Abstract:
A magnetic disk has a soft magnetic underlayer comprising a first layer containing NiFe having a concentration of iron that is at least thirty percent and not more than seventy percent; a second layer that adjoins the first layer and contains FeCoN having a concentration of iron that is greater than the second layer&#39;s concentration of cobalt, having a concentration of nitrogen that is less than the second layer&#39;s concentration of cobalt and less than three percent; and a third layer containing FeCoNi having a concentration of nickel that is less than eight percent, having a concentration of cobalt that is less than the third layer&#39;s concentration of iron and greater than the third layer&#39;s concentration of nickel, the third layer adjoining only one of the first and second layers. The first and second layers may be repeated to form a magnetically soft high B S  laminate for a disk underlayer.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims the benefit under 35 U.S.C. 120 (is a continuation-in-part) of U.S. patent application Ser. No. 10/137,030, filed May 1, 2002, which is incorporated by reference herein. Also incorporated by reference is the concurrently filed application by the same inventors, entitled: MAGNETICALLY SOFT, HIGH SATURATION MAGNETIZATION LAMINATES OF IRON-COBALT-NITROGEN AND IRON-NICKEL FOR MAGNETIC HEAD POLE LAYERS. 
    
    
     TECHNICAL FIELD 
     The present invention relates to magnetic media, for example magnetic disks or tapes for information storage systems such as disk or tape drives. 
     BACKGROUND 
     Electromagnetic transducers such as heads for disk or tape drives commonly include Permalloy (approximately Ni .81 Fe .19 ), which is formed in thin layers to create magnetic features. Permalloy is known to be magnetically “soft,” that is, to have high permeability and low coercivity, allowing structures made of Permalloy to act like good conductors of magnetic flux. Disks having a media layer that stores magnetic bits in a direction substantially perpendicular to the media surface, sometimes termed “perpendicular recording,” have been proposed to have a soft magnetic underlayer of permalloy or the like. 
     For example, an inductive head may have conductive coils that induce magnetic flux in an adjacent Permalloy core, that flux employed to magnetize a portion or bit of an adjacent media. That same inductive head may read signals from the media by bringing the core near the magnetized media portion so that the flux from the media portion induces a flux in the core, the changing flux in the core inducing an electric current in the coils. Alternatively, instead of inductively sensing media fields, magnetoresistive (MR) sensors or merged heads that include MR or giant magnetoresistive (GMR) sensors may use thinner layers of Permalloy to read signals, by sensing a change in electrical resistance of the sensor that is caused by the magnetic signal. For perpendicular recording, the soft magnetic underlayer of the disk as well as the soft magnetic core of the head may together form a magnetic circuit for flux that travels across the media layer to write or read information. 
     In order to store more information in smaller spaces, transducer elements have decreased in size for many years. One difficulty with this deceased size is that the amount of flux that needs to be transmitted may saturate elements such as magnetic pole layers, which becomes particularly troublesome when ends of the pole layers closest to the media, commonly termed pole tips, are saturated. Magnetic saturation in this case limits the amount of flux that is transmitted through the pole tips, limiting writing or reading of signals. Moreover, such saturation may blur that writing or reading, as the flux may be evenly dispersed over an entire pole tip instead of being focused in a corner that has relatively high flux density. For these reasons the use of high magnetic saturation materials (also known as high moment or high B S  materials) in magnetic core elements has been known for many years. 
     For instance, iron is known to have a higher magnetic moment than nickel, so increasing the proportion of iron compared to nickel generally yields a higher moment alloy. Iron, however, is also more corrosive than nickel, which imposes a limit to the concentration of iron that is feasible for many applications. Also, it is difficult to achieve soft magnetic properties for primarily-iron NiFe compared to primarily-nickel NiFe. Anderson et al., in U.S. Pat. No. 4,589,042, teach the use of high moment Ni .45 Fe .55  for pole tips. Anderson et al. do not use Ni .45 Fe .55  throughout the core due to problems with permeability of that material, which Anderson et al. suggest is due to relatively high magnetostriction of Ni .45 Fe .55 . 
