Patent Publication Number: US-9837118-B1

Title: Determining thermal gradient of heat-assisted magnetic recording hotspot based on timing errors

Description:
SUMMARY 
     The present disclosure is directed to determining thermal gradient of a heat-assisted magnetic recording hotspot based on timing errors. In one embodiment, a method involves writing data to a magnetic recording medium of a drive using a read/write head. The read/write head has an energy source that applies a hotspot to the magnetic recording medium while recording. During the writing, a steady-state current applied to the energy source is changed by a step value. A timing error induced by the change in the steady-state current is measured based on reading back the data. A thermal gradient of the hotspot is determined based on the step value and the timing error. 
     These and other features and aspects of various embodiments may be understood in view of the following detailed discussion and accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The discussion below makes reference to the following figures, wherein the same reference number may be used to identify the similar/same component in multiple figures. 
         FIG. 1  is a view of a slider assembly according to an example embodiment; 
         FIG. 2  is a diagram of recorded tracks according to an example embodiment; 
         FIG. 3  is a diagram illustrating mode hops in a read/write head according to an example embodiment; 
         FIG. 4  is a flowchart of a method according to an example embodiment; 
         FIGS. 5, 6 and 7  are graphs showing signals and data used in the method shown in  FIG. 4 ; 
         FIG. 8  is a graph showing experimental results obtained using the method shown in  FIG. 4 ; 
         FIG. 9  is a block diagram of an apparatus according to an example embodiment; 
         FIG. 10  is a flowchart of a method according to an example embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure generally relates to data storage devices that utilize magnetic storage media, e.g., disks. Data storage devices described herein use a particular type of magnetic data storage known heat-assisted magnetic recording (HAMR), also referred to as energy-assisted magnetic recording (EAMR), thermally-assisted magnetic recording (TAMR), and thermally-assisted recording (TAR). This technology uses an energy source such as a laser to create a small hotspot on a magnetic disk during recording. The heat lowers magnetic coercivity at the hotspot, allowing a write transducer to change magnetic orientation, after which the hotspot is allowed to rapidly cool. Due to the relatively high coercivity of the medium after cooling, the data is less susceptible to data errors due to thermally-induced, random fluctuation of magnetic orientation known as the superparamagnetic effect. 
     A HAMR device uses a near-field transducer to concentrate optical energy into the optical spot in a recording layer. The hotspot raises the media temperature locally, reducing the writing magnetic field required for high-density recording. A waveguide integrated into a read/write head can be used to deliver light to the near-field transducer. Light from a light source, such as an edge-emitting laser diode, is coupled into the waveguide through waveguide input coupler or a spot size converter. The light source may be mounted to an outside surface of the read/write head. 
     Because the bit boundaries in a HAMR device are defined by the hotspot, the characteristics of the hotspot can have a significant effect on performance. For example, the thermal gradient is the change in temperature over distance at the boundaries of the hotspot. A sharp thermal gradient, in which temperature changes a relatively large amount over a relatively small distance, is strongly correlated to performance. For example, a sharp thermal gradient results in well-defined bit boundaries in the recorded tracks, and the bits are therefore easier to detect and decode as well as being able to be placed closer together. 
     This disclosure describes techniques used to measure thermal gradient in a HAMR device. These techniques can be used in-drive, meaning they do not rely on external measuring devices and can be used during qualification testing and use of the drive. The in-drive thermal gradient measurement is based on measuring the delta in time of a transition relative to its expected location as a function of a laser power increase. If the optical power sent to the media is increased rather abruptly, the location of the written transition will shift in time. For example, a power increase causes the transition to be written earlier in time whereas a power decrease causes the transition to be written latter in time. A measurement of that shift in time as function of the power change gives rise to an estimate of the thermal gradient provided by the HAMR head. These estimates of thermal gradient can be useful in testing and controlling HAMR drives. 
     In  FIG. 1 , a block diagram shows a side view of a HAMR read/write head  102  according to an example embodiment. The read/write head  102  may also be referred to herein as a slider, write head, read head, recording head, etc. The read/write head  102  is coupled to an arm  104  by way of a suspension  106 , e.g., a gimbal. The read/write head  102  includes read/write transducers  108  at a trailing edge that are held proximate to a surface  110  of a magnetic recording medium  111 , e.g., a magnetic disk. 
     A controller  118  is coupled to the read/write transducers  108 , as well as other components of the read/write head  102 , such as heaters  114 , sensors, etc. The controller  118  may be part of general- or special-purpose logic circuitry that controls the functions of a storage device that includes at least the read/write head  102  and recording medium  111 . The controller  118  may include or be coupled to a read/write channel  119  that include circuits such as preamplifiers, buffers, filters, digital-to-analog converters, analog-to-digital converters, decoders, encoders, etc., that facilitate electrically coupling the logic of the controller  118  to the signals used by the read/write head  102  and other components. 
