Heat-assisted magnetic recording (HAMR) disk drive with disk having multiple continuous magnetic recording layers

Abstract

A heat-assisted magnetic recording (HAMR) disk drive has a disk with at least two independent data layers (RL1 and RL2), each data layer storing an independent data stream. At a high laser power both RL1 and RL2 are heated to above their respective Curie temperatures and a first data stream is recorded in both RL1 and RL2. At a lower laser power only upper RL2 is heated to above its Curie temperature and a second data stream is recorded only in RL2. The data layers are separated by a nonmagnetic spacer layer (SL) that prevents lower RL1 from being heated to above its Curie temperature at low laser power. The first and second data streams are typically asynchronous. Recorded data is read back from both data streams simultaneously as a composite readback signal. A joint Viterbi detector detects the asynchronous data streams simultaneously from the composite readback signal.

Claims

1. A heat-assisted magnetic recording (HAMR) disk drive comprising: a rotatable magnetic recording disk comprising a substrate, a first continuous non-patterned magnetic layer on the substrate comprising a substantially chemically-ordered alloy comprising Pt and one of Fe and Co and having a Curie temperature (T.sub.C1), a nonmagnetic spacer layer on the first magnetic layer, and a second continuous non-patterned magnetic layer on the spacer layer comprising a substantially chemically-ordered alloy comprising Pt and one of Fe and Co and having a Curie temperature (T.sub.C2); a laser light source capable of supplying at least two levels of laser output power; an optical channel and near-field transducer for directing radiation from the light source to the disk to heat regions of the first and second magnetic layers; a write head for applying a magnetic write field to write data bits in the heated regions of the first and second magnetic layers; a read head for generating a composite readback signal from the written data bits in the first and second magnetic layers; a carrier for supporting the write head, near-field transducer and read head; and a data detector coupled to the read head for generating from the composite readback signal independent data streams representing the written data bits in the first and second magnetic layers.

2. The disk drive of claim 1 wherein T.sub.C1 is substantially equal to T.sub.C2 and the spacer layer has a thermal conductivity and thickness to prevent the first magnetic layer from being heated to near or above T.sub.C1 when the laser is operating at a lower power level (P.sub.L).

3. The disk drive of claim 1 wherein T.sub.C1 is greater than T.sub.C2, whereby when the second magnetic layer is heated to near or above T.sub.C2 when the laser is at operating at a lower power level (P.sub.L) the first magnetic layer is not heated to near or above T.sub.C1.

4. The disk drive of claim 1 wherein T.sub.C1 is less than T.sub.C2 and the spacer layer has a thermal resistance and thickness to assure that the temperature drop across the spacer layer when the laser is operating at lower laser power P.sub.L is sufficiently large so that the first magnetic layer is not heated to near or above T.sub.C1.

5. The disk drive of claim 1 wherein at least one of the first and second magnetic layers further comprises a segregant selected from one or more of C, SiO.sub.2, TiO.sub.2, Y.sub.2O.sub.3, TaOx, ZrO.sub.2, VO.sub.x, SiC, SiN, HfC, HfO.sub.2, VN, AlN, MoO.sub.x, TiC, TiN, B, B.sub.4C, and BN.

6. The disk drive of claim 1 wherein at least one of the first and second magnetic layers is formed of a material based on the L1.sub.0 phase and having a composition of (Fe.sub.(y)Pt.sub.(100-y))X, where y is greater than or equal to 40 atomic percent and less than or equal to 60 atomic percent and the element X is selected from one or more of Ni, Au, Cu, Ge, Pd, Mn and Ag.

7. The disk drive of claim 1 wherein the alloy of at least one of the first and second magnetic layers has a composition of the form (Fe.sub.(1-x)Z.sub.x).sub.yPt.sub.(100-y), where Z is selected from Cu, Ni, Cr, Mn, B and Ag, x is greater than or equal to 1 atomic percent and less than or equal to 20 atomic percent and y is greater than or equal to 45 atomic percent and less than or equal to 55 atomic percent.

8. The disk drive of claim 7 wherein the alloy of the second magnetic layer comprises a FeCuPt alloy.

9. The disk drive of claim 7 wherein the alloy of each of the first and second magnetic layers comprises a FeCuPt alloy wherein the amount of Cu in the second magnetic layer is greater than the amount of Cu in the first magnetic layer.

10. The disk drive of claim 1 wherein the nonmagnetic spacer layer is selected from magnesium oxide (MgO), magnesium titanate (Mg.sub.2TiO.sub.4), spinel (Mg.sub.2Al.sub.3O.sub.4), barium zirconium oxide (BaZrO.sub.3), barium cerium oxide (BaCeO.sub.3), strontium titanate (SrTiO.sub.3), strontium zirconium oxide (SrZrO3), lanthanum aluminum oxide (LaAlO.sub.3), titanium nitride (TiN), titanium oxynitride (TiO.sub.xN.sub.(1-x)), silicon nitride (SiN) and carbon (C).

11. The disk drive of claim 1 wherein the nonmagnetic spacer layer is selected from Cr, W, Mo, Mn, V, Nb, Ru, Rh, Ir and alloys thereof.

