Three dimensional data storage media

A three dimensional magnetic recording media can consist of a coupling layer disposed between first and second vertically stacked recording layers. The coupling layer can provide exchange or antiferromagnetic coupling and allow the respective recording layers to be individually heat selected to different first and second coupling strengths through application of heat from a heat source.

SUMMARY

A three-dimensional magnetic recording (3DMR) media can, in accordance with some embodiments, consist of a coupling layer disposed between first and second vertically stacked recording layers. The coupling layer can allow the respective recording layers to be individually heat selected to different first and second coupling strengths through application of heat from a heat source.

DETAILED DESCRIPTION

Assorted embodiments of the present disclosure are directed to a three-dimensional magnetic recording (3DMR) medium capable of being utilized in a data storage device. Through the assorted structures and methods, at least the data density and signal-to-noise ratio of a 3DMR data storage medium can be increased.

A 3DMR medium, in some embodiments, is configured to allow for the selective switching of magnetic coupling between vertically stacked recording layers. It is noted that the term “vertically stacked” is meant as multiple layers aligned along an axis perpendicular to an air bearing surface (ABS) of the 3DMR medium.

One of the challenges in data storage media with multiple recording layers is that a recording layer with low anisotropy (Hk) will suffer from erasure due to the writing process on the high-Hk layer of adjacent tracks, which can be characterized as write inflation. In other words, it can be difficult to record vertically stacked recording layers individually. It is also challenging to keep thermal stability in vertically stacked recording layers.

Controlling the coupling, such as exchange or antiferromagnetic coupling, between vertically stacked recording layers poses another challenge. On one hand, magnetic exchange coupling may promote switching of the bottom recording layer, but on the other hand, when switching the top recording layer only, such as during a second pass above the collective recording layers, erasure of the bottom recording layer can occur.

Successful multilevel recording utilizes the independent writing of the layers, which is well controlled, and that the written bits provide well separated bit levels while maintaining SNR, thermal stability, low spacing loss, etc., at competitive recording densities. Attempts to produce such independently written two-layer recording structures by combining Co-alloy perpendicular magnetic media with conventional magnetic recording have struggled to achieve the necessary trade-offs of separable write fields while maintaining high enough anisotropy of both layers for thermal stability in a thin structure providing limited spacing loss.

Applying microwave assisted magnetic recording (MAMR) increases the effective switching field and enables the writing of higher Hk recording layers, thereby increasing the thermal stability or decreasing the necessary thickness of the recording layers employed. Applying MAMR primarily to a magnetically harder (higher He and Hk) top recording layer also further separates the maximum effective write field difference that can be applied to the top and bottom recording layers. This is because each layer has a switching field distribution from its easiest to hardest grain to switch, and it is contemplated that no part of these distributions overlap between the recording layers for completely separate switching.

In a first example embodiment, a 3DMR medium utilizes high-Hk and low-Hk materials for respective top and bottom recording layers to provide effective write contrast by writing data via different write-assisted recording schemes for the different recording layers. Such write contrast allows data writing with ultrahigh anisotropy grains for both layers, so the write inflation and thermal-stability-vs-grain-volume issues can be addressed.

In a second example embodiment, a 3DMR medium is utilized in a method for recording data on very high Hk recording layers to achieve switching contrast between the vertically stacked recording layers by using different write assisted methods, such as heat assisted magnetic recording (HAMR) and MAMR.

In a fourth example embodiment, a 3DMR medium has a tuned Hk and Curie temperature (Tc) for top and bottom recording layers and a method to address the layers selectively by starting to record the bottom layer first, and then the top layer and its adjacent tracks are selectively re-written.

In a fifth example embodiment, a 3DMR medium is configured to be written with MAMR means to increase the effective switching field and enable the writing of higher Hk recording layers, thereby increasing the thermal stability or decreasing the necessary thickness of the recording layers employed. Applying MAMR primarily to a harder top layer, with higher coercivity (Hc) and Hk, also further separates the maximum effective write field difference that can be applied to the top and bottom recording layers.

This is interesting because each recording layer has a switching field distribution from its easiest to hardest grain to switch between magnetic polarities, and it is contemplated that no part of these distributions overlap between the recording layers for completely separate switching. Applying MAMR to the top recording layer can increase the write field capability so that the entire distribution, from Hn to magnetic saturation (Hs) of the recording layer, can be moved to a higher value. Correspondingly, the softer layer Hs value, along with He and Hn values, can be increased toward the higher Hn value of the top MAMR layer, and a higher anisotropy, more stable soft layer material can be employed. It is noted that the hard layer has to be switched first.