     The illustrated read/write head  102  is configured as a HAMR device, and so includes additional components that form a hotspot  124  on the recording medium  111  near the read/write transducer  108 . These components include a laser  120  (or other energy source) and a waveguide  122 . The waveguide  122  delivers light from the laser  120  to components near the read/write transducers  108 , such as a near-field transducer that emits a tightly focused stream of energy to form the hotspot  124 . The read/write transducers  108  also include a magnetic pole that applies a magnetic field to the hotspot  124  and the surrounding area. Because of the high coercivity of the recording medium  111 , only the hotspot  124  is affected by the magnetic field due to the material being heated above the Curie temperature. Therefore, the size and shape of the hotspot  124  affects the location of magnetic transitions written to the recording medium  111 , which can affect the size and location of the bits of data defined by the transitions. 
     Small changes in the laser&#39;s power can have significant effects on the recording process, and these effects may be seen in both the downtrack and crosstrack direction of the data tracks. An example of this is shown in the diagram of  FIG. 2 , which shows two adjacent tracks  200 ,  202  according to an example embodiment. The different shaded areas in the tracks  200 ,  202  represent regions of different magnetic orientation. Circles  204 ,  206  represent a nominal hotspot size on the tracks  200 ,  202 , e.g., a hotspot size that is optimal given the desired track width and linear bit density of the tracks  200 ,  202 . Under some conditions, the thermal profile of the spot size sent to the media may shift position without a significant decrease or increase in hotspot size, causing the written transition to occur earlier (or later) than expected. This case is represented by dashed circle  208 , which indicates a momentary downtrack shift relative to the nominal hotspot  206 . This is one example of a downtrack effect, which may be induced by mode hopping or other causes. 
     Downtrack effects may also occur when the hotspot size decreases or increases, with or without a position shift. In such a case, the written transition is written later or earlier than expected because the thermal profile of the spot size has changed. This is indicated by dashed circles  210 ,  212 , which indicate a hotspot at respective lower and higher values than nominal  204 . These variations  210 ,  212  can also result in crosstrack effects, such as increasing chances of encroachment when the hotspot is too big, and making the track too narrow when the hotspot is too small. 
     The above examples can occur in the middle of writing when a laser&#39;s power increases rather abruptly, sometimes called a mode hop. The result of a mode hop is that the timing of the written data is affected. This causes errors when trying to read back the data, because the bit transitions are not written where they are supposed to be relative to previously written bits or relative to reference datum such as preambles, servo marks, etc. In  FIG. 3 , a diagram shows how mode hops can affect various parts of the data sectors. Time periods  302 - 306  are when servo sectors are being traversed, during which no writing of the media occurs. Time periods  308 - 312  represent times when data portions are being traversed, during which writing occurs. 
     Curve  314  represents the time-varying laser temperature during recording, and dashed line  316  represents a threshold temperature above which mode hops occur. Areas  318 - 312  represent effective laser power applied to the recording medium while writing the data. The written data is represented by regions  330 - 334 , which encompass two data sectors. Shaded areas  336 - 341  represent preambles/sync marks used by the decoder to detect the start of data. 
     The laser power curves  318 - 320  include transitions/steps  324 - 328  representing mode hops induced when the laser temperature  314  goes about the threshold temperature  316 . As indicated by the vertical dashed lines extending from the transitions  324 - 328 , the transitions  324 - 328  affect different regions of data recording in data regions  330 - 334 . Transitions  324 ,  325 , and  327  affect user data portions of the sectors. Transition  326  affects a preamble portion that is located between the two data sectors. Transition  328  affects a preamble portion that lies on the between the data region and a servo sector. 
     In embodiments described herein, a disk drive apparatus includes in-drive thermal gradient measurement based on measuring the delta in time of a transition relative to its expected location as a function of a power increase. If the power sent to the media is increased rather abruptly, the location of the written transition will shift in time. A power increase causes the transition to be written earlier in time whereas a power decrease causes the transition to be written latter in time. A measurement of that shift in time as function of the power change gives rise to an estimate of the thermal gradient of the HAMR head. 
     The relationship between the power change and transition shift is related by the following mathematical analysis shown below. The thermal profile of the NFT, T(x), as a function of position, x, is given in Equation (1) below, where T p , T a , and σ are the peak temperature of the NFT&#39;s profile, the ambient temperature of the air, and the standard deviation of the NFT&#39;s thermal profile respectively. 
     
