Abstract:
A CPP MR sensor interposes a tapered soft magnetic flux guide (FG) layer between a hard magnetic biasing layer (HB) and the free layer of the sensor stack. The flux guide channels the flux of the hard magnetic biasing layer to effectively bias the free layer, while eliminating instability problems associated with magnetostatic coupling between the hard bias layers and the upper and lower shields surrounding the sensor when the reader-shield-spacing (RSS) is small.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to the fabrication of a MR sensor. In particular it relates to an MR sensor in which a hard bias layer can be reduced in thickness and enhanced in effect by means of a flux guide structure. 
     2. Description of the Related Art 
     With the ever increasing areal density with which data is stored on magnetic media such as disks in hard disk drives (HDD), the magneto-resistive (MR) sensor that is used as the read-back element in the HDD is required to have a correspondingly improved spatial resolution while achieving and maintaining a reasonable signal-to-noise ratio (SNR). Referring to schematic  FIGS. 1   a ,  1   b  and  1   c , there are shown three views of a generic, prior-art current-perpendicular-to-plane (CPP) MR read head.  FIG. 1   a  illustrates the read head in a vertical cross-sectional plane parallel to its air bearing surface (ABS) plane.  FIG. 1   b  illustrates the read head from an overhead view of a horizontal cross-sectional plane through its magnetically free layer (discussed below).  FIG. 1   c  is a portion of the illustration of  FIG. 1   a , isolating the sensor stack portion of the head. 
     Referring to  FIG. 1   a , there is shown the CPP MR head, which could be a CPP-GMR head (current perpendicular-to-plane giant-magneto-resistive head), in which there is a current that passes perpendicularly to the active magnetic layers through the entire head structure and in which the resistance of the head varies in accord with the physical principles of the giant-magneto-resistive effect. Alternatively, the head could be a CPP-TMR (current perpendicular-to-plane tunneling magneto-resistive) head, in which there is a current that passes perpendicularly to the active magnetic layers through the entire head structure and in which the resistance of the head varies in accord with the physical principles of the tunneling-magneto-resistive effect. Either of these particular types of head, which are state-of-the-art read-back heads, will be well represented by the discussion that follows. 
       FIG. 1   a  shows the following physical elements of the generic prior art head. Looking vertically downward, there is first shown an upper (or top) shield ( 1 ) that protects the sensor stack ( 6 ) from extraneous magnetic fields. At the bottom of the head, there is shown a corresponding lower (or bottom) shield ( 2 ) that performs a similar task at the bottom edge of the sensor. Thus the sensor is protected by a pair of shields at some desired separation ( 3 ). 
     Hard bias (HB) magnets ( 4 ) (magnets formed of hard magnetic material) are laterally disposed to either side of the sensor stack ( 6 ). These magnets, which stabilize the magnetization of the free layer ( 8 ) are positioned between the shields ( 1 ), ( 2 ) and their magnetizations are shown as arrow ( 5 ). These hard magnetic layers are formed on underlayers ( 20 ) that promote the requisite crystalline anisotropy. The sensor stack itself ( 6 ) is typically formed as a patterned lamination of five horizontal layers, formed beneath an upper capping layer ( 18 ). An arrow ( 7 ) shows the direction of magnetization of the magnetically free layer of the sensor stack, as seen in  FIG. 1   c.    
       FIG. 1   b  is a horizontal cross-sectional slice through the two HB layers ( 4 ) and the magnetically free layer ( 8 ) of the sensor stack, as will be discussed below. 
     Referring to  FIG. 1   c , there is shown a schematic, illustration of the isolated sensor stack ( 6 ) of  FIG. 1   a  showing the following five horizontal layers: the magnetically free layer ( 8 ), showing it magnetization vector as an arrow ( 7 ); a layer ( 9 ) that is a dielectric layer that serves as a tunneling barrier layer for the TMR sensor, or is a conducting layer ( 9 ) for the GMR type sensor, a reference layer ( 10 ), a coupling layer (eg. a layer of Ru) ( 11 ), a pinned layer ( 12 ) whose magnetization is held spatially fixed by a thick layer ( 19 ) of antiferromagnetic material that also pins layer ( 10 ). The hard biasing layers ( 4 ), with longitudinal magnetization ( 5 ), provides a biasing magnetic field in the sensor stack ( 6 ) to orient the magnetization ( 7 ) of the free layer ( 8 ) in a longitudinal direction. A capping layer ( 18 ) is positioned between the free layer ( 8 ) and the upper shield ( 1 ). In forming the sensor, the stack and the hard bias layers are defined by a single etching process that insures they are at the same height. 
     In the most modern disk drives, the height at which the head flies above the rotating disk is already less than 5 nm, so the freedom of further flying height reduction to increase spatial resolution is reaching its limit. Thus, the common practice to increase the resolution is by reducing the reader-shield-spacing (RSS) ( 3 ), so the magnetic spatial resolution increases correspondingly. 
