Hard disk read-heads

In hard disk read heads, one ferromagnetic layer has its spin orientation fixed by coupling to an antiferromagnetic layer (Figure 9.13). A second ferromagnetic layer separated from the first by a nonmagnetic metal, is free to change its spin orientation when a field is applied. 

FIGURE 9.13 Two ferromagnetic layers, as in a magnetic hard disk read head, illustrating pinning hy an antiferromagnetic layer. 

The electronic and magnetic properties of nanolayers are important in devices formed from electronic materials that are more conventional. We have already discussed quantum well lasers (see Chapter 8) and giant magnetoresistance (GMR) devices used for hard disk read heads (see Chapter 9). Quantum well lasers may be an important component of light-based computers. Other possibilities include magnets with unusual properties (Section 11.2). 

In contrast to the discussion above with amorphous barriers, it is possible to use first-principles electron-structure calculations to describe TMR with crystalline tunnel barriers. In the Julliere model the TMR is dependent only on the polarization of the electrodes, and not on the properties of the barrier. In contrast, theoretical work by Butler and coworkers showed that the transmission probability for the tunneling electrons depends on the symmetry of the barrier, which has a dramatic influence on the calculated TMR values [20]. In the case of Fe(100)/Mg0(100)/Fe (100) the majority of electrons in the Fe are spin-up. They are derived from a band of delta-symmetry. In 2004 these theoretical predictions were experimentally confirmed by Parkin et al. and Yusha et al. [21, 22]. Remarkably, by 2005 TMR read heads were introduced into commercial hard disk drives. 

The second contribution spans an even larger range of length and times scales. Two benchmark examples illustrate the design approach polymer electrolyte fuel cells and hard disk drive (HDD) systems. In the current HDDs, the read/write head flies about 6.5 nm above the surface via the air bearing design. Multi-scale modeling tools include quantum mechanical (i.e., density functional theory (DFT)), atomistic (i.e., Monte Carlo (MC) and molecular dynamics (MD)), mesoscopic (i.e., dissipative particle dynamics (DPD) and lattice Boltzmann method (LBM)), and macroscopic (i.e., LBM, computational fluid mechanics, and system optimization) levels. 

V) and gradual breakdown processes [53]. The ability to produce low RA barriers reliably is a key for their application as read heads in hard disk drives. 

There are many ways of converting Is and Os to flux transitions. The simplest way is to interpret the presence of a flux transition as a 1 and the absence as a 0. Because this was the most obvious choice, the first hard disks (ST-506, ESDI types) used this method of encoding, known as Frequency Modulation (FM), and its cousin Modified FM (MFM). This worked well until techniques for increasing the track/cylinder density became almost too successful. What happened was that tracks would be placed so tightly together that at higher speeds, the read/write heads would affect not only the track immediately under the head, but the adjacent ones as well. 

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