       
         
           
             
               
                 
                   
                     T 
                     ⁡ 
                     
                       ( 
                       x 
                       ) 
                     
                   
                   = 
                   
                     
                       
                         T 
                         p 
                       
                       ⁢ 
                       
                         e 
                         
                           - 
                           
                             
                               x 
                               2 
                             
                             
                               σ 
                               2 
                             
                           
                         
                       
                     
                     + 
                     
                       T 
                       a 
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     Assuming that an injected power jump, Δ, is small, then relation between the peak temperature before, T p1 , and after, T p2 , and the power jump Δ is shown in Equation (2) below.
 
 T   p2   =T   p1 +Δ  (2)
 
     Substituting Equation 2 into Equation 1 for T p , doing some rearranging, using natural log identities, and using a small power change approximation one arrives at the shift in position of the written transition, δ, to be approximated as shown in Equations (3) and (4) below. 
                   δ   =       σ     2   ⁢       ln   ⁡     (       T   p         T   w     -     T   a         )             ⁢     A   %               (   3   )               
where
 
Δ %   =Δ/T   p   (4)
 
     The above equations describe a relationship between a power jump and a transition shift. This shift is in geometrical length, which can be converted to time based on common parameters of the drive. Next, taking the derivative of the thermal profile in Equation (1) with respect to x provides an equation for the thermal gradient 
     
       
         
           
             
               
                 
                   
                     dT 
                     dx 
                   
                   = 
                   
                     
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                             w 
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                           x 
                           w 
                         
                       
                       
                         σ 
                         2 
                       
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
     Where x w  refers to the location of the write (“w” for write). If we do some more matheimcal rearrangements, solve for x w , and subsequently evaluate that expression for T w , the temperature at writing, an expression for thermal gradient, T g , is shown in Equation (6) below. 
     
       
         
           
             
               
                 
                   
                     
                       T 
                       g 
                     
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                       [ 
                       
                         k 
                         / 
                         nm 
                       
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                         dT 
                         dx 
                       
                        
                     
                     = 
                     
                       
                         ( 
                         
                           
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                   6 
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     Plugging Equation 3 into Equation 6 yields the final relevant equation for this method 
     
       
         
           
             
               
                 
                   δ 
                   = 
                   
                     
                       
