Abstract:
A method of initializing a magnetic sensor and storage system implementing such a magnetic sensor. The method includes heating and cooling dual antiferromagnetic layers in the presence of a magnetic field.

Description:
RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 09/809,687 filed Mar. 14, 2001, now U.S. Pat. No. 6,721,146. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a sensor used in a disk drive as a magnetic read back sensor and a manufacturing method thereof. 
     DESCRIPTION OF THE BACKGROUND ART 
     Magnetic sensors used in disk drives or tape drives for reading back magnetically recorded information are usually based on thin film structures utilizing a magnetoresistive effect. In particular, recent sensors are based on a spin dependent scattering phenomenon and are generally called giant magnetoresistive (GMR) sensors or spin valve sensors. These sensors depend on having one magnetic layer, called the free layer, in which the direction of magnetization is free to move in response to the magnetic field applied to the sensor. Another layer is called the pinned layer, in which the direction of magnetization is not free to move and is perpendicular to the direction of the magnetization of the free layer when there is no applied external field. In order to achieve maximum sensitivity and linearity, it is required that the magnetization of the free layer in the absence of an applied field to be substantially parallel with the direction of the recorded track. Accordingly it is required that the magnetization in the pinned layer be substantially perpendicular to the recorded track. 
     Another requirement for the free layer is that there be longitudinal magnetic bias stabilization. Imposing a preferred magnetization direction in the free layer along the axis of the free layer parallel to the medium and perpendicular to the direction of the track insures good linearity and provides robustness to deleterious effects such as Barkhausen noise. 
     A common method of providing for pinning the pinned layer is to place a layer of antiferromagetic material, AFM, adjacent to the pinned layer. Then, at some point in the manufacture of the head, the structure is heated above the blocking temperature of the AFM, and an external field is placed on the device which is perpendicular to the recorded track direction. The blocking temperature of an AFM material is the temperature above which the material no longer has any exchange coupling strength. The structure is then cooled in the presence of the field. The applied field will orient the pinned layer in the proper direction and as the AFM cools below the blocking temperature exchange coupling will maintain the orientation of the magnetization in the pinned layer. This is the pinning process. 
     There are at least two possible techniques to provide for longitudinal biasing of the free layer. A common method is to provide two hard magnets, one on each side of the portion of the free layer which defines the track width. This is referred to as hard biasing. During the manufacture of the sensor, the direction of the magnetization in the hard magnet must be set by placing the sensors in a large magnetic field. The hard bias method has some undesirable attributes such as gradual reduction of sensitivity at track edges and is somewhat difficult to control in manufacturing. 
     Another method of providing for longitudinal biasing of the free layer is to use an AFM layer to deliver exchange coupling similar to that for the pinned layer. The principle difficulty with this approach is that the direction of magnetization in the free layer must be substantially perpendicular to the direction of magnetization in the pinned layer. Thus the steps of heating and subsequent cooling in a field would be appropriate for one of the AFM layers, but not the other. To solve this problem in the past, two different AFM materials have been used which had distinctly different blocking temperatures. The AFM layer with the highest blocking temperature was set first. Then the field angle was rotated 90° and the second AFM layer was set at a lower temperature. For reasons of magnetic performance and manufacturability there is generally one optimum AFM material which would serve for both pinned and longitudinal stabilization. However because of the requirement to have AFM materials with different blocking temperatures, the optimum choice of AFM materials was compromised. 
     SUMMARY OF THE INVENTION 
     One object of the present invention is to provide a GMR sensor which has an AFM layer for pinning the pinned layer and another AFM layer for longitudinal stabilization of the free layer. Another object is to provide a sensor which can be constructed such that the two AFM layers can have substantially the same blocking temperature. Thus, the sensor can have the two AFM layers made of the same material. Typically there is a limited number of AFM materials which would be optimum choices for both layers based on other considerations such as ease of manufacturing, sensor sensitivity, thermal stability, etc. Another object is to provide a greatly simplified manufacturing process in which the two AFM layers can be set simultaneously during a single sequence of heating and subsequent cooling in a magnetic field. Another object is to provide a disk drive having a GMR sensor using two AFM layers. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a perspective view of a slider head assembly used in magnetic recording in a disk drive. 
