Patent Publication Number: US-11024333-B2

Title: Magnetic read head structure with improved bottom shield design for better reader performance

Description:
This is a divisional application of U.S. patent application Ser. No. 16/007,014; filed on Jun. 13, 2018, which is herein incorporated by reference in its entirety, and assigned to a common assignee. 
     RELATED PATENT APPLICATION 
     This application is related to the following: U.S. Pat. No. 9,230,577; assigned to a common assignee and herein incorporated by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to an improved bottom shield design for stabilizing free layer magnetization in a single reader or dual reader (2DMR) scheme, and in particular to a bottom shield stack comprised of a non-magnetic decoupling layer, and an uppermost magnetic layer with enhanced domain stability that is formed between a bottommost magnetic layer in the bottom shield and a bottom surface of a sensor in the reader structure. 
     BACKGROUND 
     In a magnetic recording device in which a read head comprises a magnetoresistive (MR) sensor, there is a constant drive to increase recording density. One trend used in the industry to achieve this objective is to decrease the size of the MR sensor. Typically, the sensor stack has two ferromagnetic layers that are separated by a non-magnetic layer. One of the ferromagnetic layers is a reference or pinned layer wherein the magnetization direction is fixed by exchange coupling with an adjacent antiferromagnetic (AFM) pinning layer. The second ferromagnetic layer is a free layer with a magnetization that rotates in response to external magnetic fields, and is rotated towards either parallel or anti-parallel to the magnetization in the pinned layer to read out the local orientation of magnetic moment in the recording media. When passing the MR sensor over a recording medium at an air bearing surface (ABS), the free layer magnetic moment will rotate according to the local magnetic field generated by the recording media. By processing the angle of rotation as a function of location on the media, the data pattern recorded on the media can be decoded. A MR sensor may be based on a tunneling magnetoresistive effect where the two ferromagnetic layers are separated by a thin non-magnetic dielectric layer. A sense current is used to detect a resistance value, which is lowest when the moments from the two layers are parallel to each other and is the highest when the two moments are anti-parallel to each other. In a current perpendicular-to-plane (CPP) configuration, a sense current is passed from a top shield through the sensor layers to a bottom shield, or in the reverse direction. 
     A longitudinal biasing scheme is typically used in a read head design to keep the free layer in a stable orientation in the absence of the external magnetic field. Bias films of high coercivity or soft bias also known as junction shields, are abutted against the edges of the MR sensor and particularly against the sides of the free layer. By arranging for the flux flow in the free layer to be equal to the flux flow in the adjoining hard bias layer, the demagnetizing field at the junction edges of the aforementioned layers vanishes because of the absence of magnetic poles at the junction. As the critical dimensions for MR sensor elements become smaller with higher recording density requirements, the free layer becomes more volatile and more difficult to bias. Top and bottom magnetic shields with in-plane magnetization are often used to ensure the MR sensor will only respond to a local magnetic field. However, free layer magnetization is sensitive to domain wall motion in the bottom and top shield, which may lead to increased noise, reducing the signal to noise (SNR) ratio of the reader sensor and cause failure in decoding data from the media. 
     In recent years, 2DMR configurations have become attractive from an areal density improvement standpoint. However, shield stability is more difficult to control in 2DMR schemes because of a requirement to shrink reader-to-reader spacing (RRS), and in view of repeated thermal treatments during recording head fabrication that can readily change the magnetization orientation in the shields. Shield instability will directly translate into reader instability and will adversely impact SNR and bit error rate (BER). A new read head structure is needed wherein shield stability is improved while maintaining acceptable SNR and BER. 
     SUMMARY 
     One objective of the present disclosure is to provide a bottom shield design that enables improved stability in the shield and also to have improved free layer magnetization in a single reader, or in one or both readers in a 2DMR structure. 
     A second objective of the present disclosure is to provide a method of forming the bottom shield structure according to the first objective. 
