We propose a near-field optical transducer using a triangular antenna and a thin film structure (wing) to efficiently generate an optical near-field near a magnetic head. A finite-difference time-domain calculation showed that the near-field was efficiently generated at the apex of the antenna when the dimensions of the wing were optimized for efficient delivery of the surface plasmon excited on the wing to the antenna. The calculated light utilization efficiency (ratio between the absorbed power in the recording medium and the input power) was 8%. The temperature distribution on the medium, magnetic field distribution, and magnetization pattern were calculated; the proposed recording head may be capable of an areal recording density of 2.5 Tb/in.2.
© 2012 OSA
The areal density of hard disk drives has increased rapidly as the demand for large-capacity storage devices has grown. However, the areal density of conventional perpendicular magnetic recording will encounter a barrier called the super-paramagnetic limit when the areal density approaches 1 Tb/in.2. One promising way to overcome this limit is to use thermally assisted magnetic recording , which uses local heat during the writing process in order to reduce the anisotropy field of the medium. Using a medium with a high anisotropy field increases the thermal stability of the magnetic domains, whereas the temporary reduction in the anisotropy field ensures that the domains can still be written using a magnetic writer.
To achieve areal densities of more than 1 Tb/in.2, the diameter of the optical spot at the surface of the recording medium should be less than a few tens of nanometers. To generate such an optical spot, which is smaller than the diffraction limit of light, a near-field optical transducer (NFT) must be used . To generate the optical near-field efficiently, various types of highly efficient NFTs, such as a c-aperture , an e-antenna , and a lollipop transducer , have been studied. For example, the e-antenna was integrated into a magnetic head and used for writing recording bits on a bit-patterned magnetic medium at an areal density of about 1 Tb/in.2 . The lollipop NFT was also integrated into a magnetic head. The lollipop NFT was fabricated near a parabolically shaped waveguide and used to demonstrate an areal recording density of about 240 Gb/in.2 .
We previously proposed an NFT with a beaked antenna (a so-called nanobeak) [6,7]. When the antenna is illuminated with light, a strong optical near-field is generated at its apex by excitation of a localized plasmon. The possibility of 1-Tb/in.2-class writing was demonstrated using the nanobeak and a cobalt/palladium bit-patterned medium . To integrate the nanobeak into a magnetic head, we recently proposed a nanobeak NFT with a thin film structure located beside a waveguide . By using the integrated head with this NFT, both a high optical field and a high magnetic field are applied at the recording points. The size and dimensions of this NFT must be optimized to efficiently generate the optical near-field. In this article, we report on the optimum dimensions of the nanobeak NFT and the light utilization efficiency calculated using a finite-difference time-domain method. We also describe the calculated temperature distribution in a recording medium, the magnetic field distribution, and the read–write simulation results for the integrated head with the nanobeak NFT.
2. Optical modeling
Figure 1 shows a schematic of the integrated head with the nanobeak NFT. A triangular antenna is used to generate the optical near-field as in a conventional nanobeak, and a thin film metallic structure is added on top of the antenna. We call the thin film structure a wing hereafter. The wing has a rectangular region and a tapered region, and the tapered region is connected to the triangular antenna. Light from a laser diode is coupled into a waveguide having a rectangular core, and the waveguide is placed beside the wing. When the light is polarized in the direction perpendicular to the surface of the wing (x direction), an evanescent wave propagating along the surface of the core excites a surface plasmon on the wing. The surface plasmon is efficiently excited on the wing when the wave vector of the evanescent wave matches that of the surface plasmon in a manner similar to excitation of a surface plasmon on a thin metallic film using a prism coupler . The surface plasmon propagates toward the bottom of the wing and is funneled to the center of the wing by the tapered region. The surface plasmon then excites a localized plasmon at the triangular antenna, generating a strong optical near-field at its apex. A single-pole magnetic transducer is used as the magnetic head, and it is placed near the apex of the antenna. If the conventional nanobeak antenna without the wing is used, the waveguide must be placed above the nanobeak antenna. In this case, the transmission of the waveguide is reduced because the magnetic transducer is close to the waveguide and absorbs the evanescent wave. In contrast, when we use the wing, the waveguide is place beside the nanobeak antenna, and it is far from the magnetic pole. Therefore, we can reduce the influence of the magnetic transducer. The nanobeak NFT and the magnetic head are moved toward the left side against the recording medium.