         FIG. 2   a  shows a cross sectional view of the layers in a sensor which has a single pinned layer. 
         FIG. 2   b  shows a cross sectional view of the layers in a sensor which has a single pinned layer including views of the bias current direction and the shields. 
         FIG. 3  shows a cross sectional view of the layers in a sensor which has a set of antiparallel layers for the composite pinned layer. 
         FIG. 4   a  shows an exploded view of the magnetization directions in the free layer and the two magnetic layers comprising the pinned layer of a sensor while in the presence of a high external magnetic field according to an embodiment of the present invention. 
         FIG. 4   b  shows an exploded view, similar to  FIG. 4   a , of the directions of magnetization in the layers while in the presence of a medium external magnetic field. 
         FIG. 4   c  shows an exploded view, similar to  FIGS. 4   a  and  4   b , of the directions of magnetization in the layers while in the optimum field. 
         FIG. 4   d  shows a plot of the angle as a function of external field magnitude for one particular head design. 
         FIG. 5  shows a view of the layers in a sensor which has a set of antiparallel layers for the bias tabs and a simple pinned layer. 
         FIG. 6  shows a view of the layers in a sensor which has exchange biased antiparallel coupled layers for both the pinned layer and the bias tabs. 
         FIG. 7  shows a GMR sensor wherein the width of the layers in the sensor approximately defines the magnetic trackwidth. 
         FIG. 8   a  shows a cross section view of a recording device using the invented sensor. 
         FIG. 8   b  shows a top view of a recording device using the invented sensor. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In  FIG. 1  a diagram of a typical slider and head assembly is shown. The slider  101  is usually made of a rigid ceramic material such as alumina-TiC and is attached to a flexible metal suspension (not shown). A recording head  102  is constructed on the trailing surface  104  of the slider  101 . The recording head usually comprises a write element, also known as the write head, and a separate read element, also known as the read head. In addition to the recording head there are metallic pads  103  used to make electrical connections to the recording head. One side  105  of the slider is designed to be in close proximity to a disk and has air bearing features. 
       FIG. 2   a  shows some of the layers of a conventional read element  200  which utilizes the spin-valve (or GMR) effect. A layer which comprises an antiferromagnetic (AFM) material  201  is placed on a substrate layer  202 . The substrate layer can be an insulating layer or an adhesion or seed layer. The AFM material can be chosen from a range of materials including but not limited to PtMn, NiMn, IrMn, PdMn, and NiO. A pinned layer  203  is then placed on the AFM layer. A thin nonmagnetic, electrically conductive layer  204 , usually copper, is then placed on the pinned layer. A free layer  205  is then placed on the nonmagnetic layer  204 . The free layer is usually, but not limited to, a ferromagnetic alloy such as NiFe, CoFe, or CoNiFe. Finally, a pair of stabilization tabs  206  are formed on a surface of the free layer  205 . The tabs  206  define the track width and provide longitudinal magnetic stabilization for the free layer. The material in the tabs  206  may be either a hard magnetic material such as an alloy typically comprising cobalt and platinum, or an AFM material such as PtMn which exchange couples with a portion of the free layer. The view of  FIG. 2   a  is that of the sensor as presented to the disk in a disk drive. To achieve a nearly linear response it is desired that the direction of the magnetization  207  in the free layer be substantially parallel to the disk. This is true of all conventional GMR sensors used in magnetic recording. In a GMR sensor it is also desired that the magnetization of the pinned layer be substantially perpendicular to the magnetization of the free layer. Accordingly the magnetization of the pinned layer  208  is shown perpendicular to that of the free layer  207  and also perpendicular to the surface of the disk. 
       FIG. 2   b  shows the stack of sensor layers as positioned between the magnetic shields  209 . The purpose of the shields is to restrict the spatial sensitivity of the sensor to a very limited area on the disk. The sensor stack is electrically insulated from the shields by insulating layers  210 . In order to extract the electrical signal from the sensor, current  211  is passed through electrical leads  212  through the sensor stack. The direction of the current  211  is parallel to the free layer  205  and is perpendicular to the track direction. 
