IEEE TRANSACTIONS ON MAGNETICS, VOL. 41, NO. 10, OCTOBER 2005 2817

Near-Field Assisted Magnetic RecordingShintaro Miyanishi, Naoyasu Iketani, Kazuhisa Takayama, Kohsuke Innami, Ippei Suzuki, Tazuko Kitazawa,

Yasushi Ogimoto, Yoshiteru Murakami, Kunio Kojima, and Akira Takahashi

Devices Technology Research Laboratories, Sharp Corporation, Nara 632-8567, Japan

We propose a new hybrid head that generates both the magnetic field and the near field on a nano-sized region for high-density mag-netic recording. Utilizing this hybrid head, the narrowest track width of 70 nm is achieved on TbFeCo media, which proves the near-fieldassisted magnetic recording beyond the diffraction limit of an optical pickup. The hybrid head demonstrated here constitutes a break-through toward the areal density of 1 Tb/in2.

Index Terms—Hybrid head, near-field assisted magnetic recording, surface plasmon, TbFeCo.

I. INTRODUCTION

THERMAL fluctuation is a severe problem on the sta-bility of a nano-sized magnetic material to achieve the

storage density of 1 Tb/in in high-density magnetic recording[1]–[3]. The thermally assisted magnetic recording allows usto overcome this difficulty by employing magnetic media withhigh magnetic anisotropy accompanying the quite high coer-civity at room temperature. In the thermally assisted magneticrecording, the bit size (area size) is determined on the basisof the overlapped region between the magnetic field and thethermal distribution on the media. Hence, it is quite importanthow the magnetic head incorporates the heat source that raisesthe temperature of the magnetic recording media so as tomake the coercivity sufficiently lower to perform the magneticrecording. Since a focused laser-spot size in the range of0.4–1.0 m is available for the heat source, the laser-assistedmagnetic recording has attracted much attention as a break-through technology. In fact, we have already demonstratedthis type of magnetic recording on TbFeCo media with a highcoercivity by a conventional optical pickup and a magnetichead [4]–[6].

The laser-assisted magnetic recording mentioned above,however, encounters another difficulty: the optical resolutionlimit of a pickup is much larger than the spot size of 25 nmcorresponding to the bit size in 1 Tb/in . Thus, the thermallyassisted magnetic recording must exploit an alternative heatsource that confines the thermal distribution to a nano-sizedregion on the media. Recently, a near-field assisted method hasbeen emerging as a possible candidate to solve this problembecause the near field is expected to further exceed the opticalresolution limit of a pickup. Although a generator of the nearfield has been proposed as a part of a hybrid magnetic head[7], [8], there is no report on the near-field assisted magneticrecording by a hybrid magnetic head that combines the mag-netic write head and a generator of the near field.

In this paper, we propose a new hybridized magnetic headthat generates both the magnetic field and the near field on anano-sized region beyond the optical resolution limit. Here, wename the hybrid head “a surface plasmon and magnetic fieldapplicable synchronously hybridized (SMASH) head.” We in-

Digital Object Identifier 10.1109/TMAG.2005.855562

Fig. 1. Schematic illustration of the SMASH head.

troduce the structure of the SMASH head in Section II. In Sec-tion III, we elucidate how the SMASH head works, i.e., the gen-eration principle of the magnetic field and the near field. Thecapability of the SMASH head for the near-field assisted mag-netic recording is described in Section IV.

II. STRUCTURE OF THE HYBRID HEAD

The SMASH head was fabricated through a conventionalphotolithography process. Fig. 1 shows an illustration of theSMASH head. The SMASH head consists of a quartz (SiO )slider, embedded wire, side electrodes, and a protection layer.The wire consists of a sequence of Au (300 nm)/Ti (50 nm)on the SiO . The wire is narrowed and curved at the centerof the wire. The magnetic field is mainly generated aroundthe narrow part of the wire by feeding current through thewire. The structure of embedded wire was designed to releasethe heat from the narrow part of the wire during the currentsupply. Moreover, a focused laser beam on the narrow part ofthe wire through the bottom side of the slider excites near fieldradiation on the edge of the narrow part of the wire (see Fig. 2).A protrusion at the center of the head enables the narrow partof the wire to approach the surface on the magnetic media.The protection layer of SiN with the thickness of about 10 nmcovers the top surface of the slider.

Fig. 2 shows an atomic force microscopic (AFM) imagearound the narrow part of the wire on the surface of the head. A

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2818 IEEE TRANSACTIONS ON MAGNETICS, VOL. 41, NO. 10, OCTOBER 2005

Fig. 2. AFM image around the narrow port of the wire on the surface of theSMASH head. A center of curvature of a wedge is indicated the point P .

structure of a wedge into the wire forms a narrow part of itself.The wire has a narrow part with a width of 1 m and a curvewith a curvature radius of 100 nm on the top of the wedge. Acenter of curvature of a wedge is indicated a point as shownin Fig. 2. The structure of ledges is fabricated along the edgeon the wire. The height and width of ledges are about 50 nm.The structure locally enhances the magnetic field and the nearfield on the magnetic media (see below).

