SPM in Data Storage

     Atomic force microscopy and its enhancements can be useful in almost all fields where applied surface analysis or surface characterization is needed. Data storage is one of these fields. It represented mainly by magnetic recording media, in particular by hard disk drives and magneto-optic devices. Also, novel approaches in data storage technologies are being intensively developed in which not only AFM but also Scanning Tunneling Microscopy (STM) is employed.

     The principles of magnetic recording didn't change since the invention of this way of information fixation. Let us consider hard disk drives being the most commonly used data storage device in our days.

     Two main elements constitute every hard disk. These are read/write head usually called "slider" and magnetic disk resembling multilayer sandwich. The head consist of writing and reading modules being closely nearby. The writing module is a sophisticated solenoid working on the inductive principle. The reading module comprises special giant magnetoresistive or magnetoimpedance sensor (GMR-, GMI-sensor) that is a multilayer composition of magnetic and nonmagnetic layers. And the hard disk itself contains a number of seed layers, magnetic layers and protective layers on the appropriate substrate (Fig. 1).

 

Fig. 1 Schematic of magnetic recording and reading

       As well known, information on hard disk stored in terms of bits - microscopic (300 nm or less in width) areas having or not having local magnetic moment thus expressing "high" or "low" level of digital signal (Fig. 2). The writing module operates inducing local magnetic moments in bit areas of hard disk magnetic layer. Conversely, bits having remanent magnetization cause measurable change in resistance of GMR-sensor of reading module enabling to distinguish between two levels of digital signal.

 

Fig. 2.1 Topography AFM image Fig. 2.2 MFM image of recorded bits

     The progress in hard disk engineering followed by ongoing reduction of sizes of all the materials and modules involved in the process of magnetic recording. Suffice it to say, that thickness of each magnetic sublayer and protective carbon coating amounts to merely several nanometers and head hovers over disk surface at heights not exceeded 50 nm. Flying on so small heights requires extraordinary perfection of disk surface, absence of any defects and particles on it since presence of the smallest particle may result in severe damage of the surface with moving head. Moreover, the effects of nonuniform thermal expansion negligible in early stages of hard disk development nowadays interfere significantly in the mentioned processes.

     Thus monitoring of roughness and defectiveness of disk surface as well as magnetic head topography with nanometer accuracy is of vital importance in magnetic storage technology.

     Atomic Force Microscopy easily copes with these tasks along with another structure and surface analysis techniques. It was usefully applied to measure surface topography of hard disk, the topography of the slider in the region faced to hard disk surface. Reversible displacements of magnetic head layers due to thermal expansion can be observed via AFM under actual write operation conditions. It has been reported about several attempts to measure hardness of protective carbon coating (COC) in order to estimate its wear resistance. AFM scratching technique is successfully used to determine the scratch resistance of ultrathin protective coatings [1600-1602]. Using image subtraction, scratches down to a residual depth of 1 Å can be evaluated, enabling the study of the very beginning of plastic deformation [1103].

     Using cantilever with magnetic coating we acquire a powerful technique known as Magnetic Force Microscopy (MFM) for the characterization of bit structure of both hard disk and read/write head [1101, 1102, 1144, 1158, 1159, 1179]. For overview of Magnetic Force Microscopy applications to exploring and characterization of magnetic and magneto-optic materials see reports [1052, 1103, 1142].

     Magnetic force microscopy has become a powerful tool for mapping stray fields very close to the surfaces of magnetic materials since it features high lateral resolution. MFM now is the standard method to measure bit lengths and widths, and furthermore is accepted as one of the most precise techniques for the characterization of bit structure irregularities, which can be correlated with the overwrite and offtrack performance of the magnetic head as determined with regular performance testers [1101, E005]. MFM successfully employed for characterization of various magnetic carriers such as tapes [1133, 1189], longitudal [1166, 1179, 1188] and promising perpendicular recording media [1154, 1155, 1157, 1158, 1160, 1164, 1182] as well as GMR (GMI) and magnetoresistive materials for data readout [1100, 1109, 1127]. (Complete list of references to the articles devoted to MFM-related problems can be found in the Reference Collections section of our Library).

