Abstract:
A magnetoresistive stack for use in a magnetic read head has a plurality of layers including a ferromagnetic free layer, a ferromagnetic pinned layer, and an antiferromagnetic pinning layer. The pinned layer and pinning layer each have a greater number of structural grains than the free layer, which decreases a fluctuation of magnetization in the magnetoresistive stack without decreasing a spatial resolution of the magnetoresistive stack.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application claims priority from Provisional Application No. 60/317,321, filed Sep. 5, 2001 entitled “Magnetic Field Sensor with Large Pinned Layer” by T. Pokhil, O. Heinonen, and C. Hou. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to a magnetoresistive sensor for use in a magnetic read head. In particular, the present invention relates to a magnetoresistive read sensor having enhanced pinned layer magnetization and stability. 
     Magnetoresistive read sensors, such as giant magnetoresistive (GMR) read sensors, are used in magnetic data storage systems to detect magnetically-encoded information stored on a magnetic data storage medium such as a magnetic disc. A time-dependent magnetic field from a magnetic medium directly modulates the resistivity of the GMR read sensor. A change in resistance of the GMR read sensor can be detected by passing a sense current through the GMR read sensor and measuring the voltage across the GMR read sensor. The resulting signal can be used to recover the encoded information from the magnetic medium. 
     A typical GMR read sensor configuration is the GMR spin valve, in which the GMR read sensor is a multi-layered structure formed of a nonmagnetic spacer layer positioned between a ferromagnetic pinned layer and a ferromagnetic free layer. The magnetization of the pinned layer is fixed in a predetermined direction, typically normal to an air bearing surface of the GMR read sensor, while the magnetization of the free layer rotates freely in response to an external magnetic field. The resistance of the GMR read sensor varies as a function of an angle formed between the magnetization direction of the free layer and the magnetization direction of the pinned layer. This multi-layered spin valve configuration allows for a more pronounced magnetoresistive effect, i.e. greater sensitivity and higher total change in resistance, than is possible with anisotropic magnetoresistive (AMR) read sensors, which generally consist of a single ferromagnetic layer. 
     The pinned layer can be a single ferromagnetic layer or a multilayer synthetic antiferromagnet (SAF). An SAF includes a ferromagnetic reference layer and a ferromagnetic pinned layer which are magnetically coupled by a coupling layer such that the magnetization direction of the reference layer is opposite to the magnetization of the pinned layer. 
     A pinning layer is typically exchange coupled to the pinned layer to fix the magnetization of the pinned layer in a predetermined direction. The pinning layer is typically formed of an antiferromagnetic material. In antiferromagnetic materials, the magnetic moments of adjacent atoms point in opposite directions and, thus, there is no net magnetic moment in the material. 
     GMR spin valves are configured to operate in either a current-in-plane (CIP) mode or a current-perpendicular-to-plane (CPP) mode. In CIP mode, the sense current is passed through in a direction parallel to the layers of the read sensor. In CPP mode, the sense current is passed through in a direction perpendicular to the layers of the read sensor. 
     A tunneling magnetoresistive (TMR) read sensor is similar in structure to a GMR spin valve configured in CPP mode, but the physics of the device are different. For a TMR read sensor, rather than using a spacer layer, a barrier layer is positioned between the free layer and the pinned layer (or reference layer of the SAF). Electrons must tunnel through the barrier layer. A sense current flowing perpendicularly to the plane of the layers of the TMR read sensor experiences a resistance which is proportional to the cosine of an angle formed between the magnetization direction of the free layer and the magnetization direction of the pinned layer (or reference layer of the SAF). 