We calculated the distribution of the optical near-field generated by the nanobeak transducer by the finite-difference time-domain (FDTD) method using commercial software . In this calculation, we assumed that the wing and the triangular antenna were made of gold, and an experimental value given elsewhere  was used as the refractive index (n) of gold. The waveguide consisted of a Ta2O5 core and Al2O3 cladding, and light with a wavelength of 830 nm was introduced into the waveguide. The magnetic pole was made of FeCo, and both the nanobeak transducer and the magnetic pole were embedded in Al2O3. A FePt medium was placed near the recording head. The medium consisted of a 2-nm-thick carbon overcoat (n = 2.5), an 8-nm-thick FePt recording layer (n = 2.9 + i1.5 ), a 5-nm-thick MgO underlayer (n = 1.7), a 100-nm-thick Cu heat sink (n = 0.26 + i5.28), and a SiO2 substrate. The head surface was covered with a 2-nm-thick overcoat (n = 2.1), and the air gap between the head and the medium was 2 nm. The total head-medium spacing including the head overcoat, the air gap, and the medium overcoat was 6 nm. The calculation domain size was 1.5 μm in both the x and y directions, and 2 μm in the z direction. A nonuniform mesh was used, and the cell was designed to have the smallest size near the apex of the triangular antenna. The minimum cell size was 1 nm in both the x and y directions, and 0.5 nm in the z direction. Perfect matching layers were used as the boundaries of the calculation domain.
Figure 2 shows the intensity distribution of the propagation mode in the waveguide. We assumed that the Ta2O5 core extended 500 nm in the y direction and 200 nm in the x direction. The solid and dotted lines represent the distributions in the x and y directions, respectively. The inset shows the distribution on the xy plane. A strong evanescent wave was generated at the surface of the core perpendicular to the x axis. This wave excites the surface plasmon at the surface of the wing. The mode field diameter measured at 1/e2 of the peak was 580 nm in the x direction and 630 nm in the y direction.
Figure 3 shows the absorbed power distribution in the recording medium calculated in a plane 7 nm from the NFT. For this calculation, we assumed that the length of the triangular antenna (L) was 70 nm, the apex angle (θ3) was 60°, the recess depth (D) was 10 nm, and the taper angle of the tapered region (θ1) was 45°. The apex of the triangular antenna had a circular shape with a diameter of 12 nm. The height of the triangular antenna (H1) was 150 nm, the height of the tapered region (H2) was 300 nm, the total height of the transducer (H3) was 850 nm, and the width of the wing (W) was 1500 nm. The distance between the NFT and the waveguide core was 20 nm. The magnetic pole consisted of a rectangular pole and a tapered structure. The rectangular pole extended 120 nm in both the x and y directions. The height of the rectangular pole (H5) was 50 nm, the total height of the magnetic pole (H4) was 700 nm, and the taper angle (θ2) was 45°. The NFT was 30 nm from the magnetic pole. As shown in Fig. 3, light is strongly absorbed near the apex of the triangular antenna. When we defined the light utilization efficiency as the ratio between the absorbed power in the medium and the power in the waveguide, the efficiency was 8%. Here the absorbed power in the medium was calculated by integrating the absorbed power density in a region of 50 × 50 nm2.
The resonance wavelength of the localized plasmon generated at the triangular antenna depends on the length of the antenna (L). To generate a strong optical near-field, we have to adjust the length so that the resonance wavelength is tuned to the wavelength of light. Figure 4(a) shows the light utilization efficiency as a function of L. The intensity depends on the length and peaks at a length of 70 nm. This peak corresponds to the resonance condition of the localized plasmon. Note that the optimum length depends on the antenna material, the dielectric material surrounding the antenna, and the medium structure. The length must be readjusted when we change these items.
The light utilization efficiency depends on the taper angle (θ1) of the tapered region. If the angle is too small, the intensity decreases because the amount of the surface plasmon entering the tapered region is small. If the angle is too large, the intensity also decreases because the surface plasmon is not confined to the center. Therefore, an optimum angle exists. Figure 4(b) shows the light utilization efficiency as a function of the taper angle. The efficiency reaches a maximum when the apex angle is around 40°.
The surface plasmons propagating at the surface of the wing are reflected by its edges and interfere with each other inside the wing. If we optimize the dimensions of the wing, it acts as a resonator for the surface plasmon. This resonance effect increases the optical near-field intensity.
