# Patent

## Title
Interfering near field transducer having a wide metal bar feature for energy assisted magnetic recording

## Abstract

An apparatus for energy assisted magnetic recording of a storage disk include a plurality of dielectric waveguide cores configured to direct received incident light energy to a target, and a near field transducer (NFT) configured to focus light energy received from the plurality of waveguide cores and to transmit the focused light energy onto the storage disk surface to generate a heating spot on the storage disk. The NFT includes a plurality of propagating surface plasmon polariton (PSPP) elements that are energized by the light energy from the waveguide cores. Each of the PSPP elements has a plasmonic metal bar disposed above a single waveguide core in a longitudinal alignment. Each metal bar has a width at least twice the width of the heating spot generated on the storage disk.

## Background
CROSS-REFERENCE TO RELATED APPLICATION(S)
This application claims the benefit of U.S. Provisional Application Ser. No. 62/010,072 filed on Jun. 10, 2014, which is expressly incorporated by reference herein in its entirety.


BACKGROUND
High density storage disks are configured with layers of materials that provide the required data stability for storage. The magnetic properties of the media may be softened when writing to the disk to assist changing the bit state. Energy Assisted Magnetic Recording (EAMR) device or Heat Assisted Magnetic Recording (HAMR) technology provides heat that is focused on a nano-sized bit region when writing onto a magnetic storage disk, which achieves the magnetic softening. A light waveguide directs light from a laser diode to a near field transducer (NFT). The NFT couples the diffraction limited light from waveguide (WG), then further focuses the light field energy beyond diffraction limit down to a highly concentrated (nano-sized) near-field media heating spot enabling EAMR/HAMR writing to the magnetic storage disk. Inefficiencies in the NFT can have a negative impact on the power budget of the laser diode and the EAMR/HAMR system lifetime. Higher NFT efficiency allows for lower laser power demand, relieving EAMR/HAMR system requirement on the total optical power from the laser source, and results in less power for parasitic heating of the EAMR/HAMR head resulting for improved reliability.
In an NFT, plasmonic metal can be used to form SPPs (surface plasmon polaritons), which carry out the nano-focusing function beyond the light's diffraction limit. High quality plasmonic metals rely on high density free electrons which have weak mechanical robustness, and are susceptible to damages caused by thermal or mechanical stresses in an EAMR head. Under these stresses, the service lifetime of the EAMR/HAMR device is limited to NFT failure occurring at the plasmonic metal part having fine (nano-sized) features, such as at a ridge or a pin.



BRIEF DESCRIPTION OF THE DRAWINGS
Various aspects of the present invention will now be presented in the detailed description by way of example, and not by way of limitation, with reference to the accompanying drawings, wherein:
FIG. 1 shows a diagram of an exemplary hard disk drive.
FIG. 2 shows a diagram of an exemplary embodiment of a near field transducer formed with two plasmonic metal bars.
FIG. 3 shows a diagram of an exemplary embodiment of a near field transducer formed with two plasmonic metal bars and a plasmonic metal cap.
FIG. 4 shows a diagram of an exemplary embodiment of a near field transducer formed with two plasmonic metal bars and coupled to a magnetic pole.
FIG. 5 shows a diagram of an exemplary embodiment of a near field transducer formed with two plasmonic metal bars with centerlines that are offset from the dielectric waveguide core centerlines.



