Patent Publication Number: US-6219211-B1

Title: Current-pinned, current resettable soft AP-pinned spin valve sensor

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation of application Ser. No. 08/957,851 filed on Oct. 27, 1997, issued as U.S. Pat. No. 6,040,961 on Mar. 21, 2000, owned by a common assignee and having the same inventor as the present invention. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to magnetic transducers for reading information signals from a magnetic medium and, in particular, to a soft antiparallel pinned spin valve sensor, and to magnetic recording systems which incorporate such sensors. 
     2. Description of Related Art 
     Computers often include auxiliary memory storage devices having media on which data can be written and from which data can be read for later use. A direct access storage device (disk drive) incorporating rotating magnetic disks is commonly used for storing data in magnetic form on the disk surfaces. Data is recorded on concentric, radially spaced tracks on the disk surfaces. Magnetic heads including read sensors are then used to read data from the tracks on the disk surfaces. 
     In high capacity disk drives, magnetoresistive read sensors, commonly referred to as MR heads, are the prevailing read sensors because of their capability to read data from a surface of a disk at greater linear densities than thin film inductive heads. An MR sensor detects a magnetic field through the change in the resistance of its MR sensing layer (also referred to as an “MR element”) as a function of the strength and direction of the magnetic flux being sensed by the MR layer. 
     The conventional MR sensor operates on the basis of the anisotropic magnetoresistive (AMR) effect in which an MR element resistance varies as the square of the cosine of the angle between the magnetization in the MR element and the direction of sense current flow through the MR element. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in the direction of magnetization in the MR element, which in turn causes a change in resistance in the MR element and a corresponding change in the sensed current or voltage. 
     Another type of MR sensor is the giant magnetoresistance (GMR) sensor manifesting the GMR effect. In GMR sensors, the resistance of the MR sensing layer varies as a function of the spin-dependent transmission of the conduction electrons between magnetic layers separated by a non-magnetic layer (spacer) and the accompanying spin-dependent scattering which takes place at the interface of the magnetic and non-magnetic layers and within the magnetic layers. 
     GMR sensors using only two layers of ferromagnetic material separated by a layer of non-magnetic material (e.g., copper) are generally referred to as spin valve (SV) sensors manifesting the GMR effect (also referred to as SV effect). In an SV sensor, one of the ferromagnetic layers, referred to as the pinned layer, has its magnetization typically pinned by exchange coupling with an antiferromagnetic (e.g., NiO or Fe—Mn) layer. The magnetization of the other ferromagnetic layer, referred to as the free layer, however, is not fixed and is free to rotate in response to the field from the recorded magnetic medium (the signal field). In SV sensors, the SV effect varies as the cosine of the angle between the magnetization of the pinned layer and the magnetization of the free layer. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in direction of magnetization in the free layer, which in turn causes a change in resistance of the SV sensor and a corresponding change in the sensed current or voltage. IBM&#39;s U.S. Pat. No. 5,206,590 granted to Dieny et al. and incorporated herein by reference, discloses an MR sensor operating on the basis of the SV effect. 
     FIG. 1 shows a prior art SV sensor  100  comprising end regions  104  and  106  separated by a central region  102 . A free layer (free ferromagnetic layer)  110  is separated from a pinned layer (pinned ferromagnetic layer)  120  by a non-magnetic, electrically-conducting spacer  115 . The magnetization of the pinned layer  120  is fixed by an antiferromagnetic (AFM) layer  121 . Free layer  110 , spacer  115 , pinned layer  120  and the AFM layer  121  are all formed in the central region  102 . Hard bias layers  130  and  135  formed in the end regions  104  and  106 , respectively, provide longitudinal bias for the free layer  110 . Leads  140  and  145  formed over hard bias layers  130  and  135 , respectively, provide electrical connections for the flow of the sensing current I s  from a current source  160  to the MR sensor  100 . Sensing means  170  connected to leads  140  and  145  sense the change in the resistance due to changes induced in the free layer  110  by the external magnetic field (e.g., field generated by a data bit stored on a disk). 