     To reduce RSS, the thickness of the hard bias (HB) layers ( 4 ) will need to decrease as well. However, reducing the thickness of these magnetic HB layers will also reduce the amount of magnetic “charge” at the edges of the HB layers immediately adjacent to the sensor stack and facing the edges of the free layer. The fewer the magnetic charges, the less is the pinning field of the HB layers and the less effective they are at orienting the free layer magnetization. Meanwhile, with a much smaller spacing between the HB layers and the upper read shield ( 1 ), magnetostatic coupling between the HB layer and the upper shield is increased, which can rotate the magnetization of the HB layers away from their desired orientation at the free layer edges. Thus the effective field of the HB layers at the free layer is degraded by these two effects, less magnetic charge and magnetostatic coupling to the upper shield. 
     Studies (see: Y. Zhou, “Thermally Excited Low Frequency Magnetic Noise in CPP Structure MR Heads,” IEEE Trans. Magn., vol 43, pp 2187, 2007) show that a weakened HB field increases the noise produced by the sensor and will ultimately affect the reading process in high density magnetic recording. To increase the HB field for thinner HB layers, a higher HB magnetic moment and/or closer HB to free layer spacing is required. To reduce the effects of magnetostatic coupling between the HB layers and the reader shield, common practice is to increase the coercivity of the HB material (D. J. Larson et al., U.S. Pat. No. 7,061,731 B2; P. V. Chau et al., US Publ. Pat. Appl. 2005/0066514; H. S. Gill, US Publ. Pat. Appl. 2006/0114622 A1; M. M. Pinarbasi, US Publ. Pat. Appl. 2006/0087772 A1) so that the HB magnetization does not easily rotate because of the magnetostatic coupling to the upper shield. However, increasing the magnetic moment and increasing the coercivity of the HB materials are contradictory procedures according to the physics of magnetic materials. Increasing one property decreases the other. Thus, a high moment, high coercivity HB material is difficult to achieve so that it produces enough HB field on the free layer and is stable enough at narrow RSS. 
     In addition to the higher spatial resolution of the MR sensor in the down-track direction that requires a narrower RSS, higher area density of recorded data requires a higher track density so that the data tracks can be recorded more closely together. The track width will therefore be reduced, which also requires the reader to be narrower. For the conventional HB layer, as the width of the sensor stack diminishes, the distance between the HB and the stack edges does not diminish proportionally, because of the minimum interlayer distance that is required between the stack edges and the HB layer edges. This interlayer includes both a nonconductive layer that electrically isolates the HB layer from the sensor stack and an underlayer between the HB layer and the nonconductive layer to promote the crystalline growth of the HB layer in order to maintain a high coercivity. According to the prior study of Y. Zhou, cited above, this limitation on the minimum HB-to-sensor stack distance leads to a lower HB field gradient from the free layer edge to the free layer center. A lower HB gradient makes the MR sensor either have a higher noise production at the same sensitivity or a lower sensitivity in order to maintain the same noise level. Either of these alternatives leads to a lower signal-to-noise (SNR) ratio as sensor width is reduced. 
     The most ideal structure for an HB layer at narrow RSS and a narrow sensor width, is a thin and high-moment HB layer positioned as closely as possible to the free layer edge. This will only produce enough bias field to quench the self-demagnetization field of the free layer edge and, thereby, reduce random fluctuations in the free layer magnetization while producing much less field in the center of the sensor to avoid reducing sensor sensitivity. 
     To achieve a high moment in the HB layer, while still maintaining its magnetic stability, M. Arasawa et al., US Publ. Pat. Appl. 2006/0158793 A1 have suggested a dual-HB layer design as illustrated in  FIG. 2   a . A first pair of HB layers ( 13 ) are positioned at the sides of the sensor stack ( 6 ). A second pair of HB layers ( 4 ) are positioned laterally outside of layers ( 13 ). The inner HB layers ( 13 ) have a higher magnetic moment than the outer HB layers ( 4 ) and produce a high HB field in the free layer ( 8 ) of the stack ( 6 ). However, as the higher moment material of HB layer ( 13 ) usually also has a lower coercivity, the outer HB layers ( 4 ) are formed of material with lower moment but higher coercivity and are used to stabilized the inner layers ( 13 ) by applying their magnetic fields to the inner layers ( 13 ). This prior art, however, has the following limitations.