     The sensor  200  shown in  FIG. 2   a  has a pinned layer which is comprised of substantially one magnetic layer. This structure is generally called a simple spin valve or simple GMR structure. One difficulty with the simple GMR structure is that there is a noticeable demagnetization field from the pinned layer. This can cause problems with linearity and restricts the choices of magnetic biasing. These problems are greatly reduced by using a composite structure for the pinned layer comprised of two ferromagnetic layers separated by a thin nonmagnetic layer in such a way that the two ferromagnetic layers are antiparallel, AP, coupled. This composite structure is called an AP pinned substructure. The ferromagnetic material is typically an alloy comprising Ni, Fe, and Co. The thin nonmagnetic layer between the two ferromagnetic layers is usually but not limited to ruthenium. A sensor having an AP pinned substructure is called an AP pinned GMR sensor, or synthetic AFM GMR sensor. 
     A view of a typical AP pinned GMR structure is shown in  FIG. 3 .  FIG. 3  also illustrates a preferred embodiment wherein the two bias stabilization tabs  308  are constructed of an AFM material exchange coupled with selected portions of the free layer  307 . One AFM layer  301  is used for controlling the direction of the first ferromagnetic layer  303  in the AP pinned substructure  309  and a second AFM layer  308  is used for the longitudinal bias of the free layer  307 . The AP pinned substructure  309  serves as the pinned layer of the sensor. 
     In  FIG. 3 , an AFM layer  301  is placed on the substrate  302 . A first ferromagnetic layer  303  is placed on the AFM layer  301 . Then a thin nonmagnetic layer  304  is deposited on the first ferromagnetic layer  303 . A second ferromagnetic layer  305  is then placed on the thin magnetic layer  304 . The composite of layers comprising the first ferromagnetic layer  303 , the thin nonmagnetic layer  304  and the second ferromagnetic layer  305  is the AP pinned substructure. The rest of the sensor including the thin nonmagnetic layer  306 , the free layer  307 , and the bias stabilization tabs  308  serve the same function as in the simple GMR case. Because of the presence of the AP pinned substructure, the sensor in  FIG. 3  is generally referred to as an AP pinned GMR. 
     The direction of the magnetization  320  in the free layer  307  in  FIG. 3  is parallel to the surface of the disk (not shown). It is desirable that the net magnetization of the AP pinned substructure  309  is substantially perpendicular to the magnetization  320  in the free layer  307 . It is possible for the net magnetization of the AP pinned substructure  309  to be zero. In that case the magnetization of each of the ferromagnetic layers  303 , 305  in the pinned layer  309  should be substantially perpendicular to that of the free layer  307 . Thus in  FIG. 3  the magnetization  310  of the first ferromagnetic layer  303  in the AP pinned layer  309  is shown perpendicular to that of the free layer  307  and as coming out of the field of view. Because the two ferromagnetic layers  303 , 305  in the AP pinned substructure have magnetization directions  310 , 311  which are substantially at 180 degrees with respect to each other (in the absence of an external field), the direction of the magnetization  311  of the second ferromagnetic layer  305  in the AP pinned substructure  309  is also perpendicular to the direction of the magnetization  320  in the free layer  307  but is approximately 180 degrees relative to the direction of magnetization  310  of the first ferromagnetic layer  303 . This magnetization  311  is shown going into the field of view in  FIG. 3 . 
     During the manufacturing of these structures it is desirable to establish the appropriate directions of the magnetization in the first ferromagnetic layer and the free layer. In a conventional sensor two different AFM materials are chosen so that the blocking temperatures of the two are distinctly different. As an example, if the AFM with the highest blocking temperature is adjacent to the free layer, then the conventional method is to first heat the structure above the blocking temperature of both of the AFM materials. The sensor is then placed in a magnetic field along the direction of the free layer which aligns the magnetization of the free layer in the proper direction. The sensor is then cooled below the blocking temperature of this AFM layer but held above the blocking temperature of the AFM layer adjacent to the pinned layer. While the structure is held at this intermediate temperature the field is rotated 90° relative to the former direction. The assembly is then cooled to ambient temperature. It is critical that an external magnetic field be imposed on the ferromagnetic material as the AFM material cools from a temperature higher than the blocking temperature to a lower temperature. That particular layer is said to be “set” or initialized. 
