Abstract:
A magnetic sensor comprises a sensor stack and magnetic bias elements positioned adjacent each side of the sensor stack. The sensor stack and bias elements have substantially trapezoidal shapes.

Description:
BACKGROUND 
       [0001]    In a magnetic data storage and retrieval system, a magnetic recording head typically includes a reader portion having a magnetoresistive (MR) sensor for retrieving magnetically encoded information stored on a magnetic disc. Magnetic flux from the surface of the disc causes rotation of the magnetization vector of a sensing layer or layers of the MR sensor, which in turn causes a change in electrical resistivity of the MR sensor. The sensing layers are often called “free” layers, since the magnetization vectors of the sensing layers are free to rotate in response to external magnetic flux. The change in resistivity of the MR sensor can be detected by passing a current through the MR sensor and measuring a voltage across the MR sensor. Depending on the geometry of the device, the sense current may be passed in the plane (CIP) of the layers of the device or perpendicular to the plane (CPP) of the layers of the device. External circuitry then converts the voltage information into an appropriate format and manipulates that information as necessary to recover the information encoded on the disc. 
         [0002]    The essential structure in contemporary read heads is a thin film multilayer containing ferromagnetic material that exhibits some type of magnetoresistance. Examples of magnetoresistive phenomena include anisotropic magnetoresistance (AMR), giant magnetoresistance (GMR), and tunneling magnetoresistance (TMR). 
         [0003]    For all types of MR sensors, magnetization rotation occurs in response to magnetic flux from the disc. As the recording density of magnetic discs continues to increase, the width of the tracks on the disc must decrease, which necessitates smaller and smaller MR sensors as well. As MR sensors become smaller in size, particularly for sensors with dimensions less than about 0.1 micrometers (μm), the sensors have the potential to exhibit an undesirable magnetic response to applied fields from the magnetic disc. MR sensors must be designed in such a manner that even small sensors are free from magnetic noise, sufficiently stable, and provide a signal with adequate amplitude for accurate recovery of the data written on the disc. 
       SUMMARY 
       [0004]    A magnetic sensor comprises a sensor stack and magnetic bias elements positioned adjacent each side of the sensor stack. The sensor stack and bias elements have substantially trapezoidal shapes. 
         [0005]    A magnetoresistive read head comprises a first bias element and a second bias element with a magnomagnetoresistive stack positioned between the bias elements. The magnetoresistive stack and bias elements have substantially trapezoidal shapes. 
         [0006]    A magnetoresistive sensor comprises a sensor stack positioned between two magnetic bias elements. The sensor stack and bias elements have shapes that stabilize a “C” state of the sensor stack when under the influence of a bias magnetic field. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a schematic diagram of a prior art reader with rectangular bias magnets and a rectangular reader stack. 
           [0008]      FIG. 2A  is a schematic diagram showing micromagnetic magnetization patterns in a rectangular freelayer of a prior art reader design. 
           [0009]      FIG. 2B  is a schematic diagram showing a “C” type micromagnetic magnetization pattern. 
           [0010]      FIG. 2C  is a schematic diagram showing an “S” type micromagnetic magnetization pattern. 
           [0011]      FIG. 3  is a schematic diagram showing a “C” type micromagnetization pattern in a trapezoidal freelayer and bias magnets of the current reader. 
           [0012]      FIG. 4  is a schematic diagram illustrating the response of a MR sensor to the effect of a bit field source versus the distance of the sensor from the field source. 
           [0013]      FIG. 5  is a micromagnetic simulation of the response of a prior art MR sensor and an inventive MR sensor to a bit field source versus the distance of the sensors from the field source. 
           [0014]      FIG. 6A  is a schematic diagram showing an alternative embodiment of a current reader. 
           [0015]      FIG. 6B  is a schematic diagram showing an alternative embodiment of the current reader. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    A principal concern in the performance of magnetoresistive read sensors is fluctuation of magnetization in the read sensor, which directly impacts the magnetic noise of the read sensor. There are three major components of noise that decrease the SN ratio of a reader: Shot noise, Johnson noise, and thermal magnetic noise. All are related to the RA product and become increasingly disruptive to the SN ratio as the reader area decreases in size. Shot noise results from random fluctuations in electron density in an electric current and is proportional to the current I, the band width Δf, and the resistance R. The noise power, P s , in a resistor due to Shot noise in a resistor is: P s =f(IΔf RA/A). 