In a sixth example embodiment, a MAMR layer has a higher exchange value or has a coupled granular and continuous (CGC) structure at the top for MAMR transmission. MAMR frequency is Hk dependent and MAMR may apply at different frequency to each layer. MAMR can reduce effective SFD, which can have value to the bottom layer even though higher field is not needed. If frequency separation is used, Hs of the soft layer may be increased slightly above Hn of the hard layer. It may also possible to have MAMR applied only to the soft top layer, if the soft layer is written second, and the MAMR is localized to the top layer, Hs soft may be raised very much higher than Hn of the hard layer.

In a seventh example embodiment, a 3DMR medium of a MAMR system has a top magnetic recording layer with a higher switching field than conventional media, and a softer bottom recording layer with a switching field distribution with minimized overlap to the hard recording layer, also consistent with Hn high enough for thermal stability. As a non-limiting example, a 3DMR medium can have Hs˜9000 Oe, Hc˜5500 Oe, Hn˜2000 Oe. A soft recording layer can be configured with Hs˜2000 Oe, with the distribution placing Hn<0 Oe, which is clearly unstable. A multilevel 3DMR medium might have a hard layer with Hs˜12,000, Hc˜9500, Hn˜7000; and a soft layer Hs˜7000, Hc˜4500, Hn˜2000.

In an eighth embodiment, 3DMR medium has three data levels and antiferromagnetically coupled recording layers. It is contemplated that a write process will employ HAMR technology to program data to the three level 3DMR system. In exchange coupled 3DMR approaches, more bits are packed into one single dot, such as two bits per dot, which requires a 4 level configuration that poses some operational issues. For instance, a 4 level configuration can have approximately a ˜10 dB SNR loss if divided the total signal from 2 to 4 levels. A 4 level configuration also causes the write process to use more than one pass, which leads to a finite degradation in performance (data latency), or erasure conditions.

For 4 levels, gain is twice a maximum. Due to loss in SNR and uncertainty in writing, as one of the layers is physically further away from recording head, the actual data capacity gain (assume we can handle 10 dB SNR penalty) is limited. Also, the read back signal for a 4 level configuration is overlapped (physically), which contribute to the signal-to-noise loss.

In a ninth embodiment, a 3DMR medium has a first recording layer with high Tc (700-750K) and High Hk (>=70 kOe) and a second recording layer with low Tc (500-650K) and low Hk (25-60 kOe). Such a 3DMR medium can have individual recording layers selectively recorded in a first pass of writing to the bottom recording layer where the temperature of the bottom recording layer will be heated up above its Curie temperature (Tc1) and switched. However, the top recording layer and its adjacent tracks will also be erased due to the thermal spreading and lower Tc2in the top layer. Next, the top recording layer is rewritten along with its adjacent tracks while the 3DMR medium is only heated up to a temperature above Tcof the top recording layer but below the Tcof bottom recording layer, which results in only the top recording layer and the previously erased adjacent tracks are selectively rewritten.

In a tenth embodiment, a first recording layer has high Tc(800-1000 K) but low Hk (<=25 kOe) while a second recording layer has low Tc(600-750K) but high Hk (35 kOe-140 kOe). An example recording scheme writes data to the bottom recording layer with HAMR with the temperature of bottom recording layer heating up above Curie temperature and switched in combination of applied field. However, the top recording layer and its adjacent tracks will also be erased due to the lower Hk in the top recording layer. Next, the top recording layer is rewritten along with its adjacent tracks while the 3DMR medium is either not heated or only heated up well below TC2and the switching of the top recording layer is pre-dominated by magnetic field writing. As a result, the bottom recording layer is not erased due to its much larger Hk and only the top recording layer and the previously erased adjacent tracks are selectively rewritten.

In an eleventh embodiment, a 3DMR medium is configured to mitigate write inflation by having a top recording layer constructed of a high transition temperature/low anisotropy material and a bottom recording layer formed of a high anisotropy and low transition temperature. The top recording layer configuration allows for writing with or without HAMR while keeping the bottom recording layer intact due to the high anisotropy. The top recording layer can have 800-1000K Tcand less than 25 KOe Hk while the bottom recording layer has 600-750K Tcand greater than 35 kOe Hk. The writing process is expected to be HAMR for the bottom recording layer followed by rewriting the top layer and adjacent tracks with heating below the Tc of the bottom recording layer. Materials for the bottom recording layer can be FePt based granular compositions and the top layer can be CoPt based materials.

