THE STUDY OF NOVEL LIGHT DELIVERY SYSTEMS FOR HEAT-ASSISTED MAGNETIC RECORDING

Abstract

The demand for magnetic storage density increases tremendously every year. This drives the development of new techniques to increase the storage capacity in hard drives. This development is impeded by the thermal limit of magnetic media, also known as the superparamagnetic limit. This effect causes small bits to change their magnetic orientation randomly, leading to data loss. High coercivity magnetic media are required in order to overcome the superparamagnetic effect. However, this requires a higher write head magnetic field to obtain magnetic reversal of the magnetic bits. Heat assisted magnetic recording (HAMR) is a next generation technology proposed for achieving magnetic storage densities beyond 1 Tb/in2. The principle of HAMR is similar to the derivative of magneto-optical recording proposed by Katayama and Saga separately in 1999 [1, 2] and was first demonstrated by Seagate in 2006 [3]. HAMR makes writing high anisotropy media possible, facilitating the use of smaller thermally stable grains. In a typical HAMR process, the temperature of a high anisotropy medium is raised above its Curie temperature, lowering its coercivity to a value within the writable range of a magnetic field supplied by a conventional write head. However, the commercialization of HAMR faces substantial technical challenges that must be resolved before widespread adoption of the technology can commence. Foremost of these challenges is the development of a precise method of delivering light to a very small, sub-wavelength bit area with sufficient power to heat a high coercivity magnetic medium above its Curie temperature. Complex fabrication processes, low power transfer efficiency and high heat dissipation are the biggest problems faced in current HAMR light delivery systems. In this thesis a new light delivery system consisting of the nano-aperture vertical-cavity surface emitting laser (VCSEL) as a potential candidate for an alternative light delivery system in HAMR is proposed. The transmission and focusing characteristics of differently shaped nano-apertures, including the conventional square shape and unconventional shapes such as the C-shape, H-shape, T-shape and L-shape are studied via simulation, in order to find the most suitable shape to be used as a near field transducer for HAMR applications. The C-shaped nano-aperture shows the best transmission and focusing characteristics and is the strongest candidate as a near field transducer (NFT) for HAMR. The resonant wavelength of C-shaped nano-apertures is strongly affected by the storage media, placed in the near-field of the nano-aperture. The power density requirement has been found with successful HAMR demonstrations with control C-shaped nano-aperture near-field transducers fabricated on glass substrates. The C-apertures have shown localized focusing properties compared to square aperture which have low power transmission and cannot be used for successful HAMR demonstration, with the same incident power density as the C-apertures. The power density available from C-shaped nano-aperture VCSELs is comparable to the power density required for HAMR, which makes these VCSELs a strong alternative light delivery system for HAMR, with additional advantages of easy fabrication, low cost and less thermal losses inside the system.