Low flying air-bearings, "sliders," with contact sensors are used to "glide" test magnetic recording disks to be free of asperities above a predetermined height. A technical overview of the considerations necessary for accurate glide testing is illustrated by the example of an experimental flat plate PZT sensor, with electrodes divided into quadrants, to detect asperity contact. The flat plate PZT sensor detects the slider dynamic pitch, roll, and vertical vibrations of the air bearing by contact with asperities of sufficient mechanical stiffness. The sensor also detects contact by the extremely sensitive response of the resonant vibrations of the PZT/slider structure. Different linear combinations of the signal from the quadrants show mode selection based on mode symmetry. The signal response for increasing asperity interference is characterized for specific modes and a mode can be chosen by the appropriate linear combination of the signals from the quadrants. Calibration of the glide slider trailing edge flying height and roll using contact with artificial bumps of different heights is necessary for accurate glide testing. One can map the entire disk surface using the contact signal from both the air-bearing response and the bending mode response simultaneously to identify mechanically "stiff" asperities. Visualization of the mode shapes and characterization the PZT/slider structure using a laser heterodyne interferometer aid in the consistent fabrication of this class of PZT contact sensors. The considerations of sensor response, characterization of the contact signal, techniques to allow consistent PZT/slider fabrication, and calibration methods that allow the signal to be related to the test tolerances represent a technical overview of the requirements for any glide sensor technology.

As the hard disk drive industry is transitioning from longitudinal (LMR) to perpendicular (PMR) recording, a new set of reliability challenges had to be overcome. In particular, the magnetic media structure, which relies on well-segregated grains of 6-10 nm diameter, can exhibit a rough structure, with peak-to-mean amplitude of 3-6 nm. In this paper, we will discuss how this topography could affect the overall reliability of the head-disk interface. In the first part, we will illustrate the loss of adequate coverage from the overcoat on PMR media, compared to the smoother LMR media, and we will attempt to quantify the topography in terms of its deviation from a Gaussian height distribution. Particular emphasis will be given to surface outliers and their removal during the burnishing process. The second part will be devoted to the lubricant preferential migration to the grain boundaries, driven by surface tension. It will be shown by an EELS line scan that the lubricant film is indeed thicker in the valleys between the grains, in agreement with surface tension driven redistribution. Finally, we will demonstrate that the Touchdown Height (TDH) of a PMR disk is $0.5 nm higher than its LMR counterpart, owing to its enhanced nano-roughness. Once recognized, these challenges can be overcome through a careful and thorough optimization of the various processing parameters, eventually leading to an overall reliability level equal or better than LMR media.

As the magnetic recording density increases towards hundreds of Gb/in2, both the magnetic spacing and head-disk clearance decrease to < 10 nm. By one estimate, the magnetic spacing for 1 Tb/in2 is about 6 nm and the read width is ∼ 30 nm. There are at least two different approaches to achieving this. The first one is an extension of the traditional flying interface and the second is contact recording. In the former case one needs to be concerned about maintaining adequate clearance both at sea level and at higher elevation whereas in the latter case the wear and corrosion of the heads and disks may pose major challenges. In the flying regime, an accelerated test to assess the relative integrity of the head-disk interface is described here. This is accomplished by monitoring the acoustic emission, capacitance or friction between the head and the disk as the ambient pressure is reduced. The pressure at which an abrupt change in the above signals takes place is called take-off pressure (TOP). This is also known as altitude avalanche measurement. With this method it is possible to compare different disk and head designs at the full velocity of the slider. We present results correlating the TOP with disk roughness and the influence of disk lubricant. An example of how head-disk interference takes place in a disk drive will be given for an experimental 10 nm flying slider. The effects of radial flying height profile, take-off height of the disk, and the disk curvature on mechanical spacing are presented. The results of changes occurring on the air bearing surface and the disks after long term flyability test are discussed.

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