     Another type of spin valve sensors currently under development is an anti-parallel (AP)-pinned spin valve sensor. IBM&#39;s U.S. Pat. No. 5,583,725 granted to Coffey et al. and incorporated herein by reference, describes an AP-pinned SV sensor (FIG. 2) wherein the pinned layer is a laminated structure of two ferromagnetic layers separated by a non-magnetic coupling layer such that the magnetizations of the two ferromagnetic layers are strongly coupled together antiferromagnetically in an antiparallel orientation. 
     Referring to FIG. 2, there is shown a prior art AP-Pinned SV sensor  200  having a free layer  210  separated from a laminated AP-pinned layer  220  by a nonmagnetic, electrically-conducting spacer layer  215 . Free layer  210  comprises a Co layer  212  and a Ni—Fe layer  214 . The laminated AP-pinned layer  220  comprises a first ferromagnetic layer  222  and a second ferromagnetic layer  226  separated from each other by an antiparallel coupling (APC) layer  224  of nonmagnetic material. The two ferromagnetic layers  222  and  226  in the laminated AP-pinned layer  220  have their magnetization directions oriented antiparallel, as indicated by the head of the arrow  223  pointing out of the plane of the paper and the tail of the arrow  227  pointing into the plane of the paper. Antiferromagnetic (AFM) exchange biasing layers  230  and  232  formed on the lateral extensions  240  and  242  of the free layer  210 . The AFM layers  230  and  232  longitudinally bias the free layer so its magnetization in the absence of an external field is in the direction of the arrow  250 . Capping layers  260  and  262  provide corrosion resistance for the AFM layers  230  and  232 , respectively. Electrical leads  270  and  272  provide electrical connections to current source  280  and a sensing means  285 . 
     Coffey does not use a hard bias layer or an AFM layer adjacent to the pinned layer  220  for pinning the magnetization of the pinned ferromagnetic layer  220 . Consequently, Coffey avoids the problems associated with the blocking temperature and/or corrosion of many AFM materials. However, according to Coffey once the sensor geometry is completed the directions of the magnetizations of first and second pinned layers  222  and  226  are set, perpendicular to the air bearing surface (plane of the disk), by applying a sufficiently large magnetic field (about 10 KOe). That is, a large external field is used in order to ensure that the spins are all pinned in the same direction. Once the spins are all pinned in the same direction, Coffey relies on the antiferromagnetic coupling between the two pinned layers  222  and  226  to maintain the pinning. FIGS. 2 a  and  2   b  are side views showing the spins directions in the pinned layers  222  and  226  before and after applying a large external field. Before applying a large external field, the spins are randomly formed in both pinned layers  222  and  226  (FIG. 2 a ). After applying a sufficiently large external field, the directions of the magnetizations in both pinned layers  222  and  226  become set meaning that the spins in each pinned layer become uniform in their directions (FIG. 2 b ). 
     However, there are two issues present in Coffey&#39;s AP-pinned SV sensor. First, if the magnetizations directions in the pinned layers  222  and  226  becomes disoriented due to a thermal asperity (actual or near contact between the head and the disk) or other unwanted fields (such as the field generated by the write head), there is no means for applying an external field of a large magnitude in the magnetic recording system to reset the directions of the magnetizations of the pinned layers and therefore, the SV sensor becomes inoperative, either partially or completely. 
     Second, even in the absence of a thermal asperity and/or an unwanted field, the magnetizations directions of the spins in the two pinned layers  222  and  226  rotate in the presence of an unwanted field because Coffey does not provide any means for keeping the directions of magnetizations of the pinned layers perpendicular to the air bearing surface. FIG. 2 b  shows the magnetizations directions of the spins perpendicular to the air bearing surface. FIGS. 2 c  and  2   d  show the directions of the magnetizations of the spins rotated in the presence of an unwanted (external) field such that the magnetizations directions is no longer perpendicular to the air bearing surface. 
