Patent Publication Number: US-2002006021-A1

Title: Spin valve sensor with an antiferromagnetic layer between two pinned layers

Description:
[0001] The present application is a continuation-in-part of U.S. application that has been provided application Ser. No. 09/615,158 and was filed on Jul. 13, 2000. 
    
    
     
       FIELD OF THE INVENTION  
       [0002] The field of invention relates to direct access data storage, generally. More specifically, the invention relates to compensating for the effect of unwanted biasing from the pinned layer.  
       BACKGROUND  
       [0003] Hardware systems often include memory storage devices having media on which data can be written to and read from. A direct access storage device (DASD or disk drive) incorporating rotating magnetic disks are commonly used for storing data in magnetic form. Magnetic heads, when writing data, record concentric, radially spaced information tracks on the rotating disks.  
       [0004] Magnetic heads also typically include read sensors that read data from the tracks on the disk surfaces. In high capacity disk drives, magnetoresistive (MR) read sensors, the defining structure of MR heads, can read stored data at higher linear densities than thin film heads. An MR head detects the magnetic field(s) through the change in resistance of its MR sensor. The resistance of the MR sensor changes as a function of the direction of the magnetic flux that emanates from the rotating disk.  
       [0005] One type of MR sensor, referred to as a giant magnetoresistive (GMR) effect sensor, takes advantage of the GMR effect. In GMR sensors, the resistance of the MR sensor varies with direction of flux from the rotating disk and as a function of the spin dependent transmission of conducting electrons between magnetic layers separated by a non-magnetic layer (commonly referred to as a spacer) and the accompanying spin dependent scattering within the magnetic layers that takes place at the interface of the magnetic and non-magnetic layers.  
       [0006] GMR sensors using two layers of magnetic material separated by a layer of GMR promoting non-magnetic material are generally referred to as spin valve (SV) sensors. In an SV sensor, one of the magnetic layers, referred to as the pinned layer, has its magnetization direction “pinned” via the influence of exchange anisotropy with an antiferromagnetic layer. Due to the relatively high internal anisotropy field associated with the pinned layer, the magnetization direction of the pinned layer typically does not rotate from the flux lines that emanate/terminate from/to the rotating disk. The magnetization direction of the other magnetic layer (commonly referred to as a free layer), however, is free to rotate with respect to the flux lines that emanate/terminate from/upon the rotating disk.  
       [0007]FIG. 1 shows a prior art SV sensor  100  comprising a seed layer  102  formed upon a gap layer  101 . The seed layer  102  helps properly form the microstructure of the Antiferromagnetic (AFM) layer  105 . Over seed layer  102  is a free layer  103 . The Antiferromagnetic (AFM) layer  105  is used to pin the magnetization direction of the pinned layer  104 . Pinned layer  104  is separated from free layer  103  by the non magnetic, GMR promoting, spacer layer  119 . Note that free magnetic layer  103  may be a multilayer structure having two or more ferromagnetic layers.  
       [0008] A problem with structures such as the sensor  100  shown in FIG. 1, is the field biasing of the free layer  103 . Specifically, since the pinned layer  104  has a net magnetic moment with associated pole densities, flux lines  107  are produced by the pinned layer  104  that (in the example of FIG. 1) exerts a bias on the free layer  103  in the +z direction. Ideally, the free layer  103  should experience minimal bias so that its magnetization (designed to point in the +x direction) has a balanced swing in the +z and −z directions. That is, a field from the disk in the +z direction should produce a magnetization swing in the +z direction that is the same as the magnetization swing observed in the −z direction from an identically strong field from the disk in the −z direction. The bias exerted by lines  107  adversely affect the balance of this swing.  
       SUMMARY OF INVENTION  
       [0009] A multilayer structure is described having an antiferromagnetic layer between a first and second layer. The antiferromagnetic layer has antiferromagnetic coupling that helps pin the magnetization direction of the first layer and helps pin the magnetization direction of the second layer.  
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0010] The present invention is illustrated by way of example, and not limitation, in the Figures of the accompanying drawings in which:  
     [0011]FIG. 1 shows a prior art SV sensor.  
     [0012]FIG. 2 shows an SV sensor having an antiferromagnetic layer between two pinned layers.  
