Abstract:
A read head has first and second leads that are shorted to first and second shields so that the first and second shields function as lead layer extensions for the first and second leads. This permits a second read gap layer to be thinner so that a free layer structure of a spin valve sensor is located closer to a second shield layer. This increases a net imaging current field H IM  which can be employed for counterbalancing a strong sense current field H I  due to conductive layers on one side of the free layer structure. Connection of the first and second lead layers to the first and second shield layers promotes heat dissipation from the first and second lead layers and a thinner second read gap layer promotes linear read density of the head.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a read head with leads to shields shorts for permitting a thinner second read gap layer and improving read signal symmetry and, more particularly, to such a read head wherein the first and second shield layers are extensions of the first and second lead layers. 
     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, a slider that has read and write heads, 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 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 an air bearing surface (ABS) of the slider causing 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 signal fields 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. 
     An exemplary high performance read head employs a spin valve sensor for sensing the magnetic signal fields from the rotating magnetic disk. The sensor includes a nonmagnetic electrically conductive spacer layer sandwiched between a ferromagnetic pinning layer and a ferromagnetic free layer. An antiferromagnetic pinning layer interfaces the pinned layer for pinning the magnetic moment of the pinned layer 90° to an air bearing surface (ABS) wherein the ABS is an exposed surface of the sensor that faces the rotating disk. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. A magnetic moment of the free layer is free to rotate upwardly and downwardly with respect to the ABS from a quiescent or zero bias point position in response to positive and negative magnetic signal fields from the rotating magnetic disk. The quiescent position of the magnetic moment of the free layer, which is preferably parallel to the ABS, is when the sense current is conducted through the sensor without magnetic field signals from the rotating magnetic disk. If the quiescent position of the magnetic moment is not parallel to the ABS the positive and negative responses of the free layer will not be equal which results in read signal asymmetry which is discussed in more detail hereinbelow. 
     The thickness of the spacer layer is chosen so that shunting of the sense current and a magnetic coupling between the free and pinned layers are minimized. This thickness is typically less than the mean free path of electrons conducted 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 magnetic moments of the pinned and free layers are parallel with respect to one another scattering is minimal and when their magnetic moments are antiparallel scattering is maximized. An increase in scattering of conduction electrons increases the resistance of the spin valve sensor and a decrease in scattering of the conduction electrons decreases the resistance of the spin valve sensor. Changes in resistance of the spin valve sensor is a function of cos θ, where θ is the angle between the magnetic moments of the pinned and free layers. When a sense current is conducted through the spin valve sensor, resistance changes cause potential changes that are detected and processed as playback signals from the rotating magnetic disk. The sensitivity of the spin valve sensor is quantified as magnetoresistance or magnetoresistive coefficient dr/R where dr is the change in resistance of the spin valve sensor from muinimum resistance (magnetic moments of free and pinned layers parallel) to maximum resistance (magnetic moments of the free and pinned layers antiparallel) and R is the resistance of the spin valve sensor at minimum resistance. Because of the high magnetoresistance of a spin valve sensor it is sometimes referred to as a giant magnetoresistive (GMR) sensor. 
