An Experimental and Simulation Study of Touchdown Dynamics

IEEE Transactions on Magnetics (Impact Factor: 1.42). 11/2011; DOI: 10.1109/TMAG.2011.2158601
Source: IEEE Xplore

ABSTRACT Dynamic flying height technology has been widely employed for reducing the mechanical spacing between the magnetic heads and the disk. As the recording density of hard disk drives approach 1 Tbit/in2, the spacing is decreased to sub 1 nm. At such low spacing, the touchdown (TD) dynamics becomes extremely critical. First, it affects the accuracy of the spacing setting, or TD detection. Second, it affects the hard disk drive reliability, such as writing modulation, wear, instability, etc. It decides how low the slider can fly stably and reliably. In this paper, we tried to have a better understanding of the TD dynamics with several designs of experiments first. We found that there were two stages in the whole TD process. In the first TD stage, a low frequency (30-150 kHz) vibration appeared. It was a suspension mode that was excited by the lubricant on the disk. In the second stage, a high frequency vibration (200-400 kHz) appeared. It was the second pitch mode of the slider air bearing excited by the contact between the slider and the disk. Based on these experimental observations, we propose modeling and simulation procedures with a combination of a full suspension model and a simplified air bearing model. Simulation can predict these two TD frequencies very well. Therefore, it could be applied to design head/disk interface to help achieve preferred TD behaviors.

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    ABSTRACT: With the advent of thermal flying-height control (TFC) technology the physical spacing in magnetic hard disk drives is now reduced to sub-1 nm. At this spacing the tribological flyability and reliability become critical issues for the performance of the read-write head. In this paper a numerical approach is applied to study the touchdown dynamics of TFC sliders by expanding a 3-degree-of-freedom slider dynamics model to a 250-degree-of-freedom head-gimbal assembly dynamics model and considering several significant tribological effects at this ultra-low clearance (adhesion, tribo-charge, friction, contact, etc.). The simulation results show that the slider's vibration amplitude increases substantially at the beginning of touchdown, and it gets suppressed with further reduction in flying height. Analysis in the frequency domain shows the excitation of the second air-bearing pitch mode when instability occurs. The expansion of the dynamic system to include a realistic model of the suspension is shown to be important in determining the frequency of the excited mode. Adhesion force is shown to play an essential role in exciting the second air-bearing pitch mode and causing instability, while electrostatic force and friction force affect only some details of the slider's dynamics at instability. Friction force is also shown to be related to the excitation of the first air-bearing pitch mode.
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    ABSTRACT: To achieve the areal density of 1 TB/in<sup>2</sup> in hard disk drives (HDDs), the magnetic spacing between the slider and disk needs to reduce to 1 nm or smaller. At such small clearance, the contact between the slider and disk is inevitable. Slider-disk contact may induce a drastic slider response, which has a significant negative impact on the reading/writing performance. In this paper, the effect of a low-frequency vibration in the Z-direction (out-of-plane) brought by the tip of gimbal (TG) on slider dynamics is studied. The impacts of three different excitations on the TG resonance are explored. Under such excitations, the bending mode of TG can be easily excited, which consequently induces the vigorous slider vibration. The influences of the air-bearing stiffness on slider dynamics subject to TG mode are also studied. A slider designed with weaker air-bearing stiffness will suffer a stronger TG resonance than that of stiffer air bearing. These experimental and simulation results show that better TG design and more strict environmental or excitations control are required to achieve a robust head disk interface (HDI).
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    ABSTRACT: We conduct three-dimensional transient finite-element analysis to study the contact behavior during touchdown detection by a thermal flying-height control (TFC) recording head on continuous and patterned elastic-plastic layered media. The heat generated during touchdown and the plastic strain of the media are calculated in the model. We investigated key factors such as the radius of curvature of the TFC protrusion, media compositions, bit-patterned media (BPM), and the effect of planarization. Our analysis shows that when subjected to the same TFC over-push, BPM is much more likely to result in plastic deformation than the continuous media. The temperature distribution of planarized BPM with ${hbox{SiO}}_{2}$ as filling material exhibits a complex and distinctive pattern different from the one without planarization. More importantly, the maximum plastic strain of the planarized BPM is 50% larger than the one without planarization, which means that filling with ${hbox{SiO}}_{2}$ deteriorates the media's robustness to the touchdown probably due to the mismatch of thermal properties between ${hbox{SiO}}_{2}$ and recording material. This suggests the filling material must be carefully chosen to avoid the excessive plastic strain.
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