Particulate Recording Media
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domain structure. The magnetizable layer should be thought of as containing small, s i n g l e - d o m a i n m a g n e t i c units. Each one usually has two stable directions of magnetization, oppositely directed and collinear with a "preferred" axis. Each domain thus has a "binary" nature and can be switched between its two states by a sufficiently strong magnetic field; the minimum field is called the switching field. The vector sum of the magnetization directions of thèse units détermines the magnitude and direction of the magnetization intensity in the recording médium. The size of the switching units is very important; they must first of ail be small enough to permit recording the intended magnetization pattern with adéquate spatial resolution. This means that they should be no larger than the smallest feature of this pattern and should preferably be much smaller. 6 A further reason for minimizing the size of the switching units is that their discrète nature gives rise to random noise in the output signal. Decreasing the switching unit size decreases the magnitude of this noise relative to that of the desired signal.6-7 In principal, one could envision a recording System in which each domain, or magnetic unit, would encode by its magnetization direction one "bit" of digital information. In practice, finding or addressing individual domains présents problems that at this time seem insurmountable. Furthermore, actual surfaces can be anticipated to contain defects capable of deleting the information in a large number of thèse units. Thèse practical considérations dictate that actual recording Systems will be designed to hâve a large n u m b e r of domains per bit, or other important feature, of the recorded signal for some time to corne. Advances in information storage density will involve making the domains as small as possible; this will allow increases in the track density and the transition density along the tracks.
In advanced recording technology, the track width is currently about 10 ^tm, the distance between transitions (along a track) about 1 fim, and the domain dimensions substantially less than 1 fim. In addition to an appropriate domain structure, a recording médium must hâve désirable bulk magnetic properties, which are described in Figure 1. The rémanent magnetization M r is very important. It is the maximum magnetization intensity that the material can retain and therefore correlates with the field that can be available for sensing by the read head. The value of M, dépends on the saturation magnetization M s and also on how well the preferred axes of the domains are aligned with the direction in which the magnetization is being measured. This alignment is expressed by the "squareness" ratio, MJ Ms. The coercivity Hc is approximately the médian value of the switching fields of the domains in the material; it characterizes the difficulty of achieving magnetic reversai. If the coercivity is too high, available heads will not be able to adequately write or erase information. If the coercivity
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