Heat-assisted magnetic recording by a near-field transducer with efficient optical energy transfer W. A. Challener * , Chubing Peng, A. V. Itagi, D. Karns, Wei Peng, Yingguo Peng, XiaoMin Yang, Xiaobin Zhu, N. J. Gokemeijer, Y.-T. Hsia, G. Ju, Robert E. Rottmayer, Michael A. Seigler and E. C. Gage Although near-field microscopy has allowed optical imaging with sub-20 nm resolution, the optical throughput of this technique is notoriously small. As a result, applications such as optical data storage have been impractical. However, with an optimized near-field transducer design, we show that optical energy can be transferred efficiently to a lossy metallic medium and yet remain confined in a spot that is much smaller than the diffraction limit. Such a transducer was integrated into a recording head and flown over a magnetic recording medium on a rotating disk. Optical power from a semiconductor laser at a wavelength of 830 nm was efficiently coupled by the transducer into the medium to heat a 70-nm track above the Curie point in nanoseconds and record data at an areal density of 375 Tb m 22 . This transducer design should scale to even smaller optical spots. L ight can be focused by conventional far-field optics to a spot that is limited by diffraction to 0.5 l/NA where l is the wavelength and NA is the numerical aperture of the lens. The near-field scanning optical microscope 1 , on the other hand, can achieve a res- olution of 100 nm or better. Near-field microscopy typically uses either tapered optical fibres or hollow silicon cantilevers with a very small aperture that is scanned across the sample, but the optical throughput 2,3 is in the order of 1  10 24 to 1  10 25 . Using concentric grooves around an aperture in an aluminium film can substantially enhance the throughput; this has recently been used to expose marks in photoresist with a linewidth of 80 nm (ref. 4). Apertureless microscopy has achieved sub-20-nm optical resolution 5 by focusing a laser beam onto a sharp metallic tip, which concentrates the optical field onto the sample. The storage density of hard disk drives has doubled every three years since their introduction in 1955. Several major technology advances, including the development of thin-film media and record- ing heads, giant magnetoresistive readers, and perpendicular record- ing, have enabled the current areal density of 750 Tb m 22 , which is about half the theoretical limit for conventional magnetic recording materials. As the magnetic grain size is reduced to increase the storage density, the grains may become superparamagnetic and their magnetic state thermally unstable 6 . Materials with a large magnetic anisotropy, such as L1 0 FePt (ref. 7), can support grains as small as 2 to 3 nm in diameter and storage densities 8 up to 155 Pb m 22 . However, the coercivity of high-anisotropy materials is greater than the magnetic field that can be generated by a recording head. Heat-assisted magnetic recording (HAMR) overcomes this problem by heating the medium above its Curie point during record- ing to reduce its coercivity to zero 8 . Optical energy must be efficiently delivered and confined to a spot in the medium that is much smaller than the diffraction limit so that neighbouring tracks are not heated. Heating and cooling of the medium must occur within 1 ns in order to achieve the necessary data rates, to generate a large thermal gradi- ent for sharp bit edge definition, and to ensure that the recorded data are thermally stable during cooling to ambient. A planar solid immersion mirror (PSIM) is a parabolically shaped waveguide with a NA much larger than 1 that can focus light to l/4 (ref. 9). HAMR has been demonstrated with PSIMs in integrated recording heads at storage densities of 230 Tb m 22 at a wavelength of 488 nm (refs 10 and 11), and at densities of 80 Tb m 22 at a wavelength of 830 nm. Nevertheless, much smaller optical spots are necessary for HAMR to exceed the current storage densities. Results The near-field transducer. Surface plasmons (SPs) are collective oscillations of surface charge that are confined to an interface between a dielectric and a metal. When SPs are resonantly excited by an external optical field, the field amplitude in the vicinity of the surface may be orders of magnitude greater than that of the incident field. Moreover, the region of enhanced field may be tightly confined to a spot much smaller than the incident wavelength. Gold is a suitable plasmonic material for wavelengths longer than 700 nm as it is chemically inert with a relatively high melting point. At shorter wavelengths the electronic d-band transitions damp the SP effect. A gold near-field transducer (NFT) that is excited at a SP reson- ance can couple light even more efficiently into a nearby medium by including a sharp tip in its design to take advantage of the lightning rod effect 12,13 . The electric field can also be enhanced in a narrow gap between two resonant nanoparticles 14,15 . A NFT design 16 that com- bines these enhancement mechanisms is shown in Figs 1a and 4a and is called a ‘lollipop’ transducer for obvious reasons. The NFT is located at the focus of a PSIM as shown in Fig. 1b, and at resonance the surface charge oscillates along the length of the lollipop peg to gen- erate an electric field at the tip of the peg that couples energy into the medium. The peg provides the lightning rod effect for field confine- ment. A plasmonic metal beneath the recording layer acts as both a heat sink and an image plane for the electric field. The recording layer is effectively within the gap of two nanoparticles, the NFT and its image, resulting in a substantial enhancement in coupling effi- ciency and further confinement of the electric field. Seagate Technology, 1255 Waterfront Place, Pittsburgh, Pennsylvania 15222, USA. *e-mail: william.a.challener@seagate.com ARTICLES PUBLISHED ONLINE: 22 MARCH 2009 | DOI: 10.1038/NPHOTON.2009.26 NATURE PHOTONICS | VOL 3 | APRIL 2009 | www.nature.com/naturephotonics 220 © 2009 Macmillan Publishers Limited. All rights reserved.