Abstract:
A magnetic recording head consists of a write pole and a near field transducer close to the write pole that focuses light energy to a focal point. A near field transducer is positioned to receive light energy from a waveguide. The near field transducer comprises an energy-receiving end and an energy-radiating end. The energy-receiving end is located near the focal point of the waveguide and the energy-radiating end is shaped such that it is narrower closer to the write pole and wider farther from the write pole.

Description:
BACKGROUND 
     Heat assisted magnetic recording (HAMR) generally refers to the concept of locally heating a recording medium to reduce the coercivity. This allows the applied magnetic writing fields to more easily direct the magnetization during the temporary magnetic softening caused by the heat source. HAMR allows for the use of small grain media, with a larger magnetic anisotropy at room temperature to assure sufficient thermal stability, which is desirable for recording at increased areal densities. HAMR can be applied to any type of magnetic storage media including tilted media, longitudinal media, perpendicular media, and patterned media. By heating the media, the K u  or coercivity is reduced such that the magnetic write field is sufficient to write to the media. Once the media cools to ambient temperature, the coercivity has a sufficiently high value to assure thermal stability of the recorded information. Better designs are needed to increase efficiency, alignment, precision and reduced size of the local heating. 
     SUMMARY 
     A magnetic recording head comprises a write pole, a near field transducer positioned to receive light energy from an external source and positioned proximate the write pole. The near field transducer resonates to produce a heated spot on a recording medium. Asymmetries in the shape of the near field transducer proximate the recording medium result in shape anisotropies of the heated spot. 
     In another aspect, a magnetic recording head comprises a write pole, a near field transducer positioned proximate the write pole, and a recording medium disposed beneath the write pole and near field transducer. The near field transducer and recording medium form a resonant system that results in a heated spot on the recording medium. Asymmetries in the shape of the near field transducer proximate the recording medium result in shape anisotropies of the heated spot. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic representation of a waveguide and a near field transducer (NFT) proximate an associated recording medium. 
         FIG. 2A  is a perspective view of an optical near field transducer (NFT). 
         FIG. 2B  is an air bearing surface (ABS) view of a NFT disposed between a write pole and a return pole. 
         FIG. 2C  is the thermal profile under a NFT. 
         FIG. 3A  is an ABS view of a NFT with an asymmetric pin disposed between a write pole and a return pole. 
         FIG. 3B  is an ABS view of a NFT with a different asymmetric pin disposed between a write pole and a return pole. 
         FIG. 3C  is an ABS view of a NFT with a different asymmetric pin disposed between a write pole and a return pole. 
         FIG. 4  is a thermal profile under a NFT with a trapezoidal pin. 
         FIG. 5A  is a schematic cross section of a coupled nanorod (CNR) and corresponding temperature distribution. 
         FIG. 5B  is an ABS view of a CNR disposed between a write pole and a return pole. 
         FIG. 5C  is the thermal profile under a CNR. 
         FIG. 6A  is an ABS view of a CNR with asymmetric nanorods disposed between a writer pole and a return pole. 
         FIG. 6B  is an ABS view of a CNR with different asymmetric nanorods disposed between a writer pole and a return pole. 
         FIG. 7A  is the thermal profile under a CNR with asymmetric nanorods. 
         FIG. 7B  is the cross track and down track optical profiles under a CNR with asymmetric nanorods. 
         FIG. 8  is a vertical cross section of a CNR with a tapered entrance section in the gap. 
         FIG. 9A  is the optical intensity distribution of a CNR with a straight gap. 
         FIG. 9B  is the optical intensity distribution under the CNR in  FIG. 11A . 
         FIG. 10A  is the optical intensity distribution of a CNR with a tapered gap. 
         FIG. 10B  is the optical intensity distribution under the CNR in  FIG. 12A . 
