Abstract:
Magnetoresistive (MR) sensors are disclosed having mechanisms for reducing edge effects such as Barkhausen noise. The sensors include a pinned layer and a free layer with an exchange coupling layer adjoining the free layer, and a ferromagnetic layer having a fixed magnetic moment adjoining the exchange coupling layer. The exchange coupling layer and ferromagnetic layer form a synthetic antiferromagnetic structure with part of the free layer, providing bias that reduces magnetic instabilities at edges of the free layer. Such synthetic antiferromagnetic structures can provide a stronger bias than conventional antiferromagnetic layers, as well as a more exactly defined track width than conventional hard magnetic bias layers. The synthetic antiferromagnetic structures can also provide protection for the free layer during processing, in contrast with the trimming of conventional antiferromagnetic layers that exposes if not removes part of the free layer.

Description:
BACKGROUND 
     The present invention relates to magnetoresistive (MR) sensing mechanisms, which may for example be employed in information storage systems or measurement and testing systems. 
       FIG. 1  shows a prior art spin valve (SV) sensor  20  that may be used in a head of a disk drive. The sensor  20  has a magnetic pinning structure  22  that may be a permanent magnet, an antiferromagnetic material, or a synthetic antiferromagnetic structure. Pinning structure  22  may include plural layers, as is well known. For example, a synthetic antiferromagnetic structure includes a pair of ferromagnetic layers sandwiched about a very thin exchange coupling layer of ruthenium (Ru), iridium (Ir) or rhodium (Rh). The pinning structure  22  functions to pin or set the magnetization of an adjoining first ferromagnetic layer  25 , which may be termed the pinned layer. Adjoining the pinned layer  25  is an electrically conductive spacer layer  27 , which is adjoined by a second ferromagnetic layer  29  that may be termed the free layer. 
     The sensor  20  may be formed on a wafer substrate with thousands of other sensors. For the situation in which sensor  20  has the pinned layer  22  formed prior to the free layer  29 , sensor  20  may be termed a bottom SV sensor. A SV sensor in which the order is reversed, with a free layer formed prior to the pinned layer, may be termed a top SV sensor. 
     After formation of the pinned and free layers as described above, the sensor  20  is separated from other sensors on the wafer by ion beam etching (IBE) or similar removal techniques, and chromium (CR) layers  30  are formed, followed by cobalt-chrome-platinum (CoCrPt) bias layers  33  and lead layer  35 . The Cr layers provide a surface that encourages growth of bias layers  33  as a permanent or “hard” magnet. The bias layers  33  provide a magnetic field to the adjacent edges of the free layer, to reduce magnetic instability in those edges that could otherwise result in noise. 
     In operation, the leads  35  provide a sense current that flows along the conductive layer  27 , with the resistance to that flow dependent upon the relative magnetization directions of the free  29  and pinned  27  layers. The magnetization direction of the free layer  29  is designed to change due to magnetic fields from a storage medium such as a disk, while the magnetization direction of the pinned layer  29  remains constant, so that the change in current or voltage of the leads  35  indicates the magnetic field direction of the medium. 
       FIG. 2  shows another type of bottom SV sensor  50 , for which the pinning structure  22 , pinned layer  25 , conductive layer  27  and free layer  29  are essentially the same as described previously. Sensor  50  has a pair of antiferromagnetic layers  52 , however, overlapping free layer  29  in order to pin edges of the free layer to reduce magnetic instability in those edges that could otherwise result in noise. Leads  55  cause current to flow through conductive layer  27 , with the resistance to that current indicating the magnetic fields felt by the sensor. 
     The antiferromagnetic layers  52  are formed by IBE or similar trimming that may remove a small amount of the free layer  29 , which can create additional edge effects in the free layer at the edge of the antiferromagnetic layers  52 . It is also possible to have a hard bias layer overlapping the free layer, however, in this case a seed layer of Cr that is formed to encourage appropriate growth of the hard bias layer interferes with the coupling between the hard bias layer and the free layer. 
