Elsevier

Physics Letters A

Volume 391, 5 March 2021, 127129
Physics Letters A

A simple and effective semi-circle resonator system for bit-patterned HAMR

https://doi.org/10.1016/j.physleta.2020.127129Get rights and content

Highlights

  • The semi-circle resonator system for HAMR is simple without complex focusing lenses or external controls.

  • The profile of light spots can be shrunk to 20-30 nm with a good field enhancement for the high areal density.

  • The potential areal density can further improve beyond the limit size of light spots in the bit-patterned HAMR.

Abstract

The light delivery system with a light spot is the key component of heat-assisted magnetic recording (HAMR). We proposed a semi-circle resonator system as the light delivery system for HAMR. It consists of only a semi-circle resonator, a reflecting mirror, and a near-field transducer (NFT). The resonator can provide a stable enhanced optical field to excite the localized surface plasmon of NFT without complex focusing lenses or external controls. Numerical results show that the size of light spots can be 20–40 nm with a good field enhancement. Combined with bit-patterned media, the performance can be improved several times than the thin-film structure. The thermal profile of the light spot shows that the areal density of HAMR can further improve beyond the limit size of light spots, and the recording bit can be reduced to 12×12 nm2 corresponding to the areal density of about 4 Tb/in2.

Introduction

The trilemma issue of thermal stability, writability, and the grain size has constrained the increasing of the areal density of conventional magnetic recording by further reducing the grain size. To maintain the thermal stability of the small grain size, the magnetic anisotropy needs to be increased, but at the cost of increasing the switching magnetic field beyond the limit of the write head. Addressing this trilemma issue, heat-assisted magnetic recording (HAMR) with the ultra-small light spot for temporarily heating the magnetic grain has been considered as the potential technique to improve the areal data density over 1 Tb/in2 in hard disk drives. The coercivity of the magnetic grain decreases with the temperature rising at the write cycle, and the data can be written to the recording bit when the small grain is heated over the Curie temperature by the small light spot.

The light delivery system composed of laser, waveguide, and near-field transducer (NFT) plays a key role in achieving the ultra-small size of a heat source. In order to achieve the ultra-small size of light spots with high energy focusing, many approaches based on the optimizations of the light delivery system of HAMR have been explored. Firstly, the waveguide that provides high focusing energy can directly improve the excitation of localized surface plasmons (LSP) mode in NFT. A planar solid immersion mirror (PSIM) [1], a multi-layer graded refractive-index (GRIN) [2], Mach-Zehnder interferometer (MZI) [3], and a ring resonator [4] have been reported for the waveguide design. PSIM and GRIN play the role of focusing lenses to excite the SPP mode at the location of NFT. MZI has better excitation of SPP mode by controlling the light phase and coupling angle, the ring resonator requires a critical coupling condition to achieve high field enhancement. Secondly, NFT design can introduce either the rod effect [5], [6] to construct the antenna type of NFT or the feed-gap effect [7] to compose the aperture type of NFT for improving the field enhancement of light spots, such as the lollipop antenna [1], [3], taper antenna [2], droplet antenna [8], split ring aperture [9], bowtie aperture [10], E aperture [11] and so on. Finally, the impedance mismatch between the radiation resistance of NFT and magnetic media can not be ignored when considering the absorption efficiency and the field enhancement in magnetic media. For better impedance matching between the NFT and media, the impedance of NFT needs to be reduced or the impedance of waveguide and magnetic media is designed to increase [12]. However, the thermal profile of the light spot is about 50 nm in most reports, and the field enhancement in magnetic media for the light spot with a size smaller than 50 nm stays at a low value. The large thermal profile has restricted the potential density of HAMR.

For exploring methods to reduce the thermal profile, we numerically investigated a simple and effective semi-circle dielectric resonator system combined with bit-patterned media by using the commercial software Lumerical FDTD solutions. The simple resonator system consists of only a semi-circle dielectric waveguide, a gold mirror, and NFT. The proposed system is easy to integrate and manufacture. The energy of input light in the dielectric resonator can be secondarily used for the excitation LSP mode of NFT. To show the good performance of the proposed light delivery system, we studied the performance of the system for HAMR both in the thin-film magnetic media and bit-patterned media. The field enhancement in thin-film media can easily reach the normal value for the light spot size of about 20–40 nm, the enhancement can be higher in the optimized bit-patterned media structure. For the bit-patterned media, the thermal profile in the magnetic media can be reduced smaller than the profile of the light spot, which means that the areal density of HAMR can be further improved by the packed patterned media.

Section snippets

Structure and design

The waveguide in our proposed system is shown in Fig. 1(a)(c), it is made of a semi-circle waveguide and a gold mirror. The energy of input light can be recycled twice for exciting the LSP mode of NFT due to the light reflection from the gold mirror. Based on the interaction between the resonated field and the LSP field of NFT [4], the NFT is put on the top of the localized enhanced area (LEA). The resonator delivers the localized enhanced field to NFT, the distribution of the resonated field

Results and discussion

To show the performance of the proposed system for high areal density, we studied the profile of field enhancement (|E|/|E0|) in the thin-film structure. The high field enhancement of light spots means high absorption efficiency in magnetic media for a given size of light spots. The size of the light spot can be weighed by the halfwidth. To further improve the areal density of the system, we investigated the thermal profile of the bit-patterned media with different sizes of recording bits.

First

Conclusion

In conclusion, we demonstrated the performance of the semi-circle resonator system in the thin-film structure and bit-patterned media for ultra-small light spots. The semi-circle resonator system does not contain any sophisticated lens or additional control costs, and the enhanced resonated field in the simple resonator can be a good light source for exciting the LSP mode of NFT. To explore the limit areal density, we optimized the light delivery system to get the size limit of light spots, the

CRediT authorship contribution statement

Wei Chen: Conceptualization, Data curation, Investigation, Methodology, Resources, Software, Writing – original draft. Jincai Chen: Funding acquisition, Project administration, Writing – original draft, Writing – review & editing. Zongsong Gan: Writing – review & editing. Yaxiong Ma: Resources, Supervision, Validation. Ke Luo: Resources, Supervision. Zhenxing Huang: Conceptualization, Supervision. Yang He: Supervision. Ping Lu: Funding acquisition, Project administration.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

This work was supported in part by the National Natural Science Foundation of China under the Grant No.61672246, No.61272068, and No.61902243.

References (13)

  • W. Challener et al.

    Heat-assisted magnetic recording by a near-field transducer with efficient optical energy transfer

    Nat. Photonics

    (2009)
  • V. Krishnamurthy et al.

    Efficient integrated light-delivery system design for hamr: maximal optical coupling for transducer and nanowaveguide

    IEEE Trans. Magn.

    (2016)
  • J. Gosciniak et al.

    Novel mach–zehnder interferometer waveguide design as a light delivery system for heat-assisted magnetic recording

    IEEE Trans. Magn.

    (2016)
  • W. Chen et al.

    High-field enhancement of plasmonics antenna using ring resonator for hamr

    IEEE Trans. Magn.

    (2020)
  • Y.C. Martin et al.

    Strength of the electric field in apertureless near-field optical microscopy

    J. Appl. Phys.

    (2001)
  • C. Peng et al.

    Lightning rod resonance of a plasmonic near-field transducer

    Opt. Express

    (2017)
There are more references available in the full text version of this article.

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