     Another advantage of using the present invention for a sensor such as shown in  FIG. 3  is that during processing the thickness of the free layer  307  under the exchange tabs  308  is not perfectly uniform. The field strength required for the spin-flop effect is typically in the 1000 to 3000 Oe range. This field strength is higher than customary and is sufficient to overcome demagnetization effects from any nonuniformities in the free layer. 
     In a typical AP pinned GMR the material for the AFM material can be chosen from a list of AFM materials which include PtMn, NiMn, PdNn, NiO, IrMn and others. These materials must simultaneously meet the demand for good magnetic performance in addition to an acceptable blocking temperature. In general, sensors having AFM layers with higher blocking temperatures are more robust to deleterious heating effects. A conventional sensor requires two different materials with two different blocking temperatures because of the need for initialization. This need compromises both the thermal reliability of the sensor and the magnetic performance because of differences in exchange energy, etc. 
     The present invention allows the use of the same AFM material to be used for setting both the pinned layer and the bias of the free layer. Thus the material may be chosen for optimal magnetic performance. The key feature in being able to use the same AFM material is to be able to initialize the layers given the same blocking temperature. The initialization process makes use of magnetic behavior generally referred to as a spin flop effect. 
     The spin flop effect is illustrated in  FIGS. 4   a–d . In  FIG. 4   a  the important magnetic layers in the sensor are shown including the first AFM exchange layer  401  adjacent to the free layer, the free layer  402 , the second ferromagnetic layer  403  of the AP pinned substructure  409 , the first ferromagnetic layer  404  of the AP pinned substructure  409 , and the AFM material  405  adjacent to the first ferromagnetic layer  404 . The AP pinned substructure  409  serves as the pinned layer of the sensor. Also, the magnetization directions are shown. For clarity, the additional layers are not shown.  FIG. 4   a  shows the directions of magnetization when the structure has been placed in a high magnetic field. The magnetization  406  of the free layer aligns with the applied external field for most practical values of the applied field. For high values of the applied field the antiparallel coupling in the AP pinned substructure is broken and the magnetization of both the first ferromagnetic layer  408  and the second ferromagnetic layer  407  align with the applied field. This magnitude of field is not useful for initialization. As the applied field is reduced, the direction of the magnetizations  407 , 408  in the AP pinned substructure  409  will begin to rotate in order to seek the lowest energy.  FIG. 4   b  shows the case where the magnetizations in the AP pinned substructure have begun to rotate as the magnitude of the applied field is reduced. The magnetizations  407 , 408  rotate away from the direction of the applied field and in opposite directions from each other. This behavior is referred to as the spin-flop effect.  FIG. 4   c  shows that at some optimum value of the applied field the magnetization of the first ferromagnetic layer  408  will be perpendicular to the applied field and perpendicular to the magnetization of the free layer which is still aligned with the applied field. This is the value of the applied field which is used when cooling both AFM layers  401 , 405  below the blocking temperature to achieve initialization. At the optimum external field value the direction of the second ferromagnetic layer  407  need not be opposite from the magnetization of the first ferromagnetic layer  408 . However when the external field is removed the magnetization of the second ferromagnetic layer  407  will be substantially opposite from that of the first ferromagnetic layer  408 . Without using the spin-flop effect to induce the appropriate rotation it is not practical to use the optimal choice of the same material for the two AFM layers because of the difficulty with initialization. 
     An example of the angle of the magnetization in the first ferromagnetic layer  408  in the AP pinned substructure  409  as a function of field magnitude is shown in  FIG. 4   d . In this case the angle is measured from the desired direction of the final angle. The targeted angle is 90° with respect to the field direction. Therefore the desired angle as shown in  FIG. 4   d  is 0°. For this specific case, the coupling strength across the ruthenium layer is 0.5 erg/cm 2 , and the moments/area of the first and second ferromagnetic layers are 0.2 and 0.4 memu/cm 2 , respectively. In this case, the optimum field strength to achieve the spin-flop effect is approximately 2200 Oe. Other structures with different materials, thicknesses, and moments would have a different optimal external field strength. 