         (1) The prior art proposes that the inner HB layer ( 13 ) be formed of a hard magnetic material similar to that of HB layer ( 4 ), but with a higher moment, which makes the underlayer between HB ( 13 ) and the sensor stack ( 6 ) still indispensable, so the distance between ( 13 ) and ( 6 ) cannot be further reduced.   (2) The prior art specifies that the product M s t (magnetic moment, M s , times thickness, t) of the outer HB layer ( 4 ) needs to be greater than the same product of HB ( 13 ), so that HB ( 13 ) can be fully magnetized by HB ( 4 ). Although such a relationship can help maintain a full saturation of HB ( 13 ) magnetization at the edge facing FIB ( 4 ), it may not be able to prevent the magnetostatic coupling between of HB ( 13 ) to the upper shield ( 1 ) at the edge where HB ( 13 ) faces the sensor stack ( 6 ), because the HB ( 4 ) field produced in HB ( 13 ) decreases in inverse proportion to the distance from the HB ( 4 ) edge facing HB ( 13 ).   (3) The prior art also notes that HB ( 13 ) can be a soft magnetic material. However, it lacks details as to how one makes a soft HB ( 13 ), with coercivity approximately 0, to work in the structure shown in  FIG. 2   a . Since the prior art does not specify any additional features, except that HB ( 4 ) has a higher M s t, to make a soft HB ( 13 ) work, it is only logical to deduce that the prior art assumes a commonly adopted approach in forming the HB ( 13 ) on the sides of the sensor stack ( 6 ) and defining the back edges of the free layer and the HB layer in a single step etching process, leading to the top view shown schematically in  FIG. 2   b . A soft HB layer ( 13 ) as in  FIGS. 2   a  and  2   b  would be difficult to stabilize by the field of HB ( 4 ) because the soft HB ( 13 ) can have a much higher moment and a lower coercivity. Coupling to the upper shield and perturbation by external fields will be much stronger. In addition, with the same uniform stack height of the soft HB ( 13 ) and the free layer, coupling between the free layer and the HB ( 13 ) also exists during read-back and can lead to large amounts of side reading.  FIG. 3  displays simulated cross-track read-back profiles of a generic HB MR sensor and a  FIG. 2   a  type structure MR sensor with HB ( 13 ) being formed of soft material. The coupling between the free layer and the HB layer ( 13 ) leads to significant side reading, where HB ( 13 ) is rotated by the field of the medium from side tracks and the magnetization of the free layer rotates as a result of coupling with HB ( 13 ).       

     The present invention will address the problems alluded to above by achieving an optimum hard bias field on the free layer of a MR sensor stack at a narrow reader shield spacing (RSS). 
     SUMMARY OF THE INVENTION 
     A first object of this invention is to provide a hard bias (HB) layer to stabilize the free layer of an MR sensor in a CPP MR read head having a narrow shield-to-shield spacing. 
     A second object of the present invention is to provide such a CPP MR read head where the edge of the free layer is effectively pinned by the HB layer yet maintains sufficient sensitivity to produce a high signal to noise ratio (SNR). 
     A third object of this invention is to utilize a high moment, soft magnetic flux guide structure to conduct the flux produced by a hard magnetic bias layer and to locally concentrate the flux at the free layer edges by the shape anisotropy of the flux guide and to thereby produce a high field gradient from the free layer edge to its center, i.e., to achieve an ideal HB field across the entire free layer. 
     A fourth object of this invention is to achieve minimal coupling between an upper (top) shield and a hard bias layer by using a flux guide structure that is a thin layer of soft magnetic material. 
     A fifth object of this invention is to optimize the flux guide field on the free layer and to optimize the HB layer coercivity independently. 
     A sixth object of this invention is to attain all the previous objects while also achieving improved cross-track resolution during read-back. 
     These objects will be met by a CPP MR read head design in which a thin layer flux guide (FG) structure, formed of soft magnetic material having high magnetic moment, is placed between the HB layer and the adjacent edges of the free layer. 
     The FG shall have a magnetic moment of similar or higher value than the moment of the free layer. Because of its high moment and thin layer structure, out of plane magnetization of the FG is effectively quenched by the surface demagnetization field. Thus FG magnetization will only rotate in-plane, which significantly reduces the magnetostatic interaction with the reader shield at narrow RSS. 
     It is to be noted that flux guides are known in the prior art, as, for example, Gill, U.S. Pat. No. 7,237,322 who discloses a flux guide on either side of a free layer, but contacting the free layer; and Wu, U.S. Pat. No. 7,170,721, who also discloses a flux guide along one side of an MR stack. But neither of these inventions provide the characteristics of the present invention to meet its intended objects. 
     The present flux guide (FG) shall have a tapering shape, with the FG edge facing the HB layer having a longer SH (stripe height) than its edge adjacent to and facing the free layer. 
     As will be seen in schematic  FIGS. 4   a ,  4   b  and  4   c , below, the FG magnetization closer to the HB edge follows the HB field and forms a continuous magnetic flux path within the FG as the flux propagates towards the free layer and then away from the free layer. 