DETAILED DESCRIPTION
The detailed description set forth below in connection with the appended drawings is intended as a description of various exemplary embodiments and is not intended to represent the only embodiments that may be practiced. The detailed description includes specific details for the purpose of providing a thorough understanding of the embodiments. However, it will be apparent to those skilled in the art that the embodiments may be practiced without these specific details. In some instances, well known structures and components are shown in block diagram form in order to avoid obscuring the concepts of the embodiments. Acronyms and other descriptive terminology may be used merely for convenience and clarity and are not intended to limit the scope of the embodiments.
The various exemplary embodiments illustrated in the drawings may not be drawn to scale. Rather, the dimensions of the various features may be expanded or reduced for clarity. In addition, some of the drawings may be simplified for clarity. Thus, the drawings may not depict all of the components of a given apparatus.
Various embodiments will be described herein with reference to drawings that are schematic illustrations of idealized configurations. As such, variations from the shapes of the illustrations as a result of manufacturing techniques and/or tolerances, for example, are to be expected. Thus, the various embodiments presented throughout this disclosure should not be construed as limited to the particular shapes of elements illustrated and described herein but are to include deviations in shapes that result, for example, from manufacturing. By way of example, an element illustrated or described as having rounded or curved features at its edges may instead have straight edges. Thus, the elements illustrated in the drawings are schematic in nature and their shapes are not intended to illustrate the precise shape of an element and are not intended to limit the scope of the described embodiments.
The word “exemplary” is used herein to mean serving as an example, instance, or illustration. Any embodiment described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments. Likewise, the term “embodiment” of an apparatus or method does not require that all embodiments include the described components, structure, features, functionality, processes, advantages, benefits, or modes of operation.
As used herein, the term “about” followed by a numeric value means within engineering tolerance of the provided value.
In the following detailed description, various aspects of the present invention will be presented in the context of an interface between a waveguide and a near field transducer used for heat assisted magnetic recording on a magnetic storage disk.
FIG. 1 shows a hard disk drive 111 including a disk drive base 114, at least one rotatable storage disk 113 (e.g., such as a magnetic disk, magneto-optical disk), and a spindle motor 116 attached to the base 114 for rotating the disk 113. The spindle motor 116 typically includes a rotating hub on which one or more disks 113 may be mounted and clamped, a magnet attached to the hub, and a stator. At least one suspension arm 108 supports at least one head gimbal assembly (HGA) 112 that holds a slider with a magnetic head assembly of writer and reader heads. A ramp assembly 100 is affixed to the base 114, and provides a surface for tip of the suspension arm 108 to rest when the HGA 112 is parked (i.e., when the writer and reader heads are idle). During a recording operation of the disk drive 111, the suspension arm 108 rotates at the pivot 117, disengaging from the ramp assembly 100, and moves the position of the HGA 112 to a desired information track on the rotating storage disk 113. During recording, the slider is suspended by the HGA 112 with an air bearing surface of the slider that faces the rotating storage disk 113, allowing the writer head to magnetically alter the state of the storage bit. For heat assisted magnetic recording, a near field transducer (NFT) on the air bearing surface may couple light energy from a waveguide to produce a heating spot on the rotating storage disk 113 for magnetically softening the bit space.
FIG. 2 shows a diagram of an exemplary embodiment of an NFT 200 arranged at an air bearing surface (ABS) 210 of a slider which carries a magnetic head assembly. The ABS 210 is the surface of the slider facing the storage disk 113. As the slider flies over the storage disk 113, a cushion of air is maintained between the slider and the disk 113. As shown, two dielectric waveguide (WG) cores 211, 212 are each arranged to carry light energy to the NFT 200. The light energy may be generated by a common laser diode source (not shown) that may be split in half by a splitter (not shown). The dielectric waveguide cores 211, 212 may be of equal length to ensure that the combined energy wave at the ABS 210 is in substantial phase alignment for constructive interference and maximum energy emission to the storage disk 113. Alternatively, dielectric waveguide cores 211, 212 may be of unequal length such that the incident energy waves may have a particular phase difference that optimizes constructive interference and maximum energy magnitude at the ABS 210. The two waveguide cores 211, 212 are substantially linear and converge at a junction near the ABS 210 at an interior angle between 0 and 180 degrees, (e.g., approximately 90 degrees as shown in FIG. 2). The dielectric material of the waveguide core may be Ta2O5 for example.