     As a practical matter, better process consistency is achieved by first increasing the applied field to a high value and then reducing the magnitude until the optimum value is applied. However it is also possible to place the sensor directly into a field having the optimum value or to raise the field from an initial value of zero. The actual value of the applied field chosen as the optimum depends to some extent on the choice of AFM material, the thickness of the layers, the coupling strength of the nonmagnetic layer, any annealing temperature, and the geometry of the sensor. For these reasons the optimum value is usually determined by first performing a process tolerance study using the specific sensor design. 
     An alternate embodiment of this invention is shown in  FIG. 5 . In this case an AFM layer  502  is placed on the substrate  501 . A simple pinned layer  503  is deposited on the AFM layer  502 . A thin nonmagnetic layer  504  (e.g. Cu) is placed between the pinned layer  503  and the free layer  505 . The biasing tabs  510  for the free layer comprise a thin nonmagnetic layer  506  (e.g. Ru) which allows antiparallel coupling of another ferromagnetic layer  507  with a portion of the free layer. This antiparallel coupling provides the bias stabilization for the free layer. The direction  511  of the magnetization in the free layer  505  is approximately 180° relative to the magnetization  512  in the ferromagnetic layer  507  of the bias tab  510 . The ferromagnetic portion  507  of the bias tabs must be pinned and this is provided by an AFM layer  508 . The AFM layer  508  can be made of the same material as the AFM layer  502  for the pinned layer and is initialized by the same process step described above. 
     In the discussion of the spin-flop effect illustrated in  FIGS. 4   a–d , the direction of the applied external field  410  is shown along the direction of the long axis of the free layer. This is also parallel to the surface of the disk. However, for the structure shown in  FIG. 5  the direction of the initialization field  509  should be perpendicular to the desired direction of magnetization  511  of the free layer  505 . This direction aligns the magnetization  513  in the pinned layer  503  and permits the spin-flop effect to rotate the magnetization  512  of the ferromagnetic layer  507  of the bias tabs 90° relative to the applied field  509 . This must be done at the optimum external field strength. 
     Another alternate embodiment of this invention is shown in  FIG. 6 . The substrate  601 , AFM layer  602 , first ferromagnetic layer  603 , thin nonmagnetic layer  604 , second ferromagnetic layer  605 , thin nonmagnetic layer  606 , and free layer  607  are as previously described for an AP pinned sensor. The biasing tabs for the free layer comprise a thin nonmagnetic layer  608  which allows antiparallel coupling of another ferromagnetic layer  609 . This AP pinned substructure for the bias tabs was previously illustrated in  FIG. 5 . The structure in  FIG. 6  therefore has two groups of layers which are AP pinned: one group  603 ,  604 , and  605  for the pinned layer and another group  607 ,  608 , and  609  for the bias stabilization of the free layer. The ferromagnetic portion  609  of the bias tabs must be pinned and this is provided by an AFM layer  610 . The AFM layer  610  can be made of the same material as the AFM layer  602  for the pinned layer and is initialized by the same process step described above. The sensor is cooled below the blocking temperature in the presence of the optimum field to utilize the spin-flop effect. 
     For the structure in  FIG. 6  it is desirable that the spin-flop effect be operative on only one set of the AP pinned substructures. This can be achieved by such means as selecting different thicknesses for the thin nonmagnetic layers  604 ,  608 , or different thicknesses for the ferromagnetic layers  603 ,  605 ,  609 , or a combination of these. Depending on the choice of layer thicknesses it is possible to have the direction of the initialization field either along the sensor direction  611  (using the spin-flop effect on the pinned layer) or perpendicular to the disk direction  612  (using the spin-flop effect on the bias stabilization tabs). It is also possible to use an applied field direction which is not at 90° but has a magnitude which would result in the appropriate angle (see  FIG. 4   d ). This magnitude and direction could be chosen to accommodate both AP pinned substructures in  FIG. 6   a . The important feature is that a simple initialization step is required with only one externally applied field and one cooling step in the presence of this field. 
