Abstract:
A method of controlling bias supply sources for magneto-resistive transducer (MR) heads to provide essentially the same predetermined lifetime for the MR heads, by determining dependence of head lifetime on bias supply level and on head stripe temperature, and setting a bias supply level for each head based on said dependence of lifetime on bias supply level and on head temperature, to provide essentially the same predetermined lifetime for the heads.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to extending transducer head lifetime, an in particular to setting GMR head bias current in disk drives to ensure adequate GMR head lifetime. 
     BACKGROUND OF THE INVENTION 
     Many disk drives utilize giant magneto resistive (GMR) transducer heads for recording data to, and reading data from, magnetic media on data disks. The GMR head performance and lifetime is a function of the temperature of the head. The readback signal amplitude degradation in GMR heads is a function of age or stress in the heads. As recording density increases steadily, readback signal amplitude degradation in GMR heads becomes a very serious drawback. 
     Further, to meet signal to noise ratio requirements for higher density recording and readback, it is desirable to provide either higher GMR sensitivity (to offset the GMR signal degradation due to reduction of track width shrinkage in higher density recording), or a shorter stripe in the heads to maintain head sensor aspect ratio. Because developing higher sensitivity GMR requires tremendous effort and time to develop, for practical reasons, conventionally the GMR sensor stripe height is shortened to keep up with recording density increases. 
     The bias current density in the stripe film increases accordingly with the stripe height reduction. Historically as recording density increases steadily, the current density in the sensor has increased from low 10 6  A/cm 2  in early GMR programs to about 4×10 8  A/cm 2 , (in the Cu spacer layer) in the recent GMR designs. However, when the current density in the sensor stripe increases, anti-ferromagnet (AFM) de-pinning and interlayer diffusion become more severe due to the accompanied stripe temperature increase. In addition to temperature increase, electromigration and AFM de-magnetization caused by increased current density contribute significantly to amplitude degradation. For example, at stripe temperature of 168 C., a 14% amplitude loss is observed in GMR heads stressed by a 4.5 mA bias current, and a 41% amplitude loss is observed in GMR heads stressed by a 5.0 mA bias current. 
     Conventionally, the head bias current is adjusted based on temperature for optimizing head performance and GMR lifetime in disk drives. The value of the bias current used in disk drives is determined by the GMR sensor stripe temperature, without a systemic treatment of the effects of current density on GMR lifetime. 
     There is, therefore, a need for a system and method for providing correct bias current setting scheme for transducer heads in disk drives based on the resistance of each individual head measured by establishing bias current dependence of GMR resistance and GMR lifetime. 
     BRIEF SUMMARY OF THE INVENTION 
     In one embodiment, the present invention provides methods to establish dependence of head lifetime on bias current (i.e., time to failure-TTF) and head resistance specification, to meet head lifetime requirements, by taking both head (i.e., stripe) temperature and bias current into consideration. 
     The head bias current is adjusted based on individual head resistance and temperature for optimizing head performance and GMR lifetime. Generally, lower bias current is applied to GMR sensors with high resistance to avoid high sensor temperature and to prolong GMR lifetime. 
     A method is provided for determining correct bias current setting (level) in a disk drive based on the resistance of each individual head, by establishing: (1) dependence of GMR resistance on bias current, and (2) dependence of lifetime on bias current. The bias current setting processes can be programmed into each disk drive and can be different in the details for each GMR head or disk drive supplier. 
     Further, a method is provided for GMR lifetime test at the drive level (i.e., after drive assembly) based on the established bias current dependent lifetime. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features, aspects and advantages of the present invention will become understood with reference to the following description, appended claims and accompanying figures where: 
     FIG. 1 shows an embodiment of a disk drive in which the present invention can be implemented; 
     FIG. 2 shows an example flowchart of an embodiment of the steps of a method of selecting bias currents for predetermined lifetime, according to the present invention; 
     FIG. 3, shows example plots of a head signal amplitude vs. time for different bias current levels, at a fixed head temperature; 
     FIG. 4 shows example plots of head lifetime vs. bias current at different head temperatures; 
     FIG. 5 shows example plot of head temperature vs. bias current for a selected head lifetime; 
     FIG. 6 shows example plots of head temperature vs. stripe resistance at different bias current levels; 
     FIG. 7 shows an example flowchart of another embodiment of the steps of a method of selecting bias currents for predetermined lifetime, according to the present invention; 
     FIG. 8 shows an example plot of head lifetime vs. inverse head temperature; 
     FIG. 9 shows example plot of head temperature vs. bias current for a selected head lifetime; 
     FIG. 10 shows example plots of head temperature vs. stripe resistance at different bias current levels; and 
     FIG. 11 shows an example flowchart of steps of drive level lifetime testing according to another aspect of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, an example embodiment of a disk drive including GMR heads according to the present invention, is shown. The disk drive  10  comprises storage media such a data disks  12 , and a disk drive controller  14  for interfacing with a host and controlling disk drive operations including data transfer to from disks  12 , therein. The disk drive  10  further includes a head structure  16  including one or more MR heads  18  moved by a support arm of an actuator assembly  20  via a VCM  22  across tracks of one or more disks  12  for data storage and data retrieval, and tracking to maintain the head over a target position. Each disk  12  includes a servo pattern including servo bursts and sectors for system and user data, respectively, on a recording surface thereof. The disk drive  10  further includes a preamplifier  24  for amplifying the read and write signals from and to the disks  12 , respectively, and a read/write channel  26  for encoding and decoding data between user information and data written on disks  12 . The channel  26  also decodes servo track number and converts servo burst amplitudes into digital values. The disk drive  10  further includes a power driver IC  28  for driving the actuator  20  and a spindle motor  30  for rotating the disks  12 . In one example embodiment, the controller  14  includes a memory  32  microcontroller (e.g., microprocessor)  34  for controlling head bias current, and a drive control  36  for general control of the components of the disk drive  10  and interface to a measurement component  38  and a data analysis component  40 . the host system  12 . The memory  45  can include RAM and/or non-volatile (NV) memory such as EEPROM, ROM, etc. The disk drive  10  can further include memory  42  for storing other program instructions or data. 
