Plasmonic Nanocomposite Near Field Transducer with Capacitive Coupling

 Heat Assisted Magnetic Recording (HAMR) is a promising technique to extend the areal density of the magnetic disk drive beyond the current records of 1.4 Tb/in². In heat assisted magnetic recording, a near field transducer (NFT) is used to generate a very localized intensive electric field to the medium metallic magnetic grains for heating. Current NFT designs use a solid piece of a plasmonic metal of certain geometric shape for resonance mode matching. A peg of tens of nanometers dimension as a part of NFT is used to exert the electric field to the media metallic grains at optical frequencies. However, materials with excellent plasmonic properties such as gold, often have relatively low melting temperatures and can deform at elevated temperatures during operation, resulting in reliability challenges that the HAMR technology has been facing ever since its inception. To address these issues and propose a potential solution, several studies were performed in the thesis work.

In the first study, we present a capacitive nanocomposite nanostructure concept for the near field transducer (NFT) design in heat assisted magnetic recording. In this design, metal bars separated by a thin dielectric are used to form a resonant plasmonic nanostructure. The motivation for such structure design is the use of the dielectric separation for enhancing the material stability at elevated temperature so that thermally stable dielectric materials can be used without compromising much needed plasmonic properties. COMSOL Multiphysics software is used to simulate the 2D plasmonic excitation and the wave propagation to provide a detailed performance analysis on the nanocomposite structure for maintaining the plasmonic resonance. The nanostructure is composed of an array of Au rectangular gratings separated by dielectric gaps, with each grating component embedded in the dielectric medium. The resonance between the geometry and the wavelength will be illustrated. The effects of each parameter inside the geometry will be thoroughly discussed. It is shown that the capacitive nanocomposite structure could have the same heating efficiency compared with a solid piece of Au structure with certain designed geometry. 

In the second study, we extend our 2D simulation to 3D tapered NFT nanostructure using COMSOL Multiphysics software. Both the 3D untapered and the 3D tapered structure will be analyzed and discussed. It is shown that different electromagnetic wave propagation modes can appear depending on the geometry of the Au bar width. Compared with the untapered structure, the tapering NFT structure towards the air bearing surface (ABS) shows a significant focusing effect of the electromagnetic field and the focusing effect can be further enhanced with a smaller NFT peg size at the optimum wavelength. The effects of the material dielectric constants on the NFT heating efficiency will also be discussed to guide the material selection for the NFT design. In is concluded that this capacitive-coupled NFT with dielectric separation gaps and tapering yields relative high efficiency similar to that of the lollipop NFT design with the potential for significantly enhanced material thermal stability. 

In the third study, we show two attempts to fabricate a capacitive Au nanostructure. The first attempt is through the layer-by-layer self-assembly of the Au nanoparticles using Langmuir-Blodgett method followed by the evaporation of the matrix material. It turns out that the layer-by-layer self-assembly technique can form the desired nanostructure with the nanoparticles embedded in the matrix material, but the system has extremely high optical loss to conduct the plasmonic wave due to the scattering of the wave at the nanoparticle-matrix boundary. The second attempt is to apply the nanofabrication technique to fabricate the capacitive-coupled NFT structure. The fabrication progress of the sub-20 nm HSQ gratings and the optimization of the Au electrodeposition will be presented. By proposing the subsequent fabrication methods, the development and the fabrication of the final structure is seen to be promising.