Bit patterned magnetic storage medium

A magnetic storage medium comprises a plurality of discrete magnetic elements and first and second adjoining servo sectors. Each of the servo sectors comprises first and second rows of the discrete magnetic elements extending in a track direction. The second row of the discrete magnetic elements are stacked relative to the discrete magnetic elements of the first row in a cross-track direction that is perpendicular to the track direction. The discrete magnetic elements of the first servo sector are staggered in the cross-track direction relative to the discrete magnetic elements of the second servo sector.

BACKGROUND

The present disclosure relates to data storage devices and, more particularly, but not by limitation, to bit patterned magnetic storage media and magnetic storage devices utilizing the magnetic storage media.

In a conventional magnetic disc drive, data are stored on one or more discs, which are coated with a thin magnetically hard layer. The magnetic layer itself is composed of a single sheet of very fine, single-domain grains. Every information bit of data is stored by many grains. This granular recording medium is typically divided into a plurality of generally parallel data tracks, which are arranged concentrically with one another perpendicular to the disc radius.

To guarantee good signal-to-noise ratio using such conventional magnetic media, the number of grains in every bit should be above a certain level. In order to save more bits on the given disc area (i.e., to increase the areal density), the size of a single grain is decreased. This is called scaling.

It is understood that scaling is limited by the onset of superparamagnetism: if the grain size is too small, the magnetization of the grains can randomly change direction under the influence of thermal energy. At this grain size, information can no longer be stored reliably. Superparamagnetism puts an areal density limit of 0.5 Tb/in2for perpendicular recording.

This areal density limit could be exceeded through the use of bit patterned media. In a patterned medium recording, ordered arrays of discrete magnetic elements are used to store data. The magnetic elements are separated from each other by a non-magnetic material. Each of the discrete magnetic elements, or islands, is a single grain and stores one information bit.

Unlike the conventional magnetic media, which requires the alignment of many adjoining magnetic grains that must each be magnetized into the same polarity (either positive or negative) to define a bit of data, the adjacent magnetic elements of the bit patterned magnetic storage media can have the same or different magnetic polarities due to their separation from each other by the non-magnetic material. This difference should allow bit patterned media to have a much higher areal density than the conventional magnetic recording media.

The precise placement of the head relative to tracks of the media is advantageous in using high areal density recordings having bit patterned media. Unfortunately, conventional null patterns used by servo systems for controlling the head position relative to tracks of the granular magnetic medium are incompatible with bit patterned magnetic storage media.

Aspects of the present embodiments provide solutions to these and other problems.

SUMMARY

Exemplary embodiments are directed to a bit pattern magnetic storage medium. In one embodiment, the magnetic storage medium comprises first and second adjoining servo sectors. Each of the first and second servo sectors comprise first and second rows of the discrete magnetic elements extending in a track direction. The second row of the discrete magnetic elements have positions along the track direction that are substantially aligned with corresponding positions of the discrete magnetic elements of the first row. The discrete magnetic elements of the first servo sector have positions along the track direction that are offset with corresponding positions of the discrete magnetic elements of the second servo sector.

These and various other features will be apparent upon reading of the following detailed description and review of the associated drawings.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

FIG. 1is an isometric view of an exemplary disc drive100in accordance with one or more embodiments of the invention. Disc drive100includes a housing with a base102and a top cover (not shown). Disc drive100further includes a disc pack104, which is rotatably mounted to the base102. In one embodiment, the disc pack104is mounted on a spindle motor (not shown) by a disc clamp106. The disc pack104includes a plurality of individual discs108, which are mounted for co-rotation about central (spin) axis110in a direction112. In one embodiment, one or more of the discs108comprise a bit patterned recording surface114and form a magnetic storage medium, which will be described below in greater detail

Each disc108has an associated disc head slider116which is mounted to the disc drive100for communication with the patterned recording surface114. In the example shown inFIG. 1, the sliders116are supported by suspensions118which are in turn attached to track accessing arms120of an actuator122. The actuator shown inFIG. 1is of the type known as a rotary moving coil actuator and includes a voice coil motor (VCM), shown generally at124. The voice coil motor124rotates the actuator122with its attached heads116about a pivot shaft126to position the slider116and its corresponding read and/or write transducing heads (i.e., transducers)127over a desired data track of the surface114along an arcuate path128between a disc inner diameter130and a disc outer diameter132. The voice coil motor124is driven by a servo system134based on readback signals generated by the transducing head and an embedded controller in the disc drive.