     [0013]FIG. 3 shows a method that may be used to form the sensor shown in FIG. 2.  
     [0014] FIGS.  4  shows a biasing technique that may be used for an embodiment of the method shown in FIG. 3.  
     [0015]FIG. 5 a  shows a biasing technique that may be used for another embodiment of the method shown in FIG. 3.  
     [0016]FIG. 5 b  shows fields within the pinned layer and the pinned keeper layer from the setting current as well from an applied field for the technique shown in FIG. 5 a.    
     [0017]FIG. 5 c  shows the net field within the pinned layer and keeper layer produced by the fields of FIG. 5 b.    
     [0018]FIG. 6 shows a magnetic disk and activator.  
     [0019]FIG. 7 shows an air bearing surface.  
     [0020]FIG. 8 shows a direct access storage device.  
    
    
     DETAILED DESCRIPTION  
     [0021] A multilayer structure is described having an antiferromagnetic layer between a first and second layer. The antiferromagnetic layer has antiferromagnetic coupling that helps pin the magnetization direction of the first layer and helps pin the magnetization direction of the second layer.  
     [0022] These and other embodiments of the present invention may be realized in accordance with the following teachings and it should be evident that various modifications and changes may be made in the following teachings without departing from the broader spirit and scope of the invention. The specification and drawings are, accordingly, to be regarded in an illustrative rather than restrictive sense and the invention measured only in terms of the claims.  
     [0023]FIG. 2 shows sensor design  200  that improves upon the free layer  203  biasing problem discussed in the background. The SV sensor design  200  of FIG. 2 incorporates two pinned layers: pinned layer  204  and pinned keeper layer  208 . The pinned layer  204  is used similarly to prior art SV sensors having a pinned layer  204 . That is, pinned layer  204  is used to promote the GMR effect within the free layer  203  and, as such, is separated from the free layer  203  by a non magnetic spacer layer  219 .  
     [0024] Pinned layer  204  produces flux lines  207 , similar to the flux lines  107  discussed in the background with respect of FIG. 1, that (in the example of FIG. 2), exert a bias on the magnetization of the free layer  203  in the +z direction. Pinned keeper layer  208 , however, is tailored to approximately cancel out the effect of flux lines  207  within the free layer  203 .  
     [0025] As shown in FIG. 2, pinned keeper layer  208  has a magnetization direction that is antiparallel to the magnetization direction of the pinned layer  204 . The antiparallel magnetization arrangement produces pole densities  293 ,  294  on either surface of the pinned keeper layer  208  that are opposite in polarity to the pole densities  291 ,  295  produced on the same surface on the sensor  200  at the pinned layer  204 .  
     [0026] The flux lines  209  produced by the pinned keeper layer  208  are configured to approximately cancel the flux lines  207  produced by the pinned layer  204 . This substantially removes any undesired bias on the free layer  203 . As a result, the magnetization direction of the free layer  203  will be able to exhibit a balanced swing with respect to the flux that emanates/terminates from/upon the disk surface.  
     [0027] In order to substantially cancel out the flux lines  207  and  209  within the free layer  203 , considerations should be taken into account of: 1) the total magnetic moment of the pinned layer  204  and the pinned keeper layer  208 ; and 2) the distance between the free layer  203  and the pinned layer  204 ; and 3) the distance between the free layer  203  and the pinned keeper layer  208 . The total magnetic moment of each layer  204 ,  208  is determined by the thickness and material(s) of each layer  204 ,  208 .  
     [0028] In one embodiment, gap layer  201  is an Al 2 O 3  layer. Seed layer  202  is formed with 50 Å of Tantalum (Ta). Free layer  203  is formed with 50 Å of Ni 82 Fe 18 . Pinned layer  204  is a 50 Å layer of Co 90 Fe 10 . Anti Ferromagnetic layer  205  is a 200 Å layer of Platinum Manganese (PtMn). Pinned keeper layer  208  is formed with 70 Å of Co 90 Fe 10 . Cap layer  206  is formed with 50Å of Tantalum (Ta).  
