Minimum Stable Flying Height with Thermal Protrusion Actuation B. Knigge, O. Ruiz, P. Baumgart, Senior Member, IEEE 1 Abstract—With ever increasing areal densities in magnetic recording, novel methods to accurately control the clearance between slider and disk are needed. One recently introduced method is thermal protrusion of a heater element located close the read/write element at the trailing end of a slider. By applying an electric current to the heater coil, the slider’s trailing end protrudes towards the disk and can be driven into contact with sufficiently high heating power. Understanding the contact dynamics and the touch-down/take-off hysteresis is an important aspect of controling the minimum stable distance between the slider and disk. In this paper, a novel method to measure touchdown and take-off hysteresis with a single pulse applied to the heater is introduced. Furthermore, the effect of air bearing compliance as a function of thermal protrusion is investigated. By modifying the pressure profile of the air bearing surface, improved actuation efficiency can be achieved. Index Terms—Air bearing surface, hysteresis, thermal protrusion I. INTRODUCTION Latest generations of disk drives available in the market feature active control of slider-disk spacing by using a thermal heating device to produce a desired amount of localized thermal protrusion in the region of the read/write elements. The feature is referred to as “thermal flying height control” (TFC). To accurately control the clearance between the slider and disk, the amount of thermal protrusion needs to be calibrated to spacing change. This can be done using the magnetic read- back signal amplitude change (Wallace spacing loss formula). Kurita and Suk found a linear relation between clearance change and the amount of power applied to the heater coil for various heater designs [1, 2]. Typical values range from 5 - 20mW / nm. It is certainly advantageous to have Manuscript received August 20, 2006. B. Knigge is with Hitachi Global Storage Technology, San Jose Resarch Center, San Jose, CA 95120 (ph: 408-323 7254, fax: 408 323 7334, email: Bernhard.Knigge@hitachigst.com ) O. Ruiz is with Hitachi Global Storage Technology, San Jose, CA 95119, (email: Oscar.ruiz@hitachigst.com ) P. Baumgart is with Hitachi Global Storage Technology, San Jose Resarch Center, San Jose, CA 95120 (email: Peter. Baumgart@hitachigst.com ) large flying height (FH) changes with minimum amount of power applied to the heater coil. Increasing the thermal protrusion efficiency can be achieved by reducing the pressure peak around the trailing end [3]. This can be done by changing the dynamic slider pitch angle or adding a ‘pressure relief’ channel as proposed by Strom et. al. [4]. The proposed technique to investigate the touch- down take-off hysteresis is based on pulsed thermal protrusion. The contact area and contact dynamics are significantly different compared to previously established methods such as disk spin-down/spin- up or pressure pump-down/pump-up as discussed by Ambekar and Raman. [5,6]. Furthermore, the duration of the induced contacts can be controlled with high precision down to ~ 1 msec making this technique highly repeatable and less destructive than disk spin-down or pressure pump-down. II. NUMERICAL RESULTS Clearance change with thermal protrusion was simulated for various air bearing designs. To investigate the slider to disk compliance at various levels of interference, the disk surface topography was measured using a laser heterodyne interferometer. The data was then filtered and fed into a finite difference based dynamic air bearing simulator solving the generalized Reynolds equation. Figure 1 shows the air bearing pressure distribution with and without thermal protrusion. Figure 1. Air bearing pressure distribution with and without thermal protrusion The nominal flying height for this slider is 9nm without protrusion. With 8nm thermal protrusion, the maximum pressure at the trailing end increases from ~10 atm to about 19 atm and the flying height drops from 9nm to 4.5nm. Ie, the protrusion compensation of this airbearing is about ~44%. Figure 2 shows the pressure profile cross-section along the x axis at y=500um, and a cross-section along the y axis at x=1215um (near the read write