FIG. 2illustrates a simplified sectional isometric view andFIG. 3is a simplified top plan view of a portion of an exemplary patterned magnetic recording (or storage) medium140in accordance with one or more embodiments of the invention. One embodiment of the magnetic storage medium140comprises a substrate142on which the patterned recording surface, layer or film114is applied.

One embodiment of the magnetic recording medium140comprises an ordered array of patterned discrete magnetic elements or islands146in the recording layer114. In an exemplary embodiment, the term “discrete magnetic elements” means that each of the magnetic elements146are separated from each other by non-magnetic material148or gaps. In one embodiment, the term “magnetic” means ferromagnetic or ferrimagnetic. In one embodiment, the term “non-magnetic” means paramagnetic, antiferromagnetic or diamagnetic, and also includes weakly magnetic materials.

The discrete magnetic elements146can be formed using any suitable technique. The shape of the magnetic elements can be oval (FIG. 2), circular, irregular, rectangular or any other desired shape. While the discrete magnetic elements146will generally be depicted as having a circular shape, it is understood that the elements146can take on other shapes. Additionally, the size of the depicted discrete magnetic elements146in the drawings is not to scale. The discrete magnetic elements146are represented inFIG. 3and subsequent figures by the shaded portions, while the non-magnetic material148is generally represented by the non-shaded portions.

A discrete magnetic element146can be used, for example, to represent a single bit of data based on the magnetic polarity of the element. For example, a discrete magnetic element having a designated positive polarity can represent a logical zero while a magnetic element having a negative polarity, which is opposite the positive polarity, represents a logical 1. The particular orientation and direction of the domain of the magnetic elements that represents the positive or negative polarity, can be selected as desired based on the recording technique that is used. Additionally, in accordance with one embodiment, the magnetic fields generated by the magnetic elements have substantially the same magnitude.

One embodiment of the magnetic recording medium140, shown inFIG. 3, comprises one or more data tracks or segments150. Each of the data tracks150comprises an array of the magnetic elements146. In one embodiment, the data tracks include a first row152of the discrete magnetic elements146and a second row154of the discrete magnetic elements146. In one embodiment, the rows152and154extend in a lengthwise or track direction156. In one example, the track direction156is perpendicular to a cross-track direction160, as shown inFIG. 3.

A partial top plan view of the magnetic recording medium140in the form of a disc108is illustrated inFIG. 4. In accordance with this example, the data tracks150are generally concentric to the central axis110. Thus, in this example, the track direction is generally aligned with the track path that is concentric to the axis110. One embodiment of the cross-track direction160is generally aligned in the radial direction from the axis110, as shown inFIG. 4.

In one embodiment, the adjoining rows152and154of the discrete magnetic elements146of each data track150each have positions along the track direction156. In one embodiment, the positions of the elements146of the row154along the track direction156are substantially aligned with the corresponding positions of the elements146in the row152, as shown inFIGS. 3 and 4. Each of the discrete magnetic elements146of the row152are substantially aligned in the cross-track direction160with one of the discrete magnetic elements of the row154of the same track150.

In one exemplary embodiment, an element146or a position of the element146is “substantially aligned” with a corresponding element146in a given direction (e.g., cross-track direction), when a projection of the element146in the given direction overlaps at least 70% of the length of the corresponding element146measured in a direction (e.g., track direction) that is perpendicular to the given direction. For example, each element146A in the row154is substantially aligned with a corresponding element146B in the row152of the same track150in the cross-track direction160, because a projection, in the cross-track direction160, of the length LAof the element146A measured in the track direction156overlaps at least 70% (100% in this example) of the length LB, measured in the track direction156, of the corresponding element146B, as shown inFIG. 3.