     Because the FG is a soft magnetic layer, it does not require an underlayer (( 20 ) in  FIG. 1   a ) to promote crystalline anisotropy, but only an isolation layer (( 14 ) in  FIG. 4   c ) to electrically insulate it from the adjacent free layer edge. Such an insulating layer can be made thinner than the underlayers required by the generic prior art HB layers formed of hard magnetic materials. Therefore, the FG edge can be placed much closer to the free layer edge than could a HB layer of the prior art and, as a result, a more efficient HB field is applied to the free layer. 
     The HB layer ( 4 ) laterally outside the FG layer is grown with the usual procedures, including the necessary underlayers ( 20 ) required to produce crystalline anisotropy, but with much less limitation on underlayer thickness than would normally be required by the imposition of a narrow RSS. The HB layer as described in this invention can be optimized for high coercivity without much impact on the actual HB field on the free layer. At the same time, the FG layer, with its tapered shape, greatly reduces the magnetostatic coupling between the FG and the free layer through back-end magnetic charges. As a result, side reading due to FG coupling to the free layer is eliminated. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1   a  is a schematic representation of a prior-art CPP-MR read head as viewed from the ABS. 
         FIG. 1   b  is a schematic representation of the prior-art CPP-MR read head of  FIG. 1   a , now as viewed from above at a horizontal cross-sectional level taken through the free layer. 
         FIG. 1   c  is a schematic, more detailed representation of the sensor stack of the prior-art CPP-MR read head of  FIG. 1   a  as viewed from the ABS. 
         FIG. 2   a  is a schematic representation of an alternative form of prior-art CPP-MR read head, intended to overcome some of the difficulties with the read head of  FIG. 1   a , as viewed from the ABS. 
         FIG. 2   b  is a schematic representation of the prior-art CPP-MR read head of  FIG. 2   a , now as viewed from above at a horizontal cross-sectional level taken through the free layer. 
         FIG. 3  is a graphical representation comparing the performance of sensors of the types shown in  FIG. 1   a  and  FIG. 2   a.    
         FIG. 4   a  is a schematic representation of the CPP-MR read head of a first embodiment of the present invention as viewed from the ABS. 
         FIG. 4   b  is a schematic representation of the CPP-MR read head of  FIG. 4   a , now as viewed from above at a horizontal cross-sectional level taken through the free layer. 
         FIG. 4   c  is a schematic, more detailed representation of the sensor stack of the CPP-MR read head of  FIG. 4   a  as viewed from the ABS. 
         FIG. 5   a  is schematic illustration showing simulated quiescent state magnetic field vectors in a prior art sensor of the type illustrated in  FIG. 1   a.    
         FIG. 5   b  is schematic illustration showing simulated quiescent state magnetic field vectors in a sensor of the present invention as illustrated in  FIG. 4   a.    
         FIG. 6   a  is a graphical representation of the comparison between prior art sensors and the present sensor showing off-track reading performance as a function of sensor height and width. 
         FIG. 6   b  is a graphical representation of the comparison between the same prior art sensors and the present sensor of  FIG. 6   a  showing cross-track sharpness vs. 50% amplitude off-track distance. 
         FIG. 7  is a graphical representation of the noise power spectrum of a prior art sensor and the sensor of the present invention. 
         FIG. 8   a  is a schematic representation of the CPP-MR read head of a second embodiment of the present invention as viewed from the ABS. 
         FIG. 8   b  is a schematic representation of the CPP-MR read head of  FIG. 8   a , now as viewed from above at a horizontal cross-sectional level taken through the free layer. 
         FIG. 8   c  is a schematic, more detailed representation of the sensor stack of the CPP-MR read head of  FIG. 8   a  as viewed from the ABS. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Each of the preferred embodiments of this invention is a shielded read head incorporating a CPP MR sensor structure having a free layer that is longitudinally biased by a laterally disposed configuration of biasing layers formed of the hard magnetic materials Co, Fe, B, Ni, Pt, Cr, or any of their combinations, denoted hereinafter simply as hard bias layers or HB layers, whose flux is channeled through thin, tapered (generally in both depth and thickness) flux guides (denoted FG) formed of soft magnetic materials, which are the elements or the alloyed combinations of the elements Fe, Co, Ni, B, Mn, Cr, Ru or Ta. The tapered FG are interposed between the long (in depth) sides of the HB layers and the short sides of the free layer so that the field of the HB layers is “funneled”, by the tapering shape, through the FG. In this way, the HB layers bias the free layer effectively through the interposition of the FG, yet the thicker HB layers are sufficiently removed laterally from the free layer that there is no adverse effect from their interaction with the reader shields. 
     Embodiment 1 
     In what follows, for ease and consistency of description, laterally directed dimensions (in the ABS plane) will be denoted “widths,” vertically directed dimensions (in the ABS plane) will be denoted “thicknesses” and dimensions directed from the ABS plane of the sensor towards the backside will be denoted “depths.” If other terms are utilized, they will be defined as needed. For example, the depth direction will sometimes be referred to as the “stack height.” 
