Near-Field Transducer Material Q: Assumptions and Reality

In heat-assisted magnetic recording (HAMR), it is common for near-field transducers (NFTs) to comprise gold (Au), which is modeled as having textbook optical properties. The latter practice is traditionally justified by optical metrology on Au thin films employed in HAMR NFTs, reporting material quality factors comparable to the literature values from the likes of Johnson and Christy or Palik. We find models assuming that these textbook Au optical properties can recover the experimentally verified link between high thermal gradient and good recording performance; however, the models fail to reproduce experimentally observed NFT power requirements and absolute thermal gradient values. The model-experiment gap can be closed by revising the optical properties of the NFT Au in the model such that the material quality factor is reduced by a factor of 3-4. Physically, this downward revision may be attributed to the ubiquitous use of metallic adhesion layers between NFT Au and the surrounding dielectrics. The reduction in the optical quality of the NFT Au has significant implications for NFT design strategy as high thermal gradients remain attainable but the path to attaining them involves unconventional NFT design choices. It is also worth noting that the significant reduction in the material quality factor of current gold-based NFTs opens the door for the use of other materials previously scorned for being substantially optically inferior to gold.