In one embodiment, an element146is considered substantially aligned with a corresponding element146in a given direction (e.g., track or cross-track direction), when a projection of the element146in the given direction overlaps at least 80% of the length of the corresponding element146measured in a direction that is perpendicular to the given direction. In one embodiment, an element146is considered substantially aligned with a corresponding element146in a given direction (e.g., track or cross-track direction), when a projection of the element146in the given direction overlaps at least 90% of the length of the corresponding element146measured in a direction that is perpendicular to the given direction. In one embodiment, an element146is considered substantially aligned with a corresponding element146in a given direction (e.g., track or cross-track direction), when a projection of the element146in the given direction overlaps at least 95% of the length of the corresponding element146measured in a direction that is perpendicular to the given direction. In one embodiment, an element146is considered substantially aligned with a corresponding element146in a given direction (e.g., track or cross-track direction), when a projection of the element146in the given direction overlaps 100% of the length of the corresponding element146measured in a direction that is perpendicular to the given direction.

In one embodiment, the discrete magnetic elements146of a row, such as rows152and154, are substantially aligned in the track direction156.

In one embodiment, the discrete magnetic elements146of the rows152and154of a data track N have positions along the track direction156that are offset with corresponding positions of the discrete magnetic elements146of the rows152and154of the adjoining tracks N+1 and/or N−1, as shown inFIG. 3. In one exemplary embodiment, the term “offset” means not substantially aligned. Thus, for example, in one embodiment an element146or a position of the element146is offset with a corresponding element146in a given direction (e.g., track or cross-track direction), when a projection of the element146in the given direction overlaps less than 70% of the length of the corresponding element146measured in a direction (e.g., track direction) that is perpendicular to the given direction. For example, the element146C in the row154of track N is offset from the corresponding or adjacent elements146D in the row152of the adjoining track N+1, because a projection, in the cross-track direction160, of the length LCof the element146C measured in the track direction156does not overlap at least 70% of the length LD, measured in the track direction156, of the corresponding element146D, as shown inFIG. 3. In the example ofFIG. 4, the elements146of the rows152and154of the data track N have angular positions along the track direction156that are angularly offset with corresponding positions of the discrete magnetic elements146of the rows152and154of the adjoining track N−1.

The areal density of bit patterned media, such as medium140, is expected to be high, which means that the size of the discrete magnetic elements146should be small. For example, the areal density of 1 terabit/in2requires the length and width dimension of the elements146to be approximately less than thirteen nanometers. In order to correctly position the read/write head116over such small magnetic elements146, the servo system134should precisely measure the current position of the transducer(s)127, particularly the cross-track position relative to the center158of the track150between the rows152and154.

FIG. 5is a block diagram of an exemplary servo system134in accordance with one or more embodiments of the invention. As mentioned above, the servo system134operates to control a position of a transducing read head, which may by carried by the slider116(FIG. 1), relative to one of the tracks150of the magnetic recording medium140using servo sector data stored in the tracks150of the media140. The servo system134is arranged as a control loop that includes a controller170, a plant172, a servo demodulator174, and a summing junction176.

The summing junction176receives a reference position signal178and a position estimate output signal180. The reference position signal178indicates a desired head position relative to the center of a track150that is being read by the read head transducer. The summing junction176calculates the difference between the desired and estimated signals178and180to provide an error output182that is representative of a desired adjustment of the position of the head slider116.

The error output182is provided to the controller170, which in turn produces a control output184for the plant172. The plant172produces a control signal187in response to the control output184that directs the actuator122(FIG. 1) to move the slider116carrying the transducing head toward the desired position indicated by the signal178.

The plant172represents the magnetic recording system whose servo sector output signal186is a readback signal with servo specific position information. The readback signal is produced in response to the sensing of servo sector data on the recording media140, such as the disc108, by the transducing read head127of the slider116. As will be discussed below, embodiments of the servo sector include recorded position data for each track including one or more servo null or burst patterns that are used to generate a position error signal in the readback signal186that indicates a position of the head relative to a center158of the current data track150. Accordingly, the readback signal186corresponding to the servo sector can be used to obtain current position data for the transducer(s)127of the head slider including a current track and a location of the transducer(s)127relative to a center158of the current track.

The readback signal186is provided to the servo demodulator174, which includes circuitry that demodulates and decodes the position data to extract the position error signal and the current track position, which is provided in the position estimate output signal180. When the desired position of the head indicated by the reference signal178is set to zero for track center, the difference between the position estimate signal180and the reference or desired position signal178will be the position error signal once the head is positioned over the desired track. However, the desired position of the head may be an offset value from the center of the track. As a result, once the head is positioned over the desired track, the error output182may consist of a difference between a desired position error signal representative of a desired position within the track, and the actual or estimated position error signal produced by the servo demodulator174that is representative of the current position of the head relative to a center of the track.