     
    
    
     DETAILED DESCRIPTION 
     For heat assisted magnetic recording, an electromagnetic wave of, for example, visible, infrared, or ultraviolet light can be directed from the airbearing surface (ABS) of a recording head onto a surface of a data storage medium to raise the temperature of a localized area to facilitate switching. The main difficulty with HAMR has been discovering a technique that is able to conduct sufficient light energy into the storage medium to heat it by several hundred degrees, but only in the area that is desired to be recorded. If the optical spot is larger than this area, it will extend to neighboring bits and tracks on the disc, heat those areas as well, and the data recorded in those areas may be erased. Confining the optical spot to an area that is much smaller than a wavelength of light, and well below the so-called “diffraction limit” that can be achieved by standard focusing lenses, is an area of study called “near field optics” or “near field microscopy.” 
     Well known optical waveguides such as solid immersion lenses (SILs), solid immersion minors (SIMs), and mode index lenses have been proposed for use in near field optics to reduce the size of a spot on the medium that is subjected to the electromagnetic radiation. SILs, SIMs, and mode index lenses alone are not sufficient to achieve focal spot sizes necessary for high areal density recording due to diffraction limited optical effects. Metal pins and other near field transducer (NFT) designs positioned at the focal point of the waveguide are used to further concentrate the energy and direct it to a small spot on the surface of the recording medium. 
     HAMR devices can incorporate various waveguides such as mode index lenses or planar solid immersion minors or lenses to generate focused beams. An example of a parabolic planar waveguide is shown in  FIG. 1 . Edge  66  of waveguide  60  is substantially parabolic in shape. If the refractive indices of material exterior to edge  66  are less than the indices of material of waveguide  60 , waveguide  60  acts as a solid immersion lens. Electromagnetic waves  68  and  70  traveling along the longitudinal axis of waveguide  60  will be deflected at boundary  66  toward focal point  72  as shown. Diffraction gratings or other means known in the art to couple external energy into waveguide  60  can be configured to minimize radiation traveling down the center of waveguide  60  and maximize the energy reflected from parabolic edge  66 , thereby increasing the energy content of the longitudinal component of waves  68  and  70  impinging on the focal point. 
     The dimensions of the spot concentrated at focal point  72  of waveguide  60  are diffraction limited and are not sufficient for the sub-100 nm dimensions required for high areal density HAMR recording media. Near field transducers (NFTs) such as metallic pins, sphere/pin, or disc/pin combinations are required to focus the energy to acceptable sub-100 nm spot sizes. Near field transducer  58  in  FIG. 1  is an example of a disc/pin combination NFT. NFT  58  is positioned at focal point  72  of waveguide  60  where it can couple with incident waves  68  and  70  to generate surface plasmons. The fields generated by the surface plasmons on the NFT also interact with recording medium  16  and transfer electromagnetic energy into the medium as shown by arrows  78  that heat a small region  62  of recording medium  16 . 
     A perspective view of NFT  58  is shown in  FIG. 2A . NFT  58  includes head  80 , pin (sometimes referred to as “peg”)  82  and pin tip  84 . NFT  58  may be made of gold or other suitable materials known in the art such as silver or aluminum. Head  80  on NFT is generally disc-shaped, and is generally larger than pin  82 . Head  80  also may have a greater thickness than pin  82 . NFT  58  is shaped in a manner that can efficiently capture the optical energy and transfer that energy efficiently to the recording medium. Pin tip  84  ensures that the energy transferred to the medium remains confined to an area that is defined by the cross section of the pin. Traditional transducers have pins with rectangular cross sections as shown in  FIG. 2A . Transducers such as that shown in  FIG. 2A  are sometimes termed lollipop transducers. 
     A schematic ABS view of an optical transducer with a pin with a rectangular cross section is shown in  FIG. 2B  wherein write pole  36  is down-track of pin end  84  and return pole  38 . It is desired to move the optical/thermal spot generated by pin end  84  closer to write pole  36  to achieve improved recording performance. The calculated optical field intensity distributions from optical modeling showing the temperature distribution in recording medium  16  resulting from laser light from NFT  58  with rectangular pin  82  is shown in  FIG. 2C . Since the excitation laser light is incident on head  80  on the up-track side of NFT  58 , optical/thermal spot  62  under rectangular pin tip  84  is closer to return pole  38  in the up-track direction and farther away from the write pole making thermal and magnetic alignment difficult. Pin  82  could be moved closer to write pole  36  but the resonant characteristics and thermal management during writing would be altered. In addition, producing pins with extremely small dimensions is challenging. 