     The present invention provides a method to determine GMR head resistance specification to ensure head lifetime (e.g., 5 year lifetime) in disk drives. Because it is not economical to test a head for the length of the desired lifetime (e.g., 5 years), to determine if the head operates satisfactorily for that long, the present invention provides accelerated testing by using stress conditions such as stress temperatures (e.g., in an oven), and bias currents (increasing bias current increases current density and also stress temperature). A method to accelerate lifetime testing of a head having a resistance R is provided along with techniques for using the test result data to accurately estimate the head lifetime for different bias currents and ambient temperatures. As such, in one aspect the present invention allows accurate estimation of lifetime of heads, with short testing time. Accuracy of lifetime estimation is a function of the amount of test data and length of testing and there is a tradeoff between accuracy and testing time. Many heads can be tested and sufficient data collected in a shorter time to estimate actual head lifetime. 
     Conventionally head lifetime is estimated as a function of head temperature. According to the present invention, head lifetime is determined as a function of temperature of stripe  19  in the head  18  and current density (i.e., current density effects head lifetime) in the GMR head stripe  19  for more accuracy. 
     To estimate head lifetime, dependence of head lifetime on temperature and bias current (current density), and dependence of head resistance on bias current, are established. In heads  18  with different resistance, the head lifetime dependence on bias current and temperature, are not the same. Higher resistance heads heat up faster and higher because the current density in such heads is higher. Application of the same bias current to a high resistance head and to a low resistance head, results in the high resistance head heating faster then the low resistance head. 
     A high resistance head has a smaller sensor  19  size than a low resistance head. Application of the same bias current to both heads causes higher current density in the high resistance head  18  than in the low resistance head  18 , and therefore to higher Joule heating in the high resistance head relative to the low resistance head (i.e., with higher current density, there is higher Joule heating). 
     Difference in heating for the same resistance heads due to ambient temperature difference at the same bias current, causes a difference in lifetime among such heads. Further, difference in heating for different resistance heads due to Joule heating (caused by different current density) causes a difference in lifetime among such heads. And, the increase in stripe temperature due to Joule heating of bias current also increases the stripe resistance. 
     It is an objective of the present invention to provide essentially constant lifetime for all heads  18  in different disk drives  10  (different design heads have different resistances due to different resistance distribution). Accordingly, in one aspect, the present invention provides bias current settings (levels) for essentially the same lifetime for different heads  18 , based on bias current and temperature. 
     For example, several head vendors such as vendorA, vendorB and vendorC each provide GMR heads with same or different resistance. As such, for different vendors there may be different bias current settings. It is desirable to select a bias current for each head such that the heads  18  have essentially the same lifetime in disk drives (e.g., 5 years). In one example, for VendorA&#39;s heads with resistance of 40 Ohm, the bias current may be set at 5 mA; for VendorB&#39;s heads with resistance of 40 Ohm, the bias current may be set at 4.5 mA; and for VendorC&#39;s heads with resistance of 42 Ohm, the bias current may be set at 4.2 mA, whereby each head provides a 5 year lifetime. There may be different bias currents for different resistance heads for each vendor. For VendorA&#39;s heads there may be one set of bias currents, and for VendorB there maybe a different set of bias currents, to provide the same lifetime. 
     In one example, the heads that are placed in different disk drives have different resistances. Each head has a identification code that is stored in the disk drive. Then the head resistance is measured, and based on the head vendor and the head measured resistance, a lookup table (Lifetime Table) is crated based on said head test results. Using the lookup table, a head having a measured resistance of e.g. 40 Ohms from vendorA is set to a bias current of 3.5 mA, and vendorD at 40 Ohms is set to 3.8 mA, and vendorF at 45 Ohms is set at 3.0 mA. The bias current is provided such that the different heads have the same lifetime in different predicted environments. 
     As such, according to the present invention, the bias current level is selected to provide constant lifetime for the heads  18 , by taking both stripe  19  temperature and bias current density into consideration. 
     Referring to the example steps in FIG. 2, and graphs in FIGS. 3-6, according to a version of the present invention, readback signal amplitude degradation in GMR heads due to stress conditions (e.g., temperature) are measured. This is because over time amplitude of the readback signal degrades. 
     Keeping stripe temperature Tstripe the same, increasing bias current, decreases readback signal sensitivity. Example tests and measurements in Tables 1 and 2 below, show amplitude degradation of GMR heads (e.g., 7 heads of slider resistance 42 Ohms in Table 1, and 6 heads of slider resistance 42 Ohms in Table 2) stressed with different bias currents to similar Tstripe of about 168 C. for different stress durations (2 hours and 33 hours). According to Table 1, on average 14% amplitude loss (i.e., LFTAA(33)/LFTAA(0)=86%) was found in heads stressed by 4.5 mA for 33 hours, and according to Table 2, on average 41% amplitude loss (i.e., LFTAA(33)/LFTAA(0)=59%) was found in heads stressed by 5.0 mA for 33 hours. 
     