12. The disk drive of claim 1 wherein the data detector comprises a joint Viterbi detector.

13. The disk drive of claim 12 wherein the joint Viterbi detector is capable of detecting two asynchronous data streams simultaneously.

14. The disk drive of claim 1 wherein the write head is adapted to write data bits in the second magnetic layer at a frequency different from the frequency at which the data bits are written in the first magnetic layer.

15. A heat-assisted magnetic recording (HAMR) disk drive comprising: a rotatable magnetic recording disk comprising a substrate, a first continuous non-patterned magnetic layer on the substrate comprising a substantially chemically-ordered alloy comprising Fe and Pt and having a Curie temperature (T.sub.C1), a nonmagnetic spacer layer on the first magnetic layer, and a second continuous non-patterned magnetic layer on the spacer layer comprising a substantially chemically-ordered alloy comprising Fe and Pt and having a Curie temperature (T.sub.C2), wherein at least one of the first and second magnetic layers further comprises a segregant selected from one or more of C, SiO.sub.2, TiO.sub.2, Y.sub.2O.sub.3, TaOx, ZrO.sub.2, VO.sub.x, SiC, SiN, HfC, HfO.sub.2, VN, AlN, MoO.sub.x, TiC, TiN, B, B.sub.4C, and BN; a laser light source capable of supplying at least two levels of laser output power; an optical channel and near-field transducer for directing radiation from the light source to the disk to heat regions of the first and second magnetic layers; a write head for applying a magnetic write field to write data bits representing a first data stream in the first and second magnetic layers simultaneously when the laser is operating at a first power level and for applying a magnetic write field to write data bits representing a second data stream in only the second magnetic layer when the laser is operating at a power level lower than said first power level, the first and second data streams being asynchronous; a magnetoresistive read head for generating a composite readback signal from the written data bits representing the first and second asynchronous data streams; a carrier for supporting the write head, near-field transducer and read head; and a joint Viterbi detector for detecting, from the composite readback signal, the first and second asynchronous data streams simultaneously.

16. The disk drive of claim 15 further comprising a clock estimator coupled to the joint Viterbi detector for estimating the difference between the frequency of the first written data stream and the frequency of the second written data stream.

17. The disk drive of claim 15 wherein T.sub.C1 is substantially equal to T.sub.C2 and the spacer layer has a thermal conductivity and thickness to prevent the first magnetic layer from being heated to near or above T.sub.C1 when the laser is operating at a lower power level (P.sub.L).

18. The disk drive of claim 15 wherein T.sub.C1 is greater than T.sub.C2, whereby when the second magnetic layer is heated to near or above T.sub.C2 when the laser is at operating at a lower power level (P.sub.L) the first magnetic layer is not heated to near or above T.sub.C1.

19. The disk drive of claim 15 wherein T.sub.C1 is less than T.sub.C2 and the spacer layer has a thermal resistance and thickness to assure that the temperature drop across the spacer layer when the laser is operating at lower laser power P.sub.L is sufficiently large so that the first magnetic layer is not heated to near or above T.sub.C1.

20. The disk drive of claim 15 wherein the alloy of at least one of the first and second magnetic layers has a composition of the form (Fe.sub.(1-x)Z.sub.x).sub.yPt.sub.(100-y), where Z is selected from Cu, Ni, Cr, Mn, B and Ag, x is greater than or equal to 1 atomic percent and less than or equal to 20 atomic percent and y is greater than or equal to 45 atomic percent and less than or equal to 55 atomic percent.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) FIG. 1 is a top view of a heat-assisted magnetic recording (HAMR) disk drive according to the prior art.

(2) FIG. 2A depicts a sectional view, not drawn to scale because of the difficulty in showing the very small features, of an air-bearing slider for use in HAMR disk drive and a portion of a HAMR disk according to the prior art.

(3) FIG. 2B is a schematic of a conventional data sector that includes a preamble, sync word and customer data (shown encoded as low-density parity check (LDPC) code word).

(4) FIG. 3 is a sectional view showing a HAMR disk according to embodiments of the invention with multiple continuous non-patterned recording layers (RL1 and RL2) separated by a nonmagnetic spacer layer (SL) that magnetically decouples RL1 and RL2.

(5) FIG. 4A is a schematic sectional view of the multiple data layer HAMR disk according to embodiments of the invention after data is written in the tracks with high laser power showing perpendicularly magnetized regions or bits in RL1 and RL2.

(6) FIG. 4B is a schematic sectional view of the multiple data layer HAMR disk according to embodiments of the invention after data is written in the tracks with a lower laser power showing that the newly-written perpendicularly magnetized bits in RL2 are not synchronous with the previously-written perpendicularly magnetized bits in RL1 and not vertically aligned with the bits in RL1.

(7) FIG. 5 is a block diagram of a read channel for the HAMR disk drive according to embodiments of the invention.

DETAILED DESCRIPTION OF THE INVENTION

(8) FIG. 1 is a top view of a heat-assisted magnetic recording (HAMR) disk drive 100 according to the prior art. In FIG. 1, the HAMR disk drive 100 is depicted with a disk 200 with a continuous magnetic recording layer 31 with concentric circular data tracks 118. Only a portion of a few representative tracks 118 near the inner and outer diameters of disk 200 are shown.