Antiferromagnetic coupling may stabilize magnetic states in top and bottom recording layers that have opposite magnetization directions. However, such coupling can block the writability of the bottom recording layer. The magnitude of coupling, either with exchange or antiferromagnetic coupling, corresponds with a strong temperature dependency where elevated temperatures reduce coupling. It is noted that the temperature to switch off exchange coupling is significantly lower than the Curie temperature of the magnetic materials used in the recording layers.

Hence, a twelfth embodiment use exchange and antiferromagnetic (AF) coupling between two vertically stacked magnetic recording layers to take advantage of magnetic switching of bottom and top recording layers separately as well as thermal stability of states of opposite direction on top of each other. A multilayer recording media structure can be configured with an AFC interface between top and bottom recording layers. The AFC interface promotes magnetic stability of the two recording layers with opposite magnetization directions.

A 3DMR medium, in a thirteenth embodiment, can be thermally selected to employ exchange or AF coupling between recording layers. Controlling the exchange/antiferromagnetic coupling can promote magnetic switching of the first (bottom) and second (top) recording layers. Structurally, an AF or exchange interface is disposed between the recording layers and promotes stable, opposite magnetizations in the recording layers. Application of heat to the AF or exchange interfaces allows a write field to selectively write to one, or both, recording layers, which increases writing efficiency.

The assorted embodiments can be understood with a review ofFIGS. 1-15.FIG. 1represents a portion of an example a data storage system100. The system100has a rotatable data recording medium102and an adjacent data transducer104. The data transducer104can employ HAMR and/or MAMR means, which may have different structural configurations and methods of use compared to a perpendicular data bit writer. It is noted that the addition of HAMR, or MAMR, means is not limiting or required and the transducer104can have less, or more, recording assisting structures.

Generally, the medium102and the transducer104may be incorporated into a hard disc drive (HDD) or other data storage device, such as a hybrid data storage drive employing a non-volatile solid-state memory. In a HDD embodiment, multiple axially arranged recording media (discs) are separately accessed by at least one data transducer104to read data to, and store data from, a host device.

Data can be stored in a number of concentric tracks106defined along an air bearing surface108of the medium102. The data may be stored in the form of addressable user data sectors of fixed size along the tracks106. Hydrodynamic features may be provisioned on a facing surface of the transducer104to enable the transducer104to be fluidically supported in close proximity to the medium surface108by atmospheric currents established during rotation of the medium102.

The data transducer104is shown to include respective read (R), write (W) and assisting (A) elements110,112and114. The read element110may take the form of a magneto-resistive (MR) sensor. The write element112may include a write coil and one or more magnetically permeable cores. The assisting element114may have a light source, such as a laser diode, in a HAMR configuration or a frequency source, such as an oscillating material, in a MAMR configuration. A HAMR configuration may also employ a near field transistor (NFT) or other radiation beam and/or thermal energy source.

During a read operation, the read element110operates to sense a magnetization sequence written to the medium102along a portion of a selected track106. During a write operation, the assisting element114alters the magnetic properties of a “dot” portion of the rotating medium102. For example, a laser can project high powered irradiation to locally increase the temperature of the medium102. The alteration of the dot allows the write element112to direct magnetic flux into selected portions of the medium102to write a desired magnetization sequence. The transducer104is supported by an actuator arm116, which, responsive to a servo control system (not shown), radially positions the respective elements110,112and114over particular data tracks106as required.

It will be appreciated that the system represented inFIG. 1can be readily adapted for HAMR, MAMR, non-HAMR, and non-MAMR applications. For instance, the medium102can be constructed such as exchange coupled composite (ECC)+CGC perpendicular recording media.

For HAMR recording techniques generally depicted inFIG. 1, high anisotropy medium magnetic materials, such as iron-platinum (FePt) alloys, can be employed as recording layers. As noted above, a HAMR light source provides localized heating to temporarily reduce the anisotropy energy of the magnetic bit locations in the medium102so that the electromagnetic writer element112can write to the bit locations. The light beam in a HAMR application will tend to be many times smaller than its diffraction limit to avoid undesirable heating of adjacent areas, while the power level of the beam needs to be sufficiently high to accomplish the task of reducing the medium magnetic anisotropy energy.