     One principal concern in the performance of magnetoresistive read sensors is the fluctuation of magnetization in the read sensor, which directly affects the magnetic noise of the read sensor. A key determinant of the fluctuation of magnetization in the read sensor is the lateral size of the pinned layer and the pinning layer. A large pinning layer contains a greater number of structural grains than a small pinning layer. The increased number of structural grains increases the pinning field direction dispersion in the pinning layer, which decreases fluctuations of magnetization in the pinned layer. This not only decreases the magnetic noise of the read sensor, but it also decreases the variation of pinning direction from sensor to sensor and improves the long term stability of the sensor. It is important, however, to ensure that the lateral size of the free layer is not increased. The spatial resolution of the read sensor (the areal density of magnetic data it can support) is determined by the size of the free layer, and therefore a small free layer provides a higher spatial resolution than a large free layer. 
     The present invention addresses these and other needs, and offers other advantages over current devices. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is a magnetoresistive stack for use in a magnetic read head. The magnetoresistive stack has a plurality of layers including a ferromagnetic free layer, a ferromagnetic pinned layer, and an antiferromagnetic pinning layer. The pinned layer and pinning layer each have a greater number of structural grains than the free layer, which decreases a fluctuation of magnetization in the magnetoresistive stack without decreasing a spatial resolution of the magnetoresistive stack. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram describing characteristics of a pinned layer and pinning layer of the present invention. 
     FIG. 2A is a layer diagram of a sensor structure of the present invention. 
     FIG. 2B is an alternative view of a sensor structure of the present invention. 
     FIG. 3A is a layer diagram of a first embodiment of a magnetoresistive stack of the present invention. 
     FIG. 3B is a layer diagram of a second embodiment of a magnetoresistive stack of the present invention. 
     FIG. 3C is a layer diagram of a third embodiment of a magnetoresistive stack of the present invention. 
     FIG. 4A is a layer diagram of a fourth embodiment of a magnetoresistive stack of the present invention. 
     FIG. 4B is a layer diagram of a fifth embodiment of a magnetoresistive stack of the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows the effect of the number of structural grains in an antiferromagnetic pinning layer on pinning field direction dispersion and average magnetization direction. A larger pinning layer with a greater number of structural grains has a significantly greater pinning field direction dispersion than a smaller pinning layer with a fewer number of structural grains. This produces a more uniform and defined direction of magnetization in the pinned layer. 
     FIG. 2A is a layer diagram of a sensor structure  10  of the present invention. Sensor structure  10  includes a pinning layer  12 , a pinned layer  14 , a spacer/barrier layer  16 , and a free layer  18 . Pinning layer  12  is an antiferromagnetic material. Pinned layer  14  can be a single ferromagnetic layer or a multilayer synthetic antiferromagnet, and is positioned adjacent to pinning layer  12 . Free layer  18  is a ferromagnetic material. Spacer/barrier layer  16  is positioned between pinned layer  14  and free layer  18 , and is a nonmagnetic conducting material when utilized as a spacer layer in a giant magnetoresistive (GMR) stack, or is a nonmagnetic insulating material when utilized as a barrier layer in a tunneling magnetoresistive (TMR) stack. 
     The magnetization of pinned layer  14  is fixed while the magnetization of free layer  18  rotates freely in response to an external magnetic field emanating from a magnetic medium. The magnetization of pinned layer  14  is pinned by exchange coupling pinning layer  12  with pinned layer  14 . The resistance of sensor structure  10  varies as a function of an angle that is formed between the magnetization of free layer  18  and the magnetization of pinned layer  14 . 
     Pinning layer  12  and pinned layer  14  each have a significantly greater number of structural grains (and thus a significantly greater lateral size) than free layer  18 . The lateral size of free layer  18  is typically about 4 square structural grains to about 9 square structural grains. The lateral size of pinning layer  12  and pinned layer  14  is typically about 64 square structural grains to about 100 square structural grains. This allows sensor structure  10  to exhibit a significantly lower fluctuation of magnetization than if pinning layer  12  and pinned layer  14  each had a similar lateral size to free layer  18 . In addition, because free layer  18  has a significantly smaller lateral size than pinning layer  12  and pinned layer  14 , sensor structure  10  exhibits a significantly higher spatial resolution than if free layer  18  had a similar lateral size to pinning layer  12  and pinned layer  14 . 