Figure 5(a) shows the light utilization efficiency as a function of the width of the wing. The intensity increases as the width increases and saturates at about 800 nm. This saturation point is related to the mode field diameter of the waveguide. The dotted line in Fig. 2 represents the intensity distribution in the waveguide in the y direction. The intensity drops to nearly 0 at a point 400 nm from the center of the waveguide. When the wing is narrower than the mode field diameter, the efficiency increases with the wing width, but it saturates when the wing width exceeds the mode field diameter. Note that the intensity continues to increase even when the wing width is larger than 300 nm, which is the upper width of the tapered region. The efficiency at 800 nm is about twice that at 300 nm. This indicates that the surface plasmon propagating in the outer region contributes to the enhancement of the optical near-field intensity. We think that the surface plasmon propagating in the outer region is reflected by the lower edge of the rectangular region. The reflected surface plasmon is then reflected by the opposite edge of the wing and finally reaches the triangular antenna.
Figure 5(b) shows the light utilization efficiency as a function of the total height of the NFT (H3). The optical near-field is generated most efficiently when the height is around 850 nm. Because surface plasmons reflected by the upper and lower edges of the wing cause interference, the wing behaves like a Fabry–Perot resonator for the surface plasmon. When the wing height is equal to the optimum value, the intensity of the surface plasmon increases, and the optical near-field is enhanced.
When the optical near-field is absorbed by the medium, the absorbed power distribution in the recording medium depends on the apex diameter of the triangular antenna. The solid line in Fig. 6(a) represents the FWHM of the absorbed power distribution in the cross-track direction for apex diameters of 12 to 36 nm. Here the absorbed power distribution was calculated on a plane 1 nm from the medium surface. The width of the absorbed power distribution decreased linearly as the apex diameter decreased. When the apex diameter was 12 nm, the FWHM of the absorption distribution was 16 nm. The dotted line in Fig. 6(a) represents the peak absorption, which increases as the apex diameter decreases. This is because the charges in the antenna are concentrated in a smaller area, and the charge density at the apex increases. Figure 6(b) shows the light utilization efficiency as a function of the apex diameter. The efficiency increased gradually as the apex diameter decreased. When the apex diameter was 12 nm, the light utilization efficiency was 8%.
3. Media thermal simulation
The temperature distribution in the medium was calculated by solving the thermal diffusion equation. For the calculation, we assumed that the medium structure was the same as in the optical simulation, and the absorbed power distribution described above was used as the heat source. The thermal conductivities of Cu and MgO were assumed to be 200 W/m·K and 3 W/m·K, respectively. The thermal conductivity of bulk FePt is expected to be more than 70 W/m·K considering the thermal conductivities of bulk Fe and Pt. However, we suppose that the actual thermal conductivity of the FePt film is lower than the bulk value for the following reasons. First, the thermal conductivity of a sputtered thin film is typically lower than that of the bulk. Second, the FePt film used for thermally assisted magnetic recording consists of nanometer-scale grains surrounded by different materials such as carbon [14,15]. Such a nanostructure has lower thermal conductivity owing to scattering of electrons or phonons at grain boundaries [16,17]. In particular, the lateral thermal conductivity is expected to decrease greatly because of the interfaces between different materials. In this calculation, we assumed that the perpendicular thermal conductivity of the FePt film was between 5 W/m·K and 50 W/m·K, and the lateral thermal conductivity was 1/10 of the perpendicular thermal conductivity. The medium was moved at a linear velocity of 20 m/s.
Figure 7 shows the absorbed power and the temperature rise (dT) distributions in the cross-track direction. The dotted line represents the absorbed power distribution, and the outer three lines represent the dT distributions when the perpendicular thermal conductivity (κp) of FePt was 5, 20, and 50 W/m·K. Here the apex diameter of the NFT was assumed to be 12 nm, and the light power in the waveguide was adjusted so that the peak dT was 350 K. These distributions were calculated on a plane 1 nm from the medium surface. The dT distributions were broader than the absorption distribution because of thermal diffusion in the lateral direction. The FWHM values of the dT distribution were 33 nm, 38 nm, and 45 nm when κp was 5, 20, and 50 W/m·K, respectively. The optical power required to achieve the peak dT of 350 K also depends on κp. The required power in the waveguide was 1, 1.5, and 2.1 mW when κp was 5, 20, and 50 W/m·K, respectively. Note that the temperature distribution also depends on the material and the thickness of the heat sink layer. Narrower distributions can be obtained by increasing the heat sink layer thickness.
