Abstract:
A keeper layer promotes read signal symmetry of a giant magnetoresistive (GMR) read head without shunting sense current. The keeper layer is embedded in the first read gap layer of the read head and is completely insulated from the other layers of the spin valve sensor as well as from the first and second hard bias and lead layers connected to the sensor. A demagnetization field from the keeper layer opposes a demagnetization field from the pinned layer of the sensor so that fields acting on the free layer during a quiescent state of the sensor (sense current field conducted without an applied field from a rotating disk) can be balanced to a net value of zero so that the magnetic moment of the free layer maintains a parallel position with respect to the ABS.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a giant magnetoresistive (GMR) head with a keeper layer that shunts no sense current and more particularly to a keeper layer that is electrically insulated from other layers of a spin valve sensor so that sense current is not conducted through the keeper layer. 
     2. Description of the Related Art 
     The heart of a computer is an assembly that is referred to as a magnetic disk drive. The magnetic disk drive includes a rotating magnetic disk, write and read heads that are suspended by a suspension arm above the rotating disk and an actuator that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The read and write heads are directly mounted on a slider that has an air bearing surface (ABS). The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk adjacent the ABS to cause the slider to ride on an air bearing a slight distance from the surface of the rotating disk. When the slider rides on the air bearing the write and read heads are employed for writing magnetic impressions to and reading magnetic impressions from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions. 
     In recent read heads a spin valve sensor is employed for sensing magnetic fields from the rotating magnetic disk. The sensor includes a nonmagnetic conductive layer, hereinafter referred to as a spacer layer, sandwiched between first and second ferromagnetic layers, hereinafter referred to as a pinned layer, and a free layer. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. The magnetization of the pinned layer is pinned perpendicular to an air bearing surface (ABS) of the head and the magnetic moment of the free layer is located parallel to the ABS but free to rotate in response to external magnetic fields. The magnetization of the pinned layer is typically pinned by exchange coupling with an antiferromagnetic layer. 
     The thickness of the spacer layer is chosen to be less than the mean free path of conduction electrons through the sensor. With this arrangement, a portion of the conduction electrons is scattered by the interfaces of the spacer layer with the pinned and free layers. When the magnetization of the pinned and free layers are parallel with respect to one another, scattering is minimal and when the magnetization of the pinned and free layers are antiparallel, scattering is maximized. Changes in scattering alter the resistance of the spin valve sensor in proportion to sin 2 θ, where θ is the angle between the magnetizations of the pinned and free layers. In a read mode the resistance of the spin valve sensor changes proportionally to the magnitudes of the magnetic fields from the rotating disk. When a sense current is conducted through the spin valve sensor, resistance changes cause potential changes that are detected and processed as playback signals. A spin valve sensor is characterized by a magnetoresistive (MR) coefficient that is substantially higher than the MR coefficient of an anisotropic magnetoresistive (AMR) sensor. For this reason a spin valve sensor is sometimes referred to as a giant magnetoresistive (GMR) sensor. 
     It is important that the magnetic moment of the free layer be directed substantially parallel to the ABS when the sensor is in a quiescent state. The quiescent state occurs when the sense current is conducted through the sensor without an applied field from the rotating disk. The parallel position corresponds to a zero bias point on a transfer curve of the sensor. The transfer curve of the sensor can be plotted as GMR effect (ratio of change in resistance to resistance of the sensor) as a function of applied field. Applied fields from the rotating disk move the magnetic moment of the free layer up or down from the parallel position depending upon whether the applied field is positive or negative (representing ones and zeros in a digital computing scheme). This rotation, relative to the pinned magnetic moment of the pinned layer, causes scattering of spin dependent electrons at interfaces of certain layers in the sensor which results in resistance changes of the sensor. These resistance changes cause potential differences which can be processed by the processing circuitry as read signals. 
     During the quiescent state there are magnetic forces acting on the free layer that urge the magnetic moment of the free layer to rotate from the parallel position to the ABS. If the magnetic moment of the free layer is not parallel to the ABS in the quiescent state read signal asymmetry will occur which means that the potentials of the positive and negative read signals are unequal. This results in a reduced read signal. Accordingly, there is an ongoing effort to balance the magnetic forces acting on the free layer in the quiescent state. These magnetic forces are a ferromagnetic coupling field H C  exerted by the pinned layer on the free layer, sense current fields H SC  exerted by the pinned and spacer layers on the free layer and a demagnetization field H D  exerted by the pinned layer on the free layer. The ferromagnetic coupling is antiparallel to the sense current and demagnetization fields. Unfortunately, in any practical sensor scheme the combination of the sense current and demagnetization fields is greater than the ferromagnetic coupling field which results read signal asymmetry. A reduced net demagnetization field on the free layer would promote read signal symmetry. 