     The transfer curve for a spin valve sensor is defined by the aforementioned cos θ where θ is the angle between the directions of the magnetic moments of the free and pinned layers. In a spin valve sensor subjected to positive and negative magnetic signal fields from a moving magnetic disk, which are typically chosen to be equal in magnitude, it is desirable that positive and negative changes in the resistance of the spin valve read head above and below a bias point on the transfer curve of the sensor be equal so that the positive and negative readback signals are equal. When the direction of the magnetic moment of the free layer is substantially parallel to the ABS and the direction of the magnetic moment of the pinned layer is perpendicular to the ABS in a quiescent state (no signal from the magnetic disk) the positive and negative readback signals should be equal when sensing positive and negative fields that are equal from the magnetic disk. Accordingly, the bias point should be located midway between the top and bottom of the transfer curve. When the bias point is located below the midway point the spin valve sensor is negatively biased and has positive asymmetry and when the bias point is above the midway point the spin valve sensor is positively biased and has negative asymmetry. The designer strives to improve asymmetry of the readback signals as much as practical with the goal being symmetry. When the readback signals are asymmetrical, signal output and dynamic range of the sensor are reduced.            V   1     -     V   2         max                   (       V   1                   or                   V   2       )                              
     For example, +10% readback asymmetry means that the positive readback signal V I  is 10% greater than it should be to obtain readback symmetry. 10% readback asymmetry is acceptable in many applications. +10% readback asymmetry may not be acceptable in applications where the applied field magnetizes the free layer close to saturation. In these applications +10% readback asymmetry can saturate the free layer in the positive direction and will reduce the negative readback signal by 10%. An even more subtle problem is that readback asymmetry impacts the magnetic stability of the free layer. Magnetic instability of the free layer means that the applied signal has disturbed the arrangement or multiplied one or more magnetic domains of the free layer. This instability changes the magnetic properties of the free layer which, in turn, changes the readback signal. The magnetic instability of the free layer can be expressed as a percentage increase or decrease in instability of the free layer depending upon the percentage of the increase or decrease of the asymmetry of the readback signal. Standard deviation of the magnetic instability can be calculated from magnetic instability variations corresponding to multiple tests of the free layer at a given readback asymmetry. There is approximately a 0.2% decrease in standard deviation of the magnetic instability of the free layer for a 1% decrease in readback asymmetry. This relationship is substantially linear which will result in a 2.0% reduction in the standard deviation when the readback asymmetry is reduced from +10% to zero. The magnetic instability of the free layer is greater when the readback asymmetry is positive than when the readback asymmetry is negative. 
     The location of the transfer curve relative to the bias point is influenced by three major forces on the free layer of a spin valve sensor, namely a ferromagnetic coupling field H FC  between the pinned layer and the free layer, a net demagnetizing (demag) field H D  from the pinned layer, and a net sense current field H I  from all conductive layers of the spin valve except the free layer. The strongest of these forces is the net sense current field H I  from the conductive layers of the spin valve sensor. In a bottom spin valve sensor where the free layer structure is closer to the second shield layer than to the first shield layer the majority of the conductive layers is located between the free layer structure and the first shield layer. The only conductive layer between the free layer structure and the second shield layer is a cap layer typically constructed of tantalum (Ta) which has a high resistance to the sense current. Accordingly, when the sense current is conducted through the spin valve sensor the net sense current field H I  acting on the free layer structure is due to the sense current fields caused by the conductive layers between the free layer structure and the first shield layer minus the small sense current field due to the cap layer. The difference is the net sense current field which, as stated hereinabove, is the largest field acting on the free layer structure urging the magnetic moment of the free layer structure to be positioned at some angle to a zero bias position which is parallel to the ABS. 
     The sense current field needs to be counterbalanced so that the magnetic moment of the free layer will remain parallel to the ABS when the read head is in the quiescent condition. The forces available for counterbalancing are the aforementioned net demag field H D  and the ferromagnetic coupling field H FC . The net demag field H D  depends upon the type of pinned layer structure employed in the spin valve sensor. If the pinned layer structure is a single ferromagnetic layer composed of one or more ferromagnetic films the demag field H D  is higher than when an antiparallel (AP) pinned layer structure is employed. Accordingly, the single pinned layer would be advantageous for providing a greater demag field H D  for counterbalancing the net sense current field H I . However, the AP pinned layer structure is more desirable for a spin valve sensor than the single pinned layer since the AP pinned layer structure has improved thermal stability, that is, its magnetic moment retains a pinned direction at higher temperatures and fields than the single pinned layer. The AP pinned layer structure includes an antiparallel coupling layer which is located between ferromagnetic first and second AP pinned layers. Since there is partial flux closure between the first and second AP pinned layers the net demag field H D  is considerably less than a single pinned layer. This causes a greater exchange coupling between the first AP pinned layer and the pinning layer for promoting the aforementioned thermal stability. Accordingly, it would be desirable to employ the AP pinned layer structure in the spin valve sensor even though its effect of counterbalancing the net sense current field H I  is less than the single pinned layer. Typically, the ferromagnetic coupling field H FC  is antiparallel to the net demag field H D  which means that the ferromagnetic coupling field H FC  is additive with the sense current field H I . This unfortunately increases read signal asymmetry. Accordingly, there is a need for providing a read head wherein read signal asymmetry can be lessened even though an AP pinned layer structure is employed. 
