Abstract:
A method and apparatus for determining a lifetime for a medium to fail due to thermal decay of a magnetization pattern is provided. Different stress magnetic fields are applied to a write head for writing to a machine-readable medium resulting in a magnetic field on the medium. A time to failure, corresponding to each of the different stress magnetic fields, is determined, the time to failure being an amount of time for an amplitude of a signal on the medium to degrade beyond a predetermined failure criteria. A time to failure without a stress magnetic field is determined based on the corresponding times to failure determined for each of the different stress currents.

Description:
RESERVATION OF COPYRIGHT 
     This patent document contains material subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document, as it pears in the U.S. Patent and Trademark Office patent files or records, but otherwise reserves all copyright rights whatsoever. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Aspects of the invention relate to a method for estimating media thermal decay based on error rate failure criteria. 
     2. Description of Background Information 
     One of the biggest problems for high density recording media is media thermal decay. Until now, no one has proposed da feasible method to quantitatively predict media thermal decay lifetime based on real drive level failure criteria. Much of the published literature has discussed how to determine whether a medium is stable or unstable using a media stability factor, KuV/kT, where Ku is anisotropic energy of a media alloy, V is a switching volume, k is a Boltzmann constant, and T is a media temperature. However, no method has been reported that quantitatively determines the media thermal decay lifetime. Some of the reasons for this may be that the previously used failure criteria for media thermal decay is not well defined or directly related to drive performance, lifetime determination was based mostly on modeled results, and simple test acceleration and straightforward extrapolation from accelerated to non-accelerated conditions were never demonstrated. 
     Other approaches that have been proposed have included: 
     a) using critical current to detect noise peaks where the magnetic moment reaches zero. See “Thermal Effects &amp; Recording Performance at High Recording Densities”, by M. Alex &amp; D. Wachenschwanz, IEEE Trans. Mag., Vol 35, page 2796 (1999); and 
     b) using the time dependence of Hc based on the Sharrock equation to estimate decay time. See “Time Dependence of Switching Fields in Magnetic Recording Media”, by M. P. Sharrock, J. Appl. Phys., vol 15, page 6413 (1994). 
     SUMMARY 
     An embodiment of the invention is a method for determining a thermal decay lifetime of a machine-readable medium at a given temperature. A test pattern is written to a portion of the medium. An initial amiplitude of signals on the portion of the medium is measured, A first stress magnetic field is applied to the portion of the medium. After applying the first stress magnetic field, a first amplitude of signals on the portion of the medium is measured. The test pattern is rewritten to the portion of the medium. A second stress magnetic field is applied to the portion of the medium. After the applying of the second stress magnetic field, a second amplitude of signals on the portion of the medium is measured. A time to failure is determined corresponding to the first and the second stress, magnetic fields, respectively, the time to failure being an amount of time, measured from a corresponding reference time, for a respective amplitude of the signals on the medium to degrade, in relation to the initial amplitude, beyond a predetermined failure criteria. A time to failure without applying a stress magnetic field is determined based on a relationship between the corresponding times to failure determined for each of the applied stress magnetic fields and Ln(the corresponding times to failure). 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features, and advantages of the present invention are further described in the Detailed Description which follows, with reference to the drawings by way of non-limiting exemplary embodiments of the invention, wherein like reference numerals represent similar parts of the present invention throughout the several views and wherein: 
     FIG. 1 is a schematic view of a disk drive unit; 
     FIG. 2 is a detailed view of the controller shown in FIG. 1; 
     FIG. 3 is detailed view of the disk failure determiner shown in FIG. 2; 
     FIG. 4 is a flowchart for explaining the process of determining a thermal decay lifetime for a disk within a disk drive unit; 
     FIG. 5 is a flowchart for explaining the process of determining thermal decay lifetime when the disk is at an elevated temperature; 
     FIG. 6 is a schematic view of a spin stand unit; 
     FIG. 7 is a detailed view of the controller shown in FIG. 6; 
     FIGS. 8A to  8 B are a flowchart for explaining the process of determining media thermal decay in a spin stand unit; 
     FIG. 9 is a flowchart for explaining the process of determining thermal decay lifetime for a medium when the medium, in a spin stand unit, is at an elevated temperature; 
     FIG. 10 is an example of a plot of applied stress current vs. Ln(time to failure (hereinafter, TTF)) for a particular disk; 
     FIG. 11 illustrates a linear relationship between Ln(TTF) and 1(temperature of medium); and 
     FIG. 12 is an example of a plot of normalized amplitude vs. field exposure time for determining TTF. 
