Spin valve sensor with nickel oxide pinning layer on a chromium seed layer

A seed layer is located between a first read gap layer and a nickel oxide (NiO) pinning layer for improving the microstructure of the pinning layer. The improved microstructure of the pinning layer increases an exchange coupling between the pinning layer and a ferromagnetic pinned layer structure which improves the magnetoresistive coefficient (dr/R) of a spin valve sensor. The seed layer is chromium (Cr), chromium oxide (Cr.sub.2 O.sub.3) or an alloy thereof with a preferred seed layer being chromium (Cr) with a thickness less than 50 .ANG.. In another embodiment an additional seed layer of tantalum (Ta) may be employed between the chromium (Cr) seed layer and the first read gap layer for further enhancement of the magnetoresistive coefficient (dr/R).

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to a spin valve sensor with a nickel oxide
 (NiO) pinning layer on a chromium based seed layer and more particularly
 to a chromium seed layer that improves an exchange coupling between the
 pinning layer and a pinned layer of the sensor.
 2. Description of the Related Art
 A spin valve sensor is employed by a read head for sensing magnetic fields
 on a moving magnetic medium, such as a rotating magnetic disk. The sensor
 includes a nonmagnetic electrically conductive first spacer layer
 sandwiched between a ferromagnetic pinned layer and a ferromagnetic free
 layer. An antiferromagnetic pinning layer is exchange coupled to the
 pinned layer for pinning the magnetic moment of the pinned layer
 90.degree. to an air bearing surface (ABS) which is an exposed surface of
 the sensor that faces the magnetic disk. First and second leads are
 connected to the spin valve sensor for conducting a sense current
 therethrough. The magnetic moment of the free layer is free to rotate
 upwardly or downwardly with respect to the ABS from a quiescent or bias
 point position in response to positive and negative magnetic field signals
 from a rotating magnetic disk. The quiescent position, which is typically
 parallel to the ABS, is the position of the magnetic moment of the free
 layer with the sense current conducted through the sensor in the absence
 of signal fields.
 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 are scattered at the interfaces of the spacer
 layer with respect to the pinned and free layers. Changes in scattering
 changes the resistance of the spin valve sensor as a function of cos
 .theta., where .theta. is the angle between the magnetic moments of 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. The
 sensitivity of the sensor is quantified as magnetoresistive coefficient
 dr/R where dr is the change in resistance of the sensor from a minimum
 resistance, where the magnetic moments of the free and pinned layer are
 parallel, to a maximum resistance, where the magnetic moments are
 antiparallel, and R is the resistance of the sensor before the change.
 Over the years a significant amount of research has been conducted to
 improve the magnetoresistive coefficient dr/R (also referred to as GMR) of
 spin valve sensors. These efforts have increased the storage capacity of
 computers from kilobytes to megabytes to gigabytes. Some of these efforts
 have been directed to the type of material and thicknesses of the various
 layers of the spin valve sensor which is an effort of the present
 invention.
 SUMMARY OF THE INVENTION
 The present invention provides a novel seed layer for the aforementioned
 pinning layer of the spin valve sensor. The seed layer increases the
 magnetoresistive coefficient dr/R of the spin valve sensor which, in turn,
 increases the storage capacity. The seed layer is chromium based and may
 be chromium (Cr), chromium oxide (Cr.sub.2 O.sub.3) or alloys thereof. In
 a spin valve sensor without the seed layer I found the magnetoresistive
 coefficient dr/R to be 5.28% while in the same spin valve sensor with a 35
 .ANG. thick chromium seed layer I found the magnetoresistive coefficient
 dr/R to be 5.90%. Accordingly, the seed layer produced a 12% increase in
 the magnetoresistive coefficient dr/R. This was caused by an improved
 exchange coupling field between the pinning and pinned layers. It is
 believed that the seed layer improved the microstructure of the pinning
 layer which, in turn, lead to the improvement of the exchange coupling
 between the pinning and pinned layers.
 An object of the present invention is to provide a spin valve sensor with
 an improved magnetoresistive coefficient dr/R.
 Another object is to provide a seed layer for a pinning layer that improves
 an exchange coupling field between the pinning layer and a pinned layer of
 a spin valve sensor.
 Other objects and attendant advantages of the invention will be appreciated
 upon reading the following description taken together with the
 accompanying drawings.