     Moreover, one can combine AFM characterization of disk failure regions with respect to topography and subsequent characterization of magnetic structure of these regions with MFM since the procedure of substitution of nonmagnetic cantilever with magnetic one takes a few minutes. Such combination is the best tool to analyze the reasons and details of disk failure.

     As was always mentioned above Magnetic Force Microscopy can be also successfully used for characterization of magnetic head. Clear correlations between the geometry of the yoke pole tips and the emerging write field distribution were found. Scanning a magnetized tip over the GMR sensor with varying tip-sensor distance while capturing the sensor's signal provides a method to map the three-dimensional sensitivity of the sensor [1103].

     It is turned out, that application of the AFM is not limited by using it only as a surface characterization technique. Principles of scanning probe microscopy themselves are of growing importance in respect to their possible use for information storage. A cursory glance to the design of read/write system in magnetic hard disk is enough to notice the substantial resemblance with the system of data acquisition in scanning probe techniques. Very high lateral resolution of about several nanometers reached in the last years due to miniaturization of main components of AFM and improvement of data acquisition techniques, looks now quite preferable if compared with the size of "the smallest" magnetic bit in commercially available storage media taking area of about 150x150 nm² (as of middle of 2002). Due to superparamagnetic restrictions [E007] usual magnetic storage media consisting of multigrain bits will soon reach their limit of ~ 50 Gbit/in². Along with promising patterned media technology [891, 1118, 1137, E006] non-magnetic AFM- and STM-based ROM and read/write techniques are quite perspective [1464 (see a brief description in SPM-based Nanotechnology section), 1593-1596].

     Bennewitz et al. [1354] discuss the limits of pushing storage density by means of STM to the atomic scale at room temperatures. It was tentatively shown that the smallest possible bit can be coded with a single silicon atom, positioned at lattice sites along self-assembled tracks with a pitch of five atom rows. These tracks were obtained by depositing 0.4 monolayers of gold onto a Si(111) surface at 700° C with a post-anneal at 850° C, thereby forming the well-known Si(111)5x2-Au structure. All images were taken by STM with a tunnelling current of 0.2 nA and a sample bias of -2 V. The writing process consists of removing Si atoms from a preformed nearly filled lattice as has been previously performed in well-controllable manner by Dujardin et al. [1203] in the study on removing germanium atoms from Ge(111) surface. As for readout, there is no need to search in two dimensions for the location for a bit. The signal is highly predictable since all atoms have the same shape and occur on well-defined lattice sites. It has been demonstrated that 5x4=20 atoms cell containing the only bit atom represents the smallest viable one for the underlying 5x2 lattice that keeps bit interactions under control and proves experimentally an early Feynman's prediction that spacing of five atoms between bits is the smallest affordable. The remaining 19 atoms are required to prevent adjacent bits from interacting with each other, which is verified by measuring the autocorrelation. One of the fundamental limitations to devices operating on the atomic scale is speed due to the fact that the signal decreases and becomes noisier especially at room temperatures. Estimated speed by means of one probe would be of 6·10⁶ points/sec, which is respectable but still slower than today's hard disks. The future speed enhancement could be achieved in application of parallelism to such systems. Development of single atom memory is an example of finest nanotechnology.

     Investigations in the field of AFM-based data storage are held intensively by IBM. Recently IBM researchers reported new technology called "Millipede" [162, 212, 1599] which prototype was explored in early 1990s by Mamin and Rugar at the IBM Almaden Research Center. Cantilever equipped with a heater on its tip makes indentation in a plastic substrate that stands for a logical "1". Erasing of data is performed by means of heating either entire plastic card or its local region. This approach for single lever allows for reaching densities of hundreds Gbit per square inch though at the expense of relatively low data transfer rates up to 10 Mbit/s. It has been reported on the development of a novel silicon cantilever having 6,6 Mhz maximum resonant frequency [163, 1597]. Using this cantilever and a prototype of AFM recording system with new detection schemes the same speed readout of above 5 Mbit/s was achieved. In principle, using many such cantilevers working in parallel, as was implemented in "Millipede" project, one can overcome low data transfer rates according to IBM research group [212, 1598]. In spite of the fact that areal bits density with arrayed cantilevers are about 5 time less than that of the single one, the results are quite encouraging to make efforts in this promising direction. May be in near future using cheap plastic pieces of postage stamp size containing as much as a library will be a usual thing.