     According to one embodiment of the present disclosure where a reader comprises a MR sensor formed between a bottom shield and a top shield, the bottom shield structure has a stack of layers wherein a non-magnetic decoupling layer and a second magnetic layer with a magnetization saturation (Ms) value from 5 kiloGauss (kG) to 15 kG are sequentially formed on a first magnetic layer. The first magnetic layer has a thickness greater than the overlying decoupling layer and second magnetic layer, and also has a Ms value from 5 kG to 15 kG. The decoupling layer is made of a conductive material such as one or more of Cu, Al, W, Cr, Ta, Ru, Pt, and Pd, or is an insulating material including but not limited to a metal oxide that is Al 2 O 3 , SiO 2 , MgO, Ta 2 O 5 , and TiO 2 , and has a thickness that is sufficient to prevent magnetic coupling between the first and second magnetic layers. 
     The second magnetic layer is comprised of one or more of FeCo, FeCoNi, FeCoN, NiFe, NiFeW, NiFeCr, NiFeMo, or alloys thereof such as FeCoR where R is Mo, Zr, Nb, Hf, Ru, Pt, Re, Pd, or a combination thereof, and has a thickness from about 1 nm to 1 micron. In some embodiments, the second magnetic layer is a single layer with an in-plane magnetization aligned in the same direction as overlying junction shields on each side of a free layer in the MR sensor. In an alternative embodiment, the second magnetic layer is a trilayer in which an antiferromagnetic (AF) coupling layer is sandwiched between two ferromagnetic layers (FM 1  and FM 2 ) in a FM 1 /AF coupling layer/FM 2  configuration to stabilize the magnetization. In yet another embodiment, there may be an antiferromagnetic (AFM) layer inserted within the second magnetic layer to yield a FM 1 /AFM/FM 2  configuration, or inserted between the decoupling layer (DL) and second magnetic (FM) layer to give a DL/AFM/FM or DL/AFM/FM 1 /AF coupling layer/FM 2  stack of layers on the first magnetic layer. In some embodiments, the second magnetic layer may be comprised of amorphous materials such as CoTaZr to provide a smoother surface on which to fabricate the MR sensor. In preferred embodiments, the second magnetic layer has a fine grain structure to improve domain stability therein. 
     In one embodiment, the bottom shield structure of the present disclosure is formed in a single reader. According to a second embodiment, the bottom shield structure disclosed herein is employed in one or both of a bottom reader and a top reader in a 2DMR scheme. 
     The present disclosure also includes a process involving a sequence of steps to form the bottom shield including a physical vapor deposition (PVD) of the decoupling layer on the first magnetic layer, and then electroplating the second magnetic layer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a head arm assembly of the present disclosure. 
         FIG. 2  is side view of a head stack assembly of the present disclosure. 
         FIG. 3  is a plan view of a magnetic recording apparatus of the present disclosure. 
         FIG. 4  is a down-track cross-sectional view of a combined read-write head wherein a single reader is formed between top and bottom shields in the read head portion according to an embodiment of the present disclosure. 
         FIG. 5  is an ABS view of a prior art reader structure wherein a bottom shield is a single magnetic layer that is used to shield the reader from magnetic fields on the bottom shield side away from the sensor. 
         FIG. 6  is an ABS view of a 2DMR structure previously fabricated by the inventors wherein a first bottom shield and second bottom shield for shielding magnetic fields away from the sensor on the bottom shield side in a first reader and second reader, respectively. 
         FIG. 7  is a top-down view of the reader in  FIG. 5  showing the relative size of the reader and bottom shield, and a possible magnetization pattern within the bottom shield. 
         FIG. 8  is an ABS view of read head structure wherein a bottom shield is comprised of a non-magnetic decoupling layer and a second magnetic layer sequentially formed on a first magnetic layer according to an embodiment of the present disclosure. 
         FIG. 9  is a top-down view of the second magnetic layer in  FIG. 8  that has a width and height essentially equivalent to the width and height of the underlying first magnetic layer according to one embodiment of the present disclosure. 
         FIG. 10  is a top-down view of the second magnetic layer in  FIG. 8  having a width and height less than the width and height of the underlying first magnetic layer according to another embodiment of the present disclosure. 