As shown in the cross section of FIG. 2, the NFT 200 includes a plasmonic metal bar element 202, which may be disposed above a waveguide core 212 in a longitudinal direction, with centerline of the plasmonic bar element 202 approximately aligned along the centerline the waveguide core 212 surface. Similarly, a plasmonic metal bar 201 may be disposed above a waveguide core 211 as shown in FIG. 2. The optical energy from the dielectric waveguide core 211, 212 in proximity with of the plasmonic metal bar 201, 202 energizes propagating surface plasmon polaritons (PSPPs) along the surface of plasmonic metal bar 201, 202 toward the ABS 210. Thus, each plasmonic metal bar element 201, 202 may function as a PSPP element. As shown in the cross section, a gap (e.g., of about 20 nm) may exist between the plasmonic metal bar 201, 202 and the dielectric waveguide core 211, 212. Alternatively, the gap may be omitted, and the plasmonic metal bar 201, 202 may directly contact the dielectric waveguide core 211, 212, at least for a portion of the plasmonic metal bar 201, 202. The two dielectric waveguide cores 211, 212 and the entire NFT 200 may be encapsulated by a silicon oxide material. The material of the plasmonic metal bars 201, 202 may be a gold alloy, for example. Other examples of plasmonic metals that may be used to form the plasmonic metal bars 201, 202 include silver or copper alloys.
The plasmonic metal bar elements 201, 202 may be configured as shown in FIG. 2, converging at a junction above the junction of the dielectric waveguide cores 211, 212. The junction of plasmonic metal bar elements may occur on a common plane, or may be formed by overlapping one element over the other element. The junction of plasmonic metal bar elements 201, 202 may be formed at the ABS 210. For example, an NFT energy output emitter may be formed at the ABS 210 by the exposed metal bar junction at the ABS 210, from which the maximum energy is propagated across the air cushion and onto the storage disk 113 surface. The physical dimension of the emitter (i.e., the width of the exposed plasmonic metal bar junction) may be approximately equivalent to the size of the focused heating spot on the surface of the disk 113. The target size of the heating spot is dependent on the track size as the slider flies over the track, which may be about 10-70 nm wide for example. The size of the heating spot also depends on the distance between the ABS 210 and the disk 113. The focus of the heating spot may be optimized by minimizing the gap distance. The width of the plasmonic metal bar element 201, 202 may be approximately equivalent to the width of the dielectric waveguide core 211, 212 (i.e., the width of the plasmonic metal bar element 201, 202 may be slightly more wide or less wide than the width of the waveguide core 211, 212). For example, the width of the plasmonic metal bar may be within a range of about 120-350 nm, or within a more narrow range such as 150-300 nm. In an embodiment, the width of the plasmonic metal bars 201, 202 may be at least two times the width of the heating spot generated on the storage disk 113. In another embodiment, the width of the plasmonic metal bars 201, 202 may be at least three to six times the width of the heating spot generated on the storage disk 113.
To achieve the required focus of the heating spot width while using a wide plasmonic metal bar, the NFT 200 may be configured with one or more of the following features. The emitter may be configured such that the width of exposed plasmonic metal is approximately equivalent to the desired width of the heating spot. The emitter width may be controlled by lapping the ABS 210 until the width dimension for the junction of the exposed plasmonic metal bars is within the acceptable range. Also, the NFT 200 may be configured with the PSPP elements 201, 202 having unequal lengths such that the constructive interference at the ABS 210 produces the desired focus width for the heating spot.
The two-PSPP element configuration as shown in FIG. 2 may provide approximately twice as much electrical field magnitude compared with a configuration of a single PSPP element arranged perpendicular to the ABS 210, driven by a common total input power in the waveguide system. The constructive interference produced by the two PSPP elements 201, 202 allows improved efficiency of energy delivery from the laser diode source, which translates to longer service life of the EAMR/HAMR device. To optimize the efficiency of the two-PSPP element configuration, each PSPP element 201, 202 is configured with a length L which is an integer multiple of coupling length Lc from dielectric waveguide core to the PSPP element 201, 202 (e.g., for Lc of 1200 nm, the length of the PSPP element 201, 202 should be about a(1200 nm) where a is an integer value). With the PSPP element 201, 202 having a length L approximately equivalent to aLc ensures that the maximum energy transfer propagates from the PSPP element 201, 202 at the ABS 210. If the length of the PSPP element 201, 202 deviates from aLc, some of the energy wave may be lost to the dielectric waveguide core 211, 212.
FIG. 3 shows an exemplary embodiment of an NFT 300, which is a variation to the exemplary embodiment of FIG. 2, by the addition of a plasmonic metal cap 305 in the NFT 300. As shown in FIG. 3, the plasmonic metal cap 305 may be configured as a semi-circle having a straight edge substantially aligned with the ABS 310. The size of the metal cap may be, for example, 1000 nm in diameter. The thickness of the plasmonic metal cap 305 is not a significant factor in achieving the precise nano-sized heating spot, and therefore the thickness may be configured according to providing adequate heat transfer for controlling the peak temperature in the NFT 300. As an example, the plasmonic metal cap 305 may be greater than 30 nm in thickness. The metal cap 305 may be configured in shapes other than a semicircle, such as rectangular or polygonal. The plasmonic metal bar elements 301, 302 may be coupled to the plasmonic metal cap 305 above. For illustrative purpose, the plasmonic metal element 305 is depicted as transparent to reveal the metal bars 301, 302 below. As shown in the cross section, a gap (e.g., about 20 nm) may exist between the plasmonic metal bars 301, 302 and the dielectric waveguide cores 311, 312. Alternatively, the gap may be omitted, and the plasmonic metal bar 301, 302 may directly contact the dielectric waveguide core 311, 312, at least for a portion of the plasmonic metal bar 301, 302. The two dielectric waveguide cores 311, 312 and the entire NFT 300 may be encapsulated by a silicon oxide material. The material of the plasmonic metal cap 305 may be a gold alloy, for example. Other examples of plasmonic metals that may be used to form the plasmonic metal cap 305 include silver or copper alloys.