Near field transducers (NFTs) are sometimes used to meet these beam size and power level requirements.FIG. 2represents an NFT-based HAMR system120with a light source (e.g., incident laser)122, a condenser124and an NFT126. The condenser124represents a first stage and the NFT126represents a second stage. These elements cooperate to direct an electromagnetic radiation (e.g., light) beam128onto a small spot130on a medium132. It is contemplated that the spot130has a diameter of no more than about 30 nm and is provided with sufficient transmitted power as to reduce the magnetic coercivity and anisotropy energy of the medium132.

The NFT126enhances the light efficiency by combining various effects such as plasmonic resonance, surface enhancement, lightening rod collection, etc. Using an enhanced overcoat layer on the medium132with high efficiency plasmon response characteristics, as disclosed herein, can further improve the laser-medium coupling efficiency during HAMR recording operations.

For MAMR recording techniques that can be utilized in the transducer104ofFIG. 1, a particular frequency, or range of frequencies, which do not necessarily have to be microwave frequencies, excite the medium by activating one or more spin torque oscillators (STO) to allow data to be written easier and faster.

FIG. 3is a schematic depiction of various layers of an example recording medium140useful in a data storage system such as described above inFIGS. 1 and 2. Other media configurations can readily be used, including configurations with different numbers and/or orderings of layers. It will be appreciated thatFIG. 3is functional in nature and is not drawn to scale so that each of the respective layers shown inFIG. 3will have its own respective thickness. Each layer may be formed from a plurality of layers of like or different construction. Seed layers, intermediate layers, coupling layers, etc. may also be used as desired.

A base substrate142provides mechanical support for the medium140. A recording structure is formed on the base substrate142made up of a number of layers. A soft magnetic underlayer (SUL)144is formed on the substrate, such as via sputtering. The SUL144provides a return path for magnetic flux during a data recording operation. One or more intermediate layers, also referred to as interlayers (IL), can be formed on the SUL to establish a magnetic easy axis in the perpendicular direction, and may have high surface roughness to induce grain separation in the subsequently deposited recording layers. In some embodiments, a first recording layer146is positioned atop the SUL and consist of a first type of data recording, such as perpendicular recording, MAMR, HAMR, and bit patterned media (BPM).

One or more intermediate layers148are formed on the first recording layer146to separate the second recording layer150from the first recording layer146. The respective recording layers146and150may be provided with lower layers having relatively higher magnetization, anisotropy and exchange coupling levels and upper layers with softer magnetization and anisotropy. The second recording layer150, in some embodiments, is configured with a different type of data recording than the first recording layer. As such, any number of suitable recording structure configurations can be used in the recording layers146and150.

A protective overcoat layer152is formed on the topmost recording layer150. Various configurations of the protective overcoat layer152protects the medium140against environmental effects, such as inadvertent contact between the medium140and the transducer (e.g.,104inFIG. 1). The overcoat layer152can further provide increased energy coupling between the medium140and a HAMR recording system such as discussed inFIG. 2.

As a non-limiting example, two stacked recording layers which have different coercivities can be written using different write currents applied to a write head, but the same concept can be applied to any system of recording layers where a cascade of writing conditions, one for each recording layer, can be arranged such that subsequent write conditions will not destroy the information written in other layers by earlier write conditions.

Although no recording scheme or combination is required, the use of two different writing schemes, such as HAMR and MAMR, to write information on two vertically stacked magnetic layers can provide optimized write contrast and data writing performance. Because both HAMR and MAMR belong to energy-assisted magnetic recording, they can be used to write magnetic grains with ultrahigh magnetic anisotropy.FIG. 4illustrates a cross-sectional line representation of a portion of an example data storage medium170constructed and operated in accordance with various embodiments.

As shown, a first (top) recording layer172is configured to be written with a HAMR recording scheme and a second (bottom) recording layer174is configured to be written with a MAMR recording scheme. It is noted that the recording layers172and174utilize different frequencies and temperatures that provides high writing contrast and the ability to write data to a selected one recording layer.

As a non-limiting example, suppose there is a 3DMR medium with two vertically stacked magnetic layers with no interlayer exchange coupling. Each magnetic layer has a very high magnetic anisotropy (˜1×108 erg/cm3). The Curie temperatures for the two layers are made different (as shown by Tc1and Tc2inFIG. 4). The ferromagnetic resonant frequencies for the two layers are also made different (as shown by f1and f2inFIG. 4). It is noted that the stacking order of the respective layers172and174is not fixed. Under this setup, one can use HAMR to implement recording on one layer, and use MAMR on the other layer. This recording scheme allows for relatively thin magnetic layers as compared to conventional 3DMR that is based on perpendicular magnetic recording (PMR) schemes, to provide a magnetic recording system with better readback signal and lower demagnetization fields.