       
         
               
             
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 4.5 mA stress @ 135° C. oven temperature—168° C. Tstripe 
               
               
                 Phantom Preliminary ATR test 4.5 mA @ Temperature 135 C. (slider resistance 42 ohms) 
               
             
          
           
               
                   
                   
                 LFTAA(2) 
                 LFTAA(2)/ 
                 LFTAA(33) 
                 LFTAA(33)/ 
                   
                   
                   
               
               
                   
                 LFTAA(0) 
                 After 2 Hours 
                 LFTAA(0) 
                 After 33 Hours 
                 LFTAA(0) 
                 Slope 
                 Intercept 
                   
               
               
                 No. 
                 Initial 
                 135 
                 135 
                 135 
                 135 
                 135 
                 135 
                 Tstripe 
               
               
                   
               
               
                 1 
                 0.944 
                 0.867 
                 92% 
                 0.876 
                 93% 
                  0.0036 
                 0.9157 
                 166.8 
               
               
                 2 
                 0.981 
                 0.958 
                 98% 
                 0.810 
                 83% 
                 −0.0539 
                 1.0144 
                 169.4 
               
               
                 3 
                 0.925 
                 0.841 
                 91% 
                 0.780 
                 84% 
                 −0.0237 
                 0.9254 
                 165.9 
               
               
                 4 
                 0.924 
                 0.930 
                 101%  
                 0.819 
                 89% 
                 −0.0429 
                 1.0362 
                 168.1 
               
               
                 5 
                 1.203 
                 1.128 
                 94% 
                 0.991 
                 82% 
                 −0.0406 
                 0.9657 
                 164.8 
               
               
                 6 
                 1.093 
                 1.144 
                 105%  
                 1.009 
                 92% 
                 −0.0442 
                 1.0772 
                 172.1 
               
               
                 7 
                 1.365 
                 1.123 
                 82% 
                 1.059 
                 78% 
                 −0.0167 
                 0.8344 
                 167.4 
               
               
                 Average 
                   
                   
                 95% 
                   
                 86% 
                   
                   
                 167.8 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 5.0 mA stress @ 125° C. oven temperature—168° C. Tstripe 
               
               
                 Phantom Preliminary ATR test 5.0 mA @ Temperature 125 C. (slider resistance 42 ohms) 
               
             
          
           
               
                   
                   
                 LFTAA(2) 
                 LFTAA(2)/ 
                 LFTAA(33) 
                 LFTAA(33)/ 
                   
                   
                   
               
               
                   
                 LFTAA(0) 
                 After 2 Hours 
                 LFTAA(0) 
                 After 33 Hours 
                 LFTAA(0) 
                 Slope 
                 Intercept 
                   
               
               
                 No. 
                 Initial 
                 145 
                 145 
                 145 
                 145 
                 145 
                 145 
                 Tstripe 
               
               
                   
               
               
                 1 
                 1.280 
                 0.984 
                 77% 
                 0.622 
                 49% 
                 −0.1009 
                 0.8387 
                 168.3 
               
               
                 2 
                 1.133 
                 0.936 
                 83% 
                 0.794 
                 70% 
                 −0.0448 
                 0.8570 
                 162.5 
               
