Spatial and Temporal Profiling of Protrusion in Magnetic Recording Heads

Abstract

An increase in external temperature or internal heating by writing can induce protrusion or strain of thin film layers in a magnetic recording head. To profile the protrusion, the surface topography and the temperature distribution are imaged in fixed heads by scanning probe and optical microscopy. Near the poles and the shields, the protrusion is at a peak, producing a risk of head-disk interaction. The dynamics are elucidated in flying heads over a disk by measuring the amplitude of a reference pattern with the read head. The temporal profiling technique can be applied both in component testing on spinstands and reliability testing in fully integrated disk drives. For either heating or cooling of the heads, the characteristic time constant for protrusion is 100 s and reaches 95% of the plateau after 1 ms. The protrusion is linear with temperature and is reversible, implying that the strains are within the elastic limit. Furthermore, variation of the write pulse shape suggests that Joule heating is more significant than eddy current or hysteresis losses in inducing protrusion.Index Terms-Magnetic heads, magnetic recording mechanical factors, strain, thermal protrusion.

Slider dynamic motion during writer-induced head-disk contact

“…It should be stressed that we are generating more heat than is generated during normal operation, and the longer time constants may reflect the long transient associated with heating a larger head volume. There do exist previous observations of thermal protrusion that occurred over several ms (Yan 2002;Nikitin et al 2004), which support the view that the time constants are thermal and not hydrodynamic transients.…”

“…b The average magnitude of the measured peak as a function of write current is plotted. Down-track and pitch motions show an increase in magnitude, while other motions do not show an increase in the magnitude of motion with increasing current estimated from measurements of head-media spacing using the amplitude ratio method (Klassen and van Peppen 1994;Nikitin et al 2004). As the clearance increases, the time between the write gate edge and the onset of motion increases.…”

“…These effects have been demonstrated in the study of head disk contact (Knigge et al 2005(Knigge et al , 2006 and they have been modeled using solid mechanical models (Gupta et al 2001;Pust et al 2002). The results of these are that several nanometers of writer pole tip protrusion (W-PTP) can be expected at reasonably high currents (Nikitin et al 2004).…”

“…For example, the role of media roughness on touchdown-take-off hysteresis was examined using W-PTP (Knigge et al 2006). Physical properties that have shown measured changes due to W-PTP are the temperature rise in the magneto-resistive sensor, the amount of protrusion (Wang et al 2001;Nikitin et al 2004), or the level of a contact detection signal, such as acoustic emission (Knigge et al 2006).…”

“…This increase in impedance with frequency gives increasing power dissipation and protrusion. Moreover, overshoot makes an additional contribution to the increase in power with increase in frequency (Nikitin et al 2004). Thus we expect an increase in protrusion with an increase in each of these variables.…”