     In an exemplary embodiment of this invention, the cross sectional shape of pin  82  has been changed in order to move the energy in the vicinity of pin tip  84  closer to write pole  36 . One embodiment comprises a trapezoidal shape.  FIG. 3A  is a schematic ABS view of an optical transducer with pin  92 , write pole  36 , and return pole  38 . Pin  92  is disposed between write pole  36  and return pole  38 . Pin tip  94  is trapezoidal in shape, comprising first pin edge  96  and second pin edge  98 . First pin edge  96  is smaller than second pin edge  98  and is closer to write pole  36  than second pin edge  98 . In some embodiments, the optical transducer is a lollipop transducer. 
     Because first pin edge  96  is smaller in length than second pin edge  98 , a higher charge density distribution will collect at first pin edge  96  because of the lightning rod effect of conductive materials. The lightning rod effect is a natural phenomenon that occurs for any sharp geometrical feature (corner or edge) of a conductive material. Quasi-electrostatic “crowding” of electric field lines at sharp geometrical features results in significant field enhancement. This effect occurs as long as the effective curvature of the feature is much smaller than the wavelength of interest. Thus, because first pin edge  96  is a sharp geometrical feature of pin  92 , and because first pin edge  96  is located closer to write pole  36  than second pin edge  98 , the asymmetry of the optical near field will be such that the maximum in the temperature profile of the thermal spot will be moved closer to write pole  36  and coincide with the magnetic field distribution more efficiently as compared to more conventional HAMR systems. 
       FIG. 3B  shows essentially the same structure as  FIG. 3A , only pin  93  is oriented 180° from its position in  FIG. 3A . Second pin edge  98  is now closer to write pole  36  than first pin edge  96 . This orientation of pin  82  may be applicable for certain HAMR design criteria. 
       FIG. 3C  is a schematic ABS view of another embodiment of an optical transducer with pin  95 , write pole  36 , and return pole  38 .  FIG. 3C  is essentially the same structure as  FIG. 3A , only pin  95  now has straight pin edge  96 ′ and curved leading edge  98 ′, wherein the pin cross section at pin tip  94 ′ is a concave trapezoid. Curved edge  98 ′ can improve HAMR thermal and magnetic performance by focusing the thermal spot such that the hottest part of the thermal profile is moved closer to write pole  36 . 
     The performance of optical NFT  58  with pin  94  shown in  FIG. 3A  has been evaluated by optical modeling of NFT recording on magnetic media. In the model, a gold lollipop NFT with a pin with trapezoidal cross section was integrated in a recording head with a solid immersion mirror (SIM) to focus waveguide light into the NFT. The waveguide comprised a 125 nm Ta 2 O 5  core with alumina cladding. The NFT was placed in the cladding 10 nm away from the waveguide core. The magnetic medium disc comprised a 2.5 nm thick overcoat, 10 nm thick magnetic recording layer, and a 7.5 nm thick thermal barrier coating deposited on a glass substrate. The recording head had a 2.5 nm overcoat and the fly height was assumed to be 2.5 nm. The laser light had a wavelength of 920 nm. 
     The NFT dimensions, according to  FIG. 3A  are T=40 nm, W 1 =50 nm, and W 2 =10 nm. The calculated optical field intensity distributions showing the temperature distribution in recording medium  16  are shown in  FIG. 4 . In contrast to the temperature distribution shown in  FIG. 2C , spot  62  under trapezoidal pin  94  has a distinct trapezoidal shape with a trailing edge noticeably closer to write pole  36  and a leading edge at the leading edge of pin  94 . The temperature profile of the heated spot is shown as dotted line  62 . The trapezoidal shape of pin cross section  94  has clearly moved the heated spot closer to write pole  36 . The distance from the hottest part of the spot to the write pole in  FIG. 4  is about 50 nm. 