     Another problem that can occur with spin valve sensors is a loss of exchange coupling between the pinning and pinned layers when the sensor is heated by an unwanted event. The sensor can encounter elevated thermal conditions by electrostatic discharge (ESD) from an object or person, or by contacting an asperity on a magnetic disk. When this occurs the blocking temperature (temperature at which magnetic spins of the layer can be easily moved by an applied magnetic field) of the antiferromagnetic layer can be exceeded, resulting in disorientation of its magnetic spins. The magnetic moment of the pinned layer is then no longer pinned in the desired direction. 
     Efforts continue to increase the spin valve effect of GMR heads. An increase in the spin valve effect equates to higher bit density (bits/square inch of the rotating magnetic disk) read by the read head. Promoting read signal symmetry with regard to the free layer and maintaining thermal stability of the pinning layer are important factors. 
     SUMMARY OF THE INVENTION 
     I investigated the use of a keeper layer for promoting read signal symmetry and stability of the pinning layer. A keeper layer is a ferromagnetic layer that is located between the free layer and the first gap layer of the read head. It has a magnetic moment that is directed antiparallel to the magnetic moment of the pinned layer. With this arrangement a demagnetization field of the keeper layer opposes the demagnetization field from the pinned layer on the free layer. There is also a sense current field from the keeper layer that opposes the demagnetization field from the pinned layer. Unfortunately, however, the keeper layer shunts a portion of the sense current since it is electrically conductive. A shunting of the sense current equates to a loss of read signal strength. Accordingly, the keeper layer should be thin so that it will not shunt as much sense current. This is a serious restraint in designing the thickness of the keeper layer thick enough to provide sufficient demagnetization field to counterbalance the other magnetic fields acting on the free layer. Further, lessening the thickness of the keeper layer lessens the demagnetization field from the keeper layer on the pinned layer for the purpose of promoting thermal stability of the pinning layer. 
     I have found that by electrically insulating the keeper layer from the other layers of the sensor that sense current is not shunted through the keeper layer and the thickness restraint is removed from designing so that the keeper layer can be designed sufficiently thick to counterbalance other magnetic fields acting on the free layer. Accordingly, read signal symmetry can be achieved without shunting sense current. Further, the typically thicker keeper layer increases thermal stability of the pinning layer. In a preferred embodiment the keeper layer is embedded in the first gap layer. 
     An object of the present invention is to provide a keeper layer for a GMR head that does not shunt sense current. 
     Another object is to provide a keeper layer that can be designed to completely balance other magnetic forces acting on a free layer of a spin valve sensor and promote thermal stability of the pinning layer of the sensor without a restraint on its thickness. 
     Other objects and advantages of the invention will become apparent upon reading the following description taken together with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a planar view of an exemplary magnetic disk drive; 
     FIG. 2 is an end view of a slider with a magnetic head of the disk drive as seen in plane  2 — 2 ; 
     FIG. 3 is an elevation view of the magnetic disk drive wherein multiple disks and magnetic heads are employed; 
     FIG. 4 is an isometric illustration of an exemplary suspension system for supporting the slider and magnetic head; 
     FIG. 5 is an ABS view of the magnetic head taken along in plane  5 — 5  of FIG. 2; 
     FIG. 6 is a partial view of the slider and magnetic head as seen in plane  6 — 6  of FIG. 2; 
     FIG. 7 is a partial ABS view of the slider taken along plane  7 — 7  of FIG. 6 to show the read and write elements of the magnetic head; 
     FIG. 8 is a view taken along plane  8 — 8  of FIG. 6 with all material above the second pole piece removed; 
     FIG. 9 is an isometric ABS illustration of a read head which employs the present spin valve (SV) sensor; 
     FIG. 10 is an ABS illustration of a spin valve sensor without a keeper layer; 
     FIG. 11 isaview taken along plane  11 — 11  of FIG. 10; 
     FIG. 12 is an ABS illustration of the present spin valve sensor with a keeper layer insulated from the sense current I S ; and 
     FIG. 13 is a view taken along plane  13 — 13  of FIG.  12 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Magnetic Disk Drive 
     Referring now to the drawings wherein like reference numerals designate like or similar parts throughout the several views, FIGS.  1 - 3  illustrate a magnetic disk drive  30 . The drive  30  includes a spindle  32  that supports and rotates a magnetic disk  34 . The spindle  32  is rotated by a motor  36  that is controlled by a motor controller  38 . A combined read and write magnetic head  40  is mounted on a slider  42  that is supported by a suspension  44  and actuator arm  46 . A plurality of disks, sliders and suspensions may be employed in a large capacity direct access storage device (DASD) as shown in FIG.  3 . The suspension  44  and actuator arm  46  position the slider  42  so that the magnetic head  40  is in a transducing relationship with a surface of the magnetic disk  34 . When the disk  34  is rotated by the motor  36  the slider is supported on a thin (typically, 0.05 μm) cushion of air (air bearing) between the surface of the disk  34  and the air bearing surface (ABS)  48 . The magnetic head  40  may then be employed for writing information to multiple circular tracks on the surface of the disk  34 , as well as for reading information therefrom. Processing circuitry  50  exchanges signals, representing such information, with the head  40 , provides motor drive signals for rotating the magnetic disk  34 , and provides control signals for moving the slider to various tracks. In FIG. 4 the slider  42  is shown mounted to a suspension  44 . The components described hereinabove may be mounted on a frame  54  of a housing, as shown in FIG.  3 . 
