Patent Application
20140168807
The present invention relates to magnetic data recording and more particularly to a magnetic data recording system having a templated granular magnetic media and circuitry for synchronizing data recording to the templated granular magnetic media.
A key component of most computers is an assembly that is referred to as a magnetic disk drive, or hard disk drive. The magnetic disk drive includes a rotating magnetic disk, write and read heads that are suspended by a suspension arm adjacent to a surface of the rotating magnetic disk and an actuator that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The read and write heads are directly located on a slider that has an air bearing surface (ABS). When the slider rides on the air bearing, the write and read heads are employed for writing magnetic impressions to and reading magnetic impressions from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
Perpendicular magnetic recording, wherein the recorded bits are stored in a perpendicular or out-of plane orientation in the recording layer, is a promising path toward ultra-high recording densities in magnetic recording hard disk drives. One type of perpendicular magnetic recording system is a system that uses a dual layer media. The dual layer media includes a perpendicular magnetic data recording layer formed on a soft or relatively low coercivity magnetically permeable under-layer. The under-layer serves as a flux return path for the field from the write pole to the return pole of the recording head. The magnetic transitions between adjacent oppositely direct magnetized regions are detectable by the read element or head as the recorded bits.
Other variations of granular media have been explored for use in magnetic data recording systems as well. For example, granular media that can be used with thermally assisted writing, and microwave assisted writing. Media for thermally assisted recording (TAR) may include granular materials like CoPtCr, FePt, CoPt and other alloys. Media for microwave assisted writing may include granular materials like CoPtCr, FePt, CoPt and other alloys.
One type of material that can be used as a recording layer is a granular ferromagnetic cobalt (Co) alloy, such as a CoPtCr alloy, with a hexagonal-close-packed (HCP) crystalline structure having the c-axis oriented substantially out of plane or perpendicular to the plane of the recording layer. The granular cobalt alloy recording layer should also have a well-isolated fine-grain structure to produce a high coercivity (Hc) media and to reduce inter-granular exchange coupling, which is responsible for high intrinsic media noise. Enhancement of the grain segregation in the cobalt alloy recording layer can be achieved by the addition of oxides, including oxides of Si, Ta, Ti and Nb. These oxides tend to precipitate to the grain boundaries, and together with the elements of the cobalt alloy, form a non-magnetic inter-granular material.
The Co alloy recording layer has substantially out of plane or perpendicular magnetic anisotropy as a result of the c-axis of its HCP crystalline structure being induced to grow substantially perpendicular to the plane of the layer during deposition. To induce this growth of the HCP recording layer, the inter-layer onto which the recording layer is formed is also an HCP material. Ruthenium (Ru) and certain Ru alloys, such as RuCr, are non-magnetic HCP materials that can be used for the inter-layer.
The enhancement of segregation of the magnetic grains in the recording layer by the additive oxides is important for achieving high areal density and recording performance. The inter-granular oxide material not only decouples inter-granular exchange, but also exerts control on the size and distribution of the magnetic grains in the recording layer. Current disk fabrication methods achieve this segregated recording layer by growing the recording layer on a Ru or Ru-alloy interlayer that exhibits columnar growth of the Ru or Ru alloy grains. The columnar growth of the interlayer is accomplished by sputter depositing it at a relatively high sputtering pressure.
However, such a process results in a recording layer having a relatively wide variation in the size of the magnetic grains. A large grain size distribution is undesirable because it results in a variation in magnetic recording properties across the disk and because some of the smaller grains can become thermally unstable, resulting in a loss of data. There is, therefore, a need for a magnetic media having uniform grain structure, and also for a recording system that can effectively record to such a recording medium without excessive signal noise or bit error rate.
The present invention provides a method for magnetic data recording to a magnetic media configured for perpendicular magnetic data recording and having an ordered granular structure. The method includes writing an un-synchronized bootstrap servo to the magnetic media, and creating a frequency map and a phase map. A synchronized servo is then recorded to the media and the frequency map and phase map are refined.
The method can be embodied in a magnetic data recording system that includes circuitry for synchronizing the write frequency and phase to the ordered granular structure of the media.
The invention advantageously allows full advantage to be taken of a well ordered grain structure in a magnetic media, which may be a templated magnetic media having various zones each with an ordered granular structure.
The invention can be implemented in a system wherein the magnetic media has a lattice with one or more lattice vectors and wherein the grain lattice is ordered so that one of the one or more lattice vectors is kept parallel to a constant field contour of a trailing edge of a write pole of the write head and is appropriately rotated to account for skew of the write head.
In addition, the invention can be implemented in a data recording system wherein the system is configured for one or more of thermally assisted magnetic recording, microwave assisted recording and shingled magnetic recording.
In addition, the invention can be implemented in a system having electronics configured to synchronize the write frequency and write phase in a down-track direction, but not in a cross-track direction.
These and other features and advantages of the invention will be apparent upon reading of the following detailed description of preferred embodiments taken in conjunction with the Figures in which like reference numerals indicate like elements throughout.