     As noted in U.S. Pat. No. 5,606,478 to Chen et al., the use of high moment materials has also been proposed for layers of magnetic cores located closest to a gap region separating the cores. Also noted by Chen et al. are some of the difficulties presented by these high moment materials, including challenges in forming desired elements and corrosion of the elements once formed. Chen et al. state that magnetostriction is another problem with Ni .45 Fe .55 , and teach the importance of constructing of heads having Permalloy material layers that counteract the effects of that magnetostriction. This balancing of positive and negative magnetostriction with plural NiFe alloys is also described in U.S. Pat. No. 5,874,010 to Tao et al. 
     Primarily iron FeCo alloys are known to have a very high saturation magnetization but also high magnetostriction that makes them unsuitable for many head applications. That is, mechanical stress during slider fabrication or use may perturb desirable magnetic domain patterns of the head.  FIG. 7  shows a B/H loop  12  of a FeCoN layer that was formed by sputtering deposition at room temperature, the layer having a thickness of approximately 500 Å and having a composition of approximately Fe .66 Co. 28 N. 06 . The applied H-field is shown in oersted (Oe) across the horizontal axis while the magnetization of the layer is plotted in normalized units along the vertical axis, with unity defined as the saturation magnetization for a given material. The FeCoN layer has a saturation magnetization (B S ) of approximately 24.0 kilogauss and is magnetically isotropic, as shown by the single B/H loop  12 . B/H loop  12  also indicates a relatively high coercivity of about 80 oersted, which may be unsuitable for applications requiring soft magnetic properties. 
     In an article entitled “Microstructures and Soft Magnetic Properties of High Saturation Magnetization Fe—Co—N alloy Thin Films,” Materials Research Society, Spring meeting, Section F, April 2000, N. X. Sun et al. report the formation of FeCoN films having high magnetic saturation but also high magnetostriction and moderate coercivity. Sun et al. also report the formation of a thin film structure in which FeCoN is grown on and capped by Permalloy, to create a sandwich structure having reduced coercivity but compressive stress. The magnetostriction of this sandwich structure, while somewhat less than that of the single film of FeCoN, may still be problematic for head applications. Such issues would be expected to grow with increased length of a magnetostrictive layer, so that disk layers that extend many times as far as head layers would appear to be poor candidates for magnetostrictive materials. 
     SUMMARY 
     In one embodiment, a magnetic disk is disclosed, comprising a self-supporting substrate; a soft magnetic underlayer disposed over the substrate, the underlayer including a first layer containing NiFe having an atomic concentration of iron that is at least thirty percent and not more than seventy percent, a second layer that adjoins the first layer and contains FeCoN having an atomic concentration of iron that is greater than the second layer&#39;s atomic concentration of cobalt, having an atomic concentration of nitrogen that is less than the second layer&#39;s atomic concentration of cobalt and less than about three percent; and a media layer disposed over the underlayer and containing a magnetically hard material having an easy axis of magnetization oriented substantially perpendicular to both the media layer and the underlayer. 
     The underlayer may include a first plurality of layers each containing NiFe having an atomic concentration of iron that is at least about thirty percent; a second plurality of layers that is interleaved with the first plurality of layers, the second plurality of layers each containing FeCoN having an atomic concentration of iron that is greater than an atomic concentration of cobalt, and having an atomic concentration of nitrogen that is less than the atomic concentration of cobalt, the atomic concentration of nitrogen being less than eight percent. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cutaway cross-sectional view of a sandwich structure made of a primarily iron FeCoN layer affixed between a pair of FeNi layers. 
         FIG. 2  is a cutaway cross-sectional view of a laminated structure made of a plurality of primarily iron FeCoN layers interleaved with a plurality of primarily iron FeNi layers. 
         FIG. 3  is plot of a B/H loop of the laminated structure of  FIG. 2 . 
         FIG. 4  is a plot of saturation magnetization as a function of nitrogen content for (Fe .70 Co .30 )N. 
         FIG. 5  is a cutaway cross-sectional view of a magnetic disk for perpendicular recording including a soft magnetic underlayer formed of the laminated structure of  FIG. 2 . 
         FIG. 6  is a cutaway cross-sectional view of a perpendicular recording head interacting with the disk of  FIG. 5 . 