         FIG. 11  is an ABS view of an alternative embodiment where the second magnetic layer in  FIG. 8  has an AF coupling layer between two ferromagnetic (FM) layers to stabilize a magnetization direction in the upper FM layer. 
         FIG. 12  is an ABS view of another embodiment of the present disclosure wherein the second magnetic layer in  FIG. 8  is modified to include an AFM layer. 
         FIG. 13  is an ABS view of another embodiment of the present disclosure wherein the second magnetic layer in  FIG. 11  is modified to include an AFM layer that adjoins a top surface of the decoupling layer. 
         FIG. 14  is an ABS view of a 2DMR structure wherein a bottom shield in the bottom reader is comprised of a non-magnetic decoupling layer and a second magnetic layer sequentially formed on a first magnetic layer according to an embodiment of the present disclosure. 
         FIG. 15  is an ABS view of a 2DMR structure wherein a bottom shield in the top reader is comprised of a non-magnetic decoupling layer and a second magnetic layer sequentially formed on a first magnetic layer according to another embodiment of the present disclosure. 
         FIGS. 16A-16B  are plots comparing magnetic performance in terms of cumulative distribution of Barkhausen jump normalized by sensor amplitude for readers with and without the modified bottom shield structure of the present disclosure. 
         FIGS. 17A-17B  are plots comparing magnetic performance in terms of cumulative distribution of hysteresis normalized by sensor amplitude for readers with and without the modified bottom shield structure of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure is a bottom shield design that provides improved magnetic stabilization in a read head that is a single reader or has a 2DMR configuration. The more stable bottom shield ensures there is less stray field that could act on the sensor element either directly or indirectly via magnetic coupling to other magnetic parts of the reader. Improved bottom shield stability translates into improved MR sensor (free layer) stability, as well as better SNR and BER. In the drawings, the y-axis is in a cross-track direction, the z-axis is in the down-track direction, and the x-axis is in a direction orthogonal to the ABS and towards a back end of the read head. The stabilized bottom shield design described herein is not limited to a particular read head structure, and is effective even in a reader in a multiple reader structure such as 3DMR. The terms “second” and “secondary” may be used interchangeably when referring to the upper layer in the bottom shield stack of layers. In addition, the terms “read head” and “reader” are used interchangeably. 
     Referring to  FIG. 1 , a HGA  100  includes a magnetic recording head  101  comprised of a slider and a combined read-write structure formed thereon, and a suspension  103  that elastically supports the magnetic recording head. The suspension has a plate spring-like load beam  222  formed with stainless steel, a flexure  104  provided at one end portion of the load beam, and a base plate  224  provided at the other end portion of the load beam. The slider portion of the magnetic recording head is joined to the flexure, which gives an appropriate degree of freedom to the magnetic recording head. A gimbal part (not shown) for maintaining a posture of the magnetic recording head at a steady level is provided in a portion of the flexure to which the slider is mounted. 
     HGA  100  is mounted on an arm  230  formed in the head arm assembly  103 . The arm moves the magnetic recording head  101  in the cross-track direction y of the magnetic recording medium  140 . One end of the arm is mounted on base plate  224 . A coil  231  that is a portion of a voice coil motor is mounted on the other end of the arm. A bearing part  233  is provided in the intermediate portion of arm  230 . The arm is rotatably supported using a shaft  234  mounted to the bearing part  233 . The arm  230  and the voice coil motor that drives the arm configure an actuator. 
     Next, a side view of a head stack assembly ( FIG. 2 ) and a plan view of a magnetic recording apparatus ( FIG. 3 ) wherein the magnetic recording head  101  is incorporated are depicted. The head stack assembly  250  is a member to which a first HGA  100 - 1  and second HGA  100 - 2  are mounted to arms  230 - 1 ,  230 - 2 , respectively, on carriage  251 . A HGA is mounted on each arm at intervals so as to be aligned in the perpendicular direction (orthogonal to magnetic medium  140 ). The coil portion  253  of the voice coil motor is mounted at the opposite side of each arm in carriage  251 . The voice coil motor has a permanent magnet  263  arranged at an opposite position across the coil  253 . 
