Patent ID: 12254907

DETAILED DESCRIPTION

FIG.1illustrates a data storage device including a read/write head assembly120for writing data on a magnetic storage medium. Data storage device100includes magnetic media108and read/write head assembly120. Magnetic media108is a magnetic storage disc on which data bits can be recorded and read using read and write elements on read/write head assembly120. As illustrated in View A ofFIG.1, magnetic media108includes inner diameter104and outer diameter107. A number of concentric tracks (e.g., data track110) are located between inner diameter104and outer diameter107. Data may be written to or read from by read/write head assembly120at respective bit locations as magnetic media108rotates about a spindle center or magnetic media108axis of rotation112.

Read/write head assembly120is mounted on actuator arm109at an end distal to actuator axis114. Read/write head assembly120flies in close proximity above the surface of magnetic media108during magnetic media108rotation. Actuator arm109rotates during a seek operation about actuator axis114which positions read/write head assembly120over a target data track (e.g., data track110) for read and write operations.

In the example ofFIG.1, read/write head assembly120is a heat-assisted magnetic recording (HAMR) head that includes a heat source applied to a bit location on magnetic media108during recording. By temporarily heating magnetic media108during the recording process, the magnetic coercivity of the magnetic grains in magnetic media108can be selectively lowered. This reduction in the magnetic coercivity of the magnetic grains can be achieved in a tightly focused area of magnetic media108, substantially corresponding to an individual bit. The heated region is then encoded with the recorded data bit, based on the polarity of an applied magnetic write field. After cooling, the magnetic coercivity substantially returns to its pre-heated level, thereby stabilizing the magnetization for that data bit. After being recorded, such data bits can be read using a magnetoresistive read head.

Referring to View B ofFIG.1, read/write head assembly120includes, among other features, heat source132(e.g., a laser) coupled to submount assembly134. Light from heat source132is directed into waveguide138mounted to slider143. Light exiting the waveguide is focused via near-field transducer (NFT)144and applied to a bit location on magnetic media108while the bit location is subjected to a magnetic field generated by write element130. As air-bearing surface146of read/write head assembly120“flies” across the surface of magnetic media108, write element130selectively magnetizes the underlying magnetic grains of magnetic media108.

Controller106generates control signals to control power to write element130and to control the polarity of the magnetic field generated by write element130. In one example, controller106controls write element130to encode data bits of three logical states on a single pass of write element130over a data track on magnetic media108. That is, write element130encodes data bits with positive magnetic polarity (1), data bits with negative magnetic polarity (−1), and data bits with approximately zero net magnetization.

Referring to View C ofFIG.1, magnetic media108is shown to include at least two magnetic recording layers. View C illustrates magnetic media108with an upper recording layer150and a lower recording layer152. Upper recording layer150and lower recording layer152both include magnetic material FePt or an alloy thereof. An interface layer154separates upper recording layer150and lower recording layer152and may have different properties in different implementations.

In one implantation, upper recording layer150and lower recording layer152include granular material (e.g., material having magnetic grains separated from one another by a non-magnetic material). During a manufacturing process, the individual grains in upper recording layer150are grown on top of corresponding individual grains of lower recording layer152. In at least one implementation, the magnetic grains in upper recording layer150are each aligned, in a 1-to-1 configuration, with a corresponding single grain in lower recording layer152. The magnetic grains in both layers may be substantially the same in size (e.g., within +/−10% of one another) such that boundaries between the magnetic grains in lower recording layer152substantially align with boundaries between the grains in upper recording layer150(e.g., grain centers or grain edges are aligned within +/−10%).

View C illustrates4data bits that have been written in sequence during a single pass of read/write head assembly120over magnetic media108. Each data bit is represented by a pair of vertically stacked arrows that further represent multiple magnetic grains in a tightly focused area. From left to right, View C shows a sequence of data bits in the corresponding states 1, 1, 0, and −1. The ‘1’ state data bits each have a positive polarity. That is, substantially all grains in upper recording layer150and in lower recording layer152of the data bit are fixed to have a positive polarity. The ‘−1’ state data bits each have a negative polarity, meaning that substantially all grains in upper recording layer150and in lower recording layer152are fixed to have a negative polarity. The ‘0’ state data bit (e.g., in region140of View C) has a polarity of substantially zero due to each grain in upper recording layer150being fixed to have a polarity that is opposite that of a corresponding (stacked) magnetic grain in lower recording layer152. In View C, the illustrated ‘0’ state data bit140is shown to have a negative polarity in upper recording layer150and a positive polarity in lower recording layer152. While it may be the case that all grains in upper recording layer150of the data bit have negative polarity and all grains in lower recording layer152have positive polarity, it is to be appreciated that this is just one example of magnetic grain orientation that results in substantially net zero polarity within region140. For example, in some implementations, all grains in upper recording layer150of the data bit may have positive polarity and all grains in lower recording layer152may have negative polarity. This configuration would also result in substantially zero net polarity within region140.