               
                 3 
                 0.976 
                 0.882 
                 90% 
                 0.373 
                 38% 
                 −0.1859 
                 1.0320 
                 171.8 
               
               
                 4 
                 0.961 
                 0.874 
                 91% 
                 0.681 
                 71% 
                 −0.0716 
                 0.9593 
                 163.8 
               
               
                 5 
                 1.133 
                 0.898 
                 79% 
                 0.769 
                 68% 
                 −0.0406 
                 0.8207 
                 171.6 
               
               
                 6 
                 1.093 
                 1.018 
                 93% 
                 0.543 
                 50% 
                 −0.1553 
                 1.0395 
                 171.6 
               
               
                 Average 
                   
                   
                 86% 
                   
                 59% 
                   
                   
                 168.3 
               
               
                   
               
             
          
         
       
     
     Referring to FIG. 2, before assembly in disk drives  10 , during an example component level testing, GMR heads  18  are placed in ovens  37  at higher than normal oven temperatures to stress the heads  18  and measure amplitude degradation in the heads. Head stripe  19  temperatures are kept constant by adjusting the oven temperate, and different stress bias currents e.g. i 1 , i 2  and i 3  are applied to the heads for a time period. Then the bias current is discontinued and the heads are taken out of the oven, and head readback signal amplitude for each bias current is measured (step  100 ). The time-to-failure (TTF) value is selected to be the time when the readback signal amplitude of a head degrades by e.g. 15%. FIG. 3 shows example curves  50  of measured readback signal amplitude vs. TTF for heads of the same resistance maintained at a constant stress temperature (e.g., T 3 ), but at different stress bias currents i 1 , i 2  and i 3 . Each curve  60  in FIG. 3 is obtained with a different stress bias current at temperature T 3 . A log scale is used for TTF such that the data point align in a straight line for easier data analysis. In FIG. 3, i 1 &lt;i 2 &lt;i 3  (e.g., i 1  is lowest current). For the i 3  curve  50 , the head readback signal amplitude degrades by 15% in less than an hour (TTF 3 ) at very elevated stripe temperature T 3  (due to oven heating, and Joule heating caused by the bias current i 3 ). 
     In the example of FIG. 3, keeping stripe temperature constant (by adjusting oven temperature), each curve  50  shows effect of current density (bias current) on lifetime TTF. The stripe temperature is measured, and stripe temperature variation due to Joule heating caused by bias current is compensated by varying the oven temperate to maintain the stripe temperature constant. A goal is to determine upper limit of head resistance to select bias current setting to ensure that head  18  survives in a disk drive  10  for e.g. TTF=5 years. The above steps are repeated for different values of stripe temperature T (Tstripe). 
     Example FIG. 4 shows measured data of FIG. 3, wherein each curve  52  in FIG. 4 is a plot of data points in bias current vs. TTF log scale fashion, for different stress temperatures T 1 , T 2  and T 3  (step  102 ). The set of curves  50  in FIG. 3 for bias currents i 1 , i 2 , i 3 , etc., represents one curve  52  for T 3  in FIG. 4, as shown. The measurement of stripe temperature T 3 , and the data points correspond to the pairs shown in FIG. 4 (TTF 1  at i 1 , TTF 2  at i 2 , TTF 2  at i 3 , all for T 3 ). Each curve  52  in FIG. 4 is for the same head resistance, and as such the stripe temperature is different in each curve  52  due to different bias current (i.e., T 1 , T 2 , T 3 , each is stripe temperature as a combination of oven temp (Toven) and Joule heating (Tjoul) due to bias current, wherein the oven temperature is kept constant). A doffed line is drawn at 5 year TTF to intersect the extrapolated curves  52  at T 1  T 2  and T 3 , at bias current values I 1 , I 2  and I 3 , respectively. As such, as shown by a curve  54  in FIG. 5, each pair of bias current and corresponding temperature (I 1 , Ti), (I 2 , T 2 ) and (I 3 , T 3 ) represents the bias current value that provides a head lifetime of 5 years at the corresponding temperature for the heads of the same resistance (step  104 ). 
     There is a relationship between stripe resistance and stripe temperature (i.e., a TCR coefficient providing a linear relationship between the stripe resistance and stripe temperature, such that: stripe resistance=A×stripe temperature+B, wherein A and B are constants). As such, the stripe temperature can be obtained by measuring the stripe resistance, and then extrapolated to find the corresponding stripe temperature using TCR. 
     Similarly, the stripe resistance can be obtained by measuring the stripe temperature and converting the stripe temperature to the stripe resistance using TCR (step  106 ). Example FIG. 6 shows a curve  56  representing the above data points as stripe (slider) resistance R vs. bias current. The resistance values are plotted against bias current, and curve fitting is performed, showing the effect of bias current on stripe resistance (e.g., due to Joule heating). The curve fitting equation (i.e., curve  56 ) provides the head resistance specification (step  108 ). 