         FIG. 7  is plot of a B/H loop of a prior art FeCoN layer. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 1  is a cutaway cross-sectional view of a sandwich structure  20  made of an iron-cobalt-nitride (FeCoN) layer  22  affixed between a pair of iron-nickel (FeNi) layers  24  and  26 . The sandwich structure  20  is formed on a substrate  28  that provides a smooth surface promoting favorable crystallographic growth of layers  22 ,  24  and  26 . The FeCoN layer  22  has a thickness of approximately 475 Å and has a composition of approximately Fe .69 Co .30 N .01 . The FeNi layers  24  and  26  each have a thickness of approximately 25 Å and have a composition of approximately Ni .55 Fe .45 . Layers  22 ,  24  and  26  were formed by DC magnetron sputtering deposition at room temperature. Magnetron sputtering has a deposition rate that is approximately ten times faster than that of RF sputtering, which is an advantage in commercial applications such as magnetic head production. The substrate may be a silicon dioxide, alumina, chromium, tantalum or titanium, for example. 
       FIG. 2  is a cutaway cross-sectional view of a laminated structure  40  made of a plurality of primarily iron FeCoN layers  42  interleaved with a plurality of primarily iron FeNi layers  44 . The sandwich structure  20  is formed on a substrate  46  that provides a surface promoting favorable microstructural growth of layers  42  and  44 . The FeCoN layers  42  each have a thickness of approximately 475 Å and a composition of approximately Fe .69 Co .30 N .01 . The FeNi layers  42  each have a thickness of approximately 25 Å and have a composition of approximately Ni .55 Fe .45 . Layers  42  and  44  were formed by magnetron sputtering deposition on substrate  46  at room temperature. Various other compositions and thicknesses may also be suitable. For example, the FeCoN layers may have atomic concentrations of iron in a range between 50% and 80%, atomic concentrations of cobalt in a range between 17% and 50%, and atomic concentrations of nitrogen in a range between 0.01% and 3%. As another example, the NiFe layers may have atomic concentrations of iron in a range between 30% and 70%, and atomic concentrations of nickel in a range between 70% and 30%. The thickness of any of the layers may for example be in a range between a few angstroms and one hundred nanometers. 
       FIG. 3  shows B/H loops  30  and  33  for the laminated structure  40  of  FIG. 2  having an overall thickness of about 2500 Å. The laminated structure has a saturation magnetization (B S ) of approximately 2.4 tesla (T), nearly that of the single layer of FeCoN. The coercivity of the hard axis, which is defined as the applied field of the loop  30  at which the magnetization is zero, is about 4 oersted (Oe) while the coercivity of the easy axis is about 12 Oe as shown by loop  33 . The permeability is approximately 2000, and the laminate has been found to be suitable for applications such as soft magnetic underlayers for magnetic disks. 
       FIG. 4  is a plot  60  of experimentally determined saturation magnetization B S  of FeCoN for various concentrations of nitrogen gas, normalized for zero nitrogen. The plot  60  was generated using a sputtering target of Fe .70 Co .30  and varying the amount of nitrogen gas. The concentration of nitrogen in the solid layer of FeCoN has been found to be about the same as that in the gas. At the wafer formation level, the concentration of various elements can be determined by Auger Electron Spectroscopy (AES) or Electron Energy Loss Spectroscopy (EELS), while concentrations of various elements of a layer in a completed device such as a magnetic disk ca be determined by Transmission Electron Microscopy (TEM). The plot  60  has a peak saturation magnetization B S  at about 1% nitrogen, with B S  generally declining as the nitrogen content is increased above 1%. The coercivity generally increases as the nitrogen content of FeCoN layers declines from approximately 7%, however, arguing against the use of low nitrogen content FeCoN in magnetic disks. 
     A laminated structure of FeCoN/NiFe having a coercivity below 12 Oe and a B S  above 2.3 T may be desirable for applications such as soft magnetic underlayers for disks. In this case, the magnetically soft, high B S  laminate  40  used in a soft magnetic underlayer of a perpendicular recording disk may include FeCoN with a nitrogen concentration as high as about eight percent. Such a laminated soft magnetic underlayer may be formed entirely of alternating layers of FeCoN and NiFe, which, because of the high B S  compared to traditional underlayers, may have an overall thickness of about 2000 Å or less. 