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 combined read and
 write magnetic head 40 is mounted on a slider 42 that is supported by a
 suspension 44 and actuator arm 46 that is rotatably positioned by the
 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 .mu.m) 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 47 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 55, 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, which 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.sub.s conducted through the sensor causes these
 resistance changes to be manifested as potential changes. These potential
 changes are then processed as read back 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 a read head 72 which has a spin
 valve sensor 130. 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. 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
 second hard bias layer 144 and a second lead layer 146. The hard bias
 layers 140 and 144 cause magnetic flux to extend longitudinally through
 the spin valve sensor 130 for stabilizing magnetic domains of the free
 layer. 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 first and
 second shield layers 152 and 154.
 EXAMPLE 1
 In FIG. 12 there is shown a spin valve sensor 200 which includes the
 antiferromagnetic (AFM) nickel oxide (NiO) pinning layer 132, shown in
 FIG. 11. The pinning layer 132 is on the first gap layer (G1) 148, also
 shown in FIG. 11. The spin valve sensor 200 includes a nonmagnetic
 conductive spacer layer (S) 202 which is located between a ferromagnetic
 pinned layer structure 204 and a ferromagnetic free layer (F) 206. A cap
 layer 208 is on the free layer 206 to protect the free layer 206 from
 subsequent processing steps in the making of the read head. The pinned
 layer structure 204 includes a nickel iron (NiFe) first layer 210 and a
 cobalt (Co) second layer 212 with the first layer 210 being located
 between the pinning layer 132 and the second layer 212. The first layer
 210 is exchange coupled to the pinning layer 132 so that a magnetic moment
 214 of the first layer is directed perpendicular to the ABS, either toward
 or away from the ABS, as shown in FIG. 12. By exchange coupling between
 the layers 210 and 212 a magnetic moment 216 of the pinned layer 212 is
 directed parallel to the magnetic moment 214 perpendicular to and away
 from the ABS. The free layer 206 has a magnetic moment 218 which is
 directed parallel to the ABS and rotates upwardly or downwardly from that
 position in response to signal fields from a rotating magnetic disk. When
 the magnetic moment 218 is rotated upwardly in response to a signal field
 the resistance of the sensor 200 decreases and when the magnetic moment
 218 rotates downwardly in response to a signal field, the resistance
 increases.
 The thicknesses and materials of the layers of the spin valve sensor are
 425 .ANG. of nickel oxide (NiO) for the pinning layer 132, 8 .ANG. of
 nickel iron (NiFe) for the first pinned layer 210, 12 .ANG. of cobalt (Co)
 for the second pinned layer 212, 24 .ANG. of copper (Cu) for the spacer
 layer 202, 72 .ANG. of nickel iron (NiFe) for the free layer 206 and 50
 .ANG. of tantalum (Ta) for the cap layer 208. The spin valve sensor 200
 was tested for its magnetoresistive coefficient (dr/R), resistance (R) and
 ferromagnetic coupling field (H.sub.F) between the pinned layer structure
 204 and the free layer 206. The magnetoresistive coefficient (dr/R) was
 5.28%, the resistance (R) was 21 ohms/sq. and the ferromagnetic coupling
 field (H.sub.F) was 2.5 Oe.
 EXAMPLE 2
 First Embodiment of the Invention
 FIG. 13 illustrates a second example of a spin valve sensor 300 which is a
 first embodiment of the present invention. The spin valve sensor 300 is
 the same as the spin valve sensor 200 in FIG. 12 except for a seed layer
 (SL) 302 which is located between the first read gap layer 148 and the
 pinning layer 132. The seed layer 302 was 35 .ANG. of chromium (Cr). The
 spin valve sensor 300 was also tested for magnetoresistive coefficient
 (dr/R), resistance (R) and ferromagnetic coupling field (H.sub.F). The
 magnetoresistive coefficient (dr/R) was 5.9%, the resistance (R) was 18.1
 ohms/sq. and the ferromagnetic coupling field (H.sub.F) was 0.18 Oe. It
 can be seen from this testing that the seed layer 302 in FIG. 13 increased
 the magnetoresistive coefficient (dr/R) from 5.28% for the spin valve
 sensor 200 in FIG. 12 without the seed layer to 5.9% for the spin valve
 sensor 300 in FIG. 13 with the seed layer 302. This improvement was caused
 by an improved exchange coupling field between the pinning layer 132 and
 the pinned layer structure 204. It is believed that the seed layer 302
 improved the microstructure of the pinning layer 132 which, in turn,
 caused the improvement of the exchange coupling between the pinning layer
 132 and the pinned layer structure 204.