FIG. 5plots how a MAMR field enables reduced overlap of the hysteresis loops of the writable hard and soft recording layers. The MAMR layer, represented by segmented line182, has much higher Hn than the conventional hard layer, as represented by segmented line184but writes at the same field as the conventional layer when the MAMR field is on. Region186represents the overlap of hard and soft recording layers magnetic field and arrows188corresponds with the reduction of magnetic field corresponding with MAMR operation.

It is contemplated that a bottom recording layer can be constructed as a FePtX:Y granular media with the top recording layer comprising a CoPtX based materials and spacer and/or seed layers in between. A bottom recording layer could be deposited at elevated temperature to achieve the chemical ordering needed for high anisotropy while the top recording layer could be deposited at room temperature for the case of CoCrPt alloy or deposited at elevated temperatures if CoPt or FePd ordered alloys. The same sequence can be applied for cases of 3DMR with multiple sub-layers (N>=2). The concept can be applied for granular media or BPM.

FIG. 6illustrates a line representation of a portion of an example data storage medium190constructed and operated in accordance with various embodiments. As shown, each recording layer192and194has different Hk and Tc configurations. During writing of the bottom recording layer194, the temperature of bottom recording layer194will be heated up above Curie temperature and switched. However, the top recording layer192and its adjacent tracks will also be erased due to the thermal spreading and lower Tc in the top layer, as illustrated by the switching extent196that spans multiple vertically stacked data regions198.

FIG. 7plots operational data for an example data storage medium. It is noted that materials for the top and bottom recording layers can be respectively be selected to provide a tuned balance between anisotropy and Curie temperature, as shown.

FIG. 8is a line representation of a portion of an example data storage medium where a top recording layer and its adjacent tracks are rewritten. The media stack is only heated up to a temperature above Tc of the top recording layer192but below the Tc of bottom recording layer194. Thus, only the top recording layer192and the previously erased adjacent tracks are selectively rewritten, as represented by switching extents200corresponding with individual data regions198.

FIG. 9shows an example writing of data to a vertically stacked data region202with an antiferromagnetically selectable intermediate layer. The magnetically harder bottom recording layer204is recorded to a first magnetic polarity in a first run where heat is applied to deactivate the antiferromagnetic coupling (AFC) of the intermediate layer, as illustrated by the lefthand region202. Here, the magnetically softer top recording layer206is switched to the same magnetization direction as the bottom recording layer204. It is noted that the applied heat enables easier switching of the bottom recording layer204.

The righthand data region202shows how data recording to the top recording layer206occurs only when the magnetization of the top recording layer206needs to switch in the opposite direction of the bottom layer202. In such an event, a magnetic field is applied with no heat being applied to the intermediate layer, which allows the AFC promote promotes the stability of the bottom layer204.

Turning toFIG. 10, a 3DMR medium has vertically stacked recording layers204and206separated by a selectable exchange coupling (EC) intermediate layer. The exchange coupling of the intermediate layer promotes magnetic switching of the bottom recording layer204(ECC). The writing of the magnetically harder bottom recording layer204is shown by the lefthand data region210. Here, the magnetically softer top recording layer206is magnetically switched in the same magnetization direction with no heat being applied to leverage the exchange coupling and enable easier switching of the bottom recording layer204.

Recording of the top recording layer206occurs only when the magnetization of the top recording layer206needs to switch in the opposite direction compared to the bottom recording layer204. A magnetic field is applied with a transducing head activated and then the EC is switched off via the application of heat, which blocks the exchange coupling induced switching of the bottom recording layer204, as illustrated by the righthand data region210.

It is contemplated that the recording layers202and204can be configured to be accessed and written to via different recording schemes, such as perpendicular recording (PMR), microwave assisted magnetic recording (MAMR), and heat assisted magnetic recording (HAMR) recording schemes. In the case of MAMR, the respective recording layers can be configured with different resonance frequencies to allow individual selection and access.

FIG. 11shows a line representation of a portion of an example 3DMR data storage medium220arranged in accordance with various embodiments to antiferromagnetically couple a first recording layer222to a second recording layer224with a coupling layer226. As shown, the coupling layer226continuously extends between the recording layers222and224and is constructed of a non-magnetic material capable of maintaining antiferromagnetic coupling. It is noted that each recording layer222and224can have a plurality of magnetic regions228separated by non-magnetic regions230, as displayed by segmented lines, which can correspond with bit patterned media (BPM).