     SUMMARY OF THE INVENTION 
     The present invention provides a read head with a first lead layer from a spin valve sensor shorted to the first shield layer and a second lead layer from the spin valve sensor shorted to the second shield layer so that the first and second shield layers function as lead extensions for the first and second lead layers to terminals of the read head. In this manner the first and second lead layers can be significantly thinner than prior art first and second lead layers since the first and second lead layers can extend for a relatively short distance before being shorted to the first and second shield layers so that the first and second shield layers provide a large expanse of conductive material to carry the sense current to and from the read head terminals. When the first and second lead layers are thinner this causes each of the first and second lead layers to have a smaller step or rise as it extends from the spin valve sensor. Accordingly, with smaller steps the second read gap layer has less likelihood of having pin holes where it covers the steps. This means that the second read gap layer can be thinner than previous second read gap layers and still provide adequate coverage and insulation over the steps of the first and second lead layers without developing pin holes which, in turn, cause shorts between the lead layers and the shields. In the present invention, however, coverage of the step of only one of the lead layers is necessary since the other lead layer is shorted to the second shield layer. 
     The thinner second read gap layer performs three important functions, namely: (1) promotes read signal symmetry; (2) improves heat dissipation between the lead layers and the shield layers; and (3) promotes linear read density. Because of the conduction of the sense current I S  through the spin valve sensor each of the first and second shield layers produces an image current field H IM  which is exerted on the free layer structure. Since the free layer structure in a bottom spin valve sensor is located closer to the second shield layer than to the first shield layer, there is a net image current field H IM  which can be employed for counterbalancing the sense current field H I . However, when the second read gap layer is made thinner this places the free layer structure even closer to the second shield layer which will increase the net image current field H IM  for still further counterbalancing the net sense current field H I  on the free layer structure. The second advantage occurs because the lead layers are directly connected to the shield layers so that the shield layers function as heat sinks for the first and second lead layers. Further, since the second read gap layer is thinner there is still further heat dissipation between the first lead layer, which is connected to the first shield layer, and the second shield layer. In regard to the third advantage, the thinner second read gap layer decreases the read gap which is measured between the first and second shield layers so that the read head is capable of writing more bits per linear inch along a track of a rotating magnetic disk. In a preferred embodiment the invention employs a pinning layer which is made of platinum manganese (PtMn) which provides a negative ferromagnetic coupling field −H FC  which is parallel to the net demag field H D  and parallel to the net imaging field H IM  so that the sense current field is counterbalanced by three fields, namely, net demag field H D , net imaging field H IM  and ferromagnetic coupling field H FC . 
     An object of the present invention is to provide a read head wherein a net sense current field H I  acting on a free layer structure of a spin valve sensor can be more adequately counterbalanced by other magnetic fields for promoting read signal symmetry. 
     Another object is to accomplish the previous object with an antiparallel (AP) pinned layer type spin valve sensor. 
     A further object is to provide the foregoing objects along with improved heat dissipation from first and second lead layers of the read head. 
     Still another object is to accomplish the foregoing objects along with increasing linear read density of the read head. 
     Other objects and advantages of the invention will become apparent upon reading the following description taken together with the accompanying drawings. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a plan 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  of FIG. 1; 
     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 plane  5 — 5  of FIG. 2; 
     FIG. 6 is a partial view of the slider and a piggyback magnetic head as seen in plane  6 — 6  of FIG. 2; 
     FIG. 7 is a partial view of the slider and a merged magnetic head as seen in plane  7 — 7  of FIG. 2; 
     FIG. 8 is a partial ABS view of the slider taken along plane  8 — 8  of FIG. 6 to show the read and write elements of the piggyback magnetic head; 
     FIG. 9 is a partial ABS view of the slider taken along plane  9 — 9  of FIG. 7 to show the read and write elements of the merged magnetic head; 
     FIG. 10 is a view taken along plane  10 — 10  of FIGS. 6 or  7  with all material above the coil layer and leads removed; and 
     FIG. 11 is an ABS illustration of the present read head. 