FIG. 6is a simplified diagram illustrating portions of multiple data tracks150of the magnetic recording medium140in accordance with one or more embodiments of the invention. Each track150contains data sectors190, in which the magnetic elements146represent bits of data based on their magnetic polarity. The bits of data can be written and read by the transducers127of the head116(FIG. 1). In one embodiment, the data tracks150comprise one or more servo sectors192, which contain information used by the servo system134to control the position of the transducing head.

In one embodiment, the servo sector192includes one or more null patterns194, such as a first null pattern194A (PS1) and a second null pattern194B (PS2). Additionally, the servo sector192can include a gray-coded track identification196that identifies the particular track150of the medium140that the head is positioned over. The servo sector192can include other information as well.

FIG. 7is a simplified illustration of a portion of servo sectors192of three consecutive tracks150of the magnetic recording medium140, in accordance with one or more embodiments of the invention. Track N−1 comprises servo sector192(N−1), which adjoins the servo sector192(N) of track N, which adjoins the servo sector192(N+1) of track N+1. Each of the servo sectors192comprise a plurality of the discrete magnetic elements146having a positive or negative polarity. InFIG. 7and subsequent figures, the magnetic polarity of the magnetic elements146is indicated by the corresponding arrows pointing in either the upward direction or the downward direction relative to the page. For example, an arrow pointing in the upward direction can represent a positive polarity while an arrow pointing in the downward direction can represent a negative polarity. It should be understood that the actual magnetic field of the elements146could be oriented vertically or horizontally relative to the patterned recording layer114(FIG. 2).

In one embodiment, the servo sectors192include a first null pattern194A that comprises rows152A and154A of the discrete magnetic elements146. As discussed above, the rows152A and154A of the discrete magnetic elements146are substantially aligned in the track direction156. In one embodiment, the magnetic polarity of the discrete magnetic elements146of the row152A is opposite the magnetic polarity of the discrete magnetic elements146of the row154A, which forms the null pattern194A for each data track150. In one embodiment, the discrete magnetic elements in the row152A all have the same magnetic polarity. In one embodiment, the discrete magnetic elements146in the row154A all have the same magnetic polarity. As mentioned above, in one embodiment, the discrete magnetic elements146of the row152A are substantially aligned with the discrete magnetic elements146of the row154A of the same track150(i.e., N, N−1 or N+1) in the cross-track direction160, as illustrated inFIG. 7.

In accordance with one embodiment, the positions of the elements146along the track direction156of the servo sectors192of the null pattern194A of a track150are offset with the positions along the track direction156of the corresponding elements146of the adjoining tracks150, as shown inFIG. 7. That is, for instance, the projections of the magnetic elements146of the rows152A and154A of track150(N) in the cross-track direction160are offset in the track direction156relative to the discrete magnetic elements146of the adjoining servo sector192(N−1) of track150(N−1) and the discrete magnetic elements146of the servo sector192(N+1) of track150(N+1), as illustrated inFIG. 7.

In one embodiment, the magnetic polarity of the discrete magnetic elements146of the row152A matches the magnetic polarity of the discrete magnetic elements146of the adjoining row154A of the adjoining servo sector192. For instance, the magnetic polarity of the discrete magnetic elements152A of the servo sector192(N) matches the magnetic polarity of the discrete magnetic elements146of the row154A of the adjoining servo sector192(N−1), as shown inFIG. 7. Likewise, the magnetic polarity of the discrete magnetic elements154A of the servo sector192(N) matches the magnetic polarity of the discrete magnetic elements146of the row152A of the adjoining servo sector192(N+1), as shown inFIG. 7.

In one embodiment, the servo sectors192of each data track150comprises a second null pattern194B that adjoins the first null pattern194A on a down-track side195of the first null pattern194A, as shown inFIG. 7. The term “down-track side of the first null pattern,” as used herein, means in the track direction156from the first null pattern on a side that is downstream of the first null pattern as viewed by the transducer127during operation of the data storage medium. In one embodiment, the null pattern194B comprises at least one row154B of the discrete magnetic elements146that is located proximate to the center158of each track150.