However, BPM is not required as one, or both, recording layers222and224can be continuously magnetic and comprise a plurality of magnetic grains. Regardless of the magnetic configuration of the recording layers222and224, the vertically stacked orientation of the first222and second224recording layers, as measured perpendicular to the air bearing surface (ABS), allows multiple data bits232to be concurrently stored for each bit's portion of the ABS.

The 3DMR medium220inFIG. 11shows an anisotropy direction to be aligned perpendicular to the ABS. The antiferromagnetic coupling between the recording layers222and224can be tuned, along with the magnetic thickness (Mrt) of the recording layers222and224, to be about the same. The strength of antiferromagnetic coupling is relative strong such that the each recording layer grain is thermally stable when the first222and second224layer magnetizations are either in parallel or in antiparallel states. We note that although the anisotropy direction of each recording layer222and224is shown to be aligned perpendicular to the ABS, other anisotropy orientations (media magnetization direction at equilibrium states) are possible, such as longitudinal or tilted with respect to the ABS.

The antiferomagnetically coupled recording layers222and224allow for the data bits232of vertically aligned magnetic grains to be stable in at least three different logic states. In the non-limiting example ofFIG. 11, a first logic state (2) is present when vertically stacked data bits232each have a common, positive polarity and a second logic state (0) is present when stacked data bits232each have a common, negative polarity. A third logic state (1) is present when vertically stacked data bits232have different polarities.

FIGS. 12A, 12B, and 12Crespectively show block representations of an example recording portion240of a data storage medium arranged in accordance with some embodiments. As shown inFIG. 12A, the recording layers222and224can each be arranged with an easy axis of anisotropy242to be perpendicular to the ABS.FIG. 12Bconveys how the easy axis of anisotropy244for the recording layers222and224can be tilted with respect to the ABS. While not required or limiting, the easy axis of anisotropy246displayed inFIG. 12Cis oriented parallel to the ABS. The ability to orient the anisotropy (Hk) of the recording layers222and224allows the recording portion240to be tuned to accommodate a diverse variety of data storage environments and data densities.

FIG. 13plots operational data for an example data storage medium configured in accordance with some embodiments. The playback signal for the various logic states, which correspond with the logic states shown inFIG. 11, illustrate how well defined transitions are present between different bits. That is, adjacent data bits that each comprise vertically stacked magnetic polarities can be easily read and discerned due to the three distinct playback signal levels for the respective logic states.

FIG. 14graphs data associated with an example data storage medium constructed in accordance with various embodiments. The non-overlapping energy profiles of the various logic states convey the energy surface of different magnetization states and the energy barrier between the states for the coupled particles (dots). The results show that with an antiferromagnetic coupling, all three states can be thermally stable for long time.

FIG. 15is a flowchart of an example 3DMR data access routine250that is carried out in accordance with various embodiments to utilize a 3DMR data storage medium with a data transducing head, such as transducer104. The data storage medium is initially provided in step252with first and second magnetic recording layers antiferromagnetically coupled together. Step254then heats each magnetic recording layer above the respective layer's Curie temperature to lower the coercivity of each layer and allow alteration of the constituent data bit magnetization.

The heating of the magnetic recording layers can produce three different logic states depending on the magnetic polarity applied by a data writer. Step256applies a positive polarity magnetic field while the recording layers are heated, which results in a (2) logic state to be programmed with each data bit having the imparted positive polarity. A (1) logic state can be programmed with step258applying a zero, or very little, magnetic field and allowing the antiferromagnetic coupling to set the data bits to opposite polarities. Alternatively, a negative polarity magnetic field can be applied in step260on the recording layers to write a (0) logic state.

Subsequent to the writing of data in steps256,258, and/or260, heat is removed from the programmed data bits and step262proceeds to read one of three different logic states from the recording layers. That is, a programmed data bit is cooled below Curie temperature and a data reader senses the collective polarities of the vertically stacked data bits to detect one of three different logic states.

In a data writing process, both writer and a heating element is turned on. In HAMR data writing, the heating element laser diode couples light into near field transducer. When the write field is in one direction, vertically stacked magnetizations of the recording layers222and224will be aligned in the same direction. When the write field is 0, then the media dots enter to an antiparallel state, with a net 0 field/flux output. In playback, all antiparallel states are treated as one state (1). Both layer dots magnetization pointing up and down were treated as two separate states (0 and 2).