     FIG. 5 is an ABS view of the slider  42  and the magnetic head  40 . The slider has a center rail  56  that supports the magnetic head  40 , and side rails  58  and  60 . The rails  56 ,  58  and  60  extend from a cross rail  62 . With respect to rotation of the magnetic disk  34 , the cross rail  62  is at a leading edge  64  of the slider and the magnetic head  40  is at a trailing edge  66  of the slider. 
     FIG. 6 is a side cross-sectional elevation view of the merged MR head  40 , which includes a write head portion  70  and a read head portion  72 , the read head portion employing an AP pinned spin valve sensor  74  of the present invention. FIG. 7 is an ABS view of FIG.  6 . The spin valve sensor  74  is sandwiched between first and second gap layers  76  and  78 , and the gap layers are sandwiched between first and second shield layers  80  and  82 . In response to external magnetic fields, the resistance of the spin valve sensor  74  changes. A sense current I S  conducted through the sensor causes these resistance changes to be manifested as potential changes. These potential changes are then processed as readback signals by the processing circuitry  50  shown in FIG.  3 . 
     The write head portion of the merged MR head includes a coil layer  84  sandwiched between first and second insulation layers  86  and  88 . A third insulation layer  90  may be employed for planarizing the head to eliminate ripples in the second insulation layer caused by the coil layer  84 . The first, second and third insulation layers are referred to in the art as an “insulation stack”. The coil layer  84  and the first, second and third insulation layers  86 ,  88  and  90  are sandwiched between first and second pole piece layers  92  and  94 . The first and second pole piece layers  92  and  94  are magnetically coupled at a back gap  96  and have first and second pole tips  98  and  100  which are separated by a write gap layer  102  at the ABS. As shown in FIGS. 2 and 4, first and second solder connections  104  and  106  connect leads from the spin valve sensor  74  to leads  112  and  114  on the suspension  44 , and third and fourth solder connections  116  and  118  connect leads  120  and  122  from the coil  84  (see FIG. 8) to leads  124  and  126  on the suspension. 
     FIG. 9 is an isometric ABS illustration of the read head  72  shown in FIG.  6 . The read head  72  has a spin valve sensor  130  which will be described in more detail hereinafter. First and second hard bias and lead layers  134  and  136  are connected to first and second side edges  138  and  140  of the spin valve sensor. This connection is known in the art as a contiguous junction and is fully described in commonly assigned U.S. Pat. No. 5,018,037 which is incorporated by reference herein. The first hard bias and lead layers include a first hard bias layer  140  and a first lead layer  142  and the second hard bias and lead layers  136  include a hard bias layer  144  and a lead layer  146 . The hard bias layers  140  and  144  cause magnetic flux to extend longitudinally through the spin valve sensor  130  for stabilizing its magnetic domains. The spin valve sensor  130  and the first and second hard bias and lead layers  134  and  136  are located between nonmagnetic electrically insulative first and second read gap layers  148  and  150 . The first and second gap layers  148  and  150  are, in turn, located between first and second shield layers  152  and  154 . 