(9) The drive 100 has a housing or base 112 that supports an actuator 130 and a drive motor for rotating the magnetic recording disk 200. The actuator 130 may be a voice coil motor (VCM) rotary actuator that has a rigid arm 131 and rotates about pivot 132 as shown by arrow 133. A head-suspension assembly includes a suspension 135 that has one end attached to the end of actuator arm 131 and a head carrier, such as an air-bearing slider 120, attached to the other end of suspension 135. The suspension 135 permits the slider 120 to be maintained very close to the surface of disk 200 and enables it to pitch and roll on the air-bearing generated by the disk 200 as it rotates in the direction of arrow 20. The slider 120 supports the HAMR head (not shown), which includes a magnetoresistive read head, an inductive write head, the near-field transducer (NFT) and optical waveguide. A semiconductor laser 90, for example with a wavelength of 780 to 980 nm, may be used as the HAMR light source and is depicted as being supported on the top of slider 120. Alternatively, the laser may be located on suspension 135 and coupled to slider 120 by an optical channel. As the disk 200 rotates in the direction of arrow 20, the movement of actuator 130 allows the HAMR head on the slider 120 to access different data tracks 118 on disk 200. The slider 120 is typically formed of a composite material, such as a composite of alumina/titanium-carbide (Al.sub.2O.sub.3/TiC). Only one disk surface with associated slider and read/write head is shown in FIG. 1, but there are typically multiple disks stacked on a hub that is rotated by a spindle motor, with a separate slider and HAMR head associated with each surface of each disk.

(10) In the following drawings, the X direction denotes a direction perpendicular to the air-bearing surface (ABS) of the slider, the Y direction denotes a track width or cross-track direction, and the Z direction denotes an along-the-track direction. FIG. 2A is a schematic cross-sectional view illustrating a configuration example of a HAMR head according to the prior art, which is also capable of functioning as the HAMR head in this invention. In FIG. 2A, the disk 200 is depicted as a conventional single data layer disk with the HAMR recording layer 31 being a continuous non-patterned magnetic recording layer of magnetizable material with magnetized regions or bits 34. The bits 34 are physically adjacent to one another and the boundaries of adjacent bits are referred to as magnetic transitions 37. The bits are recorded in individual data sectors. FIG. 2B shows a conventional data sector that includes a preamble, sync word and customer data (shown encoded as low-density parity check (LDPC) code word). The recording layer 31 is typically formed of a high-anisotropy (K.sub.u) substantially chemically-ordered FePt alloy (or CoPt alloy) with perpendicular magnetic anisotropy. The disk includes an overcoat 36, typically formed of amorphous diamond-like carbon (DLC), and a liquid lubricant layer 38, typically a bonded perfluropolyether (PFPE).

(11) The air-bearing slider 120 is supported by suspension 135. The slider 120 has a recording-layer-facing surface 122 onto which an overcoat 124 is deposited. The overcoat 124 is typically a DLC overcoat with a thickness in the range of about 10 to 30 and whose outer surface forms the ABS of the slider 120. An optional adhesion film or undercoat (not shown), such as a 1 to 5 silicon nitride (SiN.sub.x) film, may be deposited on the surface 122 before deposition of the overcoat 124. The slider 120 supports the magnetic write head 50, magnetoresistive (MR) read head 60, and magnetically permeable read head shields S1 and S2. A recording magnetic field is generated by the write head 50 made up of a coil 56, a main magnetic pole 53 for transmitting flux generated by the coil 56, a write pole 55 with end 52, and a return pole 54. A magnetic field generated by the coil 56 is transmitted through the magnetic pole 53 to the write pole end 52 located near an optical near-field transducer (NFT) 74. The write head 50 is typically capable of operating at different clock rates so as to be able to write data at different frequencies. The NFT 74, also known as a plasmonic antenna, typically uses a low-loss metal (e.g., Au, Ag, Al or Cu) shaped in such a way to concentrate surface charge motion at a tip located at the slider ABS when light from the waveguide 73 is incident. Oscillating tip charge creates an intense near-field pattern, heating the recording layer 31. Sometimes, the metal structure of the NFT can create resonant charge motion (surface plasmons) to further increase intensity and heating of the recording layer. At the moment of recording, the recording layer 31 of disk 200 is heated by the optical near-field generated by the NFT 74 and, at the same time, a region or bit 34 is magnetized and thus written onto the recording layer 31 by applying a recording magnetic field generated by the write pole end 52.

(12) A semiconductor laser 90 is mounted to the top surface of slider 120. An optical waveguide 73 for guiding light from laser 90 to the NFT 74 is formed inside the slider 120. The laser 90 is typically capable of operating at different power levels. Materials that ensure a refractive index of the waveguide 73 core material to be greater than a refractive index of the cladding material may be used for the waveguide 73. For example, Al.sub.2O.sub.3 may be used as the cladding material and TiO.sub.2, Ta.sub.2O.sub.5 and SiO.sub.xN.sub.y as the core material. Alternatively, SiO.sub.2 may be used as the cladding material and Ta.sub.2O.sub.5, TiO.sub.2, SiO.sub.xN.sub.y, or Ge-doped SiO.sub.2 as the core material. The waveguide 73 that delivers light to NFT 74 is preferably a single-mode waveguide.