     [0029] The antiparallel magnetization arrangement between the pinned layer  204  and the pinned keeper layer  208  may be obtained by “pinning”the magnetization direction of each of these layers  204 ,  208  through the exchange anisotropy coupling exerted by the antiferromagnetic layer  205 . Exchange anisotropy is an effective field, associated with the lattice and atomic structure of an antiferromagnetic material, that causes the adjacent ferromagnetic layer moments to align preferentially in the “pinning” direction. Materials having the proper atomic and lattice structure to exhibit anisotropy coupling include IrMn, PtMn, NMn, NiO, CoO (or alloys of these materials) among others. Materials such as these may be used for antiferromagnetic layer  205 . In order to exert the anisotropy coupling associated with the antiferromagnetic layer  205  upon its neighboring pinned  204  and pinned keeper  208  layers, the sensor (or at lease the portion having the antiferromagnetic  205 , pinned  204  and pinned keeper  208  layers) may be heated above a “blocking” temperature with fields applied to the pinned  204  and pinned keeper  208  layer that are directed in the desired magnetization direction of these layers  204 ,  208 . Note that neighboring layers are layers immediately next to one another.  
     [0030] Referring to FIG. 2, a field is applied to the pinned layer  204  in the −z direction and a field is applied to the pinned keeper layer  208  in the +z direction during the fabrication of the sensor. Once the structure is heated to a temperature above the blocking temperature, the structure is cooled while sustaining the applied fields to both layers  204 ,  208 . As the structure cools below the blocking temperature, the applied fields force the antiferromagnetic coupling associated with the antiferromagnetic layer  205  to “set” in an orientation that promotes a magnetization direction in the −z direction for the pinned layer  204  and the +z direction for the pinned keeper layer  208 .  
     [0031] As such, after the sensor  200  is fully formed and installed in a DASD system, the anisotropy coupling of the antiferromagnetic layer  205  helps keep the magnetization of the pinned layer  204  “pinned” in the −z direction and the magnetization of the pinned keeper layer  208  “pinned” in the +z direction. FIG. 3 shows a methodology  300  consistent with the process discussed above. First, a multilayer structure is fabricated  301  comprising an antiferromagnetic layer between a pinned layer and a pinned keeper layer.  
     [0032] Then, the temperature of the multilayer structure is raised above the blocking temperature  302 . Next, a field is applied to pinned layer  303   a  and a field is applied to the pinned keeper layer  303   b . Alternatively, the fields may be applied  303   a ,  303   b  before the temperature is raised above the blocking temperature  302 . The fields are applied in the desired direction of magnetization for these materials, which for the example shown in FIG. 2 corresponds to the −z direction for the pinned layer  204  and the +z direction for the pinned keeper layer  208 . After the fields are applied  303   a ,  303   b  the multilayer structure is cooled  304 .  
     [0033]FIG. 4 relates to one fabrication embodiment of the method discussed with respect to FIG. 3. The sensor  400  of FIG. 4 corresponds to the basic SV sensor  200  of FIG. 2; thus, FIG. 4 relates to a processing embodiment that may be used to fabricate the basic SV sensor  200  of FIG. 2. However, the technique associated with FIG. 4 may be used with other SV sensor structures such as AP sensors and dual spin valve structures.  
     [0034] In FIG. 4, a setting current  420  in +x direction is sent through the sensor  400 . Part of this current flows through the pinned keeper layer  408 , part flows through layers to its left, and part flows through layer  406 , to the right. The net field due to this current distribution acting on pinned layer  408  depends primarily on the net current to its right and the net current to its left. The fields due to current flow on the left and the right act in opposite directions, according to Ampere&#39;s law. The current field acting on pinned layer  404  is similarly determined.  
     [0035] In FIG. 4, the bulk of the current flows through AFM layer  405 , generating flux line  421 . The current distribution is such that the current fields acting on pinned keeper  408  and pinned layer  404  are antiparallel. As seen in FIG. 4, flux line  421  creates a field in the −z direction in the pinned layer  404  and a field in the +z direction in the pinned keeper layer  408 . The minimum field strength used to set the antiferromagnetic coupling for both layers  404 ,  408  should be at or above the coercivity associated with each layer. For layers  408  formed with Co 90 Fe 10 , the coercivity is typically as low as 5.0 or as high as 30.0 Oersteds (Oe) with standard manufacturing techniques. Over time this range may change as storage densities increase.  