 The resistance (R) of the spin valve sensor 300 in FIG. 13 decreased
 approximately 3 ohms/sq. from the resistance (R) of the spin valve sensor
 200 in FIG. 12 which indicates that the seed layer 302 in FIG. 13 is
 shunting a portion of the sense current (I.sub.S). Shunting of the sense
 current decreases the magnetoresistive coefficient (dr/R), however, the
 chromium (Cr) seed layer 302 in FIG. 13 overcame this reduction in
 magnetoresistive coefficient (dr/R) and exceeded the magnetoresistive
 coefficient (dr/R) of the spin valve sensor 200 shown in FIG. 12. The
 ferromagnetic coupling field (H.sub.F) of the spin valve sensor 300 in
 FIG. 13 decreased in an excess of 2 Oe which is a desired improvement.
 EXAMPLE 3
 Second Embodiment of the Invention
 FIG. 14 illustrates a third example of a spin valve sensor 400 which is a
 second embodiment of the present invention. The spin valve sensor 400 is
 the same as the spin valve sensor 300 in FIG. 13 except for a seed layer
 (SL) 402 which is 50 .ANG. of chromium (Cr) instead of 35 .ANG. of
 chromium (Cr). The sensor 400 was tested for magnetoresistive coefficient
 (dr/R), resistance (R) and ferromagnetic coupling field (H.sub.F). The
 magnetoresistive coefficient (dr/R) was 5.62%, the resistance (R) was 17.4
 ohms/sq. and the ferromagnetic coupling field (H.sub.F) was 1.5 Oe. It can
 be seen from this testing that the magnetoresistive coefficient (dr/R) of
 the spin valve sensor 400 with a thicker chromium (Cr) seed layer 402 than
 the chromium (Cr) seed layer 302 dropped from 5.9% for the spin valve
 sensor 300 in FIG. 13 to 5.62% for the spin valve sensor 400 in FIG. 14.
 This testing shows that an increase in the thickness of the chromium (Cr)
 seed layer can result in a decrease in the magnetoresistive coefficient
 (dr/R). It is believed that this is due to an increase in the shunting of
 the sense current (I.sub.s) because of the increase in thickness of the
 chromium (Cr) seed layer 402 in FIG. 14. Accordingly, the thickness of the
 chromium (Cr) seed layer is preferably maintained below 50 .ANG.. The
 resistance (R) of 17.4 ohms/sq. of the spin valve sensor 400 in FIG. 14 as
 compared to the resistance (R) of 18.1 ohms/sq. for the spin valve sensor
 300 in FIG. 13 shows that the spin valve sensor 400 shunts more of the
 sense current through the seed layer 402 than the shunting of the sense
 current through the seed layer 302 in FIG. 13. The slight increase in
 ferromagnetic coupling field (H.sub.F) of the spin valve sensor 400
 compared to the ferromagnetic coupling field (H.sub.F) for the spin valve
 sensor in 300 is minimal and acceptable for a well-performing spin valve
 sensor.
 EXAMPLE 4
 Third Embodiment of the Invention
 FIG. 15 illustrates a fourth example of a spin valve sensor 500 which is a
 third embodiment of the present invention. The spin valve sensor 500 is
 the same as the spin valve sensor 400 in FIG. 14 except for first and
 second seed layers (SL1) and (SL2) 502 and 504. The second seed layer 504
 is located on the first read gap layer 148, the first seed layer 502 is
 located on the second seed layer 504 and the pinning layer 132 is located
 on the first seed layer 502. This embodiment of the invention shows the
 second seed layer 504 being tantalum (Ta) and the first seed layer 502
 being chromium (Cr), chromium oxide (Cr.sub.2 O.sub.3) or alloys thereof.
 For example, the first seed layer 502 may be chromium (Cr) or alloys of
 chromium (Cr), such as nickel chromium (NiCr) or nickel iron chromium
 (NiFeCr), or may be chromium oxide (Cr.sub.2 O.sub.3) or alloys thereof,
 such as nickel chromium oxide (NiCrO) or nickel iron chromium oxide
 (NiFeCrO). The second seed layer 504 of tantalum (Ta) will improve the
 texture of the first seed layer 502, which will further improve the
 microstructure of the pinning layer 132. This will, in turn, increase the
 exchange coupling between the pinning layer 132 and the pinned layer
 structure 204, which results in a further increase in the magnetoresistive
 coefficient (dr/R). The spin valve sensor 500 in FIG. 15 further differs
 from the spin valve sensor 400 shown in FIG. 14 by including a giant
 magnetoresistive (GMR) enhancement layer 506 which is located between the
 spacer layer 202 and the free layer 206. The GMR enhancement layer 506 is
 preferably 10 .ANG. of cobalt (Co) or cobalt iron (CoFe). It has been
 found that the GMR enhancement layer 506, which is sometimes referred to
 as a nanolayer, increases the magnetoresistive coefficient (dr/R) of the
 spin valve sensor.