(13) In embodiments of the HAMR disk drive of this invention the disk has multiple independent data layers, each data layer being a continuous non-patterned layer of magnetizable material. In a two-data layer embodiment, data can be stored in just one or both of the data layers. The data layers are independent in that there is no requirement that the magnetized regions or bits in one data layer be vertically aligned with the bits in the other data layers. Each data layer can store data independent and not related to the data stored in the other data layer. A first data stream is written simultaneously in both data layers by applying a magnetic field from the write head while the laser is operating at a higher power level (P.sub.H) that raises the temperature of the magnetic material in each data layer to near or above its Curie temperature. A second data stream is then written in just the upper data layer by applying a magnetic field from the write head while the laser is operating at a lower power level (P.sub.L) that raises the temperature of just the magnetic material in the upper data layer to near or above its Curie temperature. The write field will not alter the magnetized regions of the lower data layer, which will retain the previously recorded first data stream. Recorded data is read back from both data streams simultaneously with the read head coupled to a read channel that includes a joint Viterbi detector that converts the readback signal into the separate independent data streams.

(14) Each data layer is formed of high-anisotropy (K.sub.u) material so that the coercivities (H.sub.C1 and H.sub.C2) of lower and upper data layers (RL1 and RL2) are greater than the magnetic write field (H.sub.W) at normal disk drive operating temperature (i.e., approximately 275 to 335 K) when the data layers are not being written to. This assures that the recorded magnetic bits have high thermal stability. If the laser is operating at a higher power level (P.sub.H) that heats both RL1 and RL2 to above their respective Curie temperatures, data is recorded in both RL1 and RL2. If the laser is operating at a lower power level (P.sub.L) that heats only upper RL2 to above to above its Curie temperature, data is recorded only in RL2.

(15) FIG. 3 is a sectional view showing a HAMR disk 200 according to embodiments of the invention with multiple continuous non-patterned recording layers (RL1 and RL2), each having a high-K.sub.u, separated by a nonmagnetic spacer layer (SL) that magnetically decouples RL1 and RL2. The recording layers RL1 and RL2 may be comprised of a substantially chemically-ordered FePt alloy (or CoPt alloy) as proposed in the prior art, preferably with one or more segregants. The disk 200 is a substrate 201 having a generally planar surface on which the representative layers are sequentially deposited, typically by sputtering. The hard disk substrate 201 may be any commercially available high-temperature glass substrate, but may also be an alternative substrate, such as silicon, silicon-carbide, or AlMg alloy. An adhesion layer 204, typically about 10-200 nm of an amorphous adhesion material like a CrTa or NiTa alloy, is deposited on substrate 200.

(16) A heat-sink layer 206, typically about 5 to 200 nm of Ag, Al, Cu, Cr, Au, NiAl, NiTa, Ru, RuAl, W, Mo, Ta or any combination of these materials, is deposited on the adhesion layer 204. The heat-sink layer 206 facilitates the transfer of heat away from the RLs to prevent spreading of heat to regions of the RLs adjacent to where data is desired to be written, thus preventing overwriting of data in adjacent data tracks.

(17) A seed layer 208 is deposited on the heat-sink layer 206. The seed layer is typically 2-50 nm of MgO for which the 002 crystallographic orientation is chosen to determine the subsequent crystallographic orientation of the 002-oriented FePt RL1 magnetic layer whose growth is controlled by the seed layer.

(18) The perpendicular media that forms RL1 and RL2 is a high-anisotropy (K.sub.u) substantially chemically-ordered FePt alloy (or CoPt alloy) with perpendicular magnetic anisotropy. Substantially chemically-ordered means that the FePt alloy has a composition of the form Fe.sub.(y)Pt.sub.(100-y) where the subscripts are in atomic percent (at. %) and y is between about 45 and 55. Such alloys of FePt (and CoPt) ordered in L1.sub.0 are known for their high magneto-crystalline anisotropy and magnetization, properties that are desirable for high-density magnetic recording materials. The substantially chemically-ordered FePt alloy, in its bulk form, is known as a face-centered tetragonal (FCT) L1.sub.0-ordered phase material (also called a CuAu material). The c-axis of the L1.sub.0 phase is the easy axis of magnetization and is oriented perpendicular to the disk substrate. The substantially chemically-ordered FePt alloy may also be a pseudo-binary alloy based on the FePt L1.sub.0 phase, e.g., (Fe.sub.(y)Pt.sub.(100-y))X, where y is between about 40 and 60 at. % and the element X may be one or more of Ni, Au, Cu, Ge, Pd, Mn and Ag and present in the range of between about 1 and 20 at. %. While the pseudo-binary alloy in general has similarly high anisotropy as the binary alloy FePt, it allows additional control over the magnetic and other properties of the RLs. For example, Ag improves the formation of the L1.sub.0 phase and Cu reduces the Curie temperature. While the preferred material for the RLs is FePt or FePtX, the RLs may alternatively be formed of a CoPt (or a pseudo-binary CoPtX alloy based on the CoPt L1.sub.0 phase).

