Abstract:
A differential sensor for reading data from a magnetic medium is disclosed. The sensor comprises two GMR multilayer structures biased in opposite directions, such as to show the transitions between binary states recorded on the media as the media flows under the sensor. The biasing of the GMR structures can be accomplished using a synthetic-antiferromagnet.

Description:
RELATED APPLICATIONS  
     Referenced-applications  
       [0001]    This application claims the benefit of U.S. Provisional application Ser. No. 60/315,413, filed Aug. 28, 2001. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The invention relates to the field of the magnetic recording of data, and, in particular, to the recording of data on a magnetic disc. Specifically, this invention discloses a novel sensor for the reading of data from a magnetic disc.  
         BACKGROUND OF INVENTION  
         [0003]    Devices utilizing the giant magneto-resistance (GMR) effect have utility as magnetic sensors, especially as read sensors in read heads used in magnetic disc storage systems. The GMR effect is observed in thin, electrically conductive multi-layer systems having multiple magnetic layers. One sensor type that utilizes the GMR effect is the GMR multilayer. The GMR multilayer typically comprise a series of bi-layer devices, each of which comprise a thin sheet of a ferromagnetic material and a thin sheet of a non-magnetic material. The bi-layers are stacked to form a multi-layer device. The magnetization of each ferromagnetic layer in the multi-layer device is approximately orthogonal to the magnetization of adjacent ferromagnetic layers and would be oriented in a plane perpendicular to the plane of the disc. The multi-layer device is typically mounted in the read head so that the magnetic axis of the ferromagnetic layers are transverse to the direction of rotation of the disc.  
           [0004]    In operation, a sense current is caused to flow through the read head and therefore through the sensor. The magnetic flux from the disc causes a rotation of the magnetization vector in at least one of the sheets, which in turn causes a change in the overall resistance of the sensor. As the resistance of the sensor changes, the voltage across the sensor changes, thereby producing an output voltage.  
           [0005]    The output voltage produced by the sensor is affected by various characteristics of the sensor. The sense current can flow through the sensor in a direction that is parallel to the planes of the layers or stacked strips. This is known as a current-in-plane (CIP) configuration. This configuration is shown in FIG. 1, wherein the sense current is represented by arrow  8  and is shown flowing parallel to layers  9  of the sensor. Typically, the types of sensors used today for the reading of magnetically recorded data can be categorized as current-in-plane sensors.  
           [0006]    Alternatively, the sense current can flow through the sensor in a direction that is perpendicular to the planes of the layers or stacked strips that comprise the sensor. This configuration is known as a current-perpendicular-to-plane (CPP) configuration. A CPP sensor is shown schematically in FIG. 2, wherein the sense current is represented by arrow  8  and is shown flowing perpendicular to layers  9  of the sensor.  
           [0007]    The CPP sensor is interesting because of its potentially larger giant magneto-resistance (GMR) or change in resistance when a magnetic field is applied. The larger change in resistance comes about because all of the current needs to pass through every ferromagnetic/nonmagnetic/ferromagnetic (FM/NM/FM) series of interfaces and none of the current is shunted around the interfaces. Because every film and interface leads to additional resistance, it is desired to have all of the films and interfaces contribute to the overall ΔR. One such sensor is a GMR multilayer, which consists of a series of FM/NM bi-layers. Every series of interfaces is an opportunity for interfacial spin-dependent scattering and every FM material is an opportunity for bulk spin-dependent scattering.  
           [0008]    An example of a transfer curve from a CPP-GMR multilayer made of 15 bi-layers of (Cu 18Å/CoFe 10Δ) is shown in FIG. 3. In the quiescent state, the magnetization of adjacent layers in this sample are oriented 180° with respect to each other, due to RKKY (Ruderman-Kittel-Kasuya-Yosida) coupling. The Cu thickness was chosen such that the RKKY coupling between the CoFe layers would be antiferromagnetic.  
           [0009]    It can be seen from FIG. 3 that if this type of sensor is used in a magnetic recording head, it will need to be biased such that it operates in a linear region, denoted by A and B on the graph. This will be necessary to use detection and tracking schemes that depend on signal linearity. One way of biasing a GMR multilayer sensor is to place a permanent magnet (PM) nearby, such that the magnetizations of adjacent FM layers are approximately orthogonal to each other. This would be similar to applying a DC magnetic field of ˜500 Oe to the sensor shown in FIG. 3. The sensor could then be used to sense the field from the magnetic recording media.  
           [0010]    One possible CPP read head design uses a GMR multilayer that is biased into the linear operating region using a permanent magnet and which uses the shields as the current carrying leads. FIG. 4 shows a schematic representation of such a design.  
           [0011]    The transfer curve response that the head of FIG. 4 would have to perpendicular media may resemble a square wave similar to the diagram shown in FIG. 5. This type of response is difficult for a read back channel to handle due to the fact that it&#39;s impulse response contains DC components.  
           [0012]    One suggested solution to this problem is to differentiate the signal, which may result in a signal resembling that shown in FIG. 6. This would make the signal much more compatible with the read back channels used today. A problem with this solution is that the process of differentiating the signal may add high frequency noise to the read back signal.  
           [0013]    It would therefore be desirable to provide a sensor which outputs a signal compatible with contemporary read back channels without the high frequency noise.  
         SUMMARY OF INVENTION  
         [0014]    The solution disclosed herein is to make a head that effectively differentiates the flux from the media. The output from such a head may also resemble the signal shown in FIG. 6.  
           [0015]    The invention described here is a CPP-GMR design that would act as a differential read back sensor. A differential sensor could be made by biasing part of the sensor in region A shown in FIG. 3 and part of the sensor in region B shown in FIG. 3. This can be accomplished by providing a pair of GMR multilayers separated by a non-magnetic interlayer. The magnetizations of the GMR multilayers would be biased such that they point in opposite directions, for example, one pointing toward the media and one pointing away from the media. As such, when exposed to a magnetic field, the resistance of the GMR multilayers will vary inversely. 
       
