Abstract:
A magnetic recording medium for data storage uses a magnetic recording layer having at least two ferromagnetic films exchange coupled together antiferromagnetically across a nonferromagnetic spacer film. In this antiferromagnetically-coupled (AFC) recording layer the magnetic moments of the two ferromagnetic films are oriented antiparallel, and thus the net remanent magnetization-thickness product (Mrt) of the AFC recording layer is the difference in the Mrt values of the two ferromagnetic films. This reduction in Mrt is accomplished without a reduction in thermal stability of the recording medium. The lower ferromagnetic film in the AFC recording layer is a ferromagnetic CoCrFe alloy that does not require a nucleation layer between it and the Cr alloy underlayer. The medium with the CoCrFe alloy as the first or lower ferromagnetic film in the AFC recording layer has reduced intrinsic media noise.

Description:
RELATED APPLICATION 
     This application is related to application Ser. No. 09/631,908 filed Aug. 4, 2000, now U.S. Pat. No. 6,537,684, and titled “ANTIFERROMAGNETICALLY COUPLED MAGNETIC RECORDING MEDIA WITH BORON-FREE FIRST FERROMAGNETIC FILM AS NUCLEATION LAYER”, which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     This invention relates generally to magnetic recording media, and more particularly to a magnetic recording disk with an antiferromagnetically-coupled (AFC) magnetic recording layer. 
     BACKGROUND OF THE INVENTION 
     Conventional magnetic recording media, such as the magnetic recording disks in hard disk drives, typically use a granular ferromagnetic layer, such as a sputter-deposited cobalt-platinum (CoPt) alloy, as the recording medium. Each magnetized domain in the magnetic layer is comprised of many small magnetic grains. The transitions between magnetized domains represent the “bits” of the recorded data. IBM&#39;s U.S. Pat. Nos. 4,789,598 and 5,523,173 describe this type of conventional rigid disk. 
     As the storage density of magnetic recording disks has increased, the product of the remanent magnetization Mr (where Mr is measured in units of magnetic moment per unit volume of ferromagnetic material) and the magnetic layer thickness t has decreased. Similarly, the coercive field or coercivity (H c ) of the magnetic layer has increased. This has led to a decrease in the ratio Mrt/H c . The reason for this decrease is that the parameter Mrt/H c  is related to the ability of the recording head to resolve the magnetic bits at high density. Decreasing Mrt/H c  increases this ability. To achieve the reduction in Mrt, the thickness t of the magnetic layer can be reduced, but only to a limit because the stored magnetic information in the layer will be more likely to decay. This decay of the magnetization has been attributed to thermal activation of small magnetic grains (the superparamagnetic effect). The thermal stability of a magnetic grain is to a large extent determined by K u V, where K u  is the magnetic anisotropy constant of the layer and V is the volume of the magnetic grain. As the layer thickness is decreased, V decreases. If the layer thickness is too thin, the stored magnetic information will no longer be stable at normal disk drive operating conditions. 
     One approach to the solution of this problem is to move to a higher anisotropy material (higher K u ). However, the increase in K u  is limited by the point where the coercivity H c , which is approximately equal to K u /M s  (M s =saturation magnetization), becomes too great to be written by a conventional recording head. A similar approach is to reduce the M s  of the magnetic layer for a fixed layer thickness, which will reduce Mr since Mr is related to M s , but this is also limited by the coercivity that can be written. Another solution is to increase the intergranular exchange, so that the effective magnetic volume V of the magnetic grains is increased. However, this approach has been shown to be deleterious to the intrinsic signal-to-noise ratio (SNR) of the magnetic layer. 
     U.S. Pat. No. 6,280,813 describes a magnetic recording medium wherein the magnetic recording layer is at least two ferromagnetic films antiferromagnetically coupled together across a nonferromagnetic spacer film. In this type of magnetic media, referred to as AFC media, the magnetic moments of the two antiferromagnetically-coupled films are oriented antiparallel, with the result that the net remanent magnetization-thickness product (Mrt) of the recording layer is the difference in the Mrt values of the two ferromagnetic films. This reduction in Mrt is accomplished without a reduction in volume V. Therefore the thermal stability of the recording medium is not reduced. One of the ferromagnetic films is made thicker than the other, but the thicknesses are chosen so that the net moment in zero applied magnetic field is low, but nonzero. In one embodiment of the AFC medium both ferromagnetic films are sputter deposited CoPtCrB alloy films separated by a Ru spacer film that has a thickness to maximize the antiferromagnetic coupling between the two CoPtCrB films. 