     Rotation of the directions of the magnetizations in the pinned layers  222  and  226  result in read signal asymmetry, unpredictable read signal amplitude and free layer saturation. 
     Therefore there is a need for an AP-pinned SV sensor in which the directions of magnetizations in the pinned layers can be set by a rather small field, an AP-pinned SV sensor which does not use hard bias or AFM layers for pinning the pinned layers and an AP-pinned SV sensor in which the directions of magnetizations do not rotate in the presence of unwanted fields. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to disclose a current resettable AP-pinned SV sensor where the sense current may be used to reset the directions of magnetizations in the pinned layers in the event that the pinned layers become disoriented. 
     It is another object of the present invention to disclose a current-pinned AP-Pinned SV sensor wherein the sense current flowing in the layers of the SV sensor generates a pinning field for pinning the pinned layers. 
     It is a further object of the present invention to disclose an AP-pinned SV sensor, which is referred to as a Soft AP-Pinned SV sensor, wherein the antiferromagnetically coupled pinned layers comprise low magnetic coercivity materials. 
     It is still another object of the present invention to disclose a current-resettable, current-pinned soft AP-pinned SV sensor. 
     It is yet another object of the present invention to disclose an AP-pinned SV sensor wherein the antiferromagnetically coupled pinned layers comprise low anisotropy magnetic materials. 
     In accordance with the principles of the present invention, there is disclosed a Soft AP-Pinned SV sensor comprising an AP-pinned layer separated from a free ferromagnetic layer by a non-magnetic electrically conducting spacer layer. The AP-pinned layer comprises first and second ferromagnetic pinned layers separated from each other by a non-magnetic antiferromagnetically coupling layer. The first ferromagnetic pinned layer is made of low coercivity material (less than 10 Oe) such as Ni—Fe. The second ferromagnetic pinned layer further comprises first and second ferromagnetic sub-layers where the first sub-layer is preferably made of low coercivity material (less than 10 Oe) such as Ni—Fe and the second sub-layer is made of high coercivity material such as cobalt (Co). 
     The low coercivity of the first ferromagnetic pinned layer allows for: 
     (1) setting the directions of the magnetizations in the first and second pinned layers using the field generated by the sense current flowing in the soft AP-pinned SV sensor thus avoiding the use of large external field used by Coffey; and 
     (2) pinning the directions of the magnetizations of the pinned layers perpendicular to the air bearing surface using the field generated by the sense current flowing in the soft AP-pinned SV sensor thus avoiding the rotation of the pinned layers magnetizations directions in the presence of external fields. 
     Unlike Coffey&#39;s AP-pinned SV sensor in which its pinned layers magnetizations directions could not be reset in the magnetic storage device in the event of being disoriented, Applicant&#39;s soft AP-pinned SV sensor pinned layers can easily be reset using the field generated by the sense current flowing in the sensor. 
     Unlike Coffey&#39;s AP-pinned SV sensor in which pinned layers magnetizations directions could easily rotate in the presence of external fields, the pinned layers magnetizations directions in Applicant&#39;s soft AP-pinned SV sensor is pinned using the field generated by the sense current flowing in the sensor and therefore rotation of the magnetization direction is prevented. 
     Applicant&#39;s invention achieves the aforementioned results by using first ferromagnetic pinned layer which is made of soft ferromagnetic materials. Soft ferromagnetic materials are materials having low coercivity (less than 10 Oe). The low coercivity of the first ferromagnetic pinned layers allows the small field (less than 100 Oe but larger then the coercivity of the soft material) generated by the sense current to both set the directions of the magnetizations in the first and second pinned layers and also reset the directions of the magnetizations in the case that the pinned layers become disoriented. 
     Applicant&#39;s AP-pinned SV sensor is referred to as current resettable current-pinned soft AP-pinned SV sensor because it uses low coercivity ferromagnetic material in building at least the first pinned layer, the directions of the magnetizations can be reset using the field generated by the sense current (because of using a first pinned layer of low coercivity material) and the pinned layers are pinned using the field generated by the sense current (because of using low coercivity material). 