    
    
     DESCRIPTION OF DRAWINGS  
       [0016]    [0016]FIG. 1 shows a prior art CIP type sensor.  
         [0017]    [0017]FIG. 2 shows a prior art CPP type sensor.  
         [0018]    [0018]FIG. 3 shows the transfer curve from a CPP-GMR multi-layer.  
         [0019]    [0019]FIG. 4 is a schematic of a prior art CPP read head using a GMR multilayer as the sensing element and a permanent magnet for biasing.  
         [0020]    [0020]FIG. 5 shows the readback signal for the prior art CPP-GMR multilayer readback sensor of FIG. 4.  
         [0021]    [0021]FIG. 6 shows the readback signal of the differential CPP sensor of the present invention.  
         [0022]    [0022]FIG. 7( a ) shows one possible configuration of a differential CPP sensor using a SAF for biasing. FIG. 7( b ) shows the sensor of FIG. 7( a ) configured as a sensor for reading a magnetic disc.  
         [0023]    FIGS.  8 ( a - c ) show various configurations of the SAF which can be used in place of the permanent magnet of FIG. 7( a ).  
         [0024]    [0024]FIG. 9 shows a modeled response to a positive and negative pulse for a differential CPP sensor according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0025]    The sensor of the present invention consists primarily of a dual GMR multilayer wherein the two GMR multilayer structures, examples of which are well known in the art, are separated by a thin non-magnetic interlayer spacer. Thus, the GMR multilayer structures are magnetically de-coupled from each other.  
         [0026]    [0026]FIG. 7( a ) shows a schematic representation of one method of creating the desired bias configuration. Sensor  100  consists of GMR multilayer structures  102  and  104 , separated by non-magnetic de-coupling interlayer  103 . Non-magnetic interlayer  103  would commonly be composed of one of Ta, W, Ru, Al, Au or Cu, but may be any other non-magnetic material.  
         [0027]    In one embodiment, sensor  100  is biased using a pair of permanent magnets  112  and  14  separated by a thin layer of non-magnetic material  113 . In the schematic drawing of FIG. 7( a ), it can be seen that permanent magnet  12  has its magnetization pointing downward, while permanent magnet  114  has its magnetization pointing upward, such as to bias the dual multilayer structures in opposite directions. As such, when exposed to a magnetic field, the resistance of one multilayer will increase, while the resistance of the other multilayer will decrease.  
         [0028]    The new configuration of sensor and biasing structure  110  would replace sensor  10  (GMR) and biasing magnet  12  (PM) in the prior art sensor of FIG. 4, as shown in FIG. 7( b ).  
         [0029]    In another embodiment of the invention, the required biasing condition could be achieved by using a synthetic-antiferromagnet (SAF)  120  in place of the biasing magnet. The advantage of using a SAF over a permanent magnet for biasing is that there is more freedom in selecting the M R  of the individual bias magnet layers. While high coercivity permanent magnets with an M R &gt;1000 emu/cc are difficult to find, it is easy to make a strongly coupled SAF where the individual layers have an M R &gt;1400 emu/cc (pure Co). The individual layers could have the same or different materials and same or different thickness.  
         [0030]    Several possible configurations for SAF  120  are shown in FIGS.  8 ( a - c ). SAF  120  consists essentially of two sections of a ferromagnetic material,  122  and  124 , separated by a thin layer of a non-magnetic material  123 , such as ruthinium. It has been found through experimentation that the ideal width of layer  113  is between approximately 2 Å and 12 Å. The ferromagnetic layers could consist of Co, CoFe, CoNiFe, NiFe or CoFeB. The orientation of ferromagnetic material  122  and  124  could be set or stabilized by using an antiferromagnetic material or permanent magnet  126  on one or both halves of the SAF. The antiferromagnetic material could consist of PtMn, PtPdMn, IrMn or CrPtMn and the permanent magnet could consist of CoX where X can be any combination of one or more of the following materials: Pt, Ta, Cr or B.  
         [0031]    To show the operation of the novel differential sensor disclosed herein, a biased, shielded differential sensor was micromagnetically modeled and the signal output from an isolated transition is shown in FIG. 9. As a differential sensor, only transitions between states are detected. The response to the isolated transition is shown in FIG. 9 and is, as expected, a pulse.  
         [0032]    It has also been observed during modeling that the differential CPP sensor disclosed herein is not sensitive to the “neighborhood” effect. The “neighborhood” effect is when the head responds to a written track even when it is not directly under the sensor, but still under the shields. The “neighborhood” effect applies to perpendicular recording using media with a soft underlayer and a standard read head. It is thought to come about from flux traveling from the written track, through the shields, down through the sensor, through the soft underlayer and then returning to the written track. One possible explanation is that, because the sensor of the present invention is a differential sensor and immune to uniform fields, it is also immune to the “neighborhood effect”.  
         [0033]    While the present invention has been shown through the examples used in the specification, the invention is not meant to be limited thereby. It is possible to modify the basic designs shown herein without deviating from the contemplated invention, the scope of which is embodied in the following claims.