     The use of a boron-containing alloy like CoPtCrB as the ferromagnetic film composition in an AFC medium requires the use of a special onset or nucleation layer to enhance the growth of the CoPtCrB films so that the C-axis of these films is in the plane of the films. The nucleation layer, which is typically a nonferromagnetic CoCr alloy, requires still another sputtering station in the manufacturing line. In the previously cited pending application, the lower ferromagnetic film in the AFC medium is a boron-free ferromagnetic CoCr alloy that does not require a nucleation layer between it and the Cr or Cr alloy underlayer. This ferromagnetic CoCr alloy has sufficient saturation magnetization (M s ) and grain structure to produce excellent magnetic recording performance for the AFC recording layer, while also serving as a nucleation layer to induce the in-plane C-axis growth of the upper CoPtCrB ferromagnetic film through the spacer layer. 
     A continuing problem in magnetic recording media is intrinsic media noise which is a significant contributor to the overall signal-to-noise ratio in the disk drive. What is needed is an AFC medium with reduced media noise. 
     SUMMARY OF THE INVENTION 
     The invention is an AFC magnetic recording medium having at least two ferromagnetic films exchange coupled together antiferromagnetically across a nonferromagnetic spacer film. In this antiferromagnetically-coupled (AFC) recording layer the magnetic moments of the two ferromagnetic films are oriented antiparallel, and thus the net remanent magnetization-thickness product (Mrt) of the AFC recording layer is the difference in the Mrt values of the two ferromagnetic films. This reduction in Mrt is accomplished without a reduction in thermal stability of the recording medium. The lower ferromagnetic film in the AFC recording layer is a ferromagnetic CoCrFe alloy that does not require a nucleation layer between it and the Cr alloy underlayer. The AFC medium with the CoCrFe alloy as the first or lower ferromagnetic film has reduced intrinsic media noise. 
     For a fuller understanding of the nature and advantages of the present invention, reference should be made to the following detailed description taken together with the accompanying figures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a prior art schematic sectional view of an AFC magnetic recording disk as described in the previously cited pending application. 
     FIG. 2 is a schematic sectional view of an AFC magnetic recording disk according to the present invention. 
     FIG. 3 is a graph of the measured ratio of the isolated signal pulse to the measured noise (S 0 NR) for transitions recorded at different densities (thousand flux changes/inch or kfci) for several disks with the CoCrFe lower AFC film. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     General Structure of an AFC Disk 
     The magnetic recording disk of the present invention is of the type that has a magnetic recording layer made of two or more ferromagnetic films that are coupled antiferromagnetically (AF) to their neighboring ferromagnetic films by one or more nonferromagnetic spacer films. FIG. 1 illustrates the cross sectional structure of a disk  10  with an antiferromagnetically-coupled (AFC) magnetic layer  20 . 
     The disk substrate  11  is any suitable material, such as glass, SiC/Si, ceramic, quartz, or an AlMg alloy base with a NiP surface coating. The seed layer  12  is an optional layer that may be used to improve the growth of the underlayer  13 . The seed layer  12  is most commonly used when the substrate  11  is nonmetallic, such as glass. The seed layer  12  has a thickness in the range of approximately 1 to 50 nm and is one of the materials, such as Ta, CrTi, NiAl or RuAl, which are useful as seed materials for promoting the growth of subsequently deposited layers in certain preferred crystalline orientations. A pre-seed layer (not shown) may also be used between the glass substrate  11  and the seed layer  12 . The underlayer  13  is deposited onto the seed layer, if present, or otherwise directly onto the substrate  11 , and is a nonmagnetic material such as chromium or a chromium alloy, such as CrV or CrTi. The underlayer  13  has a thickness in the range of 5 to 100 nm with a typical value being approximately 10 nm. 
