The head disk spacing in state of the art hard disk drives has been successfully scaled down to a couple of nanometers and is accurately controlled, providing a workable condition for near field devices, such as near field transducers (NFT) and plasmonic sensors. Heat assisted magnetic recording (HAMR) uses an NFT to generate the localized electromagnetic field to heat the media above the Curie temperature. The coupling between the NFT and the magnetic media has a strong spacing dependency, explained by the nature of the evanescent field. In this paper, we discuss how the spacing dependency is accurately calibrated and the applications of this spacing dependency in HAMR, such as static and transient protrusion measurement and compensation and fly height variation measurement.

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