Abstract:
In one aspect, a laminated structure including a first plurality of layers containing primarily-iron FeCoN interleaved with a second plurality of layers containing primarily iron FeNi is disclosed. The structure has an easy axis of magnetization and a hard axis of magnetization, has a magnetic saturation of at least about twenty-three-thousand Gauss, and has a magnetic coercivity measured substantially along its hard axis of magnetization that is less than two Oersted. Additional elements can be added in minority concentrations to form primarily-iron FeCoN layers with increased resistivity. The laminated structure has applicability in various fields in which high saturation magnetization, magnetically soft materials are advantageous, particularly for inductive heads.

Description:
TECHNICAL FIELD 
     The present invention relates to magnetic devices, for example electromagnetic transducers of disk or tape storage systems. 
     BACKGROUND OF THE INVENTION 
     Electromagnetic transducers such as heads for disk or tape drives commonly include Permalloy (approximately Ni 0.81 Fe 0.19 ), 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. 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 sensors may use thinner layers of Permalloy to read signals, by sensing a change in electrical resistance of the MR sensor that is caused by the magnetic signal. 
     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 to be desirable. 
     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 0.45 Fe 0.55  for pole tips. Anderson et al. do not use Ni 0.45 Fe 0.55  throughout the core due to problems with permeability of that material, which Anderson et al. suggest is due to relatively high magnetostriction of Ni 0.45 Fe 0.55 . 
     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 0.45 Fe 0.55 , 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. 11 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 0.66 Co 0.28 N 0.06 . 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 very high coercivity of about 80 oersted, which is 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. 
     SUMMARY 
     In accordance with the present invention a laminated structure including a first plurality of layers containing primarily iron FeCoN interleaved with a second plurality of layers containing primarily iron FeNi is disclosed. The structure has an easy axis of magnetization and a hard axis of magnetization, has a magnetic saturation of at least about twenty-three-thousand Gauss, and has a magnetic coercivity measured substantially along its hard axis of magnetization that is less than two Oersted. 
    
    
     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 primarily iron FeNi layers. 
     FIG. 2 is a plot of B/H loops of the sandwich structure of FIG.  1 . 
     FIG. 3 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. 4 is a plot of B/H loops of the laminated structure of FIG.  3 . 
     FIG. 5 is a plot of the hard axis coercivity, the easy axis coercivity, and the anisotropy field for laminated structures of FIG. 3 having various thicknesses. 
     FIG. 6 is a plot of the hard axis coercivity, the easy axis coercivity, and the anisotropy field for samples of the laminated structure described above with reference to FIG. 3 after annealing the sample at various temperatures. 
     FIG. 7 is a plot of a hard axis loop  100  and easy axis loop  110  for a composite structure made of a layer of Fe 0.585 Ni 0.415  having a thickness of approximately 3 μm that was electroplated on a laminated seed structure of Fe 0.67 Co 0.29 N 0.04 /Fe 0.55 Ni 0.45 . 
     FIG. 8 is a cutaway cross-sectional view of a merged MR and inductive transducer employing magnetically favorable elements in accordance with the present invention. 
     FIG. 9 is a cutaway cross-sectional view of the transducer of FIG. 8 disposed in close proximity to a media. 
     FIG. 10 is a cutaway cross-sectional view of a transducer  300  designed for perpendicular recording on a media  400 . 
     FIG. 11 is a B/H loop of a prior art FeCoN layer that was formed by sputtering deposition at room temperature. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 is a cutaway cross-sectional view of a sandwich structure  20  made of a primarily iron FeCoN layer  22  affixed between a pair of primarily iron 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 500 Å and has a composition of approximately Fe 0.67 Co 0.29 N 0.04 . The FeNi layers  24  and  26  each have a thickness of approximately 50 Å and have a composition of approximately Fe 0.55 Ni 0.45 . Layers  22 ,  24  and  26  were formed by 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 shows B/H loops  30  and  33  of the sandwich structure  20 . The sandwich structure has a saturation magnetization (B S ) nearly that of the single layer of FeCoN at approximately 23.5 kilogauss. 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 8 Oe while the coercivity of the easy axis is about 18 Oe as shown by loop  33 . 