     SUMMARY 
     Magnetoresistive (MR) sensors are disclosed that offer improved mechanisms for reducing edge effects such as Barkhausen noise in a sensing layer. Such sensors include a pinned layer and a free layer with an exchange coupling layer adjoining the free layer, and a ferromagnetic layer having a fixed magnetic moment adjoining the exchange coupling layer. The exchange coupling layer and ferromagnetic layer form a synthetic antiferromagnetic structure with part of the free layer, providing bias that reduces magnetic instabilities at edges of the free layer. 
     Such synthetic antiferromagnetic structures can provide a stronger bias than conventional antiferromagnetic layers, as well as a more exactly defined track width than conventional hard magnetic bias layers. The synthetic antiferromagnetic structures can also provide protection for the free layer during processing, in contrast with the trimming of conventional antiferromagnetic layers that exposes if not removes part of the free layer. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  shows a prior art bottom spin valve (SV) sensor having hard magnetic layers that bias a free layer. 
         FIG. 2  shows a prior art bottom SV sensor having antiferromagnetic layers that bias a free layer. 
         FIG. 3  shows a bottom spin valve (SV) sensor having synthetic antiferromagnetic layers that bias a free layer. 
         FIG. 4  shows some initial steps in the formation of the sensor of  FIG. 3 . 
         FIG. 5  shows some steps in the formation of the sensor of  FIG. 3  subsequent to those shown in  FIG. 4 . 
         FIG. 6  shows some steps in the formation of a top spin valve (SV) sensor that may have synthetic antiferromagnetic layers that bias a free layer. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 3  depicts a bottom SV or giant magnetoresistive (GMR) sensor  100  in accordance with the present invention. A pinning structure  102  functions to pin or set a magnetic moment of an adjoining first ferromagnetic layer  104 , which may be termed the pinned layer. The pinned layer  104  in this figure has a magnetic moment directed away from the viewer, as shown by X marks  105 . The pinning structure  102  may include a permanent or hard magnetic layer, an antiferromagnetic layer, or a synthetic antiferromagnetic structure that adjoins a hard magnetic layer or antiferromagnetic layer. 
     Such a synthetic antiferromagnetic structure may include a pair of ferromagnetic layers sandwiched about a nonferromagnetic layer, such as a very thin exchange coupling layer of ruthenium (Ru), iridium (Ir) or rhodium (Rh). The ferromagnetic layers may be made of nickel-iron (NiFe) and a seed layer of nickel-iron-chromium (NiFeCr) may be formed prior to forming the synthetic antiferromagnetic structure. 
     Adjoining the pinned layer  104  is an electrically conductive spacer layer  106 , which is adjoined by a second ferromagnetic layer  108  that may be termed the free layer. The pinned layer  104  and the free layer  108  may each be formed of NiFe or other materials known in the art of MR sensors, and may each have a thickness in a range between less than 5 angstroms (Å) and greater than 30 Å. The spacer layer  106  may be formed of copper (Cu), gold (Au) or silver (Ag) or other materials known in the art, and may have a thickness in a range between less than 15 Å and greater than 30 Å. 
     Adjoining the free layer  108  is a coupling layer  110  made of a nonferromagnetic, electrically conductive material. In one embodiment, the coupling layer  110  is made of material such as ruthenium (Ru), iridium (Ir) or rhodium (RH), and has a thickness that may be in a range between about 5 Å and about 10 Å. Bias layers  111  and  112 , which may be formed of ferromagnetic material to a thickness between about 5 Å and about 30 Å, adjoin the coupling layer  110  adjacent to edges  113  and  114  of the free layer  108 , the bias layers having a magnetic moment fixed in a direction substantially perpendicular to that of the pinned layer  104 . Antiferromagnetic layers  115  and  116  or other pinning structures adjoin the bias layers  111  and  112 , pinning the magnetic moments of the bias layers  111  and  112  in a direction shown by arrows  117  and  118 . Coupling layer  110 , bias layers  111  and  112 , and antiferromagnetic layers  115  and  116  together form bias structures for edge portions of the free layer  108 . 