     A media layer  158  is disposed over the underlayer  155 , the media layer having an easy axis of magnetization that is substantially perpendicular to a major surface  153  of the medium. A thin, physically hard overcoat  156  separates the media layer  158  from the medium surface  153 . The medium  150 , which may for example be a rigid disk, is moving relative to the head in a direction shown by arrow  159 . The head  100  may be spaced from the medium  150  by a nanoscale air bearing, or the head may be in frequent or continuous contact with the medium during operation. The word nanoscale as used herein is meant to represent a size that is most conveniently described in terms of nanometers, e.g., between about one nanometer and about two hundred nanometers. 
       FIG. 5  is a cutaway cross-sectional view of a portion of a medium  150  such as a disk designed for perpendicular storage of data that is written and read by a relatively moving head containing an electromagnetic or electrooptical transducer. The medium  150  includes a substrate  152  that, for the case in which the medium is a disk for a hard disk drive, may be made of glass, aluminum or other materials. The substrate  152  may be textured, or an optional texture layer  151  may provided that promotes favorable growth of a soft magnetic underlayer  155 . For example, a substrate  152  of an aluminum-magnesium (AIMg) alloy may be plated with a layer of nickel phosphorous (NiP) to a thickness of about 15 microns that increases the hardness of the substrate and provides a surface suitable for polishing to provide a desired roughness or texture. 
     A soft magnetic underlayer  155  has been formed of interleaved layers of FeCoN  162  and NiFe  160  similar to that described above, formed to an overall thickness that is in a range between about 1000 Å and 4000 Å. The NiFe layers  160  may contain Ni x Fe (1-X) , wherein 0.3≦X≦0.7 and FeCoN layers  162  may contain Fe y Co z N (1-Y-Z) , wherein 0.5≦Y≦0.8 and 0&lt;(1-Y-Z)≦0.03. The underlayer may be thinner than is conventional for perpendicular media, for example less than 2000 Å, due to the relatively high B S  of over 2.3 T. The coercivity of the underlayer  155  may be in a range between about twelve oersted and two oersted. 
     The soft magnetic underlayer  155  may alternatively contain a plurality of magnetic layers each containing FeCoN that are interleaved with a plurality of much thinner nonmagnetic layers. For example, the underlayer may include a plurality of magnetic layers each containing FeCoN having an atomic concentration of iron that is greater than its atomic concentration of cobalt, and having an atomic concentration of nitrogen that is less than eight percent and less than the atomic concentration of cobalt, the underlayer including a plurality of nonmagnetic layers that are interleaved with the magnetic layers, each of the nonmagnetic layers having a thickness that is less than one-tenth that of an adjoining layer of the magnetic layers. The coupling between adjacent magnetic layers that is provided by the nonmagnetic layers may reduce noise in the underlayer that may otherwise reduce signal integrity. 
     As an example, the nonmagnetic layers may have a thickness in a range between about eight angstroms and twelve angstroms, although a greater or smaller thickness is possible. The FeCoN layers may each have a thickness in a range between about one hundred angstroms and five hundred angstroms, although a greater or smaller thickness is possible. The underlayer may be thinner than is conventional for perpendicular media, for example less than 2000 Å, due to the relatively high B S  of over 2.3 T. The coercivity of the underlayer  155  may be in a range between about twenty oersted and two oersted. 
     In one embodiment, the nonmagnetic layers may be chromium or ruthenium, formed to a thickness in a range between about eight angstroms and twelve angstroms so that adjacent magnetic layers are exchange coupled in an antiparallel orientations. This may be termed antiferromagnetic exchange coupling. In this embodiment the underlayer may have a substantially zero net magnetic moment, provided that an even number of magnetic layers of equal thickness is formed, or that the overall thickness of the layers having one magnetic orientation is substantially equal to the overall thickness of the magnetic layers having the opposite orientation. 