     With reference to  FIG. 3 , the head stack assembly  250  is incorporated in a magnetic recording apparatus  260 . The magnetic recording apparatus has a plurality of magnetic media  140  mounted to a spindle motor  261 . For every magnetic recording medium, there are two magnetic recording heads arranged opposite one another across the magnetic recording medium. The head stack assembly and actuator except for the magnetic recording heads  101  correspond to a positioning device, and support the magnetic recording heads, and position the magnetic recording heads relative to the magnetic recording medium. The magnetic recording heads are moved in a cross-track of the magnetic recording medium by the actuator. The magnetic recording head records information into the magnetic recording media with a perpendicular magnetic recording (PMR) writer element (not shown) and reproduces the information recorded in the magnetic recording media by a magnetoresistive (MR) sensor element (not shown). 
     Referring to  FIG. 4 , magnetic recording head  101  comprises a combined read-write head previously fabricated by the inventors and disclosed in related U.S. Pat. No. 9,230,577. The down-track cross-sectional view is taken along a plane formed orthogonal to the ABS  30 - 30 , and that bisects the main pole layer  14 . The read head is formed on a substrate  80  that may be comprised of AlTiC (alumina+TiC) with an overlying dielectric layer  81  that is made of a dielectric layer such as alumina. The substrate is typically part of a slider formed in an array of sliders on a wafer. After the combined read head/write head is fabricated, the wafer is sliced to form rows of sliders. Each row is typically lapped to afford an ABS before dicing to fabricate individual sliders that are used in a magnetic recording device. A bottom shield  1  is formed on dielectric layer  81 . 
     A magnetoresistive (MR) element also known as MR sensor  2  is formed on bottom shield  1  at the ABS  30 - 30  and typically includes a plurality of layers that are described later with regard to  FIG. 4 . A top magnetic shield  15  is formed on the MR sensor. Layer  82  adjoins the backside of the MR sensor, and layer  83  contacts the backsides of the bottom shield and top shield. An insulation layer  8  and a top shield (S 2 B) layer  9  are sequentially formed on the top shield. Note that the S 2 B layer  9  may serve as a flux return path (RTP) in the write head portion of the combined read/write head. Thus, the portion of the combined read/write head structure formed below layer  9  in  FIG. 4  is typically considered as the read head. 
     The present disclosure anticipates that various configurations of a write head (PMR writer) may be employed with the read head portion. In the exemplary embodiment, magnetic flux  70  in main pole layer  14  is generated with flowing a current through bucking coil  60   b  and driving coil  60   d  that are below and above the main pole layer, respectively, and are connected by interconnect  61 . Magnetic flux  70  exits the main pole layer at pole tip  14   p  at the ABS  30 - 30  and is used to write a plurality of bits on magnetic media  140 . Magnetic flux  70   b  returns to the main pole through a trailing loop comprised of trailing shields  17 ,  18 , PP3 shield  26 , and top yoke  18   x . Layer  41  adjoins top yoke  18   x . There is also a leading return loop for magnetic flux  70   a  that includes leading shield  11 , leading shield connector (LSC)  33 , S 2  shield connector (S 2 C)  32 , return path  9 , and back gap connection (BGC)  62 . The magnetic core may also comprise a bottom yoke  35  below the main pole layer. Dielectric layers  10 ,  13 ,  19 ,  22 ,  37 - 39 ,  43 , and  45  are employed as insulation layers around magnetic and electrical components. A protection layer  27  covers the PP3 trailing shield and is made of an insulating material such as alumina. Above the protection layer and recessed a certain distance u from the ABS  30 - 30  is an optional cover layer  29  that is preferably comprised of a low coefficient of thermal expansion (CTE) material such as SiC. Overcoat layer  28  is formed as the uppermost layer in the write head. 