The term “substantially zero polarity” is used herein to refer to regions where the individual grains have true substantially net zero polarity (e.g., each grain has an upper layer portion with a polarity opposite in magnitude and sign of that of a corresponding lower layer portion). However, “substantially zero polarity” is also intended to encompass the scenario where the magnetic and/or physical characteristics of the two recording layers are tuned such that the reader on read/write head assembly120detects substantially zero polarity in a given region when the true net polarity of the region is somewhat greater or less than substantially zero. Since upper recording layer150is closer in proximity to read/write head assembly120than lower recording layer152, lower recording layer152may contribute less to the readback signal than upper recording layer150. Consequently, there exist scenarios where the net polarity of the region could be zero but where the reader nevertheless detects a non-zero signal. To adjust for this, some implementations of the disclosed technology may provide for running of lower recording layer152to have an Mrt (magnetization saturation multiplied by recording layer thickness) that is greater than the Mrt of upper recording layer150. This would ensure that the reader detects substantially zero net polarity in the zero state regions. In these cases, the “zero state” regions on magnetic media108may have a true polarity biased toward that of lower recording layer152(due to its greater Mrt), but an effective polarity that is detected by the reader as being zero or substantially zero. This tuning of Mrt may be performed with respect to any of the implementations disclosed herein.

According to the various implementations disclosed herein, the three logical states illustrated in View C may all be written, in entirety, during a single pass of the read/write head above rotating magnetic media108.

The herein disclosed “zero state” write techniques may also be utilized to insert small areas of neutral polarity between positive and negative data bits in either a conventional recording process (e.g., one that performs binary recording) or a recording process using the 3-state recording techniques disclosed herein. By inserting small areas of zero net polarity along the boundaries between data bits, signal-to-noise can be dramatically improved. These small areas of zero net polarity may be referred to as the zero-insertion width (ZIW).

FIG.2illustrates an example magnetic media200that may facilitate writes of zero-state polarity to localized regions of magnetic grains in a HAMR device. Magnetic media200includes at least lower recording layer202, upper recording layer204, and antiferromagnetic coupling (AFC) layer206. In this example, lower recording layer202and upper recording layer204both comprise a recording material (e.g., FePt or an alloy thereof) and may have the same or different magnetic characteristics (e.g., Curie temperature, coercivity, anisotropy). AFC layer206is a metal insertion layer that facilitates a weak antiferromagnetic coupling between upper recording layer204and lower recording layer202. In the absence of an applied magnetic field, the antiferromagnetic coupling causes magnetic grains in upper recording layer204to align with the opposite polarity along the interface to AFC layer206, as shown in region208.

InFIG.2, timesteps t0, t1, and t2illustrate states of a heat source and of a write field applied by a write element that are effective to write corresponding logical states 1, −1 and 0 (from left to right) on the data bits of magnetic media200. At time t0, the heat source is in the “ON” state and the applied write field has a positive polarity. As magnetic media200cools in the presence of the applied positive polarity magnetic field, the magnetization in upper recording layer204and lower recording layer202align to the same direction as the applied write field as it cools and maintains the positive polarity due to the high anisotropy of the FePt grains within the individual layers.

At time t1, the heat source is in the “ON” state and the applied write field is switched to a negative polarity. As magnetic media200cools in the presence of the applied negative polarity magnetic field, the magnetization in upper recording layer204and lower recording layer202align to the same direction as the applied write field as it cools and maintains negative polarity due to the high anisotropy of the FePt grains within the individual layers.

At time t2, the heart source is left in the “ON” state, but the write field is switched off. In this case, the magnetic grains in the underlying region are heated but not subjected to a magnetic field. As the grains in this region cool, there is a small region of AFC coupled magnetic domain (e.g. opposing polarity grains) that start to form near the interface to AFC layer206. As cooling continues, the opposing polarity domain grows within these grains and establishes fully opposing magnetization states between upper and lower layers which is stable at operating temperature due to the high anisotropy of the FePt grains. This effect drives the net magnetization of the individual grains within the corresponding data bit close to zero.

Whereas the implementation ofFIG.2utilizes a weak AFC coupling field and corresponding heat only and no applied field to create regions of net zero polarity (e.g., region208),FIG.3toFIG.10illustrate alternate examples that rely on differences in magnetic characteristics between the two stacked magnetic recording layers to generate regions of zero state polarity. Among other characteristics, these examples provide for a lower Curie temperature in one of the magnetic recording layers than in the other magnetic recording layer. In some examples, the upper recording layer is the layer with the lower Curie temperature. In other examples, the lower recording layer is the layer with the lower Curie temperature.