In one embodiment, the second null pattern194B includes rows152B and154B of the discrete magnetic elements146that are offset in the cross-track direction160relative to the rows152A and154A of the null pattern194A, as shown inFIG. 7. That is, for example, the projections of the magnetic elements146of the rows152B and154B in the track direction156are offset in the cross-track direction160relative to the positions of the discrete magnetic elements146of the rows152A and154A of the null pattern194A along the cross-track direction160, as illustrated inFIG. 7.

In one embodiment, the magnetic elements146of each row152B of the null pattern194B are offset in the cross-track direction160relative to the magnetic elements146of an adjoining row154B of the null pattern194B, as shown inFIG. 7. That is, for example, the magnetic elements146of the row152B each have positions along the track direction156that are offset relative to the corresponding positions of an adjoining row154B.

In one embodiment, the magnetic elements146of each row152B of the null pattern194B are substantially aligned in the cross-track direction160relative to the magnetic elements146of an adjoining row154B of the null pattern194B, as shown inFIG. 7. For example, each of the magnetic elements146have a position along the track direction156that is substantially aligned with a corresponding position of one of the discrete magnetic elements146of an adjoining row152B.

In one embodiment, the discrete magnetic elements146of the row152B of the null pattern194B each have the same magnetic polarity. In one embodiment, the discrete magnetic elements146of the row154B of the null pattern194B each have the same magnetic polarity. In one embodiment, the magnetic polarity of the discrete magnetic elements146of the row152B is opposite that of the discrete magnetic elements146of the adjoining row154B that are substantially aligned in the cross-track direction160relative to the magnetic elements146of the row152B. In one embodiment, the magnetic polarity of the discrete magnetic elements146of the row152B is opposite that of the discrete magnetic elements146of the adjoining row154B that are offset in the cross-track direction160relative to the magnetic elements146of the row152B.

The slider116(FIG. 1) carries a read head and a write head, each of which include a transducer127respectively for reading the data bits represented by the magnetic polarity of the magnetic elements146, and setting the magnetic polarity of the magnetic elements146to write the data bits. The read and write heads each carry a transducer127for performing the desired read or write operation.FIG. 8is a simplified illustration of a read or read/write head204positioned over the medium140that includes the null pattern194A described above.

The read head204travels over the recording medium140and generates the readback signal186in response to the sensing of the magnetic fields of the discrete magnetic elements146that are in close proximity to the head204(e.g., directly below the head). InFIG. 8, ε represents the position of the head204(e.g., center of the head) relative to the desired location (e.g., the center) within the track identified by the reference position signal178(FIG. 5), which here is the center158of the track N.

Coarse head position adjustments can initially be made to place the head204within the desired track N using the gray-coded track identifications196(FIG. 6). Following such a coarse head position adjustment, the observable range of the head position ε is generally limited to that provided in Equation 1.
εε[−0.5Wtrack,0.5Wtrack]  Eq. 1

For the given ε range in Eq. 1, the readback signal of the null pattern194in Track N is

y⁡(k)=α1⁡(ɛ)T⁢y1⁡(k)+α2⁡(ɛ)T⁢y2⁡(k)Eq.⁢2
where y1(k) and y2(k) represent the readback signal respectively generated by the first row152and the second row154of the desired track N at column k (explained below), and α1(ε) and α2(ε) are the portions of the width of the head (Whead) that overlap the first row152and the second row154of track N, respectively. T represents the size of the magnetic elements146. k represents the column of the servo sector over which the transducer127is positioned. The column k corresponds to the stacked pairs of the magnetic elements146in rows152and154of the servo sector192.

Because of the stacked formation of the magnetic elements146in the rows152and154of the null pattern194A, {y1(k)} and {y2(k)} result in a substantially time invariant readback signal186as the head204passes over the null pattern194A and when the head204is within a given track150and when the read signal186is properly sampled. When the head204is centered within a given track150(i.e., ε=0), the readback signal186is that is generated by the head204is zero as it passes over the null pattern194A moving from the first pair (i.e., first column k=1) of the magnetic elements146of the rows152and154to the last pair (i.e., last column k=i) of the magnetic elements146. When the position of the head204overlaps adjoining tracks150, the readback signal186that is generated as the head204passes over the null pattern194A is generally time variant due to the staggered magnetic elements146of the adjoining rows152and154.