    
    
     DETAILED 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 spindle motor  36  that is controlled by a motor controller  38 . A slider  42  has a combined read and write magnetic head  40  and is supported by a suspension  44  and actuator arm  46  that is rotatably positioned by an actuator  47 . 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  are moved by the actuator  47  to 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 spindle 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 spindle motor drive signals for rotating the magnetic disk  34 , and provides control signals to the actuator 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 a piggyback magnetic head  40 , which includes a write head portion  70  and a read head portion  72 , the read head portion employing a spin valve sensor  74  of the present invention. FIG. 8 is an ABS view of FIG.  6 . The spin valve sensor  74  is sandwiched between nonmagnetic electrically insulative first and second read gap layers  76  and  78 , and the read gap layers are sandwiched between ferromagnetic 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  70  of the magnetic head  40  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. An insulation layer  103  is located between the second shield layer  82  and the first pole piece layer  92 . Since the second shield layer  82  and the first pole piece layer  92  are separate layers this head is known as a piggyback head. 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. 10) to leads  124  and  126  on the suspension. 
     FIGS. 7 and 9 are the same as FIGS. 6 and 8 except the second shield layer  82  and the first pole piece layer  92  are a common layer. This type of head is known as a merged magnetic head. The insulation layer  103  of the piggyback head in FIGS. 6 and 8 is omitted. 
     The Invention 
     The present read head  600 , shown in FIG. 11, includes a spin valve sensor  602  which is located between first and second gap layers (G 1  and G 2 )  604  and  606  and the first and second gap layers are located between first and second shield layers (S 1  and S 2 )  608  and  610 . The spin valve sensor  602  includes a nonmagnetic electrically conductive spacer layer  612  which is located between an AP pinned layer structure  614  and a free layer structure  616 . The AP pinned layer structure  614  has an antiparallel coupling layer (APC)  618  which is located between a ferromagnetic first AP pinned layer (AP 1 )  620  and a ferromagnetic second AP pinned layer (AP 2 )  622 . The first AP pinned layer  620  is exchange coupled to an antiferromagnetic (AFM) pinning layer  624  so that a magnetic moment  626  of the first AP pinned layer is pinned perpendicular to the ABS, such as into the sensor as shown in FIG.  11 . By a strong antiparallel coupling between the first and second AP pinned layers  620  and  622  the second AP pinned layer  622  has a magnetic moment  628  which is antiparallel to the magnetic moment  626 . In this embodiment the second AP pinned layer  622  is thicker than the first AP pinned layer  620  so that a net demagnetizing (demag) field H D  from the AP pinned layer structure  614  on the free layer structure  616  will be antiparallel to the magnetic moment  628 . 
     A first seed layer (SL 1 )  630  is located on the first gap layer  604 , a second seed layer (SL 2 )  632  is located on the first seed layer  630  and a third seed layer (SL 3 )  634  is located on the second seed layer  632 . These seed layers, which influence the microstructures of subsequent spin valve sensor layers formed thereon, in combination with a predetermined thickness of the spacer layer  612 , establish a negative ferromagnetic coupling field −H FC , which is in the same direction as the net demag H D  on the free layer structure  616 . 
     The free layer structure  616  includes a ferromagnetic free layer (F)  636  and a ferromagnetic nanolayer (NL)  638 . The free layer structure  616  has a magnetic moment  640  which is parallel to the ABS when there is readback symmetry and this direction can be from left to right as shown in FIG. 11. A cap layer  642  may be on the free layer structure  616  for protecting it from subsequent processing steps. When a signal field from a rotating magnetic disk rotates the magnetic moment  640  into the sensor it becomes more antiparallel to the magnetic moment  628  which increases the resistance of the spin valve sensor and when a signal field rotates the magnetic moment  640  out of the sensor it becomes more parallel to the magnetic moment  628  which decreases the resistance of the head. When the sense current I S  is conducted through the spin valve sensor these changes in resistance cause potential changes which are processed as playback signals by the processing circuitry  50  in FIG.  3 . 