     FIG. 2B is an alternative view of sensor structure  10  showing its orientation relative to an air bearing surface (ABS). By elongating pinned layer  14  in a direction parallel to the ABS, sense current shunting through pinned layer  14  is reduced. By elongating pinned layer  14  in a direction perpendicular to the ABS, the shape anisotropy of pinned layer  14  will tend to align the magnetization of pinned layer  14  in the direction perpendicular to the ABS. 
     FIG. 3A is a layer diagram of a first embodiment of a giant magnetoresistive (GMR) stack  20  of the present invention. GMR stack  20  has a current-in-plane (CIP) geometry and includes a pinning layer  22 , a pinned layer  24 , a spacer layer  26 , a free layer  28 , permanent magnets  30 A and  30 B, and contacts  32 A and  32 B. Pinning layer  22  is an antiferromagnetic material. Pinned layer  24  can be a single ferromagnetic material or a multilayer synthetic antiferromagnet, and is positioned adjacent to pinning layer  22 . Free layer  28  is a ferromagnetic material. Spacer layer  26  is a nonmagnetic conducting material, and is positioned between pinned layer  24  and free layer  28 . Permanent magnets  30 A and  30 B are each positioned adjacent to pinned layer  24  and to a corresponding side of spacer layer  26  and free layer  28 . Contacts  32 A and  32 B are positioned adjacent to permanent magnets  30 A and  30 B, respectively, and to a corresponding side of free layer  28 . 
     The magnetization of pinned layer  24  is fixed while the magnetization of free layer  28  rotates freely in response to an external magnetic field emanating from a magnetic medium. The magnetization of pinned layer  24  is pinned by exchange coupling pinning layer  22  with pinned layer  24 . Permanent magnets  30 A and  30 B stabilize free layer  28  and provides proper bias. Contacts  32 A and  32 B provide a sense current through GMR stack  20 . The resistance of GMR stack  20  varies as a function of an angle that is formed between the magnetization of free layer  28  and the magnetization of pinned layer  24 . The GMR signal produced by GMR stack  20  is generated by the sense current flowing parallel to the layers of GMR stack  20 . 
     Pinning layer  22  and pinned layer  24  each have a significantly greater number of structural grains (and thus a significantly greater lateral size) than free layer  28 . The lateral size of free layer  28  is typically about 4 square structural grains to about 9 square structural grains. The lateral size of pinning layer  22  and pinned layer  24  is typically about 64 square structural grains to about 100 square structural grains. This allows GMR stack  20  to exhibit a significantly lower fluctuation of magnetization than if pinning layer  22  and pinned layer  24  each had a similar lateral size to free layer  28 . In addition, because free layer  28  has a significantly smaller lateral size than pinning layer  22  and pinned layer  24 , GMR stack  20  exhibits a significantly higher spatial resolution than if free layer  28  had a similar lateral size to pinning layer  22  and pinned layer  24 . 
     GMR stack  20  would also function similarly if permanent magnets  30 A and  30 B were replaced by antiferromagnetic exchange tabs coupled to the outer regions of free layer  28 . 
     FIG. 3B is a layer diagram of a second embodiment of a GMR stack  20 ′ of the present invention. GMR stack  20 ′ is similar to GMR stack  20  of FIG.  3 A. Spacer layer  26 ′, however, differs from spacer layer  26  of GMR stack  20  in that spacer layer  26 ′ has a similar lateral size to pinned layer  24  (instead of free layer  28 ). Spacer layer  26 ′ is a nonmagnetic conducting material and separates permanent magnets  30 A and  30 B from pinned layer  24 . This prevents direct exchange coupling between permanent magnets  30 A and  30 B and pinned layer  24 , which can disturb spin structure in pinned layer  24 . 