     Referring now to schematic  FIGS. 4   a ,  4   b  and  4   c , there is shown, first, in  FIG. 4   a , an ABS view of a first embodiment of the present invention. Reader shields ( 1 ) (top or upper shield) and ( 2 ) (bottom shield) are formed above and below the CPP MR sensor stack ( 6 ) that is patterned to accept the short substantially vertical edges of the flux guides ( 16 ) on either side of the substantially vertical edges free layer ( 8 ). The sensor stack is a vertical lamination of horizontal layers that will be described below. The reader-shield-separation (RSS) is between approximately 10 and 40 nm. 
     The hard bias (HB) layers ( 4 ) are adjacent to the longer, slightly sloped lateral edges of the FG layers, as will be described in greater detail with reference to  FIGS. 4   b  and  4   c . The HB layers are typically formed on crystal growth-enhancing underlayers ( 20 ), such as layers of Ti, Cr, Al, Mg or their combinations or combinations with O, formed to between approximately 3 and 10 nm in thickness, and formed beneath the HB layers and between the HB layers and the FG. 
     Referring next to  FIG. 4   c , there is shown a more detailed ABS view of the patterned MR sensor stack ( 6 ) showing the FG layers ( 16 ) abutting the narrow (in width) patterned free layer ( 8 ), which has a width of between approximately 10 and 100 nm and a thickness between approximately 2 and 10 nm. Arrows show the magnetizations of the FG layers and the free layer. A similarly narrow in width patterned conducting or dielectric barrier layer ( 9 ) is formed beneath the free layer, depending on whether the sensor stack is GMR or TMR, respectively. 
     Because the free and conductive or barrier layers are patterned by etching away their deposited layers material to either side, the remaining lamination exhibits an exposed upper surface that now extends symmetrically to each side of the patterned free and barrier layers. Going vertically downward, this remaining portion of the sensor stack includes, respectively, a reference layer ( 10 ), a coupling layer ( 11 ), that is preferably a layer of Ru, and a pinned layer ( 12 ). A pinning layer ( 19 ), typically of an antiferromagnetic material, is formed between the pinned layer and the bottom shield, pinning both the pinned ( 12 ) and reference ( 10 ) layers. 
     The flux guide layer ( 16 ) is separated from the horizontal exposed upper surface reference layer and from the vertically etched sides of the free and barrier layers by an insulating layer ( 14 ) that is formed to a thickness between approximately 1 and 3 nm. A capping layer ( 18 ) (shown in  FIG. 4   a ), formed of a material such as Ta, provides electrical contact between the free layer ( 8 ) and the top shield ( 1 ). However, the FG layers are electrically insulated by layer ( 14 ) from the passage of current through this CPP configuration, as current passes vertically downward only through the capping layer and free layer and the layers beneath them. As can be seen in the figure, the sides of the FG farthest from the free layer is patterned to be co-planar with the plane formed by the outer lateral edges of the remaining layers of the sensor stack. The inner sides of the HB layers as shown in  FIG. 4   a  are substantially conformal to the shape of the plane formed by the outer lateral edges of the remaining layers and the FG layer. The HB layers, however, are separated from the lateral sides by the underlayer ( 20 ), as shown in  FIG. 4   a.    
     Looking now at schematic  FIG. 4   b , there is shown a horizontal cross-sectional view of the structure in  FIG. 4   a  taken at the level of the free layer. There are seen the two HB layers ( 4 ), the free layer ( 8 ) and the two FG layers ( 16 ) that are disposed between the HB layers and the free layer. The free layer ( 8 ), shown here as substantially square, has a depth dimension (in the ABS-to-back edge direction) of between approximately 10 to 100 nm, corresponding to the same approximate lateral width in the ABS dimension. The FG is tapered, with a larger (in area and particularly in its depth dimension) outer edge adjacent to the inner edge of the HB layer and a smaller (vertical) inner edge adjacent to the free layer and barrier layer. The width of the FG in the ABS plane is similar to that of the free layer, namely between approximately 10 and 100 nm and the depth of the inner edge of the FG is between approximately 10 and 100 nm. A small portion ( 25 ) along the back edge of the FG, having the same depth as the free layer, is etched towards the HB layer. The FG layers taper down from a maximum height where they are adjacent to the HB layers, to a minimum height, where they are adjacent to the free layer edges. The product of the FG thickness, t, facing FL and FG magnetic moment, M s , (actually, the product is M(FG) s t(FG)) needs to be equal to or larger than the corresponding product M s t of the free layer ( 8 ) (actually M(FL) s t(FL)) where M s  is the symbol for the magnetic moment and the parentheses contain the relevant structure of the flux guide or the free layer and t is the symbol for thickness of the flux guide edge or the free layer. 