FIG.3illustrates aspects of an example HAMR device300that writes regions of zero-state polarity by leveraging different thermal magnetic characteristics of an upper recording layer and a lower recording layer to selectively cause certain magnetic reversals to be isolated to the upper recording layer while causing other magnetic reversals to simultaneously occur in both the upper recording layer and in the lower recording layer. InFIG.3, the lower recording layer has a higher Curie temperature than the upper recording layer (e.g., the layer closest to the write element). For this reason, lower recording layer is referred to as high Tc layer302while the upper recording layer is referred to as low Tc layer304.

As write element306flies above rotating underlying magnetic media308, laser311heats a tightly localized underlying region of magnetic media308. Laser311generates a thermal profile312that moves along a data track while data is being recorded to the track. Thermal profile312varies according to a heat gradient having a highest temperature underlying near-field transducer (NFT)320and temperature that decreases with distance from NFT320. While thermal profile312moves along the plane of magnetic media308, higher temperature zone314exists near the center of thermal profile312(e.g., at least partially underlying NFT320) while lower temperature zone316trails higher temperature zone314.

Magnetic media308has characteristics such that higher temperature zone314is within a temperature range sufficient to facilitate magnetic reversals in both low Tc layer304and high Tc layer302, in the presence of an applied magnetic field. In contrast, lower temperature zone316is within a temperature range sufficient to facilitate magnetic reversals in low Tc layer304but not in high Tc layer302in the presence of an applied magnetic field. Because of this, a magnetic grain in low Tc layer304can be recorded for a longer period of time (e.g., as it passes beneath NFT320) than its corresponding (stacked) magnetic grain in high Tc layer302. That is, magnetic grains in low Tc layer304can be recorded when passing through both higher temperature zone314and lower temperature zone316. In contrast, magnetic grains in high Tc layer302can be recorded only when passing through higher temperature zone314.

By example and without limitation, grains in a region324may be initially recorded at a positive polarity state when passing through higher temperature zone314(which simultaneously causes magnetic reversals in underlying region326). Once region324moves into lower temperature zone316, grains within region324may, in some examples, still be recorded without affecting the polarity of the grains in underlying region326. If, for example, the polarity of the magnetic field is switched as region324moves from higher temperature zone314to lower temperature zone316, region324may have data bits that are fixed in a magnetic state opposite that of the underlying grains in region326.

InFIG.3, low Tc layer304and high Tc layer302are separated from one another by break layer310. In some examples, break layer310is a non-magnetic layer thick enough to fully decouple low Tc layer304from high Tc layer302at room temperature. Break layer310may, for example, comprise a dielectric material, ruthenium, platinum, chromium or cobalt-chrome.

Similar to an AFC coupling layer (e.g., AFC layer206ofFIG.2), break layer310is, ideally, a material that provides upper and lower interface characteristics that promote L10lattice growth within the top layer (e.g., low Tc layer304).

In some examples, break layer310is a magnetic layer which may include the same base material as the upper recording layer and the lower recording layer (e.g., FePt) and may serve as a good template for growth of the grains of the upper recording layer. Additionally, break layer310may be granular such that it grows on magnetic grains in the lower recording layer with a 1:1 alignment such that the boundaries between its grains are substantially aligned with the boundaries between grains in the lower recording layer, high Tc layer302. Consequently, the upper recording layer can then be grown such that its magnetic grain boundaries align between upper and lower layers (e.g., because the magnetic grains naturally align with underlying magnetic grains and the grains may be of the same size due to similarities in material composition). When looking at magnetic materials (FePt), these ideal characteristics are easier to satisfy than in non-magnetic materials.

FIG.3is an example of magnetic media with an upper recording layer with lower Curie temperature and lower recording layer with a higher Curie temperature. In some examples, the upper recording layer is the layer with the higher Curie temperature and lower recording layer with lower Curie temperature.

FIG.4illustrates plot400showing characteristics of a HAMR device media suitable for implementing the techniques discussed with respect toFIG.3. The HAMR device includes a magnetic media (not shown) with a structure the same or similar to that shown inFIG.3, including dual recording layers, where the lower recording layer further from the write element has a higher Curie temperature than the upper recording layer closer to the write element. The lower recording layer and the upper recording layer are separated by a break layer that may have characteristics the same or similar to those discussed with respect toFIG.3.