     A spin valve sensor  200  without a keeper layer is shown in FIGS. 10 and 11. The sensor  200  is formed on the first read gap layer  202  and may include a seedlayer  204  of tantalum (Ta), a free layer  206  of nickel iron (NiFe), a spacer layer  208  of copper (Cu), a pinned layer  210  of cobalt (Co), an antiferromagnetic pinning layer  212  of nickel manganese (NiMn) and a cap layer  214  of tantalum (Ta). The tantalum (Ta) seedlayer  204  is used for the purpose of epitaxial growth of the free layer  206  when it is sputtered on the seedlayer. This causes the crystalline structure of the free layer to simulate the crystalline structure of the tantalum seedlayer  204  and also prevents any possible contamination of aluminum oxide (Al 2 O 3 ) typically employed for the first gap layer  202 . Exemplary directions of the magnetic moments of the magnetic layers are magnetic moment  216  of the free layer is to the right parallel to the ABS and the magnetic moment  218  of the pinned layer is downward and perpendicular to the ABS. With these magnetic moments the sense current I S  should be from left to right through the sensor. When the sense current I S  is being conducted through the sensor without the application of any applied fields from a rotating magnetic disk the sensor is in a quiescent state. In a quiescent state the magnetic moment  216  of the free layer should be parallel to the ABS so that a bias point on a transfer curve of the free layer  206  is at a zero position. In order for the magnetic moment  216  of the free layer to be maintained parallel to the ABS it is important that the combined magnetic fields on the free layer in the quiescent state do not rotate the magnetic moment  216  from its parallel position. 
     The magnetic moments imposed on the free layer  206  during the quiescent state is illustrated in FIG. 11. A ferromagnetic coupling field H FC(P)  is directed in a downward direction since the magnetic moment  218  of the pinned layer is directed in a downward direction, sense current fields H FC(P+S) , caused by the sense current Is conduction through the pinned and spacer layers  210  and  208  is directed downwardly and a demagnetization field H D(P)    220  from the pinned layer  210  is directed upwardly. It is difficult to counterbalance the sense current fields H SC(P+S)  and the demagnetization field H D(P)  with the ferromagnetic coupling field H FC(P)  in order to achieve a net zero field on the free layer  206 . 
     If a ferromagnetic keeper layer is interposed between the first gap layer  202  and the seedlayer  204  the magnetic moment of this layer would be directed antiparallel to the magnetic moment  218  of the pinning layer so that demagnetization fields from the keeper layer and the pinned layer  210  oppose one another. This would help to reduce the net demagnetization field HD on the free layer  206 . Unfortunately, the ferromagnetic keeper layer shunts some of the sense current thereby reducing the read signal. Therefore, there is a limitation on the thickness of the keeper layer for the purpose of reducing the effect of the demagnetization field of the pinning layer  210  on the free layer  206 . 
     In FIGS. 12 and 13 I have provided a spin valve sensor  300  which has a keeper layer  302  that does not shunt the sense current I S . The keeper layer  302  is embedded in the first read gap layer  304  of the read head. The first gap layer  304  is typically aluminum oxide (Al 2 O 3 ) and may be 500 Å thick. The keeper layer  302  is sputter deposited with any suitable masking such as bilayer photoresist lift-off processing. Located on top of the keeper layer  302  is an insulation layer  306  which may be sputter deposited on the keeper layer  302  and the first gap layer  304 . In essence, the insulation layer  306  forms, along with the gap layer  304 , a read gap thickness for the read head. The insulation layer may be any suitable material such as aluminum oxide (Al 2 O 3 ), silicon dioxide (SiO 2 ), silicon nitride (SiN) or nickel oxide (NiO). A preferred thickness range for the insulation layer  306  is 50 Å to 150 Å with the preferred thickness being 100 Å. The preferred material for the keeper layer is nickel iron (NiFe) with a thickness range of 10 Å to 40 Å with a preferred thickness of 20 Å. 
