Abstract:
A perpendicular writer includes a write coil and a main pole having a pole tip for conducting magnetic flux to write data to a magnetic medium. The pole tip includes a plurality of magnetic layers that are magnetically coupled and biased so that their magnetic moment orientations are substantially parallel to an external surface when no write current is applied to the write coil. The pole tip is partially embedded in the yoke, such that portions of the yoke surrounding the pole tip help direct magnetic flux to the pole tip. The pole tip extends beyond the yoke, with a first end located outside the yoke and a second end located within the yoke.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     None. 
     FIELD OF THE INVENTION 
     The present invention relates generally to the field of electronic data storage and retrieval, and more particularly to a device for emitting magnetic flux, such as a perpendicular magnetic writer for suppressing on-track erasure and increasing switching speed. 
     BACKGROUND OF THE INVENTION 
     Perpendicular recording can support much higher linear density than longitudinal recording due to lower demagnetizing fields in recorded bits, which diminish as linear density increases. To provide decent writeability, double layer media are used. The double layer perpendicular media consist of a high coercivity, thin storage layer with perpendicular to-plane anisotropy and a soft magnetic keeper (underlayer) having in-plane anisotropy and relatively high permeability. 
     A magnetic head for perpendicular recording generally consists of two portions, a writer portion for writing magnetically-encoded information on a magnetic media (disc) and a reader portion for retrieving magnetically-encoded information from the media. The reader portion typically consists of a bottom shield, a top shield, and a sensor, often composed of a magnetoresistive (MR) material, positioned between the bottom and top shields. Magnetic flux from the surface of the disc (media) causes rotation of the magnetization vector of a sensing layer of the MR sensor, which in turn causes a change in electrical resistance of the MR sensor. The change in resistance of the MR sensor can be detected by passing a current through the MR sensor and measuring a voltage across the MR sensor. External circuitry then converts the voltage information into an appropriate format and manipulates that information as necessary to recover the data that was encoded on the disc. 
     The writer portion of the magnetic head for perpendicular recording typically consists of a main pole and a return pole which are magnetically separated from each other at an air bearing surface (ABS) of the writer by a non-magnetic gap layer, and which are magnetically connected to each other in a region at a distance away from the ABS. Positioned at least partially between the main and return poles are one or more layers of conductive coils encapsulated by insulating layers. The ABS is the surface of the magnetic head immediately adjacent to the perpendicular medium. The writer portion and the reader portion are often arranged in a merged configuration in which a shared pole serves as both the top or bottom shield of the reader portion and the return pole of the writer portion. 
     To write data to the magnetic media, an electrical current is caused to flow through the conductive coil, thereby inducing a magnetic field across the write gap between the main and return poles. The main and return poles are made of soft magnetic materials. Both the main and return pole may generate magnetic field in the media during recording when the write current is applied to the coil. However, the main pole produces a much stronger write field than the return pole by having a much smaller cross-sectional area at the ABS. A magnetic moment of the main pole should be oriented along an easy axis parallel to the ABS when the main pole is in a quiescent state, namely without a current field from the write coil. When the magnetic moment does not return to an orientation parallel to the ABS after being subjected to one or multiple excitations of the write current field, the main pole is not stable. In an unstable pole, the orientation of the magnetic moment generally remains nonparallel to the ABS even after current to the write coil is turned off. Thus, the main pole in the quiescent state may still emit a magnetic flux and may deteriorate or even erase data from the disc. Further, an unstable pole results in increased switching time when a write current is applied. In a perpendicular head, the main pole is a predominant source of remanent magnetism due to a strong shape anisotropy perpendicular to the ABS. 
     Accordingly, there is a need for a writer with minimal remanent magnetization when the write current is switched off. Such a stable writer will reduce switching time, increase data rate of the disc drive, and prevent unintentional erasing on perpendicular media after the write current is turned off. 
     BRIEF SUMMARY OF THE INVENTION 
     A device for emitting magnetic flux, such as perpendicular writer, includes a write coil and a pole. The pole includes a yoke and a pole tip connected to the yoke. The yoke is formed of magnetic material. The pole tip includes a plurality of magnetic layers, each layer having a first side and a second side. The plurality of magnetic layers are magnetically coupled and biased so that their magnetic moment orientations are substantially parallel to an external surface of the pole tip when no write current is applied to the write coil. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a top perspective view of a disc drive. 
         FIG. 2  is a cross-sectional view of a portion of the transducer of the present invention. 