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                   7 
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     Because T w  and T a  are known quantities, the transition shift δ (in nm) is affected only in response to a power change Δ sent to the media. Therefore, by inducing a change in power while recording an measuring transition shift, the results are plugged into Equation (7) to estimate T g . A feature in the preamp can be implemented to accurately adjust the power sent to the media by a known amount while recording. Measuring the transition shift, which again is the shift in nm (or time) relative to the expected location of the transition, can also be determined by a feature in the current channel that measures disturbances in the read channel. 
     A hard disk read channel can generate errors of the read-back waveform by comparing it to the expected waveform. It can then feed these errors into a timing loop which is able to extract just the timing portion of these errors (as opposed to amplitude errors, for example). A read channel feature is able to quantify the size of the timing errors by comparing them to a programmable threshold. If the timing errors are greater than the threshold, the read channel will output a flag. The greater the threshold where non-zero flags are generated the larger the timing shift on the media is. In this example, a flag&gt;0 indicates a timing error and flag=0 indicates no timing error, however other conventions may be used. The size of the timing error is the largest threshold value which still indicates a timing error, e.g., flag&gt;0 in this example. While the value of the flag may also have some meaning (e.g., a flag value&gt;1 indicating more or greater timing errors than flag=1), for purposes of this example, only the threshold region where the flags transition between zero and one are considered. The drive controller can be configured to calibrate timing error values into an actual physical shift in nm. 
     For example, the calibration to determine mapping between timing error and physical shift may involve associating first and second different write precompensation values with different first and second non-return-to-zero, inverted (NRZI) data patterns. The first and second different write precompensation values cause a predetermined phase shift to be written into test data that uses the first and second NRZI data patterns. The test data is used to determine a response of the storage device to the predetermined phase shift. An example of inducing a predetermined phase shift in this way is disclosed in commonly owned U.S. patent application Ser. No. 15/233,298, filed Aug. 10, 2016, which is incorporated by reference in its entirety. The phase shift corresponds to a percentage of a bit cell. Therefore once, the bit cell size is known, the bit shift in nanometers can be determined. The bit cell size is 2πrf/ω, where r is the track radius, f is the data frequency, and ω is the disk rotation speed. Therefore, the timing error induced by the predetermined phase shift can be measured, and thereby converted to a distance shift. With that calibration, the read channel can therefore count how many errors are occurring and how large they are, thereby obtaining δ in Equation 6. 
     If a HAMR drive is configured to controllably increase (or decrease) the laser current rather abruptly, this in turn increases (or decreases) the power sent to the media rather abruptly (reflected in the Δ %  value). By calibrating a timing loop error detector, the value of δ can be measured. Measuring allows δ facilitates estimating T g  using the generally know values of T w  and T a . Further, δ can be measured at multiple A % , which can be fit into a curve, The slope of the curve is proportional to T g . 
     In  FIG. 4 , a flowchart shows a procedure according to an example embodiment. The device is calibrated  500  to map timing error parameter into a distance shift value, e.g., in nm. As represented by block  501 , a step function is varied over a desired range (e.g., −7% to 7% in step sizes of 1%). For each step function, the recording medium is written to  502  (e.g., written to the same or different sectors, tracks, etc.) with a unique pattern that is advantageous for this calibration, where the laser current sent to the head has the step function in it. 
     In  FIG. 5  a graph shows the output of a photodetector (e.g., a photodiode or other a device which measure the laser&#39;s output light) where a rather abrupt shift  600  is seen in the output of the photodiode from a steady-state value  602 . The steady-state photodiode value corresponds to the steady-state current applied to the laser just before the shift  600 . The optical power being sent to the recording media has an abrupt shift that corresponds to the shift  600 , resulting in a change in timing of the bit transitions being written to the media. 
     As indicated by loop limit  503 , preamp timing error threshold parameter is set for a desired range of threshold values (e.g., 0 to 30 in step sizes of 1). For each timing threshold selected at block  503 , some or all of the sectors and/or tracks that have this laser current step function written are read back  504 . When reading back the data, the timing threshold error flags are stored  505  together with the corresponding threshold values. After exiting the loop  503 , the stored flags and threshold values are used the find  506  the highest threshold value that registers a value greater than zero. This is illustrated in the graph of  FIG. 6 , which shows a plurality of flags as a function of a plurality of thresholds for a particular step value. 
     The value of interest in the graph in  FIG. 6  is the highest timing error threshold value that still registers a flag&gt;0. In this example, the threshold value that satisfies this criterion is 22. This means that at that power jump, the flag indicates a timing error as large as 22, which can be converted to a shift in time or in physical units such as nm via a calibration method. In reference again to  FIG. 4 , the time threshold value is converted  507  to a distance, and is stored together with the corresponding step value used with this iteration. Outer loop  501  continues with the next laser step value, during which blocks  504 - 507  repeat with this step value. 
