An important aspect of the development of mankind has been the ability to store information. Ancient cave paintings, hieroglyphs on pyramids in Egypt, the eight-century archival scrolls of the Shosoin treasury in Nara (Japan), or medieval book printing remind us of the strong desire of mankind to preserve knowledge and information for future generations. The past century has seen the development of a plethora of new techniques for storing huge amounts of data, in particular in connection with computer systems. As of today, the most commonly used re-writable data storage technologies are (i) semiconductor-based integrated circuit memories, that are often volatile, (ii) magnetic storage, that is non-volatile and uses different magnetization patterns in magnetic media, and (ii) optical storage, that is also non-volatile and uses magnetoptical and phase-change media. In these two lectures, we will discuss the materials aspects of advanced magnetic data storage and of advanced optical data storage.
The key magnetic recording components of hard-disk drives are magnetic media (where information bits are storde) and recording heads (which write to and read from the media). Magnetic media comprise a thin-film structure consisting of several non-magnetic and magnetic thin films, capped with a thin (sub-5-nm) carbon film coating, and a thin (sub-1-nm) lubricant layer. The information is stored in the CoCr-based thin film located close to the disk surface in the form of magnetic domains with perpendicular magnetic orientation, which are also called bits. The bits which are produced or written with a magnetic recording head that utilizes a write head to produce a varying magnetic field and write bits to the media. The bits are read back with the magnetic reading head that utilizes the giant magntoresistance effect to register changes in the magnetic orientation of the written bits. The dramatically increasing capacity of hard-disk drives has been achieved by improving the bit areal density to preserve constant volume, weight, and power demands of storage. This required several breakthroughs in materials science that allowed the introduction of new technologies, such as the giant magnetoresistance and magnetic tunnel junction heads and the corresponding magnetic recording media (nanoengineered composite magnetic multilayers).
Two different classes of re-writable optical storage media were initially used, namely magneto-optical and phase-change media. In recent years most re-writable products have used phase-change materials, which consist of chalcogenide alloys of Ge, Sb, and Te. The storage concept of phase-change materials is based on the rapid switching between amorphous and crytalline phases for millions of cycles by applying appropriate heat pulses (optical or electrical). The optical absorption edge of the material shifts to longer wavelength. Amorphization is achieved by a short heating pulse (10 ns) above the melting temperature of 600 C with subsequent rapid cooling, while recrystallization is achieved by a slightly longer heat pulse (100 ns) below the melting temperature but above the glass-transition temperature of 200 C. The challenges for ultrahigh-density storage are the reproducible writing of ultrahigh-density bit patterns on the one hand and the writing/recording optics on the other hand. The amorphous and the crystalline state of phase-change materials also exhibit a pronounced difference in electrical resistivity that forms the basis for nonvolatile memory applications.
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