     Referring to  FIG. 5 , an ABS view is shown of a portion of the read head including bottom shield  1 , top shield  15 , junction shields  21 , and the MR sensor having a lower layer  2   d , middle free layer  2   f , and upper layer  2   h  that is formed on a center section of the bottom shield. Sidewalls  2   s  connect a bottom surface  2   b  with the top surface  2   t  of the MR sensor. There is a non-magnetic isolation layer  40  formed along the sidewalls  2   s  and on portions of the bottom shield that are not covered by the MR sensor. Magnetization  21   m  in the junction shields is primarily responsible for providing longitudinal biasing to the free layer. Magnetization  15   m  in the top shield stabilizes the magnetization direction in the junction shields through exchange coupling. Layer  2   h  comprises at least a capping layer, and layer  2   d  includes a reference layer with a fixed magnetization direction, and a non-magnetic spacer (not shown) between the reference layer and free layer  2   f . Layer  2   d  may also include a bottommost seed layer, and an antiferromagnetic (AFM) layer such as IrMn or another Mn alloy may be formed on a side of the reference layer that faces away from the free layer to pin the magnetization direction in the reference layer. Shield-to-shield spacing is depicted as RSS 1 . In other embodiments, the AFM layer (not shown) may be recessed behind the MR sensor stack or embedded in a back portion of bottom shield  1  to satisfy reduced RSS 1  requirements. The non-magnetic spacer may be comprised of one or more metal oxides, metal oxynitrides, or metal nitrides to provide a tunneling magnetoresistive effect. 
     A conventional 2DMR structure is shown in  FIG. 6  wherein a first reader such as the reader structure in  FIG. 5  is used as a bottom reader. Above first top shield  15  is an isolation (dielectric) layer  25  that magnetically and electrically isolates the first reader from a second (top) reader that includes a second MR sensor  3  comprised of bottom layer  3   d , free layer  3   f , and upper layer  3   h  where layers  3   d ,  3   f , and  3   h  may have the same composition and function as layers  2   d ,  2   f , and  2   h , respectively. The second MR sensor is formed between a second bottom shield  50  with magnetization  50   m  and a second top shield  55 . A second insulation layer  48  adjoins the sidewalls of the second MR sensor. Junction shields  31  formed on the second insulation layer have a magnetization  31   m  and are employed to provide a longitudinal bias to the free layer  3   f  magnetization direction. Magnetization  31   m  is in the same direction as magnetization  55   m  in the second top shield because of ferromagnetic coupling. Preferably, the second MR sensor is aligned above the first MR sensor such that center plane  44  bisects each of the aforementioned MR sensors. The center plane is orthogonal to the ABS. Here, reader-reader spacing RRS is depicted as the down-track distance from the center of the first reader to the center of the second reader. 
     Referring to  FIG. 7 , a top-down view of MR sensor  2  on bottom shield  1  is shown with overlying layers removed. The MR sensor has width a, which is substantially smaller width than width w of the bottom shield, and height b substantially less than height h of the bottom shield. Moreover, the magnetic domains  1   a - 1   d  in the bottom shield may form a closed loop such that a first domain  1   a  proximate to front side  1   f  at the ABS  30 - 30  has a cross-track direction for magnetization  1   m , and a back portion with domain  1   c  proximate to backside  1   e  has a magnetization  1   m  anti-parallel to that in the first domain. Side domains  1   b ,  1   d  adjacent to sides  1   s   1 ,  1   s   2 , respectively, have anti-parallel magnetizations that are orthogonal to the ABS in order to form a closed loop for magnetization  1   m  in a counterclockwise direction. 
     According to a first embodiment of the present disclosure depicted in  FIG. 8 , the bottom shield in a single reader is modified to have a trilayer stack  1   x  wherein first magnetic (bottom shield) layer  1  is retained from the prior art, and a non-magnetic decoupling layer  4  with a thickness t 1  from 1 nm to 0.5 micron, and a second magnetic layer  5  with a thickness t 2  from 1 nm to 0.5 micron are sequentially formed thereon. 
     The first magnetic layer  1  has a thickness t greater than 0.5 micron, and preferably substantially greater than the second magnetic layer  5 . The decoupling layer  4  is made of a conductive material such as one or more of Cu, Al, W, Cr, Ta, Ru, Pt, and Pd, or is an insulating material including but not limited to a metal oxide that is Al 2 O 3 , SiO 2 , MgO, Ta 2 O 5 , or TiO 2 , and has a thickness that is sufficient to prevent exchange coupling between the first magnetic layer and second magnetic layer. 