Plot400illustrates thermal characteristics of the media layers as well as recording temperatures employed within a HAMR device. In plot400, horizontal line420illustrates a magnitude of applied magnetic write field (Ha) as the layers of the media undergo changes in magnetic anisotropy and temperature. A first line422illustrates trends in the thermal characteristics for the high Tc layer and a second line424illustrates trends in the thermal characteristics for the low Tc layer. A point labeled “Tc_high” marks the Curie temperature of the high Tc layer (lower layer in the example ofFIG.4) and a point labeled “Tc_low” marks the Curie temperature of the low Tc layer (upper layer in the example ofFIG.4). Similarly, a point labeled “Tr_high” marks the recording temperature of the high Tc layer (lower layer) and a point labeled “Tr_low” marks the recording temperature of the low Tc layer (upper layer). Tc and Tr values for individual layers are highly correlated and the Tr value is offset from the Tc value depending on the magnitude of applied magnetic write field, Ha. However, since both upper layer and lower layer “see” the same field magnitude, the difference in Tr values between the two layers will remain essentially constant regardless of applied magnetic write field value.

For each of the high Tc layer (lower layer) and the low Tc layer (upper layer), there exists a distinct temperature range in which magnetic reversals can occur in the presence of applied magnetic field (Ha). Between Tr_high and Tc_high, magnetic reversals are possible for the high Tc layer (lower recording layer). Between Tr_low and Tc_low, magnetic reversals are possible for the low Tc layer (upper recording layer).

These temperature ranges depend upon the anisotropy (Hk) and the Curie temperature of the material in each recording layer. In general, magnetic reversals of individual grains cannot occur above a layer's Curie temperature. As the material cools down below the Curie temperature, the magnetic moment of the material gradually increases and, at the same time, the magnetic field required to flip the direction of the moment from its current orientation also increases as the media anisotropy increases. Therefore, if the layer is in the presence of a magnetic field when its temperature drops below the Curie temperature of the layer, the layer will be magnetized in the direction of the applied write field and the layer's magnetic moment will increase, thus locking in the magnetization direction as the layer cools down to operating temperatures. If the direction of the applied write field is then reversed while the same layer continues to cool, the developed magnetic moment then switches to the direction of the newly applied write field provided that the layer's anisotropy (Hk) has not yet increased beyond the strength of the applied write field (e.g., applied write field as represented by line420inFIG.4).

If a given one of the layers has cooled enough that the layer's Hk is larger than the applied write field at the time of the field reversal, the moment will not be switched, and the previous magnetization direction is “frozen in”. If, however, the temperature is still high enough that the Hk of the material is still less than the applied write field, then whatever moment has developed will switch to the new applied write field direction.

Given that for any magnetic material, it is possible to readily determine a corresponding temperature range in which magnetization reversals are possible, it is also possible to select materials for magnetic recording layers that allow for matching of these temperature ranges to temperature zones within a thermal profile created by a recording head in a HAMR device to realize 3-state recording techniques.

For example, a top-down thermal profile410created by the HAMR write element includes higher temperature zone412bounded by a contour line at the temperature Tr_high and a lower temperature zone414bounded by a contour line at the temperature Tr_low. When a magnetic grain is heated to temperature Tr_high, magnetic reversals may be realized in both the high Tc layer and the low Tc layer. When a magnetic grain is heated to the temperature Tr_low, magnetic reversals may be realized in the low Tc layer but not in the high Tc layer. Therefore, as a data bit travels through thermal profile412, both recording layers can be written at Tr_high. However, by the time the data bit reaches Tr_low, the magnetization of the high Tc layer is “locked in” while the magnetization of the low Tc layer is still subject to change.

FIG.5A-5Dillustrate operations performed by a HAMR device having the characteristics described with respect toFIGS.3and4. That is, the HAMR device includes a magnetic media with dual recording layers including a low Tc layer502and a high Tc layer504separated by a break layer510. The HAMR device includes a magnetic media508that generates a thermal profile512with characteristics the same or similar to that described with respect toFIG.4relative to temperature zones in which reversals are possible for each of the two layers. This thermal profile512includes a higher temperature zone514and a lower temperature zone506. An outer edge of the higher temperature zone514corresponds to a recording temperature Tr_high and an outer edge of the lower temperature zone506corresponds to a recording temperature Tr_low, where Tr_low and Tr_high may be defined as inFIG.4.

FIG.5Aillustrates a cross-sectional view of media layers during a first example recording operation500for writing a zero-state data bit in a HAMR device. Here a first localized region ‘A’ is passing through higher temperature zone514of the media and is cooling to temperature Tr_high while a positive polarity field518is applied. Since temperature Tr_high is sufficient to facilitate magnetic reversals in both low Tc layer502and high Tc layer504of region ‘A’, magnetic grains are positively polarized in both layers.