         FIG. 3  is a perspective view of one embodiment of a main pole of the present invention. 
         FIG. 4  is a partial perspective view of another embodiment of a main pole of the present invention. 
         FIG. 5  is a partial perspective view of another embodiment of a main pole of the present invention. 
         FIG. 6  is a partial perspective view of another embodiment of a main pole of the present invention. 
         FIG. 7  is a partial perspective view of another embodiment of a main pole of the present invention. 
         FIG. 8  is a partial perspective view of another embodiment of a main pole of the present invention. 
         FIG. 9  is a partial perspective view of another embodiment of a main pole of the present invention. 
         FIG. 10  is a partial cross-sectional view of another embodiment of the writer of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a top perspective view of disc drive  12 , which includes voice coil motor (VCM)  13 , actuator arm  14 , suspension  16 , flexure  18 , slider  20 , head mounting block  22 , and disc or media  24 . Slider  20  is connected to the distal end of suspension  16  by flexure  18 . Suspension  16  is connected to actuator arm  14  at head mounting block  22 . Actuator arm  14  is coupled to VCM  13 . As shown on the right side of  FIG. 1 , disc  24  has a multiplicity of tracks  26  and rotates about axis  28 . 
     During operation of disc drive  12 , rotation of disc  24  generates air movement which is encountered by slider  20 . This air movement acts to keep slider  20  aloft a small distance above the surface of disc  24 , allowing slider  20  to fly above the surface of disc  24 . VCM  13  is selectively operated to move actuator arm  14  around axis  30 , thereby moving suspension  16  and positioning the transducing head (not shown) carried by slider  20  over tracks  26  of disc  24 . Proper positioning of the transducing head is necessary for reading and writing data on concentric tracks  26  of disc  24 . 
       FIG. 2  is a cross-sectional view of an embodiment of a magnetic writer  36  of the present invention. Medium  24  for recording comprises thin storage layer  32  having high coercivity and perpendicular anisotropy (the magnetization is held in a direction substantially normal to the surface of medium  24 ) and soft magnetic underlayer or keeper  34  having high permeability and in-plane orientation of the easy axis. In an exemplary embodiment, writer  36  comprises main pole  38  and return pole  40 , connected to each other by back gap closure  42  at a distal end and separated from each other by write gap  46  at the ABS. At least one write coil  44  positioned proximate main pole  38  conducts current around main pole  38 , thereby intermittently inducing a magnetic field in main pole  38 . 
     Transducer main pole  38  serves as a trailing pole for the given direction of motion  47  of medium  24 . Magnetization transitions on medium  24  are recorded by trailing edge  49  of main pole  38 . Main pole  38  includes laminated main pole tip  45  partially embedded in yoke  48 . Main pole  38  includes a first end including main pole tip  45  and a second, opposite end. In one embodiment, the first end of main pole  38  defines a plane at the air bearing surface. Yoke  48  has a first end, a second end, and four sides (see  FIG. 3 ). In the illustrated embodiment, main pole tip  45  is attached to the first end of yoke  48 . However, main pole tip  45  could also be attached to any of the four sides of yoke  48 . In an exemplary embodiment, magnetic layer  50 , shown here as the top layer of main pole tip  45 , contains trailing edge  49 . Generally, the ABS surface of main pole tip  45  is also an external surface of the transducer. In an exemplary embodiment, main pole tip  45  has a submicron width at the ABS, to provide recording of ultra-narrow tracks on medium  24 . In an exemplary embodiment, main pole  38  is at least partially embedded in yoke  48 . The portions of yoke  48  surrounding main pole tip  45  help to direct the magnetic flux from relatively wide yoke  48  to relatively narrow pole tip  45 . The proposed structure of main pole tip  45  increases the uniaxial anisotropy of magnetic layers  50  and  54 , thereby rendering main pole  38  more magnetically stable. Lamination of only pole tip  45  of main pole  38  may lead to savings in production costs and materials as well as an efficient writer  36 . Flux easily travels through the bulk magnetic material of yoke  48  without disruption from lamination interfaces in the length of yoke  48 . 
     To write data to perpendicular magnetic medium  24 , a time-varying write current is caused to flow through coil  44 , which in turn produces a time-varying magnetic field through main pole tip  45  and return pole  40 . Medium  24  is then passed by the ABS of writer  36  at a predetermined distance such that medium  24  is exposed to the magnetic field. With perpendicular writer  36 , the soft magnetic keeper  34  of magnetic medium  24  in essence acts as a third pole of the writer. 