     Arrows ( 15 ) show the flux lines beginning adjacent to the HB layer and funneling in the direction towards the free layer, then funneling away from the free layer, through the FG layer on the opposite side. The tapering shape of the FG confines the magnetic flux originating in the HB layer within itself, due to its shape anisotropy. There is minimal flux leakage beyond the FG structure itself and the flux is effectively focused towards the free layer edge as shown. The flux emerges from the FG only at its tapered end adjacent to the free layer, at which position the HB layer field is locally produced. Thus, the free layer is biased by the closely positioned FG ( 16 ), while the actual FG magnetization focusing direction is oriented by the HB field which emerges from outside the FG layers. At narrower RSS, although the HB layer thickness must be correspondingly reduced and the magnetic flux it produces will be less, the FG collects and focuses the HB flux in the stack height (depth) direction. With a high enough ratio of the FG height facing the HB layer to the FG height facing the free layer, an optimal biasing of the free layer can still be achieved even with the weaker HB field. Considering commonly used FIB and FG materials, the height ratio should be at least 2 to achieve a significantly higher field at the free layer edge. Meanwhile, a FG to free layer edge spacing of less than the free layer thickness is needed to produce enough field on the free layer with the thin layer FG. 
     The FG as a soft magnetic layer does not require an underlayer to promote crystalline anisotropy, but only an isolation layer ( 14 ) to electrically insulate it from the free layer edge. Such an insulating layer can be made thinner than the underlayers required by the generic HB layers formed of hard magnetic materials. Therefore, the FG edge can be placed much closer to the free layer edge than could a HB layer of the prior art and, as a result, a more efficient HB field is applied to the free layer. 
     The HB layer ( 4 ) outside the FG layer is grown with the usual procedures, including the necessary underlayers ( 20 ) required to produce crystalline anisotropy, but with much less limitation on underlayer thickness than would normally be required by the imposition of a narrow RSS. The HB layer as described in this invention can be optimized for high coercivity without much impact on the actual HB field on the free layer. At the same time, the FG layer, with its tapered shape, greatly reduces the magnetostatic coupling between the FG and the free layer through back-end magnetic charges. As a result, side reading due to FG coupling to the free layer is eliminated. 
     To fabricate the MR sensor structure of  FIGS. 4   a ,  4   b  and  4   c , the procedures can be, but are not limited to, the following steps. The sensor stack ( 6 ) can be formed with the pattern shown by applying a partial top-down etching to the deposited sensor stack that stops in the barrier/conductive layer ( 9 )., so that only the free layer ( 8 ) and some depth into the barrier/conduction layer ( 9 ) are patterned by the etching process. Layers ( 10 ), ( 11 ) and ( 12 ) remain intact after this etching process. Following the etching process, a thin insulating layer ( 14 ) is deposited over the etched surface to form an electrical isolation layer on top of which the FG layer is deposited as a refill of the etched portion of the free layer. As shown in  FIG. 4   a , the refilled MR stack can be exposed to a second etching. The MR stack with the refilled FG layer are etched sequentially in this second etching to form the final patterned sensor structure with two side walls. After the second etching, the under-layers required for the HB layer growth are deposited on the etched surfaces and followed by the HB layer depositions. 
     The  FIG. 4   b  type sensor SH structure can also be formed by a two-step etching process, where the first step etching forms the free layer back edge and the tapering slope of the FG ( 16 ) simultaneously and the second step forms the HB back edge and the FG top edge simultaneously. The first step etching can also extend to a certain depth ( 25 ) laterally (towards the HB layer) into the FG to form a flat edge of the same position as the free layer back edge to accommodate process variations. This depth shall not exceed the magnetic exchange length of the FG material and should be preferably less than 50 nm. 
     Referring next to  FIG. 5   a , there is shown simulated quiescent state magnetization (arrows) of the free layer ( 8 ) for a generic prior art MR sensor with a HB layer ( 4 ) as in  FIG. 1   a .  FIG. 5   b  shows magnetizations of the free layer ( 8 ) and the FG ( 16 ) in a FG enhanced HB sensor having the same free layer properties as in  FIG. 5   a . We consider a free layer of dimension 40 nm by 40 nm by 5 nm thickness.  FIG. 5   b  has a FG of the same material and thickness as the free layer.  FIG. 5   b  also shows a clear FG magnetization (arrows) focusing towards the free layer edges. 