         [0017]    Johnson noise results from thermal fluctuations in electron density in a conductor regardless of whether a current is flowing and is proportional to the temperature T, band width Δf, and the resistance R. The noise power P j  in a resistor due to Johnson noise is: P j =f(TΔf RA/A). 
         [0018]    Thermal magnetic noise results from thermally induced magnetic fluctuations in the sensing layers of the reader and is proportional to the temperature T; band width Δf; the reader bias field to the free ferromagnetic layer H bias ; the magnetic moment of the freelayer M sf ; and the volume of the freelayer, V free . The noise power, P mag , in a resistor due to thermal magnetic noise is: P mag =f(TΔf/H 2   bias M sf V free ). 
         [0019]    The RA product of a CPP or TMR sensor is an intrinsic value depending on the material. As the sensor area decreases, the resistance as well as the Shot noise and Johnson noise levels increase. The thermal magnetic noise level varies inversely as the free layer volume of the sensor and also increases accordingly as the sensor area decreases. The resistance increase problem can be overcome with a shunt resistor, but the reader loses signal amplitude. From a reader performance standpoint, it is advantageous to maximize the reader area while maintaining a small reader footprint at the ABS. 
         [0020]    RTN noise is an additional noise component to the reader outpoint signal. RTN noise originates from the existence of two remanent magnetization patterns in the sensor that are energetically close enough and have a low energy barrier such that thermal activation can cause oscillation between the two states. Each magnetization pattern (termed “C” state and “S” state) has a different resistance that adds noise to the sensor output signal. Thus there is an additional challenge to stabilize the “C” state or “S” state in addition to maximizing reader area while maintaining a small reader footprint at the ABS. 
         [0021]    The reader disclosed herein reduces the above mentioned noise levels for a given recording geometry as well as permitting a higher playback amplitude. 
         [0022]      FIG. 1  shows prior art reader  10 , which includes rectangular reader stack  20 , rectangular bias elements  22  and  24 , and nonmagnetic spacers  26  and  28 . Reader stack  20  includes magnetic and nonmagnetic layers, including at least one free layer. Stack  20  has a reader width W R  and a stripe height H S . Each of bias elements  22  and  24  has a width W Bias . Widths W R  and W Bias  are uniform from air bearing surface ABS to the top of reader  10 . Spacers  26  and  28  are nonmagnetic, and may be, for example, metal or ceramic. 
         [0023]      FIG. 2A  is a schematic diagram showing a top view of micromagnetic magnetization patterns in free layer FL of reader stack  20  in prior art sensor  10 . Magnetization in bias elements  22  and  24  is indicated by arrows  30  and  32 , respectively. Arrow  40  depicts primary magnetization in free layer FL of sensor stack  20  resulting from bias magnets  22  and  24 . The micromagnetic magnetization patterns in free layer FL of sensor stack  20  are preferably parallel to the borders close to bias magnets  22  and  24  due to demagnetization effects as shown by arrows  42 ,  44 ,  46  and  48 . The magnetization in free layer FL exists in two states that are energetically close and that change from one to another as a result of thermal activation. A “C” state is shown in  FIG. 2B  comprising magnetization vectors  40 ,  42  and  46 . Another “C” state can be represented by vectors  44 ,  40  and  48 . An alternate state designated an “S state”, is shown in  FIG. 2C  comprising magnetization vectors  42 ,  40  and  48 . Another “S” state can be represented by vectors  44 ,  40  and  46 . Changing magnetization resulting from thermally activated fluctuations between the “C” and “S” states results in RTN noise. 
         [0024]    The inventive reader disclosed herein stabilizes the “C” state at the expense of the “S” state and minimizes RTN noise.  FIG. 3  shows reader  110 , which includes sensor stack  120 , permanent magnet bias elements  122  and  124 , and spacers  126  and  128 . Sensor stack  120  and bias elements  122  and  124  have trapezoidal shapes, that, as shown by micromagnetization vectors  142 ,  140  and  146  in free layer FL of sensor stack  120 , stabilize the “C” state when under the influence of bias magnetization vectors  130  and  132 . The dimensions of trapezoidal sensor stack  120  are reader base width W RB , reader top width W RT , and stripe height H S . The dimensions of this aspect of the invention are base width W RB  of about 20 nm, top width W RT  of about 40 nm, and height H S  of about 30 nm. In another aspect, reader top width W RT  is at least 10 percent wider than reader base width W RB . 