     FIG. 3C is a layer diagram of a third embodiment of a magnetoresistive stack  20 ″ of the present invention. GMR stack  20 ″ is similar to GMR stack  20  of FIG.  3 A. GMR stack  20 ″, however, differs from GMR stack  20  in that GMR stack  20 ″ includes separator layers  27 A and  27 B. Separator layer  27 A is positioned between permanent magnet  30 A and pinned layer  24 , and separator layer  27 B is positioned between permanent magnet  30 B and pinned layer  24 . Separator layers  27 A and  27 B are a nonmagnetic conducting material and prevent direct exchange coupling between permanent magnets  30 A and  30 B and pinned layer  24 , which can disturb spin structure in pinned layer  24 . 
     FIG. 4A is a layer diagram of a fourth embodiment of a GMR stack  40  of the present invention. Magnetoresistive stack  40  has a current-perpendicular-to-plane (CPP) geometry and includes a pinning layer  42 , a pinned layer  44 , a spacer layer  46 , a free layer  48 , permanent magnets  50 A and  50 B, and contacts  52 A and  52 B. Pinning layer  42  is an antiferromagnetic material. Pinned layer  44  can be a single ferromagnetic material or a multilayer synthetic antiferromagnet, and is positioned adjacent to pinning layer  42 . Free layer  48  is a ferromagnetic material. Spacer layer  46  is a nonmagnetic conducting material, and is positioned between pinned layer  44  and free layer  48 . Permanent magnets  50 A and  50 B are each positioned adjacent to pinned layer  44  and to a corresponding side of spacer layer  46  and free layer  48 . Contacts  52 A and  52 B are positioned adjacent to free layer  48  and pinning layer  42 , respectively. 
     The magnetization of pinned layer  44  is fixed while the magnetization of free layer  48  rotates freely in response to an external magnetic field emanating from a magnetic medium. The magnetization of pinned layer  44  is pinned by exchange coupling pinning layer  42  with pinned layer  44 . The resistance of GMR stack  40  varies as a function of an angle that is formed between the magnetization of free layer  48  and the magnetization of pinned layer  44 . The GMR signal produced by GMR stack  40  is generated by a sense current flowing perpendicularly through the layers of GMR stack  40 . 
     Pinning layer  42  and pinned layer  44  each have a significantly greater number of structural grains (and thus a significantly greater lateral size) than free layer  48 . The lateral size of free layer  48  is typically about 4 square structural grains to about 9 square structural grains. The lateral size of pinning layer  42  and pinned layer  44  is typically about 64 square structural grains to about 100 square structural grains. This allows GMR stack  40  to exhibit a significantly lower fluctuation of magnetization than if pinning layer  42  and pinned layer  44  each had a similar lateral size to free layer  48 . In addition, because free layer  48  has a significantly smaller lateral size than pinning layer  42  and pinned layer  44 , GMR stack  40  exhibits a significantly higher spatial resolution than if free layer  48  had a similar lateral size to pinning layer  42  and pinned layer  44 . 
     GMR stack  40  would also function similarly if permanent magnets  50 A and  50 B were replaced by antiferromagnetic exchange tabs coupled to the outer regions of free layer  48 . 
     FIG. 4B is a layer diagram of a fifth embodiment of a tunneling magnetoresistive (TMR) stack  40 ′ of the present invention. TMR stack  40 ′ is similar to GMR stack  40  of FIG.  4 A. Barrier layer  46 ′, however, differs from spacer layer  46  of GMR stack  40  in that barrier layer  46 ′ is a nonmagnetic insulating material (instead of a nonmagnetic conducting material). The TMR signal produced by TMR stack  40 ′ is generated by a sense current flowing perpendicularly through the layers of TMR stack  40 ′. 
     For both GMR stack  40  and TMR stack  40 ′, permanent magnets  50 A and  50 B can be separated from pinned layer  44  using techniques similar to those in FIGS. 3B and 3C. 
     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.