Spin valve sensor with magnetic and nonmagnetic layers for improving asymmetry and softness of a free layer structure

1. Field of the Invention

The present invention relates to a spin valve sensor with magnetic and nonmagnetic layers for improving asymmetry and softness of a free layer structure and, more particularly, to a magnetic keeper layer for providing flux closure for a pinned layer structure and a nonmagnetic layer for counterbalancing sense current fields and improving the softness of the free layer structure.

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 pinned 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 minimum 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. In order to improve the sensitivity of the spin valve sensor a soft magnetic material, such as nickel iron (NiFe), is employed as the free layer. It has been found, however, that when a free layer structure employs a cobalt based layer in addition to the nickel iron (NiFe) free layer that the magnetoresistive coefficient dr/R increases when the cobalt based layer is located between and interfaces the nickel iron (NiFe) free layer and a copper (Cu) spacer layer. 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.

Readback asymmetry is defined as V 1 - V 2 max  ( V 1     or     V 2 ) .

For example, +10% readback asymmetry means that the positive readback signal V 1 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, of course, 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. Magnetic instability of the free layer is greater when the readback asymmetry is positive. Accordingly, the magnetic instability of the free layer is greater when the readback asymmetry is positive than when the readback asymmetry is negative. A positive readback asymmetry can be improved by changing thickness of the magnetic layers and/or changing the sense current, however, a change of one of these parameters can change other parameters making it a trial and error process to reduce the asymmetry. More importantly, however, is that the channel electronics of the disk drive as well as specific thicknesses of the magnetic layers are designed to satisfy other magnetic considerations. Since these values are set it is manifest that there is a need to deal with the many magnetic influences on the free layer of the AP pinned spin valve so that the net value of these influences on the free layer can be reduced to virtually zero thereby reducing the asymmetry to virtually zero.

The location of the transfer curve relative to the bias point is influenced by four 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 demag field H D from the pinned layer, a sense current field H I from all conductive layers of the spin valve except the free layer, a net image current field H IM from the first and second shield layers. The strongest magnetic force on the free layer structure is the sense current field H I . In an exemplary bottom spin valve sensor where the free layer is closer to the second gap layer than it is to the first gap layer the majority of the conductive layers are below the free layer structure between the free layer structure and the first gap layer. The amount of conductive material in this region is further increased if the pinning layer is metal instead of an oxide, such as nickel oxide (NiO). Accordingly, when the sense current is conducted through the sensor the conductive layers below the free layer structure causes a sense current field on the free layer structure which is minimally counterbalanced by a typical cap layer made of tantalum (Ta) on top of the free layer structure. Accordingly, there is a strong-felt need to counterbalance the strong sense current field exerted by the conductive layers of the spin valve sensor below the free layer structure in a bottom spin valve sensor. Further, the pinned layer structure below the free layer structure in a bottom spin valve sensor exerts a demagnetizing field on the free layer structure which needs to be counterbalanced to improve asymmetry of the spin valve sensor. There is a strong-felt need to counterbalance the sense current and demagnetizing fields and optimize the sense current, the pinning layer structure and the type of material of the pinning layer while still obtaining the desired readback symmetry of the spin valve sensor.

The present invention provides a nonmagnetic conductive layer that interfaces the top of the free layer structure in a bottom spin valve sensor for the purpose of producing a sense current field which counterbalances, to the extent desirable, a sense current field from conductive layers below the free layer structure. In the preferred embodiment the nonmagnetic conductive layer is copper which also improves the softness of the free layer structure so that the magnetic moment of the free layer structure is more responsive in its rotation to magnetic field signals from the rotating magnetic disk. The invention further includes a magnetic keeper layer with the nonmagnetic conductive layer located between the keeper layer and the free layer structure so that the magnetic moment of the keeper layer is isolated from the free layer structure. The keeper layer provides flux closure, to the extent desired, for the demagnetizing field from the pinned layer structure below the free layer structure in the bottom spin valve sensor. In a preferred embodiment the keeper layer is nickel iron (NiFe). In a still further embodiment a second nonmagnetic conductive layer is provided with the keeper layer located between the first and second nonmagnetic conductive layers. The second nonmagnetic conductive layer isolates the keeper layer from a tantalum (Ta) cap layer so as to improve the softness of the keeper layer and provides an additional sense current field for counterbalancing the sense current fields exerted by the conductive layers below the free layer structure.

