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The behavior of a thin lubricant film under a flying head has been studied by examining the lubricant redistribution on the disk after flying, using an HDI-SRA instrument. Lubricant depletion tracks were observed on the disk surfaces and, more interestingly, the lubricant film was found to exhibit a periodic thickness modulation in the downtrack direction. The wavelength of the lubricant thickness modulation is found to increase linearly with the disk linear velocity, and depends weakly on the lube type and lube bonding ratio. The amplitude of the modulation grows slowly with flying time. Both negative-pressure air bearing pico sliders and catamaran-style positive-pressure nano sliders generate similar patterns on lubricant films. The frequency of the thickness modulation is in the range of 40-50 kHz, and is attributed to interactions of the disk lubricant with the slider roll mode. In addition, for highly-bonded lubricant films, a much finer lubricant modulation pattern can be seen with a frequency of 196 kHz, which is very close to the frequency of the pitch mode of the slider. These results indicate that the lubricant thickness modulations on-the disk are generated by slider-disk dynamic interactions, and are due to slider body motions.