(19) FePt L1.sub.0 phase based thin films exhibit strong perpendicular anisotropy, which potentially leads to small (e.g., 3-9 nm in diameter) thermally stable grains for ultrahigh density magnetic recording. To fabricate small grain FePt L1.sub.0 media some form of segregant to separate grains can be used as an integral part of the magnetic recording layer. Thus in the HAMR disk 200, the RLs may also typically include a segregant, such as one or more of C, SiO.sub.2, TiO.sub.2, Y.sub.2O.sub.3, TaOx, ZrO.sub.2, VO.sub.x, SiC, SiN, HfC, HfO.sub.2, VN, AlN, MoO.sub.x, TiC, TiN, B, B.sub.4C, and BN that forms between the FePt grains and reduces the grain size. The segregant concentration by volume in the RL can be from 10 to 50%, with the remainder being FePt alloy.

(20) The spacer layer (SL) may be a nonmagnetic material that is thermally conductive, provided it is thick enough to ensure that RL1 and RL2 are magnetically decoupled. In addition, the SL needs to set the growth correctly for RL2 so that it has the correct magnetocrystalline orientation and good grain microstructure. Materials that may be used for the SL include but are not limited to dielectric materials such as magnesium oxide (MgO), magnesium titanate (Mg.sub.2TiO.sub.4), spinel (Mg.sub.2Al.sub.3O.sub.4), barium zirconium oxide (BaZrO.sub.3), barium cerium oxide (BaCeO.sub.3), strontium titanate (SrTiO.sub.3), strontium zirconium oxide (SrZrO3), lanthanum aluminum oxide (LaAlO.sub.3), titanium nitride (TiN), titanium oxynitride (TiO.sub.xN.sub.(1-x)), silicon nitride (SiN) and carbon (C). Other suitable materials include body-centered cubic (bcc) metals such as Cr, W, Mo, Mn, V, Nb and alloys thereof, and other metals such as Ru, Rh, Ir and alloys thereof. The material of the SL is selected so that its known thermal conductivity, in combination with its thickness, provides the desired amount of heat flow to the lower RL1 when the laser is at higher power level P.sub.H while preventing or reducing heat flow to the lower RL1 when the laser is at lower power level P.sub.L. The typical thickness of the SL may be between about 1 nm and 20 nm, depending on the thermal conductivity of the SL material.

(21) A protective overcoat (OC) may deposited on RL2, preferably to a thickness between about 1-5 nm. The OC may be a layer of amorphous carbon, like amorphous diamond-like carbon (DLC). The amorphous carbon or DLC may also be hydrogenated and/or nitrogenated, as is well-known in the art. On the completed disk, a liquid lubricant, like a perfluorpolyether (PFPE), is coated on the OC.

(22) In a first embodiment, RL1 and RL2 may have substantially the same Curie temperature (T.sub.C1T.sub.C2) and substantially the same coercivity (H.sub.C1H.sub.C2), for example by having substantially the same chemical composition, while the SL has a thermal conductivity and thickness that prevents RL1 from being heated to near or above its Curie temperature (T.sub.C1) when the laser is operating at a lower power level (P.sub.L). As used herein, substantially the same Curie temperature indicates that the two values are within 3 percent of one another (20 K). As used herein, substantially the same coercivity indicates the two values are within 10 percent of one another. It is understood that the coercivity depends on the magnetic field exposure time and temperature. Coercivities are typically measured at room temperature with long field sweep rates, even though the coercivity under recording conditions is the more relevant parameter. For example, RL1 and RL2 may each be formed of Fe.sub.50Pt.sub.50 with one or more segregants of C, BN and SiO.sub.2 and with a thickness between about 5 and 15 nm. RL1 and RL2 would then each have a Curie temperature of about 740 K and a coercivity of about 50 kOe. The SL may be formed of any of the materials listed above, such as MgO or BaZrO.sub.3 with a thickness between about 3 and 10 nm. In this example it is assumed the write head generates a magnetic write field H.sub.W of approximately 10 kOe and the laser generates a higher power level P.sub.H of 10 mW and a lower power level P.sub.L of 7 mW, with a 5% coupling efficiency with the output of the NFT, which can be used to write independent data to the top and bottom layers. A higher laser power level P.sub.H of 10 mW will increase the temperature of both RL1 and RL2 to more than 740 K, which will reduce H.sub.C1 and H.sub.C2 to approximately 0 Oe so that data will be written to both RL1 and RL2 by the write field H.sub.W as the media cools. This is depicted in the schematic sectional view FIG. 4A which shows the perpendicularly magnetized regions or data bits when a data stream is written simultaneously in the data tracks in both RL1 and RL2. However, a lower laser power level P.sub.L of 7 mW will still increase the temperature of the upper RL2 to over 740 K, which will reduce H.sub.C2 to approximately 0 Oe, but the SL will prevent the lower RL1 from being heated to near or above T.sub.C1 so that H.sub.C1 will not be reduced below the write field H.sub.W. Thus the write field H.sub.w will not alter the magnetized regions of RL1. This is depicted in the schematic sectional view FIG. 4B which shows that the newly-written perpendicularly magnetized data bits in RL2 are not necessarily vertically aligned with the previously-written perpendicularly magnetized data bits in RL1 when a different data stream is written in the data track of just RL2. This is because the writing of bits to the upper RL2 with lower laser power level P.sub.L is not synchronized with the previously-written bits in RL1. Even if it is desired to write both data layers at the same data densities so as to have the bits vertically aligned, the natural fluctuations in disk velocity during writing make this alignment extremely difficult to achieve. Also, the bits may intentionally be written to the upper and lower data layers different frequencies. For example, it may be desirable to write the lower RL1 data layer at rds the frequency at which the upper RL2 data layer is written because of the inherently lower resolution of the lower RL1 data layer. If the data layers are intentionally written at different frequencies, the width of the bits in RL2 will be different from the width of the bits in lower RL1.