     Both of the first magnetic layer  1  and second magnetic layer  5  have a Ms value from 5 kG to 15 kG. Each of the first and second magnetic layers are comprised of one or more of FeCo, FeCoNi, FeCoN, NiFe, NiFeW, NiFeCr, NiFeMo, or alloys thereof such as FeCoR where R is Mo, Zr, Nb, Hf, Ru, Pt, Re, Pd, or a combination thereof. Preferably, a material is selected for the second magnetic layer that provides a stable domain in a cross-track direction (magnetization  5   m ) even in the presence of external magnetic fields, and is not susceptible to domain wall motion proximate to the sensor that leads to instability. Moreover, a material with a fine grain structure or amorphous character such as CoTaZr is beneficial in forming a smooth top surface  5   t  on which to build a MR sensor with better film uniformity, which in turn yields improved performance. Top shield  15 , and junction shields  21  are generally comprised of CoFe, CoFeNi, CoFeN, or NiFe, or a combination thereof. In some embodiments, one or both of the top shield and junction shields may be comprised of stacks of magnetic materials separated by one or more non-magnetic materials (such as Ru, Cr, Rh, Ir, Mo, Re, and Os) that can provide antiferromagnetic coupling between the adjacent magnetic layers. 
     In the exemplary embodiment shown in  FIG. 8 , the second magnetic layer  5  is a single layer with in-plane magnetization  5   m  aligned in the same cross-track direction as magnetization  21   m  in overlying junction shields, and as magnetization  2   m  in free layer  2   f . Magnetization  15   m  is ferromagnetically coupled to magnetization  21   m  that provides a bias field to stabilize free layer magnetization. Because of decoupling layer  4 , domain wall motion that tends to form in the first magnetic layer  1  has no destabilization effect on magnetization in the second magnetic layer. Note that RSS 1  is maintained compared with the prior art single reader in  FIG. 5 . However, a total thickness of bottom shield  1   x  is increased by (t 1 +t 2 ) compared with the bottom shield in  FIG. 5  if the thickness of the first magnetic layer is constant. 
     An important feature is that the first magnetic layer  1  is a greater down-track distance from free layer  2   f  in the embodiment shown in  FIG. 8  than in the prior art MR sensor in  FIG. 5 . Accordingly, stray fields that may arise due to domain wall motion in the first magnetic layer have a significantly smaller effect on free layer magnetization in view of the greater separation from the MR sensor in  FIG. 8 . 
     Referring to  FIG. 9 , a top-down view of the second magnetic layer  5  and MR sensor  2  is shown with overlying layers removed according to an embodiment of the present disclosure. In a preferred embodiment, the shape of the second magnetic layer including the width w between side  5   s   1  and side  5   s   2  and height h between a front side  5   f  at the ABS  30 - 30  and a backside  5   e  is optimized independent of the width and height of the underlying first magnetic layer for better shield stability. 
     As an example, in an alternative embodiment depicted in  FIG. 10 , height h 1  and width w 1  of the second magnetic layer  5  are less than height h and width w, respectively, of the first magnetic layer  1 . However, height h 1  is preferably at least 5× greater than height b of the MR sensor, and width w 1  is preferably at least 5× greater than width a of the MR sensor in order to prevent stray fields (from domain wall motion) in the first magnetic layer from disrupting magnetization in the free layer. 