     Another NFT suitable for HAMR application is a coupled nanorod. A coupled nanorod (CNR) comprises a pair of rod-like structures proximate each other, such that, when illuminated with electromagnetic energy of a proper wavelength, will resonate and generate an intense optical spot at the end of the CNR located between the ends of both rods. An example of a vertical cross section of a CNR is shown in  FIG. 5A . CNR  114  comprises rods  116 ,  116 ′,  118 , and  118 ′ separated by gap  120 . When acting as a NFT in a HAMR transducer, CNR  114  is placed at a focal point of a waveguide such as focal point  58  in  FIG. 3 . CNR  114  resonates and surface plasmons on ends  117  and  119  of rods  116 ,  116 ′,  118  and  118 ′ respectively, heat recording media proximate the ABS  56 . 
     The temperature profiles under ends  117  and  119  are schematically shown by temperature curves  122  and  124  respectively. The sum of curves  122  and  124  is shown as curve  125 . The highest temperature is directly under gap  120  between the nanorods. Although CNR  114  is shown in  FIG. 5A  comprising 4 nanorods, CNR structures can be constructed of two or more nanorods as necessary. For HAMR applications, CNR NFT assemblies are typically thin film structures made of gold, silver, aluminum, or other materials known in the art. 
     A schematic ABS view of CNR  114  is shown in  FIG. 5B  wherein write pole  36  is down-track of ends  117  and  119  of CNR  114  and return pole  38 . It is critical to move the optical/thermal spot closer to write pole  36  to achieve improved recording performance. A calculated thermal profile resulting from optical modeling showing the temperature distribution in recording media  16  from laser light from CNR  114  with rectangular ends  117  and  119  is shown in  FIG. 5C . Optical/thermal spot  62  is closer to return pole  38  in the up track direction and farther away from writer pole  36  making thermal and magnetic alignment difficult. As discussed earlier, the thermal asymmetry results from the excitation laser light impinging on the up track side of CNR  114 . 
     In an exemplary embodiment of this invention, the cross sectional shapes of rods  116 ,  116 ′,  118  and  118 ′ have been changed in order to move the energy in the gap closer to write pole  36 . One embodiment comprises introducing gap  121  asymmetry in the down-track direction towards the write pole as shown in  FIG. 6A .  FIG. 6A  is a schematic ABS view of CNR  114  showing asymmetric nanorod ends  117  and  119 , write pole  36 , and return pole  38 . 
     First and second asymmetric nanorod tips  117  and  119  include first angled edge  126 , second angled edge  128 , and opposite edges  130  and  132 . First angled edge  126  and second angled edge  128  are angled such that first and second nanorod tips  117  and  119  gradually grow farther apart away from pole  36 . Opposite edges  130  and  132  are not angled. In another embodiment shown in  FIG. 6B , edges  131  and  133  are also angled to create trapezoidal cross sections. Other asymmetrical cross sections of nanorods  116  and  118 , not shown, are also included in this invention. 
     The NFT performance of CNR  114  with an asymmetrical gap has been evaluated by optical modeling of NFT recording on magnetic media. In the model, a coupled gold nanorod NFT was integrated in a recording head with a solid immersion mirror (SIM) to focus waveguide light into the NFT. The waveguide comprised a 125 nm Ta 2 O 5  core with alumina cladding. The NFT was placed in the cladding 10 nm away from the waveguide core. The magnetic medium disc comprised a 2.5 nm thick overcoat, 10 nm thick magnetic recording layer, and a 7.5 nm thick thermal barrier coating deposited on a glass substrate. The recording head had a 2.5 nm overcoat and the fly height was assumed to be 2.5 nm. The laser light had a wavelength of 920 nm. 