     The AFC magnetic layer  20  is made up of two ferromagnetic films  22 ,  24  separated by a nonferromagnetic spacer film  26 . The nonferromagnetic spacer film  26  thickness and composition are chosen so that the magnetic moments  32 ,  34  of adjacent films  22 ,  24 , respectively, are AF-coupled through the nonferromagnetic spacer film  26  and are antiparallel in zero applied field. The two AF-coupled films  22 ,  24  of layer  20  have magnetic moments that are oriented antiparallel, with the upper film  22  having a larger moment. The ferromagnetic films  22 ,  24  are made of a CoPtCrB alloy with 4 to 20 atomic percent (at. %) platinum, 10 to 23 at. % chromium and 2 to 20 at. % boron. The nonferromagnetic spacer film  26  is ruthenium (Ru). 
     Because the first ferromagnetic film  24  of the AFC magnetic layer  20  is a boron-containing CoPtCrB alloy, a very thin (typically 1 to 5 nm) Co alloy onset or nucleation layer  14  is deposited on the underlayer  13 . The nucleation layer  14  has a composition selected to enhance the growth of the hexagonal close-packed (HCP) CoPtCrB alloy of film  24  so that its C-axis is oriented in the plane of the film. The proper crystalline structure of the first CoPtCrB film  24  in turn enhances the growth of the second CoPtCrB film  22 , through the Ru spacer film  26 , to also have its C-axis in-plane. If the CoPtCrB film  24  were grown directly on the Cr alloy underlayer  13  without a nucleation layer, then it would not grow with its C-axis in the plane of the film, which would result in poor recording performance. It is well known that the presence of boron is important for achieving small grains in the recording layer, which is necessary for high performance media. Therefore, the nucleation layer  14  allows use of boron-containing alloys as the recording layer. The nucleation layer  14  typically is a nonferromagnetic Co alloy, such as a CoCr alloy with Cr&gt;=31 at. %. This CoCr composition produces a phase which is nonferromagnetic or slightly ferromagnetic. 
     The AF coupling of ferromagnetic films via a nonferromagnetic transition metal spacer film, like the structure of layer  20  in FIG. 1, has been extensively studied and described in the literature. In general, the exchange coupling oscillates from ferromagnetic to antiferromagnetic with increasing spacer film thickness. This oscillatory coupling relationship for selected material combinations is described by Parkin et al. in “Oscillations in Exchange Coupling and Magnetoresistance in Metallic Superlattice Structures: Co/Ru, Co/Cr and Fe/Cr”,  Phys. Rev. Lett ., Vol. 64, p. 2034 (1990). The material combinations include ferromagnetic films made of Co, Fe, Ni, and their alloys, such as Ni—Fe, Ni—Co, and Fe—Co, and nonferromagnetic spacer films such as Ru, chromium (Cr), rhodium (Rh), iridium (Ir), copper (Cu), and their alloys. For each such material combination, the oscillatory exchange coupling relationship has to be determined, if not already known, so that the thickness of the nonferromagnetic spacer film is selected to assure antiferromagnetic coupling between the two ferromagnetic films. The period of oscillation depends on the nonferromagnetic spacer material, but the strength and phase of the oscillatory coupling also depends on the ferromagnetic material and interfacial quality. 
     For this AFC structure of layer  20  the orientations of the magnetic moments  32 ,  34  of adjacent films  22 ,  24 , respectively, are aligned antiparallel and thus add destructively. The arrows  32 ,  34  represent the moment orientations of individual magnetic domains that are directly above and below one another across the AF coupling film  26 . 
     While FIG. 1 is shown for an AFC magnetic layer  20  with a two-film structure and a single spacer film, the AFC disk may have additional ferromagnetic films with AF-coupling spacer films between the ferromagnetic films. 
     Structure of an AFC Disk with a CoCrFe Lower Ferromagnetic Film 
     High performance commercially available disks using glass substrates and CoPtCrB single-layer magnetic layers can require up to six layers. These layers are a pre-seed layer on the glass substrate (not shown in FIG.  1 ), the seed layer, the underlayer, the nonferromagnetic (or slightly ferromagnetic) CoCr nucleation layer, the CoPtCrB magnetic layer, and the protective overcoat. Common manufacturing sputtering tools, such as the Circulus M12, currently have only seven stations available for actual sputter deposition, assuming the use of two heating stations and one cooling station. Since the AFC magnetic layer replaces the single magnetic layer with three layers, the total number of sputtering cathodes needed to create an AFC disk is eight. This number is larger than is currently available on the Circulus M12 configured as described above. Other types of manufacturing sputter tools may also have a limited number of sputtering cathodes, which makes implementation of AFC media difficult. 