         [0025]    The trapezoidal geometry shown in  FIG. 3  offers an increased reader area and resulting RA product at no expense to the reader footprint at the ABS. In this aspect of the present invention, the increased width W Bias  of trapezoidal bias elements  122  and  124  at the ABS increases the bias field in that vicinity. In another aspect, by extending the height H Bias  of the bias magnets beyond reader stripe height H S , the “C” micromagnetic magnetization pattern is enhanced and RTN noise is minimized. 
         [0026]      FIG. 4  is a plot showing the response of MR sensor  110  due to the field from a very narrow track (called micro-track) on a recording medium as a function of the distance r of sensor  110  from the bit. A normalized peak magnetic field strength detected by the sensor from the narrow track is plotted on the Y axis and the relative separation r of the sensor from the bit is plotted on the X axis. The signal is greatest when the sensor is directly on the bit at X=0. As the separation between MR sensor  110  and the bit increases, the signal strength decreases rapidly, that is, it decays. The curve is plotted to indicate a 1/r 2  relationship between signal strength and separation r. The distance between two positions on the media, at which the signal strength decreases 50% from its maximum, is known as MT50. The distance between two positions on the media, at which the signal decreases to 10% of its maximum, is known in as MT10. The ratio MT10/MT50 is an indication of the ability of sensor  110  to detect magnetic fields from adjacent tracks that distort the sensing signal. 
         [0027]    Since trapezoidal sensor stack  120  is about 10% wider than rectangular sensor stack  20 , it is helpful to know how the cross track signal profile changes between the two sensors. Micromagnetic modeling of cross track signal strength from the same micro-track on the two sensor geometries gave the results shown in  FIG. 5 . The FIG. shows signal strength as a function of distance from the micro-track center on a recording medium for sensor  10  and sensor  110 . The two curves almost superimpose, indicating that increasing the top width (and area) of trapezoidal sensor  120  has not affected sensor cross-track performance. MT10/MT50 of both sensors  10  and  110  are about the same. 
         [0028]      FIGS. 6A and 6B  are schematic illustrations of two alternative aspects of the present reader.  FIG. 6A  shows reader  110   a,  which includes sensor stack  120   a,  permanent bias magnet elements  122   a  and  124   a  and spacers  126   a  and  128   a.  Sensor stack  120   a  and bias elements  122   a  and  124   a  have shapes that, as shown by micromagnetization vectors  142   a,    140   a,  and  146   a  in free layer FL of sensor stack  120   a,  stabilize the “C” state when under the influence of bias magnetization vectors  130   a  and  132   a.  In one embodiment the sensor stack and permanent bias magnets are curved designs. The dimensions of sensor stack  120   a  are reader base width W RBa , reader top width W RTa  and stripe height H Sa . The dimensions of this aspect of the invention are base width W RBa  of about 20 nm, top width W RTa  of about 40 nm, and height H Sa  of about 30 nm. In another aspect, reader top width W RTa  is at least 10 percent wider than reader base width W RBa . 
         [0029]    The geometry shown in  FIG. 6A  offers an increased reader area and resulting RA product at no expense to the reader footprint at the ABS. In this aspect of the present invention, the increased width, W Biasa  of bias elements  122   a  and  124   a  at the ABS increases the bias field in that vicinity. In another aspect, by extending the height H Biasa  of the bias magnets beyond the reader stripe height H Sa , a “C” micromagnetization pattern is enhanced and RTN noise is minimized. 
         [0030]      FIG. 6B  shows reader  110   b,  which includes sensor stack  120   b,  permanent magnet bias elements  122   b  and  124   b  and spacers  126   b  and  128   b.  Sensor stack  120   b  and bias elements  122   b  and  124   b  have shapes that, as shown by micromagnetization vectors  142   b,    140   b  and  146   b  in freelayer FL of sensor stack  120   b,  stabilize the “C” state when under the influence of bias magnetization vectors  130   b  and  132   b.  The dimensions of sensor stack  120   b  are reader base width W RBb , reader top width W RTh  and stripe height H Sb . The dimensions of this aspect of the invention are base width W RBb  of about 20 nm, top width W RTh  of about 40 nm, and height H Sb  of about 30 nm. In another aspect, reader top width W RTh  is at least 10 percent wider than reader base width W RBb . The geometry shown in  FIG. 6B  offers an increased reader area and resulting RA product at no expense to the reader footprint at the ABS. In this aspect of the present invention, the increased width W Biasb  of bias elements  122   b  and  124   b  at the ABS increases the bias field in that vicinity. 
         [0031]    While the invention has been described with reference to an exemplary embodiment(s), it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment(s) disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.