(23) In a second embodiment, lower RL1 has a higher Curie temperature than upper RL2 (T.sub.C1>T.sub.C2). In one example, both RL1 and RL2 may each have a thickness between about 5 and 15 nm. Lower RL1 may be formed of a chemically-ordered Fe.sub.50Pt.sub.50 alloy with one or more segregants. Upper RL2 may be formed of (Fe.sub.(1-x)Cu.sub.x).sub.50Pt.sub.50, where x is greater than or equal to 1 at. % and less than or equal to 20 at. %, and preferably greater than or equal to 5 at. % and less than or equal to 10 at. %. The SL may be formed of any of the materials listed above, such as MgO or BaZrO.sub.3, with a thickness between about 2 and 10 nm. The addition of Cu to FePt, preferably by substitution for Fe, decreases the Curie temperature with only a small decrease in magnetic anisotropy. These FeCu.sub.xPt materials can be selected to have a high anisotropy field of 80-100 kOe or above for the lower RL1. Upper RL2 can have an anisotropy field of 70-90 kOe, which will yield a coercivity that is also substantially higher than the write field H.sub.W of 10 kOe. Because upper RL2 contains Cu and lower RL1 does not, T.sub.C1 is greater than T.sub.C2. A substantial difference in Curie temperatures of RL1 and RL2 will assure that only the magnetizations of the bits in upper RL2 will be switched in the presence of H.sub.w when the laser is at lower power level P.sub.L. Generally this means there will also be a substantial difference in coercivities of RL1 and RL2 (H.sub.C1 greater than H.sub.C2). For example, by appropriate selection of the amount of Cu this results in upper RL2 with H.sub.C2 of 30 kOe and T.sub.C2 of 630 K. Lower RL1 will have H.sub.C1 of 50 kOe and T.sub.C1 of 740 K. Thus in the example of this second embodiment a higher laser power level P.sub.H of 10 mW will increase the temperature of both RL1 and RL2 to more than 740 K, which will reduce H.sub.C1 and H.sub.C2 to approximately 0 Oe so that data will be written to both RL1 and RL2 by the write field H.sub.W. This is depicted in the schematic sectional view FIG. 4A which shows the perpendicularly magnetized regions or bits when a data stream is written simultaneously in the data tracks in both RL1 and RL2. However, a lower laser power level P.sub.L of 5 mW will increase the temperature of the upper RL2 to approximately 630 K, which will reduce H.sub.C2 to approximately 0 Oe, but this temperature is below T.sub.C1 of 740 K and the SL will further limit the temperature in RL1, so H.sub.C1 will not be reduced below H.sub.W. Thus H.sub.w will not alter the magnetized regions of RL1. This is depicted in the schematic sectional view of FIG. 4B which shows that the newly-written perpendicularly magnetized bits in RL2 are not vertically aligned with the previously-written perpendicularly magnetized bits in RL1 when a different data stream is written in the data track of just RL2. This is because the writing of bits to the upper RL2 with lower laser power level P.sub.L is not synchronized with the previously-written bits in RL1. Also, the writing of bits to the upper RL2 may be at a different frequency so the width of the bits in RL2 may be different from the width of the bits in lower RL1. The two data layers are thus independent of one another.

(24) In the example of this second embodiment, lower RL1 is the chemically-ordered Fe.sub.50Pt.sub.50 binary alloy without Cu. However, RL1 could also be a (Fe.sub.(1-x)Cu.sub.x).sub.50Pt.sub.50 alloy provided it contains less Cu than upper RL2 to assure that T.sub.C1 is greater than T.sub.C2.