     Certain materials can serve the dual purpose of acting as the lower ferromagnetic film in the AFC layer as well as facilitating the in-plane C-axis growth of the second CoPtCrB film through the Ru spacer layer. This allows a combination of the nucleation layer and the lower ferromagnetic film of the AFC layer into one layer and therefore only one sputtering cathode is required. This reduces the total number of sputtered layers in the AFC disk structure, thereby overcoming potential manufacturability problems. The previously cited pending application has demonstrated that ferromagnetic Co 78 Cr 22 , which has a saturation magnetization (M s ) of 425 emu/cc, acts as both an onset or nucleation layer and the lower film in the AFC structure. 
     In the present invention a CoCrFe alloy acts as both an onset or nucleation layer and the lower film in the AFC recording and also significantly improves the signal-to-noise ratio of the AFC media. This is a surprising result because the addition of Fe to Co alloys is known to add intergranular exchange coupling, which is generally considered to be deleterious to recording performance. As explained in the previously cited &#39;813 patent, it is advantageous to use a granular ferromagnetic material with isolated magnetic grains, i.e., no intergranular exchange coupling, to lower the media noise because it has been shown that intergranular exchange coupling increases the media noise. Thus it is unexpected that the use of a CoCrFe lower film would decrease the media noise in the AFC media. 
     FIG. 2 shows the preferred structure of the AFC disk  10 ′ according to the invention. The thicknesses and compositions for the various layers in this preferred embodiment are as follows: 
     Pre-seed layer: Cr 50 Ti 50  (20-50 nm) 
     Seed Layer  12 : Ru 50 Al 50  (8-20 nm) 
     Underlayer  13 : Cr 80 Ti 20  (6-20 nm) 
     Lower AFC Film  24 ′ directly on underlayer  13 : 
     Co (100-x-y) Cr x Fe y  with 2&lt;x&lt;25 and 2&lt;y&lt;30 
     Spacer Layer  26 : Ru or Cr (0.4-1.0 nm) 
     Top AFC Film  22 : Co (100-y-x-z) Pt y Cr x B z  with 6&lt;y&lt;25, 10&lt;x&lt;25, 6&lt;z&lt;15. 
     In the preferred embodiment the CoCrFe alloy that serves as the bottom AFC film without the need for a special nucleation layer has an Fe composition between approximately 2 and 30 at. % and a Cr composition between approximately 2 and 25 at. %. The Fe concentration is determined primarily by the thickness and Mrt desired for the bottom AFC film. The at. % of Cr and Fe determines the Mrt of the CoCrFe film for a given thickness. The upper Fe limit is the amount around which the hcp structure of the CoCrFe alloy is lost and thus the subsequent in plane C-axis growth of the upper CoPtCrB film is compromised. The upper Cr limit of approximately 25 at. % is the amount around which the CoCrFe alloy becomes nonferromagnetic. The desired thickness of the CoCrFe film is determined by optimum film growth and recording performance, and is typically between approximately 0.5 and 3.5 nm. 
     FIG. 3 shows the measured ratio of the isolated signal pulse to the measured noise (S 0 NR) for transitions recorded at different densities (thousand flux changes/inch or kfci) for several disks. These media were all grown under the same growth conditions, with the same thicknesses and alloys used for both the underlayers and the top AFC film. The top AFC film has an Mrt=0.39 emu/cm 2 . The composite Mrt is listed in the legend and is obtained by subtracting the Mrt of the lower AFC film from the Mrt of the top AFC film. FIG. 3 shows that the AFC media with the CoCrFe alloy as the lower film has significantly higher S 0 NR than the AFC media using the CoCr alloy described in the previously cited pending application as the lower film. FIG. 3 also shows that the improvement in S 0 NR is obtained in the AFC media with the CoCrFe lower film even compared to AFC media with the CoCr lower film with several different Mrt values. 
     While the present invention has been particularly shown and described with reference to the preferred embodiments, it will be understood by those skilled in the art that various changes in form and detail may be made without departing from the spirit, scope, and teaching of the invention. Accordingly, the disclosed invention is to be considered merely as illustrative and limited in scope only as specified in the appended claims.