     After all step values have been tested, loop  501  exits and the stored data are used to find  508  a thermal gradient function. An example of this function is shown in the graph of  FIG. 7 , which is a plot of estimated transition shift in units of distance as a function of laser power step change. A mathematical fit can be used to determine an equation similar to Equation (7), with the slope being inversely proportional to T g . 
     It will be understood that there may be many variations in the method shown in  FIG. 4 . For example, the setting  501  of step functions and writing  502  may occur within its own loop, such that n-segments of data (e.g., n-tracks) are all written at once with different step function values. Thereafter, each of the segments is read back in turn to perform the other parts  503 - 508  of the procedure. Other steps may be optional. For example, if the timing error relationship to distance shift is known, then the operation in block  500  may not need to be performed every time the other operations are performed. 
     A method as shown in  FIG. 4  was run on four drives, and their estimated thermal gradients were compared with the values provided by a lab test. The result is shown in the graph of  FIG. 8 . The graph shows that the estimation compares favorably with more direct form of measurement in the lab test. 
     In  FIG. 9 , a block diagram illustrates a hard disk drive apparatus  1000  according to an example embodiment. Control logic circuit  1002  of the drive  1000  includes a system controller  1004  that processes read and write commands and associated data from a host device  1006 . The host device  1006  may include any electronic device that can be communicatively coupled via host interface  1005  to store and retrieve data from a data storage device, e.g., a computer, peripheral card, etc. The data controller  1004  is coupled to a read/write channel  1008  that reads from and writes to a surface of a magnetic disk  1010 . 
     The read/write channel  1008  generally converts data between the digital signals processed by the data controller  1004  and the analog signals conducted through one or more read/write heads  1012  during read operations. To facilitate the read operations, the read/write channel  1008  may include analog and digital circuitry such as preamplifiers, filters, decoders, digital-to-analog converters, timing-correction units, etc. The read/write channel  1008  also provides servo data read from servo wedges  1014  on the magnetic disk  1010  to a servo controller  1016 . The servo controller  1016  uses these signals to provide a voice coil motor control signal  1017  to an actuator  1018 . The actuator  1018  moves an arm  1020  upon which the read/write heads  1012  are mounted in response to the voice coil motor control signal  1017 . 
     The disk drive  1000  is a HAMR device, and therefore the read/write heads  1012  include an energy source (e.g., laser diode) that heats the magnetic disk  1010  when recording. A HAMR laser controller  1023  sends a current to activate the laser diode when recording. As will be described below, the HAMR laser controller  1023  includes the ability to shift a steady-state write current being applied to the laser during recording, resulting in a corresponding jump in optical power applied to the disk  1010 . 
     The disk drive  1000  includes a thermal gradient detector  1024  that can estimate the thermal gradient of hotspots written to the disk  101  via the laser. The thermal gradient detector  1024  applies a laser current shift  1026  while writing test data to one or more segments (e.g., tracks sectors) of the disk  1010 . When reading back the test data, a loop disturbance detector  1028  detects a timing error  1030 . The thermal gradient detector  1024  uses this timing error along with the current shift  1026  to estimate thermal gradient  1032 . The system controller  104  can use this thermal gradient data  1024  for, among other things, evaluation and calibration of the drive during qualification testing, performance testing during use of the drive, etc. The detector  1024  allows an in-drive measurement of the thermal gradient, as opposed to the spinstand measurement. A drive measurement is faster and cheaper, and therefore can provide large scale feedback. The estimated thermal gradient  1032  provides an additional metric of performance, one which is a good predictor of performance (e.g., bit error rate). The detector  1024  can also be used in the field to predict health of a fielded drive, e.g., when a drive is about to fail, is in need of a calibration and/or other changes. 
     In reference now to  FIG. 10 , a flowchart illustrates a method according to an example embodiment. The method involves writing  1100  data to a magnetic recording medium of a drive using a HAMR read/write head, e.g., one that has an energy source that applies a hotspot to the magnetic recording medium while recording. During the writing, a steady-state current applied to the energy source is changed  1101  by a step value. After writing is finished  1102 , the data is read back  1103  and a timing error induced by the change in the steady-state current is measured  1105 . A thermal gradient of the hotspot is determined based on the step value and the timing error. The step value and timing error may be converted to respective temperature changes and distance shifts as part of determining  1105  the thermal gradient. 
     The various embodiments described above may be implemented using circuitry, firmware, and/or software modules that interact to provide particular results. One of skill in the arts can readily implement such described functionality, either at a modular level or as a whole, using knowledge generally known in the art. For example, the flowcharts and control diagrams illustrated herein may be used to create computer-readable instructions/code for execution by a processor. Such instructions may be stored on a non-transitory computer-readable medium and transferred to the processor for execution as is known in the art. The structures and procedures shown above are only a representative example of embodiments that can be used to provide the functions described hereinabove. 
     The foregoing description of the example embodiments has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the embodiments to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. Any or all features of the disclosed embodiments can be applied individually or in any combination are not meant to be limiting, but purely illustrative. It is intended that the scope of the invention be limited not with this detailed description, but rather determined by the claims appended hereto.