     Referring next to  FIG. 6   a , there is shown a simulated read-back signal cross-track profile comparison between a prior art HB and an FG enhanced HB of the present invention. Free layers of 40 nm track width (TW) by 40 nm depth (or stack height, SH) and free layers of 30 nm by 30 nm are considered. From the profiles in  FIG. 6   a  the tapered FG enhanced HB as in  FIG. 5   b  shows similar cross-track profiles as prior art conventional HB as in  FIG. 5   a , where the 50% track width (TW) is approximately the same. No side reading as shown in  FIG. 3  is seen. However, FG HB combination of the present invention shows a lower tail in the cross-track profile than the conventional prior art HB with no FG at about 20 nm to 25 nm off-track distance. The y-axis in  FIG. 6   b  is “sharpness ratio”, the 10% amplitude off-track distance (i.e., the distance at which the read-back amplitude is 10% of the maximum amplitude) divided by the 50% amplitude off-track distance, which is a measure of the reading characteristics of the sensor. For higher track density, smaller side reading is preferred, which means a smaller sharpness ratio is preferred. 
     As  FIG. 6   b  shows, for both the 30 nm and 40 nm free layer sizes, the FG HB structure of the present invention exhibits better sharpness than the conventional prior art HB structure at the same 50% width. The better sharpness of the FG HB structure is the result of the FG layer being closer to the free layer edge, so that the side track flux can flow partially into the FG magnetization and cause less free layer response. However, such partial medium flux conduction by the FG does not cause any HB field degradation on the free layer due to the much larger size of the FG which permits it to dissipate the medium flux over a large area. 
     Referring to  FIG. 7  there is shown the simulated noise power spectrum from the sensor having a 40 nm by 40 nm free layer and both a HB layer with and without the FG of the present invention. The HB layer with the FG shows a first peak frequency that is approximately 1 GHz higher than the convention sensor without the FG. The FG supplied sensor also has a noise floor at &lt;2 GHz (see the vertical dashed line) that is about 3 dB lower than the conventional sensor. Both of these results indicate an effectively higher HB field on the free layer and the effectiveness of the FG structure in conducting and focusing the HB flux to bias the free layer more effectively than the prior art sensor without the FG. 
     Embodiment 2 
     Referring now to schematic  FIGS. 8   a ,  8   b  and  8   c , there is shown, first, in  FIG. 8   a , an ABS view of a second embodiment of the present invention, in  FIG. 8   b  there is shown a top view (cross-section between the shields) of  FIG. 8   a  and in  FIG. 8   c  there is shown a more detailed view of the sensor stack ( 6 ) components and the positioning of the FG layer ( 16 ) and its magnetization vectors (arrows). 
     Reader shields ( 1 ) (top or upper shield) and ( 2 ) (bottom shield) are formed above and below (RSS separation approximately 10 to 40 nm) the CPP MR sensor stack ( 6 ) that is patterned to accept the short edges of the flux guides (FG) ( 16 ) on the etched slopes of the sensor stack. These etched slopes can extend through more than two layers of the MR stack. It is noted that the width of the free layer ( 8 ) is between approximately 10 and 100 nm and the widths of the layers ( 9 )-( 12 ) below the free layer are greater, corresponding to the slope of the sides. Unlike the patterned shape of  FIG. 4   a , where the free ( 8 ) and barrier ( 9 ) layers were etched vertically downward to expose a laterally extending surface of the reference layer ( 10 ), in this embodiment, the sides of the free ( 8 ) and barrier ( 9 ) layers are both sloped by etching as shown, to produce a first and second sloped portion of the lateral sides of the MR stack. The FG ( 16 ) has a tapering shape with a first and second sloped portion that conformally follows the dual slopes of the sensor stack. The FG has an opposite lateral side with a single slope further away from the sensor edge, towards the HG layer ( 4 ). The FG ( 16 ) and the sensor stack are separated by an insulator layer ( 14 ) whose thickness is between approximately 1 and 3 nm. During operation, the electrical sensing current only flows through the free layer ( 8 ) and the layers beneath, while the FG layers are insulated from the current path. The HB layer ( 4 ) exists on both sides of the sensor stack and FG, being positioned laterally outside the FG edge. The FG ( 16 ) can be in direct contact with the HB ( 4 ) and exchange coupled to it when the FG is the outermost layer after sensor stack patterning. Alternatively, the HB can be separated from the sensor stack and FG with necessary underlayers (( 20 ) in  FIG. 8   c ) formed between them. 
     As seen in  FIG. 8   b , the FG ( 16 ) also has a tapering shape in the SH (depth) direction. The tapering shape in both thickness and SH (depth) creates a funneling of the magnetic flux through the FG between the HB layers and the free layer, as can be seen by the arrows in  FIG. 8   b  and  FIG. 8   c . As already noted above, in the description of the first embodiment, the product of the FG ( 16 ) thickness, t, facing FL and FG magnetic moment, M s , (actually, M(FG) s t(FG)) needs to be equal to or larger than the corresponding product M s t of the free layer ( 8 ) (actually M(FL) s t(FL)) where M s  is the symbol for the magnetic moment and the parentheses contain the relevant structure of the flux guide or the free layer and t is the symbol for thickness of the flux guide edge or the free layer, both of which are between approximately 2 and 10 nm). 
