Dynamic microwaviness measurements of super smooth disk media used in magnetic hard disk drives

Recent technological advances in magnetic storage suggest the feasibility of extremely high-density magnetic recording up to 1 terabit per square inch (1 Tbit=10¹² bits) areal densities. Modelling indicates that approximately 3 nanometers (nm) of physical head-disk spacing is required for such high recording densities. When the recording slider is flying at such ultra low spacing over a high-speed rotating disk, it is experiencing disturbances from various different sources and of a wide frequency range. These disturbances may cause the recording slider to vibrate significantly, a condition that is known as fly height modulation (FHM), which may result in data loss. A significant source of excitation is from the surface irregularities of the rotating disk and is termed dynamic microwaviness. The term dynamic microwaviness has been introduced recently to differentiate from regular topographical features that are measured statically. In this paper, the procedure for making reliable dynamic microwaviness measurements of disk media used in hard disk drive (HDD) systems is described. Furthermore, such measurements are performed on different super smooth magnetic disks that are intended for extremely high recording densities using non-contact laser vibrometry. The root-cause of the dynamic microwaviness is investigated by measuring disk topographical features under static conditions and the interaction with system dynamics. It is found that dynamic microwaviness is primarily due to topographical features of spatial wavelengths ranging from 58.8 to 250 μm, and secondarily due to system dynamic effects.