     For example, for the same bias current, stripe temperature T 1  is translated to stripe resistance R 1  using said relationship. (i.e., curve  56 ). For a desired constant lifetime (e.g., 5 years), following the curve  56  provides the constant lifetime. For a stripe resistance R 1 , selecting a bias current of I 1 , achieves a 5 year head lifetime. For a stripe resistance R 2 , selecting a bias current of I 2 , achieves a 5 year head lifetime. Using a measured stripe resistance of R 1  (i.e., head resistance based on oven temperature and Joule heating), then the bias current is set to I 1  to achieve the desired head lifetime (e.g., 5 years). If the stripe resistance is R 2 , then the bias current is set to I 2  to achieve the desired head lifetime (e.g., 5 years). As can be seen, for the case of higher resistance R 1  (i.e., R 1 &gt;R 2 ), a lower bias current I 1  (i.e., I 1 &lt;I 2 ) is selected to obtain the same lifetime as for the lower resistance R 2 . 
     In another example, a group of heads  18  with the same resistance Ra, are tested. The effective stripe temperature T is combination of oven temperature (To) and Joule heating (Tj) due to bias current (i.e., T=To+Tj). The increase in stripe temperature due to Joule heating of bias current increases the stripe resistance to the hot resistance HRa. In another group of heads  18  with resistance Rb, the hot resistance due to Joule heating is HRb. If Rb&gt;Ra, the increase in resistance in the second group of heads due to Joule heating is more than the resistance in the first group of heads. 
     For calibration, an environment/ambient temperature for testing is specified. Because the stripe temperature is combination of oven heating and Joule heating, an environment temperature that the head will most likely experience in the disk drive in actual use, is selected to obtain a calibration curve for TCR (coefficient of resistance) for translating measures stripe temperature to resistance, described above. For different groups of heads, there are different TCRs. 
     As such, a relationship between head lifetime and head resistance is provided by this technique. Generally, the higher the head resistance, the shorter the lifetime because of higher stripe temp due to Joule heating. Therefore, it is preferable to not use high relative resistance in the heads, wherein the boundary for the head resistance for guarantee of desired lifetime is determined. When the heads  18  are installed in the disk drives  20 , at the drive level, the bias current is adjusted according to the predicted ambient drive temperature, and the resistance of each head  18 . The combination of ambient temperature and current density determines head lifetime. 
     In the above examples, the lifetime values obtained for accelerated oven testing are extrapolated to obtain actual lifetime values for the heads. For, example, in FIG. 4, a portion of the curve  52  is extrapolated, to the desired lifetime (e.g., 5 year TTF). The measurement of stripe temperature, T 3 , and the data points correspond to the pairs shown in FIG. 3 (TTF 1  at i 1 , TTF 2  at i 2 , TTF 2  at i 3 , all for temperature T 3  for example). A head is tested in the oven for e.g. two days to obtain the (i 1 , TTFL), (i 2 , TTF 2 ) and (i 3 , TTF 3 ) data points for T 3  (and of other temperatures and bias currents) below the 5 year TTF. Then curve fitting to the data points allows extrapolation of the curve  52  to the 5-year TTF. 
     For the example in FIG. 3, typically a head  18  is stress tested by heating in an oven to a constant temperature (e.g., T 3 ) with bias currents (e.g., i 1 ) for one day, then removed and readback signal amplitude of the head is measured (e.g., 10% degradation). Then the head  18  is put back in the oven and stress testing is continued for two additional days under the same conditions as before (same temp (e.g., T 3 ) and bias current (e.g., i 1 )). The head  18  is then removed and the head readback signal amplitude degradation measured again (e.g., 30% degradation), providing a second data point. If a Failure Threshold is selected at 15% amplitude degradation, there is one data point for 10% degradation, and another for 30% degradation, allowing extrapolation/interpolation to find the bias current for 15% degradation. The test can be longer and more frequent for more data points to obtain more accurate results (tradeoff is between test time and accuracy). 
     For a set of data points, a polynomial that fits the data is determined, and then extrapolated to determine actual lifetime under actual field conditions. By taking joule heating due to current density into consideration, as well as ambient temperature, the estimate of actual lifetime is even more accurate according to the present invention. 
     Referring to the example steps in FIG. 7, and graphs in FIGS. 8-10, according to another example embodiment of the present invention, readback signal amplitude degradation in GMR heads  18  due to stress conditions (e.g., stress temperature and stress bias current) are measured. The TTF at different stress temperatures (e.g., T 1 , T 2 , T 3 ) and with different bias currents (e.g. I 1 , I 2 , I 3 ) are measured (step  110 ). A plot of the measured TTF values on a log scale vs. stripe temperature T is shown in FIG. 8 (step  112 ). The, 1/T scale is used here because of the relationship between lifetime and temperature, wherein TTF=f(e Q/kT ) such that LnTTF=f (Q/kT) where Q is the “activation energy” and k is the Boltzman constant. 
     As shown in FIG. 8, curves  60  for each bias current are extrapolated until they intercept the 5-year TTF dotted line (step  114 ), and the associated values T for each intercept point (on horizontal axis) is determined for the 5 year lifetime (step  116 ). 
     Next, using the above data values, the stripe temperature T for each value of bias current is determined as shown by the example curve  62  in FIG. 9 (step  118 ) (the bias currents I 1 , I 2  and I 3  in FIG. 8 are as in FIG.  9 ). The stripe temperature data is then converted to stripe resistance based on said TCR relationship, as shown in FIG. 10 (step  120 ). Then, curve fitting (e.g., polynomial fitting) to obtain a curve fit  64  as shown in FIG. 10 (step  122 ). The fitting equation (i.e., curve  64 ) provides the head resistance specification. The bias current values can be selected based on the specification curve  64  in FIG.  10 . 