     A closed path for magnetic flux from writer  36  to medium  24  travels from main pole  38  through storage layer  32  of medium  24  to soft magnetic keeper  34  and returns to writer  36  through return pole  40 , again passing through storage layer  32 . To ensure that the magnetic field does not write data on this return path, the surface area of return pole  40  at the ABS is preferably substantially larger than the surface area of main pole tip  45  at the ABS. Thus, the strength of the magnetic field affecting storage layer  32  under return pole  40  will not be sufficient to overcome a nucleation field of storage layer  32 . In a preferred embodiment, the thickness of main pole tip  45  is between about 0.05 and about 1 micrometer. The total cross-sectional area at the ABS of return pole  40  is preferably greater than 10 times and more preferably greater than 100 times the total cross-sectional area of all the magnetic layers of main pole tip  45 . 
     In an exemplary embodiment, main pole tip  45  has a multilayer structure. Further, in one embodiment, main pole tip  45  is partially embedded in yoke  48 . Multilayer main pole tip  45  preferably comprises magnetic layer  50 ; non-magnetic spacer layer  52 ; and magnetic layer  54  (shown here as an underlayer). 
     This multilayer pole structure induces anisotropy in both magnetic layers  50  and  54  parallel to the external surface or ABS, thereby enhancing the magnetic stability of main pole tip  45  while suppressing on-track erasure and increasing switching speed and ultimately increasing the data recording rate and reliability. When top magnetic layer  50  is coupled with magnetic underlayer  54  across non-magnetic layer  52  according to the present invention, the properties of the coupled multilayer system improve the performance of main pole tip  45  compared with a main pole tip made of a single layer of high magnetic moment material. While the layers of main pole tip  45  are illustrated as planar layers, it is contemplated that they may follow other contours. Additionally, the illustrations are not rendered to scale. 
     Any suitable ferromagnetic materials may be used for magnetic layer  50  and magnetic underlayer  54 . The materials for each layer may be the same or they may be different. The materials are preferably magnetically soft, with a preferred coercivity less than about 5 Oersted and more preferably less than about 1 Oersted. The chosen materials preferably have well defined magnetic anisotropy, meaning that they have a stable orientation of the easy axis of magnetization parallel to the ABS. In a preferred embodiment, magnetic layers  50  and  54  are made of CoFe, CoNiFe, FeCoN, CoNiFeN, FeAlN, FeTaN, FeN, NiFe (e.g. Ni 80 Fe 20 , Ni 45 Fe 55 , etc.), NiFeCr, NiFeN, CoZr, CoZrNb, FeAlSi, a permalloy, CoZrTa or another suitable material. Magnetic layers  50  and  54  can be of any suitable thickness for use in writer  36 ; they are preferably each between about 0.01 and about 1 micrometer thick, and more preferably between about 0.1 to about 0.5 micrometer thick. 
     Non-magnetic spacer  52  may be composed of any non-magnetic material which is mechanically and chemically compatible with the magnetic materials used for top magnetic layer  50  and magnetic underlayer  54 . In an exemplary embodiment, non-magnetic spacer  52  between top magnetic layer  50  and magnetic underlayer  54  results in formation of an antiferromagnetic (AFM) exchange coupling between top magnetic layer  50  and magnetic underlayer  54 . This coupling reinforces the anisotropy of top magnetic layer  50  and magnetic underlayer  54  oriented parallel to the ABS, resulting in a more stable main pole tip  45  due to a reduction in magnetic energy of the parallel state. 
     According to the RKKY (Ruderman-Kittel-Kasuya-Yosida) interaction, non-magnetic spacer  52  composed of some materials induces a periodic coupling between top magnetic layer  50  and magnetic underlayer  54 . The coupling alternates characteristics between antiferromagnetic and ferromagnetic as a function of the thickness of non-magnetic spacer  52  and depends upon the crystallographic orientation of the material of non-magnetic spacer  52 . Suitable non-magnetic materials for non-magnetic spacer  52  include, for example, copper, ruthenium, gold, tantalum, aluminum, rhodium, chromium, copper-silver alloys, nitride, carbide and various oxides, including aluminum oxide and silicon dioxide. Preferred non-magnetic materials are those which provide for antiferromagnetic exchange coupling between the adjacent magnetic layers, such as copper, ruthenium, gold, rhodium, chromium, and copper-silver alloys. 