FIG.5Billustrates a second example recording operation501following that ofFIG.5Aand recording operation501. Here, the media has rotated slightly such that the read-write head has shifted in the down-track position relative to magnetic media508and the heat element is now positioned over another localized region ‘B’. Since positive write field518is still being applied, the magnetic moment of the grains within low Tc layer502and high Tc layer504of region ‘B’ are again rotated to align with the positive write field. At this point in time, localized region ‘A’ that was previously written per the operations illustrated inFIG.5Ais now located within cooler temperature zone506of thermal profile512. The temperature of region ‘A’ is cooling toward the temperature Tr_low, which is sufficient to facilitate magnetic reversals in low Tc layer502but not in high Tc layer504. Thus, at Tr_low, the grains in the upper layer of region ‘A’ have the potential to be overwritten (e.g., flipped and locked in). However, since the field direction has not actually changed, this region maintains its positive polarity.

FIG.5Cillustrates a third example recording operation503following that ofFIG.5B. Here, the media has again rotated slightly such that read-write head has shifted in the down-track direction of magnetic media508. The heat element is now positioned over another localized region ‘C’. At this point in time, the direction of the applied write field520is switched to a negative polarity. Region ‘C’ which is passing through higher temperature zone515is magnetized (at Tr_high) such that grains in both the low Tc layer502and high Tc layer504are rotated to match the direction of the now-negative applied write field520.

At this same point in time, localized region ‘B’ that was previously written per the operations illustrated inFIG.5Bis now passing through lower temperature zone506of thermal profile512. The temperature of region ‘B’ approaches Tr_low which is sufficient to facilitate magnetic reversals in low_Tc layer502but not in high_Tc layer504. Thus, at Tr_low, the grains in upper layer of region ‘B’ have the potential to be overwritten. Since the direction of the applied write field has changed, the grains in region ‘B’ of low_Tc layer are flipped from the positive direction to the negative direction (as shown) without affecting the polarity of grains in high_Tc layer504. At this point in time, region ‘A’ has positive polarity (e.g., a 1 bit value), region ‘B’ has net zero polarity (e.g., a 0 bit value), and region ‘C’ has negative polarity (e.g., a −1 bit value).

FIG.5Dillustrates a fourth example recording operation505following that ofFIG.5C. Here, the media has again rotated slightly such that read-write head has shifted in the down-track direction of the magnetic media, and the heat element is now positioned over another localized region ‘D’. Since negative write field520is still being applied, the magnetic moment of the grains within low Tc layer502and high Tc layer504of region ‘D’ are again rotated to align with the negative write field. At this point in time, localized region ‘C’ that was previously written per the operations illustrated inFIG.5cis now located within cooler temperature zone506of thermal profile512. The temperature of region ‘A’ is cooling toward the temperature Tr_low, which is sufficient to facilitate magnetic reversals in low Tc layer502but not in high Tc layer504. Thus, at Tr_low, the grains in the upper layer of region ‘C’ have the potential to be overwritten (e.g., flipped and locked in). However, since the field direction has not actually changed, this region maintains its negative polarity.

FIG.6and plot600illustrates operations performed by a HAMR device having the characteristics described with respect toFIGS.5A-5D. That is, the HAMR device includes a magnetic media608with dual recording layers including low Tc layer602and high Tc layer604, separated by break layer610. Plot600ofFIG.6includes thermal profile606of magnetic media608in the down-track direction.FIG.6describes a scenario whereby there is no vertical temperature gradient (in the z-direction) in magnetic media608. In other words, for a particular down-track location on the x-axis of plot600, the temperature of high Tc layer604is substantially the same as the temperature of low Tc layer602.

On the y-axis of plot600, Tr1represents the recording temperature of high Tc layer604and Tr2represents the recording temperature of low Tc layer602. In this example, the temperature of the grains in low Tc layer602and the temperature of the grains in high Tc layer604(e.g., the media temperature in a vertical, or cross-track direction) are substantially the same. However, as described inFIGS.5A-5C, and as observed in plot600, the temperature of magnetic media608changes in the down-track direction, due to the heating profile of near-field transducer612. Because NFT612heats magnetic media608beneath it, the media temperature (e.g., y-axis in plot600) decreases away from NFT612location.

Region ‘B’ inFIG.6represents a region with zero net polarity (e.g., a 0-bit value), the formation of which is described in relation toFIG.5C, above. In this region, the grains in high Tc layer604are already “frozen-in’ in a positive direction since the media temperature in region ‘B’ is lower than the recording temperature (Tr1) for that layer. Therefore, grains in this region will not respond to the now negative applied write field, Ha. The grains in low Tc layer602, however, have a temperature that is higher than the recording temperature (Tr2) for that layer and the grains can respond to the negative applied write field, Ha and flip to a negative direction.