An object of the present invention is to provide a pair of layers between a free layer structure and a cap layer of a spin valve sensor for improving playback asymmetry as well as improving soft magnetic properties of the free layer structure.

Another object is to provide a copper layer on top of a free layer structure in a spin valve sensor for at least partially counterbalancing sense current fields from conductive layers below the free layer structure and for improving soft magnetic properties of the free layer structure in combination with a magnetic keeper layer on top of the copper layer for providing at least partial flux closure for a pinned layer structure below the free layer structure.

Still another object is to provide a second copper layer on top of the keeper layer in the preceding object for the purpose of improving soft magnetic properties of the keeper layer and providing an additional sense current field for at least partially counterbalancing sense current fields from conductive layers below the free layer structure.

Other objects and attendant advantages of the invention will be appreciated upon reading the following description taken together with the accompanying 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;

FIG. 11 is an isometric ABS illustration of a read head which employs an AP pinned spin valve (SV) sensor;

FIG. 12 is an ABS illustration of a first embodiment of the present invention;

FIG. 13 is an ABS illustration of a second embodiment of the present invention;

FIG. 14 is an ABS illustration of a third embodiment of the present invention; and

FIG. 15 is an ABS illustration of a fourth embodiment of the present invention.

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.

FIG. 11 is an isometric ABS illustration of the read head 72 shown in FIGS. 6 or 8 . The read head 72 includes the present spin valve sensor 130 which is located on an antiferromagnetic (AFM) pinning layer 132 . A ferromagnetic pinned layer in the spin valve sensor 130 , which is to be described hereinafter, is pinned by the magnetic spins of the pinning layer 132 . The AFM pinning layer may be 425 Å of nickel oxide (NiO). 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 134 include a first hard bias layer 140 and a first lead layer 142 and the second hard bias and lead layers 136 include a second hard bias layer 144 and a second lead layer 146 . The hard bias layers 140 and 144 cause magnetic fields to extend longitudinally through the spin valve sensor 130 for stabilizing the magnetic domains therein. The AFM pinning layer 132 , 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 read gap layers 148 and 150 are, in turn, located between ferromagnetic first and second shield layers 152 and 154 .

In all embodiments the pinning layers 213 in FIGS. 12 and 13 and the pinning layer 424 in FIGS. 14 and 15 may be located directly upon the first read gap layer 148 shown in FIG. 1 . If the pinning layer is nickel oxide (NiO), as shown, the pinning layer also provides additional thickness to the read gap. However, it should be understood that the pinning layer may be other materials such as platinum manganese (PtMn), nickel manganese (NiMn), or iridium manganese (IrMn). It should be understood that the percentage of each element in each of the alloys nickel iron (NiFe) and cobalt iron (CoFe) layers in each of the embodiments in FIGS. 12-15 may be any value provided the total percentage for each alloy equals 100%. It should be understood that the cobalt iron (CoFe) layers in the embodiments may be cobalt (Co) or the cobalt (Co) layers may be cobalt iron (CoFe). While copper (Cu) is preferred for each of the layers 228 in FIGS. 12 and 13 and 438 in FIGS. 14 and 15, it should be understood that another nonmagnetic conductive layer may be substituted therefor.

It can be seen that the nonmagnetic conductive layer 228 or 438 and the magnetic conductive keeper layer 230 or 440 of the present invention provide flexibility for counterbalancing sense current and demagnetizing fields so that the sense current I S and the thicknesses of the layers below the free layer structure can be optimized for improving performance of the spin valve sensor. While an AP pinned layer structure has been shown for each of the embodiments in FIGS. 12-15 it should be understood that the pinned layer structure may be a single pinned layer structure which may be a single ferromagnetic layer or multi-ferromagnetic layers without an antiparallel coupling layer. When the pinned layer structure is a single ferromagnetic layer it should be understood that the demagnetizing field will be greater than the net demagnetizing field from an AP pinned layer structure. Accordingly, for single pinned layer structures the employment of the layers provided by the present invention may be even more important for counterbalancing the demagnetizing field than simply counterbalancing the smaller net demagnetizing field of the AP pinned layer structure.

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.