     Preferably the FG should have a SH facing the HB that is at least two times that of the SH of the FG facing the free layer. Spacing between the FG and the free layer edges is preferably equal to or less than the free layer thickness. 
     Referring next to  FIG. 8   c , there is shown the patterned CPP MR sensor stack ( 6 ) with the FG layers ( 16 ) conformally abutting its sloping patterned lateral sides. Arrows show the longitudinal magnetizations of the FG layers and the free layer. A similarly patterned conducting or dielectric barrier layer ( 9 ) is formed beneath the free layer, depending on whether the sensor stack is GMR or TMR, respectively. 
     Beneath the free and barrier layers, and progressively wider than those two layers, there are formed, respectively, a reference layer ( 10 ), a coupling layer ( 11 ), preferably a layer of Ru, and a pinned layer ( 12 ). A pinning layer ( 19 ) is formed between the pinned layer and the bottom shield, pinning both the pinned and reference layers. The flux guide layer ( 16 ) is separated from the reference layer and from the sides of the free and barrier layers by an insulating layer ( 14 ). A capping layer ( 18 ) provides electrical contact between the free layer and the top shield. However, the FG layers are electrically insulated by layer ( 14 ) from the passage of current through this CPP configuration, as current passes vertically downward only through the capping layer and free layer and the layers beneath them. 
     Looking now at schematic  FIG. 8   b , there is shown a horizontal cross-section taken through the read head at the level of the free layer. There are seen the HB layers ( 4 ), the free layer ( 8 ) and the FG layers ( 16 ) between the HB layers and the free layer. The FG layers taper down from a maximum depth and thickness where they are adjacent to the HB layers, to a minimum depth and thickness, where they are adjacent to the free layer edges. Note that the maximum sized edge is greater than the corresponding edge in the first embodiment, which did not conformally abut the full side of the HB layer. Arrows ( 15 ) show the flux lines beginning adjacent to the HB layer and funneling in the direction towards the free layer and then funneling from the free layer towards the opposite side FG layer. The tapering shape of the FG confines the magnetic flux originating in the HB layer within itself, due to its shape anisotropy. There is minimal flux leakage beyond the FG structure itself and the flux is effectively focused towards the free layer edge as shown. The flux emerges from the FG at its tapered end adjacent to the free layer, at which position the HB layer field is locally produced. Thus, the free layer is biased by the closely positioned FG ( 16 ), while the actual FG magnetization focusing direction is oriented by the HB field which emerges from outside the FG layers. 
     The HB layer ( 4 ) laterally outside the FG layer is grown with the usual procedures, including the necessary underlayers ( 20 ) required to produce crystalline anisotropy, but with much less limitation on underlayer thickness than would normally be required by the imposition of a narrow RSS. The HB layer as described in this invention can be optimized for high coercivity without much impact on the actual HB field on the free layer. 
     The fabrication of the second embodiment is easier than that of the first embodiment but the second embodiment may have a greater shield coupling effect due to the thicker FG layer. This may still be a viable solution for achieving a stronger biasing field on the free layer at a narrow RSS and thin HB condition, with both the thickness and SH tapering of the FG layer. 
     To fabricate the MR sensor structure of  FIGS. 8   a ,  8   b  and  8   c , the procedures can be, but are not limited to, the following steps. A first patterning of the sensor stack that forms two slopes on the sides. Afterwards, a thin layer ( 14 ) is deposited on the sloped sides to form insulation between the sensor and the FG ( 16 ) and the FG is deposited on this layer as a refill of the etched slopes and forms a tapering shape towards the free layer edge. The refilled MR stack is then exposed to a second etching to form the final patterned sensor structure with two sidewalls. After this second etching process, the HB layer is directly deposited onto the etched surfaces or deposited on an underlayer that is first formed. A first step back edge etching (forming sloped edges as shown in  FIG. 8   b ) forms the back edge of the free layer and the tapering slope of the FG simultaneously. This first step back edge etching can also extend for a certain distance (( 25 ) in  FIG. 8   b ) into the FG to form a flat edge in the FG at the same position as the back edge of the free layer to accommodate process variations. This distance shall not exceed the magnetic exchange length of the FG material and should preferably be &lt;50 nm. Finally, a second step of back edge etching forms the back edge of the HB layer and the FG top edge simultaneously. 
     As is finally understood by a person skilled in the art, the preferred embodiments of the present invention are illustrative of the present invention rather than limiting of the present invention. Revisions and modifications may be made to methods, materials, structures and dimensions employed in forming and providing a CPP MR sensor in which a soft magnetic flux guide is interposed between a hard magnetic bias layer and the free layer of the sensor, while still forming and providing such a device and its method of formation in accord with the spirit and scope of the present invention as defined by the appended claims.