    
    
     DESCRIPTION 
     A practical method to experimentally determine a quantitative lifetime for media thermal decay should include several elements. The failure criteria used has to be correlated with real drive performance. A simple and practical stress method for acceleration should be well established because the media signal for real products will not show any noticeable decay during the test period without being stressed. In an embodiment of the invention, the magnetic field produced by passing a small current through the write head is chosen to do the stress (hereinafter, stress current). The magnetic field stress does not need to come from an electromagnet, but can result from another magnetic field creation method, for example, a permanent magnet. In order to estimate lifetime of the media, time dependence of a testing parameter should be clearly identified. Extrapolation from a stressed to a non-stressed condition should be simple and should be experimentally established. 
     Two different approaches will be described, a drive level approach and a component level approach. 
     For drive level testing, FIG. 1 shows an example of a disk drive unit. The disk drive unit includes a disk  100 , a controller  102 , which includes a processor and a memory, and a read write head  104 , the movement of which is controlled by controller  102 . 
     FIG. 2 is a functional diagram explaining the functions performed by the controller  102 . The functions are performed via software or firmware in the described embodiment; however, the functions may be performed by hardware or a combination of hardware, software and firmware. The controller  102  includes stress applier  201  for applying a stress current and resulting magnetic field to the read/write head  104 , error measuring mechanism  202 , which receives error information from the read write head  104 , and a disk failure determiner  204 , which analyzes the error measurements and determines when disk failure would occur. 
     FIG. 3 shows a functional diagram of the disk failure determiner  204 . Disk failure determiner  204  includes a plotter  302  to plot the Ln (TTF) vs. the respective applied stress current, and least squares fit mechanism  304  to apply a least squares fit line to the plotted Ln TTF) to determine the Ln (TTF) without applying any stress current. Plotter  302  maybe implemented, for example, via software or firmware on the processor within the controller  102 . 
     For drive level testing, values of mean square error (MSE) are used as a failure indicator due to an excellent correlation of MSE with error rate. Due to the use of simple stress acceleration and extrapolation, thermal decay lifetime in drives can be easily determined. FIG. 4 provides a flowchart explaining the process of determining media thermal decay lifetime. 
     At act A 400 , a data pattern is written to a track on the disk. 
     At A 401 , stress applier  201  applies different stress currents, one at a time, to the write head  104  of the disk drive unit  100  in order to apply a stress magnetic field to the disk. 
     At A 402 , after a respective one of the stress currents has been applied, a value of MSE is determined by the error measuring mechanism  202  after different revolutions of the disk. under stress until a predefined MSE failure point is reached. The data pattern is rewritten to the disk after each respective application of a stress current. 
     At A 404 , the error measuring mechanism  202 , records or saves a corresponding time to failure (TTF) at each of the applied stress currents, each TTF indicates an amount of time until a respective MSE failure point was reached. 
     At A 406 , plotter  302  plots, at least internally via the processor and associated memory, the Ln (TTF) vs. the stress current. Such a plot reveals a linear graph, as can be seen by FIG.  10 . 
     At act A 408 , the Ln (TTF) at zero stress is determined from a least squares fit line passing through the plotted Ln (TTF) vs. stress current. 
     FIG. 10 is an example of a plot of applied stress current vs. Ln(TTF) for a particular disk. The horizontal axis of FIG. 10 represents the applied stress current in milliamps (ma) and the vertical axis represents the Ln(TTF). As is easy to see from FIG. 10, after several points on the graph are determined, a least squares fit line can be determined. Thus, the Ln(TTF) at zero stress can easily be extrapolated and subsequently, TTF at zero stress is easily determined. 
     The above mentioned process can be repeated at different temperatures in order to estimate media thermal decay lifetime under adverse conditions. For example, the disk drive unit and disk can be tested in a 45° C. oven. FIG. 5 shows a flowchart for such a procedure. 
     At act A 500  the disk unit, including the disk, is placed in an oven set for a temperature of, for example, 45° C. 
     At act A 502 , acts A 400 -A 408  are repeated in order to determine the Ln (TTF) at zero stress from a least squares fit line for a disk at an elevated temperature, such as 45° C. 
     The lifetime of thermal decay of a media at any operational temperature can be determined by using the above procedure to determine the TTF at zero stress for at least two different temperatures. As can be seen, FIG. 11 illustrates a linear relationship between Ln(TTF) and 1/(Temperature of Media (hereinafter, 1T Media )), known as the Arrhenius law. As T Media  increases, the TTF decreases, and subsequently, as 1/T Media , increases, TTF increases. The horizontal axis of FIG. 11 represents values of 1/T Media , while the vertical axis represents Ln(TTF). FIG. 11 shows a least squares fit line passing through or near the plotted points. Using the least squares fit line, a TTF at any temperature for a particular disk can be determined. 
     The following explains component level testing using a spin stand test. FIG. 6 shows an example of an embodiment in a spin stand test unit. The embodiment includes controller  602  which includes a processor and a computer memory. The controller  602  is attached to a moveable read/write head  604  for reading and writing to a media, such as a disk  600 . 