     Electrically conductive leads  120  and  121  abut edges  113  and  114 , contacting the pinned layer  104 , spacer layer  106  and free layer  108 . Leads  120  and  121  may be formed of a layer of gold (Au) sandwiched by a pair of tantalum (Ta) layers. In operation, the leads  120  and  121  provide a sense current that flows at least along the spacer layer  106 , with the resistance to that flow dependent upon the relative directions of the magnetic moments of the free  108  and pinned  104  layers. The magnetization direction of the free layer  108  is designed to change in response to magnetic fields from a storage medium such as a disk, while the magnetization direction of the pinned layer  104  remains constant, so that a change in sense current or voltage between the leads indicates a change in the magnetic field of the medium. 
     Portions of the free layer  108  located near edges  113  and  114  and overlapped by the bias layers  111  and  112  are coupled to the bias layers, with magnetic moments directed substantially opposite to those of the bias layers. Magnetic domains of those edge portions of the free layer  108  are thus stabilized by coupling to bias layers  111  and  112 , reducing Barkhausen noise in the sensor  100 . The magnetic stabilization provided to edge portions of free layer  108  by bias structures can be stronger than that provided by prior art antiferromagnetic or hard magnetic bias mechanisms. Moreover, the portion of the free layer that is not overlapped by the bias layers  111  and  112  may define a sensing width (sometimes called a trackwidth) of sensor  100  more exactly than in prior art sensors. 
       FIG. 4  shows some initial steps in forming the sensor  100  of  FIG. 3 . On a wafer substrate  133  that may be made of alumina-titanium-carbide (Al 2 O 3 TiC) or silicon carbide (SiC), for example, first pinning structure  102  has been formed by sputtering or other vacuum deposition techniques. Pinning structure  102  may include a first antiferromagnetic layer made of material such as PtMn, PtPdMn, IrMn, FrMnRh, FeMn or other materials that have antiferromagnetic properties. Pinning structure  102  may also a include synthetic antiferromagnetic structure formed of a pair of ferromagnetic layers separated by an exchange coupling layer, such as a less than 10 Å thick layer of Ru or Rh. A seed layer of NiFeCr or other materials may be formed prior to forming pinning structure  102 . Atop pinning structure  102 , first (pinned) ferromagnetic layer  104 , electrically conductive (spacer) layer  106 , and second (free) ferromagnetic layer  108  have been formed by similar techniques. 
     Exchange coupling layer  110  is then formed atop free layer  108 , followed by formation of a third ferromagnetic layer  150 , and formation of a second antiferromagnetic layer  152  made of material such as PtMn, PtPdMn, IrMn, FrMnRh, FeMn, NiO or other materials that have antiferromagnetic properties, all of which may be formed by sputtering or other vacuum deposition techniques. In order to create individual sensors from the above-described layers that coat the wafer  133 , masks such as conventional bi-layer mask  155  are defined by photolithography. Mask  155  has outer edges  157  and  158  and inner edges  162  and  163 , that are used to define outer edges and trackwidth of the sensor, respectively. Outer edges of the sensor, including edges  113  and  114  of the free layer  108 , are defined by IBE or similar directional etching that removes uncovered sensor layers. Leads  120  and  121  are then created by sputtering or other vacuum deposition techniques, with a portion of leads  120  and  121  extending to meet inner edges  162  and  163 , respectively. Mask  155  may be coated with a layer of the material  166  used to form the leads, but overhangs of the bi-layer mask  155  allow access by a solvent, dissolving mask  155  and lifting off material  166  as well. 
     In  FIG. 5 , the mask has been removed, exposing an inner portion of layer  152 . Inner portion of layers  150  and  152  are then removed by IBE or similar directional etching, leaving coupling layer  110  protecting free layer  108 , as shown in  FIG. 3 . 