     In a preferred embodiment, the thickness of non-magnetic spacer  52  is chosen to induce an antiferromagnetic coupling between top magnetic layer  50  and magnetic underlayer  54 . If the thickness in the proximity corresponding to the first antiferromagnetic peak results in a non-magnetic spacer which is too thin to be practical, then the thickness of non-magnetic spacer  52  in the proximity corresponding to the next antiferromagnetic peak can be used, and so on. In one embodiment, Cu or Ru spacer  52  has a thickness of about 1 to about 200 Angstroms (Å), more preferably a thickness of about 3 to about 30 Å, and most preferably a thickness of about 6 to about 25 Å. 
     Any suitable material may be used for yoke  48 , which is used in one embodiment to increase efficiency of writer  36 , as well as enhance the anisotropy and structural and magnetic integrity of multilayer main pole tip  45 . A length of main pole tip  45  is preferably about 0.02 to about 4.0 micrometers. Thus, yoke  48  is preferably recessed from the ABS by a distance of about 0.02 to about 4.0 micrometers so that yoke  48  does not contribute to an increased track width of main pole tip  45  at the ABS. A narrow track width of main pole tip  45  at the ABS allows for high track density recording and a narrow thickness of main pole tip  45  reduces skew-related side writing effects. In one embodiment, a length of yoke  48  is preferably about 8 to about 50 times the length of main pole tip  45 . 
     In one embodiment, yoke  48  is made of a magnetic material such as CoNiFe, FeCoN, CoNiFeN, FeAlN, FeTaN, FeN, NiFe (e.g. Ni 80 Fe 20 , Ni 45 Fe 55 , etc.), NiFeCr, NiFeN, CoZr, CoZrNb, CoZrTa, FeAlSi, or other suitable materials. The chosen material preferably has well defined magnetic anisotropy, meaning that it has well defined easy and hard magnetic axes. The material is preferably magnetically soft, with a preferred coercivity less than about 5 Oersted and more preferably less than about 1 Oersted. Yoke  48  preferably has a relatively large magnetic permeability more than about 500, and more preferably more than about 1000. 
     In an exemplary embodiment, the portions of yoke  48  surrounding main pole tip  45  are shown as tapered wedges; however, they can also embody other configurations, such as graduated layers, for example. Additionally, while portions of yoke  48  are illustrated as being positioned above and below main pole tip  45 , they can also be disposed on either side of main pole tip  45  in a case where yoke  48  is wider than main pole tip  45  in a lateral dimension. 
     Any suitable magnetic material may be used for back gap closure  42 . In a preferred embodiment, back gap closure  42  is constructed of a soft magnetic material such as CoNiFe, NiFe, Ni 80 Fe 20 , Ni 45 Fe 55 , NiFeCr, CoZr, FeN, FeAlSi, or other suitable materials. 
       FIG. 3  is a partial perspective view of an embodiment of a main pole of the present invention, viewed from a bottom of the pole tip. Main pole  38  includes main pole tip  45  partially embedded in yoke  48 . Main pole tip  45  is preferably centered on yoke  48  to most efficiently conduct the flux flowing from yoke  48  and through pole tip  45 . Main pole tip  45  of the present invention has first magnetic layer  50  with a first magnetic moment orientation and second magnetic layer  54  with a second magnetic moment orientation. While the terms “first” and “second” are used for discussion purposes, it is to be understood that the order of the layers may be reversed or otherwise altered. In an exemplary embodiment of main pole tip  45 , the magnetic moment orientations or anisotropies of magnetic layers of  50  and  54  are fixed in a direction parallel to the bottom or external surface of the pole tip when the write current is off, thereby reducing unwanted erasure by reducing the remanent magnetization. This directional bias can be accomplished by means including but not limited to the choices of materials for the magnetic and non-magnetic layers, the thicknesses of the magnetic and non-magnetic layers, the application of stress or magnetostriction, the directional deposition of the materials of the magnetic and non-magnetic layers with low glancing angles, and the use of antiferromagnets or permanent magnets. By orienting the moments of magnetic layers  50  and  54  in a direction parallel to the ABS, for example, residual magnetization and the remanence charge left on the tip  45  of main pole  38  of perpendicular writer  36  are minimized when the writing current is off. 