     FIG. 7 illustrates the functional elements of the controller  602 . In the illustrated embodiment the functional elements include software or firmware; however, the functional elements may be implemented in hardware or a combination of hardware, software and firmware. 
     As shown in FIG. 7, the controller  602  includes a stress applier  702  for applying different stress currents, one at a time to a write head  604  for applying the stress current to a disk. 
     TTF determiner  704  determines the amount of amplitude signal on the disk for each of the applied stress currents and a corresponding amount of exposure time. TTF determiner plots, at least internally, for each applied stress current, the determined normalized amplitude value vs. the corresponding exposure time. 
     FIG. 12 is an example of a plot of normalized amplitude vs. field exposure time for a particular disk, where normalized amplitude is represented along the vertical axis and field exposure time, in microseconds, is represented along the horizontal axis. Line  1200  represents normalized amplitude vs. field exposure time when the stress current is 12 ma, at line  1202  the stress current is 11 ma, at  1204  the stress current is 10 ma, and at  1206  the stress current is 9 ma For this particular disk, a normalized amplitude of 0.85 or less indicates a disk failure. In FIG. 12 the dashed line indicates the failure point for this particular disk. 
     An amount of amplitude degradation is determined for each applied stress current using different exposure times. Based on the exposure times, TTF determiner determines the TTF for each applied stress current based on a linear relationship of normalized amplitude vs. field exposure, as shown in FIG.  12 . The point at which each of the lines  1200 ,  1202 , 1204  and  1206  cross the dashed line indicates the TTF for each respective stress current. Thus, the TTF for each respective stress current can be determined, for example, by reading the field exposure time at the point at which each respective line  1200 ,  1202 ,  1204  and  1206  crosses the dashed line. 
     Media failure determiner  706  determines the TTF without a stress current applied based on a linear relationship between the Ln (TTF) vs. stress current, as previously shown in FIG.  10 . Thus, by plotting LN(TTF) vs. each respective stress current, TTF at zero stress current can be determined, as previously described. 
     FIGS. 8A to  8 B show a flowchart for explaining a process for determining TTF at component level in a spin stand test unit. 
     At A 800 , a data pattern is written to a reference track and a data track. 
     At A 802 , an initial amplitude of the data track is measured. 
     At A 804 , stress applier  702  applies different stress currents, one at a time, to the data track via a read/write head  604 , thereby generating a magnetic field on the data track of the disk. 
     At A 806 , the amplitude of the reference track is measured. Because no stress current was applied to the reference track, ideally no amplitude degradation should be observed; however, if amplitude degradation is observed, the measured amplitude of the data track should be calibrated accordingly. For example, if a 4% amplitude degradation of the reference track is observed, the measured amplitude of the data track is adjusted or calibrated to be increased by 4%. 
     At A 808 , for each applied stress current, the amplitude of the data track is measured and divided by the initial amplitude of the data track to provide normalized amplitude. TTF determiner  704  plots the corresponding field exposure time as it relates to normalized amplitude. See FIG.  12 . 
     At A 810 , TTF determiner  704  determines TTF for each stress current applied based on a linear relationship of an amount of amplitude degradation corresponding to an amount of exposure time at each applied stress current. For example, TTF determiner  704  determines TTF by plotting field exposure time vs. normalized amplitude, as shown, for example, in FIG.  12 . In FIG. 12, the TTF for each respective stress current can be determined by determining the point at which each respective line crosses a predefined failure point, for example the dashed line of FIG.  12 . 
     At A 812  plot Ln(TTF) vs. each applied stress current. See FIG.  10 . 
     At A 814 , determine Ln(TTF) at zero stress, and subsequently, TTF at zero stress, from the least squares fit line. 
     FIG. 9 is a flowchart for determining TTF under adverse operating conditions in which the temperature of the medium is raised. At act A 910  the temperature of the medium is elevated by, for example placing the medium in an oven using a temperature, such as 45° C. 
     In act A 912  acts A 800 -A 814  are repeated in order to determine a TTF without stress at the elevated temperature. 
     Using a TTF at a first temperature, which is, for example an ambient temperature, and a TTF at an elevated temperature, a TTF at any temperature can be determined by plotting the Ln (TTF) vs. 1/T media  based on the data collected at ambient temperature and an elevated temperature, as shown in FIG.  11 . 
     While the invention has been described by way of example embodiments, it is understood that the words which have been used herein are words of description, rather than words of limitation. Changes may be made, within the purview of the appended claims, without departing from the scope and spirit of the invention in its broader aspects. Although the invention has been described herein with reference to particular structures, materials, and embodiments, it is understood that the invention is not limited to the particulars disclosed. 
     The invention extends to all equivalent structures, mechanisms, acts, and uses, such as are within the scope of the appended claims.