Gapped Near Field Transducer Design and Performance for Heat Assisted Magnetic Recording
Creators: name: Chow, Cheng-Ming
Publisher: name: Carnegie Mellon University
Year: 2020
Subjects: subject: 90699 Electrical and Electronic Engineering not elsewhere classified
subject: FOS: Electrical engineering, electronic engineering, information engineering
subject: FOS: Electrical engineering, electronic engineering, information engineering

description: Achieving storage densities greater than 1 Tb/in2 in hard disk drives (HDDs) requires the adoption of new recording technologies to replace perpendicular magnetic recording(PMR), with heat assisted magnetic recording (HAMR) being the leading contender. In this dissertation, a split pole/gapped near field transducer (NFT) recording head design is presented, motivated by the need to reduce transition curvature. An analysis of the expected impact of manipulating the write field gradient using the split pole and thermal pro?les using the gapped NFT on transition curvature is performed through simulation using finite element modeling simulation. A crosstrack effective write field difference of 2.2 kOe from track center to track edge for a 40 nm wide track was measured, along with an average crosstrack write field gradient of 110 Oe/nm. Thermal pro?le curvature reduction was demonstrated, accompanied by increased crosstrack spot size, by changing the opticalgap width of the gapped NFT and media interlayer thickness. Areal density capability (ADC) was measured for three sets of thermal and write field pro?les produced by ourdesign using micromagnetic modeling. Our results showed the ADC for the wider but flatter thermal pro?le to be slightly higher versus a narrower but more curved thermal pro?le at1.16 Tb/in2 and 1.10 Tb/in2, respectively. However, better linear density with worse track pitch resulted from the straighter but wider versus the more curved, narrower thermal pro?le, and is consistent with the trade off between reduced curvature but increased crosstrack widthof the thermal pro?le. A dual nanowire (NW) NFT design is presented as a possible configuration for realizing a gapped NFT. Two axisymmetric surface plasmon modes in phase opposition are shown to convert to a gap mode as the wire separation is reduced. A novel method for excitingaxisymmetric surface plasmon modes on NWs using a nearby standing wave in a dielectric waveguide cavity is demonstrated. This method can achieve a total coupling efficiency of 9% power into a 70 by 70 nm Au NW with a power efficiency of 55% between the waveguide power and the power coupled into the NW and is shown capable of driving the dual NW NFT. Thermal pro?le and efficiency analysis of hotspots generated in recording media areperformed for the device at two different NW sizes. At an optimal length, a 70 nm by 70 nm dual NW device can couple 0.65% of waveguide power to the recording region in the media, while the NWs absorb 9%. Thermal efficiency of the device was measured at below 0.20. The challenges of high temperature applications of Au NWs as NFTs are discussed. Potential strategies for overcoming the low thermal efficiency including increasing the thermalconductivity of the dielectric cladding and introducing adhesion layers that reduce thermal interface resistance are also presented. These strategies result in the potential for improving the thermal efficiency by a factor of 10.Finally, a photonic device design for controlling the excitation of the axisymmetric NW mode using thermo-optic phase shifters is presented. The device consists of an air/Si3N4/SiO2 dielectric rib waveguide, Au NW, and Cr/Au thermal tuners. Expected output behavior, fabrication process ow, and measurement setup for the experimental device are discussed. A testing methodology is presented which can bene?t future researchers requiring phase controlof standing waves interacting with optical components. Inclusion of power monitors for the counter-propagating waves is suggested for future device iterations based on this analysis. Optical grating efficiencies and waveguide losses are presented for fabricated components. Preliminary results demonstrated the capability to fabricate optical gratings with coupling efficiencies of 10%, approximately half that demonstrated through simulation. Optical losses in the Si3N4 waveguides were measured to be 30 dB. We attribute this to a high extinction coefficient of the Si3N4 ?lm, which was determined to be<i> k = </i>7 x 10<sup>-5</sup>.