     [0036] The current distribution for a particular sensor structure is function of the resistivity of each layer and the thickness (i.e., width along the y axis) of each layer within the sensor  400 . The individual layer resistances may be tailored to achieve a current distribution which produces the desired fields at the positions of the pinned ( 404 ) and pinned keeper ( 408 ) layers. For example, to increase the downward field acting on pinned layer  404 , free layer  403  may be made thinner, or of a higher resistivity material, so that a greater fraction of the current flows to the right of pinned layer  404 . Alternatively, the upward field acting on pinned keeper  408  may be increased by decreasing the thickness of layer  406 . In the embodiments mentioned above, the resistance of the antiferromagnetic layer  405  could be less than the combined resistance of the sensor  400  regions outside the antiferromagnetic layer  405  to force most of the setting current  420  to flow in the antiferromagnetic layer  405 .  
     [0037] The coercivities of the pinned and pinned keeper layers will be unequal, in general. It is desirable to adjust the current distribution such that the fields acting on the pinned and pinned keeper layers overcome the individual layer coercivities. For example, if the pinned layer  404  has a higher coercivity than the pinned keeper layer  408 , layer  406  may be thickened to increase the downward field on pinned layer  404  while decreasing the upward field on pinned keeper  408 . Similarly, spacer layer  419  may be thickened if the coercivity of the pinned keeper layer  408  is higher than the pinned layer  404 .  
     [0038]FIG. 5 a  relates to another embodiment of the method shown with respect to FIG. 3. In FIG. 5 a , similar to FIG. 4, a setting current  520  is used to apply a field in the pinned  504  and pinned keeper  508  layers. In FIG. 5, the bulk of the current flows through the free layer  503 , and generates flux line  530 . In the embodiment of FIG. 5, it is the current field differential between pinned layer  504  and pinned keeper  508  which is made large, rather than the fields themselves. In addition to the current fields, another field is applied in order to properly orient the fields within the two layers  504 ,  508 . This other applied field may be an external applied field.  
     [0039] Referring to FIGS. 5 a  and  5   b , the field within the pinned layer  504  that results from setting current  520  is represented by vector H 530 . Also, the field within the pinned keeper layer  508  that results from setting current  520  is represented by vector H 531 . An applied external field is represented by vector H external . Note that, as seen in FIG. 5 a , the setting current  520  is such that both fields H 530 , H 531 , created by the setting current  520  are oriented in the same direction (e.g., the +z direction).  
     [0040] Furthermore, of the two fields H 530 , H 531  that result from the setting current  520 , one field has a stronger intensity than the other. In the example of FIGS. 5 a  and  5   b , field H 531 , is stronger than field H 530 . In order to form one field stronger than another field with a setting current  520 , the setting current  520  may be partly confined outside the antiferromagnetic layer  505  (e.g., outside the multilayer structure  555  formed by the pinned layer  504 , antiferromagnetic layer  505  and pinned keeper layer  508 ).  
     [0041] By partly confining the setting current through the sensor outside the antiferromagnetic layer  505 , the stronger field (e.g., H 531 ) may be formed in the layer further from the confined setting current (e.g., layer  508 ) and the weaker field (e.g., H 530 ) may be formed in the layer closer to the confined setting current (e.g., layer  504 ). Thus, in the embodiment of FIG. 5 a , the resistivity of each of the various sensor  500  layers may be configured to confine the setting current  520  to flow substantially on one side of the multilayer structure  555  having the pinned  504 , AFM  505  and pinned keeper  508  layers such that the field strength continually increases as the distance from the side of the multilayer structure having the substantial amount of setting current increases. Specifically, in the sensor embodiment  500  of FIG. 5 a , the setting current  520  substantially flows to the “left” of position y 1 .  
     [0042] The setting current may be partly confined in this manner by tailoring the resistivity of each of the layers within the sensor  500  and their associated thickness appropriately. For example, a thicker free layer  503  comprised of CoFe and/or NiFe promotes current confinement to the left of y 1  as does an antiferromagnetic  505  layer comprised of a highly resistive material (e.g., an oxide such as NiO or CoO).  