     According to another embodiment shown in  FIG. 11 , there is a modified bottom shield  1   x - 1  wherein a second magnetic layer  5 - 1  has a trilayer stack  5   a / 5   b / 5   c  in which bottom ferromagnetic (FM) layer  5   a  and top FM layer  5   c  are anti-ferromagnetically (AF) coupled through middle AF coupling layer  5   b . Thus, magnetization  5   m   1  in FM layer  5   c  is aligned parallel to magnetization  21   m  in the overlying junction shields, and is stabilized through AF coupling with FM layer  5   a  that has magnetization  5   m   2  anti-parallel to magnetization  5   m   1 . AF coupling layer  5   b  is typically Ru, but may also be one of Rh, RhRu, Mo, Re, Os, or Ir. FM layer  5   a  is decoupled from magnetization (not shown) in first magnetic layer  1  because of the intermediate decoupling layer  4 . Thus, bottom shield  1   x - 1  has a  1 / 4 / 5 - 1  stack of layers. 
     In another embodiment illustrated in  FIG. 12 , the bottom shield is modified with the insertion of AFM layer  6  between the decoupling layer  4  and bottom FM layer  5   a  in the second magnetic layer  5 - 1  to give a bottom shield  1   x - 2  with a  1 / 4 / 6 / 5 - 1  stack of layers. AFM layer  6  is advantageously used to pin magnetization  5   m   2  in FM layer  5   a , which in turn further stabilizes magnetization  5   m   1  in FM layer  5   c  because of AF coupling between FM layers  5   a ,  5   c . The AFM layer is preferably one of IrMn, PtMn, PdMn, NiMn, OsMn, RuMn, RhMn, RuRhMn, or MnPtPd. 
     In yet another embodiment shown in  FIG. 13 , the bottom shield is modified with the insertion of AFM layer  6  within the second magnetic layer to give a configuration  5 - 2  having a  5   a / 6 / 5   c  stack of layers where bottom FM layer  5   a  and top FM layer  5   c  are pinned and aligned in the same cross-track direction through contact with AFM layer  6 . The resulting bottom shield  1   x - 3  has a  1 / 4 / 5   a / 6 / 5   a  stack of layers. In this case, magnetization  5   m   1  is stabilized because of FM layer  5   c  contact with the AFM layer  6  (also known as the AFM pinning layer) rather than indirectly through antiferromagnetic coupling with FM layer  5   a  as in the previous embodiment. Alternatively, layer  5   a  may be made of a non-magnetic material. 
     The present disclosure also encompasses an embodiment wherein the bottom shield in a 2DMR configuration has enhanced stabilization because of incorporating one of the previously described bottom shield designs  1   x ,  1   x - 1 ,  1   x - 2 , or  1   x - 3 . For example,  FIG. 14  depicts a 2DMR design wherein the reader with MR sensor  2  and bottom shield  1   x  described in  FIG. 8  serves as the bottom reader. All layers including MR sensor  3  in the top reader are retained from the 2DMR structure shown in  FIG. 6 . Only the bottom shield in the bottom reader is modified to provide additional stability to junction shield magnetization  21   m  and to free layer magnetization  2   m . Furthermore, RRS is maintained from the previously described 2DMR structure. 
     Referring to  FIG. 15 , the present disclosure anticipates an embodiment where the bottom shield in the second read head (top reader) is stabilized with a modified bottom shield  50   x  having a  50 / 51 / 52  stack of layers where decoupling layer  51  has the same properties and composition, and serves the same function as decoupling layer  4  in previous embodiments, and second magnetic layer  52  with magnetization  52   m  is essentially equivalent in composition and function to previously described second magnetic layer  5 . In embodiments where thickness t 4  of first magnetic layer  50  in bottom shield  50   x  is equal to thickness t 3  of the first magnetic layer (bottom shield) in the previous 2DMR embodiment, BER may be degraded because RRS 2 &gt;RRS. However, in preferred embodiments, thickness t 4  is reduced so that RRS 2  ( FIG. 15 )=RRS ( FIG. 14 ), without significantly compromising BER in the second reader while enabling improved stabilization of magnetization  31   m  in junction shields  31 , and of magnetization  3   m  in free layer  3   f . In all embodiments described herein, one or both of first magnetic layer  1  and first magnetic layer  50  continue to protect the overlying MR sensor from stray (external) magnetic fields. 