FIG. 9is a flowchart illustrating a method of using the null pattern of the medium140to adjust the position of the read head204relative to the medium, in accordance with one or more embodiments of the invention. In one embodiment of the method, a magnetic storage medium140, which is formed in accordance with one or more embodiments of the invention described above, is provided at step210. In one embodiment, the magnetic storage medium140comprises a plurality of discrete magnetic elements146and adjoining servo sectors, such as192(N) and192(N−1) (FIGS. 4 and 7). In one example, each of the servo sectors192include a row152of the discrete magnetic elements extending in a track direction156and a row154of the discrete magnetic elements154, which are substantially aligned with the discrete magnetic elements146of the row152in a cross-track direction160. In one example, the discrete magnetic elements146of the servo sector192(N) are offset in the cross-track direction160relative to the discrete magnetic elements146of the servo sector192(N−1).

At step212, a readback signal186is generated using the transducer127responsive to relative movement between the transducer and the magnetic elements146of at least one of the servo sectors192(N) and192(N−1). When the medium140is in the limn of a disc108, the disc108rotates relative to the slider116causing the relative movement between the magnetic elements146of the servo sectors192and the transducer127.

At step214, a demodulated position signal (180) corresponding to the position of the head204relative to the target track150is generated based on the readback signal samples. In one embodiment of step214, the demodulated position signal is generated based on a summation of the readback signal samples213.

At step216, the transducer127is moved relative to the magnetic storage medium140based on the position signal180. For the magnetic storage device in the form of the disc drive100(FIG. 1), the transducer127is moved along the arc128responsive to the position signal180to place the read or write transducer127over the desired track150. The desired read or write operation within the track150can then begin on the data sectors190of the track150.

In one embodiment of the method, the magnetic recording medium140includes both the null pattern194A and the null pattern194B described above with reference toFIG. 7. In one embodiment, step212comprises generating the readback signal samples213corresponding to the discrete magnetic elements146of the null pattern194A, adjusting the readback signal samples by multiplying the readback signal samples by a digital reference and summing the adjusted readback signal samples to form the sum A. The digital reference waveform is defined as −1k (where k=1 to n and n is the maximum number of samples of the null pattern194A) when we have only one sample per element146. For higher sample rates the digital waveform will be accordingly defined as an alternating series of m/2 (1)'s and m/2 (−1)'s, where m is the number of samples per every element period. Additionally, the step212comprises generating the readback signal samples corresponding to the discrete magnetic elements146of the null pattern194B, adjusting the readback signal samples by multiplying the readback signal samples by a digital reference and summing the adjusted readback signal samples to form the sum B. At step214, the demodulated position signal is generated based on the sums A and B. When the sum A is zero, it is known that the transducer127is positioned in the center158of the track150. However, when the sum A is non-zero and the transducer is not positioned in the center158of the track150. The sum A and B is used to determine the magnitude of the distance and the direction on which the transducer127is located.

Another embodiment of the invention is directed to a method of forming the null pattern194A on the medium140, which will be described with reference toFIG. 8and the flowchart ofFIG. 10. At220of the method, the discrete magnetic elements146of the medium140are each set to the same magnetic polarity, as indicated at220. In one embodiment, this is accomplished by applying a magnetic field to the medium140that aligns the magnetic domains of the elements146in the same direction. Although not illustrated inFIG. 8, at this stage, the magnetic polarity arrows of each of the elements146would point in the same direction.

Next, at222, the read/write head223carried by the slider116(FIG. 1) is calibrated using the medium140in accordance with conventional techniques. This calibration of the head223includes writing test patterns to the elements146, which are used to determine the offset distance DOin the cross-track direction160between the read transducer127R and the write transducer127W of the head223, which are shown in phantom. The offset distance DO, which typically spans multiple tracks150, is not shown to scale inFIG. 8. The offset distance DOis used to establish a location of the write transducer127W from the location of the read transducer127R, which is determined through the reading of the magnetic elements146.

At224, the write transducer127W is positioned between two adjoining tracks150, such as the location represented by line226, using the location of the read transducer127R and the offset distance DO. At228, a write operation commences that reverse the polarity of the magnetic elements146in the rows154and152that adjoin the location226of the write transducer127W. At230, the location of the write transducer127is moved in the cross-track direction160a distance equal to twice the distance DTseparating adjoining tracks150. Steps228and230are then repeated until the pattern194A is formed on the medium140.