Hard-drive read heads

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. 

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. 

Like the CD-ROM drives, the CD-R drives have relatively low performance (when compared with optical or hard drives). The seek times are on the order of a few hundred miUiseconds, whereas the maximum data rate for a 4X speed drive is about 600 kilobytes/s. The seek time is slow because the CD-R drives spin the disks in constant linear velocity (CLV) mode as defined in the Red Book standards. Constant linear velocity means that the disk rotation speed varies with the radius at which the read head is positioned in such a way as to ensure that the linear velocity is constant with radius. 

Even though the effect is small, magnetoresistance is used for sensing magnetic fields and for read heads on magnetic hard drives. 

Figure 20.23 Schematic diagram of a hard disk drive that uses the perpendicular magnetic recording medium also shown are inductive write and magnetoresistive read heads.

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. 

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