     FIG. 3 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 500 Å and a composition of approximately Fe 0.67 Co 0.29 N 0.04 . The FeNi layers  42  each have a thickness of approximately 50 Å and have a composition of approximately Fe 0.55 Ni 0.45 . 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 70%, atomic concentrations of cobalt in a range between 25% and 40%, and atomic concentrations of nitrogen in a range between 0.4% and 11%. As another example, the FeNi layers may have atomic concentrations of iron in a range between 50% and 70%, and atomic concentrations of nickel in a range between 50% 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. 4 shows B/H loops  50  and  55  of the laminated structure  40  having eight layers made of FeCoN interleaved with eight layers made of FeNi. The laminated structure has a saturation magnetization (B S ) of approximately 23.5 kilogauss and a permeability of approximately 2500. The laminated structure has a hard axis coercivity of about 1.0 oersted, as shown by loop  50 , and has an easy axis coercivity of about 4 oersted, as shown by loop  55 . The well-defined uniaxial anisotropy and low hard axis coercivity of the laminated structure are suitable for many head applications, such as inductive transducer pole layers and pole tips. 
     The laminated structure was found to be under tensile stress of approximately 450 MPa, versus the compressive stress of approximately −300 MPa noted above for the prior art sandwich structure of Permalloy and FeCoN. The resistivity of the novel laminated structure was measured to be approximately 35 μΩcm, versus 55 μΩcm reported for the prior art sandwich structure of Permalloy and FeCoN. Both of these measurements indicate that the novel laminated structure has a substantially different microstructure than the prior art sandwich structure of Permalloy and FeCoN. A different laminated structure of FeCoN/FeNi having Fe 0.2 Ni 0.8  layers in place of Fe 0.55 Ni 0.45  layers was fabricated, but surprisingly was not as magnetically soft as the higher moment laminated structure characterized in FIG.  4 . 
     FIG. 5 shows a plot of the hard axis coercivity  60 , the easy axis coercivity  64 , and the anisotropy field  66  for laminated structures thicknesses that are varied based upon the numbers of layers contained in the laminated structures. 
     FIG. 6 is a plot of the hard axis coercivity  80 , the easy axis coercivity  84 , and the anisotropy field  86  for samples of the laminated structure described above with reference to FIG. 3 after annealing at various temperatures. A 600 Oe field was maintained in a direction essentially perpendicular to the easy axis of the film layers during annealing. The hard axis coercivity  80  and the easy axis coercivity  84  were essentially unchanged after annealing at up to 250° C. Also observed was that the moment of the samples did not change even after 250° C. annealing. These results suggest that the phase composition of the structure is stable at least to that temperature. The temperature of magnetic recording heads typically does not exceed 100° C., indicating that the laminated structure has good thermal stability for use in pole layers and pole tips. The anisotropy field  86  changes sign at about 180° C., indicating a switch of the hard and easy axis of uniaxial anisotropy at that temperature. 
     For use in a longitudinal recording head having a submicron nonferromagnetic gap separating first and second pole tips, the laminated structure can be sputtered onto the gap on the second or tailing pole tip, with the remainder of that pole tip formed by electroplating. For example, after magnetron sputtering several layers of Fe 0.67 Co 0.29 N 0.04  interleaved with several layers of Fe 0.55 Ni 0.45  to form a laminated structure having a thickness on the order of 400 nm, a conventional plating seed layer such as Permalloy can be sputter-deposited, followed by electroplating with Permalloy or predominantly-iron NiFe to a thickness of 1-3 μm or more. 
     Alternatively, such a novel laminated structure can be used as a seed layer for electroplating high moment materials such as Fe 0.585 Ni 0.415  or CoNiFe to create the remainder of a pole layer. FIG. 7 shows hard axis loop  100  and easy axis loop  110  for a composite structure made of a layer of Fe 0.585 Ni 0.415  having a thickness of approximately 3 μm that was electroplated on a laminated seed structure of Fe 0.67 Co 0.29 N 0.04 /Fe 0.55 Ni 0.45 . The composite structure may have an overall saturation magnetization of approximately 20 kilogauss, with the laminated seed portion having a saturation magnetization of about approximately 23.5 kilogauss. 