     In one exemplary embodiment, the material of magnetic layer  50 , which is proximate trailing edge  49 , is made of a material with a higher saturation magnetic moment than the material of magnetic underlayer  54 . Because the strength of the write field in the media is proportional to the magnetic moment of the main pole material, it is desirable to use a material with a high magnetic moment (or high flux density saturation) for construction of main pole tip  45  for ultra high track density recording. When the magnetic moment of the main pole material is increased, a track width and thickness of the main pole tip can be reduced for increasing the storage capacity of the disc drive while reducing skew-related effects. In an exemplary embodiment, magnetic layers  50  and  54  are antiferromagnetically exchanged coupled through nonmagnetic layer  52 . The antiferromagnetic coupling of the high magnetic moment material of top magnetic layer  50  by lower magnetic moment material  54  leads to greater overall stability in main pole tip  45  while retaining the high writability and high data rate advantages of using the high magnetic moment material. 
       FIG. 4  is a partial perspective view of another embodiment of a main pole tip of the present invention. In the illustrated embodiment of main pole tip  45  having trailing edge  49 , anisotropy in magnetic layers  50  and  54  parallel to the plane of the ABS is induced by antiferromagnetic pinning layer or anisotropy inducing magnetic layer  62  contacting magnetic layer  50 . Magnetic layer  62  induces the magnetic moment orientations of magnetic layers  50  and  54  into orientations substantially parallel to the ABS in the absence of a write current. Anisotropy inducing layer  62  is composed of an antiferromagnet or permanent magnet, for example. Examples of suitable antiferromagnets include Cr, NiO, MnO, IrMn, PtMn, NiMn, IrMnX, PtMnX, and NiMnX; where X represents a third element. Examples of suitable permanent magnets include Co; CoCr; CoPt; CoCrPt; MFe 2 O 4 , where M represents any one of several metallic elements; Fe 3 O 4 ; AB 12 O 19 , where A is a divalent metal such as Ba, Sr, or Pb, and B is a trivalent metal such as Al, Ga, Cr or Fe; and M 3 Fe 5 O 12 , where M is a rare earth ion such as Sm, Eu, Gd or Y. 
       FIG. 5  is a partial perspective view of another embodiment of a main pole tip of the present invention. In the illustrated embodiment of main pole tip  45 , anisotropy inducing layers  62  are disposed adjacent to the magnetic layer  50  and magnetic layer  54 . This configuration is useful where stronger pinning coupling is desirable to induce the orientations of the magnetic moments of magnetic layers  50  and  54  into directions parallel to the ABS. 
       FIG. 6  is a partial perspective view of another embodiment of a main pole tip of the present invention. In this embodiment, anisotropy inducing layers  62  are disposed on one or both sides of main pole  38 , spaced apart from main pole tip  45 , but in close proximity so that the magnetic fields of anisotropy inducing layers  62  act upon magnetic layers  50  and  54  in the absence of a write current. Those skilled in the art will appreciate that anisotropy inducing layers  62  could be disposed above or below main pole  38 . In these embodiments, a distance between main pole tip  45  and each layer  62  is between about 10 nm and about 80 nm. These configurations are especially useful when it is desirable to keep the surface area of pole tip  45  as small as possible for increasing efficiency and reducing skew-related effects. In an exemplary embodiment, a cross sectional area of main pole tip  45  at the ABS is less than about 10,000 nm 2 . The anisotropy inducing layers  62  switch the direction of the magnetic moments in main pole tip  45  into directions parallel to the ABS after the writing current is switched off, thereby decreasing or eliminating on-track erasure. In one embodiment, the longitudial biasing field provided by the biasing layers  62  is between about 5 Oe and about 2000 Oe, which is generally larger than the coercivity of the materials of writer  36  and generally smaller than the perpendicular field generated by coils  44 , resulting in a decrease in on-track erasure while resulting in minimal interference with the recording process. 
       FIG. 7  is a partial perspective view of another embodiment of a main pole tip of the present invention. Main pole tip  45  of  FIG. 7  is similar to main pole tip  45  of  FIG. 3 , except that the embodiment illustrated in  FIG. 7  includes additional magnetic layers  66 ,  70 ,  74  and  78  and additional non-magnetic layers  64 ,  68 ,  72  and  76 . In the illustrated embodiment, each magnetic layer  50 ,  54 ,  66 ,  70 ,  74  and  78  is antiferromagnetically coupled to an adjacent magnetic layer so that the moments of adjacent layers align anti-parallel to each other. In one exemplary embodiment, a magnetic moment gradient from trailing edge  49  is created by disposing magnetic layers of higher moment closer to trailing edge  49  and lower moment layers further away from trailing edge  49 . While the multilayer structure of  FIG. 7  leads to better performance uniformity than the structure of  FIG. 3 , due to the stronger overall levels of coupling between the magnetic layers  50 ,  54 ,  66 ,  70 ,  74  and  78 , a large magnetic field is needed to saturate main pole  38  during the writing process. 