For a filler understanding of the nature and advantages of this invention, as well as the preferred mode of use, reference should be made to the following detailed description read in conjunction with the accompanying drawings which are not to scale.
a is an enlarged top down view of a portion of a magnetic media having ordered grains with write synchronization with a head traveling without skew;
b is an enlarged top down view of a portion of a magnetic media having ordered grains with write synchronization with a head traveling with skew
The following description is of the best embodiments presently contemplated for carrying out this invention. This description is made for the purpose of illustrating the general principles of this invention and is not meant to limit the inventive concepts claimed herein.
Referring now to
At least one slider 113 is positioned near the magnetic disk 112, each slider 113 supporting one or more magnetic head assemblies 121. As the magnetic disk rotates, slider 113 moves radially in and out over the disk surface 122 so that the magnetic head assembly 121 can access different tracks of the magnetic disk where desired data are written. Each slider 113 is attached to an actuator arm 119 by way of a suspension 115. The suspension 115 provides a slight spring force which biases slider 113 against the disk surface 122. Each actuator arm 119 is attached to an actuator means 127. The actuator means 127 as shown in
During operation of the disk storage system, the rotation of the magnetic disk 112 generates an air bearing between the slider 113 and the disk surface 122 which exerts an upward force or lift on the slider. The air bearing thus counter-balances the slight spring force of suspension 115 and supports slider 113 off and slightly above the disk surface by a small, substantially constant spacing during normal operation.
The various components of the disk storage system are controlled in operation by control signals generated by control unit 129, such as access control signals and internal clock signals. Typically, the control unit 129 comprises logic control circuits, storage means and a microprocessor. The control unit 129 generates control signals to control various system operations such as drive motor control signals on line 123 and head position and seek control signals on line 128. The control signals on line 128 provide the desired current profiles to optimally move and position slider 113 to the desired data track on disk 112. Write and read signals are communicated to and from write and read heads 121 by way of recording channel 125.
The magnetic recording layer structure 206 is formed as a plurality of grains 205 that are preferably separated from one another by non-magnetic oxide boundaries 207. When a magnetic bit is recorded to the media, the magnetization of the magnetic grain 205 is aligned in a direction either up or down as indicated by arrows 209 in
The magnetic grains 205 of the recording layer 206 can be a material such as an ordered L10 Fe—P, chosen for its high magnetic anisotropy and moderately high Curie temperature Tc. The magnetic grains 205 can include other structures as well. For example in a thermally assisted recording system, the grains 205 of the recording layer 206 can include an exchange coupling layer within the grain (not shown) which helps to maintain thermal stability of the grain at normal operating temperature, but which allows the magnetization of the grains 205 to be switched at elevated temperatures during recording.
The cap layer 214 can be constructed of a highly exchange coupled magnetic alloy which may be an alloy containing Co, Cr and Pt. More particularly, the cap layer 214 can be a Co—Cr—Pt alloy having 50 to 80 atomic percent Co, 10 to 20 atomic percent Cr and 10 to 20 atomic percent Pt. The cap layer 214 may also include small amounts of one or more of B, Ta, Ru, W and Ti.
a, 4b and 5 are enlarged top down views of a magnetic media 400 with a highly ordered, uniform size grain structure.
The well ordered, uniform shape of the grains 802 can be achieved through a templated media growth, by growing the grains on a substrate of pre-patterned nucleation sites. An example of such a process for producing well ordered, uniform grains 802 is described in United States Patent Application US 2011/013169, filed Dec. 16, 2009 and published Jun. 16, 2011, which is incorporated herein by reference.
A challenge that arises as a result of having a very well ordered grain structure can be understood with reference to
With reference to
The present invention includes methods for synchronizing the magnetic recording on highly ordered template granular media. In this way a recording system is created that utilizes a granular medium with improved effective media noise. Furthermore, the present invention enables larger grains relative to conventional continuous media.
As those skilled in the art will appreciate a magnetic bit is recorded using a write head having a write pole, which generally has a trapezoidal shape, with the bit size being primarily determined by the trailing edge of the write pole. An example of a magnetic footprint resulting from such a magnetic write pole is shown as dashed line 406 in
Since the data tracks on the disk are circular, the templated lattice can be partitioned into zones to keep the down-track lattice pitch close to the optimal pitch of the templating process.
Write Synchronization:
The write synchronization process requires learning how the underlying grain lattice varies with radius and angle around the disk. Key parameters for the recording system include the ideal write frequency F and the ideal write phase P for all points on the disk for nominal disk spindle frequency ν0.
A process according to a first embodiment of the invention is illustrated with reference to
After the initial bootstrap servo has been recorded, a frequency map can be created in a step 804 and a phase map can be created in a step 806. Application of the frequency F and phase P learning algorithm at selected radial locations is used to create an initial map F(r,θ) of frequencies and map of phases P(r,θ), where r and θ denote the radius and azimuthal angle relative to a servo mark on the course bootstrap servo pattern.
During the operation of the disk drive 100 (
F
—
w=(ν/ν0)*F(r—r+RW_off(r—r),θ—r+2*pi*ν*(t+t0)+RW_separation(r—r),
and
P
—
w=P(r—r+RW_off(r—r),θ—r+2*pi*ν*(t+t0)+RW_separation(r—r).