     Different magnetic directions of the pinned layer  105  and the bias layers  113  and  114  may be set by using antiferromagnetic pinning materials having different blocking temperatures for the pinned and bias layers, and changing the magnetic field direction while annealing at the different blocking temperatures. That is, a magnetic field is applied in a first direction as the temperature is reduced from above the blocking temperature of the higher blocking temperature materials, and then a magnetic field is applied in a second direction as the temperature is reduced from above the blocking temperature of the lower blocking temperature materials. 
     For the case in which pinning structure  102  includes an antiferromagnetic layer with a relatively low blocking temperature (e.g., IrMn, FrMnRh or FeMn) and antiferromagnetic layers  115  and  116  include an antiferromagnetic material with a relatively high blocking temperature (e.g., PtMn or PtPdMn), the applied magnetic field is oriented along the direction desired for bias layers  111  and  112  as the temperature is reduced from above the high blocking temperature, and then the applied magnetic field is oriented along the direction desired for pinned layer  104  as the temperature is reduced from above the low blocking temperature. 
     For the case in which pinning structure  102  includes an antiferromagnetic layer with a relatively high blocking temperature (e.g., PtMn or PtPdMn) and antiferromagnetic layers  115  and  116  include an antiferromagnetic material with a relatively low blocking temperature (e.g., IrMn, FrMnRh or FeMn), the applied magnetic field is oriented along the direction desired for pinned layer  104  as the temperature is reduced from above the high blocking temperature, and then the applied magnetic field is oriented along the direction desired for bias layers  111  and  112  as the temperature is reduced from above the low blocking temperature. 
     For some sensors it may be desirable to employ a material with a high blocking temperature in a layer formed prior to forming the free and pinned layers, to avoid damage to the free and pinned layers by fixing the magnetization with high temperature annealing prior to forming the free and pinned layers. The sensor  100  may be part of a magnetic head that includes a write element that may be previously or subsequently formed. Alternatively, the sensor may be used for measuring or testing magnetic fields. 
       FIG. 6  shows some initial steps in forming another sensor  200  in accordance with the present invention. In this embodiment, a Cr seed layer  202  is formed over a wafer substrate  201 , the Cr layer  202  encouraging favorable crystallographic formation of CoCrPt hard magnetic layers  203  and  205  having a magnetic moment in a longitudinal direction shown by arrows  218 . The Cr layer  202  may be formed to a thickness of between 50 Å and 300 Å, for example, and hard magnetic layers  203  and  205  may be formed to a thickness of between 5 Å and 200 Å, for example. The Cr layer  202  and hard magnetic layer  203  are then trimmed, for example by IBE, and a nonferromagnetic, electrically insulating or highly resistive spacer layer  204  is formed between the hard magnetic layers  203  and  205 . A first soft or free ferromagnetic layer  206  is then formed adjoining the hard magnetic layers  203  and  205 . Alternatively, instead of employing a hard magnet for biasing, layers  202  may be antiferromagnetic and layers  203  may be formed of a ferromagnetic bias layer and an electrically conductive, nonferromagnetic exchange coupling layer, on which the free layer may be formed and coupled to the bias layer. 
     An electrically conductive spacer layer  208  is then formed on the free layer  206 , followed by a second soft or pinned ferromagnetic layer  210 . An electrically conductive, nonferromagnetic exchange coupling layer  212  is then formed, followed by another ferromagnetic layer  214 . An antiferromagnetic layer  216  is then formed which is used to fix the magnetic moment of the ferromagnetic layer  214  in a lateral direction, with the pinned layer  210  having a magnetic moment in an opposite direction, as shown by X-marks  219 . A bi-layer mask  220  and then leads  222  are formed, with excess lead material  225  removed by dissolving the mask  220 . The resulting bottom SV sensor has improved biasing of the free layer for reduced Barkhausen noise, as well as an accurately defined trackwidth. 
     Although we have focused on teaching the preferred embodiments of an improved magnetoresistive sensor, other embodiments and modifications of this invention will be apparent to persons of ordinary skill in the art in view of these teachings. Therefore, this invention is limited only by the following claims, which include all such embodiments and modifications when viewed in conjunction with the above specification and accompanying drawings.