Nanoscale Heat Conduction in Data Storage Technology

The magnetic data storage industry has followed a similar density (and data rate) improvement curve as the semiconductor technology (Moore's Law) for the past decade. However, whether the storage densities will continue to increase at this rate and be able to keep up with the improvements in processor technology is under a near term threat resulting from the fundamental physics up on which the hard disk drives are based. It is expected that novel, more unconventional technological solutions become necessary to overcome limitations, however, many of these technologies rely heavily on heating and energy transport at extremely short time and length scales. It is widely believed that further advances in high-technology data storage systems will be difficult, if not impossible, without rigorous treatment of the nano-scale energy transport. The nano-scale heat transfer research effort at Data Storage System Center (DSSC) has been focused on three interwoven areas of thermal design, failure analysis, and metrology of micro/nano-devices and structures relevant to data storage technologies. In this presentation, underlying physics and fundamentals of heat transport at nanoscale will be discussed. In addition, applications of the nanoscale heat transfer to the thermal analyses of the magnetic and phase change optical data storage technologies will be presented. The problem facing the HDD industry is due to the continued shrinkage of the dimensions of the bit cells on the disk. Each of these individual magnetic data bit cells is comprised of a collection of smaller crystalline grains of a thin magnetic film, each of which is uniformly magnetized. In order to maintain adequate signal-to-noise ratio for reliable data recording and retrieval, it is necessary to keep the number of such grains per bit cell adequately large. This has required the size of the grains to be reduced with increasing AD. The superparamagnetic effect becomes important when the grain volume V is so small, that the inequality KuV/kBT >40 can no longer be met [1]. Here Ku is the material’s magnetic crystalline anisotropy energy density, kB is the Boltzmann’s constant, and T is absolute temperature. When the above inequality is not satisfied, thermal energy demagnetizes the individual grains and the stored data bits will not be stable. Therefore, as we make the grain diameter smaller with increasing AD, we reach a regime for a given material Ku and T, such that reliable data storage is no longer feasible. With the current pace of miniaturization, some experts believe the industry could reach this barrier as early as 2005. Data storage opportunities using nanoscale thermal effects arise within system architectures that overcome the superparamagnetic effect by using thermally assisted writing schemes (e.g., hybrid thermomagnetic disk recording systems, thermally assisted scanned probe magnetic recording systems, and cantilever probe based thermomechanical recording systems) [2]. The data storage industry’s ambition to improve AD and data rates has pushed the characteristic length and timescales of these devices and processes down to the nanometer and picoseconds range, respectively. If a small quantity of heat is generated over a period of time less than the thermal time constant of the magnetic element or device, then heat travels only a few nanometers away during the heating pulse. The energy must therefore be absorbed by a very small volume, which dramatically increases the temperature in the device, structure, or system. Such heating phenomena will be vastly underestimated by continuum theory and could melt the magnetic element, otherwise damage it, or cause long-term reliability concerns. The fundamentals of energy transport (by phonon, electron, lubricant molecules and gas molecules) in nanostructures and nanoscale components of the data storage devices/systems are not well understood at present. Further advances in the relevant high-density data storage systems will be difficult, if not impossible, without rigorous treatment of energy transport at extremely short time and length scales. This manuscript shows that the fundamental knowledge developed through nanoscale thermal characterization and research can be used to realize the technological opportunities and addresses the technological threats in data storage devices and systems.