III. MAGNETIC AND NEAR FIELDS

A. Magnetic Field

We estimate the magnetic field around the cross section of thewire of the SMASH head from one of the Maxwell equations

(1)

where , and are the magnetic field, the line element alongthe path , and the current, respectively. The intensity of themagnetic field around the wire is expressed as follows:

, where is an outer path length around the cross section(see the inset of Fig. 3).

Fig. 3 indicates the dependence of the magnetic fieldaround the embedded wire on the SMASH head. decreaseswith length , and is 35.7 kA/m (447 Oe) on the surface ofthe wire, where the is 100 mA and the is 2.80 m at thepart of the narrow. The configuration of the embedded wire,which is similar to a half turn coil, focused the magnetic fluxat the point .

B. Near Field

1) Finite-Difference Time-Domain (FDTD) Simula-tion: Fig. 4 shows a model for a FDTD simulation [9] tocalculate the distribution of the electric field around the narrowwire on the SMASH head. The simulation area is the cubeof 600 nm. The size of calculation mesh is set 10 nm. Anabsorbing boundary condition was set on the outer boundaryof the simulation area. In this model, the wire part has samelayout as mentioned above in Section II. The top configurationof the wire has a slit with a width of 200 nm and ledges alongthe edge of a slit, where the size of the ledges utilizes theresult of the observation on the SMASH head by AFM image.

Fig. 3. Dependence of the magentic field on the path length around the narrowpart of the embeded wire in the SMASH head.

Fig. 4. Model for a FDTD simulation for the distribution of the electric fieldaround the narrow wire part on the SMASH head.

The light is incident on the bottom side of the simulation areacorresponding to the backside of the wire of the SMASH head.The wavelength of the light is 650 nm and the intensity of theelectric field is 1 V/m. The polarization of the light is parallelto the axis, as shown in Fig. 4.

Fig. 5(a) and (b) shows results of the distribution of thesteady-state electric field on the plane and the planeof the near field by the FDTD simulation, respectively. The

plane indicates a cutting plane on the center of the slit,where the dashed lines mean the cross section of the wire part.According to the result of the time-resolved distribution of theelectric field, surface plasmon is excited and propagated on theboundary between the metal layers and the part of the quartzslider during the irradiation of light from the backside of thenarrow part of the wire. In the steady state, the electric field

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Fig. 5. Images of distribution of electric field at the narrow metal part on theSMASH head useing FDTD simulation. (a) Distribution of electric field on theY Z plane in Fig. 4. (b) Distribution of electric field on theXY plane in Fig. 4.

is excited on the edge of the narrow wire in Fig. 5(a). Thislocalized electric field is named the localized surface plasmonor the near field. The plane indicates an upper plane 10 nmabove the ledges of the slit, where the dashed lines indicate theconfiguration of the top view of ledges on the edge of the slit.The electric field is excited on the curved edge of the end of theslit in Fig. 5(b). The maximum intensity of the excited electricfield is about 0.45 V/m on the plane. The propagation rateof the energy through the SMASH head is about 20% on 10 nmabove the ledges of the slit. An increase of the distance fromthe top of ledges exponentially decreases the intensity of theelectric field. In addition, we confirmed the dependence of thegeneration of the near field on the polarization of the irradiatedlight. The electric field is selectively excited on the edge ofthe ledges, when the edge line is normal to the direction ofpolarization.

From the result of the FDTD simulation, we found that thestructure of the embedded narrow wire on the SMASH head iseffective to localize the surface plasmon on the edge of the wire.Moreover, the excited area of the near field is determined by theconfiguration on the edge of the narrow wire, where the nearfield is remarkably generated around ledges on the wire.

Fig. 6. (a) AFM image on the surface of the SMASH head. (b) Near-fieldimage at the expanded region, which is indicated by a dashed square in (a).

2) Near-Field Scanning Optical Microscope (NSOM) Obser-vation: We employed an NSOM to experimentally confirm thegeneration of the near field on the SMASH head. The near fieldis observed by the detection of the scattering of the transformedfar field, where a tip of an AFM probe changes the near field intothe far field. We thus obtain the distribution of the near field bythe scanning of the probe on the surface of the sample. By thescanning height dependence of the scattered intensity, we dis-tinguished the near field and the far field since the near fieldexponentially decreases with the distance from the point of thestanding near field.

In order to generate the near field on the SMASH head, wefocused the laser beam with circular polarization on the wirefrom the backside of the head using the optical head. The spec-ification of the optical head was as follows: wavelength is 650nm, NA is 0.65, and a spot diameter is around 1 m.

Fig. 6(a) and (b) shows an AFM image and an NSOM imageon the surface of the SMASH head, respectively. The scanningarea in Fig. 6(b) is corresponds to the enclosed area by thedashed line in Fig. 6(a). The focused laser spot is located on thepoint , as indicated in Fig. 6(a). It is found that the near field islocally generated around the top of the wedge in Fig. 6(b). Theirradiation of the circular polarized laser excites the near fieldon the whole edge on the curved ledge. The excited area of thenear field on the SMASH head coincides with that obtained bythe FDTD simulation [see Fig. 5(b)].