     The spin valve sensor  300  further includes a seedlayer  308 , a free layer  310 , a spacer layer  312 , a pinned layer  314 , an antiferromagnetic (AFM) pinning layer  316  and a cap layer  318 . The seedlayer  308  is preferably tantalum (Ta) with a thickness range of 10 Å to 50 Å with a preferred thickness of 30 Å, the free layer is preferably nickel iron (NiFe) with a thickness range of 30 Å to 60 Å with a preferred thickness of 45 Å, the spacer layer is preferably copper (Cu) with a thickness range of 18 Å to 30 Å with a preferred thickness being 22 Å, the pinned layer is preferably cobalt (Co) with a thickness range of 10 Å to 40 Å with a preferred thickness being 20 Å, the pinning  316  is preferably nickel manganese (NiMn) with a thickness range of 200 Å to 300 Å with a preferred thickness of 250 Å, and the cap layer is preferably tantalum (Ta) with a thickness range of 30 Å to 70 Å with a preferred thickness of 50 Å. Other suitable antiferromagnetic materials for the pinning layer  316  may be iridium manganese (IrMn), platinum manganese (PtMn), platinum palladium manganese (PtPdMn) and ruthenium rhodium manganese (RuRhMn). It should be noted that the keeper layer  302  is preferably the same thickness as the pinned layer  314  so that a demagnetization field from the keeper layer completely counterbalances a demagnetization field from the pinned layer  314  on the free layer  310 . It should further be noted that the keeper layer  302  may be wider than the other layers  308 ,  310 ,  312 ,  314 ,  316  and  318  ofthe spin valve sensor  300  so that the portions  320  and  322  of the first insulation layer  306  are substantially planarized for construction of the hard bias and lead layers  134  and  136  shown in FIG.  9 . It is preferred that the keeper layer  302  extend 0.5 μm on each side edge of the seedlayer  308  for this purpose. 
     The magnetic moment  324  of the free layer is shown parallel to the ABS and to the right. Optionally, the magnetic moment  324  could be directed to the left. The magnetic moment  326  of the pinned layer is shown directed perpendicular to the ABS in the downward direction. Optionally, the direction of the magnetic moment  326  could be directed upwardly away from the ABS. For the directions of the magnetic moments  324  and  326  in FIG. 12 the sense current Is should be directed from left to right, as shown. In the quiescent state various fields will be exerted on the free layer  310  by the pinned layer  314 , the keeper layer  302  and the spacer layer  312 , which are shown in FIG.  13 . 
     FIG. 13 is a side view of the sensor taken along plane  13 — 13  of FIG. 12. A ferromagnetic coupling field H FC(P)  is exerted on the free layer  310  by the pinned layer  314 . This field is directed in a downward direction because the magnetic moment  326  of the pinned layer is directed in a downward direction. Sense current fields H FC(P+S)  are exerted on the free layer  310  by the pinned and spacer layers  314  and  312 . Because of the direction of the sense current I S  the sense current fields are directed upwardly on the free layer  310 , as shown. A demagnetization field  328  is exerted on the free layer  310  by the pinned layer  314  and a demagnetization field  330  is exerted on the free layer  310  by the keeper layer  302 . It should be noted that these demagnetization fields are in opposite directions and, if equal, would completely counterbalance each other providing a net demagnetization field of zero on the free layer  310 . The net demagnetization field H NET D(P-K)  is shown with some value which may be necessary if the ferromagnetic coupling HFC and the sense current field H FC(P+S)  do not completely counterbalance one another. In a preferred embodiment the ferromagnetic coupling field H FC(P)  and the sense current fields H FC(P+S)  completely counterbalance one another and the demagnetization fields  328  and  330  completely counterbalance one another so as to produce a net demagnetization field H NETD(P-K)  of zero. With this arrangement there are no net fields on the free layer  310  in the quiescent state that urge the magnetic moment  324  of the free layer to move from its parallel position with respect to the ABS. With this scheme the thicknesses of the keeper and pinned layers  302  and  314  would be equal with a preferred material for each being nickel iron (NiFe). It should be noted that there is no sense current field from the keeper layer  302  on the free layer  310  since there is no sense current I S  conducted through the keeper layer  302 . 
     It should be noted that the demagnetization fields from the keeper layer  302  is also imposed on the pinned layer  314 . This enhances the pinning of the magnetic moment  326  of the pinned layer in the downward direction which direction is necessary for the spin valve effect to occur. Should the sensor encounter a high temperature, due to electrostatic discharge (ESD) or contact with an asperity on the rotating magnetic disk, the pinning layer  316  may be heated at or above its blocking temperature which allows its magnetic spins to easily rotate in the presence of extraneous fields. When this occurs the demagnetization field from the keeper layer  302  keeps the magnetic moment  326  of the pinned layer directed downwardly so that when the pinning layer  316  cools off its magnetic spins will align with the magnetic spins of the pinned layer back to the original direction. Accordingly, the keeper layer  302  serves a double function of promoting read signal symmetry as well as stabilizing the pinning layer  316 . 
     Clearly, other embodiments and modifications of this invention will occur readily to those of ordinary skill in the art in view of these teachings. Therefore, this invention is to be limited only by following claims, which include all such embodiments and modifications when viewed in conjunction with the above specification and accompanying drawings.