The region of media with zero net polarity (e.g., region B inFIG.6) is referred to as the zero-insertion width (ZIW)614and its dimension is determined by both the vertical temperature gradient in magnetic media608and the delta between Tr1and Tr2. In some examples ZIW614is equivalent to about half the standard grain size of grains in magnetic media608. For example, ZIW614may be on the order of about 3 nm to about 4 nm.

FIG.6describes a scenario whereby there is no vertical temperature gradient in magnetic media608. Thus, ZIW614is defined by the delta between Tr1and Tr2. In a scenario where there is no vertical temperature gradient in the media and where Tr1=Tr2then ZIW614is substantially zero and it may not be possible to write a region with zero net polarity. Thus, the recording temperature of each layer may be tuned to define the dimension of the ZIW (e.g., ZIW614).

FIG.7and plot700further illustrate operations performed by a HAMR device having the characteristics described with respect toFIGS.5A-5D. That is, the HAMR device includes a magnetic media708with dual recording layers including upper recording layer702and lower recording layer704, separated by break layer710.FIG.7describes a scenario whereby there is a vertical temperature gradient in magnetic media708. Plot700ofFIG.7includes thermal profile706of lower recording layer704in the down-track direction and thermal profile716of upper recording layer702in the down-track direction.

On the y-axis of plot700, Tr1represents the recording temperature of lower recording layer704and Tr2represents the recording temperature of upper recording layer702. In this example, the recording temperature of lower recording layer704is approximately the same as the recording temperature of upper recording layer702. In the scenario described inFIG.7, the temperature of the grains in upper recording layer702are however, higher than the temperature of the grains in lower recording layer704. The temperature difference between upper recording layer702and lower recording layer704is represented by temperature delta718. Temperature delta718is substantially constant across the down-track direction. Temperature delta718between lower recording layer704and upper recording layer702may be present due to a variety of reasons that are not illustrated inFIG.7, including, but not limited to, the presence of a heat sink layer adjacent to magnetic media708, or a cap layer on top of magnetic media708. In some examples, the presence of a heat sink layer may induce a thermal profile in magnetic media708.

The temperature of magnetic media708also changes in the cross-track direction (z-direction inFIG.7), due to the heating profile of near-field transducer712. Because NFT712heats magnetic media708beneath it, the media temperature (e.g., y-axis in plot700) decreases away from NFT712location.

Zero-insertion width (ZIW)714is also illustrated inFIG.7. Like the scenario described in relation toFIG.6, ZIW714is a region with zero net polarity. In the example ofFIG.7, although each of lower recording layer704and upper recording layer702have substantially the same Tr, the temperature at which grains are “frozen in” is different for each layer due to the temperature gradient that is present in the vertical (z) direction. The presence of a temperature gradient in the vertical (z) direction allows for upper recording layer702and lower recording layer704to have similar or substantially the same recording temperature and still maintain a region of zero net polarity (e.g., ZIW714). Thus, in the scenario described inFIG.7, the dimension of ZIW714may be determined by considering the vertical temperature gradient within magnetic media708.

One factor that may be considered when writing zero-state grains in dual-layer media is the interaction effect between the two magnetic media layers. A reliable zero-state grain is formed when there is little or no magnetostatic interaction between the upper recording layer (e.g., upper recording layer702) and the lower recording layer (e.g., lower recording layer704). If there is a strong magnetostatic interaction field from, for example, the upper recording layer during the switching process, then the lower recording layer may be susceptible to demagnetization or destabilization of magnetization from the upper recording layer field.

FIG.8is an illustration of a single magnetic grain in a HAMR device including dual layer recording media during a recording process of a zero-state grain. The HAMR device includes a recording head with a heat element that heats a magnetic media according to a thermal profile that moves along a data track during a write process.FIG.8illustrates magnetic media808with dual recording layers including high Tc layer804and low Tc layer802, break layer810, magnetostatic interaction field814, applied magnetic field816and magnetization state818. Magnetostatic interaction field814is formed in response to applied magnetic field816.FIG.8illustrates each of high Tc layer804and low Tc layer802having approximately the same thickness. In some scenarios, high Tc layer804may be thicker than low Tc layer802and in other scenarios, high Tc layer804may be thinner than low Tc layer802.

In the example ofFIG.8, high Tc layer804is just below the recording temperature, and magnetization state818is “freezing in” in the direction of a previously applied write field (e.g., in a positive direction). Low Tc layer802is in the process of switching to the direction of now applied magnetic field816. As applied magnetic field816is switched to the negative direction, magnetostatic interaction field814from low Tc layer802opposes magnetization state818of high Tc layer804near break layer810. Because high Tc layer804is still in the process of “freezing in” (since the media temperature is just below the recording temperature of high Tc layer804) magnetostatic interaction field814may destabilize high Tc layer804and cause it to switch to the same direction as low Tc layer802(e.g., to a negative direction). This could prevent formation of the stable zero-state grain.