2820 IEEE TRANSACTIONS ON MAGNETICS, VOL. 41, NO. 10, OCTOBER 2005

Fig. 7. Schematic illustration of a near-field assisted magnetic recordingsystem.

Fig. 8. MFM image of a far-field assisted magnetic recording on a TbFeComedium.

IV. NEAR-FIELD ASSISTED MAGNETIC RECORDING

Fig. 7 is a schematic illustration of the near-field assistedmagnetic recording system. An optical head is located abovethe SMASH head in order to focus onto a backside of the partof the narrow wire through the quartz slider. The specificationof the optical head is the same as described in Section III-B2.During irradiation by the laser, the generated near field is local-ized on the edge of the ledges of the wire. The magnetic field isgenerated at the same area by flowing current through the wire.Therefore, the SMASH head enables near-field assisted mag-netic recording on the magnetic media from a side with all com-ponents on one side of the medium.

We employed TbFeCo for the near-field assisted magneticrecording media because of the tunability of the magnetic prop-erties. We can tune the temperature dependence of the mag-netization and the coercivity by changing its composition. ATbFeCo film with a thickness of 50 nm was deposited after theformation of Al underlayer on a disk. The magnetic propertiesof the TbFeCo medium were as follows: coercive force is 1.9

Fig. 9. (a) MFM image of a near-field assisted magnetic recording on aTbFeCo medium. (b) Magnified image of enclosed area by dashed line in (a).

kOe, a magnetization is 200 emu/cc, and the easy axis of mag-netization is perpendicular to the surface of the media.

The near-field assisted magnetic recording was performedwith a spin-stand tester equipped with the SMASH head and theTbFeCo medium. A write alternating current for the laser-as-sisted magnetic recording was 200 mA .

Before the near-field magnetic recording, we examined theconventional magnetooptical (MO) recording by the SMASHhead with the aim of confirming the generation of the mag-netic field. The focused laser spot was intentionally withdrawnfrom the narrow part of the wire. The heated region on themedium elongates along the tangential direction with increasingthe linear velocity on the disk. In this case, we set the linear ve-locity to 2.8 m/s and an irradiating laser power for the writeprocess of 5 mW. The heated region on the medium is extendedinto the generation area of the magnetic field applied by theSMASH head. The MO recording was performed at the overlapbetween magnetic field and thermal distribution on the medium.Fig. 8 shows a magnetic force microscopic (MFM) image ofthe MO recording on the medium. The recording patterns showcrescent marks with a mark length of 2.8 m. We found that arecording track width was about 1.7 m, corresponding to theheated region on the medium.

We attempted contact recording to obtain the best perfor-mance of the near-field assisted magnetic recording since theincreasing the space between the head and the medium expo-nentially decreases the near field radiation in the medium. The

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irradiating laser power for the write process was 8 mW. The laserspot was focused on the narrow part of the wire. The linear ve-locity was 0.6 m/s. We performed the near-field assisted mag-netic recording during the dragging of the SMASH head on themedia. Fig. 9(a) and (b) shows MFM images of the near-fieldmagnetic recording on the TbFeCo medium. The scanning areain Fig. 9(b) corresponds to the enclosed area by the dashed linein Fig. 9(a). Recording tracks were observed in the erased regionon the medium. We found that the continual lines are recordedon the medium. A track width of 70 nm was less than opticalresolution.

V. DISCUSSION

We accomplished new thermal-assisted magnetic recordingof a very narrow track width less than the optical resolution.The results of NSOM observation indicated that the near field islocally generated around the narrow wire on the SMASH headduring the irradiation by the optical head. The result of the ob-servation is consistent with that of the FDTD simulation. Thesize of the excited area for the near field on the SMASH headcorresponds to the width of the narrow recording track. Joule’sheat, which is less than 1 mW during feeding the current throughthe narrow wire, does not influence thermal-assisted magneticrecording. The embedded wire of the SMASH head releasesthe heat to the quartz and the other metal parts. The Joule’sheating area that is about 1.0 m , corresponding to the sizeof the narrow wire. A recording track width of 70 nm wouldbe unattainable by the Joule’s heating. Therefore, we confirmedthat our proposed SMASH head is promising for the near-fieldassisted magnetic recording.

VI. CONCLUSION

In this paper, we proposed a new hybridized magnetic headthat generates both the magnetic field and the near field on anano-sized region beyond the optical resolution limit. Togetherwith the FDTD simulation and the NSOM observation, weelucidated the generation of the near field on the SMASH head.Utilizing the SMASH head, the near-field assisted magneticrecording has been accomplished on TbFeCo media with the

narrowest track width of 70 nm, which would be unattainableby the conventional optical pickup. The advantage of the simplestructure of the SMASH head would be more highlighted whenit is combined with another magnetic (read) head. We believethat the SMASH head demonstrated here will pave the way tothe higher density magnetic recording beyond 1 Tb/in .

ACKNOWLEDGMENT

The authors are grateful to colleagues in SHARP, at variousdivisions and distant sites, who generously contributed to thiswork. Although too numerous to acknowledge here, their workis wholeheartedly appreciated.

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Manuscript received January 7, 2005.