4. Read/write simulation
To check the performance of the proposed recording head, we calculated the magnetization pattern on the recording medium using the temperature distribution for κp = 5 W/m·K described above.
We first calculated the distribution of the magnetic field generated by the single-pole transducer using the finite element method. The dotted line in Fig. 8 represents the magnetic field distribution calculated on a plane 8 nm from the magnetic pole. For this calculation, we assumed that the dimensions of the magnetic pole were the same as in the optical modeling. The magnetic field represents the effective magnetic field defined by Hx + Hy + Hz, where Hx, Hy, and Hz are the magnetic field components in the x, y, and z directions, respectively. The peak field was 17.8 kOe.
The solid line in Fig. 8 represents the dT distribution in the down-track direction for κp = 5 W/m·K. When the NFT and the magnetic head move toward the left side in Fig. 1, the data transition point is on the right side of the dT distribution, and it is located at a point where the thermal gradient is a maximum (indicated by the arrow). In this distribution, the maximum thermal gradient was obtained at a point 10 nm from the NFT. At this point, the thermal gradient was 14 K/nm when the peak dT was 350 K, and the magnetic field was 11.4 kOe.
The magnetization patterns on the magnetic medium were calculated by solving the Landau–Lifshitz–Gilbert equation. We assumed that the average grain size of the recording layer (D) was 4.5 nm, and the grain size distribution (σD/D) was 10%. At room temperature, the average anisotropy field (Hk) was 44 kOe with a distribution (σHk/Hk) of 5%, the anisotropy energy (Ku) was 17.9 Merg/cm3, the saturation magnetization (Ms) was 820 emu/cm3, the coercivity was 16.5 kOe, and the intergranular exchange was 0.2 erg/cm2. We assumed that the temperature dependence of Ms is expressed by a Brillouin function with J = 0.85, and Ku satisfies , where T is the temperature. The Curie temperature was assumed to be 600 K. Figure 9(a) shows the magnetization pattern at a linear density of 1382 kfci (bit length = 18 nm), and Fig. 9(b) shows the pattern at a linear density of 2764 kfci (bit length = 9 nm). We can see clear recording bits at each linear density. At a linear density of 1382 kfci, the track width was 27 nm, and the estimated signal to noise (S/N) ratio of the read-out signal was 14.1 dB when the read-sensor width was 18 nm and the shield gap was 20 nm. Here the read-out signal was calculated by integrating the magnetization in the sensor area, and the S/N ratio was estimated by the Fourier transform of f(x), where f(x) is the signal intensity as a function of the down-track position. This linear density corresponds to a 2 T signal at an areal recording density of 2.5 Tb/in.2. The estimated S/N ratio satisfies the requirement for recording devices. In this simulation, we assumed that κp = 5 W/m·K. If the actual value is larger than this, the thermal distribution becomes broader, as described above, and the recording density decreases. In this case, the same density will be achieved, for example, by reducing the apex diameter or increasing the thickness of the heat sink layer.
We proposed a near-field optical transducer using a triangular antenna and a thin film structure to efficiently generate an optical near-field near a magnetic head. A finite-difference time-domain calculation showed that a strong optical near-field is generated at the apex of the triangular antenna by optimizing the dimensions of the triangular antenna and the thin film structure. When a FePt recording medium was placed near the near-field optical transducer with an apex diameter of 12 nm, the full width at half-maximum of the absorption power distribution in the recording medium was 16 nm, and the light utilization efficiency, defined as the ratio between the absorbed power in the medium and the power in the waveguide, was about 8%. The full width at half-maximum of the calculated temperature distribution in the cross-track direction was 33–45 nm when the perpendicular thermal conductivity of the FePt recording layer was 5–50 W/m·K, and the lateral thermal conductivity was 1/10 of the perpendicular thermal conductivity. The optical power required to achieve a temperature rise of 350 K was 1–2.1 mW.
The magnetization patterns on the recording medium were also calculated by solving the Landau–Lifshitz–Gilbert equation. It is expected that recording at an areal density of 2.5 Tb/in.2 can be achieved when the apex diameter of the near-field optical transducer is 12 nm and the perpendicular thermal conductivity of the recording layer is 5 W/m·K.
Some of this work was conducted as a part of the “Development of Nanobit Technology for Ultra-high Density Magnetic Recording (Green IT project)” of NEDO.
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