FIG.9illustrates a single magnetic grain in a in a HAMR device including dual layer recording media during a recording process of a zero-state grain where the magnetostatic interaction field is minimized. The HAMR device includes a recording head with a heat element that heats a magnetic media according to a thermal profile that moves along a data track during a write process.FIG.9illustrates dual layer recording media908including average high Tc layer904, average low Tc layer902, break layer910, magnetostatic interaction field914, applied magnetic field916and magnetization states918A,918B,919A, and919B. In some examples, a capping layer (not shown) may be disposed on average low Tc layer902.

Magnetostatic interaction field914is formed due to the upper layer magnetization switching in the down direction in response to applied magnetic field916. Average high Tc layer904includes a first surface906and a second surface905. First surface906is at break layer910. Average low Tc layer902also includes a first surface907and a second surface909. First surface907interface resides next to break layer910. High Tc layer904has average Curie temperature Tcavg_highand low Tc layer902has average Curie temperature Tcavg_low, as shown in plots922and920, respectively. The temperature delta between Tcavg_lowand Tcavg_highis between about 10 C and about 100 C.

In the scenario described inFIG.9, each of average high Tc layer904and average low Tc layer902comprise a gradient in Curie temperature. Plots920and922represent example positive Curie temperature gradients in average low Tc layer902and average high Tc layer904, respectively. In the example of plot920, average low Tc layer902has a temperature range of Tc1at a first surface907to Tc2at a second surface909. Average high Tc layer904has a temperature range of Tc3at a second surface905to Tc4at a first surface906. In the example ofFIG.9, Tc4is greater than Tc3and Tc2is greater than Tc1. Said another way, average low Tc layer902has a higher Curie temperature at second surface909and a lower Curie temperature at first surface907. Average high Tc layer904has a higher Curie temperature at first surface906and a lower Curie temperature at second surface905.

FIG.9illustrates a snapshot in time just after applied magnetic field916is switched from an initial positive up direction to the down direction and the grain temperature is just below the recording temperature of the of high Tc layer904. Therefore, the temperature, and magnetization states918A and918B are “freezing in” in the direction of a previously applied write field (e.g., in a positive direction). In the scenario and snapshot in time illustrated inFIG.9, because Tc4is higher than Tc3, magnetization state918A will have a larger magnetization moment than magnetization state918B since918A recording temperature is larger and more “frozen in” than the lower magnetization state918B which is just above the current grain temperature.

In the example ofFIG.9, the Low Tc layer902is in the process of switching to the direction of now applied magnetic field916. In the example illustrated inFIG.9, because Tc2is higher than Tc1, magnetization state919B “freezes in” at a higher temperature than magnetization state919A. As applied magnetic field916is switched to the negative direction, magnetostatic interaction field914from low Tc layer902opposes magnetization state918A and919B of high Tc layer904near break layer910.

Including a Curie temperature gradient in each of average high Tc layer904and average low Tc layer902may assist in reducing magnetostatic interaction field914which may subsequently reduce the demagnetizing effect of magnetostatic interaction field914. Reducing the demagnetizing effect of magnetostatic interaction field914reduces the likelihood that magnetization states918A and918B will switch back to the negative direction. This may subsequently assist in forming a more stable zero-state grain

The manner in which magnetostatic interaction field914is reduced may be explained by considering the Curie temperature at surface906of average high Tc layer904and the Curie temperature at surface907of average low Tc layer902(Tc4and Tc1, respectively). In the example ofFIG.9, average low Tc layer902is on top of average high Tc layer904. Low Tc layer902has a lower Curie temperature (Tc1) at surface907. In contrast, high Tc layer904has a higher Curie temperature (Tc4) at surface906. During zero-state grain recording, average high Tc layer904is set in the positive magnetization direction. Because of the Curie temperature gradient, high Tc layer904has high anisotropy magnetization state918A close to surface906and break layer910. In contrast, low Tc layer902has low anisotropy magnetization state919A close to surface907and break layer910, due to the graded Curie temperature within the layer. So, low Tc layer902generates a weak moment at surface907and high Tc layer904is difficult to demagnetize at surface907due the relatively high anisotropy magnetization state918A there. Because of this, magnetostatic interaction field914is weak and thus, average high Tc layer904is not easily destabilized. This lack of destabilization of average high Tc layer904may lead to an improvement in formation of a stable zero-state grain and a subsequent improvement in SNR.

