Abstract:
Various embodiments generally relate to a magnetic sensor, and more specifically to a magnetoresistive read head sensor. In one such exemplary embodiment, a magnetic sensor comprises a sensor stack and magnetic bias elements positioned adjacent opposite sides of the sensor stack. At least one of the bias elements has a non-rectangular shape, such as substantially trapezoidal or parallelogram shapes having non-perpendicular corners.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This is a continuation-in-part application of U.S. patent application Ser. No. 12/547,832, filed Aug. 26, 2009, entitled “TRAPEZOIDAL READER FOR ULTRA HIGH DENSITY MAGNETIC RECORDING.” 
    
    
     SUMMARY 
     Various embodiments of a magnetic sensor comprises a sensor stack and magnetic bias elements positioned adjacent each side of the sensor stack. At least one bias element has non-rectangular shapes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a prior art reader with rectangular bias magnets and a rectangular reader stack. 
         FIG. 2A  is a schematic diagram showing micromagnetic magnetization patterns in a rectangular free layer of a prior art reader design. 
         FIG. 2B  is a schematic diagram showing a “C” type micromagnetic magnetization pattern. 
         FIG. 2C  is a schematic diagram showing an “S” type micromagnetic magnetization pattern. 
         FIG. 3  is a schematic diagram showing a “C” type micromagnetization pattern in a trapezoidal free layer and bias magnets of a reader of the invention. 
         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. 
         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. 
         FIG. 6A  is a schematic diagram showing an alternative embodiment of a reader of the invention. 
         FIG. 6B  is a schematic diagram showing an alternative embodiment of a reader of the invention. 
         FIG. 7  is a schematic diagram of an embodiment of the reader of the invention having a non-rectangular parallelogram free layer and non-rectangular parallelogram bias magnets. 
         FIG. 8  is a schematic diagram of an embodiment of the reader of the invention having a non-rectangular parallelogram free layer and trapezoidal bias magnets. 
     
    
    
     DETAILED DESCRIPTION 
     A 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). 
     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). 
     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 free layer M sf ; and the volume of the free layer, 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 ). 
     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. 
     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. 
     The reader disclosed herein reduces the above mentioned noise levels for a given recording geometry as well as permitting a higher playback amplitude. 
       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. 
       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. 
     The reader of the invention makes use of a non-rectangular shaped sensor stack (and free layer) and non-rectangular shaped magnetic bias elements to stabilize either the “C” shape or the “S” shape. In embodiments shown in  FIGS. 3 ,  6 A, and  6 B, the inventive reader disclosed herein stabilizes the “C” state at the expense of the “S” state and minimizes RTN noise. In embodiments shown in  FIGS. 7 and 8 , the inventive reader stabilizes the “S” state at the expense of the “C” shape 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 . 
     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. 
       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. 
     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. 
       FIGS. 6A and 6B  are schematic illustrations of various exemplary 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 . 
     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 embodiment, 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. 
       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 free layer 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 RTb  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 RTb  of about 40 nm, and height H Sb  of about 30 nm. In another aspect, reader top width W RTb  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 embodiment, the increased width W Biasb  of bias elements  122   b  and  124   b  at the ABS increases the bias field in that vicinity. 
       FIGS. 7 and 8  are schematic illustrations of two alternative aspects of the inventive reader that make use of a non-rectangular parallelogram sensor stack (and free layer) to stabilize the “S” shape at the expense of the “C” shape.  FIG. 7  shows reader  110   c , which includes sensor stack  120   c , permanent bias magnet elements  122   c  and  124   c  and spacers  126   c  and  128   c . Sensor stack  120   c  and bias elements  122   c  and  124   c  have parallelogram shapes that, as shown by micromagnetization vectors  144   c ,  140   c , and  146   c  in free layer FL of sensor stack  120   c , stabilize the “S” state when under the influence of bias magnetization vectors  130   c  and  132   c . In this embodiment the adjacent sides of bias element  122   c  and sensor stack  120   c  are parallel to one another and separated by spacer  126   c . Similarly, the adjacent sides of sensor stack  120   c  and bias element  124   c  are parallel to one another and separated by spacer  128   c.    
     As illustrated in  FIG. 7 , the width of sensor stack  120   c  is less than the widths of bias elements  122   c  and  124   c . In other embodiments, the relative widths may differ. 
     In  FIG. 7  the height of bias elements  122   c  and  122   d  is greater than the reader stripe height of sensor stack  120   c . This helps to enhance a “S” micromagnetization pattern and reduce RTN noise. 
       FIG. 8  shows reader  110   d , which includes sensor stack  120   d , permanent magnet bias elements  122   d  and  124   d  and spacers  126   d  and  128   d . Sensor stack  120   d  has a non-rectangular parallelogram shape, while bias elements  122   d  and  124   d  have trapezoidal shapes. As shown by micromagnetization vectors  144   d ,  140   d  and  146   d  in free layer FL of sensor stack  120   d , these shapes stabilize the “S” state when under the influence of bias magnetization vectors  130   d  and  132   d . Bias elements  122   d  and  124   d  have the same shape, but bias element  124   d  is inverted with respect to bias element  122   d . In other words, bias elements are arranged in a reciprocal relationship. The base of bias element  122   d  of the ABS is smaller than the base of bias element  124   d . The right side of bias element  122   d  is parallel to and spaced from the left side of sensor stack  120   d  by spacer  126   d . Similarly, the left side of bias element  124   d  is parallel to and spaced from sensor stack  120   d  by spacer  128   d.    
     Both the embodiments with a substantially trapezoidal sensor stack ( FIGS. 3 ,  6 A, and  6 B) and the embodiments with a non-rectangular parallelogram sensor stack ( FIGS. 7 and 8 ) help to reduce magnetic noise by stabilizing, either the “C” state or the “S” state. The trapezoidal sensor stack embodiments also help to reduce electronic noise, reduce resistance and resistance distribution. The parallelogram sensor stack embodiments offer advantages of easier fabrication, and will not increase reader width distribution. 
     While the disclosure 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 discussed technology. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the embodiments without departing from the essential scope thereof. Therefore, it is intended that the present disclosure 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.