     Electroplating directly onto the laminated structure of Fe 0.67 Co 0.29 N 0.04 /Fe 0.55 Ni 0.45  can improve results compared to first depositing a Permalloy seed layer on the laminated structure before electroplating. In addition to providing a more simplified fabrication process, electroplating directly atop the laminated FeCoN/FeNi seed layer can yield better quality materials. For example, deposition of the laminated FeCoN/FeNi seed on a sloped surface, which may occur for instance near an apex region of a trailing pole layer, may produce a lower density film, perhaps due to an orientation mismatch between the growth surface and the growth direction. Such films may then be susceptible to etching by acid treatment prior to electroplating, causing the high Bs laminate in some cases to be rough with nanometer scale voids. Electroplating directly onto this laminate can fill in these voids, yielding a high quality magnetic layer. For the case in which a Permalloy seed layer is first deposited atop the laminated FeCoN/FeNi structure, pin holes in the Permalloy may allow etching of the laminated FeCoN/FeNi structure, which subsequent electroplating does not necessarily fill. In this case voids can remain in the laminated FeCoN/FeNi structure, reducing the quality and durability of the overall magnetic structure. 
     In one embodiment, the laminated structure of Fe 0.67 Co 0.29 N 0.04 /Fe 0.55 Ni 0.45  was formed in a Nordiko 9950 physical vapor deposition (PVD) system at room temperature. The distance between the substrate and the target was about 7 cm or less, and the power was set to 3.5 kW. The Fe 0.67 Co 0.29 N 0.04  layers were formed to a thickness of about 500 Å, and the Fe 0.55 Ni 0.45  layers were formed to a thickness of about 50 Å. 
     Instead of alloys of primarily-iron FeCoN, an additional element can be added that increases resistivity. For example, FeCoRhN/NiFe laminated thin films have been fabricated to have similar magnetic performance to the FeCoN/NiFe laminates described above, but with increased resistivity. More generally, alloys containing (Fe A Co B N C ) (1−Y) X Y , where X is an element from the group including V, Cr, Ir, Ni, Rh, Mo, W, Nb and Ti, having atomic concentrations of A≧0.5, B≦0.5, 0≦C≦0.2, 0≦X≦0.1, with A+B+C=1, are believed to offer favorable magnetic performance. 
     Referring now to FIG. 8, a cutaway cross-sectional view of a merged MR and inductive transducer  140  employing magnetically favorable elements in accordance with the present invention is depicted. Although a merged transducer is depicted, the invention may instead be used for example in a separate inductive transducer, in a piggyback head or in other applications in which magnetically soft, high saturation magnetization materials are desirable. The transducer is formed on a wafer substrate  141 , which may contain Al 2 O 3 , AlTiC, Si, SiC or other conventional materials. A first magnetically soft shield layer  142  is disposed atop the substrate  141 . A first read gap layer  144  composed of electrically insulating, nonmagnetic material such as Al 2 O 3  is disposed on shield layer  142 . 
     A MR sensor  146  is disposed atop the first read gap layer  144 , and a second read gap layer  148  composed of electrically insulating, nonmagnetic material such as Al 2 O 3  is disposed on the MR sensor. The MR sensor  146  may include a single layer of anisotropic magnetoresistive (AMR) material such as Permalloy, or the sensor may contain plural or multiple layers of sensor materials as is known to form a spin valve (SV) sensor, giant magnetoresistive (GMR) sensor, dual stripe magnetoresistive (DSMR) sensor or other known types of sensing mechanisms. In other embodiments, such an MR sensor may involve spin-dependent tunneling (SDT) sensor. The MR sensor  146  may be trimmed to leave an insulating layer  149  formed of a dielectric such as Al 2 O 3  distal to a media-facing surface  170 . 