The symbol t in the above equations is the time after the write process has started and t0 represents any latencies between the time when θ_r was determined and the time when the write process started.
Next, in a step 808 a synchronized servo is written. The information of F and P found above in steps 802-806 is used to improve the servo by rewriting the servo in a synchronized fashion. This may be done, for instance, by using the coarse servo on track (n−1) to write a high quality synchronized servo at track n, and so forth. In step 810 a refined frequency map is generated and in step 812 a refined phase map is generated. Then, in a decision step 814 a determination is made as to whether the change in frequency F is greater than an allowable change dF or whether a the change in phase P is greater than an allowable change in phase dF. If the answer to either of these questions is yes, then the process returns to reiterate steps 810 and 812. If the answer to either of these questions is no, then the process can terminate 816. Once F(r,θ) and P(r,θ) have been determined for several locations, knowledge of the original template layout can be used to extrapolate F and P to all radii and angles. This also determines the run-out between the templated pattern and the disk rotation axes, which will be discussed further herein below. The above, therefore, describes an iterative process for determining an optimal frequency F and optimal phase P that is synchronized with the grain lattice of a highly ordered templated media.
Several embodiments are available for an algorithm for learning frequency F and phase P at a particular disk location r and θ. One such embodiment is illustrated with reference to
The written patterns could also be a Pseudo-Random Bit Sequence (PRBS) sequences. In that case the phase that has the lowest bit error rate (BER) would be picked as P=P1. The algorithm may be applied during servo writing. The algorithm may also be applied during other operations of the drive, for instance, in cases of data recovery from drives with corrupted servo regions. In this case, the servo capability may be rebuilt from the underlying template lattice by applying the algorithm. Hence, the present application can be useful for enhancing the robustness of high-value data, such as data from flight recorders.
In a step 1002, a pattern is recorded with B in the range of [B0−Db:dB:B0+dB]. This pattern can be recorded as square waves. Then, in a step 1004, a spectra is measured and a sideband frequency is found. Then, in a step 1006 B1 is determined by finding a crossing point of sideband and B in the spectra. In a step 1008 a pattern is recorded using the B1 adjusted phase in the range of P=P0−dP:dP:P0+dP]. Again the pattern can be recorded as square waves. Then in
The present invention also provides a means for determining F(r,θ) and P(r,θ), as well as the run-out, for the whole disk using a finite number of measurements at different r and θ values. The number of servo wedges that are written is chosen based on the knowledge of the maximum expected run-out that is typical for the drive manufacturing. The number of servo wedges is picked appropriately with a minimum of two servo wedges within each zone. For example, current server drives may have 300-500 servo sectors, which generally would provide sufficient resolution for implementing the invention. In the absence of run-out, the lattice frequency F would be constant between consecutive servo wedges at the same radius. However, run-out causes F to vary periodically with θ, but with jumps in F and P, when a zone boundary is crossed. For determining run-out, it is sufficient to evaluate the behavior of F. An example of how the F information looks when zone boundaries are crossed is given in the
An algorithm and method are described herein for determining the effective run-out of the template. It is noted that the dependence of the lattice frequency on θ depends on many parameters, including:
LO=natural spatial period of the lattice
L=lattice period as seen by the moving head
α=expansion fraction
φ=lattice skew at template radius R
n=zone index
dZn=separation between zones n and n−1 in the radial direction
R0=radius for first zone index, as measured from center of the template
R=radial location of the write head, as measured from center of template
r=radial location of the write head, as measured from the center of disk rotation
ν0=disk rotational frequency
rro=distance between center of disk rotation axis and center of template axis
θ0=angle at which run-out is maximal.
These parameters are further illustrated with reference to
The parameter R can be found as:
R=(r̂2+rrô2+2*r*rro*cos(θ−θ0))̂0.5
The parameter L can be found as:
L=(LO/cos(φ(R))*(R/(Rn+dZn/2))
The parameter F(r,θ) can be found as:
F(r,θ)=2*π*ν*r/L.
The above equations can be used in the following way to determine R, rro and θ0 by fitting the F(r,θ) measurement as exemplified by the disk schematic of
Since the layout of the template is known, the fits of F for a single r determine, in principle, the frequency F(r,θ) and zone boundary locations for the rest of the disk. The measurements can be further refined by re-measuring repeatedly at different radii until F and P have been determined with sufficient precision for the whole disk. The degree of accuracy depends on how write synchronization is implemented and in particular how frequently the optimal write phase is updated throughout the write process. As an example, we assume the write phase error can be at most 10% (1−sigma). If data is written in blocks of 4086 Bytes, then ((δF*32768)̂0.5<0.1. If the error is divided evenly, then the requirement is δP<0.07 and δF<2e−6. The procedure discussed above is iterated until the fit errors fall below these values. The fitting procedure can be modified to account for known RRO and distortions in the template. This amounts to adding correction terms to R, L, and n in the equations above as part of the fit.
While various embodiments have been described above, it should be understood that they have been presented by way of example only and not limitation. Other embodiments falling within the scope of the invention may also become apparent to those skilled in the art. Thus, the breadth and scope of the invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.