     The above, as well as additional objects, features, and advantages of the present invention will become apparent in the following detailed written description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a fuller understanding of the nature and advantages of the present invention, as well as the preferred mode of use, reference should be made to the following detailed description read in conjunction with the accompanying drawings. 
     FIG. 1 is an air bearing surface view of a prior art SV sensor; 
     FIG. 2 is an air bearing surface view a prior art AP-Pinned SV sensor; 
     FIGS. 2 a - 2   d  are side views of the AP-pinned layer of the SV sensor of FIG. 2; 
     FIG. 3 is a simplified block diagram of a magnetic recording disk drive system incorporating the present invention; 
     FIG. 4 is an air bearing surface view, not to scale, of the Soft AP-Pinned SV sensor of the present invention; and 
     FIG. 5 is an air bearing surface view, not to scale, of an alternate embodiment of the Soft AP-Pinned SV sensor of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The following description is the best embodiment presently contemplated for carrying out the present invention. This description is made for the purpose of illustrating the general principles of the present invention and is not meant to limit the inventive concepts claimed herein. 
     Referring now to FIG. 3, there is shown a disk drive  300  embodying the present invention. As shown in FIG. 3, at least one rotatable magnetic disk  312  is supported on a spindle  314  and rotated by a disk drive motor  318 . The magnetic recording media on each disk is in the form of an annular pattern of concentric data tracks (not shown) on disk  312 . 
     At least one slider  313  is positioned on the disk  312 , each slider  313  supporting one or more magnetic read/write heads  321  where the head  321  incorporates the MR sensor of the present invention. As the disks rotate, slider  313  is moved radially in and out over disk surface  322  so that heads  321  may access different portions of the disk where desired data is recorded. Each slider  313  is attached to an actuator arm  319  by means of a suspension  315 . The suspension  315  provides a slight spring force which biases slider  313  against the disk surface  322 . Each actuator arm  319  is attached to an actuator means  327 . The actuator means as shown in FIG. 3 may be a voice coil motor (VCM). The VCM comprises a coil movable within a fixed magnetic field, the direction and speed of the coil movements being controlled by the motor current signals supplied by controller  329 . 
     During operation of the disk storage system, the rotation of disk  312  generates an air bearing between slider  313  (the surface of slider  313  which includes head  321  and faces the surface of disk  312  is referred to as an air bearing surface (ABS)) and disk surface  322  which exerts an upward force or lift on the slider. The air bearing thus counter-balances the slight spring force of suspension  315  and supports slider  313  off and slightly above the disk surface by a small, substantially constant spacing during normal operation. 
     The various components of the disk storage system are controlled in operation by control signals generated by control unit  329 , such as access control signals and internal clock signals. Typically, control unit  329  comprises logic control circuits, storage means and a microprocessor. The control unit  329  generates control signals to control various system operations such as drive motor control signals on line  323  and head position and seek control signals on line  328 . The control signals on line  328  provide the desired current profiles to optimally move and position slider  313  to the desired data track on disk  312 . Read and write signals are communicated to and from read/write heads  321  by means of recording channel  325 . 
     The above description of a typical magnetic disk storage system, and the accompanying illustration of FIG. 3 are for representation purposes only. It should be apparent that disk storage systems may contain a large number of disks and actuators, and each actuator may support a number of sliders. 