     The present invention allows establishing bias current dependence of lifetime and head resistance specification to meet lifetime requirements with both stripe temperature and bias current taken into consideration. This is because head resistance is a function of bias current (e.g., FIG.  6 ), and because head lifetime is a function of bias current (e.g., FIG. 3) and head temperature (e.g., FIGS.  4 - 5 ). 
     To obtain the bias setting algorithm (e.g., fitting curves  56  and  64  in; example FIGS. 6 and 10 above, respectively), the GMR heads  18  are tested at the component level (i.e., before they are in the disk drives  10 ) as described. To ensure disk drive reliability, it is desirable to predict head lifetime in the disk drive. The present invention provides accelerated testing by using stress temperatures (e.g., in an oven), and bias currents (increasing bias current increases current density and also stress temperature). 
     The effect of oven temperature, and temperature due to Joule heating of current density (current effect) are distinct. As shown and described in relation to FIGS. 2-6, above, a technique is used to establish a bias algorithm using current bias dependence of the head resistance at the component level (i.e., testing to separate out bias current effect and temperature effect, to obtain bias dependent resistance), whereby a bias current is selected for a desired lifetime. Then, with the head  18  in the disk drives  10  (i.e., drive level test described further below), using the acceleration factors already established based on the component level tests, a reliability test is performed to determine how reliable the head is. 
     As such, dependence of lifetime on stress current is established as a curve (e.g., FIGS.  3  and  8 ). Then based on the dependence curves, an acceleration condition is applied to the head during the drive level testing. Preferably, during the drive level testing, temperature is also raised, but not as much as component level test above, and the bias current is also increased in order to accelerate the test when the head is in the disk drive. 
     The methods for measuring (establishing) bias current dependence of GMR head lifetime (e.g., ATR lifetime) and bias current dependence of resistance for GMR heads, according to the present invention, allow proper bias current setting for each individual head in the disk drive to ensure GMR reliability (e.g., predictable lifetime). Head performance can be frequently monitored during operation in customer environment and the bias current can be automatically adjusted downward if GMR readback signal degrades below certain limits (the disk drive preamp provides the bias current to the head wherein the current level is controlled by the drive controller firmware). These limits can be loaded into the disk drive and are usually different in details for each GMR supplier. In one embodiment, the disk drive preamp  24  provides the bias current to the head  18 . The level of bias current provided by the preamp  24  is controlled by the firmware in disk drive channel ASIC  26  or controller  14  (e.g., processor  34  or drive control  36 ). Said Lifetime Table and bias current settings/levels are stored in the disk drive for setting the bias current as described. 
     Lifetime can be defined by various techniques such as Amplitude Thermal Robustness (ATR), which according to the present invention includes a technique to determine current dependence lifetime in addition to temperature. ATR test algorithms and extrapolation formulas according to the present invention can be included in the disk drive programming (e.g., in Diag-script or super-command script). A drive level lifetime test can be conducted by including additional disk drives in reliability demonstration test (RDT) or ongoing reliability test (ORT). Disk drive level verification before mass production is important as a further check of the component level results above. 
     Determining said bias current dependence of lifetime and bias current dependence of resistance for GMR heads, enables GMR AIR lifetime test in the disk drive by providing test acceleration factors and extrapolation formulas to determine GMR ATR lifetime from current stressed condition to nominal operating condition in the field. Drive level ATR test can provide an essentially final and conclusive verification to guarantee GMR reliability lifetime in the field. In one example, ATR performance is only conducted at GMR component level (i.e., GMR head not in disk drive) using oven temperatures as high as e.g. 150 C. (this procedure may not be applicable for drive level tests due to high stress temperatures). Bias current stress with moderate temperature increase is the desirable method to; perform GMR drive level (i.e., GMR head in disk drive) ATR lifetime test for lifetime determination. 
     When performing component level and drive level AIR lifetime test GMR stress factors/conditions must be selected, and the GMR lifetime extrapolation determined from the stress conditions to the nominal operating conditions. Both high GMR stripe temperature (Tstripe) and high bias current (Ibias) are used for component level GMR ATR stress tests. At the component level, temperature stress up to e.g. 150 C. (Toven) with nominal Ibias can be used as the major test, acceleration factor due to simplicity in said extrapolation, described above. 
     Effects induced by bias current stress include not only intrinsic current, but also temperature increase through Joule heating, wherein Tstripe is a function of I 2 ×R, such that I is the bias current, and R is resistance of the MR sensor in the head. Calculation and data collection allow separation of pure bias current effect from temperature effect due to combined oven temperature and Joule heating. To accelerate GMR amplitude decay (i.e., time to failure) in disk drives, higher read bias current than nominal operating current, is applied at maximum allowed drive environmental temperature (e.g., 55 C. oven temperature). By providing a process/algorithm to select proper bias current stress factors for determining lifetime of the head, and a process/algorithm to extrapolate test GMR lifetime to nominal conditions for each head family (e.g., head vendor) in the disk drive firmware, ATR test and lifetime determination can be performed at drive level. Various example methods for determining bias dependence of ATR lifetime are described herein. 
