Absolute head media spacing measurement in situ

For the nanometer-spaced head disk interface, the demand for precision flying height (FH) measurement increases. The conventional optical FH testing technology is facing physical challenges, and the industry is seeking the alternative technology for nanometer FH test. This work proposes to use logarithmic harmonic ratio of reproduced waveform versus testing frequency to derive absolute head media spacing (HMS) in situ. The linearization process on logarithmic harmonic ratio is conducted to eliminate the gap loss term. After linearization, the calculated spacing is measured from the sensor surface to the bottom of the recording layer. If the fixed spacing between head and media is known, the physical FH can be measured through this method.