     FIG. 4 shows an air bearing surface (ABS) view of the soft AP-pinned SV sensor  400  according to the preferred embodiment of the present invention. The soft AP-pinned SV sensor  400  comprises a substrate  450  which can be any suitable material, including glass, semiconductor material, or a ceramic material, such as alumina (Al 2 O 3 ). The seed layer  440 , formed over the substrate  450 , is any layer deposited to modify the crystallographic texture or grain size of the subsequent layers, and may not be needed depending on the substrate material. If used, the seed layer may be formed of tantalum (Ta), zirconium (Zr) or Al 2 O 3 . The soft AP-pinned SV sensor  400  further comprises a free ferromagnetic layer  410  separated from an AP-pinned layer  420  by a non-magnetic electrically conducting spacer layer  415 . The free layer  410  may further comprises a first and second free sub-layers  490  and  492 . respectively. The first sub-layer  490  is preferably made of Co and the second free sub-layer  492  is preferably made of Ni—Fe. If free layer  410  is made of single layer, then it may be made of a single layer of Co or a single layer of Ni—Fe. The spacer layer  415  is preferably made of copper (Cu) although it may also be made of gold (Au) or silver (Ag). The AP-pinned layer  420  further comprises a first ferromagnetic pinned layer  422  separated from a second ferromagnetic pinned layer  430  by a non-magnetic antiferromagnetically coupling layer  424 . Layer  424  allows the first pinned layer  422  and the second pinned layer  430  to be strongly coupled together antiferromagnetically. The second pinned layer  430  further comprises first and second ferromagnetic sub-layers  426  and  428 . In the preferred embodiment of the present invention, the first pinned layer  422  is made of magnetically soft material such as Ni—Fe. First pinned sub-layer  426  is preferably made of low coercivity (less than 10 Oe) material such as NiFe and the second pinned sub-layer  428  is made of generally high coercivity (greater than 100 Oe) material such as a thin layer of Co. The first and second pinned layers  422  and  430  have their magnetization directions oriented antiparallel, as indicated by arrows  421  (arrow&#39;s head pointing into the plane of the paper) and  423  (arrow&#39;s head pointing out of the plane of the paper). The antiparallel alignment of the magnetizations of the two pinned layers  422  and  430  is due to an antiferromagnetic exchange coupling through the layer  424 , preferably made of ruthenium (Ru) although it may also be made of other materials such as iridium (Ir) and rhodium (Rh). 
     The SV sensor  400  further comprises antiferromagnetic (AFM) layers  403  and  405  formed on lateral extensions  411  and  413 , respectively, of the free layer  410  for the purpose of longitudinally biasing the free layer  410 . Layers  403  and  405  are preferably made if NiMn although they may also be made of FeMn. In the absence of an applied field, the free layer  410  has its magnetization direction in the direction shown by arrow  412 , i.e., generally perpendicular to the magnetizations directions  421  and  423  of the pinned layers  422  and  430  and parallel to the ABS. Leads  460  and  465 , formed over the AFM layers  403  and  405 , respectively, provide electrical connection between the SV sensor  400  and a sense current  470  and a sensing means  480 . Leads  460  and  465  are preferably made of tantalum. 
     Sensing means  480  comprises a recording channel for reading information sensed by the SV sensor  400 . The recording channel may be either analog or digital and is preferably made of a digital channel such as partial-response maximum likelihood as is known in the art. In the preferred embodiment, a magnetic signal in the medium is sensed by the sensing means  480  detecting the change in resistance, deltaR, as the magnetization of the free layer  410  rotates in response to the applied magnetic signal from the recorded medium. 
     Referring back to FIG. 4, in the preferred embodiment of the present invention, the first pinned layer  422  and the first sub-layer  426  are made of soft (low coercivity) ferromagnetic such that the coercivity of the layers  422  and  426  are less than 10 Oe. The low coercivity of the first pinned layer  422  which is the farthest from the spacer  415 , allows the pinned layer  422  magnetization direction be set by the magnetic field from the sense current I s  flowing in the SV sensor  400 . The sense current supplied by current source  470  flows through the layers of SV sensor  400  and consequently, the sense current flowing in all the layers in the SV sensor  400  (except for the portion that flows in the first pinned layer  422  itself) provides a magnetic field at the first pinned layer  422  in the range of 10-100 Oe. Since this field is larger than the coercivity of the first pinned layer  422 , the field orients and pins the magnetization of the first pinned layer  422  in the direction of the field from the sense current (i.e. essentially perpendicular to the ABS surface). The strong antiferromagnetic coupling through anti-parallel coupling layer  424  pins the directions of the magnetizations of the first and second sub-layers  426  and  428  antiparallel to the first pinned layer  422  magnetization direction. The first pinned sub-layer  426  is preferably made of low coercivity (less than 10 Oe) NiFe while the second sub-layer  428  is made of high coercivity cobalt to ensure a large GMR coefficient. 