     The readback signal amplitudes include spin spin-stand amplitude (i.e., Guzik), R(H) amplitude (i.e., Quasi-static tester), or drive level (i.e., selfscan). To determine bias current dependence of maximum allowed resistance specification, a correlation (key point) between: (1) maximum allowed Tstripe (stripe temperature) to guarantee a desired GMR lifetime (e.g., 5 years) and (2) bias (bias current), are determined first, as described above and detailed further below. 
     Thus, a method of determining bias current dependence of ATR failure (TTF) includes the steps of keeping constant stripe temperature by adjusting stress current and stress environmental temperature, to achieve similar GMR resistance at various stress conditions. By keeping stripe temperature constant (e.g., adjusting oven temperature), the bias current dependence of ATR lifetime is determined directly. Normalized GMR readback signal amplitude degradation as a function of time is determined using several different bias currents Ibias, and environmental temperature combinations, to keep constant stripe temperature. Similar amplitude degradation values can be generated at different stripe temperatures. 
     Based on the amplitude failure criteria (i.e. desired lifetime based on amplitude degradation), Ln(TTF) at each Ibias is obtained and bias current dependence of GMR lifetime is determined for each desired stripe temperature. For example, bias current (Ibias) dependence of ATR lifetime is extracted for three different stripe temperatures (T or Tstripe) 180 C., 165 C. and 150 C. Using that data, various combinations of Tstripe and Ibias for stressing GMR heads to a selected ATR lifetime are derived (e.g., FIGS. 4-5,  8 - 9 , show examples of such combinations/relationships of Tstripe and Ibias for stressing GMR heads to e.g. 5 year ATR lifetime). Said Tstripe vs. Ibias relationships are used to extrapolate ATR lifetime from one Ibias to another, and are further used to determine GMR bias current stress level and to extrapolate GMR lifetime from stressed condition to nominal condition. 
     In the example shown in FIG. 8, Ln (TTF) vs. 1/Tstripe is generated using GMR heads with constant resistance and several component level oven stress temperatures. The maximum allowed Tstripe for e.g. 5 year GMR lifetime, determined based on constant resistance GMR, can then be adjusted based on the relationships established in FIG. 9 for different bias currents. Then as shown in example of FIG. 10, relationships between Tstripe and GMR resistance-for various bias currents, are established. As such, according to the present invention, as shown in examples of FIG. 6 (i.e., curve  56 ) and FIG. 10 (i.e., curve  64 ), head resistance spec, Rs at different bias currents are determined by taking both stripe temperature and bias current effects into consideration. 
     The above example methods allow obtaining bias current dependence of ATR lifetime using constant resistance GMR and different stress bias currents. Then, maximum allowed Tstripe for e.g. 5 year ATR lifetime for different bias currents is obtained. The maximum allowed Rs specification for various bias currents is also determined. Then extrapolation formula (e.g., polynomial curve fit) of lifetime (e.g., Ln(TTF)) for different Ibias at various Tstripe is determined from the Tstripe vs. Ibias relationships described. This extrapolation formula is used for drive level bias stress and lifetime extrapolation. 
     For drive level ATR life time testing, bias current dependence of ATR lifetime is established (i.e., an ATR resistance specification at various bias currents at component level), and then :bias current dependence of ATR resistance specification, and bias current dependence of ATR lifetime, are pre-loaded into disk drive for each vendor&#39;s heads. Therefore, the dependence of head resistance on bias current, and dependence of ATR lifetime on bias current, determined above, are pre-loaded into the disk drive for each vendor&#39;s heads. For each head vendor, the head resistance is measured for each head in a disk drive and then for a desired lifetime, proper bias current is selected based on the measured resistance and pre-loaded bias current dependence of ATR resistance specification. The pre-loaded bias current dependence of ATR lifetime is used for drive level ATR test. 
     A version of a Drive Level ATR test is shown by example steps in flowchart of FIG.  11 . Failure criteria such as record/playback error rates (e.g., Minimized Square Error (MSE) or Bit Error Rate (BER)) can, be used as the indices in the disk drive for drive level tests. Failure criteria of MSE and BER are determined based on requirements of particular disk drive platform. A set of disk drives  10  that pass internal MSE or BER specification are selected (step  130 ). The disk drives are tested for a period of time at elevated temperatures (e.g., at the same elevated temperature that is applied to the RTD and ORT drive testing), wherein the test bias currents can be chosen to be e.g. the same as head component head testing (step  132 ). The test is interrupted and the MSE or BER (instead of readback signal amplitude in component testing) are measured under normal operating conditions (step  134 ). The test can be restarted again to obtain more measurements by repeating steps  132  and  134 . Then values of MSE or BER for the heads in the disk drives are obtained as a function of time, and the values of MSE or BER degradation vs. time, provide Time-to-Failure (step  136 ). The same analytical technique as used in the component testing is applied to obtain resistance and current specification (step  138 ). 
     Stress levels, including bias current, head slider temperature and sensor stripe temperature selected for ATR test acceleration are determined as described. For example, the Ln(TFF) vs. bias current plot for each GMR vendor is determined using component level testing above. The stripe temperature can be calculated using the following example relation: 
     