FIG.10illustrates a single magnetic grain in a in a HAMR device including dual layer recording media during a recording process of a zero-state grain where the magnetostatic interaction field is minimized. The HAMR device includes a recording head with a heat element that heats a magnetic media according to a thermal profile that moves along a data track during a write process.FIG.10illustrates dual layer recording media1008comprising average high Tc layer904, average low Tc layer1004, break layer1010, magnetostatic interaction field1014, applied magnetic field1016and magnetization states1018A,1018B,1019A, and1019B. Magnetostatic interaction field1014is formed in response to applied magnetic field1016. Average high Tc layer1002includes a first surface1007and a second surface1009. First surface1007is at break layer1010. Average low Tc layer1004also includes a first surface1006and a second surface1005. First surface1006is at break layer1010. High Tc layer1002has average Curie temperature Tcavg_highand low Tc layer1004has average Curie temperature Tcavg_low, as shown in plots1020and1022, respectively. The temperature delta between Tcavg_lowand Tcavg_highis between about 10 C and about 100 C.

In the scenario described inFIG.10, each of average high Tc layer1002and average low Tc layer1004comprise a gradient in Curie temperature. Plots1022and1020represent example negative Curie temperature gradients in average low Tc layer1004and average high Tc layer1002, respectively. In the example of plot1022, average low Tc layer1004has a temperature range of Tc7at a first surface1006to Tc8at a second surface1005. Average high Tc layer1002has a temperature range of Tc5at a second surface1009to Tc6at a first surface1007. In the example ofFIG.10, Tc8is greater than Tc7and Tc6is greater than Tc5. Said another way, average low Tc layer1004has a higher Curie temperature at second surface1005and a lower Curie temperature at first surface1006. Average high Tc layer1002has a higher Curie temperature at first surface1007and a lower Curie temperature at second surface1009.

FIG.10illustrates a snapshot in time just after applied magnetic field1016is switched from an initial positive up direction to the down direction and the grain temperature is just below the recording temperature of high Tc layer1002. Therefore, the temperature, and magnetization states1019A and1019B are “freezing in” in the direction of a previously applied write field (e.g., in a positive direction). In the scenario and snapshot in time illustrated inFIG.10, because Tc6is greater than Tc5, magnetization state1019A will have a larger magnetization moment than magnetization state1019B since magnetization state1019A recording temperature is larger and magnetization state1019A is more “frozen in” than the lower magnetization state1019B, which is just above the current grain temperature.

In the example ofFIG.10, low Tc layer1004is in the process of switching to the direction of now applied magnetic field1016. In the example illustrated inFIG.10, because Tc8is higher than Tc7, magnetization state1018B “freezes in” at a higher temperature than magnetization state1018A. As applied magnetic field1016is switched to the negative direction, magnetostatic interaction field1014from low Tc layer1004opposes magnetization state1019A and1019B of high Tc layer1002near break layer1010.

Including a Curie temperature gradient in each of average high Tc layer1002and average low Tc layer1004may assist in reducing magnetostatic interaction field1014which may subsequently reduce the demagnetizing effect of magnetostatic interaction field1014. Reducing the demagnetization effect of magnetostatic interaction field1014reduces the likelihood that magnetization states1019A and1019B will switch back to the negative direction. This may subsequently assist in forming a more stable zero-state grain.

The manner in which magnetostatic interaction field1014is reduced may be explained by considering the Curie temperature at surface1007of average high Tc layer1002and the Curie temperature at surface1006of average low Tc layer1004(Tc6and Tc7, respectively). In the example ofFIG.10, average high Tc layer1002is on top of average low Tc layer1004. Low Tc layer1004has a lower Curie temperature (Tc7) at surface1006. In contrast, high Tc layer1002has a higher Curie temperature (Tc6) at surface1007. During zero-state grain recording, average high Tc layer1002is set in the positive magnetization direction. Because of the Curie temperature gradient, high Tc layer1002has high anisotropy magnetization state1019A close to surface1007and break layer1010. In contrast, low Tc layer1004has low anisotropy magnetization1018A close to surface1006and break layer1010, due to the graded Curie temperature within the layer. So, low Tc layer1004generates a weak moment at surface1006and high Tc layer1002is difficult to demagnetize at surface1007due the relatively high anisotropy magnetization state1019A there. Because of this, magnetostatic interaction field1014is weak and thus, average high Tc layer1002is not easily destabilized. This lack of destabilization of average high Tc layer1002may lead to an improvement in formation of a stable zero-state grain and a subsequent improvement in SNR.

Various examples have been presented for the purpose of illustration and description. These and other examples are within the scope of the following claims.