     In one embodiment, the nonmagnetic layers are made of a metal oxide or nitride that induces antiparallel magnetostatic coupling between a pair of adjacent magnetic layers. As an example, the metal oxide or nitride is Al X O (1-X) , Ta Y O (1-Y)  or Al Z N (1-Z) . In another embodiment, to induce antiparallel magnetostatic coupling between a pair of adjacent magnetic layers, the nonmagnetic layer can be made of a metal such as Cu, Ti, Ta or NiCr. 
     A media layer  158  is disposed over the underlayer  155 , the media layer having an easy axis of magnetization that is substantially perpendicular to a major surface  153  of the medium  150 . The media layer  158  may be formed of a single layer or of multiple layers, for example of cobalt based magnetic alloy layers interleaved with platinum group nonmagnetic layers to enhance perpendicular anisotropy. A nonmagnetic exchange decoupling material may be contained in the media layer or layers to decouple magnetic grains for reducing noise. A nonmagnetic decoupling layer  164 , which also serves as a seed layer for the media layer  158 , is disposed between the underlayer  155  and the media layer, and may contain for example chromium (Cr) or titanium (Ti). A thin, physically hard overcoat  156  of diamond-like carbon (DLC), tetrahedral-amorphous carbon (ta—C), silicon carbide (SiC) or the like separates the media layer  158  from the medium surface  153 . Although not shown, a thin lubricant layer may coat the medium surface  153 . 
       FIG. 6  is a cutaway cross-sectional view of a magnetic head  100  interacting with the medium  150 , which is moving relative to the head in a direction shown by arrow  159 . The head  100  has a medium-facing surface  166  disposed adjacent to the medium surface  153 . The head  100  may be spaced from the medium  150  by a nanoscale air bearing, or the head may be in frequent or continuous contact with the medium during operation. The word nanoscale as used herein is meant to represent a size that is most conveniently described in terms of nanometers, e.g., between about one nanometer and about two hundred nanometers. The head  100  in this embodiment is designed for perpendicular recording on the medium  150 , and includes a laminated write pole layer  101  that terminates adjacent to the medium-facing surface in a first pole tip  170 , which may sometimes be called a write pole tip. The write pole layer  101  may be formed of a plurality of layers of FeNi interleaved with a plurality of layers of FeCoN, similar to laminate  40  described above. 
     A soft magnetic layer  188  adjoins the write pole layer  101  but terminates further from the medium-facing surface  166  than the first pole tip  170 , layers  101  and  188  combining to form a write pole. Another soft magnetic layer  178  is magnetically coupled to the write pole layer  101  in a region that is removed from the medium-facing surface and not shown in this figure, and is magnetically coupled to the write pole layer  101  adjacent to the medium-facing surface by a soft magnetic pedestal  175 . The pedestal  175  may serve to deflect magnetic flux from traveling exactly perpendicular to the media layer  158 , so that perpendicularly oriented bits in the media layer can flip more easily. For this purpose write pole tip corner  171  may be spaced a similar distance from the pedestal as it is from the soft underlayer  155 , e.g., on the order of  50 - 200  nm. The soft magnetic layer  178  and pedestal  175  may be considered to form a return pole layer that terminates adjacent to the medium-facing surface in a second pole tip  180 . At least one electrically conductive coil section may be disposed between layers  101  and  178  and another coil section disposed upstream of layer  188 , to induce magnetic flux in the pole layers. 
     Although not apparent in this view, the return pole tip  180  may have an area that is at least two or three orders of magnitude greater than that of the write pole tip  170 . Alternatively, another return pole layer and return pole tip may additionally be provided, for example between the write pole layer and a MR sensor. The write pole tip  170  may have a substantially trapezoidal shape that has a maximum track width at a trailing corner  171 . The trailing corner  171  of the write pole tip  170  may be approximately equidistant from soft magnetic underlayer  155  and soft magnetic pedestal  175  in this embodiment, to deflect magnetic flux from the write pole. The write pole layer  170  may have a B S  that is between about 2.35 T and 2.45 T, while the soft magnetic pedestal  175  may have a B S  that is substantially less, e.g., less than 2.0 T. A magnetoresistive or magnetooptical sensor may also be included with the head, such a sensor not shown in this view.