     Exemplary thicknesses for the first and second gap layers  604  and  606  are  200  Å of aluminum oxide (Al 2 O 3 ) for the first gap layer  604  and 100 Å of aluminum oxide (Al 2 O 3 ) for the second gap layer  606 . Exemplary thicknesses and materials for the spin valve sensor  602  are 30 Å of aluminum oxide (Al 2 O 3 ) for the first seed layer  630 , 30 Å of nickel manganese oxide (NiMnO) for the second seed layer  632 , 35 Å of tantalum (Ta) for the third seed layer  634 , 150 Å of platinum manganese (PtMn) for the pinning layer  624 , 17 Å of cobalt iron (CoFe) for the first AP pinned layer  620 , 8 Å of ruthenium (Ru) for the antiparallel coupling layer  618 , 20 Å of cobalt iron (CoFe) for the second AP pinned layer  622 , 21 Å of copper (Cu) for the spacer layer  612 , 15 Å of cobalt iron (CoFe) for the nanolayer  638 , 15 Å of nickel iron (NiFe) for the free layer  636  and 50 Å of tantalum (Ta) for the cap layer  642 . 
     First and second lead layers  650  and  652  and first and second hard bias layers  654  and  656  are electrically connected to first and second side edges of the spin valve sensor. The first and second lead layers  650  and  652  may be constructed of tantalum (Ta) and the hard bias layers (HB) are constructed of a hard magnetic material such as cobalt platinum chromium (CoPtCr) which is also electrically conductive. The hard bias layers  654  and  656  provide longitudinal biasing of the free layer structure  616  for promoting a desirable single magnetic domain structure. 
     The invention electrically connects (shorts) the first and second lead layers  650  and  652  to the first and second shield layers  608  and  610 . The first lead layer  650  may be shorted to the first shield layer  608  by a via  658  which is simply a hole filled with a conductive material such as copper which electrically connects the first lead layer  650  to the first shield layer  608 . Accordingly, the first shield layer  608  becomes a lead layer extension for the first lead layer  650  all the way to a read head terminal such as that shown at  104  in FIG.  2 . It can be seen that the second read gap layer  606  insulates the first lead layer  650  from the second shield layer  610 . The second lead layer  652  is shorted to the second shield layer  610  by any suitable means such as terminating the second read gap layer  606  at  660  so that the second shield layer directly interfaces the second lead layer  652  at  662 . Accordingly, the second shield layer now functions as a lead layer extension for the second lead layer  652  to the other terminal  106  shown in FIG.  2 . 
     Since the first and second shield layers  608  and  610  carry most of the sense current I S  to the terminals the first and second lead layers can be thinner than prior art first and second lead layers so that their steps at  664  and  666  can be less. Since there is less step coverage for the second read gap layer  606  it can be thinner than prior art second read gap layers so that the free layer structure  616  can be located closer to the second shield layer  610 . This will increase an image current field  668  from the second shield layer on the free layer structure  616  and reduce an image current field  670  from the first shield layer on the free layer structure. Accordingly, there is a stronger net image current field H IM  which can be employed for counterbalancing the net sense current field H I . 
     When the sense current I S  is conducted through the spin valve sensor  602  it can be seen that the net sense current field H I , which is due to the conductive layers below the free layer structure minus the cap layer  642  above the free layer structure, is directed through the free layer structure  616  and thence out of the page. This is the largest of the fields acting on the free layer structure  616  affecting its biasing and must be counterbalanced for read signal symmetry. Since the second AP pinned layer  622  is thicker than the first and second AP pinned layers  620  and  622  there is a net demag field H D  through the free layer structure  616  which is directed into the page. When a platinum manganese (PtMn) pinning layer  624  is employed there is a negative ferromagnetic coupling field H FC  from the second AP pinned layer  622  on the free layer structure  616  which is directed out of the page. The invention locates the center of the free layer structure closer to the second shield layer  610  than the first shield layer  608  (D 1 &lt;D 2 ) so that there is a net image current field H IM  which works in cooperation with the ferromagnetic coupling field H FC  and the demag field H D  to counterbalance the sense current field H I . In the preferred embodiment the second gap layer  606  is thinner than the first gap layer  604  so as to place the magnetic center of the free layer structure  616  closer to the second shield layer  610  and thereby increase the net image current field H IM  on the free layer structure for improving read signal symmetry. 
     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 the following claims, which include all such embodiments and modifications when viewed in conjunction with the above specification and accompanying drawings.