     Three nanorod dimensions were modeled. Referring to  FIG. 6A , the dimensions were T′=40 nm, W 1 ′=55 nm, and W 2 ′=35 nm; T′=40 nm, Wi=45 nm, and W 2 ′=25 nm; and T′=30 nm, Wi=35 nm and W 2 ′=15 nm. In each case the optical intensity distributions were shifted in the down-track direction closer to the write pole due to the focusing effect of the asymmetric gap. The calculated optical field intensity and resulting temperature distributions, in the magnetic layer for the nanorod with T′=40 nm, W 1 ′=45 nm and W 2 ′=26 nm are shown in  FIG. 7A . The cross-track and down-track intensity distributions are shown in  FIG. 7B . The cross track intensity distribution, as shown by the dotted line, is symmetric about the center. The non-uniform field distribution in the gap is clearly evident in  FIG. 7A  as compared to the field distribution in the case of a rectangular gap shown in  FIG. 5C . The distance from the hottest part of the spot to the write pole in  FIG. 7A  is about 60 nm. The down-track intensity distribution in  FIG. 7B  shows a definite peak shift toward the writer pole as indicated by arrows DT. The full width at half maximum (FWHM) of the cross-track profile was 35 nm. FWHM decreased from 45 nm to 25 nm as W 2  decreased from 35 nm to 15 nm respectively. In addition, the down-track optical gradient increased when the spot size decreased with a narrower gap. This will benefit HAMR recording. 
     Calculations have shown that the propagation constants of plasmon modes in the gap increase exponentially as the gap width decreases. Thus, in a gap with decreasing width, the plasmons will be bent toward the smaller gap dimension as they travel from the top of the CNR toward ABS  56 . As such, the asymmetric gap has a focusing effect. However, calculations also show that the plasmon propagation distance in a gap is proportional to the gap width. For a 5 nm gap, for example, the propagation distance is less than one micron. This problematic feature can be countered by the addition of a tapered gap along the gap plasmon propagation direction to the CNR. That is, along the vertical direction from the top of the CNR toward the ABS. 
     A vertical cross section of CNR  134  with a tapered gap is shown in  FIG. 8 . CNR  134  comprises nanorods  136  and  138  with nanorod ends  142  and  144 . Gap  140  with sides  146  and  148  is wide at the top and narrows to sides  150  and  152  at the bottom proximate the ABS. Plasmon loss in the narrow gap is offset by the larger plasmon energy in the upper gap. An added advantage of a tapered gap is that improved impedance matching between the exitation source (preferably a waveguide) and the NFT is possible by the design variable offered by the variable gap. 
     Optical field intensity calculations have been made for coupled nanorods with straight and tapered entrances to the gap. The optical intensity of a vertical cross section of CNR  114  ( FIG. 5A ) is shown in  FIG. 9A . The dimensions of CNR  114  are W 3 =20 nm, T 1 =100 nm and T 2 =175 nm.  FIG. 9B  shows the optical intensity distribution at the ABS from CNR  114 . The optical intensity of a vertical cross section of CNR  134  with the tapered gap ( FIG. 8 ) is shown in  FIG. 10A . The dimensions of CNR  134  are W 4 =20 nm, W 5 =195 nm, T 3 =90 nm and T 4 =170 nm.  FIG. 10B  shows the optical intensity distribution at the ABS from CNR  134 . The energy of CNR  134  is distinctly confined to the gap in  FIGS. 10A and 10B . Both spots at the ABS in  FIGS. 9B and 10B  are similar in size, however, the peak intensity is more than doubled when a tapered entrance is used. 
     Finally, in both pin type (eg. lollipop) and gap type (eg. CNR) NFTs with asymmetric pins and rods that move the optical spot closer to the leading edge of the writer pole, the thickness of the NFT can be increased by as much as a few hundred nanometers without affecting writing performance. The asymmetric pins and rods also cause the shape and size of the spots to be more efficient. Finally, the increased gold thickness aids in dissipating heat energy thereby reducing the NFT temperature. 
     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the scope of the invention. The implementations described above and other implementations are within the scope of the following claims.