       
           Tstripe =((( Rs   —   stress−Rs   —   amb )/ Rs   —   amb )−1)/ TCR+Tambient   
       
     
     wherein: 
     Rs_stress: GMR resistance measured by BHV (buffer head voltage) at component level stress condition, 
     Rs_amb: GMR resistance measured at ambient using low bias current, 
     TCR: Thermal coefficient of sensor resistance (Each vendor&#39;s TCR number is recorded into the disk drive memory), 
     Tambient: drive ambient temperature during Rs_amb measurement. 
     The disk drive ATR lifetime test can be performed using additional disk drives in parallel with RTD or ORT drive testing to take advantage of the oven time for these two tests. The total stress time for this test can be adjusted by dialing in estimates of stress current to the head according to the Ln(TTF) vs. Ibias correlation at RDT (or ORT) oven temperature. Further, for example, 50 to 100 disk drives from each product line can be used for this test to obtain sufficient data for statistical analysis. The simplest implementation for the ATR lifetime test includes go-no-go type test, in which the stress bias current is selected so that test time equals to ORT or RDT test time (i.e., lifetime under stressed condition). Then disk drive MSE or BER is checked before and after the stress at nominal operating operation condition. 
     More sophisticated lifetime test, i.e., lifetime determination, includes measuring drive MSE or BER after different stress durations until the indices reach failure point. In that case, less aggressive stress levels can be chosen. By extrapolating the ATR lifetime from stressed to non-stressed condition using the algorithms stored in disk drive memory from the component level testing, the drive ATR lifetime for the tested GMR head can be determined at disk drive level. 
     Algorithms for selecting acceleration factors, Tstripe estimation, and test execution can be stored in the disk drive. The stored algorithms can be shipped with disk drives and can be protected from unauthorized access. As necessary, upon ATR drive failures, a stored test can be executed using built-in codes on surviving heads (e.g., low resistance heads). The remaining lifetimes can be estimated by subtracting usage time elapsed from the guaranteed 5 year lifetime. 
     The present invention allows performing ATR lifetime test in drive level, as described, to guarantee GMR reliability even if ATR resistance specification is defined based on thorough reliability studies at component level. A disk drive level ATR verification test can be used to prove reliability associated with platform related differences, i.e., bias setting accuracy, worst case drive temperature, excessive writing, etc. 
     The methods described herein for choosing stress factor, stress time, and lifetime extrapolation basis, make drive level lifetime test feasible and meaningful. The conventional temperature stress using nominal bias requires 5 years to prove the drive reliability. According to the present invention, reduction in lifetime due to intrinsic current effect is included in the overall lifetime determination because conventional head lifetime determination using only temperature effect is not accurate. 
     The present invention has been described in considerable detail with reference to certain preferred versions thereof; however, other versions are possible. Therefore, the spirit and scope of the appended claims should not be limited to the description of the preferred versions contained herein.