In modern computers, the hard disk encodes information in the form of a linear array of planar ferromagnetic regions, or bits. The performance has recently improved with the discovery of the Giant Magnetoresistance (GMR) effect allowing a smaller assembly (read head), to scan the magnetic data. The ferromagnetic bits are written into (and read from) the disk surface, which is uniformly coated with a ferromagnetic film having a small coercive field. This is a "soft" ferromagnet, so that a small imposed magnetic field B can easily establish the ferromagnetic magnetization M along the direction of the applied B field. Both writing and reading operations are accomplished by the "read head".
The density of information that can be stored in a magnetic disk is fundamentally limited by the minimum size of a ferromagnetic "domain". Ferromagnetism is a cooperative nanophysical effect requiring a minimum number of atoms: below this number the individual atomic magnetic moments remain independent of each other. It is estimated that this "super-paramagnetic limit" is on the order of 100 Gb/in2.
The practical limit, however, has historically been the size of the "read head", as sketched in Figure 1.5, which on the one hand impresses a local magnetic field B on the local surface region to create the magnetized domain, and then also senses the magnetic field of the magnetic domain so produced. In present technology, the linear bits are about 100 nm in length (M along the track) and have widths in the range 0.3 - l.OjLim. The ferromagnetic domain magnetization M is parallel or anti-parallel to the linear track.
The localized, perpendicular, magnetic fields B that appear at the junctions between parallel and anti-parallel bits are sensed by the read head. The width of the transition region between adjacent bits, in which the localized magnetic field is present, is between 10 and 100 nm. The localized B fields extend linearly across the track and point upward (or down) from the disk surface, as shown in Figure 1.5.
The state-of-the-art magnetic field sensor is an exquisitely thin sandwich of magnetic and non-magnetic metals oriented vertically to intercept the fringe B field between adjacent bits. The total thickness of the sensor sandwich, along the direction of the track, is presently about 80 nm, but this thickness may soon fall to 20 nm. The GMR detector sandwich is comprised of a sensing soft ferromagnetic layer of NiFe alloy, a Cu spacer, and a "magnetically hard" Co ferromagnetic film. A sensing current is directed along the sandwich in the direction transverse to the track, and the voltage across the sandwich, which is sensitive to the magnetic field in the plane of the sandwich, is measured. In this read-head sandwich the Cu layer is about 15 atoms in thickness! The sensitivity of this GMR magnetoresistive sensor is presently in the vicinity of 1% per Oersted.
Writing the magnetic bits is accomplished by an integrated component of the "read head" (not shown in Figure 1.5) which generates a surface localized magnetic field B parallel or antiparallel to the track. The local surface magnetic field is produced inductively in a closed planar loop, resembling an open-ended box, of thin magnetic film, which is interrupted by a linear gap facing the disk, transverse to the track.
These "read head" units are fabricated in mass, using methods of the silicon lithographic microtechnology. It is expected that even smaller sensor devices and higher storage densities may be possible with advances in the silicon fabrication technology.
These units, which have had a large economic impact, are a demonstration of nanotechnology in that their closely controlled dimensions are in the nm range. The mechanism of greatly enhanced magnetic field sensitivity in the Giant Magneto-resistance Effect (GMR) is also fully nanophysical in nature, an example of (probably unexpectedly) better results at the quantum limit of the scaling process.
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