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Write-induced pole-tip-protrusion (WPTP) and its effect on head-disk interface (HDI) clearance are studied. Drives are placed in a vacuum chamber and the contact altitude is used in this study together with triple harmonic method. It is found that write-induced PTP is a function of the write-signal frequency and the write duration of the head. The magnitude of the WPTP is on the order of nanometers. For a state-of-the-art HDI design with nominal flying height around 10 nm, WPTP can reduce the HDI clearance dramatically. It can cause write modulation and altitude failure. In order to improve the HDI reliability, WPTP must be taken into account.