     [0043] Alternate sensor embodiments may be designed to confine the setting current partly to the “right” of multilayer structure  555 . Note that in still other sensor embodiments, the setting current may flow in the pinned  504  and pinned keeper  508  layers, provided the difference in field magnitude between fields H 530 , H 531 , and the uniformity in field direction of fields H 530 , H 531  is not substantially disturbed.  
     [0044] In yet other embodiments, considerable current may flow through the antiferromagnetic layer  505  provided there is sufficient current outside the antiferromagnetic layer to properly bias layers  504 ,  506 . That is, if the resistance of the antiferromagnetic layer  505  is sufficiently greater than the combined resistance of the sensor  500  through the regions on either side of the antiferromagnetic layer  505  (e.g., the region to the left of the antiferromagnetic layer  505  or the region to the right of the antiferromagnetic layer  505  as seen in FIG. 5 a ) layers  504  and  508  may be properly biased as in FIG. 5 c.    
     [0045]FIG. 5 c  shows the resultant field within the pinned layer H pinned  and the pinned keeper layer H keeper . H pinned  is the vector addition of H 530 and H external  of FIG. 5 b . H keeper  is the vector addition of H 531  and H external  of FIG. 5 b . The magnitude of H external  should be greater than one field (e.g., H 530 ) produced by the setting current yet smaller than the other field (e.g., H 531 ) produced by the setting current. This relationship will force the resultant field H pinned within the pinned layer  504  to be antiparallel to the resultant field H keeper  within the pinned keeper layer  508 . When the proper resultant fields are established in the pinned  504  and pinned keeper layers  508 , the sensor  500  may be cooled from a temperature above the antiferromagnetic blocking temperature to a temperature below this temperature in order to properly orient the antiferromagnetic coupling. Typical blocking temperatures range from 200 to 400° C. Note, however, that the blocking temperature is a function of material and other physical parameters (e.g., lattice structure) which may affect this range from embodiment to embodiment.  
     [0046] Still other embodiments of FIG. 3 may differ slightly from those discussed above by incorporating a hard magnetic layer (e.g., a material exhibiting permanent magnet characteristics such as a high coercivity) as either the pinned layer or pinned keeper layer or as both layers. The hard magnetic layer(s) may have its magnetization direction permanently set by an applied field that is greater than the layer&#39;s coercivity.  
     [0047] When the magnetization direction of the hard magnetic layer is permanently set, the temperature of the sensor may be lowered from above the antiferromagnetic blocking temperature to below it. Note that in order to employ this approach the curie temperature of the hard magnetic layer should be greater than the blocking temperature of the antiferromagnetic coupling field. This is true in most cases since typical hard magnetic materials have curie temperatures above 500° C.  
     [0048] Referring to FIGS. 2, 4 and  5 , it is important to note that the gap layer  201 ,  401 ,  501  may be comprised of any oxide layer used within MR structures such as NiMnO, NiMgO 2 , and Al 2 O 3 among others. Furthermore, seed layer  202 ,  402 ,  502  may be formed with magnetic materials such as a Co based alloy (e.g., CoFe) or non magnetic materials such as Ta or Cu. Note that if magnetic seed layers  302 ,  402  are used, the effect of its associated pole density and corresponding magnetic field (if any or if noticeable) on the biasing of the free, pinned and pinned keeper layers may have to be accounted for in the design of the sensor  300 ,  400 . Cu, Au, Ag or Ru may be used for the non magnetic and spacer layer  219 ,  419 ,  519 . Free layer  203 ,  403 ,  503  is typically comprised of CoFe or NiFe or alloys thereof. Note that consistent with the skills of those who practice in the art, embodiments employing CoFe and NiFe are not limited solely to Co 90 Fe 10  and Ni 82 Fe 18 . That is, element percentages may vary consistent with the general formulations: Co x Fe x-1 and Ni   x Fe x-1 .  
     [0049] Referring now to the drawings wherein like reference numerals designate like or similar parts throughout the several views, FIGS.  6 - 8  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 slider  42  with a combined read and write magnetic head  40  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. 8. 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 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.