The NFT 200 and NFT 300 embodiments do not have to be limited to two interfering PSPP elements as shown in FIGS. 2 and 3. In an alternative embodiment, N (a positive integer) PSPP elements interfere at the ABS, which may provide approximately N times increase of the electrical field magnitude, driven by a common total input power in the waveguide system. The value of N can increase beyond 2 or 3, until other parasitic interferences within the three dimensional layout the EAMR head becomes a limiting factor. For N≧3, the PSPP elements may be arranged in a three dimensional configuration (i.e., not all NPTs must exist in a common two dimensional plane).
The number of plasmonic metal bars may vary with respect to the number of dielectric waveguide cores. For example, as an alternative aspect to the NFT 200 and NFT 300 shown in FIGS. 2 and 3, two metal bars may be disposed side-by-side above a single waveguide, with the total width of the metal bars being approximately equivalent to the width of the waveguide.
FIG. 4 illustrates and exemplary embodiment in which a magnetic pole 407 for the writer head may be coupled to the plasmonic metal cap 405, as a variation of NFT 300 shown in FIG. 3. Plasmonic metal bars 401, 402 are disposed above waveguide cores 411, 412. A thin diffusion barrier 408 may be disposed between the pole 407 and plasmonic metal cap 405 to prevent diffusion between the ferrous material of the magnetic pole and the alloy material in the plasmonic metal cap 405. The plasmonic metal cap 405 serves as a heat sink and a light block for the magnetic pole 407. In an alternative embodiment, the pole 407 may be recessed from the ABS 410. The pole 407 need not be centered above the plasmonic metal cap 405 of NFT 400.
FIG. 5 illustrates an exemplary embodiment of an NFT 500 in which the centerline 521, 522 of the plasmonic metal bars 501, 502 is offset from the centerline 531, 532 of the dielectric waveguide cores 511, 512. With this NFT feature, additional control of the additive interference of the light energy at the ABS 510 is provided to achieve the desired nano-sized focus width for the heating spot on the storage disk 113 while using wider PSPP elements 501, 502. As shown, each of the two plasmonic metal bars 501, 502 is disposed longitudinally along its respective waveguide core 511, 512. At the junction of the two metal bars 501, 502, the metal is exposed at the ABS 510 to form an emitter for the laser, as described above with respect to FIG. 2. The embodiment shown in FIG. 5 differs with the embodiment shown in FIG. 2, however, in that the junction of the waveguide cores 511, 512 is not exposed to the ABS 510 (i.e., the outer edge of the waveguide core junction is recessed from the ABS 510), while the offset junction of plasmonic metal bars 501, 502 extends to the ABS 510 to form the emitter. The width of the emitter may be greater than or equal to the width of the heating spot on the storage disk 113. In one embodiment, the additive interference of the two metal bars 501, 502 may focus the laser spot to a width that is less than the width of the emitter.
The NFT 500 embodiment shown in FIG. 5, with the waveguide core junction recessed from the ABS 510, may produce an energy wave with side lobes from optical energy leakage from the waveguide that are weak enough not to pass through the NFT 500 to the ABS 510. By preventing the transmission of the side lobes, alternative track interference on the storage disk 113 can be alleviated. The NFT 500 embodiment shown in FIG. 5 may be configured with or without a plasmonic metal cap as described with respect to FIG. 3.
The embodiments described above employ a plasmonic metal bar having a robust width that can better withstand the harsh service duty conditions while still capable of delivering a precise focus of the heating spot required at the storage disk surface, compared to much smaller dimensioned plasmonic elements used in typical EAMR/HAMR devices.
The various aspects of this disclosure are provided to enable one of ordinary skill in the art to practice the present invention. Various modifications to exemplary embodiments presented throughout this disclosure will be readily apparent to those skilled in the art, and the concepts disclosed herein may be extended to other devices. Thus, the claims are not intended to be limited to the various aspects of this disclosure, but are to be accorded the full scope consistent with the language of the claims. All structural and functional equivalents to the various components of the exemplary embodiments described throughout this disclosure that are known or later come to be known to those of ordinary skill in the art are expressly incorporated herein by reference and are intended to be encompassed by the claims. Moreover, nothing disclosed herein is intended to be dedicated to the public regardless of whether such disclosure is explicitly recited in the claims. No claim element is to be construed under the provisions of 35 U.S.C. §112(f) unless the element is expressly recited using the phrase “means for” or, in the case of a method claim, the element is recited using the phrase “step for.”

