This paper presents a nonlinear dynamic analysis of the head-disk interface by including intermolecular adhesion forces for sub-5-nm flying air-bearing sliders. Experimental evidence shows that one of the major roadblocks in achieving ultralow flying heights is the stability of the head-disk interface. It is found that the inclusion of intermolecular forces between the slider and disk in modeling the head-disk interface leads to dynamic instability of the slider. A parametric study is conducted showing the dependence of stability/instability on the variables. By understanding the effect each parameter has on stability, we can achieve air-bearing surface and disk morphology system design guidelines. From this study, it is found that the head-disk interface can become unstable due to intermolecular forces below a flying height of about 6 nm. However, from the results of the parametric study, it is shown that a head-disk interface can be designed such that it maximizes stability, although the instability cannot be attenuated completely. By minimizing the intermolecular adhesion forces and the flying-height modulation, and by maximizing the air-bearing stiffness and damping, we achieve maximum stability. Also, it is found that the stiffening effect of the air-bearing film increases the stability. The implications of this study are that the head-disk interface stability is dramatically compromised in the sub-6-nm flying-height regime and that the glide height of "super-smooth" disks will not only be a function of the disk's morphology, but also the intermolecular adhesion force induced instability of the slider.
A Laser Doppler Vibrometer (LDV) was used to measure flying height modulation (FHM) of sliders with sub-10 nm flying-heights (FH). It was found that a precise trigger, averaging, and suitable filtering are key to successfully measuring FHM by LDV. Also, more accurate results can be obtained from the LDV velocity output as opposed to the displacement output. The FHM's of a 7-nm FH slider flying over three different disks were measured. One of the disks had higher roughness and waviness values (disk A) than the other two (disks B and C). Disks B and C had the same super-smooth substrate but different lubricants and carbon overcoats. It was observed that this slider flew steadily over disk A and disk C, but it could not fly over disk B. The repeatable part of the FHM of the slider flying over disk A and disk C was about 0.45 nm and 0.37 nm (RMS), respectively, in the frequency range between 20 kHz and 300 kHz. Also, for disk C, the dependence of FHM on RPM was investigated, and it was found that at the design condition (7200 RPM) the FHM (peak-to-peak) was minimized for this particular slider/disk system. However, we do need to consider the ratio of FHM to FH. Increasing RPM increases FHM due to the disk surface topography and decreasing air-bearing modal frequencies, but the ratio of FHM to FH stays relatively constant. Decreasing RPM increases FHM due to intermittent contacts and excitation of the air-bearing.Index Terms-Flyability, flying height modulation, head/disk interface, LDV.
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