     Another embodiment is shown in  FIG. 7 . The sensor shown in  FIG. 7  differs from the sensors shown in the prior Figures in that the physical width  720  of the layers approximately defines the magnetic trackwidth of the sensor. A first AFM layer  703  provides exchange coupling with the first ferromagnetic layer  704  of the AP pinned substructure  707 . The nonmagnetic layer  705  and a second ferromagnetic layer  706  complete the AP pinned substructure  707  which serves as the pinned layer of the sensor. A nonmagnetic layer  708  usually of copper separates the second ferromagnetic layer  706  of the AP pinned substructure  707  from the free layer  709 . A third ferromagnetic layer  711  is separated from the free layer  709  by third nonmagnetic layer  710 . The third nonmagnetic layer may be formed from a number of materials such as Ru or Ta. Ru or Ta is preferred over Cu for layer  710  to minimize possible GMR behavior of the free layer  709 , nonmagnetic layer  710 , ferromagnetic layer  711  combination. A second AFM layer  712  is exchanged coupled with the third ferromagnetic layer  711 . The stack of layers forming the sensor is connected to electrical leads  713  and is insulated with an insulating material  702  from the magnetic shields  701 . The direction of the sense current  715  is indicated. Also the direction  730  of the external field required to take advantage of the spin-flop effect in indicated. One novel feature of the sensor in  FIG. 7  is that bias stabilization of the free layer is accomplished by weakly coupling the third ferromagnetic layer  711  through the third nonmagnetic layer  710 . Additionally, the width of the third ferromagnetic layer is approximately the width of the free layer. The width  720  of all the layers in  FIG. 7  are shown to be equal. In practice some variation of widths will occur because of processing artifacts. 
     The process of using the spin-flop effect to achieve a simplified initialization process allows sensors having the same material for the AFM layers to be practical. However initialization using the spin-flop effect could also be used on sensors having two different AFM materials. The only requirement is that the blocking temperature of both AFM layers must be initially exceeded and that the final temperature after the cooling step in the presence of the external field be below the blocking temperature of both materials. 
     For the purposes of this invention it is immaterial whether the magnetization of the free layer (e.g.  309  in  FIG. 3 ) is pointing left or right. It is also immaterial whether the magnetization ( 310  in  FIG. 3 ) of the first ferromagnetic layer ( 303  in  FIG. 3 ) is into or out of the field of view. It is also immaterial if during the construction of the sensor, the pinned layer is deposited first and then the free layer is deposited or whether the order of layers is reversed. However, it is important that the magnetizations of the two ferromagnetic layers (e.g.  310 ,  311  in  FIG. 3 ) be substantially opposite to each other after the removal of the external field. 
       FIGS. 8   a  and  8   b  show the present invention as used in a magnetic recording disk drive. The magnetic recording disk  802  is rotated by drive motor  804  with hub  806 , which is attached to the drive motor. The disk comprises a substrate, a magnetic layer, an optional overcoat layer such as carbon, and typically a lubricant layer such as a perfluoropolyether. The substrate is typically either aluminum, glass, or plastic. Some disk drives are designed such that the slider  810  comes to rest on the disk when the disk drive is stopped. In other disk drives, the slider is lifted off of the disk surface when the disk drive is turned off. The latter is preferable when the surfaces of the slider and the disk are designed to have very low roughness. This is advantageous for designs requiring frequent or continuous contact between the slider and the disk during normal operation. 
     A recording head assembly  808  is formed on the trailing surface of a slider  810 . The slider has a trailing vertical surface  809 . The recording head assembly usually comprises a separate write element along with the GMR read sensor. The slider  810  is connected to the actuator  812  by means of a rigid arm  814  and a suspension  816 . The suspension  816  provides a force which pushes the slider toward the surface of the recording disk  802 . 
     While the invention has been described above in connection with preferred embodiments thereof and as illustrated by the drawings, those with skill in the art will readily recognize alternative embodiments of the invention can be easily produced which do not depart from the spirit and scope of the invention as defined in the following claims.