(25) In a third embodiment, lower RL1 has a lower Curie temperature than upper RL2 (T.sub.C1<T.sub.C2). In order to reduce thermal degradation of the NFT, it is desirable to keep the operating laser powers P.sub.H and P.sub.L as low as possible. Lowering T.sub.C1 reduces the power P.sub.H required to write the lower layer and can increase the useable lifetime of the NFT. In one example, both RL1 and RL2 may each have a thickness between about 5 and 15 nm. Lower RL1 may be formed of (Fe.sub.(1-x)Cu.sub.x).sub.50Pt.sub.50, where x is greater than or equal to 1 at. % and less than or equal to 20 at. %, and preferably greater than or equal to 5 at. % and less than or equal to 10 at. %. Upper RL2 will have less Cu than RL1 and may be formed of (Fe.sub.(1-x)Cu.sub.x).sub.50Pt.sub.50, where x is greater than or equal to 1 at. % and less than or equal to 20 at. %, and preferably less than or equal to 10 at. %. The addition of Cu to FePt, preferably by substitution for Fe, decreases the Curie temperature with only a small decrease in magnetic anisotropy. Because the Cu content of the RL1 is higher than RL2, T.sub.C1 is lower than T.sub.C2. The SL may be formed of any of the materials listed above, such as MgO or BaZrO.sub.3, with a thickness between about 2 and 10 nm. Generally this means there will also be a substantial difference in coercivities of RL1 and RL2 (H.sub.C1 is less than H.sub.C2). For example, by appropriate selection of the amount of Cu this results in upper RL2 with H.sub.C2 of 50 kOe and T.sub.C2 of 740 K. Lower RL1 will have H.sub.C1 of 40 kOe and T.sub.C1 of 690 K. The thermal resistance of the SL depends on the thickness and thermal conductivity of the SL material. The thermal resistance of the SL is selected so that the temperature drop across the SL during recording of RL2 with laser power P.sub.L is sufficiently large so that the temperature of RL1 does not exceed T.sub.C1. A sufficient thermal resistance of SL will ensure that RL1 stays sufficiently below T.sub.C1 so that only the magnetizations of the bits in upper RL2 will be switched in the presence of H.sub.w when the laser is at lower power level P.sub.L. Thus in the example of this third embodiment a laser power level P.sub.H of 9 mW will increase the temperature of both RL1 and RL2 to more than 740 K, which will reduce H.sub.C1 and H.sub.C2 to Approximately 0 Oe so that Data Will be written to both RL1 and RL2 by the write field H.sub.W. This is depicted in the schematic sectional view FIG. 4A which shows the perpendicularly magnetized regions or bits when a data stream is written simultaneously in the data tracks in both RL1 and RL2. A lower laser power level P.sub.L of 7 mW will still increase the temperature of the upper RL2 to approximately 740 K, which will reduce H.sub.C2 to approximately 0 Oe. However, the large thermal resistance of the SL will prevent RL1 from exceeding 690K, so H.sub.C1 will not be reduced below H.sub.W. Thus H.sub.w will not alter the magnetized regions of RL1. This is depicted in the schematic sectional view of FIG. 4B which shows that the newly-written perpendicularly magnetized bits in RL2 are not vertically aligned with the previously-written perpendicularly magnetized bits in RL1 when a different data stream is written in the data track of just RL2. This is because the writing of bits to the upper RL2 with lower laser power level P.sub.L is not synchronized with the previously-written bits in RL1. Also, the writing of bits to the upper RL2 may be at a different frequency so the width of the bits in RL2 may be different from the width of the bits in lower RL1. The two data layers are thus independent of one another.

(26) While Cu is the preferred material for substitution of a portion of the Fe to reduce the Curie temperature, other materials that may be used include Ni, Cr, Mn, B and Ag. Thus the RLs may be formed of a material having a composition of the form (Fe.sub.(1-x)Z.sub.x).sub.yPt.sub.(100-y), where Z is selected from Cu, Ni, Cr, Mn, B and Ag, x is between 1 and 20 and y is between about 45 and 55. The addition of Ni to create FeNiPt alloys, like FeCuPt alloys, also decreases the magnetocrystalline anisotropy and Curie temperature. The properties of FeCuPt alloys have been described by J. Ikemoto et al., Control of Curie Temperature of FePt(Cu) Films Prepared From Pt(Cu)/Fe Bilayers, IEEE Transactions on Magnetics, Volume 44, Issue 11, November 2008, pp. 3543-3546 and S. D. Willoughby, Electronic and magnetic properties of Fe.sub.1-xCu.sub.xPt, J. Appl. Phys. 95, 6586 (2004). The properties of FeNiPt alloys have been described by J. Thiele et al., Temperature dependent magnetic properties of highly chemically ordered Fe.sub.55-xNi.sub.xPt.sub.45L1.sub.0 films, J. Appl. Phys. 91, 6595 (2002).

(27) In addition to or as an alternative to the use of Cu, Ni, Cr, Mn, B or Ag to substitute for a portion of the Fe to reduce the Curie temperature of the FePt alloy, certain segregants can be added to reduce the Curie temperature. In general the use of pure carbon segregant produces the highest Curie temperatures in FePt media (740 K and greater). The use of nearly every other segregant, such as BNSiO.sub.2, will reduce the Curie temperature. (See O. Hovorka et al., The Curie temperature distribution of FePt granular magnetic recording media, Appl. Phys. Lett. 101, 052406 (2012)). Thus, in one embodiment the segregant in RL1 contains more carbon than the segregant in RL2 to assure that T.sub.C1 is greater than T.sub.C2. In another embodiment the segregant in RL2 contains more carbon than the segregant in RL1 to assure that T.sub.C2 is greater than T.sub.C1.

(28) Another technique to reduce the Curie temperature is to deposit off-stoichiometry Fe.sub.(50-x)Pt.sub.(50-x) media in which x=1 to 10 or 10 to 1. Tc is reduced whenever x is not equal to zero.

(29) Increasing the difference between P.sub.L and P.sub.H will increase the likelihood that RL2 can be written without altering the magnetic configuration of RL1. The SL thermal resistance is proportional to the product of the thermal resistivity of the SL multiplied by its thickness. Increasing the thickness of the SL will require an increase in P.sub.H, but will potentially reduce the readback signal of RL1. The thermal resistivity is a property of the materials used in the SL, the defect density of the SL, and the interface thermal properties of the SL with RL1 and RL2. The choice of spacer layers with high thermal resistance, such as BaZrO.sub.3 or SiO.sub.2, will require an increase in P.sub.H and enable spacer layer thicknesses below 5 nm.