## Claims

1. An apparatus for energy assisted magnetic recording of a storage disk, comprising:
a plurality of dielectric waveguide cores configured to receive incident light energy from an energy source and direct the incident light energy to a target; and
a near field transducer formed at the air bearing surface and configured to focus the light energy received from the plurality of waveguide cores and to transmit the focused light energy onto the storage disk surface to generate a heating spot on the storage disk, the near field transducer comprising:
a plurality of propagating surface plasmon polariton (PSPP) elements that are energized by the light energy from the waveguide cores, wherein each of the PSPP elements comprises a plasmonic metal bar disposed above a surface of a single waveguide core in a longitudinal alignment with the waveguide core, each metal bar having a width at least twice the width of the heating spot generated on the storage disk.

2. The apparatus of claim 1, further comprising a plasmonic metal cap disposed above the PSPP elements and coupled to the PSPP elements.

3. The apparatus of claim 2, wherein the plasmonic metal cap is configured with a straight edge aligned with the air bearing surface.

4. The apparatus of claim 2, wherein the plasmonic cap is configured with a thickness sufficient for a heat sink to control peak temperature of the near field transducer.

5. The apparatus of claim 2, wherein the plasmonic cap is configured with a variable thickness.

6. The apparatus of claim 2, wherein the plasmonic cap is configured with a flat surface for coupling to the PSPP elements.

7. The apparatus of claim 2, further comprising a magnetic pole disposed above the plasmonic cap.

8. The apparatus of claim 1, wherein a gap exists between each of the plurality of PSPP elements and the corresponding surface of the respective waveguide core.

9. The apparatus of claim 1, wherein
each of the waveguide cores and corresponding PSPP elements is substantially linear and comprises a first end and a second end,
the first ends of all PSPP elements are connected together at a junction point near the air bearing surface with at least a portion of the junction exposed on the air bearing surface.

10. The apparatus of claim 1, wherein more than one PSPP element is disposed along at least one of the plurality of waveguide cores.

11. The apparatus of claim 10, wherein the plurality of waveguide cores and corresponding PSPP elements are configured in a three dimensional arrangement with respect to the air bearing surface.

12. The apparatus of claim 1, wherein the plurality of PSPP elements provides constructive interference of the incident light energy at the target.

13. The apparatus of claim 1, wherein the width of the PSPP element is approximately equivalent to the width of the waveguide core.

14. The apparatus of claim 1, wherein each of the PSPP elements is disposed above a waveguide core such that the centerline of the PSPP element is offset from the centerline of the waveguide core.

15. A magnetic storage disk drive, comprising:
a rotatable magnetic storage disk;
a laser diode;
a plurality of dielectric waveguide cores configured to receive incident light energy from the laser diode and direct the incident light energy to a target; and
a near field transducer formed at the air bearing surface and configured to focus the light energy received from the plurality of waveguide cores and to transmit the focused light energy onto the storage disk surface to generate a heating spot on the storage disk, the near field transducer comprising:
a plurality of propagating surface plasmon polariton (PSPP) elements that are energized by the light energy from the waveguide cores, wherein each of the PSPP elements comprises a plasmonic metal bar disposed above a surface of a single waveguide core in a longitudinal alignment with the waveguide core, each metal bar having a width at least twice the width of the heating spot generated on the storage disk.