     A second magnetically soft shield layer  150  is disposed atop the second read gap layer  148 , the second shield layer also serving in this merged transducer as a first write pole  150 . In an alternative embodiment, a first write pole is separated from the second shield layer. The layer  150  may be formed of Permalloy or other materials having higher saturation magnetization, including magnetically soft, primarily-iron NiFe or FeXN, where X is an element such as Ta, Rh, Al, etc. The layer  150  may be electroplated and then polished to form a smooth surface, upon which a first laminated FeCoN/FeNi structure  151  can be formed. The laminated structure  151  includes layers of primarily-iron FeCoN interleaved with layers of primarily iron FeNi, and provides a magnetically soft high moment material upon which a submicron nonferromagnetic gap layer  152  is formed. The gap layer  152  separates the first write pole  150  from a second write pole layer  160 , and magnetic flux communicated between the pole layers  150  and  160  can fringe out from the gap layer to write magnetic pattern on an adjacent media. 
     A second laminated FeCoN/FeNi structure  162  is formed on the gap layer  152 . The laminated structure  162  includes layers of primarily-iron FeCoN interleaved with layers of primarily-iron FeNi, and provides a second magnetically soft high moment layer adjoining the nonferromagnetic gap layer  152 . Having the laminated FeCoN/FeNi structure  162  adjoining a trailing edge of the gap layer  152  allows sharply defined, high density magnetic patterns to be written onto an adjacent media. Materials from which gap layer  152  can be made include dielectric materials such as Al 2 O 3  or SiO 2  or metals such as chromium or tantalum. The gap layer  152  can serve as a seed layer promoting favorable deposition of the laminated FeCoN/FeNi structure  162 . 
     An electrically conductive coil  155  is formed on an insulating layer  157  for inductively writing and/or reading signals via the pole layers. The coil is surrounded by baked photoresist  154  that provides insulation between coil sections and also provides a sloped surface that allows the pole layers  150  and  160  to be separated by several microns adjacent the coil  155  and less than one micron adjacent the media-facing surface  170 . In another embodiment, a second pole layer can be substantially flat, with the magnetic core brought close to the gap by an additional magnetic layer, which may be termed a pedestal, adjoining either or both of the pole layers. In yet another embodiment, such a pedestal can be formed adjoining a second pole layer that curves in a similar fashion as pole layer  160 , with the pedestal and pole layer stitched together adjacent to the media-facing surface  170 . 
     The second laminated FeCoN/FeNi structure  162  is formed in a plurality of layers, beginning with a sputter-deposited layer of primarily-iron NiFe, which may be formed to a thickness of 20 Å-50 Å and at room temperature. Primarily-iron NiFe is defined in the present invention to be an alloy having a greater atomic concentration of iron than nickel, such as Fe 0.55 Ni 0.45 . Alternatively, the layer  162  may be formed on a layer of Cr having a similar thickness, in order to promote favorable growth of subsequently formed Fe 0.55 Ni 0.45 . In this case, or for the situation in which the gap layer  152  is made of Cr, it may be possible to form an initial layer of the laminated FeCoN/FeNi structure  162  of primarily-iron FeCoN. After formation of at least three layers of primarily-iron FeCoN interleaved with at least three layers of primarily-iron FeNi, a remainder  166  of pole layer  160  may be formed by electroplating. The electroplated layer  166  may be formed of Permalloy or other materials having higher saturation magnetization, including magnetically soft, primarily-iron NiFe or CoNiFe. Alternatively, pole layer  160  may be formed entirely of a magnetically soft laminated FeCoN/FeNi structure having high saturation magnetization, which may be feasible due to the relatively high deposition rate of approximately 22 Å/s afforded by magnetron sputtering, so that for example a laminated structure a few microns in thickness can be formed in less than one hour. 