(30) The absorption properties of RL1 and RL2 also affect the required values for P.sub.L and P.sub.H Thicker recording layers absorb proportionally more light and lower the required power to reach Tc. A larger RL2 thickness relative RL1 will require a lower P.sub.L relative to P.sub.H. A practical difference in the thickness is that the thickness of RL2 is 5-25% greater than the thickness of RL1. A larger RL2 thickness will decrease the power P.sub.L required to write only RL2. In another embodiment, the thickness of RL1 is 5-25% greater than the thickness or RL2. In this embodiment P.sub.H is required to be decreased relative to P.sub.L, but the NFT lifetime is increased due to the lower maximum required operating laser power.

(31) FIG. 5 is a block diagram of an example read channel for converting the asynchronous readback signals from the two data layers of the HAMR disk into the separate data streams of customer data. The recorded customer data for each data layer RL1 and RL2, in the form of LDPC code words, is detected by the MR read head as a composite analog readback signal. Because of the lower resolution of the data from the lower RL1 data layer, the lower layer may employ a lower code-rate in the LDPC correction code so as to have more redundancy to make up for the lower quality signal from the lower RL1 data layer.

(32) The clock can operate at a frequency different from the data frequencies fc1 and fc2 of RL1 and RL2, respectively, but preferably operates at fc2, the frequency of the upper data layer RL2 which provides a stronger higher-resolution signal because it is closer to the MR read head and would typically be written at a higher data rate. The composite analog signal detected by the read head is amplified by a preamplifier (not shown) and filtered by an analog continuous-time filter (not shown) before being sampled and converted to digital format and sent to a digital equalizer.

(33) The digital equalizer provides an output y.sub.k(t) for each sample k that is representative of the composite analog input signal at that sample time. In this example, each equalized sample depends on 2 bits from RL2 and slowly varying amounts of 5 bits from RL1. The equalizer's target for the upper layer RL2 is 1+3.0D where D represents a delay of one clock cycle. The upper layer RL2 provides a taller, narrower bit response than the lower layer RL1, which has a shorter, wider bit response because of the increased separation from the read head. The bit in RL1 is depicted at five positions because the sampling phase of the signal from RL1 may slowly slip in timing since the data is written at a different clock rate or a slightly different velocity than the data in RL2.

(34) The read channel includes a joint Viterbi detector, i.e. a Viterbi detector capable of decoding two mixed bit streams into two separate bit streams. The joint Viterbi detector assumes the input signal to have the following form:
y.sub.k(t)=1.0a.sup.U.sub.k+0.3a.sup.U.sub.k1+c1(t)a.sup.L.sub.k2+c2(t)a.sup.L.sub.k1+c3(t)a.sup.L.sub.k+c4(t)a.sup.L.sub.k+1+c5(t)a.sup.L.sub.k+2
where 1.0 and 0.3 are coefficients for the upper RL2 data stream, c1 through c5 are coefficients for the lower RL1 bit stream, a.sup.U.sub.k is the bit value (+/1) of the kth sample for the upper layer and a.sup.L.sub.k is the bit value (+/1) of the kth sample for the lower layer.

(35) The branches in the joint Viterbi detector must describe all possible values of y.sub.k, which can take 2.sup.2+5 levels. Branches begin and end on states, so this requires 2.sup.(21)+(51)=32 states. The data frequencies fc1 and fc2 may be chosen to be different and, in any case, are very difficult to make precisely equal because the data is written in the RLs at slightly different velocities. For this reason the coefficients (c1, c2, c3, c4, c5) on the branches must be varied slowly at the beat frequency between the two layers. Thus a clock estimator estimates slight variations in fc1 relative to fc2 and controls a cyclic coefficients estimator for the c1 through c5 coefficients used by the joint Viterbi detector. The cyclic coefficients estimator can use a look-up table to extract new coefficients based on the known difference in clock frequencies. The detector is reset every 1-bit of timing slip and a single clock cycle (and thus one data bit) is omitted from the lower frequency data stream. With this method of varying the Viterbi coefficients based on timing differences between the two asynchronous data streams, the joint Viterbi detector is capable of detecting two asynchronous data streams simultaneously.

(36) The joint Viterbi detector outputs the two separate data streams, which are sent to respective upper (RL2) and lower (RL1) LDPC decoders and output as the two data streams of previously recorded customer data. Typically soft information is passed back and forth between the joint Viterbi detector and the LDPC decoders in iterative fashion until there is convergence to a solution. This approach is commonly referred to as iterative detection with belief propagation. This is suggested in FIG. 5 by the bi-directional arrows on the lines between the joint Viterbi detector and the LDPC decoders.

(37) While the present invention has been particularly shown and described with reference to the preferred embodiments, it will be understood by those skilled in the art that various changes in form and detail may be made without departing from the spirit and scope of the invention. Accordingly, the disclosed invention is to be considered merely as illustrative and limited in scope only as specified in the appended claims.