     In the preferred embodiment of the present invention, the thickness of the Co layer  428  is in the range of approximately 1-20 Å and the thickness of the first pinned layer  422  and first pinned sub-layer  426  are in the range of 10-100 Å. 
     FIG. 5 shows a soft AP-pinned SV sensor  500  which is an alternative embodiment of the present invention. SV sensor  500  comprises end regions  502  and  506  separated from each other by a central region  504 . In this embodiment, the free layer  510  is formed on the seed layer  540 . Free layer  510  is separated from AP-pinned layer  520  by the spacer  515 . Free layer  510  may comprise first and second free sub-layers  592  and  594 , respectively. The AP-pinned layer  520  comprises first ferromagnetic pinned layers  522  separated from a second ferromagnetic pinned layer  530  by an anti-parallel coupling layer  524 . The second pinned layer  530  further comprises first and second pinned sub-layers  526  and  528 , respectively. The free layer  510 , spacer layer  515  and the AP-pinned layer  520  are all formed in the central region  504 . Hard bias layers  560  and  565  formed in the end regions  502  and  506 , respectively, longitudinally bias the free layer  510 . Leads  570  and  575  formed over said hard bias layers  560  and  565 , respectively, provide electrical connection between the SV sensor  500  and the sense current  590  and the sensing means  580 . 
     In this alternative embodiment, free layer  510  may be made of a single layer of Co or Ni—Fe. Alternatively, free layer  510  may be a laminated structure having a layer of cobalt ( 592 ) and a layer of NiFe ( 594 ); spacer  515  may be CU, Au or Ag; anti-parallel coupling layer  524  may be Ru, Rh or Ir; first pinned layer  522  is made of low coercivity (less than 10 Oe) ferromagnetic material such as NiFe; first pinned sub-layer  526  is made of low coercivity ferromagnetic material; second pinned sub-layer  528  is made of high coercivity cobalt; hard bias layers  560  and  565  are made of CoPtCr; and leads  570  and  575  are made of tantalum. 
     In the absence of an external field (such as a field generated by the information stored on a magnetic storage disk, the free layer  510  magnetization direction  511  is perpendicular to the pinned layers magnetizations directions (parallel to the ABS). In the presence of the magnetic field generated by the sense current  590  flowing in conductive layers of the SV sensor  500 , the pinned layers  522  and  526  magnetizations directions will be in the direction of arrows  521  (into the plane of the paper, i.e., perpendicular to the ABS) and  523  (out of the plane of the paper, i.e., perpendicular to the ABS). 
     In this embodiment, since first pinned layer  522  which is the farthest from the free layer  510  is made of soft (low coercivity, less than 10 Oe) ferromagnetic material such as NiFe, the field generated by the sense current I s  flowing in the SV sensor  500  can be used to set the magnetization direction of the first pinned layer  522  and make the spins to uniformly point to the same desired direction (direction of arrow  521 ). Once the magnetization direction of the first pinned layer  522  is set, the second pinned layer  530  magnetization direction will also be set uniformly, in the opposite direction (direction of the arrow  523 ) as a result of strong antiferromagnetic coupling between the two pinned layers. 
     In the event that during the disk operation, the pinned layer  520  become disoriented (i.e., magnetization direction becomes random) as a result of electrical or mechanical disturbances, once the disturbance has diminished, then the field generated by the sense current I s  will automatically reset the magnetization directions of the pinned layer  520  due to the fact that the first pinned layer  522  is made of low coercivity material. 
     While the present invention has been particularly shown and described with reference to the preferred embodiments, it will be understood by those skilled in the art that various changes in form and detail may be made without departing from the spirit, scope, and teaching of the invention. Accordingly, the disclosed invention is to be considered merely as illustrative and limited in scope only as specified in the appended claims.