     After formation of second pole layer  160 , that layer may be masked and trimmed by a directional etching process such as IBE to define a trailing pole tip. The etching may be designed to also cut into the first pole layer  150 , creating a leading pole tip that is aligned with the trailing pole tip. A protective coating layer  168  of Al 2 O 3 , diamond like carbon (DLC) or other hard materials is then formed on what will become a trailing end  175  of the head, after which the wafer substrate  141  and formed layers are diced into thousands of heads. A protective coating  168  has also been formed on the media-facing surface  170  of the transducer. The media-facing surface  170  is formed along one die edge. One should note that the MR sensor  146  may be formed after the formation of the inductive core that includes write poles  150  and  160 , affording higher temperature processing of the write poles. In an alternative embodiment, sensing is performed inductively with the same transducer elements that are used to write magnetic patterns on the media, without the need for a MR sensor. 
     FIG. 9 shows the merged transducer  140  disposed in close proximity to a media  200  which is moving relative to the head as shown by arrow  202 , from a leading end to the trailing end  175  of the head. The media  200  may be a disk or tape, for example, which includes a media layer or layers  205  disposed atop a substrate  208 , with an overcoat layer  210  protecting the media layer  205 . The write poles  150  and  160  form a magnetic circuit or loop to encourage the flow of magnetic flux across the write gap layer  152 . An electrical current flowed through the coil  155  induces a magnetic flux in the write layers that travels across the nonferromagnetic gap  152  to write a magnetic bit in the media layer  205 . The MR sensor  146  can read magnetic bits that have been written on the media. 
     FIG. 10 is a cutaway cross-sectional view of a transducer  300  designed for perpendicular recording on a media  400 . The transducer  300  is formed on a wafer substrate  302 , and has a generally U-shaped ferromagnetic core  305  that includes a relatively large flux return pole layer  308 , a smaller write pole layer  312 , and a magnetic stud layer  310  connecting the pole layers  308  and  312 . A conductive coil including three layers in this embodiment winds around the magnetic stud  310 . An MR sensor  320  is disposed adjacent a media-facing surface  325  between a magnetically soft shield layer  318  and the write pole layer  312 . In this design as well as the previous embodiment a thin protective coating layer may be disposed on the media-facing surface  325 , protecting the MR sensor  320  from damage and corrosion. 
     The media  400  may be a disk or tape, for example, which includes a media layer or layers  402  disposed atop a magnetically soft underlayer  404 , which in turn is disposed atop a substrate  408 . The underlayer  404  and core  305  form a circuit or loop for magnetic flux  420  that writes information on the media layers  402 . The media  400  moves past the transducer  300  as shown by arrow  422 , so that a leading end of the head encounters a portion of the media before a trailing end  375  of the head, the leading end formed by an edge of substrate  302  that is not shown in this drawing to instead illustrate the magnetic elements. The MR sensor  320  can read magnetic bits that have been written on the media  400 . 
     The write pole layer  312  in this embodiment includes a first layer  321  that has been formed primarily by electroplating, and a second section  322  that is made of a laminated FeCoN/FeNi structure, each layer of the structure  322  including an atomic concentration of iron that exceeds 50%. In this type of perpendicular head the magnetic flux  420  is concentrated adjacent the relatively small write pole layer  312  and is spread out adjacent the relatively large flux return pole layer  308 . The trailing end of the write pole layer  312  is the last magnetic element encountered by the media  400  that moves past the transducer  300 . Having a magnetically soft, high magnetic saturation structure  322  disposed at the trailing end of the write pole layer  212  allows concentrated magnetic flux  420  adjacent to that structure  322  to leave a sharp and lasting magnetic pattern on the media  400 . The underlayer  404  may also include magnetically favorable elements containing FeCoN/FeNi layers. 
     Although the present disclosure has focused on teaching the preferred embodiments, other embodiments and modifications of this invention will be apparent to persons of ordinary skill in the art in view of these teachings. For example, the primarily-iron laminates of FeCoN/FeNi can be employed in various devices that benefit from high saturation magnetization, magnetically soft materials, such as magnetic sensors, magnetic force microscopes, magnetic switches, etc. Therefore, this invention is to be limited only by the following claims, which include all such embodiments and modifications when viewed in conjunction with the above specification and accompanying drawings.