     Although not shown, the present disclosure anticipates that both bottom shields  1 ,  50  in the 2DMR structure shown in  FIG. 6  may be replaced with a modified bottom shield to provide enhanced stability to free layer magnetization  2   m  and free layer magnetization  3   m , respectively. In other words, the 2DMR configuration shown in  FIG. 14  may be modified with the replacement of bottom shield  50  with bottom shield  50   x  from the  FIG. 15  embodiment. 
     The present disclosure also encompasses a method of fabricating a modified bottom shield  1   x ,  1   x - 1 ,  1   x - 2 , or  1   x - 3  described previously. Note that the process flow for forming a MR sensor with junction shields on a bottom shield is found in related U.S. Pat. No. 9,230,577. With regard to  FIG. 8 , the first magnetic layer  1  is formed on a substrate such as dielectric layer  81  ( FIG. 4 ) by an electroplating method, for example, to give a thickness t from 0.5 to 5 microns. Thereafter, the decoupling layer  4  is preferably deposited at the desired thickness t 1  with a physical vapor deposition (PVD) process at a temperature from 25° C. to 250° C. Second magnetic layer may be electroplated on the decoupling layer, or optionally deposited by a PVD method, depending on the thickness t 2 , composition, and desired film uniformity. In some embodiments, bottom shield layers  4  and  5  may be formed in the same chamber with two sequential PVD steps without breaking a vacuum to minimize process time. 
     To demonstrate the benefits of the modified bottom shield described herein, an experiment was performed to compare a MR sensor stabilized using a conventional bottom shield with a MR sensor that is stabilized with a bottom shield  50   x  described previously with respect to  FIG. 15 . In particular, a 2DMR structure was fabricated with a top sensor in a second reader (R 2 ) formed on a bottom shield having a trilayer ( 50 / 51 / 52 ) stack of layers according to an embodiment of the present disclosure, and a bottom MR sensor in a first reader (R 1 ) formed on a conventional bottom shield consisting of a 2 micron thick NiFe layer. A plurality of 2DMR devices was formed on two different wafers before being probed with a quasi static tester where a resistance across a MR sensor is measured while scanning with an external magnetic field. 
       FIGS. 16A-16B  are plots showing the cumulative distribution of Barkhausen jump normalized by sensor amplitude when an external magnetic field is scanned from −600 Oe to +600 Oe. Threshold is defined as the ratio of Barkhausen jump to the sensor amplitude. The plots are showing the percentage of heads that show jump values less than the threshold. In both wafer  1  ( FIG. 16A ) and wafer  2  ( FIG. 16B ), the second reader (top reader) clearly has significantly better distribution than the first reader (bottom reader). Note that the cumulative distribution encompasses results from a plurality of MR sensor devices across each wafer. 
       FIGS. 17A-17B  are plots showing the cumulative distribution of hysteresis as a function of sensor amplitude when an external magnetic field is scanned from −600 Oe to +600 Oe. Here, threshold is defined as the cumulative distribution of hysteresis as a function of sensor amplitude. Again, for both wafer  1  ( FIG. 17A ) and wafer  2  ( FIG. 17B ), R 2  has significantly better performance than R 1 . Accordingly, R 2  is expected to have SNR that is substantially improved over SNR for R 1 . More importantly, R 2  will have better stability after excitation by a magnetic field. 
     In summary, we have disclosed a scheme for improved stabilization in a MR sensor wherein a bottom shield has improved stability because of a multilayer stack wherein an uppermost magnetic layer adjoining junction shields in the MR sensor is stabilized compared with a conventional single layer bottom shield because of decoupling from a bottommost magnetic layer that is prone to domain wall motion (instability). Moreover, in some embodiments additional stability is provided through AF coupling between the upper magnetic layer and a middle magnetic layer in the multilayer stack, and through insertion of an AFM pinning layer in the multilayer stack. When the improved bottom shield design is incorporated in one or both of a first reader (bottom reader) and a second reader in a 2DMR structure, or in a single reader structure, an enhanced sensor performance is realized in terms of better signal to noise ratio, which helps to improve bit error rate (BER) performance. Moreover, improved sensor stability is also realized because of the improved shield stability. 
     While this disclosure has been particularly shown and described with reference to, the preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of this disclosure.