 EXAMPLE 5
 Fourth Embodiment of the Invention
 FIG. 16 illustrates a fifth example of a spin valve sensor 600 which is a
 fourth embodiment of the present invention. The spin valve sensor 600
 includes a nonmagnetic conductive spacer layers 602 which is located
 between an antiparallel (AP) pinned layer structure 604 and a free layer
 structure 606. The free layer structure 606 includes a GMR layer 608 which
 is the same as the GMR layer 506 in FIG. 15 and a free layer (F) 610, the
 GMR layer 608 being located between the spacer layer 602 and the free
 layer 610. A magnetic moment 612 of the free layer 610 is directed
 parallel to the ABS, either from right to left or from left to right, as
 shown in FIG. 16. The GMR layer 608 has a magnetic moment (not shown)
 which is parallel to the magnetic moment 612.
 The AP pinned layer structure 604 includes an antiparallel (AP) coupling
 layer 614 which is located between first and second antiparallel pinned
 layers (AP1) and (AP2) 616 and 618. The structure 604 further includes an
 interlayer of nickel iron (NiFe) 620 which is located between the pinning
 layer 132 and the first AP pinned layer 616. The nickel iron (NiFe) layer
 620 is exchange coupled to the pinning layer 132 and has a magnetic moment
 622 which is pinned by the pinning layer 132 in a direction perpendicular
 to the ABS, either away from or toward the ABS, as shown in FIG. 16. By
 exchange coupling with the layer 620 the first AP pinned layer 616 has a
 magnetic moment 624 which is directed parallel to the magnetic moment 622.
 By antiparallel coupling between the second AP pinned layer 618 and the
 first AP pinned layer 616 the second AP pinned layer 618 has a magnetic
 moment 626 which is directed antiparallel to the magnetic moment 624. When
 the magnetic moment 612 of the free layer 610 rotates upwardly from the
 ABS in response to a signal field the resistance of the spin valve sensor
 decreases and when the magnetic moment 612 rotates downwardly with respect
 to the ABS in response to a signal field the resistance of the spin valve
 sensor increases. A cap layer 628 is located on the free layer 610 for its
 protection from subsequent processing steps in the making of the read
 head.
 The sensor 600 employs a seed layer 630, which is chromium (Cr), chromium
 oxide (Cr.sub.2 O.sub.3) or an alloy thereof, between the first read gap
 layer 148 and the pinning layer 132. The seed layer (SL) 630 is the same
 as the seed layer 502 in FIG. 15 and has a thickness which is preferably
 less than 50 .ANG.. Exemplary thicknesses and materials for the layers are
 10 .ANG. of nickel iron (NiFe) for the layer 620, 24 .ANG. of cobalt (Co)
 or cobalt iron (CoFe) for the layer 616, 8 .ANG. of ruthenium (Ru) for the
 layer 614, 24 .ANG. of cobalt (Co) or cobalt iron (CoFe) for the layer
 618, 24 .ANG. of copper (Cu) for the layer 602, 10 .ANG. of cobalt (Co) or
 cobalt iron (CoFe) for the GMR enhancement layer 608, 70 .ANG. of nickel
 iron (NiFe) for the free layer 610 and 50 .ANG. of tantalum (Ta) for the
 cap layer 628.
 EXAMPLE 6
 Fifth Embodiment of the Invention
 FIG. 17 is a sixth example of a spin valve sensor 700 which is a fifth
 embodiment of the present invention. The embodiment 700 in FIG. 17 is the
 same as the embodiment 600 in FIG. 16 except first and second seed layers
 (SL1) and (SL2) 702 and 704 are employed with the second seed layer 704
 being on the first read gap layer 148, the first seed layer 702 being on
 the second seed layer 704 and the pinning layer 132 being on the first
 seed layer 702. The first seed layer 702 is the same as the seed layer 630
 shown in FIG. 16. The second seed layer 704 is tantalum (Ta) which will
 improve the microstructure of the first seed layer 702 which will, in
 turn, improve the microstructure of the pinning layer 132. This will
 result in an improved exchange coupling between the pinning layer 132 and
 the AP pinned layer structure 604 which will further increase the
 magnetoresistive coefficient (dr/R).
 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 the accompanying drawings.