     In order to make it easier to saturate main pole  38 , different materials can be chosen for non-magnetic layers  52 ,  64 ,  68 ,  72  and  76  to selectively determine the strength of coupling between the magnetic layers in one embodiment. For example, a conductive material such as a transition metal such as copper, ruthenium, gold, rhodium, or chromium, for example, can be used in non-magnetic layers  52 ,  68  and  76  to promote a relatively strong anti-parallel exchange coupling between magnetic layers  52  and  54 , between magnetic layers  66  and  70 , and between magnetic layers  74  and  78 . Other non-magnetic materials, including transition metals of certain thicknesses, will cause only a weak antiferromagnetic exchange coupling between the two magnetic layers on either side of the non-magnetic layer. For example, an electrically insulating material such as tantalum, aluminum oxide, nitride, carbide, or silicon dioxide, for example, can be used for non-magnetic layers  64  and  72  to separate the sets of strongly coupled magnetic layers. In another example, Ru with a thickness of about 5 Å to about 10 Å is used in non-magnetic layers  52 ,  68  and  76  to promote a relatively strong anti-parallel exchange coupling between magnetic layers  52  and  54 , between magnetic layers  66  and  70 , and between magnetic layers  74  and  78 . Ru with a thickness of about 12 Å to about 18 Å, which causes only a weak antiferromagnetic exchange coupling between the two magnetic layers on either side of the non-magnetic layer, is used in non-magnetic layers  64  and  72  to separate the sets of strongly coupled magnetic layers. With such configurations, each magnetic layer is antiferromagnetically coupled to an adjacent magnetic layer, but main pole  38  is easier to saturate during the writing process because the overall coupling strength of main pole tip  45  is decreased. This leads to higher writer efficiency while reducing on-track erasure. 
       FIG. 8  is a partial perspective view of yet another embodiment of a main pole tip of the present invention. Non-magnetic layer materials  64  and  72  couple but do not antiferromagnetically couple the magnetic layers on either side of the non-magnetic layers. Therefore, the moments of magnetic layers  54  and  66  point in the same direction, and the moment of magnetic layers  70  and  74  point in the same direction in the illustrated example. However, the magnetic layers are still arranged so that each pair of magnetic layers is antiferromagnetically coupled. For example, magnetic layers  50  and  54  are antiferromagnetically coupled by non-magnetic layer  52 ; magnetic layer  66  and  70  are antiferromagnetically coupled by non-magnetic layer  68 ; and magnetic layer  74  and  78  are antiferromagnetically coupled by non-magnetic layer  76 . Since each magnetic layer has only one of its surfaces relatively strongly antiferromagnetically exchange coupled, the overall effective coupling strength of main pole tip  45  is reduced, making it easier to saturate main pole  38  during the writing process by requiring a smaller magnetic field, thereby leading to greater efficiency. 
       FIG. 9  is a partial perspective view of another embodiment of a main pole tip of the present invention. Main pole tip  45  of  FIG. 9  is similar to main pole tip  45  of  FIG. 7  except that the embodiment of  FIG. 9  includes a trapezoidal shape of main pole tip. In this embodiment a magnetic moment gradient from trailing edge  49  is created by disposing wider layers of magnetic materials closer to trailing edge  49  and narrower layers of magnetic materials further away from trailing edge  49 . Thus, a gradient can be created even if each magnetic layer is made of materials with similar magnetic moment values. 
       FIG. 10  is a partial cross-sectional view of another embodiment of the writer of the present invention. Writer  82  of  FIG. 10  is similar to writer  36  of  FIG. 2 , except that the embodiment illustrated in  FIG. 10  includes an additional return pole  40  connected to main pole  38  by an additional back closure  42 . This configuration reduces side erasure because the flux flowing through main pole  38  is directed to both return poles  40 , effectively reducing the side flux effects by half. Writer  82  thereby increases writer efficiency while reducing both remanent erasure and side erasure effects. 
     Although the present invention has been described with reference to exemplary embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.