBL  305  and dual magnetic layer  306 . The tests show single CoPtCrTaO media has much higher corrosion potential than the single CoPtCrSiO media. Different methods applied to reduce the corrosion potential of the single layer CoPtCrTaO media without hurting recording performance has been unsuccessful. The CoPtCrTaO bottom layer and CoPtCSiO top layer combine recording advantages of having a CoPtCrTaO bottom layer and superior corrosion performance of the CoPtCSiO media. 
         [0041]      FIG. 7  is corrosion current dependence of CoPtCrTaO thickness as a percentage of total CoPtCrSiO/CoPtCrTaO in a bi-layer media. The figure shows that the smaller the current, the better the corrosion resistivity. The corrosion current of single layer CoPtCrTaO (100%) media is one order of magnitude higher than single layer CoPtCrSiO media (0%). Therefore, the corrosion current is significantly reduced as the thickness ratio of CoPtCrTaO layer decreased. As the current dependence of the thickness ratio is exponential, a thin CoPtCrSiO top layer can improve the corrosion resistivity. At a 50% bi-layer, the current is almost a third of a single layer CoPtCrTaO media. Therefore, the dual-layer recording layer  306  improves the corrosion resistivity significantly. 
         [0042]    Overcoat  307  is made of a diamond like carbon (DLC). It protects the magnetic recording layer from scratching and corrosion. Typically, overcoat  307  is between 1 nm and 5 nm. Of course, the thinner the overcoat, the closer a magnetic head will fly over the media. Generally, less distance between the head and media improves the recording and reading characteristics of a recording system. 
         [0043]    The present invention is not limited by the previous embodiments or examples heretofore described. Rather, the scope of the present invention is to be defined by these descriptions taken together with the attached claims and their equivalents.